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.     

Blood flow imaging by cine magnetic resonance

A technique for measuring blood flow by whole body nuclear magnetic resonance is described. This method uses imaging gradient profiles that combine even echo rephasing with a field echo sequence to overcome the problem of signal loss from flowing blood. The flow velocity component in any desired direction may be measured by appropriate gradient profile modifications, producing velocity dependent phase shifts that can be displayed by phase mapping. The sequence allows for fast repetition so that flow information may be acquired rapidly from many points in the cardiac cycle and has been used in this mode to observe and measure blood flow in the heart chambers and great vessels. Flow measurements in the femoral artery were also carried out using the same technique; these were compared with similar measurements obtained by Doppler ultrasound. The technique can readily be applied using standard imaging equipment and should prove useful in the clinical assessment of many diseases of the cardiovascular system.     

Echo-planar high-resolution flow velocity mapping

A technique for the very rapid measurement of blood flow with high spatial resolution is described. The method combines the previously validated technique of phase velocity mapping and echo-planar principles. The relatively small diameter of blood vessels enables a high-resolution echo-planar flow measurement to be made with as few as 16 echoes such that the method can be incorporated into a near standard NMR scanner. Two sequence variations are tested and validated in vitro and one is used to demonstrate in vivo blood flow measurement. The results are shown to compare well with a previously validated less rapid method. The technique should enhance the potential of NMR flow imaging by enabling sudden changes in flow to be studied. It should also simplify the measurement of blood flow in small mobile vessels such as the coronary arteries.     

Measurement of aortic blood flow with MR imaging: comparative study with Doppler US

An innovative magnetic resonance imaging technique was applied to the measurement of blood flow in the abdominal aorta. The technique combines selective excitation and visualization from an orthogonal view. The distance that fluid has moved is directly visualized. The blood flow velocity at every 50 msec throughout the cardiac cycle was measured in a short time (about 4 minutes) using electrocardiographic gating and repeated excitations in each cycle. Measurements were compared with those obtained by Doppler ultrasound (US) as a reference. The pulsatile change of flow velocity in the cycle correlated well with the Doppler US recording. Two flow velocity indexes, peak flow velocity and the velocity integral, also showed good correlation (r = .98 for both). This method is applicable for clinical use and is useful for measurement of high flow rates, as found in arteries.     

Three-dimensional time-of-flight magnetic resonance angiography using spin saturation

A three-dimensional Fourier transform magnetic resonance imaging technique is presented. This procedure can be used to selectively detect flowing material such as blood in arteries and veins. Since flow is detected in a manner in which velocity-induced phase shifts are compensated, signal loss arising from complex flow and turbulence is minimized. The flow image is sensitive to all velocity components of flow. Applications of this technique are limited, however, to relatively straight vessels having appreciable flow. Examples of application of this technique to healthy and diseased carotid arteries are shown.     

Nonsubtractive spiral phase contrast velocity imaging.

Phase contrast velocity imaging is a standard method for accurate in vivo flow measurement. One drawback, however, is that it lengthens the scan time (or reduces the achievable temporal resolution) because one has to acquire two or more images with different flow sensitivities and subtract their phases to produce the final velocity image. Without this step, non-flow-related phase variations will give rise to an erroneous, spatially varying background velocity. In this paper, we introduce a novel phase contrast velocity imaging technique that requires the acquisition of only a single image. The idea is to estimate the background phase variation from the flow-encoded image itself and then have it removed, leaving only the flow-related phase to generate a corrected flow image. This technique is sensitive to flow in one direction and requires 50% less scan time than conventional phase contrast velocity imaging. Phantom and in vivo results were obtained and compared with those of the conventional method, demonstrating the new method's effectiveness in measuring flow in various vessels of the body. Magn Reson Med 42:704-713, 1999.     

Differential flow imaging by NMR

A new method for spatially resolved NMR flow measurements, named differential flow imaging (DFI), is introduced and experimentally verified. The DFI technique is based on the fact that flow velocity in any direction may cause a pixel position shift in the phase-encoding direction of a 2DFT NMR image. In this method two flow-influenced magnitude images are obtained by properly encoding and/or compensating the flow velocity. A spatial map of the desired component of the flow velocity can consequently be calculated from these two images. Since the DFI technique uses only the magnitude information of the complex images, it is not sensitive to systematic phase errors in contrast to other methods which are based on the phase measurements. On the other hand, the DFI technique can be combined with the phase measurement methods to perform multidimensional flow measurements in a shorter data acquisition time when the phase errors are small or corrected.     

