Real-time black-blood MRI using spatial presaturation

A real-time interactive black-blood imaging system is described. Rapid blood suppression is achieved by exciting and dephasing slabs outside the imaging slice before each imaging excitation. Sharp-profiled radio frequency saturation pulses placed close to the imaging slice provide good blood suppression, even in views containing slow through-plane flow. In vivo results indicate that this technique improves endocardial border definition during systole in real-time cardiac wall-motion studies. Phantom and animal results indicate that this technique nearly eliminates flow artifacts in real-time intravascular studies. J. Magn. Reson. Imaging 2001;13:807-812.     

Split‐acquisition real‐time CINE phase‐contrast MR flow measurements

The temporal and spatial resolution of real‐time phase‐contrast magnetic resonance (PCMR) is restricted by the need to acquire two interleaved phase images. In this article, we propose a split‐acquisition real‐time CINE PCMR technique, where the acquisition of flow‐encoded and flow‐compensated data is divided into separate blocks. By comparing magnitude images, automatic matching of data in cardio‐respiratory space allows subtraction of background phase offsets. Thus, the data is acquired in real‐time but with phase correction originating from a different heart beat. This effectively doubles the frame rate, allowing either higher temporal or spatial resolution. Two split‐acquisition sequences were tested: one with high‐temporal resolution and one with high‐spatial resolution. Both sequences showed excellent agreement in stroke volumes in 20 adults when validated against cardiac‐gated PCMR and interleaved real‐time PCMR (cardiac gated: 95.2 ± 20.0 mL, interleaved real‐time: 96.2 ± 20.7 mL, high‐temporal resolution: 95.6 ± 20.1 mL, high‐spatial resolution: 95.5 ± 20.4 mL). In six children, the high‐spatial resolution sequence provided more accurate flow measurements than interleaved real‐time PCMR, when compared with cardiac‐gated PCMR (cardiac gated: 20.6 ± 7.6 mL, interleaved real‐time: 24.3 ± 9.2 mL, high‐spatial resolution: 20.8 ± 7.8 mL), due to the increased spatial resolution. The matching technique is shown to be accurate (truth: 94.6 ± 21.8, split‐acquisition: 95.0 ± 21.9 mL) and quantitative image quality (signal‐to‐noise ratio, velocity‐to‐noise ratio and edge sharpness) is acceptable. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Assessing arterial blood flow and vessel area variations using real-time zonal phase-contrast MRI

PURPOSE: To measure peripheral artery function using a real-time phase-contrast (PC)-MRI sequence with tailored image-processing algorithms for flow computation. MATERIALS AND METHODS: An approach to real-time flow measurements was developed based on two-dimensional spatially selective excitation pulses and consecutive tailored processing of the data to derive blood flow and vessel area variations. The data acquisition strategy allows for flow measurements at high spatial and temporal resolutions of 1 mm(2) and 50 msec, respectively. In postprocessing the vessel area is automatically extracted using correlation measures in conjunction with morphological image operators. By means of in vitro and in vivo validations, it is shown that the current methods provide accurate and reproducible measurements of flow and vessel area variations. RESULTS: In vitro the comparison between the lumen area measured with the presented method and the values obtained by caliper gauge measurement showed a difference of 3.4% +/- 3.4% (mean +/- 2 SD). Similarly, the comparison between the stroke volumes determined with the presented method and by stopwatch and bucket measurements yielded a difference of 6.1% +/- 2.1%. In vivo the results from the real-time measurements for lumen area and stroke volume were compared with those from a gated PC-MRI technique with differences of 4.8% +/- 14% and 3.0% +/- 24.7%, respectively. CONCLUSION: The presented method constitutes a reliable tool set for quantifying the variations of blood flow and lumen area in the superficial femoral artery during reactive hyperemia and for studying their correlation with cardiovascular risk factors.     

