Magnetic Resonance in Medicine

Yeh, Ernest N.; Stuber, Matthias; McKenzie, Charles A.; Botnar, Rene M.; Leiner, Tim; Ohliger, Michael A.; Grant, Aaron K.; Willig‐Onwuachi, Jacob D.; Sodickson, Daniel K.

doi: 10.1002/mrm.20517pmid: 15968671

The use of self‐calibrating techniques in parallel magnetic resonance imaging eliminates the need for coil sensitivity calibration scans and avoids potential mismatches between calibration scans and subsequent accelerated acquisitions (e.g., as a result of patient motion). Most examples of self‐calibrating Cartesian parallel imaging techniques have required the use of modified k‐space trajectories that are densely sampled at the center and more sparsely sampled in the periphery. However, spiral and radial trajectories offer inherent self‐calibrating characteristics because of their densely sampled center. At no additional cost in acquisition time and with no modification in scanning protocols, in vivo coil sensitivity maps may be extracted from the densely sampled central region of k‐space. This work demonstrates the feasibility of self‐calibrated spiral and radial parallel imaging using a previously described iterative non‐Cartesian sensitivity encoding algorithm. Magn Reson Med 54:1–8, 2005. © 2005 Wiley‐Liss, Inc.

Sutton, Bradley P.; Ciobanu, Luisa; Zhang, Xiaozhong; Webb, Andrew

doi: 10.1002/mrm.20531pmid: 15968672

Parallel imaging techniques using arrays of mutually decoupled coils have become standard on almost all clinical imaging systems. Such techniques also have great potential for high‐field magnetic resonance (MR) microscopy, where measurement times are usually long and susceptibility artifacts can be severe. However, it is technically very challenging to design efficient high‐frequency phased arrays for small‐diameter, vertical‐bore magnets, especially since standard decoupling methods, such as impedance mismatched preamplifiers, cannot be easily integrated. A four‐coil phased array was constructed for microimaging at 600 MHz, and sensitivity encoding (SENSE) and generalized autocalibrating partially parallel acquisitions (GRAPPA) reconstructions of spin‐echo and echo‐planar images of the mouse brain were performed to reduce imaging time and susceptibility artifacts, respectively. Magn Reson Med 54:9–13, 2005. © 2005 Wiley‐Liss, Inc.

Gröhn, Heidi I.; Michaeli, Shalom; Garwood, Michael; Kauppinen, Risto A.; Gröhn, Olli H. J.

doi: 10.1002/mrm.20536pmid: 15968651

The feasibility of performing quantitative T1ρ MRI in human brain at 4 T is shown. T1ρ values obtained from five volunteers were compared with T2 and adiabatic Carr–Purcell (CP) T2 values. Measured relaxation time constants increased in order from T2, CP‐T2, T1ρ both in white and gray matter, demonstrating differential sensitivities of these methods to dipolar interactions and/or proton exchange and diffusion in local microscopic field gradients, which are so‐called dynamic averaging (DA) processes. In occipital lobe, all relaxation time constants were found to be higher in white matter than in gray matter, demonstrating contrast denoted as an “inverse transverse relaxation contrast.” This contrast persisted despite changing the delay between refocusing pulses or changing the magnitude of the spin‐lock field strength, which suggests that it does not originate from DA, as might be induced by the presence of Fe, but rather is related to dipolar interactions in the brain tissue. Magn Reson Med 54:14–19, 2005. © 2005 Wiley‐Liss, Inc.

Deichmann, Ralf

doi: 10.1002/mrm.20552pmid: 15968665

A sequence for the acquisition of high‐resolution T1 maps, based on magnetization‐prepared multislice fast low‐angle shot (FLASH) imaging, is presented. In contrast to similar methods, no saturation pulses are used, resulting in an increased dynamic range of the relaxation process. Furthermore, it is possible to acquire data during all relaxation delays because only slice‐selective radiofrequency (RF) pulses are used for inversion and excitation. This allows for a reduction of the total acquisition time, or scanning with a reduced bandwidth, which improves the signal‐to‐noise ratio (SNR). The method generates quantitative T1 maps with an in‐plane resolution of 1 mm, slice thickness of 4 mm, and whole‐brain coverage in a clinically acceptable imaging time of about 19 s per slice. It is shown that the use of off‐center RF pulses does not result in imperfect inversion or magnetization transfer (MT) effects. In addition, an improved fitting algorithm based on smoothed flip angle maps is presented and tested successfully. Magn Reson Med 54:20–27, 2005. © 2005 Wiley‐Liss, Inc.

