Continuously moving table time‐of‐flight angiography of the peripheral veins

Time‐of‐flight (TOF) MR angiography allows for noninvasive vessel imaging. To overcome the limited volumetric coverage of standard TOF techniques, the aim of this study was to investigate the combination of TOF and continuously moving table (CMT) acquisitions for peripheral vein imaging based on image subtraction. Two acquisition strategies are presented: a simple two‐step method based on 2‐fold CMT acquisition and an advanced one‐step method requiring only one continuous scan. Image quality of both CMT TOF techniques was evaluated by semiquantitative image grading and by signal‐to‐noise ratio and contrast‐to‐noise ratio analysis for veins of the upper and lower leg in 10 healthy volunteers. Results were compared to a standard stationary two‐dimensional (2D) TOF multistation acquisition. Image grading revealed good image quality for both CMT TOF methods, thereby confirming the feasibility of axial 2D CMT TOF to assess the veins of the lower extremities during a single scan. Quantitative evaluation showed no significant difference in signal‐to‐noise ratio and contrast‐to‐noise ratio compared to the stationary experiment. Additional measurements in three patients with postthrombotic changes and varicosities demonstrated the clinical applicability of the presented methods. CMT TOF provides promising results and permits the detection of various pathologic changes of the venous system. Magn Reson Med 63:1219–1229, 2010. © 2010 Wiley‐Liss, Inc.     

Compressed sensing in hyperpolarized 3He Lung MRI

In this work, the application of compressed sensing techniques to the acquisition and reconstruction of hyperpolarized ³He lung MR images was investigated. The sparsity of ³He lung images in the wavelet domain was investigated through simulations based on fully sampled Cartesian two‐dimensional and three‐dimensional ³He lung ventilation images, and the k‐spaces of 2D and 3D images were undersampled randomly and reconstructed by minimizing the L1 norm. The simulation results show that temporal resolution can be readily improved by a factor of 2 for two‐dimensional and 4 to 5 for three‐dimensional ventilation imaging with ³He with the levels of signal to noise ratio (SNR) (～19) typically obtained. The feasibility of producing accurate functional apparent diffusion coefficient (ADC) maps from undersampled data acquired with fewer radiofrequency pulses was also demonstrated, with the preservation of quantitative information (mean ADC_cs ～ mean ADC_full ～ 0.16 cm² sec^?1). Prospective acquisition of 2‐fold undersampled two‐dimensional ³He images with a compressed sensing k‐space pattern was then demonstrated in a healthy volunteer, and the results were compared to the equivalent fully sampled images (SNR_cs = 34, SNR_full = 19). Magn Reson Med 63:1059–1069, 2010. © 2010 Wiley‐Liss, Inc.     

The influence of temporal resolution in determining pharmacokinetic parameters from DCE‐MRI data

We investigated the influence of the temporal resolution of dynamic contrast‐enhanced MRI data on pharmacokinetic parameter estimation. Dynamic Gd‐DTPA (Gadolinium‐diethylene triamine pentaacetic acid) enhanced MRI data of implanted prostate tumors on rat hind limb were acquired at 4.7 T, with a temporal resolution of ～5 sec. The data were subsequently downsampled to temporal resolutions in the range of 15 sec to 85 sec, using a strategy that involves a recombination of k‐space data. A basic two‐compartment model was fit to the contrast agent uptake curves. The results demonstrated that as temporal resolution decreases, the volume transfer constant (K^trans) is progressively underestimated (～4% to ～25%), and the fractional extravascular extracellular space (v_e) is progressively overestimated (～1% to ～10%). The proposed downsampling strategy simulates the influence of temporal resolution more realistically than simply downsampling by removing samples. Magn Reson Med 63:811–816, 2010. © 2010 Wiley‐Liss, Inc.     

Robust water/fat separation in the presence of large field inhomogeneities using a graph cut algorithm

Water/fat separation is a classical problem for in vivo proton MRI. Although many methods have been proposed to address this problem, robust water/fat separation remains a challenge, especially in the presence of large amplitude of static field inhomogeneities. This problem is challenging because of the nonuniqueness of the solution for an isolated voxel. This paper tackles the problem using a statistically motivated formulation that jointly estimates the complete field map and the entire water/fat images. This formulation results in a difficult optimization problem that is solved effectively using a novel graph cut algorithm, based on an iterative process where all voxels are updated simultaneously. The proposed method has good theoretical properties, as well as an efficient implementation. Simulations and in vivo results are shown to highlight the properties of the proposed method and compare it to previous approaches. Twenty‐five cardiac datasets acquired on a short, wide‐bore scanner with different slice orientations were used to test the proposed method, which produced robust water/fat separation for these challenging datasets. This paper also shows example applications of the proposed method, such as the characterization of intramyocardial fat. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Globally optimal,minimum stored energy,double‐doughnut superconducting magnets

