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EFFECTIVE RADIATION DOSE TO STAFF MEMBERS DUE TO MYOCARDIAL PERFUSION SPECT IMAGING: TRACKING THE EXPOSURE FROM PREPARATION TO PATIENT RELEASE

EFFECTIVE RADIATION DOSE TO STAFF MEMBERS DUE TO MYOCARDIAL PERFUSION SPECT IMAGING: TRACKING THE... Abstract The aim of the work was to tack the radiation exposure from dose preparation down to patient release during nuclear cardiac imaging. A total of 111 patients were recruited and external exposure was measured using a standard calibrated survey meter. Five phases have been determined including dose preparation, injection, post-injection, patient setup and patient release. An average effective dose of 0.87 ± 0.2, 8.1 ± 2.0, 12.3 ± 2.1, 8.7 ± 1.9 and 7.1 ± 1.3 μSv were recorded as staff external exposure due to patient injection, post-injection (rest), recovery phase (stress), patient setup and patient release, respectively. On average, patient-to-staff effective dose rate coefficient was 0.04 ± 0.007 μSv MBq/m2 h. Post-stress phase is the most critical time point where staff members receive the highest radiation dose. The study has mapped the relative contribution of radiation doses and can also be used as simple prediction model in facility design and local rules as well as safety measure. INTRODUCTION Nuclear based myocardial perfusion imaging (MPI), x-ray cardiac computed tomography (CT), interventional cardiology procedures are among those diagnostic modalities that account for a significant level of patient radiation exposure in medicine(1). Myocardial perfusion imaging using Tc-99m labeled tracers such as sestamibi and tetrofosmin has received an increasing interest due to their superior image quality, diagnostic performance and possibility of performing gated myocardial perfusion single photons emission computed tomography (SPECT) imaging(2–4). MPI is invaluable in diagnosing and managing coronary artery disease; however, it accounts for ~10% of the radiation burden to the US population(5). Among the various procedures involving the use of ionizing radiation, MPI alone was found to be responsible for over 22% of the total effective dose to patients from all medical imaging procedures in the USA(6). Furthermore, the contribution for patient doses varies between countries but the relative risk of myocardial examination on personnel doses can be regarded as important. While the cumulative dose due to cardiac imaging represents a relatively high radiation burden especially in longitudinal and follow up studies, it contributes to a significant dose level to occupationally exposed radiation workers. Normally in myocardial perfusion SPECT studies, patients are undergoing two injections in the same day (i.e 1-day protocol) or alternatively two injections in two different days (i.e. 2-day protocol). For Tc-99m setamibi or tetrofosmin rest/stress imaging, the patient effective doses were found in the range of 9–13 mSv(7). This would also impact the occupationally exposed radiation worker since they are in many instances in close contact to injected patients during the time course of the imaging procedure. There have been several approaches to reduce patient effective dose due to the internally administered myocardial perfusion radiotracers including the use of stress-only imaging(8), multi-focal collimator technology(9), manoeuvre in the imaging protocol and/or injected dose(10), semiconductor detectors with high detection sensitivity(11, 12), wide beam or iterative reconstruction accompanied with resolution recovery that is commercially available in clinical SPECT or SPECT/CT clinical scanners(13, 14). These dose reduction techniques would definitely play a significant role in reducing staff radiation exposure as well. However, in dedicated cardiac imaging laboratories, high patient throughput represents a potential source of elevated radiation exposure to technical, nursing and medical staff. Tracking the staff dose in cardiac clinic from preparation down to patient injection and imaging setup as well as during patient release require further and elaborate investigation as which time point(s) has/have the highest level of exposure. This would provide more insights on staff dose profile over the time period during which patient gets into the department, undergo scanning examination, and released outside the clinic. It will also be helpful to organize the imaging team and their distribution so that minimal or reduced exposure levels on an individual basis can be achieved. Therefore, the aim of this study was to track down and assess the external exposure to radiation workers in nuclear cardiac laboratory during the whole procedure of myocardial perfusion SPECT imaging. MATERIALS AND METHODS Study design One hundred eleven patients were prospectively and randomly selected. Patients recruited in the study were those with suspected or known coronary artery disease. They were referred to our department to undergo myocardial perfusion imaging. Patient population are described in Table 1 along with other demographics, injected activities and type of stressors. Table 1. Patient characteristics and study parameters. Total =111 Stress = 56 Rest = 55 Total Male Female Male Female No. 45 11 43 12 111 Age (y) 58 ± 11 62 ± 9 60 ± 10 Height (cm) 168.5 ± 8.4 170.7 ± 6.1 169.5 ± 7.4 Weight (kg) 86.2 ± 17.4 86.7 ± 17.4 86.4 ± 17.3 kg Injected radioactivity, MBq (mCi) 684.5 ± 77.7 MBq (18.5 ± 2.1 mCi) 669.7 ± 88.8 MBq (18.1 ± 2.4 mCi) 673.4 ± 81.4 MBq (18.2 ± 2.2 mCi) Body mass index (BMI) (kg/m2) 30.2 ± 4.4 29.7 ± 5.2 29.9 ± 4.81 Body surface area (BSA) (m2) 1.96 ± 0.2 1.96 ± 0.2 2.0 ± 0.22 Treadmill exercise 39 — 39 Pharmacological stress 17 — 17 Total =111 Stress = 56 Rest = 55 Total Male Female Male Female No. 45 11 43 12 111 Age (y) 58 ± 11 62 ± 9 60 ± 10 Height (cm) 168.5 ± 8.4 170.7 ± 6.1 169.5 ± 7.4 Weight (kg) 86.2 ± 17.4 86.7 ± 17.4 86.4 ± 17.3 kg Injected radioactivity, MBq (mCi) 684.5 ± 77.7 MBq (18.5 ± 2.1 mCi) 669.7 ± 88.8 MBq (18.1 ± 2.4 mCi) 673.4 ± 81.4 MBq (18.2 ± 2.2 mCi) Body mass index (BMI) (kg/m2) 30.2 ± 4.4 29.7 ± 5.2 29.9 ± 4.81 Body surface area (BSA) (m2) 1.96 ± 0.2 1.96 ± 0.2 2.0 ± 0.22 Treadmill exercise 39 — 39 Pharmacological stress 17 — 17 Table 1. Patient characteristics and study parameters. Total =111 Stress = 56 Rest = 55 Total Male Female Male Female No. 45 11 43 12 111 Age (y) 58 ± 11 62 ± 9 60 ± 10 Height (cm) 168.5 ± 8.4 170.7 ± 6.1 169.5 ± 7.4 Weight (kg) 86.2 ± 17.4 86.7 ± 17.4 86.4 ± 17.3 kg Injected radioactivity, MBq (mCi) 684.5 ± 77.7 MBq (18.5 ± 2.1 mCi) 669.7 ± 88.8 MBq (18.1 ± 2.4 mCi) 673.4 ± 81.4 MBq (18.2 ± 2.2 mCi) Body mass index (BMI) (kg/m2) 30.2 ± 4.4 29.7 ± 5.2 29.9 ± 4.81 Body surface area (BSA) (m2) 1.96 ± 0.2 1.96 ± 0.2 2.0 ± 0.22 Treadmill exercise 39 — 39 Pharmacological stress 17 — 17 Total =111 Stress = 56 Rest = 55 Total Male Female Male Female No. 45 11 43 12 111 Age (y) 58 ± 11 62 ± 9 60 ± 10 Height (cm) 168.5 ± 8.4 170.7 ± 6.1 169.5 ± 7.4 Weight (kg) 86.2 ± 17.4 86.7 ± 17.4 86.4 ± 17.3 kg Injected radioactivity, MBq (mCi) 684.5 ± 77.7 MBq (18.5 ± 2.1 mCi) 669.7 ± 88.8 MBq (18.1 ± 2.4 mCi) 673.4 ± 81.4 MBq (18.2 ± 2.2 mCi) Body mass index (BMI) (kg/m2) 30.2 ± 4.4 29.7 ± 5.2 29.9 ± 4.81 Body surface area (BSA) (m2) 1.96 ± 0.2 1.96 ± 0.2 2.0 ± 0.22 Treadmill exercise 39 — 39 Pharmacological stress 17 — 17 Imaging workflow in our clinic starts by scheduling patients in two consecutive days of time period less than a week to ensure minimal or no haemodynamic changes or cardiac events were taken place. We receive a Mo99/Tc99m generator every 2 weeks with calibration activity of 50 GBq. Normally, the scheduling system has on average 15 patients per day including treadmill exercise, pharmacological stress or rest imaging for patients who had already undergone a stress examination. Radioactivity elution, tracer radiolabeling and dispending are performed by the medical physicist. In our SPECT laboratory, nurses are responsible for providing instructions and imaging logistics once the patient arrives to the clinic. They are also