Flow velocity quantification in human coronary arteries with fast,breath-hold MR angiography

Measurement of coronary artery flow velocities has, until now, largely required the use of invasive technologies. The authors have implemented a breath-hold magnetic resonance (MR) angiography technique for depicting the coronary arteries and for quantifying flow velocities. The method was tested in flow phantoms and then applied to a series of subjects: 11 subjects were studied at rest, and four were studied before and during pharmacologic stress induced by intravenous adenosine. Flow velocities at rest in the midportion of the right coronary artery were 9.9 cm/sec ± 3.5 (n = 12); in the proximal left anterior descending coronary artery, they were significantly higher, measuring 20.5 cm/sec ± 5.2 (n = 6). With adenosine, flow velocities typically increased at least fourfold. The authors conclude that noninvasive measurement of coronary artery flow velocities is feasible with MR angiography; this method may prove useful for determining the physiologic significance of coronary artery stenosis.     

Proximal coronary artery stenosis: Three-dimensional MRI with fat saturation and navigator echo

The purpose of this study was to compare the diagnostic value of MR angiography (MRA) with conventional contrast angiography in coronary artery disease. Thirty-five patients underwent MRA and coronary angiography within 4 hours. Of these, three patients were investigated twice: once before and once after balloon angioplasty. The pulse sequence was a cardiac-triggered, single-slab, three-dimensional gradient-echo sequence, employing a spin-echo navigator echo measurement to track the variation of the diaphragm during the scan. The following segments of the coronary arteries were included in this prospective study: left main coronary artery, proximal and middle left anterior descending, proximal and middle left circumflex, proximal and middle right coronary artery, and intermediate branch, if present. In total, 176 segments were classified as normal or having a stenosis of less than 50% and as having a stenosis of more than 50%. Five patients were excluded because of lack of cooperation. Over all, 45 of 54 stenoses were detected and interpretable by MRA. Sensitivity, specificity, and positive and negative predictive values of MRA for detecting significant stenoses were 83%, 94%, 87%, and 93%, respectively. MRA identified significant stenoses within the major coronary arteries with a high degree of accuracy. Sensitivity and specificity are higher compared with exercise tests or scintigraphy or top of the precise localization.     

MR imaging of coronary artery flow in isolated and in vivo hearts.

Methods for imaging flow in coronary arteries with magnetic resonance (MR) imaging techniques are demonstrated in isolated heart preparations and live animal models. Coronary artery flow was first imaged with a flow-compensated gradient-echo pulse sequence in isovolumic and working perfused rat hearts and then in vivo. A bolus tracking technique was used to measure flow velocity in the coronary arteries. Ultrafast gradient-echo imaging techniques were then applied, with high resolution obtained by combining the information from several cardiac cycles. A stimulated-echo pulse sequence was demonstrated as a method for performing coronary angiography by flow tagging in isovolumic perfused hearts. This report describes the results of coronary flow MR imaging in isolated rat hearts and live mice and rats. The general approach has proved useful in evaluating new methods for coronary MR angiography and should permit well-controlled studies of pathologic conditions. This ability to image coronary flow in isolated hearts and in small animals should permit integrated MR studies of coronary flow, myocardial perfusion, myocardial metabolism, and cellular ionic status.     

MR imaging of poststenotic flow phenomena: experimental studies.

Poststenotic flow patterns were analyzed in a flow phantom with a 1.5-T magnetic resonance (MR) imager, with use of different MR imaging and MR angiographic pulse sequences. Spin-echo, fast field-echo, two-dimensional inflow (multiple single-section technique), and flow-adjusted-gradient sequences were applied. For the spin-echo sequences, modulus and phase images were reconstructed from each data set. The length of the region of poststenotic changes in signal amplitude and phase measured at a constant flow rate increased with stenosis grade. Likewise, the length of the region of poststenotic changes measured at a constant stenosis grade increased with flow rate. Moreover, the results depended on the alignment of the flow direction with the readout gradient. Comparison of modulus and phase images allowed discrimination of turbulent and nonturbulent flow, which yields additional information on stenosis grade in clinical MR angiography.     

