Atherosclerotic plaque components in human aortas contrasted by ex vivo imaging using fast spin-echo magnetic resonance imaging and spiral computed tomography.

RATIONALE AND OBJECTIVES: Imaging techniques that distinguish atherosclerotic plaque components may be useful in identifying the nature of the atherosclerotic lesion and determining the best method of treatment for obstructive vascular mining the best method of treatment for obstructive vascular disease. This study compares fast spin-echo (FSE) magnetic resonance (MR) and spiral computed tomography (CT) images of excised human atherosclerotic aortas to determine which imaging technique provides the best contrast between plaque components ex vivo. METHODS: Aortas were imaged using four FSE sequences in MR with and without frequency-selective fat saturation, and using spiral CT without contrast. The average signal intensity of a region of calcification, thrombosis, fatty plaque, and normal vessel wall was measured on all images and compared. RESULTS: The use of fat saturation pulses in MR did not significantly alter the signal from atherosclerotic plaque for the sequences used. Proton density-weighted FSE sequences that collected early echoes were better than other FSE sequences and CT at differentiating calcification from all soft tissues. T2-weighted FSE sequences that collected later echoes were best at soft-tissue discrimination. CONCLUSIONS: The FSE techniques used were superior to nonenhanced spiral CT in discriminating plaque components ex vivo, including calcification.

Enhancement of tumor oxygenation and radiation response by the allosteric effector of hemoglobin, RSR13.

Prior studies using pO(2) microelectrodes have shown that RSR13, an allosteric modifier of hemoglobin, increases tissue oxygenation in vivo. Recently, measurements of tissue oxygenation have been performed by many investigators using blood oxygen level-dependent magnetic resonance imaging (BOLD MRI). In this study, we tested the hypothesis that the BOLD MRI signal ratio in tumors will change after administration of RSR13. NCI-H460 human lung carcinoma cells were used as a xenograft in athymic nude mice. Mice with 1-cm(3) tumors in the flank were anesthetized and mounted on the MRI apparatus, and various doses of RSR13 were administered intraperitoneally (i.p.). MR images were then acquired at 10-min intervals for up to 60 min after injection. The effect of RSR13 on tumor response was studied using the same mouse xenograft model with tumor growth delay measurements. RSR13 increased the MRI signal ratio [Intensity(t)/Intensity(t = 0)] in a dose-dependent manner, with maximum increases occurring 30 min after RSR13 was administered. An RSR13 dose of 200 mg/kg proved to be optimum. Since the MRI signal ratio has been shown previously to be linearly related to tissue oxygenation, the changes in the MRI signal ratio can be attributed to changes in tumor oxygen levels. Using a 200-mg/kg dose of RSR13, with a 10-Gy dose of radiation administered to tumors 30 min later, enhancement of radiation-induced tumor growth delay by RSR13 was observed (enhancement factor = 2.8). Thus our MRI results support and verify the previously reported RSR13-induced increase in tumor oxygenation obtained using pO(2) microelectrodes. Based upon these results and other previous studies, the mechanism of enhancement of the effect of radiation by RSR13 probably involves an increase in tumor oxygenation.

Regional measurements of pulmonary edema by using magnetic resonance imaging.

A three-dimensional magnetic resonance imaging (MRI) method to measure pulmonary edema and lung microvascular barrier permeability was developed and compared with conventional methods in nine mongrel dogs. MRIs were obtained covering the entire lungs. Injury was induced by injection of oleic acid (0.021-0.048 ml/kg) into a jugular catheter. Imaging followed for 0.75-2 h. Extravascular lung water and permeability-related parameters were measured from multiple-indicator dilution curves. Edema was measured as magnetic resonance signal-to-noise ratio (SNR). Postinjury wet-to-dry lung weight ratio was 5.30 +/- 0.38 (n = 9). Extravascular lung water increased from 2.03 +/- 1.11 to 3.00 +/- 1.45 ml/g (n = 9, P < 0.01). Indicator dilution studies yielded parameters characterizing capillary exchange of urea and butanediol: the product of the square root of equivalent diffusivity of escape from the capillary and capillary surface area (D1/2S) and the capillary permeability-surface area product (PS). The ratio of D1/2S for urea to D1/2S for butanediol increased from 0.583 +/- 0.027 to 0.852 +/- 0.154 (n = 9, P < 0.05). Whole lung SNR at baseline, before injury, correlated with D1/2S and PS ratios (both P < 0.02). By using rate of SNR change, the mismatch of transcapillary filtration flow and lymph clearance was estimated to be 0.2-1.8 ml/min. The filtration coefficient was estimated from these values. Results indicate that pulmonary edema formation during oleic acid injury can be imaged regionally and quantified globally, and the results suggest possible regional quantification by using three-dimensional MRI.

