Purpose A projection onto convex sets reconstruction of multiplexed sensitivity encoded MRI (POCSMUSE) is developed to reduce motion-related artifacts, including respiration artifacts in abdominal imaging and aliasing artifacts in interleaved diffusion weighted imaging (DWI). Theory Images with reduced artifacts are reconstructed with an iterative POCS procedure that uses the coil sensitivity profile as a constraint. This method can be applied to data obtained with different pulse sequences and k-space trajectories. In addition, various constraints can be incorporated to stabilize the reconstruction of ill-conditioned matrices. Methods The POCSMUSE technique was applied to abdominal fast spin-echo imaging data, and its effectiveness in respiratory-triggered scans was evaluated. The POCSMUSE method was also applied to reduce aliasing artifacts due to shot-to-shot phase variations in interleaved DWI data corresponding to different k-space trajectories and matrix condition numbers. Results Experimental results show that the POCSMUSE technique can effectively reduce motion-related artifacts in data obtained with different pulse sequences, k-space trajectories and contrasts. Conclusion POCSMUSE is a general post-processing algorithm for reduction of motion-related artifacts. It is compatible with different pulse sequences, and can also be used to further reduce residual artifacts in data produced by existing motion artifact reduction methods.

A technique suitable for diffusion tensor imaging (DTI) at high field strengths is presented in this work. The method is based on a periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) k-space trajectory using EPI as the signal readout module, and hence is dubbed PRO-PELLER EPI. The implementation of PROPELLER EPI included a series of correction schemes to reduce possible errors associated with the intrinsically higher sensitivity of EPI to off-resonance effects. Experimental results on a 3.0 Tesla MR system showed that the PROPELLER EPI images exhibit substantially reduced geometric distortions compared with single-shot EPI, at a much lower RF specific absorption rate (SAR) than the original version of the PROPELLER fast spin-echo (FSE) technique. For DTI, the self-navigated phase-correction capability of the PROPELLER EPI sequence was shown to be effective for in vivo imaging. A higher signal-to-noise ratio (SNR) compared to single-shot EPI at an identical total scan time was achieved, which is advantageous for routine DTI Key words: PROPELLER imaging; EPI; geometric distortions; specific absorption rate; diffusion tensor imagingThe importance of diffusion tensor imaging (DTI) in white matter diseases is now well recognized by the clinical neurology community. It has several applications, including the detection of pathologically induced alterations in neural fiber architecture resulting from multiple sclerosis (1), traumatic axonal injury (2), adrenoleukodystrophy (3), or tumors (4,5). The current implementation of DTI often uses single-shot echo-planar imaging (EPI) as the signal readout module following diffusion-weighted (DW) magnetization preparation. Due to strong susceptibility effects from the air-tissue interface, however, the EPI images show severe geometric distortions that are prominent especially near the skull base (6,7). As a consequence, image mapping methods based on DTI, such as fractional anisotropy maps or neural fiber tractograms, are inherently prone to errors in regions such as the frontal lobe near the frontal sinus and optic chiasm in the central brain base.Reductions in geometric distortions can be accomplished via a decrease in the total data acquisition time following the RF excitation pulse, so as to reduce influences from off-resonance spins. A typical method to achieve this purpose is the multishot EPI or segmented EPI technique, which splits the series of gradient-echo acquisitions into several TRs, at the expense of possible motion artifacts (8). In DW imaging (DWI) using multishot EPI, navigator phase correction is further needed because of phase inconsistencies in the presence of involuntary motion sensitized by the DW gradients (6,7,9). Alternatively, imaging methods based on spin-echo acquisitions are intrinsically immune to off-resonance effects due to the refocusing functions of the 180°pulses. One way to achieve distortion-free DTI images is to use a periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELL...

