Surface coil phased arrays for high-resolution imaging of the carotid arteries

A three-coil comparison for MR angiography

The purpose of this work was to compare intracranial magnetic resonance angiography (MRA) image quality using three different radiofrequency coils. The three coil types included a reduced volume quadrature birdcage coil with endcap, a commercially available quadrature birdcage head coil, and a four-element phased-array coil. Signal-to-noise ratio (SNR) measurements were obtained from comparison studies performed on a uniform cylindrical phantom. MRA comparisons were performed using data acquired from 15 volunteers and applying a thick-slab three-dimensional time-of-flight sequence. Analysis was performed using the signal difference-to-noise ratio, a quantitative measure of the relative vascular signal. The reduced-volume endcap and phased-array coils, which were designed specifically for imaging the intracranial volume of the head, improved the image SNR and vascular detail considerably over that obtained using the commercially available head coil. The endcap coil configuration provided the best vascular signal overall, while the phased-array coil provided the best results for arteries close to the coil elements.

Closed contour edge detection of blood vessel lumen and outer wall boundaries in black-blood MR images

Quantitative measurements of the blood vessel wall area may provide useful information of atherosclerotic plaque burden, progression and/or regression. Magnetic resonance imaging is a promising technique for identifying both luminal and outer wall boundaries of the human blood vessels. Currently these boundaries are primarily defined manually, a process viewed as labor intensive and subject to significant operator bias. Fully automated post-processing techniques used for identifying the lumen and wall boundaries, on the other hand, are also problematic due to the complexity of signal features in the vicinity of the blood vessels. The goals of this study were to develop a robust, automated closed contour edge detection algorithm, apply this algorithm to high resolution human carotid artery images, and assess its accuracy, and reproducibility. Our algorithm has proven to be sensitive to various contrast situations and is reasonably accurate and highly reproducible.

Hemodynamics of human carotid artery bifurcations: computational studies with models reconstructed from magnetic resonance imaging of normal subjects

PURPOSE

The precise role played by hemodynamics, particularly wall shear stress, in the development and progression of vascular disease remains unclear, in large part because of a lack of in vivo studies with humans. Although technical challenges remain for noninvasively imaging wall shear stresses in humans, vascular anatomy can be imaged with sufficiently high resolution to allow reconstruction of three-dimensional models for computational hemodynamic studies. In this paper we present an entirely noninvasive magnetic resonance imaging (MRI) protocol that provides carotid bifurcation geometry and flow rates from which the in vivo hemodynamics can be computed. Maps of average, oscillatory, and gradients of wall shear stress are presented for two normal human subjects, and their data are compared with those computed for an idealized carotid bifurcation model.

METHODS

An MRI protocol was developed to acquire all necessary image data in scan times suitable for patient studies. Three-dimensional models of the carotid bifurcation lumen were reconstructed from serial black blood MR images of two normal volunteers. Common and internal carotid artery flow rate waveforms were determined from MRI phase-contrast velocity imaging in the same subjects and were used to impose fully developed velocity boundary conditions for the computational model. Subject-specific time-resolved velocities and wall shear stresses were then computed with a finite element-based Navier-Stokes equation solver.

RESULTS

Models reconstructed from in vivo MRI of two subjects showed obvious differences in branch angle, bulb size and extent, and three-dimensional curvature. Maps of a variety of wall shear stress indices showed obvious qualitative differences in patterns between the in vivo models and between the in vivo models and the idealized model. Secondary, helical flow patterns, induced primarily by the asymmetric and curved in vivo geometries, were found to play a key role in determining the resulting wall shear stress patterns. The use of in vivo flow rate waveforms was found to play a minor but noticeable role in some of the wall shear stress behavior observed.

CONCLUSIONS

Conventional "averaged" carotid bifurcation models mask interesting hemodynamic features observed in realistic models derived from noninvasive imaging of normal human subjects. Observation of intersubject variations in the in vivo wall shear stress patterns supports the notion that more conclusive evidence regarding the role of hemodynamics in vascular disease may be derived from such individual studies. The techniques presented here, when combined with subject-specific MRI measurements of carotid artery plaque thickness and composition, provide the tools necessary for entirely noninvasive, prospective, in vivo human studies of hemodynamics and the relationship of hemodynamics to vascular disease.