Single-breath-hold venous or arterial flow-suppressed pulmonary vascular MR imaging with phased-array coils

A method for acquiring pulmonary vascular magnetic resonance (MR) images with either venous or arterial flow suppression is described. The proposed method only marginally increases the overall imaging time compared with conventional flow-suppression techniques. This enables an acquisition to be completed within a single breath hold with some selectivity as to flow direction. Instead of applying a spatially selective presaturation pulse before each radio-frequency (RF) excitation pulse, the flow presaturation pulse is applied once every 16-20 RF excitation pulses. To avoid image artifacts and to maintain a steady state, each presaturation pulse interval is followed by a normal imaging segment but with data acquisition turned off. Overall imaging time is increased by two TR intervals for each presaturation segment. For a 256 × 128 matrix acquisition, venous flow presaturation increases overall imaging time by approximately 14 TR intervals, while arterial flow suppression increases imaging time by 10 TR intervals.     

Aortoiliac disease: Two-dimensional inflow MR angiography with lipid suppression

A magnetic resonance (MR) imaging strategy, SLIP (spatially separated lipid presaturation), which can be incorporated into existing MR imaging and MR angiographic techniques, has been developed to suppress lipid signal. The authors report the clinical application of this technique, with a triple comparison of two-dimensional inflow MR angiography, with and without SLIP, and x-ray angiography in patients with aortoiliac disease. SLIP improved visualization of arterial segments, with 50 of 63 (79%) arterial segments visualized versus 41 of 63 (65%) for non-SLIP MR angiography. The SLIP strategy aids in the depiction of slow or turbulent flow, because the lipid signal is suppressed while the intravascular signal is left undisturbed. Image quality improves because of the combination of decreased background lipid signal intensity and use of the maximum-intensity-projection algorithm. Compared with x-ray arteriography, non-SLIP MR angiography had a sensitivity and specificity of 60% and 56%, respectively, for detection of lesions with 50%–100% diameter reduction, while SLIP MR angiography had a sensitivity and specificity, respectively, of 53% and 67%.     

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.     

MR characterization of blood flow in native and grafted internal mammary arteries

In the postoperative patient with anginal symptoms, differentiation between bypass graft compromise and nonischemic causes has until now been accomplished only by means of x-ray angiography. A noninvasive test is clearly desirable. The authors used a cine phase-contrast (PC) magnetic resonance (MR) imaging technique to characterize blood flow in native and grafted internal mammary arteries (IMAs). Ten volunteers and 15 patients who had recently undergone IMA coronary artery bypass grafting were imaged. Cine PC MR imaging was performed in the transaxial plane at the level of the pulmonary artery bifurcation. Flow in both IMAs was quantified and expressed as a percentage of cardiac output measured in the ascending aorta. In the 15 patients, flow analysis was performed in both the native and grafted IMAs. In the volunteers, IMA blood flow ranged from 2.1% to 4.3% of cardiac output on the left (mean, 3.5%) and 2.1% to 5.1% (mean, 3.5%) on the right. There was considerable intersubject variability, with coefficients of variation of 10.7% for the left and 12.3% for the right IMA. Intrasubject variability was limited, with estimated common standard deviations of 0.45% of cardiac output (range, 0.2%–1.1%) for the left and 0.39% (range, 0.1%–0.6%) for the right IMA. Flow in grafted IMAs was identified in 13 of 15 patients. In one of two patients without demonstrable IMA graft flow, cardiac catheterization confirmed lack of flow. IMA graft flow varied from 28 to 164 mL/min (mean, 80.3 mL/min). This study shows the feasibility of using cine PC MR imaging as a quantitative method of evaluating blood flow in IMA coronary artery bypass grafts.     

Lumen definition in MR angiography.