The importance of ray pathlengths when measuring objects in maximum intensity projection images.

It is important to understand any process that affects medical data. Once the data have changed from the original form, one must consider the possibility that the information contained in the data has also changed. In general, false negative and false positive diagnoses caused by this post-processing must be minimized. Medical imaging is one area in which post-processing is commonly performed, but there is often little or no discussion of how these algorithms affect the data. This study uncovers some interesting properties of maximum intensity projection (MIP) algorithms which are commonly used in the post-processing of magnetic resonance (MR) and computed tomography (CT) angiographic data. The appearance of the width of vessels and the extent of malformations such as aneurysms is of interest to clinicians. This study will show how MIP algorithms interact with the shape of the object being projected. MIP's can make objects appear thinner in the projection than in the original data set and also alter the shape of the profile of the object seen in the original data. These effects have consequences for width-measuring algorithms which will be discussed. Each projected intensity is dependent upon the pathlength of the ray from which the projected pixel arises. The morphology (shape and intensity profile) of an object will change the pathlength that each ray experiences. This is termed the pathlength effect. In order to demonstrate the pathlength effect, simple computer models of an imaged vessel were created. Additionally, a static MR phantom verified that the derived equation for the projection-plane probability density function (pdf) predicts the projection-plane intensities well (R(2)=0.96). Finally, examples of projections through in vivo MR angiography and CT angiography data are presented.

Functional cardiovascular evaluation by magnetic resonance imaging.

The pursuit of capabilities for the evaluation of functional aspects of cardiovascular disease by MRI has resulted in the development and implementation of a number of interesting techniques that can be performed on a conventional scanner. Some currently available techniques emphasize the production of anatomically accurate images representing different phases of the cardiac cycle; others demonstrate physical changes within the acquired data that reflect motion, such as blood flow. Magnitude data from spin-echo and gradient-echo sequences can be used to produce dynamic images of the cardiovascular system. Phase data can be used to generate flow-based images reflecting the movement of blood protons. These techniques can be applied in the evaluation of ventricular function, valve function, or functional abnormalities in either congenital cardiovascular disease or great artery disease.

Quantitative 3D VUSE pulmonary MRA.

The purposes of this study were to quantitatively evaluate a free-breathing three-dimensional (3D) variable angle uniform signal excitation (VUSE) magnetic resonance angiography (MRA) technique in normal volunteers, to demonstrate breathold 3D VUSE MRA in a normal volunteer, and to investigate the ability of the free-breathing 3D VUSE MRA technique to quantify differential flow in lung transplant patients. A free-breathing 3D VUSE MRA pulse sequence was run on the right lungs of 15 normal volunteers and both lungs of eight single or double lung transplant patients. A breathold scan was also used on one volunteer. No contrast agents were used. Normal lung MRA images were analyzed for maximum level of branching observed and minimum distance between distal vessels seen and the pleura. In patients, differential flow was determined with a program that counted the number of MRA pixels over a threshold signal level in each lung. These values were compared to radionuclide perfusion (Q) scan results. Average observed branching order in normal lung images was 5.9 +/- 0.7. Average distance between the most peripheral vessels seen and the pleura was 0.9 cm. Differential blood flow measured by pulmonary MRA was well correlated with that measured by Q scan (R2 = 0.84, p < 0.005). In addition to providing good visualization of normal pulmonary vessels, this technique was demonstrated to provide accurate estimates of differential blood flow in lung transplant patients free of serious lung scarring.

Evaluation of linear diaphragm-chest expansion models for magnetic resonance imaging motion artifact correction.

The efficacy of Fourier analysis, autoregressive with exogenous input (ARX) and adaptive models to estimate diaphragm position from respiratory belt signal (a measure of chest expansion) was evaluated for the purpose of correcting respiratory motion artifacts in magnetic resonance imaging (MRI). Respiratory belt signal and diaphragm position data were obtained simultaneously during one-dimensional MRI scans with sampling intervals of 100 ms for 128 s (1280 samples). The models were trained using the first 512 data samples for the Fourier method and the first 640 samples for the ARX and adaptive methods. The remaining samples were used as a test set for evaluating the models. Both ARX and adaptive methods produced more accurate results than the Fourier method as reflected by the normalized mean square error (NMSE) and correlation coefficient (R) between the estimated and actual diaphragm position during normal breathing (P < 0.05). However, all three models had difficulty modeling diaphragm positions during breathing plateaus.