Vignaud, Alexandre; Maître, Xavier; Guillot, Geneviève; Durand, Emmanuel; de Rochefort, Ludovic; Robert, Philippe; Vivès, Véronique; Santus, Robin; Darrasse, Luc

doi: 10.1002/mrm.20576pmid: 15968677

Burns, Melissa A.; Taylor, Jennifer L.; Wu, Chin‐Lee; Zepeda, Andrea G.; Bielecki, Anthony; Cory, David; Cheng, Leo L.

doi: 10.1002/mrm.20523pmid: 15968646

High‐resolution magic angle spinning (HRMAS) NMR spectroscopy has proven useful for analyzing intact tissue and permitting correlations to be made between tissue metabolites and disease pathologies. Extending these studies to slow‐spinning methodologies helps protect tissue pathological structures from HRMAS centrifuging damage and may permit the study of larger objects. Spinning sidebands (SSBs), which are produced by slow spinning, must be suppressed to prevent the complication of metabolic spectral regions. In this study human prostate tissues, as well as gel samples of a metabolite mixture solution, were measured with continuous‐wave (CW) water presaturation on a 14.1T spectrometer, with HRMAS spinning rates of 250, 300, 350, 600, and 700 Hz, and 3.0 kHz. Editing the spectra by means of a simple minimum function (Min(A, B, …, N) for N spectra acquired at different but close spinning rates) produced SSB‐free spectra. Statistically significant linear correlations were observed for metabolite concentrations quantified from the Min(A, B, …, N)‐edited spectra generated at low spinning rates, with concentrations measured from the 3 kHz spectra, and also with quantitative pathology. These results indicate the empirical utility of this scheme for analyzing intact tissue, which also may be used as an adjunct tool in pathology for diagnosing disease. Magn Reson Med 54:34–42, 2005. © 2005 Wiley‐Liss, Inc.

Schmitz, Jonathan E.; Kettunen, Mikko I.; Hu, De‐En; Brindle, Kevin M.

doi: 10.1002/mrm.20529pmid: 15968678

Proton MRS detection of cellular lipid accumulation has been suggested as a noninvasive method for detecting apoptosis or programmed cell death (PCD) in vivo. The spectral changes that have been observed in apoptotic cells include a general increase in lipid signals and a specific increase in the ratio of the lipid methylene‐to‐methyl peak intensities. These changes were investigated here following drug‐induced apoptosis, both in vitro with a murine lymphoma cell line (EL‐4) and in vivo following implantation of these cells to form subcutaneous tumors. Fluorescence microscopy and flow cytometric measurements with a lipophilic dye revealed an accumulation of cytoplasmic lipid droplets in isolated EL‐4 cells undergoing etoposide‐induced apoptosis. 1H MR spectra (both diffusion‐weighted (DW) and unweighted) showed an increase in lipid signals. However, the methylene/methyl peak ratio showed only minimal changes. Localized in vivo spectroscopy of EL‐4 tumors also showed an increase in lipid signals, including a signal from polyunsaturated lipid at 2.8 ppm, after 16–24 h of drug treatment. Again there was no significant change in the methylene/methyl peak ratio. This study confirms that MRS‐detectable lipids accumulate in tumor cells undergoing apoptosis, and therefore may be usable as a marker for the noninvasive detection of tumor cell apoptosis in the clinic. Magn Reson Med 54:43–50, 2005. © 2005 Wiley‐Liss, Inc.