Factors affecting blood vessel lumen definition for two-dimensional and three-dimensional inflow magnetic resonance (MR) imaging methods are considered. Vessel definition is affected (a) by the amount of dephasing of the blood in the vessels, both for uncompensated and velocity-compensated gradients; (b) by the image reconstruction technique (normal Fourier reconstruction when asymmetric echoes are collected or a maximum-intensity projection technique in post-processing); (c) by loss of signal due to T2* dephasing; (d) by misregistration; (e) by vessel wall motion; and (f) by partial-volume effects. The first two factors were found to dominate for resolution on the order of 1 mm3. To overcome these dephasing problems, the authors developed asymmetric echo, velocity-compensated sequences with TEs as short as 4.8 msec. The data were then reconstructed with an iterative partial Fourier algorithm, enabling improved lumen definition to be obtained in phantoms and in vivo.     

Pulse sequence strategies for vascular contrast in time-of-flight carotid MR angiography

A systematic evaluation in healthy volunteers of the relative efficacy of various techniques for background suppression to improve two-dimensional (2D) and three-dimensional (3D) time-of-flight magnetic resonance angiography of the cervical carotid arteries was performed. Conventional 2D and 3D FISP (fast imaging with steady-state precession) sequences with flow compensation were compared with modifications of these sequences, including a tracking saturation pulse (2D), prolonged absolute TEs for fat suppression based on T2* decay (2D and 3D), frequency-selective saturation of fat (2D and 3D), in-plane spatial saturation (2D), and magnetization transfer contrast (2D and 3D). The tracking saturation pulse and slight overlap of the excitation sections provided uniform background suppression without impairing depiction of the morphology of the cervical carotid arteries. Frequency-selective fat saturation was the most effective background suppression scheme among the 2D and 3D techniques but was occasionally compromised by local field inhomo-geneities. Magnetization transfer contrast provided little suppression of stationary tissues in the neck because of the intrinsic limitations of the coil. In-plane spatial saturation yielded the highest background suppression but reduced apparent arterial diameters and could not be implemented in a 3D version. The T2* decay method not only reduced the apparent size of the vessels but also their signal intensity.     

Clinical comparison of three-dimensional MP-RAGE and FLASH techniques for MR imaging of the head.

Three-dimensional (3D) MP-RAGE (magnetization-prepared rapid gradient-echo) imaging was evaluated as a high-resolution 3D T1-weighted brain imaging technique for patients with suspected neurologic disease. Fourteen patients were studied. In five, 3D MP-RAGE images were compared with 3D FLASH (fast low-angle shot) images. Signal difference--to-noise ratios and T1 contrast were not statistically different for 3D MP-RAGE images as opposed to 3D FLASH images. Advantages intrinsic to the application of 3D MP-RAGE sequences include decreased imaging time and decreased motion artifact. With this technique, it is possible to perform a relatively motion-insensitive, T1-weighted screening brain study with voxel resolution of 1.0 x 1.4 x 2.0 mm or smaller, in an imaging time of 5.9 minutes or less--permitting offline (poststudy) reconstruction of high-resolution images in any desired plane.     

High-speed black blood imaging of vessel stenosis in the presence of pulsatile flow.

Stenosis phantoms were created to study the ability of "black blood" methods to image a vessel stenosis in the presence of pulsatile flow. Black blood images were acquired with a modified TurboFLASH (fast low-angle shot) method that eliminates flow signal by applying a set of prepulses before segmented data acquisition. With this high-speed approach, imaging can be completed within 16 seconds. This technique was compared with conventional spin-echo black blood, gradient-echo black blood, and gradient-echo bright blood methods. Loss of flow signal, which extended beyond the site of the stenosis, was seen on the gradient-echo bright blood images. The pattern of signal loss varied with the type of stenosis. Flow voids were achieved with spin-echo black blood imaging; however, substantial ghosting artifacts were seen. With gradient-echo black blood imaging, it was difficult to eliminate all flow signal, particularly for in-plane flow. The modified TurboFLASH method produced high-quality black blood images in a fraction of the time needed for spin-echo imaging. It showed no ghosting artifacts even in the presence of pulsatile flow.     