looking after the patient, ensuring patient compliance and adherence to prior instructions, and fixing an intravenous line. Nuclear technologists also share the nursing staff some responsibilities during patient preparation, instructions, stress testing in addition to their duties of imaging and guidance for release. Stress testing was performed using either treadmill exercise or pharmacological stress agents. Pharmacological stress was conducted using dipyridamole or dobutamine infusion. Rest injection was carried out on a different day normally few days after the stress testing using the same administered radioactivity. The dose was administered while the patient was in a sitting position with a syringe shield used to protect the radiation worker from radiations emitted from the syringe. Injected activities for stress and rest studies are summarized in Table 1. Data processing and protocol adjustment are determined by the medical physicist. During the exercise test, the nuclear cardiologist does oversee the patient, monitor electrocardiographic parameters, supervise the stress and recovery portions of the test, ensure patient stability and no adverse reactions are present. All staff are monitored by thermoluminescence dosemeter (TLD) worn on laboratory coats in the trunk region. The TLD is read every 3 months and results are carefully interpreted, analyzed and archived by a certified radiation safety officer. Exposure measurements It has been determined that there are five major time points where radioactivity or patient is in contact or close proximity to one of the medical, nursing or technical staff. These time points were dose preparation, radiotracer injection, patient recovery, patient setup for imaging and finally during patient release. We attempted to measure the dose rate during radiotracer preparation while the medical physicist is handling the radioactivity, in some instances eluting Tc-99m generator, performing the radiolabelling step, and finally withdrawing the radiotracer for final patient injection. These measurements were also taken at a distance of half-meter and then corrections were made to account for the exact distance between the operator and radioactivity behind the lead shielding in the hot laboratory. The second time point was noted at the onset of patient injection while the third one was captured during patient recovery, removing ECG electrodes after stress testing or immediately after rest dose injection. The fourth measurement was chosen during patient setup for imaging, where the technologist is escorting the patient and giving instructions not to move, fixing ECG electrodes and other relevant information for sake of obtaining images of high quality. The last time point was recorded at the end of the imaging session and technologist removing the ECG electrodes, assisting patient to get out of the imaging bed and guidance to the clothes changing room. All measurements were carried out at a distance of 1 m, If not stated otherwise, from the radioactive material or injected patient with a standard survey mater (Radalert 100 X™, IMI-International Medcom, USA) that is calibrated annually in the National Institute of Standards, Giza Governorate, Egypt. The survey meter specification and energy response curve is demonstrated in Table 2 and Figure 1, respectively. Table 2. The survey meter specification including detector material, display, linearity, operating range, calibration and gamma sensitivity. Courtesy of IMI-International Medcom, Inc. Detector Halogen-quenched Geiger-Mueller detector (LND 712) Mica end window density is 1.5–2.0 mg/cm2 side wall is 0.012′ #446 stainless steel. Detects alpha, beta, gamma and X-radiation. Display 4-digital liquid crystal display with mode indicators. Operating range: μSv/h: 0.000–1100 mR/h: 0.000–110 CPM: 0–350 000 CPS: 0–3500 Total: 0–9 999 000 counts Calibration Cesium-137 (gamma) Gamma Sensitivity 1000 CPM/mR/h referenced to Cs-137 Detector Halogen-quenched Geiger-Mueller detector (LND 712) Mica end window density is 1.5–2.0 mg/cm2 side wall is 0.012′ #446 stainless steel. Detects alpha, beta, gamma and X-radiation. Display 4-digital liquid crystal display with mode indicators. Operating range: μSv/h: 0.000–1100 mR/h: 0.000–110 CPM: 0–350 000 CPS: 0–3500 Total: 0–9 999 000 counts Calibration Cesium-137 (gamma) Gamma Sensitivity 1000 CPM/mR/h referenced to Cs-137 Table 2. The survey meter specification including detector material, display, linearity, operating range, calibration and gamma sensitivity. Courtesy of IMI-International Medcom, Inc. Detector Halogen-quenched Geiger-Mueller detector (LND 712) Mica end window density is 1.5–2.0 mg/cm2 side wall is 0.012′ #446 stainless steel. Detects alpha, beta, gamma and X-radiation. Display 4-digital liquid crystal display with mode indicators. Operating range: μSv/h: 0.000–1100 mR/h: 0.000–110 CPM: 0–350 000 CPS: 0–3500 Total: 0–9 999 000 counts Calibration Cesium-137 (gamma) Gamma Sensitivity 1000 CPM/mR/h referenced to Cs-137 Detector Halogen-quenched Geiger-Mueller detector (LND 712) Mica end window density is 1.5–2.0 mg/cm2 side wall is 0.012′ #446 stainless steel. Detects alpha, beta, gamma and X-radiation. Display 4-digital liquid crystal display with mode indicators. Operating range: μSv/h: 0.000–1100 mR/h: 0.000–110 CPM: 0–350 000 CPS: 0–3500 Total: 0–9 999 000 counts Calibration Cesium-137 (gamma) Gamma Sensitivity 1000 CPM/mR/h referenced to Cs-137 Figure 1. View largeDownload slide The energy response curve of the Radalert 100x™ survey meter used in the study. Courtesy of IMI-International Medcom, Inc. Figure 1. View largeDownload slide The energy response curve of the Radalert 100x™ survey meter used in the study. Courtesy of IMI-International Medcom, Inc. Then distance correction was made to reflect the proper dose rate that should be used in the calculation of the integral cumulative dose. The correction decay factor (Rt) was computed using the following formula(15): Rt=1.44xT1/2tx[1−e−0.693tT1/2] (1) where T1/2 is the tracer half life time and t is the estimated total time of stay. The total estimated effective dose was calculated using the following formula: Doserate×Staytime×Decaycorrection(Rt) (2) The dose rate is the measure recorded by the Radalert 100x survey meter in units of μSv/h. Stay time was estimated based on measurement of the average period spent on every point of significant exposure (i.e. the five time points highlighted above) and Rt does account for the source decay during exposure period. Although the decay of the radionuclide (i.e Tc-99m, T1/2 = 6.02 h) was negligible in most exposure periods, it was incorporated in the calculation scheme to ensure high precision of the measurements (Table 3). Table 3. Details of exposure dose levels recorded for the entire patient population. Dose rate (μSv/h) Time elapsed (average, min) Average cumulated dose* (μSv) Dose reduction factor (Rt) During injection (stress or rest) 26.5 ± 4.5 0.5 0.87 ± 0.2 0.997 Post-injection (rest) 26.0 ± 4.2 4 8.1 ± 2.0 0.994 Recovery phase (stress) 25.9 ± 4.2 7 12.3 ± 2.1 0.991 Patient setup (stress or rest) 21.9 ± 4.4 6 8.7 ± 1.9 0.992 Patient release (stress or rest) 21.4 ± 4.4 5 7.1 ± 1.3 0.993 Dose rate (μSv/h) Time elapsed (average, min) Average cumulated dose* (μSv) Dose reduction factor (Rt) During injection (stress or rest) 26.5 ± 4.5 0.5 0.87 ± 0.2 0.997 Post-injection (rest) 26.0 ± 4.2 4 8.1 ± 2.0 0.994 Recovery phase (stress) 25.9 ± 4.2 7 12.3 ± 2.1 0.991 Patient setup (stress or rest) 21.9 ± 4.4 6 8.7 ± 1.9 0.992 Patient release (stress or rest) 21.4 ± 4.4 5 7.1 ± 1.3 0.993 *The cumulated effective absorbed dose was corrected for a distance of 0.5 m using the inverse square law. As the exact distance could be variable due to movement of the patient and/or the worker we estimated a distance of 0.5 m as a good compromise in many steps of measurements. Table 3. Details of exposure dose levels recorded for the entire patient population. Dose rate (μSv/h) Time elapsed (average, min) Average cumulated dose* (μSv) Dose reduction factor (Rt) During