Parallel imaging has been demonstrated to reduce the encoding time of MR spectroscopic imaging (MRSI). Here we investigate up to 5-fold acceleration of 2D proton echo planar spectroscopic imaging (PEPSI) at 3T using generalized autocalibrating partial parallel acquisition (GRAPPA) with a 32-channel coil array, 1.5 cm 3 voxel size, TR/TE of 15/2000 ms, and 2.1 Hz spectral resolution. Compared to an 8-channel array, the smaller RF coil elements in this 32-channel array provided a 3.1-fold and 2.8-fold increase in signal-to-noise ratio (SNR) in the peripheral region and the central region, respectively, and more spatial modulated information. Comparison of sensitivityencoding (SENSE) and GRAPPA reconstruction using an 8-channel array showed that both methods yielded similar quantitative metabolite measures (P > 0.1). Concentration values of N-acetyl-aspartate (NAA), total creatine (tCr), choline (Cho), myo-inositol (mI), and the sum of glutamate and glutamine (Glx) for both methods were consistent with previous studies. Using the 32-channel array coil the mean Cramer-Rao lower bounds (CRLB) were less than 8% for NAA, tCr, and Cho and less than 15% for mI and Glx at 2-fold acceleration. At 4-fold acceleration the mean CRLB for NAA, tCr, and Cho was less than 11%. In conclusion, the use of a 32-channel coil array and GRAPPA reconstruction can significantly reduce the measurement time for mapping brain metabolites. Key words: proton echo planar spectroscopic imaging; PEPSI; MR spectroscopic imaging; parallel MRI; 32-channel phase array; GRAPPA MR spectroscopic imaging (MRSI) plays important roles in both clinical diagnosis and biomedical research. One of the main challenges of the conventional MRSI techniques is the lengthy data acquisition time, a result of the many phase-encoding steps required for complete spatial encoding. Several methods have been proposed to reduce scanning time using reduced or weighted k-space acquisition (1). Other methods acquire multiple (typically two to four) individually phase-encoded spin echoes within a single RF excitation to reduce encoding time (2). However, because the acquisition of multiple echoes requires shortened echo spacing, such a method is characterized by a limited spectral resolution. Alternatively, it is possible to acquire all the spatial information in a single shot using fast imaging readout modules, and to encode the spectral information by incrementing the spectral evolution time in separate RF excitations (3-6). In this way, spatial resolution is independent of scanning time, thus high spatial resolution can be achieved. However, the disadvantage of such approaches is the time-consuming spectral encoding process required to achieve high spectral resolution and bandwidth. Proton echo planar spectroscopic imaging (PEPSI) (7-9) uses an oscillating readout gradient to simultaneously acquire spatial and spectral information in a single RF excitation. PEPSI yields spectral resolution that approximates that of conventional MRSI and enables a reduction in the encoding tim...

SUMMARY:There have been numerous reports documenting the graphic reconstruction of 3D white matter architecture in the human brain by means of diffusion tensor MR tractography. Different from other reviews addressing the physics and clinical applications of DTI, this article reviews the computational principles of tractography algorithms appearing in the literature. The simplest voxel-based method and 2 widely used subvoxel approaches are illustrated first, together with brief notes on parameter selection and the restrictions arising from the distinct attributes of tract estimations. Subsequently, some advanced techniques attempting to offer improvement in various aspects are briefly introduced, including the increasingly popular research tracking tool using HARDI. The article explains the inherent technical limitations in most of the algorithms reported to date and concludes by providing a reference guideline for formulating routine applications of this important tool to clinical neuroradiology in an objective and reproducible manner.ABBREVIATIONS CC ϭ corpus callosum; DTI ϭ diffusion tensor imaging; FA ϭ fractional anisotropy; HARDI ϭ high angular resolution diffusion imaging A t the beginning of the 21st century, the radiologic community witnessed the tremendous technical progress toward noninvasive investigations of the white matter architecture in the central nervous system. Thanks to the advancement of DTI plus the ever-growing computer technology of the socalled tractography methods, a comprehensive visualization of the white matter fiber bundles and their relationships to tumors (Fig 1) can be readily displayed without the need for any surgery.1 Needless to say, the potential implications in presurgical planning are huge, particularly if the related technology can be executed objectively and automatically on a daily basis, with the reliability approaching a level that is highly acceptable in routine practice.This article attempts to provide a pedagogic overview of the aspects of diffusion MR tractography that clinical neuroradiologists may find helpful for routine use. Because an overview of data acquisition via DTI and potential clinical applications has been provided in several review articles, 1-3 only the computational means to perform the tractography will be introduced here. In addition, only selective methods will be mentioned, because detailed comprehensive explanations about the massive amount of newly proposed algorithms are neither necessary nor insightful. At the same time, the ultimate restrictions originating from the computation methods will be addressed. With the basic concepts understood, extensions to more sophisticated algorithms would ideally be easily comprehensible. Finally, we conclude with a suggestive guideline for optimal use of diffusion MR tractography in clinical neuroradiology, emphasizing particularly the preparatory work that ought to be done before routine application. The mathematics in this entire article, though inevitable, will be kept at a minimum to ensure readabili...

