Ultra-fast imaging using low flip angles and fids

A new ultra-fast imaging technique that does not place extreme demands on the speed of the gradient system is described. When used with comparable MRI systems, the rotating ultra-fast imaging sequence (RUFIS) can acquire images 4 to 5 times faster than gradient-moment nulled EPI and more than twice as fast as DUFIS, OUFIS, or BURST techniques. Because the technique uses free induction decays instead of echoes, it can be made particularly insensitive to effects of motion, flow, and diffusion. Preliminary images of turbulent flow are presented to demonstrate this insensitivity. However, with appropriate encoding, flow effects may be imaged.     

MRI phase contrast velocity and flow errors in turbulent stenotic jets

PURPOSE: To clarify the use of MRI phase contrast (PC), as an alternative to Doppler echocardiography, when measuring high-velocity turbulent jets associated with stenotic valvular disease. MATERIALS AND METHODS: In vivo PC aortic stroke volume (SV) was compared with ventricular SV in 31 patients with moderate to severe aortic stenosis (AS). Two in vitro pipe experiments were conducted to evaluate errors in steady stenotic and nonstenotic turbulent flows. RESULTS: The average in vivo error in SV was -24% in the left-ventricular (LV) outflow tract (LVOT) and -41% in the aortic root. Errors were most prominent in patients with the highest Doppler peak velocities. In vitro nonstenotic flow experiments showed accurate flow measurement with an average error of 1.8%. Significant errors were found in the in vitro stenotic flow, which reduced with shorter echo times (TE): average error -166/-67/-25/-13/-8.8% for TEs of 4.8/4.0/3.3/2.2/2.0 msec. In both the in vivo and in vitro stenotic experiments the errors were associated with signal loss in the flow-compensated magnitude image. CONCLUSION: Signal loss is associated with flow errors in stenotic jets. Current clinically available PC pulse sequences with TE >2 msec may not accurately quantify flow for severe lesions.     

Three-dimensional projection imaging with half the number of projections

This study demonstrates a sampling scheme for three-dimensional projection imaging with half the number of projections. The angular distribution of projections is designed so that the oversampling of the low spatial frequencies, in tandem with a partial Fourier algorithm, can be used to recreate purposely missing projections. The performance of the sampling scheme and its associated reconstruction algorithm are illustrated with computer-simulated as well as experimental data sets. We find that this technique produces images of comparable quality to the conventional projection imaging scheme. Although a 30% loss of signal-to-noise ratio (SNR) results from its use, the algorithm should prove useful for applications where robustness against motion artifacts and reduced T₂* signal loss are desired.     

One-shot spatially resolved velocity imaging

For quantitative velocity measurement, we have developed a technique that acquires full velocity spectra without cardiac gating. After a cylindrical excitation restricts imaging to one spatial dimension, data are acquired while an oscillating gradient is played out. After each excitation, an image of velocity versus spatial location is obtained. For a given spatial location, a series of these images can be used to form an image of velocity versus time. Acquisition times are much shorter than for phase-contrast imaging or Fourier-encoded velocity imaging, obviating the need for cardiac gating. Although a two-shot version of this technique has been presented previously, we have developed a one-shot version that offers higher temporal resolution for a given velocity resolution and superior off-resonance properties.     

Frequency resolved single-shot MR imaging using stochastic k-space trajectories

A new single-shot stochastic imaging technique with a random k-space path that provides very selective filtering with respect to chemical shift or off-resonance signals of the investigated tissue is proposed. It is demonstrated that in stochastic imaging only on-resonance compartments are visible whereas frequency shifted compartments cancel to noise that is distributed over the whole image. This method can be used as a single-shot chemical shift selective imaging technique and allows to calculate frequency resolved spectra for each spatial position of the image based on a single signal aquisition. The single-shot stochastic imaging sequence makes high demands on the gradient system and the theoretical k-space trajectory is distorted by imperfect gradient performance. Therefore an additional k-space guided imaging technique that uses the true, measured k-space trajectory to correct artifacts generated by eddy currents and delay times of the rapid switched gradients is presented. In vitro and in vivo measurements demonstrate the successful implementation of single-shot stochastic imaging on a conventional MR scanner with unshielded gradient systems.     