injection (stress or rest) 26.5 ± 4.5 0.5 0.87 ± 0.2 0.997 Post-injection (rest) 26.0 ± 4.2 4 8.1 ± 2.0 0.994 Recovery phase (stress) 25.9 ± 4.2 7 12.3 ± 2.1 0.991 Patient setup (stress or rest) 21.9 ± 4.4 6 8.7 ± 1.9 0.992 Patient release (stress or rest) 21.4 ± 4.4 5 7.1 ± 1.3 0.993 Dose rate (μSv/h) Time elapsed (average, min) Average cumulated dose* (μSv) Dose reduction factor (Rt) During injection (stress or rest) 26.5 ± 4.5 0.5 0.87 ± 0.2 0.997 Post-injection (rest) 26.0 ± 4.2 4 8.1 ± 2.0 0.994 Recovery phase (stress) 25.9 ± 4.2 7 12.3 ± 2.1 0.991 Patient setup (stress or rest) 21.9 ± 4.4 6 8.7 ± 1.9 0.992 Patient release (stress or rest) 21.4 ± 4.4 5 7.1 ± 1.3 0.993 *The cumulated effective absorbed dose was corrected for a distance of 0.5 m using the inverse square law. As the exact distance could be variable due to movement of the patient and/or the worker we estimated a distance of 0.5 m as a good compromise in many steps of measurements. Patient-to-staff dose coefficient has some important implications in radiation protection measurements and calculations including shielding, radiation worker or staff exposure, risk analysis and others. It can really account for patient self-absorption in comparison to point source dose rate calculations. The unit of this coefficient can be μSv m2/MBq h and therefore the measured dose rate (μSv/h taken at 1 m) was divided by the injected radioactivity to generate the patient-to-staff dose coefficient. This has been carried out for every patient to obtain the average value of the population recruited including data variation. A commercially available dual head gamma camera (E.CAM, Siemens Medical Solutions, USA) equipped with parallel hole high resolution collimator was used in all imaging procedures. The SPECT imaging protocol consisted of 64 projections, 20 s/view, and matrix size of 64 × 64. The SPECT images were also synchronized with ECG-gating trigger so that functional parameters can be derived and measured. No attenuation or scatter correction was applied. Statistical analysis All data were represented as mean±sd. Paired student t-test was used to check the significance of difference between exposure pre and post-imaging at confidence level of 5%. The Statistical Package for the Social Sciences (SPSS Inc., Chicago, IL, USA, version 20) was used in data analysis while Microsoft® Excel (version 2013) was used in data plotting and graphing. RESULTS Data analysis of the 111 patients revealed that patients spent on average 116 ± 43 min in the department throughout the whole process starting from dose injection until clinic discharge. The injected activity averaged over the whole population was 677.1 ± 81.4 MBq (18.3 ± 2.2 mCi). Table 3 and Figure 3 summarize all measurements recorded for staff exposure while handling patients undergoing myocardial perfusion SPECT imaging. Time elapsed during patient injection was on average 30 s for rest myocardial perfusion imaging. After rest injection, technical staff spends on average 4 min for sake of cannula flushing, and instructions for the uptake period. In case of stress injection, the medical team was found to stay ~7 min observing patient recovery, removing ECG, and sending patient to the waiting room. Data acquisition and patient preparation for the imaging session was on average 6 min while 4 min was noted as exposure period during which patient is released. These later two instances shared technologist responsibility in escorting patient, fixing and removing ECG electrodes, position adjustment as well as letting patient to get out of the bed. The staff member responsible for injection was found to receive, on average, an effective dose of 0.87 ± 0.15 μSv per patient as shown in Table 3, while a dose of 12.3 ± 2.1 μSv for the nursing staff assisting in patient recovery, removing ECG electrodes and guiding patient to the waiting room (Figure 2). The uptake time was to some extent variable among patients measuring 80 ± 20 min. When calling patient for imaging and letting him/her lying on the scanner bed for proper positioning, the technologist was found to receive an average effective dose of 8.7 ± 1.9 μSv and similar dose of 7.1 ± 1.3 μSv at release after the imaging procedure. Due to the increased time of stay after stress dose injection, the exposure was significantly higher in stress than in rest (mean difference of 4.2 ± 0.4 μSv, p < 0.05) as can be seen in Figure 2. The dose rate measured when patients end their imaging session was on average 21.5 ± 4.6 μSv/h at a distance of 1 m. Figure 2. View largeDownload slide Average cumulated dose to radiation workers due to five-different phases of myocardial perfusion SPECT imaging including tracer injection, post-stress and post-rest injections, patient setup for imaging and at time of release. Figure 2. View largeDownload slide Average cumulated dose to radiation workers due to five-different phases of myocardial perfusion SPECT imaging including tracer injection, post-stress and post-rest injections, patient setup for imaging and at time of release. Measuring the exposure rate immediately after injection at a distance of 1 m enabled to calculate patient-to-staff effective dose rate coefficient. On average, this estimate was 0.04 ± 0.007 μSv MBq/m2 h. It was found to correlate moderately with body surface area whereas mildly with body mass index. Both measures had a negative correlation with the patient-to-staff effective dose rate coefficient as shown in Figures 3 and 4. Figure 3. View largeDownload slide Correlation of patient-to-staff dose coefficient with body surface area. Figure 3. View largeDownload slide Correlation of patient-to-staff dose coefficient with body surface area. Figure 4. View largeDownload slide Correlation of patient-to-staff dose coefficient with body mass index. Figure 4. View largeDownload slide Correlation of patient-to-staff dose coefficient with body mass index. DISCUSSION This study focused on individual steps that contribute to staff radiation exposure while handling cardiac patient who refereed to myocardial perfusion SPECT imaging. The study was conducted in relatively busy nuclear cardiac laboratory that performs on average a number of 15–20 patients per day. Therefore, the study aimed to localize which time point(s) contribute maximally or minimally to the total radiation burden during this particular examination. The experiment was designed to screen the whole procedure since injection, patient handling post-injection and then towards patient setup, imaging session and final release. There were significant differences that appeared among those phases and minimal dose was recorded during patient injection which took ~30 s. We routinely use a syringe shield during patient injection, which was found very useful in potentially minimizing the radiation dose. The time of decay during exposure of the staff though not significant was also considered to avoid any source of error improving certainty of measurements. The results highlighted that patient injection during exercise is the most important hot spot in nuclear cardiac imaging. In this time period, the staff members supervise patient injection till end of the study and successful release to outside of the exercise room. The time frame during which this process took place was ~7 min resulting in a dose of 12.3 ± 2.1 μSv. This duration could be significantly longer as some patients might need further assistance to recover. In this case the cumulated dose to the radiation worker would also increase. Patient preparation for imaging and release processes were the fourth and fifth phases, respectively. In these two phases the radiation workers were also noted to receive a relatively high dose. The dose received before was slightly but significantly higher than the dose received after the imaging procedure. This obviously was due to the longer time (and also dose rate) the technologist spends in patient positioning, attaching ECG electrodes, and other instructions to avoid patient motion during image acquisition. The study also enabled to measure