Recent advances in the treatment of cerebral gliomas have increased the demands on noninvasive neuroimaging for the diagnosis, therapeutic planning, tumor monitoring, and patient outcome prediction. In the meantime, improved magnetic resonance (MR) imaging techniques have shown much potentials in evaluating the key pathological features of the gliomas, including cellularity, invasiveness, mitotic activity, angiogenesis, and necrosis, hence, further shedding light on glioma grading before treatment. In this paper, an update of advanced MR imaging techniques is reviewed, and their potential roles as biomarkers of tumor grading are discussed.

Metabolite T 2 is necessary for accurate quantification of the absolute concentration of metabolites using long-echo-time (TE) acquisition schemes. However, lengthy data acquisition times pose a major challenge to mapping metabolite T 2 . In this study we used proton echo-planar spectroscopic imaging (PEPSI) at 3T to obtain fast T 2 maps of three major cerebral metabolites: N-acetyl-aspartate (NAA), creatine (Cre), and choline (Cho). We showed that PEPSI spectra matched T 2 values obtained using single-voxel spectroscopy (SVS). Data acquisition for 2D metabolite maps with a voxel volume of 0.95 ml (32 ؋ 32 image matrix) can be completed in 25 min using five TEs and eight averages. A sufficient spectral signal-to-noise ratio (SNR) for T 2 estimation was validated by high Pearson's correlation coefficients between logarithmic MR signals and TEs (R 2 ‫؍‬ 0.98, 0.97, and 0.95 for NAA, Cre, and Cho, respectively). In agreement with previous studies, we found that the T 2 values of NAA, but not Cre and Cho, were significantly different between gray matter (GM) and white matter (WM; P < 0.001). The difference between the T 2 estimates of the PEPSI and SVS scans was less than 9%. Consistent spatial distributions of T 2 were found in six healthy subjects, and disagreement among subjects was less than 10%. In summary, the PEPSI technique is a robust method to obtain fast mapping of metabolite Key words: proton echo-planar spectroscopic imaging; singlevoxel spectroscopy; T 2 relaxation time; cerebral metabolites; gray/white matter difference Estimation of the relaxation times of metabolites is necessary for accurate quantification of metabolite concentrations using long-echo-time (TE) acquisition schemes (1-3). Given the T2 relaxation time, metabolite signals acquired at different TEs can be extrapolated to obtain the signal at TE ϭ 0 and thus estimate the concentration of the metabolite (4). Differences in T2 decay are negligible only for short-TE methods (with TE below 10 ms). In several pathological conditions, such as edema and ischemic stroke (5-8), interactions between cerebral metabolites and macromolecules and proteins are known to modify the biochemical environments of the metabolites, which in turn alters the motion-sensitive T2 relaxation time. Accurate estimation of metabolite T2 is therefore especially important in clinical applications of magnetic resonance spectroscopy (MRS) to ascertain whether changes in metabolite MR signals are derived from fluctuations in the metabolite concentration or from changes in the metabolite relaxation time (2,9). In addition to providing information about metabolite concentration, T2 values may give complementary information about metabolite behavior, as has been suggested by studies of brain tumor (9), ischemic stroke (5-7,10), virus infection (11), drug abuse (12), and other neurological disorders (13-15).The T2 relaxation times of metabolites can be measured by collecting multiple spectra over a range of TEs and then fitting the MR signals as a function of TE (1-3,5,10). Se...