Evaluation of variable-angle uniform signal excitation, tilted optimized nonsaturating excitation, and flat radio-frequency pulses in free-breathing non-contrast-enhanced pulmonary MR angiography.

PURPOSE: To improve the visualization of distal pulmonary vessels at magnetic resonance (MR) angiography so effects of pathologic lung conditions (eg, emboli) on circulation may be more easily observed. MATERIALS AND METHODS: Radio-frequency pulses with flip angles that were uniform (flat), linearly increasing (tilted optimized nonsaturating excitation [TONE]), or nonlinearly increasing (variable-angle uniform signal excitation [VUSE]) were used at pulmonary MR angiography in 15 healthy volunteers. Three-dimensional fast imaging with steady-state precession was performed with free breathing. Statistical analysis of signal-to-noise ratios (S/Ns) was performed. RESULTS: At examinations in both lungs, a higher S/N was achieved with VUSE pulses than with TONE pulses, and with both VUSE pulses and TONE pulses than with flat pulses. Relative dispersion was best with VUSE pulses in all lobes of both lungs. Results were statistically significant (P < .01) in 19 of 27 comparisons in the right lung. CONCLUSION: Higher S/N and better signal uniformity were achieved with VUSE pulses than with TONE or flat pulses at pulmonary MR angiography.

Effects of acceleration, jerk, and field inhomogeneities on vessel positions in magnetic resonance angiography.

Blood flow and magnetic field inhomogeneities lead to distortions in MR angiography (MRA) images that present added risk for stereotactic neurosurgical applications. These effects are demonstrated in an MRA image of a model of cerebrovasculature. Analysis of the effects of velocity, acceleration, jerk, and field inhomogeneities on vessel position is presented; results are used to predict vessel shifts for several cerebral blood vessels. The actual encoded position for flowing spins is shown to be a moment-weighted average position. Maximum shift of 3.11 mm was reduced to 0.05 mm when velocity compensation was added. Velocity compensation applied specifically in the phase-encoding direction reduces flow-dependent shifts to the point that they can be safely ignored even if acceleration and jerk are present. Those prescribing and using MRA images for stereotactic applications must be aware of whether compensation is actually applied along the phase-encoding axis when a flow-compensated sequence is used.

Variable-angle uniform signal excitation (VUSE) for three-dimensional time-of-flight MR angiography.

A spatially asymmetric RF pulse that improves the uniformity of blood signal intensity and vascular contrast in three-dimensional (3D) MR angiography (MRA) is presented. The pulse, called variable-angle uniform signal excitation (VUSE), was designed to provide uniform signal response and improved contrast for blood flowing through a 3D imaging volume during a FLASH sequence. The VUSE excitation profile was optimized on the basis of the number of pulses seen by the blood, which varied with the velocity of through-plane flow, repetition time, and slab thickness with the maximum flip angle at the flow exit constrained at 90 degrees. The theoretical results show that the optimal RF pulse gives more uniformity for flow signal than does a linear ramp excitation profile or a Gaussian pulse combined with a presaturation pulse. After truncation and filtering of the VUSE pulse in the time domain, the general shape of the VUSE RF excitation profile is maintained, but the maximum flip angle is reduced. The arteries of the neck in a healthy volunteer were imaged with the VUSE pulse, a constant flip angle (flat) pulse, and a linear ramp pulse in flow-compensated 3D MRA sequences. The VUSE pulse produced the most uniform signal as evidenced by the lowest relative dispersion of signal along the left vertebral artery (18.0 versus 26.4 to 23.6 for the other studies). F-distribution tests also showed that the signal distribution obtained with the VUSE pulse in a 3D FLASH sequence was statistically different from that for the flat and the linear ramp pulses.

Comparison of projection algorithms used for the construction of maximum intensity projection images.

OBJECTIVE: Four methods of producing maximum intensity projection (MIP) images were studied and compared. MATERIALS AND METHODS: Three of the projection methods differ in the interpolation kernel used for ray tracing. The interpolation kernels include nearest neighbor interpolation, linear interpolation, and cubic convolution interpolation. The fourth projection method is a voxel projection method that is not explicitly a ray-tracing technique. The four algorithms' performance was evaluated using a computer-generated model of a vessel and using real MR angiography data. The evaluation centered around how well an algorithm transferred an object's width to the projection plane. RESULTS: The voxel projection algorithm does not suffer from artifacts associated with the nearest neighbor algorithm. Also, a speed-up in the calculation of the projection is seen with the voxel projection method. Linear interpolation dramatically improves the transfer of width information from the 3D MRA data set over both nearest neighbor and voxel projection methods. Even though the cubic convolution interpolation kernel is theoretically superior to the linear kernel, it did not project widths more accurately than linear interpolation. A possible advantage to the nearest neighbor interpolation is that the size of small vessels tends to be exaggerated in the projection plane, thereby increasing their visibility. CONCLUSION: The results confirm that the way in which an MIP image is constructed has a dramatic effect on information contained in the projection. The construction method must be chosen with the knowledge that the clinical information in the 2D projections in general will be different from that contained in the original 3D data volume.