the dose rate immediately after injection and thus we were able to calculate the patient-to-staff effective dose rate coefficient. The value revealed was significantly lower than point source measurements accounting for 48% reduction due to photon absorption and patient self-attenuation. This piece of information has meaningful use and implications in site planning, staff exposure as well as shielding calculations(16). There have been several attempts to reduce the radioactive dose injected into myocardial perfusion SPECT patients. One of those was the use of resolution recovery incorporated into the iterative algorithms. A number of publications reported that a reduction of the injected dose can be realized while able to maintain image quality in standard acquisition or with minimal loss of diagnostic accuracy(17). These methods can be classified as software or hardware-based methods. The former can be seen in resolution recovery algorithms and collimator characteristics modeling whereas the later can be observed in the use of different collimator system or detectors with high detection efficiency(9, 11–14). This would have a definite impact on the dose received by radiation workers since a significant portion of the exposure levels can be reduced. Further looking into our data revealed that on average one cardiac stress study deliver an effective dose of ~29 μSv to the working staff, while a dose of 24 μSv was recorded for cardiac rest examination bearing in mind the variable distances recoded for each step and also the diversity of the medical/technical team across the whole procedure. Stated another way, if one person is committed to perform all steps of SPECT MPI for a given patient he would receive an average effective dose of 29 and 24 μSv in stress and rest study respectively. Furthermore, handling patient after tracer injection represents ~42% of the total dose received in one cardiac stress study. Although this dose is not received by a single individual, it is extremely important to consider it in staff roster as well as any radiation safety measures related to the lab. If we assume one physician is scheduled to perform 10 patients for exercise testing, then the dose expected is going to be on average 123 μSv if the post-injection duration extended to ~7 min. However, this measure could be larger in case some patients would need further assistance or the recovery period is prolonged. Patient escorting and preparation for imaging as well as patient release are also important phases of considerable attention. Our results showed a slightly higher dose during patient setup due to some relevant factors including instructions, positioning, attaching ECG to the chest wall. This time frame was estimated on average as 6 and 5 min for patient setup and release respectively. However, it can be relatively longer in some patients who have fluctuation in cardiac rhythms, atrial fibrillation or in severally ill cohorts. A recent study showed that the exit release dose due to patients injected with three PET radiotracers for neuroendocrine diseases was median of 4.8, 9.5 and 8.8 μSv/h while median dose of 9.4 and 4.9 μSv/h at the level of the sternum for SPECT-based radiopharmaceuticals(18). These values are lower than that reported in this study as the release dose was around 20 μSv/h pinpointing the importance of carefully looking at patients undergoing MPI in busy nuclear medicine departments. These higher values are probably due to the point that our dosing protocol might be slightly higher than other institutions. Overall, staff exposure in the hot lab was estimated as 15 and 10 μSv for the first and second week of the generator. These measurements were a bit challenging as the technologist was in close contact to the lead glass shield handling the radioactivity. As stated above, measurements were performed at 0.5 m but corrections were made so that the actual distance was made at 0.3 m, again as a reasonable trade-off accounting for several movements of the radiation worker during dose preparation. However, another limitation of the study was the absence of dose extremity measurements due to the unavailability of the suitable dosemeters. Patient body surface area and body mass index are widely used in many clinical measures. The former showed a moderate negative correlation with the patient-to-staff effective dose rate coefficient while the later was poorly correlated. This observation might be related to the fact that increasing the surface area of the source could have a relatively large emission rate whereas patients with high BMI have a significant self-attenuation and thus the correlation was not substantially high. This information tells us how the two metrics could have an impact on staff radiation dose based on patient habitus. Recently, the safety measures in our lab have adopted the use of lead aprons by the nursing and medical staff during cardiac exercise testing. Therefore, our future research work would investigate the efficacy of this on the total exposure level as well its implications on a day-by-day practice in addition to staff comfortability in a short and long term perspectives. CONCLUSION This report provides quantitative information on the distribution of radiation burden in nuclear cardiac laboratory. Assisting patient post-stress either during treadmill exercise or pharmacological stress is the most critical time points where staff members receive the highest radiation dose. New imaging technologies including sophisticated reconstruction algorithms and/or highly sensitive radiation detectors would help in minimizing the injected radioactive dose and hence reduction in staff exposure. The study has mapped the relative contribution of radiation doses to staff members due to myocardial perfusion imaging and would be efficiently used to plan ahead the distribution of tasks in the clinic. It can also be used as simple prediction or simulation model in facility design, local rules refinement and ultimately as safety measures in nuclear medicine cardiac laboratory. ACKNOWLEDGEMENTS This work has been presented in part at the Annual Meeting of the Society of Nuclear Medicine and Molecular Imaging (SNMMI), 11–15 June 2016. It was also selected as one of the top 10 posters in Radiation Dosimetry track. REFERENCES 1 Cousins , C. , Miller , D. L. , Bernardi , G. et al. . ICRP Publication 120: radiological protection in cardiology . Ann. ICRP 42 , 1 – 125 ( 2012 ). Google Scholar Crossref Search ADS 2 Acampa , W. , Cuocolo , A. , Petretta , M. et al. . 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For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

EFFECTIVE RADIATION DOSE TO STAFF MEMBERS DUE TO MYOCARDIAL PERFUSION SPECT IMAGING: TRACKING THE EXPOSURE FROM PREPARATION TO PATIENT RELEASE

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Abstract