Phase-contrast MR angiography with reduced acquisition time: New concepts in sequence design

A new acquisition scheme for three-dimensional (3D) phase-contrast MR angiography reduces by 33% the measurement time for a data set sensitive to flow in all three orthogonal directions. Background suppression is achieved by acquiring a flow-compensated data set and three data sets flow encoded in the three orthogonal directions, with subsequent complex subtraction. The data are acquired in an interleaved fashion, eliminating misregistration artifacts due to patient motion between measurements sensitive to different flow directions. A standard maximum-intensity-projection algorithm is applied to the combined 3D data set to obtain angiographic projections sensitive to all three orthogonal flow directions. The theory and implementation of the method are described and examples of its application to the intracranial and abdominal circulation are provided.     

T1-weighted spoiled gradient-echo MR imaging of focal hepatic lesion: comparison of in-phase vs opposed-phase pulse sequence

The goal of our prospective study was to compare quantitatively and qualitatively in-phase and opposed-phase T1-weighted breath-hold spoiled gradient-recalled-echo (GRE) MR imaging technique for imaging focal hepatic lesion. Thirty-eight patients with 53 focal hepatic lesions had in-phase (TR = 12.3 ms, TE = 4.2 ms) and opposed-phase (TR = 10.1 ms, TE = 1.9 ms) GRE (flip angle = 30°, bandwidth ± 32 kHz, matrix size 256 × 128, one signal average) MR imaging at 1.5 T. Images were analyzed quantitatively by measuring the lesion-to-liver contrast and for lesion detection. In addition, images were reviewed qualitatively for lesion conspicuity. Quantitatively, lesion-to-liver contrast obtained with in-phase (3.22 ± 1.86) and opposed-phase pulse sequence (3.72 ± 2.32) were not statistically different (Student's t-test). No difference in sensitivity was found between in-phase and opposed-phase pulse sequence (31 of 53, sensitivity 58 % vs 30 of 53, sensitivity 57 %, respectively). Two lesions not seen with opposed-phase imaging were detected with in-phase imaging. Conversely, one lesion not seen on in-phase imaging was detected on opposed-phase imaging so that the combination of in-phase and opposed-phase imaging yielded detection of 32 of 53 lesions (sensitivity 60 %). Qualitatively, lesion conspicuity was similar with both techniques. However, in-phase images showed better lesion conspicuity than opposed-phase images in 9 cases, and opposed-phase images showed better lesion conspicuity than in-phase images in 7 cases. No definite advantage (at a significant level) emerged between in-phase and opposed-phase spoiled GRE imaging. Because differences in lesion conspicuity and lesion detection may be observed with the two techniques in individual cases, MR evaluation of patients with focal hepatic lesion should include both in-phase and opposed-phase spoiled GRE imaging. Received 30 October 1996; Revision received 6 January 1997; Accepted 8 January 1997     

MR flow quantification using race: Clinical application to the carotid arteries

Real time MR flow quantification was performed with real time acquisition and evaluation of motion (RACE) in a rigid phantom under steady flow conditions and in the common carotid arteries of 43 subjects aged 24–78 years. Hemodynamic information included the intraluminal velocity distribution during the complete cardiac cycle, the distensibility of the arterial wall, and age-dependent changes of the flow curves. Systolic peak velocities of 51 ± 33 cm/s and time-averaged volume flow rates of 4.3 ± 2.0 ml/s were measured in healthy subjects. Flow rates below 3.0 ml/s and the observation of abnormal flow patterns indicated stenoses greater than 70% in the region of the bifurcation (sensitivity: 83.3%; specificity: 93.7%; accuracy: 71.4%). Improvements may be achieved from a combination with MR angiography, providing both functional and morphologic vascular information noninvasively within one observer-independent examination. MR imaging, therefore, has a strong potential for the diagnosis of critical stenoses in symptomatic patients.     

MR imaging of knee hyaline cartilage: Evaluation of two- and three-dimensional sequences

Magnetic resonance (MR) imaging of cadaveric knees was performed to determine optimal sequences for visualization of hyaline cartilage. Full-thickness cartilage lesions ranging in diameter from 1 to 5 mm and a partial-thickness cartilage lesion 15 mm in diameter were created in the femoral articular surfaces of three cadaveric knees. The knees were then imaged with a 1.5-T imager with various two-dimensional and high-resolution three-dimensional (3D) techniques. After imaging, the knee specimens were sectioned for evaluation. Measurements of cartilage thickness in fast spin-echo images correlated best with those in the gross specimen. Diffuse areas of cartilage thinning were also most accurately identified with fast spinecho images. Small, focal cartilage defects were best delineated in 3D SPGR (spoiled GRASS [gradient-recalled acquisition in the steady state]) images.     