Three-dimensional MR imaging of the coronary arteries: preliminary clinical experience.

A three-dimensional (3D) magnetization-prepared (MP) rapid gradient-echo (RAGE) and 3D RAGE technique was used to image the coronary arteries in healthy volunteers and patients with known disease. Each sequence produced images of volumes partitioned into 16 thin sections with differing blood-fat-myocardium contrast. The two types of images were subtracted to null fat signal, thus producing a third image set that showed flowing blood. Total imaging time was about 17 minutes. In the volunteers, the 3D MP-RAGE and subtraction images consistently showed the morphology of the right coronary artery. The left main and left anterior descending arteries were also well seen. The circumflex artery was less consistently identified. Of the 17 diseased coronary artery segments identified at catheterization, 16 had altered signal intensity (narrowing, occlusion, reduced contrast-to-noise ratio, irregularity) on the subtraction images, while 13 had altered signal intensity on the 3D MP-RAGE images. The results indicate that this 3D MP-RAGE and 3D RAGE technique has potential utility as a screening method for coronary heart disease.

Evaluation of radiofrequency pulses and contrast agent doses for use in 3D pulmonary magnetic resonance angiography.

Ten healthy volunteers were imaged with breath-hold, three-dimensional (3D) time-of-flight (TOF) magnetic resonance angiography (MRA) using single-variable-angle uniform signal excitation (VUSE), double-VUSE, and flat radiofrequency (RF) pulses with various doses of contrast agent. The ability of each technique to display pulmonary vasculature was evaluated. Images were segmented to isolate lungs, and maximum intensity projections (MIPs) were computed. All MIPs were assigned an image quality (IQ) rating, and signal-to-noise ratios (SNRs) were measured in pulmonary vessels. Without contrast agent, subsegmental vessels were displayed in single- and double-VUSE images while no vessels were visible in flat images. With equal doses of contrast agent, SNRs and IQ ratings were comparable for images obtained with VUSE and flat pulses. In addition, single-VUSE pulses produced more uniform signal from vessels than flat pulses in contrast-enhanced images. The results indicate that non-contrast-enhanced 3D TOF pulmonary MRA with VUSE RF pulses may be a useful screening tool. In addition, contrast-enhanced 3D TOF MRA with VUSE pulses may be useful as a stand-alone technique for assessing the pulmonary vasculature or as an adjunct to contrast-enhanced 3D TOF MRA with flat pulses. J. Magn. Reson. Imaging 10:929-938, 1999.

Estimation and visualization of regional and global pulmonary perfusion with 3D magnetic resonance angiography.

The purposes of this work were to estimate regional and global pulmonary perfusion and display pulmonary vasculature in 10 postoperative lung transplant patients using breath-hold, contrast-enhanced (0.2 mmol/kg, Gd DTPA-BMA, Omniscan, Nycomed, Inc., Princeton, NJ), three-dimensional (3D) magnetic resonance angiography (MRA) with specially designed double-variable-angle uniform signal excitation (VUSE) radio frequency (RF) pulses. Double-VUSE scans imaged both lungs simultaneously during contrast agent injection and provided both qualitative and quantitative information about pulmonary perfusion. Double-VUSE pulses clearly displayed healthy and diseased vessels. There was a strong correlation between contrast-enhanced double-VUSE MRA flow estimates and those measured from nuclear scans for global or whole lung (R(2) = 0.95; P = 0.000002) and upper, central, and lower thirds of the lung (R(2) = 0.89, 0.92, and 0.86, respectively; P < 0.001 for each region). In conclusion, 3D MRA using VUSE pulses in combination with a contrast agent is a valuable tool for the assessment of pulmonary perfusion that simultaneously acquires data for both the qualitative display of pulmonary vessels and the quantification of regional and global differential pulmonary blood flow.

Relative quantification of pulmonary edema with noncontrast-enhanced MRI.