Shared velocity encoding: a method to improve the temporal resolution of phase-contrast velocity measurements

Phase-contrast magnetic resonance imaging (PC-MRI) is used routinely to measure fluid and tissue velocity with a variety of clinical applications. Phase-contrast magnetic resonance imaging methods require acquisition of additional data to enable phase difference reconstruction, making real-time imaging problematic. Shared Velocity Encoding (SVE), a method devised to improve the effective temporal resolution of phase-contrast magnetic resonance imaging, was implemented in a real-time pulse sequence with segmented echo planar readout. The effect of SVE on peak velocity measurement was investigated in computer simulation, and peak velocities and total flow were measured in a flow phantom and in volunteers and compared with a conventional ECG-triggered, segmented k-space phase-contrast sequence as a reference standard. Computer simulation showed a 36% reduction in peak velocity error from 8.8 to 5.6% with SVE. A similar reduction of 40% in peak velocity error was shown in a pulsatile flow phantom. In the phantom and volunteers, volume flow did not differ significantly when measured with or without SVE. Peak velocity measurements made in the volunteers using SVE showed a higher concordance correlation (0.96) with the reference standard than non-SVE (0.87). The improvement in effective temporal resolution with SVE reconstruction has a positive impact on the precision and accuracy of real-time phase-contrast magnetic resonance imaging peak velocity measurements.     

Feasibility of small bowel flow rate measurement with MRI

Objective:

To evaluate whether it is feasible to measure the segmental flux of small bowel content using MR phase‐contrast (PC) pulse sequences.

Materials and methods:

Using a phantom the accuracy of flux measurements was validated. Afterwards, 10 volunteers were included in a prospective clinical trial. To provide standardized small bowel distension, all volunteers ingested four equal portions of 400 mL of water doped with 5 mL gadoterate and 5.8 g Metamucil mite. The MR protocol covering the sagittal cross‐sections of the small bowel included several two‐dimensional (2D) PC sequences with a velocity encoding of 7 cm/second at a temporal resolution of 0.55 second. As proof of concept time‐dependent flux was measured after intravenous (i.v.) administration of a spasmolytic agent in one volunteer.

Results:

Phantom measurements resulted in an excellent correlation between pump and PC measured flow rates (R = 0.999). Time‐resolved small bowel flux was successfully measured in distended small bowel loops of all volunteers. A mean flow rate of 0.188 mL/second (standard deviation ± 0.144 mL/second) was documented. The flux plots presented a sinus wave‐like shape with regular aboral and oral flow. A spasmolytic effect both on flux and motility could be shown with residual flux despite complete arrest of small bowel motility.

Conclusion:

PC MRI allows time‐resolved in vivo measurement of small bowel flux in single well‐distended bowel loops filled with gadolinium‐doped aqueous solution. J. Magn. Reson. Imaging 2010;32:345–351. © 2010 Wiley‐Liss, Inc.     

Real-time cardiac MRI at 3 tesla.

Real-time cardiac and coronary MRI at 1.5T is relatively "signal starved" and the 3T platform is attractive for its immediate factor of two increase in magnetization. Cardiac imaging at 3T, however, is both subtly and significantly different from imaging at 1.5T because of increased susceptibility artifacts, differences in tissue relaxation, and RF homogeneity issues. New RF excitation and pulse sequence designs are presented which deal with the fat-suppression requirements and off-resonance issues at 3T. Real-time cardiac imaging at 3T is demonstrated with high blood SNR, blood-myocardium CNR, resolution, and image quality, using new spectral-spatial RF pulses and fast spiral gradient echo pulse sequences. The proposed sequence achieves 1.5 mm in-plane resolution over a 20 cm FOV, with a 5.52 mm measured slice thickness and 32 dB of lipid suppression. Complete images are acquired every 120 ms and are reconstructed and displayed at 24 frames/sec using a sliding window. Results from healthy volunteers show improved image quality, a 53% improvement in blood SNR efficiency, and a 232% improvement in blood-myocardium CNR efficiency compared to 1.5T.     

Real-time MRI of joint movement with trueFISP

PURPOSE: To develop a technique for dynamic magnetic resonance imaging (MRI) of joint motion based on a combination of real-time TrueFISP (fast imaging with steady state precession) imaging with surface radiofrequency (RF) coils.MATERIALS AND METHODS: The metacarpal, elbow, tarsal, and knee joint of five volunteers and the knees of four patients were examined with a real-time TrueFISP sequence during movement of the joints.RESULTS: All examined joints could be assessed under dynamic conditions with high image contrast and high temporal resolution.CONCLUSION: Dynamic MRI of joints with TrueFISP is feasible and can provide information supplemental to static joint examinations.     