High temporal resolution phase contrast MRI with multiecho acquisitions.

Velocity imaging with phase contrast (PC) MRI is a noninvasive tool for quantitative blood flow measurement in vivo. A shortcoming of conventional PC imaging is the reduction in temporal resolution as compared to the corresponding magnitude imaging. For the measurement of velocity in a single direction, the temporal resolution is halved because one must acquire two differentially flow-encoded images for every PC image frame to subtract out non-velocity-related image phase information. In this study, a high temporal resolution PC technique which retains both the spatial resolution and breath-hold length of conventional magnitude imaging is presented. Improvement by a factor of 2 in the temporal resolution was achieved by acquiring the differentially flow-encoded images in separate breath-holds rather than interleaved within a single breath-hold. Additionally, a multiecho readout was incorporated into the PC experiment to acquire more views per unit time than is possible with the single gradient-echo technique. A total improvement in temporal resolution by approximately 5 times over conventional PC imaging was achieved. A complete set of images containing velocity data in all three directions was acquired in four breath-holds, with a temporal resolution of 11.2 ms and an in-plane spatial resolution of 2 mm x 2 mm.     

Quantitative measurement of high flow velocities by a spin echo MR technique.

A new method of flow measurement using a spin echo (SE) technique has been developed on the basis of the flow effect that at high velocities signal intensity decreases linearly with increasing flow velocity. Flow velocity is calculated from the signal intensity ratio of the flowing material in two images with the same imaging parameters but different echo times. The linear relationship between the signal intensity and flow velocity was examined with a steady flow phantom. When assessed with steady flows in the phantom, flow velocities calculated by this method were in good agreement with velocities measured by a flow meter. This method was used with ECG gating to measure the blood flow of the right common carotid artery of a healthy volunteer. The measured peak flow velocity and the pattern of flow velocities during systole correlated well with the results obtained by Doppler ultrasound.     

Some results of high-flow-velocity NMR imaging using selection gradient

A simple and new flow velocity measurement technique using conventional spin-echo sequence is proposed and its applications to a preclinical result are presented. This technique utilizes the phase velocity encoding effect due to 180 degrees rf and its corresponding selection gradient. This phase encoding and its phase velocity relations have been obtained by numerical solution of the Bloch equation. A flow velocity measurement obtained with a volunteer using this proposed technique indicates close agreement with other previously measured values.     

Accelerated phase-contrast cine MRI using k-t SPARSE-SENSE

Phase-contrast (PC) cine MRI is a promising method for assessment of pathologic hemodynamics, including cardiovascular and hepatoportal vascular dynamics, but its low data acquisition efficiency limits the achievable spatial and temporal resolutions within clinically acceptable breath-hold durations. We propose to accelerate PC cine MRI using an approach which combines compressed sensing and parallel imaging (k-t SPARSE-SENSE). We validated the proposed 6-fold accelerated PC cine MRI against 3-fold accelerated PC cine MRI with parallel imaging (generalized autocalibrating partially parallel acquisitions). With the programmable flow pump, we simulated a time varying waveform emulating hepatic blood flow. Normalized root mean square error between two sets of velocity measurements was 2.59%. In multiple blood vessels of 12 control subjects, two sets of mean velocity measurements were in good agreement (mean difference = -0.29 cm/s; lower and upper 95% limits of agreement = -5.26 and 4.67 cm/s, respectively). The mean phase noise, defined as the standard deviation of the phase in a homogeneous stationary region, was significantly lower for k-t SPARSE-SENSE than for generalized autocalibrating partially parallel acquisitions (0.05 ± 0.01 vs. 0.19 ± 0.06 radians, respectively; P < 0.01). The proposed 6-fold accelerated PC cine MRI pulse sequence with k-t SPARSE-SENSE is a promising investigational method for rapid velocity measurement with relatively high spatial (1.7 mm × 1.7 mm) and temporal (～35 ms) resolutions.     