Abstract The aim of the work was to tack the radiation exposure from dose preparation down to patient release during nuclear cardiac imaging. A total of 111 patients were recruited and external exposure was measured using a standard calibrated survey meter. Five phases have been determined including dose preparation, injection, post-injection, patient setup and patient release. An average effective dose of 0.87 ± 0.2, 8.1 ± 2.0, 12.3 ± 2.1, 8.7 ± 1.9 and 7.1 ± 1.3 μSv were recorded as staff external exposure due to patient injection, post-injection (rest), recovery phase (stress), patient setup and patient release, respectively. On average, patient-to-staff effective dose rate coefficient was 0.04 ± 0.007 μSv MBq/m2 h. Post-stress phase is the most critical time point where staff members receive the highest radiation dose. The study has mapped the relative contribution of radiation doses and can also be used as simple prediction model in facility design and local rules as well as safety measure. INTRODUCTION Nuclear based myocardial perfusion imaging (MPI), x-ray cardiac computed tomography (CT), interventional cardiology procedures are among those diagnostic modalities that account for a significant level of patient radiation exposure in medicine(1). Myocardial perfusion imaging using Tc-99m labeled tracers such as sestamibi and tetrofosmin has received an increasing interest due to their superior image quality, diagnostic performance and possibility of performing gated myocardial perfusion single photons emission computed tomography (SPECT) imaging(2–4). MPI is invaluable in diagnosing and managing coronary artery disease; however, it accounts for ~10% of the radiation burden to the US population(5). Among the various procedures involving the use of ionizing radiation, MPI alone was found to be responsible for over 22% of the total effective dose to patients from all medical imaging procedures in the USA(6). Furthermore, the contribution for patient doses varies between countries but the relative risk of myocardial examination on personnel doses can be regarded as important. While the cumulative dose due to cardiac imaging represents a relatively high radiation burden especially in longitudinal and follow up studies, it contributes to a significant dose level to occupationally exposed radiation workers. Normally in myocardial perfusion SPECT studies, patients are undergoing two injections in the same day (i.e 1-day protocol) or alternatively two injections in two different days (i.e. 2-day protocol). For Tc-99m setamibi or tetrofosmin rest/stress imaging, the patient effective doses were found in the range of 9–13 mSv(7). This would also impact the occupationally exposed radiation worker since they are in many instances in close contact to injected patients during the time course of the imaging procedure. There have been several approaches to reduce patient effective dose due to the internally administered myocardial perfusion radiotracers including the use of stress-only imaging(8), multi-focal collimator technology(9), manoeuvre in the imaging protocol and/or injected dose(10), semiconductor detectors with high detection sensitivity(11, 12), wide beam or iterative reconstruction accompanied with resolution recovery that is commercially available in clinical SPECT or SPECT/CT clinical scanners(13, 14). These dose reduction techniques would definitely play a significant role in reducing staff radiation exposure as well. However, in dedicated cardiac imaging laboratories, high patient throughput represents a potential source of elevated radiation exposure to technical, nursing and medical staff. Tracking the staff dose in cardiac clinic from preparation down to patient injection and imaging setup as well as during patient release require further and elaborate investigation as which time point(s) has/have the highest level of exposure. This would provide more insights on staff dose profile over the time period during which patient gets into the department, undergo scanning examination, and released outside the clinic. It will also be helpful to organize the imaging team and their distribution so that minimal or reduced exposure levels on an individual basis can be achieved. Therefore, the aim of this study was to track down and assess the external exposure to radiation workers in nuclear cardiac laboratory during the whole procedure of myocardial perfusion SPECT imaging. MATERIALS AND METHODS Study design One hundred eleven patients were prospectively and randomly selected. Patients recruited in the study were those with suspected or known coronary artery disease. They were referred to our department to undergo myocardial perfusion imaging. Patient population are described in Table 1 along with other demographics, injected activities and type of stressors. Table 1. Patient characteristics and study parameters. Total =111 Stress = 56 Rest = 55 Total Male Female Male Female No. 45 11 43 12 111 Age (y) 58 ± 11 62 ± 9 60 ± 10 Height (cm) 168.5 ± 8.4 170.7 ± 6.1 169.5 ± 7.4 Weight (kg) 86.2 ± 17.4 86.7 ± 17.4 86.4 ± 17.3 kg Injected radioactivity, MBq (mCi) 684.5 ± 77.7 MBq (18.5 ± 2.1 mCi) 669.7 ± 88.8 MBq (18.1 ± 2.4 mCi) 673.4 ± 81.4 MBq (18.2 ± 2.2 mCi) Body mass index (BMI) (kg/m2) 30.2 ± 4.4 29.7 ± 5.2 29.9 ± 4.81 Body surface area (BSA) (m2) 1.96 ± 0.2 1.96 ± 0.2 2.0 ± 0.22 Treadmill exercise 39 — 39 Pharmacological stress 17 — 17 Total =111 Stress = 56 Rest = 55 Total Male Female Male Female No. 45 11 43 12 111 Age (y) 58 ± 11 62 ± 9 60 ± 10 Height (cm) 168.5 ± 8.4 170.7 ± 6.1 169.5 ± 7.4 Weight (kg) 86.2 ± 17.4 86.7 ± 17.4 86.4 ± 17.3 kg Injected radioactivity, MBq (mCi) 684.5 ± 77.7 MBq (18.5 ± 2.1 mCi) 669.7 ± 88.8 MBq (18.1 ± 2.4 mCi) 673.4 ± 81.4 MBq (18.2 ± 2.2 mCi) Body mass index (BMI) (kg/m2) 30.2 ± 4.4 29.7 ± 5.2 29.9 ± 4.81 Body surface area (BSA) (m2) 1.96 ± 0.2 1.96 ± 0.2 2.0 ± 0.22 Treadmill exercise 39 — 39 Pharmacological stress 17 — 17 Table 1. Patient characteristics and study parameters. Total =111 Stress = 56 Rest = 55 Total Male Female Male Female No. 45 11 43 12 111 Age (y) 58 ± 11 62 ± 9 60 ± 10 Height (cm) 168.5 ± 8.4 170.7 ± 6.1 169.5 ± 7.4 Weight (kg) 86.2 ± 17.4 86.7 ± 17.4 86.4 ± 17.3 kg Injected radioactivity, MBq (mCi) 684.5 ± 77.7 MBq (18.5 ± 2.1 mCi) 669.7 ± 88.8 MBq (18.1 ± 2.4 mCi) 673.4 ± 81.4 MBq (18.2 ± 2.2 mCi) Body mass index (BMI) (kg/m2) 30.2 ± 4.4 29.7 ± 5.2 29.9 ± 4.81 Body surface area (BSA) (m2) 1.96 ± 0.2 1.96 ± 0.2 2.0 ± 0.22 Treadmill exercise 39 — 39 Pharmacological stress 17 — 17 Total =111 Stress = 56 Rest = 55 Total Male Female Male Female No. 45 11 43 12 111 Age (y) 58 ± 11 62 ± 9 60 ± 10 Height (cm) 168.5 ± 8.4 170.7 ± 6.1 169.5 ± 7.4 Weight (kg) 86.2 ± 17.4 86.7 ± 17.4 86.4 ± 17.3 kg Injected radioactivity, MBq (mCi) 684.5 ± 77.7 MBq (18.5 ± 2.1 mCi) 669.7 ± 88.8 MBq (18.1 ± 2.4 mCi) 673.4 ± 81.4 MBq (18.2 ± 2.2 mCi) Body mass index (BMI) (kg/m2) 30.2 ± 4.4 29.7 ± 5.2 29.9 ± 4.81 Body surface area (BSA) (m2) 1.96 ± 0.2 1.96 ± 0.2 2.0 ± 0.22 Treadmill exercise 39 — 39 Pharmacological stress 17 — 17 Imaging workflow in our clinic starts by scheduling patients in two consecutive days of time period less than a week to ensure minimal or no haemodynamic changes or cardiac events were taken place. We receive a Mo99/Tc99m generator every 2 weeks with calibration activity of 50 GBq. Normally, the scheduling system has on average 15 patients per day including treadmill exercise, pharmacological stress or rest imaging for patients who had already undergone a stress examination. Radioactivity elution, tracer radiolabeling and dispending are performed by the medical physicist. In our SPECT laboratory, nurses are responsible for providing instructions and imaging logistics once the patient arrives to the clinic. They are also looking after the patient, ensuring patient compliance and adherence to prior instructions, and fixing an intravenous line. Nuclear technologists also share the nursing staff some responsibilities during patient preparation, instructions, stress testing in addition to their duties of imaging and guidance for release. Stress testing was performed using either treadmill exercise or pharmacological stress agents. Pharmacological stress was conducted using dipyridamole or dobutamine infusion. Rest injection was carried out on a different day normally few days after the stress testing using the same administered radioactivity. The dose was administered while the patient was in a sitting position with a syringe shield used to protect the radiation worker from radiations emitted from the syringe. Injected activities for stress and rest studies are summarized in Table 1. Data processing and protocol adjustment are determined by the medical physicist. During the exercise test, the nuclear cardiologist does oversee the patient, monitor electrocardiographic parameters, supervise the stress and recovery portions of the test, ensure patient stability and no adverse reactions are present. All staff are monitored by thermoluminescence dosemeter (TLD) worn on laboratory coats in the trunk region. The TLD is read every 3 months and results are carefully interpreted, analyzed and archived by a certified radiation safety officer. Exposure measurements It has been determined that there are five major time points