John A. Roberts

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. J. Magn. Reson. Imaging 2000;11:458–468.

Carotid Magnetization-Prepared Rapid Acquisition With Gradient-Echo Signal Is Associated With Acute Territorial Cerebral Ischemic Events Detected by Diffusion-Weighted MRI

Background— Carotid intraplaque hemorrhage has been associated with symptomatic stroke and can be accurately detected with magnetization-prepared rapid acquisition with gradient-echo (MPRAGE). Currently, there are no studies analyzing carotid MPRAGE signal and territorial ischemic events defined by diffusion restriction in the acute setting. Our aim was to determine the association of carotid MPRAGE signal with acute territorial ischemic events using carotid MPRAGE and brain diffusion tensor imaging. Methods and Results— After the addition of the MPRAGE sequence to the neck MR angiographic protocol, 159 patients with suspected acute stroke were evaluated with both brain diffusion tensor imaging and carotid MPRAGE sequences over 2 years, providing 318 carotid artery and paired brain images for analysis. Forty-eight arteries were excluded due to extracarotid sources of brain ischemia and 4 were excluded due to carotid occlusion. Two hundred sixty-six arteries were eligible for data analysis. Carotid MPRAGE-positive signal was associated with an acute cerebral territorial ischemic event with a relative risk of 6.4 (P<0.001). The relative risk of a diffusion tensor imaging-positive territorial ischemic event with carotid MPRAGE-positive signal was increased in mild, moderate, and severe stenosis categories (10.3, P<0.001; 2.9, P=0.01; and 2.2, P=0.01, respectively). Conclusions— In the workup of acute stroke, carotid MPRAGE-positive signal was associated with an increased risk of territorial cerebral ischemic events as detected objectively by brain diffusion tensor imaging. The relative risk of stroke was increased in all carotid stenosis categories but was most elevated in the mild stenosis category.Background —Carotid intraplaque hemorrhage has been associated with symptomatic stroke and can be accurately detected with Magnetization-Prepared Rapid Acquisition with Gradient-Echo (MPRAGE). Currently, there are no studies analyzing carotid MPRAGE signal and territorial ischemic events defined by diffusion restriction in the acute setting. Our aim was to determine the association of carotid MPRAGE signal with acute territorial ischemic events using carotid MPRAGE and brain diffusion tensor imaging (DTI). Methods and Results —After the addition of the MPRAGE sequence to the neck MRA protocol, 159 patients with suspected acute stroke were evaluated with both brain DTI and carotid MPRAGE sequences over 2 years, providing 318 carotid artery and paired brain images for analysis. 48 arteries were excluded due to extracarotid sources of brain ischemia and 4 were excluded due to carotid occlusion. 266 arteries were eligible for data analysis. Carotid MPRAGE positive signal was associated with an acute cerebral territorial ischemic event with a relative risk of 6.4 (p<0.001). The relative risk of a DTI positive territorial ischemic event with carotid MPRAGE positive signal was increased in mild, moderate and severe stenosis categories (10.3 p<0.001, 2.9 p=0.01, and 2.2 p=0.01 respectively). Conclusions —In the workup of acute stroke, carotid MPRAGE positive signal was associated with an increased risk of territorial cerebral ischemic events as detected objectively by brain DTI. The relative risk of stroke was increased in all carotid stenosis categories, but was most elevated in the mild stenosis category.

Carotid MPRAGE Signal Is Associated with Acute Territorial Cerebral Ischemic Events Detected by Diffusion Weighted MRI