The application of phase shifts in NMR for flow measurement

A brief overview of the history of the application of phase shifts in NMR, and in particular NMR imaging, is presented. The imaging methods include direct phase mapping, Fourier flow imaging (where the flow data are Fourier transformed into one dimension of an image), and alternative methods, where flow-related phase shifts are utilized for flow measurement from the magnitude of the signal. A discussion then follows of the principal errors that can affect the accuracy of the various flow imaging techniques, with particular reference to the phase mapping methods that have been used extensively in our institution. The results from a number of experiments are included to illustrate the extent of the errors and methods of removing or minimizing these effects are suggested.     

Rapid quantitation of high-speed flow jets.

Flow jets containing velocities up to 5-7 m/s are common in patients with congenital defects and patients with valvular disease (stenosis and regurgitation). The quantitation of peak velocity and flow volume in these jets is clinically significant but requires specialized imaging sequences. Conventional 2DFT phase contrast sequences require lengthy acquisitions on the order of several minutes. Conventional spiral phase contrast sequences are faster, but are highly corrupted by flow artifacts at these high velocities due to phase dispersion and motion during the excitation and readout. A new prospectively gated method based on spiral phase contrast is presented, which has a sufficiently short measurement interval (<4 ms) to minimize flow artifacts, while achieving high spatial resolution (2 x 2 x 4 mm(3)) to minimize partial volume effects, all within a single breathhold. A complete single-slice phase contrast movie loop with 22 ms true temporal resolution is acquired in one 10-heartbeat breathhold. Simulations indicate that this technique is capable of imaging through-plane jets with velocities up to 10 m/s, and initial studies in aortic stenosis patients show accurate in vivo measurement of peak velocities up to 4.2 m/s (using echocardiography as a reference).     

Comparison of velocity-encoded MR imaging and fluid dynamic modeling of steady and disturbed flow.

The contrast of flow-encoded magnetic resonance (MR) images obtained in vivo and the accuracy of velocity measurements are complicated by the presence of complex flow states. The effects of complex flow states on MR flow-encoded images were studied and quantitative flow information was obtained with an MR phase-subtraction technique. Regions of complex flow, including flow stagnation and separation and laminar flow, could be clearly identified on the phase images. The MR imaging velocity measurements were validated by comparison with numerical simulation results for three-dimensional velocity distributions. The velocity MR images and the profiles obtained from the simulation generally agreed well for flow rates of 660 and 1,680 mL/min. This agreement lends support to both the fluid dynamic model and the physical basis of the phase imaging technique and establishes the validity of flow-encoded phase imaging as an in vivo flow quantitation method, especially under low Reynolds number flow conditions.     

Pulsed-injection method for blood flow velocity measurement in intraarterial digital subtraction angiography

The pulsed-injection method for measuring the velocity of blood flow in intraarterial digital subtraction angiography is described. With this technique, contrast material is injected at a pulsing frequency as high as 15 Hz, so that two or more boluses can be imaged simultaneously. The velocity of flow is determined by measuring the spacing between the boluses and multiplying it by the pulsing frequency. Results of tests with phantoms correlate well with flow measurements obtained with a graduated cylinder for velocities ranging from 8 to 60 cm/sec. The potential of the method for time-dependent velocity measurement has been demonstrated with simulated pulsatile flows.     

Breath-hold MR measurements of blood flow velocity in internal mammary arteries and coronary artery bypass grafts

Breath-hold velocity-encoded cine MR (VENC-MR) imaging is a feasible method for measuring phasic blood flow velocity in small vessels that move during respiration. The purposes of the current study are to compare breathhold VENC-MR measurements of flow velocities in the internal mammary arteries (IMA) with nonbreath-hold measurements and to characterize the systolic and diastolic flow velocity curves in a cardiac cycle in native IMA and IMA grafts. Flow velocity in 30 native IMA and 8 IMA grafts were evaluated with a breath-hold VENC-MR sequence with K-space segmentation and view-sharing reconstruction(TR/TE=16/9 msec, VENC=100 cm/s). In 10 native IMA, nonbreathhold VENC-MR images were acquired as well for comparison. Breath-hold VENC-MR imaging showed significantly higher systolic and diastolic peak velocities in native IMA (43.1 cm/second ± 15.0 and 10.0 cm/second ± 4.8), in comparison to those of nonbreath-hold VENC-MR imaging (27.6 cm/second ± 10.2 and 7.3 cm/second ± 3.9, P<.05). The diastolic/systolic peak velocity ratio in the IMA grafts (.88 ± .41) was significantly higher than that in native IMA (.24 ± .08, P<.01). Interobserver variability in the flow velocity measurement was less than 4%. Breath-hold VENC-MR imaging demonstrated higher peak flow velocity in the IMA than nonbreath-hold VENC-MR imaging. This technique is a rapid and effective method for the noninvasive assessment of blood flow velocity in IMA grafts.     