where radioactivity or patient is in contact or close proximity to one of the medical, nursing or technical staff. These time points were dose preparation, radiotracer injection, patient recovery, patient setup for imaging and finally during patient release. We attempted to measure the dose rate during radiotracer preparation while the medical physicist is handling the radioactivity, in some instances eluting Tc-99m generator, performing the radiolabelling step, and finally withdrawing the radiotracer for final patient injection. These measurements were also taken at a distance of half-meter and then corrections were made to account for the exact distance between the operator and radioactivity behind the lead shielding in the hot laboratory. The second time point was noted at the onset of patient injection while the third one was captured during patient recovery, removing ECG electrodes after stress testing or immediately after rest dose injection. The fourth measurement was chosen during patient setup for imaging, where the technologist is escorting the patient and giving instructions not to move, fixing ECG electrodes and other relevant information for sake of obtaining images of high quality. The last time point was recorded at the end of the imaging session and technologist removing the ECG electrodes, assisting patient to get out of the imaging bed and guidance to the clothes changing room. All measurements were carried out at a distance of 1 m, If not stated otherwise, from the radioactive material or injected patient with a standard survey mater (Radalert 100 X™, IMI-International Medcom, USA) that is calibrated annually in the National Institute of Standards, Giza Governorate, Egypt. The survey meter specification and energy response curve is demonstrated in Table 2 and Figure 1, respectively. Table 2. The survey meter specification including detector material, display, linearity, operating range, calibration and gamma sensitivity. Courtesy of IMI-International Medcom, Inc. Detector Halogen-quenched Geiger-Mueller detector (LND 712) Mica end window density is 1.5–2.0 mg/cm2 side wall is 0.012′ #446 stainless steel. Detects alpha, beta, gamma and X-radiation. Display 4-digital liquid crystal display with mode indicators. Operating range: μSv/h: 0.000–1100 mR/h: 0.000–110 CPM: 0–350 000 CPS: 0–3500 Total: 0–9 999 000 counts Calibration Cesium-137 (gamma) Gamma Sensitivity 1000 CPM/mR/h referenced to Cs-137 Detector Halogen-quenched Geiger-Mueller detector (LND 712) Mica end window density is 1.5–2.0 mg/cm2 side wall is 0.012′ #446 stainless steel. Detects alpha, beta, gamma and X-radiation. Display 4-digital liquid crystal display with mode indicators. Operating range: μSv/h: 0.000–1100 mR/h: 0.000–110 CPM: 0–350 000 CPS: 0–3500 Total: 0–9 999 000 counts Calibration Cesium-137 (gamma) Gamma Sensitivity 1000 CPM/mR/h referenced to Cs-137 Table 2. The survey meter specification including detector material, display, linearity, operating range, calibration and gamma sensitivity. Courtesy of IMI-International Medcom, Inc. Detector Halogen-quenched Geiger-Mueller detector (LND 712) Mica end window density is 1.5–2.0 mg/cm2 side wall is 0.012′ #446 stainless steel. Detects alpha, beta, gamma and X-radiation. Display 4-digital liquid crystal display with mode indicators. Operating range: μSv/h: 0.000–1100 mR/h: 0.000–110 CPM: 0–350 000 CPS: 0–3500 Total: 0–9 999 000 counts Calibration Cesium-137 (gamma) Gamma Sensitivity 1000 CPM/mR/h referenced to Cs-137 Detector Halogen-quenched Geiger-Mueller detector (LND 712) Mica end window density is 1.5–2.0 mg/cm2 side wall is 0.012′ #446 stainless steel. Detects alpha, beta, gamma and X-radiation. Display 4-digital liquid crystal display with mode indicators. Operating range: μSv/h: 0.000–1100 mR/h: 0.000–110 CPM: 0–350 000 CPS: 0–3500 Total: 0–9 999 000 counts Calibration Cesium-137 (gamma) Gamma Sensitivity 1000 CPM/mR/h referenced to Cs-137 Figure 1. View largeDownload slide The energy response curve of the Radalert 100x™ survey meter used in the study. Courtesy of IMI-International Medcom, Inc. Figure 1. View largeDownload slide The energy response curve of the Radalert 100x™ survey meter used in the study. Courtesy of IMI-International Medcom, Inc. Then distance correction was made to reflect the proper dose rate that should be used in the calculation of the integral cumulative dose. The correction decay factor (Rt) was computed using the following formula(15): Rt=1.44xT1/2tx[1−e−0.693tT1/2] (1) where T1/2 is the tracer half life time and t is the estimated total time of stay. The total estimated effective dose was calculated using the following formula: Doserate×Staytime×Decaycorrection(Rt) (2) The dose rate is the measure recorded by the Radalert 100x survey meter in units of μSv/h. Stay time was estimated based on measurement of the average period spent on every point of significant exposure (i.e. the five time points highlighted above) and Rt does account for the source decay during exposure period. Although the decay of the radionuclide (i.e Tc-99m, T1/2 = 6.02 h) was negligible in most exposure periods, it was incorporated in the calculation scheme to ensure high precision of the measurements (Table 3). Table 3. Details of exposure dose levels recorded for the entire patient population. Dose rate (μSv/h) Time elapsed (average, min) Average cumulated dose* (μSv) Dose reduction factor (Rt) During injection (stress or rest) 26.5 ± 4.5 0.5 0.87 ± 0.2 0.997 Post-injection (rest) 26.0 ± 4.2 4 8.1 ± 2.0 0.994 Recovery phase (stress) 25.9 ± 4.2 7 12.3 ± 2.1 0.991 Patient setup (stress or rest) 21.9 ± 4.4 6 8.7 ± 1.9 0.992 Patient release (stress or rest) 21.4 ± 4.4 5 7.1 ± 1.3 0.993 Dose rate (μSv/h) Time elapsed (average, min) Average cumulated dose* (μSv) Dose reduction factor (Rt) During injection (stress or rest) 26.5 ± 4.5 0.5 0.87 ± 0.2 0.997 Post-injection (rest) 26.0 ± 4.2 4 8.1 ± 2.0 0.994 Recovery phase (stress) 25.9 ± 4.2 7 12.3 ± 2.1 0.991 Patient setup (stress or rest) 21.9 ± 4.4 6 8.7 ± 1.9 0.992 Patient release (stress or rest) 21.4 ± 4.4 5 7.1 ± 1.3 0.993 *The cumulated effective absorbed dose was corrected for a distance of 0.5 m using the inverse square law. As the exact distance could be variable due to movement of the patient and/or the worker we estimated a distance of 0.5 m as a good compromise in many steps of measurements. Table 3. Details of exposure dose levels recorded for the entire patient population. Dose rate (μSv/h) Time elapsed (average, min) Average cumulated dose* (μSv) Dose reduction factor (Rt) During injection (stress or rest) 26.5 ± 4.5 0.5 0.87 ± 0.2 0.997 Post-injection (rest) 26.0 ± 4.2 4 8.1 ± 2.0 0.994 Recovery phase (stress) 25.9 ± 4.2 7 12.3 ± 2.1 0.991 Patient setup (stress or rest) 21.9 ± 4.4 6 8.7 ± 1.9 0.992 Patient release (stress or rest) 21.4 ± 4.4 5 7.1 ± 1.3 0.993 Dose rate (μSv/h) Time elapsed (average, min) Average cumulated dose* (μSv) Dose reduction factor (Rt) During injection (stress or rest) 26.5 ± 4.5 0.5 0.87 ± 0.2 0.997 Post-injection (rest) 26.0 ± 4.2 4 8.1 ± 2.0 0.994 Recovery phase (stress) 25.9 ± 4.2 7 12.3 ± 2.1 0.991 Patient setup (stress or rest) 21.9 ± 4.4 6 8.7 ± 1.9 0.992 Patient release (stress or rest) 21.4 ± 4.4 5 7.1 ± 1.3 0.993 *The cumulated effective absorbed dose was corrected for a distance of 0.5 m using the inverse square law. As the exact distance could be variable due to movement of the patient and/or the worker we estimated a distance of 0.5 m as a good compromise in many steps of measurements. Patient-to-staff dose coefficient has some important implications in radiation protection measurements and calculations including shielding, radiation worker or staff exposure, risk analysis and others. It can really account for patient self-absorption in comparison to point source dose rate calculations. The unit of this coefficient can be μSv m2/MBq h and therefore the measured dose rate (μSv/h taken at 1 m) was divided by the injected radioactivity to generate the patient-to-staff dose coefficient. This has been carried out for every patient