Images acquired using the TrueFISP technique (true fast imaging with steady-state precession) are generally believed to exhibit T 2 /T 1 -weighting. In this study, it is demonstrated that with the widely used half-flip-angle preparation scheme, approaching the steady state requires a time length comparable to the scan time such that the transient-state response may dominate the TrueFISP image contrast. Two-dimensional images of the human brain were obtained using various phase-encoding matrices to investigate the transient-state signal behavior. Contrast between gray and white matter was found to change significantly from proton-density-to T 2 /T 1 -weighted as the phase-encoding matrix size increased, which was in good agreement with theoretical predictions. It is concluded that TrueFISP images in general exhibit T 2 /T 1 -contrast, but should be more appropriately regarded as exhibiting a transient-state combination of proton-density and T 2 /T 1 contrast under particular imaging conditions. Interpretation of tissue characteristics from TrueFISP images in clinical practice thus needs to be exercised with caution. Magn Reson Med 48:684 -688, 2002.

The advantage of increased signal-to-noise ratio (SNR) efficiency in balanced steady-state free precession (SSFP) imaging (also denoted as true fast imaging in steady-state precession (TrueFISP), balanced fast field-echo (FFE), or fast imaging employing steady-state acquisition (FIESTA) by various manufacturers) has made this technique attractive for clinical applications. Examples of such applications include (but certainly are not limited to) cardiac imaging (1,2), angiography (3,4), gastrointestinal imaging (5,6), and fetal imaging (7). In certain situations, the signal from fat protons is a major source of interference that hinders our ability to interpret the image unambiguously. This is understood because fat has a higher T 2 /T 1 value compared to parenchymal tissues, which corresponds to bright steady-state signals on SSFP images (8). Therefore, for SSFP imaging applications intended to highlight fluids with large T 2 /T 1 values, such as angiography, myelography, and MR cholangiopancreatography (MRCP), it is essential to eliminate the fat signals.Fat suppression in SSFP imaging can be accomplished by using frequency-selective RF pulses in every TR, similarly to the conventional approach used in spin-echo imaging (5). This method increases TRs that are ordinarily short in generic SSFP sequences, and thus increases total scan time by a noticeable factor. Alternatively, magnetization preparation during the steady state, which refers to the addition of one fat-suppression pulse every several TRs, has also been shown to be effective (9). The latter method is advantageous in that the scan time is not significantly increased, which is beneficial for 3D examinations.Other methods, such as linear combination SSFP (10) and fluctuating equilibrium MR (11), have been proposed that make use of the SSFP spectral profiles manipulated by different RF phase schemes to selectively reconstruct different spectral species. For 2D imaging, the use of a single fat-suppression RF pulse followed by a centric-ordered SSFP readout should serve the same purpose well, with the exception that the resulting image contrast is inevitably altered to proton-density weighting due to the transient-state signal behavior (12).In a recent work, it was shown that SSFP images exhibit spin-echo-like behavior, such that spin isochromats at similar resonant frequencies show phase coherence at either 0°or 180°relative to the RF pulses at the time TR/2, the nominal TE in SSFP imaging (13). For off-resonance species, such as fat relative to water, the SSFP angle (i.e., the precession phase angle for the spin isochromats within one TR in the rotating frame) can be manipulated by adjusting the center reference frequency, which in turn determines the directional location for phase coherence in the rotating frame (13). This property leads naturally to the use of in-phase and out-of-phase images for Dixon addition/subtraction to achieve fat-water separation in SSFP imaging (14). In this study, we demonstrate the feasibility of separating fat and water sig...