Pulmonary edema is a debilitating effect of acute respiratory distress syndrome. The ability to measure it noninvasively with high sensitivity and in three dimensions could be useful in not only detection but also in assessment and guidance of treatment. To this end, a three-dimensional MRI pulse sequence to measure the formation of edema was developed and tested. Another sequence was tested to measure blood flow in distal pulmonary arteries. Pulmonary edema was induced in nine dogs via venous injections of oleic acid. Edema was verified by wet-to-dry weight ratio (5.30 +/- .38) and extra-vascular lung water at baseline (2.03 +/- 1.12 ml/g dry lung weight) versus postinjury (3.00 +/- 1.45 ml/g) (P < .005). The signal-to-noise ratio within the lungs increased from 5.47 +/- 1.00 at baseline to 7.51 +/- 1.96 (P < .005), and the time course of edema formation was resolved. Results from MR phase-contrast blood flow measurements were variable. The authors conclude that the three-dimensional scan provides a sensitive relative quantification of pulmonary edema formation without the use of contrast agents or ionizing radiation.

Coronary arteries: three-dimensional MR imaging with fat saturation and magnetization transfer contrast.

Magnetic resonance imaging of the coronary arteries is a particularly difficult task because of the small size of the vessels and the motion of the heart during the cardiac and respiratory cycles. The authors developed a non-breath-hold three-dimensional (3D) technique to accomplish this goal. Imaging was performed with voxel sizes of 1.50-4.50 mm3. This allows for excellent multiplanar reconstruction to view the coronary arteries from any angle. The short echo time usually makes blood isointense with surrounding tissue since inflow enhancement is weak with a thick-slab 3D method. This problem is overcome by applying fat saturation and magnetization transfer contrast techniques to suppress the signals of fat and myocardium surrounding the coronary arteries. Respiratory motion artifacts are reduced by taking four acquisitions and averaging the data. The authors acquired the first 3-10 cm of both the left and right coronary arteries in most cases in 7-10 minutes with single slab coverage. Acquisition of multiple slabs should further increase the length of coverage of the coronary arteries. Further improvements will occur when respiratory gating is used.

Magnetic resonance coronary artery imaging.

Coronary artery imaging with magnetic resonance imaging carries the potential for a noninvasive, essentially risk-free screening test for those at risk for coronary heart disease. Many physiologic and anatomic challenges including cardiac and respiratory motion and the small size, tortuosity, and variable flow characteristics of the coronary arteries hamper effort to achieve this goal. This article reviews the efforts of several research groups to surmount these difficulties through the use of 2D and 3D techniques; spin echo, gradient echo, and ultrafast sequences; saturation pulses; and contrast agents. Promising results have been and continue to be reported although no obvious optimal solution has yet been determined.

Electrocardiograph-independent, "wireless" cardiovascular cine MR imaging.

A new electrocardiograph (ECG)-independent, "wireless" gating technique for cine magnetic resonance (MR) imaging was evaluated in 23 cases of cardiovascular disease; in each case, standard ECG-dependent image loops were available for comparison. The ECG-independent strategy references cine MR imaging data retrospectively to inherent periodic changes in MR signal related to the cardiac cycle. With a "double-section" method, both timing data reflecting such changes and imaging data can be acquired simultaneously. "Artificial R waves" are extracted from the timing data acquired with a projection approach. The ECG-independent image loops were diagnostic in 91% of cases. Their overall image quality was at least equal to that of available ECG-dependent versions in only 39% of cases, but this proportion increased to 53% if cases with suboptimal imaging orientations for monitoring of the motion-dependent signal changes were excluded. Orientation appeared to be the primary technical limitation associated with this ECG-independent technique; however, poor ventricular function also significantly impaired performance. Improvement in the performance of the ECG-independent strategy is anticipated with technical advances.

MRI of coronary arteries: 2D breath-hold vs 3D respiratory-gated acquisition.

OBJECTIVE: Respiratory motion degrades MR images of the coronary arteries. The purpose of this study was to assess and compare two methods of reducing the effects of respiration in coronary artery MRI. Single-slice 2D imaging with breath-holding and a respiratory-gated 3D technique were used. MATERIALS AND METHODS: A comparison was made in 10 normal subjects between a 2D multiple breath-holding approach and a 3D technique with and without retrospective respiratory gating in imaging the coronary arteries. RESULTS: Respiratory gating improved the image quality in 76% of the 3D images. Both the 2D and the 3D approaches were capable of visualizing the proximal parts of the coronary arteries, with comparable vessel length and diameter. The image quality was somewhat better for images obtained by breath-holding in 83% of the vessels, probably due to less blurring by remnant respiratory motion and higher inflow contrast. CONCLUSION: The 2D breath-holding approach reveals a better image quality. However, the 3D respiratory-gated acquisition is less operator dependent, faster, and less strenuous for patients.