to obtain the average value of the population recruited including data variation. A commercially available dual head gamma camera (E.CAM, Siemens Medical Solutions, USA) equipped with parallel hole high resolution collimator was used in all imaging procedures. The SPECT imaging protocol consisted of 64 projections, 20 s/view, and matrix size of 64 × 64. The SPECT images were also synchronized with ECG-gating trigger so that functional parameters can be derived and measured. No attenuation or scatter correction was applied. Statistical analysis All data were represented as mean±sd. Paired student t-test was used to check the significance of difference between exposure pre and post-imaging at confidence level of 5%. The Statistical Package for the Social Sciences (SPSS Inc., Chicago, IL, USA, version 20) was used in data analysis while Microsoft® Excel (version 2013) was used in data plotting and graphing. RESULTS Data analysis of the 111 patients revealed that patients spent on average 116 ± 43 min in the department throughout the whole process starting from dose injection until clinic discharge. The injected activity averaged over the whole population was 677.1 ± 81.4 MBq (18.3 ± 2.2 mCi). Table 3 and Figure 3 summarize all measurements recorded for staff exposure while handling patients undergoing myocardial perfusion SPECT imaging. Time elapsed during patient injection was on average 30 s for rest myocardial perfusion imaging. After rest injection, technical staff spends on average 4 min for sake of cannula flushing, and instructions for the uptake period. In case of stress injection, the medical team was found to stay ~7 min observing patient recovery, removing ECG, and sending patient to the waiting room. Data acquisition and patient preparation for the imaging session was on average 6 min while 4 min was noted as exposure period during which patient is released. These later two instances shared technologist responsibility in escorting patient, fixing and removing ECG electrodes, position adjustment as well as letting patient to get out of the bed. The staff member responsible for injection was found to receive, on average, an effective dose of 0.87 ± 0.15 μSv per patient as shown in Table 3, while a dose of 12.3 ± 2.1 μSv for the nursing staff assisting in patient recovery, removing ECG electrodes and guiding patient to the waiting room (Figure 2). The uptake time was to some extent variable among patients measuring 80 ± 20 min. When calling patient for imaging and letting him/her lying on the scanner bed for proper positioning, the technologist was found to receive an average effective dose of 8.7 ± 1.9 μSv and similar dose of 7.1 ± 1.3 μSv at release after the imaging procedure. Due to the increased time of stay after stress dose injection, the exposure was significantly higher in stress than in rest (mean difference of 4.2 ± 0.4 μSv, p < 0.05) as can be seen in Figure 2. The dose rate measured when patients end their imaging session was on average 21.5 ± 4.6 μSv/h at a distance of 1 m. Figure 2. View largeDownload slide Average cumulated dose to radiation workers due to five-different phases of myocardial perfusion SPECT imaging including tracer injection, post-stress and post-rest injections, patient setup for imaging and at time of release. Figure 2. View largeDownload slide Average cumulated dose to radiation workers due to five-different phases of myocardial perfusion SPECT imaging including tracer injection, post-stress and post-rest injections, patient setup for imaging and at time of release. Measuring the exposure rate immediately after injection at a distance of 1 m enabled to calculate patient-to-staff effective dose rate coefficient. On average, this estimate was 0.04 ± 0.007 μSv MBq/m2 h. It was found to correlate moderately with body surface area whereas mildly with body mass index. Both measures had a negative correlation with the patient-to-staff effective dose rate coefficient as shown in Figures 3 and 4. Figure 3. View largeDownload slide Correlation of patient-to-staff dose coefficient with body surface area. Figure 3. View largeDownload slide Correlation of patient-to-staff dose coefficient with body surface area. Figure 4. View largeDownload slide Correlation of patient-to-staff dose coefficient with body mass index. Figure 4. View largeDownload slide Correlation of patient-to-staff dose coefficient with body mass index. DISCUSSION This study focused on individual steps that contribute to staff radiation exposure while handling cardiac patient who refereed to myocardial perfusion SPECT imaging. The study was conducted in relatively busy nuclear cardiac laboratory that performs on average a number of 15–20 patients per day. Therefore, the study aimed to localize which time point(s) contribute maximally or minimally to the total radiation burden during this particular examination. The experiment was designed to screen the whole procedure since injection, patient handling post-injection and then towards patient setup, imaging session and final release. There were significant differences that appeared among those phases and minimal dose was recorded during patient injection which took ~30 s. We routinely use a syringe shield during patient injection, which was found very useful in potentially minimizing the radiation dose. The time of decay during exposure of the staff though not significant was also considered to avoid any source of error improving certainty of measurements. The results highlighted that patient injection during exercise is the most important hot spot in nuclear cardiac imaging. In this time period, the staff members supervise patient injection till end of the study and successful release to outside of the exercise room. The time frame during which this process took place was ~7 min resulting in a dose of 12.3 ± 2.1 μSv. This duration could be significantly longer as some patients might need further assistance to recover. In this case the cumulated dose to the radiation worker would also increase. Patient preparation for imaging and release processes were the fourth and fifth phases, respectively. In these two phases the radiation workers were also noted to receive a relatively high dose. The dose received before was slightly but significantly higher than the dose received after the imaging procedure. This obviously was due to the longer time (and also dose rate) the technologist spends in patient positioning, attaching ECG electrodes, and other instructions to avoid patient motion during image acquisition. The study also enabled to measure the dose rate immediately after injection and thus we were able to calculate the patient-to-staff effective dose rate coefficient. The value revealed was significantly lower than point source measurements accounting for 48% reduction due to photon absorption and patient self-attenuation. This piece of information has meaningful use and implications in site planning, staff exposure as well as shielding calculations(16). There have been several attempts to reduce the radioactive dose injected into myocardial perfusion SPECT patients. One of those was the use of resolution recovery incorporated into the iterative algorithms. A number of publications reported that a reduction of the injected dose can be realized while able to maintain image quality in standard acquisition or with minimal loss of diagnostic accuracy(17). These methods can be classified as software or hardware-based methods. The former can be seen in resolution recovery algorithms and collimator characteristics modeling whereas the later can be observed in the use of different collimator system or detectors with high detection efficiency(9, 11–14). This would have a definite impact on the dose received by radiation workers since a significant portion of the exposure levels can be reduced. Further looking into our data revealed that on average one cardiac stress study deliver an effective dose of ~29 μSv to the working staff, while a dose of 24 μSv was recorded for cardiac rest examination bearing in mind the variable distances