Repeat cerebral blood volume assessment with first-pass MR imaging

The feasibility of performing multiple first-pass studies with dynamic, contrast material-enhanced magnetic resonance (MR) imaging was evaluated in a cat model of acute middle cerebral artery (MCA) ischemia. Two dynamic series of SSFP (steady-state free precession) images were acquired in each animal (n = 5) with a conventional 1.5-T imager. The initial first-pass study was acquired at 60 minutes after MCA occlusion, and the second study at 70 minutes, with each performed during an intravenous bolus injection of a 0.5 mmol/kg dose of gadoteridol. In both first-pass studies, differentiation of normal and ischemic gray and white matter was highly statistically significant. At a threshold of P<.01, no statistically significant difference in the peak signal intensity between the first and second studies was noted. A difference between the two studies in the recovery to baseline was seen, presumably due to T1 effects. First-pass MR studies can be repeated within the time frame of a single clinical examination, expanding their utility.     

Blood flow modeling in carotid arteries with computational fluid dynamics and MR imaging

RATIONALE AND OBJECTIVES: The authors' goal was to develop a noninvasive method for detailed assessment of blood flow patterns from direct in vivo measurements of vessel anatomy and flow rates. MATERIALS AND METHODS: The authors developed a method to construct realistic patient-specific finite element models of blood flow in carotid arteries. Anatomic models are reconstructed from contrast material-enhanced magnetic resonance (MR) angiographic images with a tubular deformable model along each arterial branch. A surface-merging algorithm is used to create a watertight model of the carotid bifurcation for subsequent finite element grid generation, and a fully implicit scheme is used to solve the incompressible Navier-Stokes equations on unstructured grids. Physiologic boundary conditions are derived from cine phase-contrast MR flow velocity measurements at two locations below and above the bifurcation. Vessel wall compliance is incorporated by means of fluid-solid interaction algorithms. RESULTS: The method was tested on imaging data from a healthy subject and a patient with mild stenosis. Finite element grids were successfully generated, and pulsatile blood flow calculations were performed. Computed and measured velocity profiles show good agreement. Flow patterns and wall shear stress distributions were visualized. CONCLUSIONS: Patient-specific computational fluid dynamics modeling based on MR images can be performed robustly and efficiently. Preliminary validation studies in a physical flow-through model suggest that the model is accurate. This method can be used to characterize blood flow patterns in healthy and diseased arteries and may eventually help physicians to supplement imaging-based diagnosis and predict and evaluate the outcome of interventional procedures.     

Effect of contrast dilution on contrast-enhanced MR angiography of the aorta and renal arteries

The aim of this study was to investigate the effect of gadolinium chelate dilution on vascular enhancement in contrast-enhanced two-dimensional (2D) MR subtraction angiography of aorta and renal arteries. Twenty patients were prospectively included. 2D subtraction MR angiography consisted of successive multisection breathhold GRE acquisitions of 16 s (2D FLASH, TR/TE 72/4, flip angle 60 degrees) obtained in the coronal plane before and after intravenous bolus administration of 0.1 mmol/kg BW gadolinium chelate. Patients underwent both diluted and undiluted gadolinium chelate administration in a random order. The data were studied both qualitatively and quantitatively on source and maximum intensity projection images. The length of renal arteries opacified was found not to differ significantly according to contrast dilution. The contrast enhancement percentage was not significantly modified according to the dilution used, but the time to peak enhancement was observed to be longer with the diluted contrast. Qualitatively, the best MR images were those obtained when undiluted contrast was injected first (chi2, P = 0.01). At a dosage of gadolinium chelate 0.1 mmol/kg BW, undiluted contrast 2D MR subtraction angiography seems to be more appropriate for studying diseases of the aorta and renal arteries than a similar diluted dose.