Background— Carotid intraplaque hemorrhage has been associated with symptomatic stroke and can be accurately detected with magnetization-prepared rapid acquisition with gradient-echo (MPRAGE). Currently, there are no studies analyzing carotid MPRAGE signal and territorial ischemic events defined by diffusion restriction in the acute setting. Our aim was to determine the association of carotid MPRAGE signal with acute territorial ischemic events using carotid MPRAGE and brain diffusion tensor imaging. Methods and Results— After the addition of the MPRAGE sequence to the neck MR angiographic protocol, 159 patients with suspected acute stroke were evaluated with both brain diffusion tensor imaging and carotid MPRAGE sequences over 2 years, providing 318 carotid artery and paired brain images for analysis. Forty-eight arteries were excluded due to extracarotid sources of brain ischemia and 4 were excluded due to carotid occlusion. Two hundred sixty-six arteries were eligible for data analysis. Carotid MPRAGE-positive signal was associated with an acute cerebral territorial ischemic event with a relative risk of 6.4 (P<0.001). The relative risk of a diffusion tensor imaging-positive territorial ischemic event with carotid MPRAGE-positive signal was increased in mild, moderate, and severe stenosis categories (10.3, P<0.001; 2.9, P=0.01; and 2.2, P=0.01, respectively). Conclusions— In the workup of acute stroke, carotid MPRAGE-positive signal was associated with an increased risk of territorial cerebral ischemic events as detected objectively by brain diffusion tensor imaging. The relative risk of stroke was increased in all carotid stenosis categories but was most elevated in the mild stenosis category.Background —Carotid intraplaque hemorrhage has been associated with symptomatic stroke and can be accurately detected with Magnetization-Prepared Rapid Acquisition with Gradient-Echo (MPRAGE). Currently, there are no studies analyzing carotid MPRAGE signal and territorial ischemic events defined by diffusion restriction in the acute setting. Our aim was to determine the association of carotid MPRAGE signal with acute territorial ischemic events using carotid MPRAGE and brain diffusion tensor imaging (DTI). Methods and Results —After the addition of the MPRAGE sequence to the neck MRA protocol, 159 patients with suspected acute stroke were evaluated with both brain DTI and carotid MPRAGE sequences over 2 years, providing 318 carotid artery and paired brain images for analysis. 48 arteries were excluded due to extracarotid sources of brain ischemia and 4 were excluded due to carotid occlusion. 266 arteries were eligible for data analysis. Carotid MPRAGE positive signal was associated with an acute cerebral territorial ischemic event with a relative risk of 6.4 (p<0.001). The relative risk of a DTI positive territorial ischemic event with carotid MPRAGE positive signal was increased in mild, moderate and severe stenosis categories (10.3 p<0.001, 2.9 p=0.01, and 2.2 p=0.01 respectively). Conclusions —In the workup of acute stroke, carotid MPRAGE positive signal was associated with an increased risk of territorial cerebral ischemic events as detected objectively by brain DTI. The relative risk of stroke was increased in all carotid stenosis categories, but was most elevated in the mild stenosis category.

Automatic detection of three-dimensional vascular tree centerlines and bifurcations in high-resolution magnetic resonance angiography

Objectives:We sought to develop a simple and robust algorithm capable of automatically detecting centerlines and bifurcations of a three-dimensional (3D) vascular bed. Materials and Methods:After necessary preprocessing, an appropriate cost function is computed for all vessel voxels and Dijkstras minimum-cost-path algorithm is implemented. By back tracing all the minimum-cost paths, centerlines and bifurcation are detected. The detected paths are then split into segments between adjacent nodes (bifurcations or vessel end-points) and smoothed by curve fitting. Results:Application of the algorithm to both simulated 3D vessels and 3D magnetic resonance angiography (MRA) images of an actual intracranial arterial tree produced well-centered vessel skeletons. Quantitative assessment of the algorithm was performed. For the simulated data, the root mean square error for centerline detection is about half a voxel. For the human intracranial MRA data, the sensitivity, positive predictive value (PPV), and accuracy of bifurcation detection were calculated for different cost functions. The best case gave a sensitivity of 91.4%, a PPV of 91.4%, and an RMS error of 1.7 voxels. Conclusions:To the extent that imperfections are eliminated from the segmented image, the algorithm is effective and robust in automatic and accurate detection of centerlines and bifurcations. The cost function and algorithm used are demonstrated to be an improvement over similar algorithms in the literature.

In vivo and ex vivo measurements of the mean ADC values of lipid necrotic core and hemorrhage obtained from diffusion weighted imaging in human atherosclerotic plaques

To determine the apparent diffusion coefficient (ADC) values of lipid and hemorrhage in atherosclerotic plaque in human carotid arteries in vivo and compare the values obtained from ex vivo carotid endarterectomy specimens.