Characterization of atherosclerotic plaque components by high resolution quantitative MR and US imaging

High resolution MRI at 3 T and US imaging at 50 MHz were used for atherosclerotic plaque characterization. For 14 excised segments of human arteries, conventional MR and US images, quantitative MR T2 maps, US integrated attenuation (IA) maps, and histologic sections were produced and compared. The MR T2 and US attenuation mean values estimated in selected regions of interest were related with tissue type as identified on histologic sections. Significant distinction between media or collagen and lipid or collagen lipidic plaque was achieved with both techniques (MR: P < .001; US: P < .01). Significant distinction was obtained between media and collagen (P < .0001) and between iliac and aortic media (P < .05) with MR T2 but not with IA. MR and US native and parametric images, with different sensitivities to tissue type, provide complementary information useful for quantitative plaque characterization.

On the nature and reduction of plaque-mimicking flow artifacts in black blood MRI of the carotid bifurcation

Cardiac-gated black blood MRI of the carotid artery bifurcation in normal human subjects shows signal within the lumen suggesting wall thickening or atherosclerotic plaque. This signal was believed to be artifactual, arising from complex flow patterns present at the carotid bifurcation. Computer simulation of the hemodynamics and black blood multislice image acquisition in a model of the carotid bifurcation showed that these artifacts arise from spins recovering their signal within the slow, recirculating flow of the carotid bulb. The computed hemodynamics also suggested that these artifacts could be minimized or eliminated entirely by gating the acquisition of slices in the most artifact-prone region of the carotid bulb within a 250-ms window after peak systole. Application of these predictions to studies of normal volunteers showed that, in most cases, these flow artifacts in black blood MRI can be eliminated simply by altering the phase of the cardiac cycle to which the image acquisition is gated. The observation that the size and placement of the saturation slabs had little effect on these artifacts suggested that, in those cases in which recirculation persists throughout the cardiac cycle, either inversion-recovery or presaturation within the bulb itself would be required to suppress them.

An expandable intravenous RF coil for arterial wall imaging

An intravenous (iv) radiofrequency (RF) coil is proposed as a means of obtaining high resolution images of artery wall. The anatomic positioning of peripheral artery/vein pairs was investigated and a phantom mimicking the iliac artery/vein pair was constructed. Imaging results, comparing iv coils with external coils, demonstrated a potential 15- to 20-fold increase in signal-to-noise ratio (SNR) with iv coils. The SNR benefit was measured over a cylindrical volume, adjacent to the coil, and typical of artery position. Prototype expandable iv coils were constructed of Cu-Be loops and introduced via an 8-Fr catheter. The effects of local and remote iv coil tuning were investigated and local tuning was found to provide significant SNR benefits. The in vivo performance of iv RF coils was demonstrated in a porcine animal model. The iv coils were found to be an excellent alternative to intraarterial coils.

Theory and application of array coils in MR spectroscopy

The theory and application of array coils are reviewed in the context of phased array spectroscopy. The optimization of the signal-to-noise ratio from an array of coils is developed by considering the efficiency of a phased array transmit coil. This approach avoids the need to consider noise correlation, and should be useful in future considerations of transmit phased array coils for MR spectroscopy. Methods to characterize array coil performance, including fields and coupling are briefly summarized, along with methods to minimize the effects of mutual inductance. The signal-to-noise advantages of array coils over single coils are examined for both planar and cylindrical arrays. Numerical simulations of planar arrays of 2 x 2, 4 x 4 and 8 x 8 elements and constant overall dimension are compared to a single coil of the same size. The results demonstrate a significant improvement in sensitivity near the array coil. Although the benefits of the array decrease as a function of distance from the array, the array sensitivity never drops below that of a single coil with the same overall dimensions, or that of a single element of the array. Similar results are obtained for a sixteen element cylindrical array, which is compared to a standard quadrature birdcage coil using both computational methods and phantom measurements. The phased array techniques reviewed are demonstrated with proton spectroscopic images of the brain.

Serial magnetic resonance imaging of experimental atherosclerosis detects lesion fine structure, progression and complications in vivo

A major problem in the study of lesions of atherosclerosis is the difficulty of imaging noninvasively the lesions and following their progression in vivo. To address this problem, we have developed advanced magnetic resonance techniques to noninvasively and serially image advanced lesions of atherosclerosis in the rabbit abdominal aorta. Both lumen and wall were imaged with high resolution. Progression of disease, resulting in increase in lesion mass, decrease in arterial lumen, or stenosis, and intralesion complications, can be detected. Images acquired in vivo correlate with the fine structure of the lesions of atherosclerosis, including the fibrous cap, necrotic core, and lesion fissures, as verified by gross examination, dissection microscopy, and histology. The ability to noninvasively identify the features of atherosclerotic plaques, has significant implications for determining risks and benefits associated with different therapeutic approaches.