MR measurement of pulsatile pressure gradients

A magnetic resonance (MR) imaging method for evaluating pulsatile pressure gradients in laminar blood flow is presented. The technique is based on an evaluation of fluid shear and inertial forces from cardiac-gated phase-contrast velocity measurements. The technique was experimentally validated by comparing MR and manometer pressure gradient measurements performed in a pulsatile flow phantom. Analyses of random noise propagation and sampling error were performed to determine the precision and accuracy of the method. The results indicate that a precision of 0.01–0.03 mmHg/cm and an accuracy of better than 8% can be achieved by using standard clinical pulse sequences in tubes exceeding 6 mm in diameter. The authors conclude that MR measurement of pressure gradients is feasible and that additional hemodynamic information may be derived from conventional phase-contrast imaging studies.     

Blood flow volume quantification of cerebral ischemia: comparison of three noninvasive imaging techniques of carotid and vertebral arteries

OBJECTIVE: Determination of blood flow volume is useful in assessing ischemic cerebrovascular disease. We compared the blood flow volume measurement of three noninvasive imaging techniques, namely color velocity imaging quantification, spectral Doppler imaging quantification, and MR phase-contrast flow quantification, to see how well the flow values determined by each technique agreed with one another. SUBJECTS AND METHODS: Flow volume quantification was tested experimentally using a flow simulator and by the three techniques in the vertebral and internal carotid arteries of 40 patients with histories of cerebral ischemia. In the flow simulation study, the flow values in each technique were compared with the phantom flow by the Wilcoxon's signed rank test. In the patient study, the flow values between each paired technique were compared by paired t test. The significance level was taken at p less than 0.05. RESULTS: Flow volumes were measured by color velocity imaging quantification. MR phase-contrast flow quantification agreed with the phantom flow simulation within the tested range, and spectral Doppler imaging quantification values were significantly overestimated. In patients, a large variation of the blood flow volume was obtained between each technique (p < 0.05). Among them, spectral Doppler imaging quantification showed the highest flow values in the vessels (internal carotid arteries, 312.6 mL/min; vertebral arteries, 112.0 mL/min), followed by color velocity imaging quantification (internal carotid arteries, 216.8 mL/min; vertebral arteries, 58.1 mL/min) and MR phase-contrast flow quantification (internal carotid arteries, 169.1 mL/min; vertebral arteries, 66.5 mL/min). CONCLUSION: Blood flow volume measurements determined by the three noninvasive imaging techniques on the same vessel can differ widely, and spectral Doppler imaging quantification consistently overestimated the flow volume. It is, therefore, essential that the same technique, preferably color velocity imaging quantification or MR phase-contrast flow quantification, be used for clinical follow-up investigations in the future.     

多普勒血流超声医学图像的动态三维重建

目的：将超声医学图像三维重建技术与多普勒血流成像技术结合起来，实现超声血流图的动态三维重建。方法：通过对多普勒血流图的彩色编码方式的研究，利用多普勒血流图中Color Bar的信息，解决了从原始多普勒血流图中分离解剖结构和功能信息的问题，实现了心腔内血流的动态三维重建及与心脏解剖结构的三维融合显像。结果：对临床人体实验获取的超声血流图像进行三维重建，所得重建图中，血流信息与解剖结构之间的相互关系正确，与心脏解剖生理情况相符，证明了方法的可行性和有效性。结论：结合超声医学图像三维重建技术和多普勒成像技术，实现超声医学图像功能三维重建，提供更多的医学信息，是超声医学成像技术的发展方向，具有巨大的应用前景。