Validation of an arterial tortuosity measure with application to hypertension collection of clinical hypertensive patients

BackgroundHypertension may increase tortuosity or twistedness of arteries. We applied a centerline extraction algorithm and tortuosity metric to magnetic resonance angiography (MRA) brain images to quantitatively measure the tortuosity of arterial vessel centerlines. The most commonly used arterial tortuosity measure is the distance factor metric (DFM). This study tested a DFM based measurement’s ability to detect increases in arterial tortuosity of hypertensives using existing images. Existing images presented challenges such as different resolutions which may affect the tortuosity measurement, different depths of the area imaged, and different artifacts of imaging that require filtering.MethodsThe stability and accuracy of alternative centerline algorithms was validated in numerically generated models and test brain MRA data. Existing images were gathered from previous studies and clinical medical systems by manually reading electronic medical records to identify hypertensives and negatives. Images of different resolutions were interpolated to similar resolutions. Arterial tortuosity in MRA images was measured from a DFM curve and tested on numerically generated models as well as MRA images from two hypertensive and three negative control populations. Comparisons were made between different resolutions, different filters, hypertensives versus negatives, and different negative controls.ResultsIn tests using numerical models of a simple helix, the measured tortuosity increased as expected with more tightly coiled helices. Interpolation reduced resolution-dependent differences in measured tortuosity. The Korean hypertensive population had significantly higher arterial tortuosity than its corresponding negative control population across multiple arteries. In addition one negative control population of different ethnicity had significantly less arterial tortuosity than the other two.ConclusionsTortuosity can be compared between images of different resolutions by interpolating from lower to higher resolutions. Use of a universal negative control was not possible in this study. The method described here detected elevated arterial tortuosity in a hypertensive population compared to the negative control population and can be used to study this relation in other populations.

The need for phase-encoding flow compensation in high-resolution intracranial magnetic resonance angiography

To demonstrate that the time delay between phase and frequency encoding and the presence of pulsatile blood flow in high‐resolution time‐of‐flight (TOF) imaging of the intracranial arteries (especially near the circle of Willis) can distort the appearance of blood vessels and result in a cross‐hatch—appearing artifact in surrounding tissue.

Comparing Performance of Centerline Algorithms for Quantitative Assessment of Brain Vascular Anatomy

Attributes like length, diameter, and tortuosity of tubular anatomical structures such as blood vessels in medical images can be measured from centerlines. This study develops methods for comparing the accuracy and stability of centerline algorithms. Sample data included numeric phantoms simulating arteries and clinical human brain artery images. Centerlines were calculated from segmented phantoms and arteries with shortest paths centerline algorithms developed with different cost functions. The cost functions were the inverse modified distance from edge (MDFEi), the center of mass (COM), the binary‐thinned (BT)‐MDFEi, and the BT‐COM. The accuracy of the centerline algorithms were measured by the root mean square error from known centerlines of phantoms. The stability of the centerlines was measured by starting the centerline tree from different points and measuring the differences between trees. The accuracy and stability of the centerlines were visualized by overlaying centerlines on vasculature images. The BT‐COM cost function centerline was the most stable in numeric phantoms and human brain arteries. The MDFEi‐based centerline was most accurate in the numeric phantoms. The COM‐based centerline correctly handled the “kissing” artery in 16 of 16 arteries in eight subjects whereas the BT‐COM was correct in 10 of 16 and MDFEi was correct in 6 of 16. The COM‐based centerline algorithm was selected for future use based on the ability to handle arteries where the initial binary vessels segmentation exhibits closed loops. The selected COM centerline was found to measure numerical phantoms to within 2% of the known length. Anat Rec, 2012.

Motion-insensitive carotid intraplaque hemorrhage imaging using 3D inversion recovery preparation stack of stars (IR-prep SOS) technique

Carotid artery imaging is important in the clinical management of patients at risk for stroke. Carotid intraplaque hemorrhage (IPH) presents an important diagnostic challenge. 3D magnetization prepared rapid acquisition gradient echo (MPRAGE) has been shown to accurately image carotid IPH; however, this sequence can be limited due to motion‐ and flow‐related artifact. The purpose of this work was to develop and evaluate an improved 3D carotid MPRAGE sequence for IPH detection. We hypothesized that a radial‐based k‐space trajectory sequence such as “Stack of Stars” (SOS) incorporated with inversion recovery preparation would offer reduced motion sensitivity and more robust flow suppression by oversampling of central k‐space.

Reducing morphological variability of the cervical carotid artery in serial magnetic resonance imaging using a head and neck immobilization device

To evaluate how well a head and neck immobilization device performed in reducing lumen morphology variability in repeated MR imaging of the carotid artery.