The use of the minimum stored energy current density map–based methodology of designing closed‐bore symmetric superconducting magnets was described recently. The technique is further developed to cater for the design of interventional‐type MRI systems, and in particular open symmetric magnets of the double‐doughnut configuration. This extends the work to multiple magnet domain configurations. The use of double‐doughnut magnets in MRI scanners has previously been hindered by the ability to deliver strong magnetic fields over a sufficiently large volume appropriate for imaging, essentially limiting spatial resolution, signal‐to‐noise ratio, and field of view. The requirement of dedicated interventional space restricts the manner in which the coils can be arranged and placed. The minimum stored energy optimal coil arrangement ensures that the field strength is maximized over a specific region of imaging. The design method yields open, dual‐domain magnets capable of delivering greater field strengths than those used prior to this work, and at the same time it provides an increase in the field‐of‐view volume. Simulation results are provided for 1‐T double‐doughnut magnets with at least a 50‐cm 1‐ppm (parts per million) field of view and 0.7‐m gap between the two doughnuts. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

In vivo assessment of wall shear stress in the atherosclerotic aorta using flow‐sensitive 4D MRI

Our purpose was to correlate atherogenic low wall shear stress (WSS) and high oscillatory shear index (OSI) with the localization of aortic plaques. Flow‐sensitive four‐dimensional MRI was used to acquire three‐dimensional blood flow in the aorta of 62 patients with proven aortic atherosclerosis and 31 healthy volunteers. Multiplanar data analysis of WSS magnitude and OSI in 12 wall segments was performed in analysis planes distributed along the aorta. Disturbed WSS and OSI were defined as areas exposed to low WSS magnitude and high OSI beyond individual 15% thresholds. Planewise analysis revealed a good correlation (r = 0.85) of individual low WSS magnitude but not of high OSI with plaque distribution. Although plaques occurred only rarely in the ascending aorta, the incidence of low WSS magnitude and high OSI was similar to findings in other aortic segments where plaques occurred more frequently. Case‐by‐case comparisons of plaque location and critical wall parameters revealed a shift of atherogenic WSS magnitude (78% of all cases) and OSI (91%) to wall segments adjacent to the atheroma. Our results indicate that the predictive value of WSS for plaque existence depends on the aortic segment and that locations of critical wall parameters move to neighboring segments of regions affected by atherosclerosis. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Efficient implementation of hardware‐optimized gradient sequences for real‐time imaging

This work improves the performance of interactive real‐time imaging with balanced steady‐state free precession. The method employs hardware‐optimized gradient pulses, together with a novel phase‐encoding strategy that simplifies the design and implementation of the optimized gradient waveforms. In particular, the waveforms for intermediate phase‐encode steps are obtained by simple linear combination, rather than separate optimized waveform calculations. Gradient waveforms are redesigned in real time as the scan plane is manipulated, and the resulting sequence operates at the specified limits of the MRI gradient subsystem for each new scan‐plane orientation. The implementation provides 14‐25% improvement in the sequence pulse repetition time over the vendor‐supplied interactive real‐time imaging sequence for similar scan parameters on our MRI scanner. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

The role of MRI in assessment of asymmetrical breast densities

This study included 86 women presented with asymmetric breast densities, seen on either routine screening or diagnostic mammogram.     

Gliosarcoma of the Cerebellar Hemisphere: a Case Report and Review of the Literature

Gliosarcoma is a rare central nervous system tumor usually located in the supratentorial area. Here we report a rare case of a gliosarcoma that developed in the cerebellar hemisphere in a 70-year-old woman. Computed tomography (CT) and magnetic resonance imaging (MRI) of the brain revealed an infratentorial mass of which radiological features were similar to those of glioblastoma. The tumor was diagnosed by pathology as a gliosarcoma. Though rare, gliosarcoma should be considered in the differential diagnosis of infratentorial tumors with radiological features of glioblastoma or metastasis in elderly patients.     

PET、EEG和MRI对儿童难治性癫癎病灶定位的对比研究

目的　探讨正电子发射断层扫描 (PET)、脑电图 (EEG)和磁共振成像 (MRI)在儿童难治性癫疒间 病灶定位方面的诊断价值。方法　对 2 2例难治性癫疒间 患儿分别进行 18F FDGPET、EEG和MRI检查 ,并对癫疒间 病灶的检出率及一致性进行比较。结果　PET显像异常率 90 9% (2 0例 ) ,其中低代谢灶 7例 ,高代谢灶 12例 ,低代谢灶和高代谢灶共存 1例。EEG异常率 81 8% (18例 ) ,在病灶诊断上与PET完全一致为 36 .4 % ,部分一致为36 .4 % ,完全不一致为 2 7.2 %。MRI异常率占 4 0 % ,与PET完全一致为 2 5 % ,完全不一致为 75 %。MRI与PET、EEG比较完全一致率 2 0 % ,部分一致率 5 % ,完全不一致率 75 %。结论　PET显像对儿童难治性癫疒间 病灶的探测敏感性高于EEG和MRI ,在病灶诊断上PET与EEG一致性高于PET与MRI。