Trabesinger, Andreas H.; Meier, Dieter; Dydak, Ulrike; Lamerichs, Rolf; Boesiger, Peter

doi: 10.1002/mrm.20544pmid: 15968673

Analytical methods are used to characterize the response of the strongly coupled two‐spin system of citrate to point‐resolved spectroscopy (PRESS)‐based sequences at 3 T. The signal output is analyzed line by line, as well as in the Cartesian product operator basis. Patterns with a periodicity of 80.9 ms are identified. Furthermore, it is shown that at TE = n · 80.9 ms (n ∈ {0,1,2,…}), the spin evolution can be described without direct reference to strong coupling terms. The theoretical results are found to be in good agreement with in vivo experiments. These results can be used to design protocols for prostate MRS and MRSI at 3 T, and give guidelines for optimizing spin‐echo‐based acquisition schemes for detecting two‐spin systems at arbitrary field strengths. Magn Reson Med 54:51–58, 2005. © 2005 Wiley‐Liss, Inc.

Weber, Ralph; Wegener, Susanne; Ramos‐Cabrer, Pedro; Wiedermann, Dirk; Hoehn, Mathias

doi: 10.1002/mrm.20532pmid: 15968679

Focal cerebral ischemia was induced in rats and followed with high‐resolution MRI methods for a chronic period of 10 weeks. Two weeks after stroke induction and at the end of the observation period, conventional histological analysis was combined with immunohistochemical staining for macrophages and with Prussian blue staining for the detection of ferric iron. In the late chronic phase, a patchy hypointensity was observed in the ischemic caudoputamen exclusively on T 2*‐weighted (T 2*W) images, with no change in quantitative T1 and T2 relaxation time maps. This characteristic MRI pattern is different from hemorrhagic transformations (HTs) at earlier time points (2 weeks post stroke induction), which became apparent on images of all three imaging sequences. The exclusive T 2*‐sensitive hypointensity colocalized with iron‐positive macrophages in the lesion territory at this time. These iron‐containing macrophages were found predominantly around blood vessels in the ischemic tissue, and interpreted as the result of a phagocytotic incorporation of red blood cells leaking from slowly degrading vessels. The present investigation demonstrates the sensitivity of heavily T 2*W 3D MRI for observing the inflammatory response in the chronic phase after stroke, without prior systemic labeling of the blood‐borne macrophages by iron oxide nanoparticles. Magn Reson Med 54:59–66, 2005. © 2005 Wiley‐Liss, Inc.

Mancuso, Anthony; Zhu, Aizhi; Beardsley, Nancy J.; Glickson, Jerry D.; Wehrli, Suzanne; Pickup, Stephen

doi: 10.1002/mrm.20545pmid: 15968647

An artificial tumor method was developed to study cells inside the sensitive volume of an NMR spectrometer during growth and apoptosis. The tumor was composed of a 50:50 mixture of tightly packed porous‐collagen and nonporous‐polystyrene microspheres. The porous collagen served as a growth surface for the tumor cells, and the nonporous polystyrene served as a structural support to limit compression of the packed bed during perfusion. The microspheres were held between two porous polyethylene discs that were tightly sealed inside the NMR perfusion chamber. The new method was evaluated with two cell types: a mouse mammary tumor line (EMT6/SF) and a human glioma line (SF188). The results indicate that for both lines, ∼109 metabolically active cells could be sustained for at least 1 week in the 12‐cm3 artificial tumor. Further, cells undergoing chemotherapy‐induced apoptosis (which is known to cause detachment of cells from their surroundings) were retained in the artificial tumor. In preliminary 31P NMR studies, glioma cells treated with temozolomide (TMZ) exhibited reduced phosphocholine (PCh) levels relative to glycerophosphocholine (GPC) and diphosphodiester (DPDE) levels. They also exhibited sharply reduced oxygen consumption and TCA cycle 13C labeling, while they retained glycolytic activity. These metabolic changes are consistent with those that would be expected during mitochondrially‐mediated apoptosis. Magn Reson Med 54:67–78, 2005. © 2005 Wiley‐Liss, Inc.