recoded for each step and also the diversity of the medical/technical team across the whole procedure. Stated another way, if one person is committed to perform all steps of SPECT MPI for a given patient he would receive an average effective dose of 29 and 24 μSv in stress and rest study respectively. Furthermore, handling patient after tracer injection represents ~42% of the total dose received in one cardiac stress study. Although this dose is not received by a single individual, it is extremely important to consider it in staff roster as well as any radiation safety measures related to the lab. If we assume one physician is scheduled to perform 10 patients for exercise testing, then the dose expected is going to be on average 123 μSv if the post-injection duration extended to ~7 min. However, this measure could be larger in case some patients would need further assistance or the recovery period is prolonged. Patient escorting and preparation for imaging as well as patient release are also important phases of considerable attention. Our results showed a slightly higher dose during patient setup due to some relevant factors including instructions, positioning, attaching ECG to the chest wall. This time frame was estimated on average as 6 and 5 min for patient setup and release respectively. However, it can be relatively longer in some patients who have fluctuation in cardiac rhythms, atrial fibrillation or in severally ill cohorts. A recent study showed that the exit release dose due to patients injected with three PET radiotracers for neuroendocrine diseases was median of 4.8, 9.5 and 8.8 μSv/h while median dose of 9.4 and 4.9 μSv/h at the level of the sternum for SPECT-based radiopharmaceuticals(18). These values are lower than that reported in this study as the release dose was around 20 μSv/h pinpointing the importance of carefully looking at patients undergoing MPI in busy nuclear medicine departments. These higher values are probably due to the point that our dosing protocol might be slightly higher than other institutions. Overall, staff exposure in the hot lab was estimated as 15 and 10 μSv for the first and second week of the generator. These measurements were a bit challenging as the technologist was in close contact to the lead glass shield handling the radioactivity. As stated above, measurements were performed at 0.5 m but corrections were made so that the actual distance was made at 0.3 m, again as a reasonable trade-off accounting for several movements of the radiation worker during dose preparation. However, another limitation of the study was the absence of dose extremity measurements due to the unavailability of the suitable dosemeters. Patient body surface area and body mass index are widely used in many clinical measures. The former showed a moderate negative correlation with the patient-to-staff effective dose rate coefficient while the later was poorly correlated. This observation might be related to the fact that increasing the surface area of the source could have a relatively large emission rate whereas patients with high BMI have a significant self-attenuation and thus the correlation was not substantially high. This information tells us how the two metrics could have an impact on staff radiation dose based on patient habitus. Recently, the safety measures in our lab have adopted the use of lead aprons by the nursing and medical staff during cardiac exercise testing. Therefore, our future research work would investigate the efficacy of this on the total exposure level as well its implications on a day-by-day practice in addition to staff comfortability in a short and long term perspectives. CONCLUSION This report provides quantitative information on the distribution of radiation burden in nuclear cardiac laboratory. Assisting patient post-stress either during treadmill exercise or pharmacological stress is the most critical time points where staff members receive the highest radiation dose. New imaging technologies including sophisticated reconstruction algorithms and/or highly sensitive radiation detectors would help in minimizing the injected radioactive dose and hence reduction in staff exposure. The study has mapped the relative contribution of radiation doses to staff members due to myocardial perfusion imaging and would be efficiently used to plan ahead the distribution of tasks in the clinic. It can also be used as simple prediction or simulation model in facility design, local rules refinement and ultimately as safety measures in nuclear medicine cardiac laboratory. ACKNOWLEDGEMENTS This work has been presented in part at the Annual Meeting of the Society of Nuclear Medicine and Molecular Imaging (SNMMI), 11–15 June 2016. It was also selected as one of the top 10 posters in Radiation Dosimetry track. REFERENCES 1 Cousins , C. , Miller , D. L. , Bernardi , G. et al. . ICRP Publication 120: radiological protection in cardiology . Ann. ICRP 42 , 1 – 125 ( 2012 ). Google Scholar Crossref Search ADS 2 Acampa , W. , Cuocolo , A. , Petretta , M. et al. . 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Health Phys. 107 , 60 – 72 ( 2014 ). Google Scholar Crossref Search ADS PubMed 17 Slomka , P. J. , Dey , D. , Duvall , W. L. et al. . Advances in nuclear cardiac instrumentation with a view towards reduced radiation exposure . Curr. Cardiol. Rep. 14 , 208 – 216 ( 2012 ). Google Scholar Crossref Search ADS PubMed 18 Zhang-Yin , J. , Dirand , A. S. , Sasanelli , M. et al. . Equivalent dose rate 1 meter from neuroendocrine tumor patients exiting the nuclear medicine department after undergoing imaging . J. Nucl. Med. 58 , 1230 – 1235 ( 2017 ). Google Scholar Crossref Search ADS PubMed © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

Journal

Radiation Protection Dosimetry – Oxford University Press

Published: Dec 1, 2018

References

  • ICRP Publication 120: radiological protection in cardiology
  • Tetrofosmin imaging in the detection of myocardial viability in patients with previous myocardial infarction: comparison with sestamibi and Tl-201 scintigraphy
  • Sestamibi SPECT in the detection of myocardial viability in patients with chronic ischemic left ventricular dysfunction: comparison between visual and quantitative analysis
  • Quantitative thallium-201 and technetium 99m sestamibi tomography at rest in detection of myocardial viability in patients with chronic ischemic left ventricular dysfunction
  • NCRP Report No. 160, Ionizing Radiation Exposure of the Population of the United States, medical exposure—are we doing less with more, and is there a role for health physicists?
  • Exposure to low-dose ionizing radiation from medical imaging procedures
  • Importance of consideration of radiation doses from cardiac imaging procedures and risks of cancer
  • Estimating the reduction in the radiation burden from nuclear cardiology through use of stress-only imaging in the United States and Worldwide
  • Dose reduction in half-time myocardial perfusion SPECT-CT with multifocal collimation
  • Opportunities for improvement on current nuclear cardiology practices and radiation exposure in Latin America: findings from the 65-country IAEA Nuclear Cardiology Protocols cross-sectional Study (INCAPS)
  • Reduced isotope dose and imaging time with a high-efficiency CZT SPECT camera
  • Reduced isotope dose with rapid SPECT MPI imaging: initial experience with a CZT SPECT camera
  • New reconstruction algorithm allows shortened acquisition time for myocardial perfusion SPECT
  • Wide beam reconstruction for half-dose or half-time cardiac gated SPECT acquisitions: optimization of resources and reduction in radiation exposure
  • AAPM Task Group 108: PET and PET/CT shielding requirements
  • Dose equivalent rate constants and barrier transmission data for nuclear medicine facility dose calculations and shielding design
  • Advances in nuclear cardiac instrumentation with a view towards reduced radiation exposure
  • Equivalent dose rate 1 meter from neuroendocrine tumor patients exiting the nuclear medicine department after undergoing imaging