Fast proton spectroscopic imaging using the sliced k-space method

The use of one-shot imaging methods for proton spectroscolpic imaging (¹H-SI) is examined. In particular the acquisition of K_x × K_y × N_t data points by means of N_t excitations, each acquiring a K_x, × K_y, k-space slice, is advocated. A number of strategies for realising this experiment, and combining it with water suppression and volume-selection are proposed. The practical implementation at 4.7 T for ¹H-Sl of the rat brain is described. Experimental results from a 32 × 32 spatial matrix with N_t = 64 are presented. Spectra obtained from volumes as low as 3.5 μl and within measuring times of as little as 3.8 min are shown. In these choline, creatine/phosphocreatine and N-acetylaspartate are all clearly visible.     

Magnetic resonance velocity imaging using a fast spiral phase contrast sequence

Time-resolved velocity imaging using the magnetic resonance phase contrast technique can provide clinically important quantitative flow measurements in vivo but suffers from long scan times when based on conventional spin-warp sequences. This can be particularly problematic when imaging regions of the abdomen and thorax because of respiratory motion. We present a rapid phase contrast sequence based on an interleaved spiral k-space data acquisition that permits time-resolved, three-direction velocity imaging within a breath-hold. Results of steady and pulsatile flow phantom experiments are presented, which indicate excellent agreement between our technique and through plane flow measurements made with an in-line ultrasound probe. Also shown are results of normal volunteer studies of the carotids, renal arteries, and heart.     

Reduced phase encoding in spectroscopic imaging

The effect of different spatial-encoding (k-space) sampling distributions are evaluated for magnetic resonance spectroscopic imaging (MRSI) using Fourier reconstruction. Previously, most MRSI studies have used square or cubic k-space functions, symmetrically distributed. These studies examine the conventional k-space distribution with spherical distribution, and 1/2 k-space acquisition, using computer simulation studies of the MRSI acquisition for three spatial dimensions and experimental results. Results compare the spatial response function, Gibbs ringing effects, and signal contamination for different spatial-encoding distribution functions. Results indicate that spherical encoding, in comparison with cubic encoding, results in a modest improvement of the re sponse function with approximately equivalent spatial resolution for the same acquisition time. For spin-echo acquired data, reduced acquisition times can readily be obtained using 1/2 k-space methods, with a concomitant reduction in signal to noise ratio.     

Spectrally weighted twisted projection imaging: Reducing T2 signal attenuation effects in fast three-dimensional sodium imaging

A scheme for the reduction of T₂ signal attenuation effects in three-dimensional twisted projection imaging is presented. By purposely reducing the sample density at the high spatial frequencies, a considerable reduction in readout time is achieved. The reduction in readout time leads to decreased T₂ signal attenuation which translates into improved signal-to-noise ratio (SNR). The SNR improvement is achieved without decreasing the image's resolution since the point spread function depends on the sample weighting as well as the T₂ attenuation. The results indicate that SNR improvements of up to 40% can be achieved using the proposed scheme.     

Quantification of the pulse wave velocity of the descending aorta using axial velocity profiles from phase-contrast magnetic resonance imaging.

The pulse wave velocity (PWV) of aortic blood flow is considered a surrogate for aortic compliance. A new method using phase-contrast (PC)-MRI is presented whereby the spatial and temporal profiles of axial velocity along the descending aorta can be analyzed. Seventeen young healthy volunteers (the YH group), six older healthy volunteers (the OH group), and six patients with coronary artery disease (the CAD group) were studied. PC-MRI covering the whole descending aorta was acquired, with velocity gradients encoding the in-plane velocity. From the corrected axial flow velocity profiles, PWV was determined from the slope of an intersecting line between the presystolic and early systolic phases. Furthermore, the aortic elastic modulus (Ep) was derived from the ratio of the brachial pulse pressure to the strain of the aortic diameter. The PWV increased from YH to OH to CAD (541 +/- 94, 808 +/- 184, 1121 +/- 218 cm/s, respectively; P = 0.015 between YH and OH; P = 0.023 between OH and CAD). There was a high correlation between PWV and Ep (r = 0.861, P < 0.001). Multivariate analysis showed that age and CAD were independent risk factors for an increase in the PWV. Compared to existing methods, our method requires fewer assumptions and provides a more intuitive and objective way to estimate the PWV.     