Real-time blood flow imaging using autocalibrated spiral sensitivity encoding.

A novel spiral phase contrast (PC) technique was developed for high temporal resolution imaging of blood flow without cardiac gating. An autocalibrated spiral sensitivity encoding (SENSE) method is introduced and used to reconstruct PC images. Numerical simulations and a flow phantom study were performed to validate the technique. To study the accuracy of the flow measurement in vivo, a high-resolution cardiac experiment was performed and a subset of undersampled SENSE reconstructed data were reconstructed. Good agreement between the velocity measurement from the fully-sampled and undersampled data was achieved. Real-time experiments were performed to measure blood velocity in the ascending aorta and aortic valve, and during a Valsalva maneuver. The results demonstrate the potential of this technique for real-time flow imaging.     

Analysis of systematic and random error in MR volumetric flow measurements

The spatial aspects of error in 2D MR cine phase-velocity mapping are considered in order to define acquisition strategies which will minimize error in measuring volumetric flow. Error was separated into two categories: systematic and random. Potential sources of systematic error examined were intravoxel phase dispersion (IVPD), partial volume effects, misalignment of flow axis and flow-encoding gradients, and improper choice of vessel voxels for flux calculations. Random error was addressed using analysis of propagation of variance. Analytical expressions for sources of error were derived; and computer models were used to test the analytical models. Flow phantom studies examining error in MR volumetric flow measurements were performed and compared with error predicted by the analytical models. Expected error in several clinical situations of interest was then derived to find appropriate acquisition strategies. Spatial resolution, signal to noise ratio, velocity sensitivity and the ratio of the modulus of moving isochromats to that of static isochromats were found to be the most important parameters in controlling error and were found to cause competing effects with respect to systematic and random error.     

Coronary flow and coronary flow reserve measurements in humans with breath-held magnetic resonance phase contrast velocity mapping

Objective evidence for coronary lesion significance can be obtained with ischemic stress testing. Since flow-limiting stenoses have already undergone compensatory vasodilatation to maintain flow, the response to vasoactive stimulation is dampened. The degree of response limitation is reflected by the coronary flow reserve (CFR). Absolute volume flow rates can be accurately and noninvasively measured with MRI techniques. The purpose was to assess the ability to measure coronary volume flow rate noninvasively, and characterize the effect of pharmacologic stress on coronary flow quantitatively by using ultrafast, breath-held segmented k-space phasecontrast-MR imaging (PC-MRI). Ten healthy volunteers were examined by using ultrafast breath-held PC-MRI. Coronary volume flow rates were measured in the anterior descending coronary artery (LAD) at rest and following intravenous administration of dipyridamole. CFR was determined based on these data. Mean LAD volume flow rates increased from 38 ± 11 ml/min before application of dipyridamole to 169 ± 42 ml/min. The mean CFR amounted to 5.0 ± 2.6 (median = 4.15). This study demonstrates the feasibility of breath-held PC-MRI to noninvasively quantify coronary volume flow rates over the cardiac cycle. Pharmacologically induced changes in volume flow rate and thus CFR can be quantitated.     

Large field-of-view real-time MRI with a 32-channel system.

The emergence of parallel MRI techniques and new applications for real-time interactive MRI underscores the need to evaluate performance gained by increasing the capability of MRI phased-array systems beyond the standard four to eight high-bandwidth channels. Therefore, to explore the advantages of highly parallel MRI a 32-channel 1.5 T MRI system and 32-element torso phased arrays were designed and constructed for real-time interactive MRI. The system was assembled from multiple synchronized scanner-receiver subsystems. Software was developed to coordinate across subsystems the real-time acquisition, reconstruction, and display of 32-channel images. Real-time, large field-of-view (FOV) body-survey imaging was performed using interleaved echo-planar and single-shot fast-spin-echo pulse sequences. A new method is demonstrated for augmenting parallel image acquisition by independently offsetting the frequency of different array elements (FASSET) to variably shift their FOV. When combined with conventional parallel imaging techniques, image acceleration factors of up to 4 were investigated. The use of a large number of coils allowed the FOV to be doubled in two dimensions during rapid imaging, with no degradation of imaging time or spatial resolution. The system provides a platform for evaluating the applications of many-channel real-time MRI, and for understanding the factors that optimize the choice of array size.