Measurement of fluid-shear rate by fourier-encoded velocity imaging

A new technique for estimating the blood fluid shear rate at the vessel wall is presented. The technique uses Fourier-encoded velocity imaging to determine the velocity distribution within a spatial element (voxel) that straddles the bloodvessel wall interface. By appropriate processing, the velocity distribution (1) can determine the location of the wall-blood interface within the voxel and (2) estimate the velocity profile across the spatial extent of the voxel. From this information, accurate estimates of fluid shear rate may be obtained. Simulations are presented to illustrate this technique and to show the effects of various error sources, including differences in proton densities between blood and wall tissues and flow-related signal changes. Experimental evidence obtained for steady flow in straight tubes is also presented in support of the technique. The mean error in the experimental shear rate estimates found using the proposed technique was -15%. This represents a significant improvement over estimates obtained by extrapolation of the velocity profile over multiple voxels (mean error of -73%).     

速度向量成像技术对胎儿正常左心室内外膜扭转运动的初步研究

目的运用速度向量成像(VVI)技术定量研究胎儿正常左心室的心内、外膜扭转运动特点。方法采用Siemens Sequoia C512超声诊断仪及VVI技术对30例胎儿心脏正常的左室心肌旋转角度和速度进行定量分析。结果无论心内膜或心外膜,左心室基底部或心尖部收缩期旋转方向无一定规律性。无论心内、外膜,心尖部旋转速度的绝对值高于基底部[(收缩期心内膜:(90±36)°/s对(59±23)°/s,心外膜:(36±17)°/s对(31±17)°/s,P(0.05),(舒张期心内膜:(71±37)°/s对(49±22)°/s,心外膜:(29±14)°/s对(25±14)°/s,均P(0.05)]。无论基底部、心尖部,心内膜旋转速度和角度均高于心外膜,[(收缩期基底部(59±23)°/s对(31±17)°/s,(4.10±2.74)°对(2.35±1.66)°;心尖部:(71±37)°/s对(29±14)°/s,(5.72±2.90)°对(2.80±1.53)°;(舒张期基底部(49±22)°/s对(25±14)°/s,心尖部:(71±37)°/s对(29±14)°/s,均P(0.05)]。结论速度向量成像技术是一种安全有效评价胎儿正常左心室内、外膜的扭转运动的方法。     

组织速度显像技术定量估测先天性心脏病右心室容量负荷过重时右心室功能

目的：探讨组织多普勒显像技术对先天性心脏病(先心病)右心室容量负荷过重时右心室功能评估价值。方法：采集18例正常儿童，28例房间隔缺损(ASD)患者和14例Amplatzer介入治疗后患者心尖四腔心右室长轴切面和腹部剑突下右室短轴切面组织多普勒图像，应用室壁运动速度定量分析技术测量长轴方向右室侧壁基底段、中段、室间隔基底段、短轴方向右室游离壁中段收缩期平均峰值速度(V)、平均峰值应变率(SR)和平均峰值应变(S)并进行比较。28例ASD患者行右心导管检查测定右室等容收缩期最大压力上升速率(max dp/dt)作为右室收缩性能的金标准。结果：ASD长轴方向右室侧壁基底段、中段收缩期V、SR、S均与正常对照组比较差异有显著性意义。室间隔基底段室壁运动速度指标也高于正常，但差异无显著性意义。短轴切面右室游离壁中段收缩期V、SR和S与正常比较差异无显著性意义。14例ASD介入治疗后右室侧壁基底段、中段V、SR和S明显下降。与dp/dt相关分析表明，右室壁基底段和中段V、SR和S与dp／dt显著相关(基底段r分别是0．3398；0．4315；0．7055；中段r分别是0．3051；0．3692；0．5081)。结论：组织速度显像技术可定量估测先心病右室室壁运动功能，长轴方向右室侧壁基底段、中段收缩期平均峰值应变是无创性定量估测右室收缩功能的良好指标。     

彩超定量组织速度成像对肥厚型心肌病舒张运动的评价

目的：探讨定量组织速度成像技术评价肥厚型心肌病患者局部心肌舒张功能以及同步性运动的价值。方法：采集32例肥厚型心肌病患者及38例正常人的动态组织多普勒图像后,将心尖部左室三个切面测得的六个室壁共十二个节段之间的速度指标和时间指标与正常组进行比较。包括Ve、Va、TQ-E、TQ-Q、a（A峰结束至下一个E峰起点时间）、b（S峰持续时间）,以及TQ-E/TQ-Q。心功能指标：包括Ve/Va以及Tei指数。此外还分析了左室内同步性指标,包括Inter-TQ-E（同一节段6个壁间的差值）,Intra-TQ-E（同一壁内2个节段间的差值）以及Max-△TQ-E（左心室12个位点间的最大差值）。结果：正常人和肥厚型心肌病患者部分节段的TQ-E以及TQ-E/TQ-Q存在显著性差异（P〈0.05）。肥厚型心肌病患者Inter-TQ-E,Inter-TQ-E及Max-TQ-E均明显延长。肥厚型心肌病患者速度指标Ve,Va较正常人均有降低,尤其是Ve和正常组比较有非常显著差异（P〈0.01）。而Tei指数,Va/Ve均明显增高。结论：QTVI可定量评价肥厚型心肌病的舒张功能及室壁非同步运动,为临床准确评价其左室舒张功能提供了一种新方法。     

Coronary vessel-wall and lumen imaging using radial k-space acquisition with MRI at 3 Tesla

This study investigates the feasibility of imaging the coronary lumen and vessel-wall, using MRI with a radial k-space trajectory at 3 T. Such radial trajectories offer the advantage of greater vessel sharpness than traditional Cartesian trajectories. This field strength offers an increased signal-to-noise ratio (SNR) compared with 1.5 T, which compensates for the slight SNR reduction due to the radial sequence. Images of the coronary lumen were acquired for seven healthy volunteers. In ten volunteers the vessel wall was scanned, with blood suppression using oblique-slab adiabatic re-inversion. Scans were performed during free breathing, using prospective respiratory navigator-gating. Coronary lumen scans had SNR of 16.0±1.9 and contrast-to-noise ratio (CNR) of 10.3±2.1, showing acceptable image quality. Vessel wall images showed good image quality, with mean SNR of 16.6±2.0/5.8±2.8/10.1±2.2 for vessel wall/lumen/epicardial fat. The wall-blood CNR was 10.7±2.7, and wall-fat CNR was 6.5±2.5. It is concluded that radial gradient-echo imaging at 3 T is a promising method for coronary vessel-wall imaging, and is also feasible for imaging the coronary lumen.     

Fast dynamic imaging using two reference images

This paper presents a fast dynamic imaging method which is characterized by the acquisition of two high-resolution reference images and a sequence of low-resolution dynamic data sets. Image reconstruction is accomplished using a generalized series based algorithm. Experimental results demonstrate that dynamic images with high temporal resolution can be obtained while maintaining excellent spatial resolution. This method will be useful for a variety of dynamic imaging applications including contrast-enhanced dynamic imaging and functional brain studies.     

Optimized parallel imaging for dynamic PC‐MRI with multidirectional velocity encoding

Phase contrast MRI with multidirectional velocity encoding requires multiple acquisitions of the same k‐space lines to encode the underlying velocities, which can considerably lengthen the total scan time. To reduce scan time, parallel imaging is often applied. In dynamic phase contrast MRI using standard generalized autocalibrating partially parallel acquisitions (GRAPPA), several central k‐spaces for autocalibration of the reconstruction (autocalibrating signal lines (ACS)) are typically acquired, separately for each velocity direction and each cardiac timeframe, for calculating the reconstruction weights. To further accelerate data acquisition, we developed two methods, which calculated weights with a substantially reduced number of ACSl lines. The effects on image quality and flow quantification were compared to fully sampled data, standard GRAPPA, and time‐interleaved sampling scheme in combination with generalized autocalibrating partially parallel acquisitions (TGRAPPA). The results show that the two proposed methods can clearly improve scan efficiency while maintaining image quality and accuracy of measured flow or myocardial tissue velocities. Compared to TGRAPPA, the proposed methods were more accurate in evaluating flow velocity. In conclusion, the proposed reconstruction strategies are promising for dynamic multidirectionally encoded acquisitions and can easily be implemented using the standard GRAPPA reconstruction algorithm. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.