Ty A. Cashen

Method for Improving the Accuracy of Quantitative Cerebral Perfusion Imaging

To improve the accuracy of dynamic susceptibility contrast (DSC) measurements of cerebral blood flow (CBF) and volume (CBV).

Quantitative CBV measurement from static T1 changes in tissue and correction for intravascular water exchange

The steady‐state (SS) approach has been proposed to measure quantitative cerebral blood volume (CBV). However, it is known that the CBV value in SS (CBVSS) is subject to error resulting from the effects of water diffusion from the intra‐ to extravascular space. CBVSS measurements were simulated in both fast‐ and no‐water‐exchange limits, and compared with measured CBVSS values to determine which limiting case is appropriate. Twenty‐eight patients were scanned with a segmented Look‐Locker echo‐planar imaging (LL‐EPI) sequence before and after the injection of 0.1 mmol/kg of a T1‐shortening contrast agent. Signal changes and T1 values of brain parenchyma and the blood pool were measured pre‐ and postcontrast. These signal changes and T1 values, in combination with the simulated results, were used to estimate water‐exchange rates. We found that the intra‐ to extravascular water‐exchange rates in white matter (WM) and gray matter (GM) were 0.9 and 1.6 s–1, respectively. With these water‐exchange rates, the fast‐water‐exchange limit of the CBV values showed good agreement with the simulation (r = 0.86 in WM, and 0.78 in GM). The CBV values with the correction for water‐exchange effects were recalculated as 2.73 ± 0.44 and 5.81 ± 1.12 of quantitative cerebral blood water volume (%) in WM and GM, respectively. Magn Reson Med, 2006.

Advanced noninvasive imaging of spinal vascular malformations.

Spinal vascular malformations (SVMs) are an uncommon, heterogeneous group of vascular anomalies that can render devastating neurological consequences if they are not diagnosed and treated in a timely fashion. Imaging SVMs has always presented a formidable challenge because their clinical and imaging presentations resemble those of neoplasms, demyelination diseases, and infection. Advancements in noninvasive imaging modalities (MR and CT angiography) have increased during the last decade and have improved the ability to accurately diagnose spinal vascular anomalies. In addition, intraoperative imaging techniques have been developed that aid in the intraoperative assessment before, during, and after resection of these lesions with minimal and/or optimal use of spinal digital subtraction angiography. In this report, the authors review recent advancements in the imaging of SVMs that will likely lead to more timely diagnoses and treatment while reducing procedural risk exposure to the patients who harbor these uncommon spinal lesions.

4D radial contrast-enhanced MR angiography with sliding subtraction

A method is presented for high spatial and temporal resolution 3D contrast‐enhanced magnetic resonance angiography. The overall technique involves a set of interrelated components suited to high‐frame‐rate angiography, including 3D cylindrical k‐space sampling, angular undersampling, asymmetric sampling, sliding window reconstruction, pseudorandom view ordering, and a sliding subtraction mask. Computer simulations and volunteer studies demonstrated the utility of each component of the technique. Angiograms of one hemisphere of the intracranial vasculature were acquired with a pixel size of 1.1 × 1.1 × 2.8 mm and a frame rate of 0.35 sec based on a temporal resolution of 3.5 sec. Such a 3D time‐resolved, or “4D,” technique has the potential to noninvasively acquire diagnostic quality images of certain anatomic regions with a frame rate fast enough to not only ensure the capture of an uncontaminated arterial phase, but even demonstrate contrast bolus flow dynamics. Clinical applications include noninvasive imaging of arteriovenous shunting, which is demonstrated with a patient study. Magn Reson Med 58:962–972, 2007.

Radial sliding-window magnetic resonance angiography (MRA) with highly-constrained projection reconstruction (HYPR).

Sufficient temporal resolution is required to image the dynamics of blood flow, which may be critical for accurate diagnosis and treatment of various intracranial vascular diseases, such as arteriovenous malformations (AVMs) and aneurysms. Highly‐constrained projection reconstruction (HYPR) has recently become a technique of interest for high‐speed contrast‐enhanced magnetic resonance angiography (CE‐MRA). HYPR provides high frame rates by preferential weighting of radial projections while maintaining signal‐to‐noise ratio (SNR) by using a high SNR composite. An analysis was done to quantify the effects of HYPR on SNR, contrast‐to‐noise ratio (CNR), and temporal blur compared to the previously developed radial sliding‐window technique using standard filtered backprojection or regridding methods. Computer simulations were performed to study the effects of HYPR processing on image error and the temporal information. Additionally, in vivo imaging was done on patients with angiographically confirmed AVMs to measure the effects of alteration of various HYPR parameters on SNR as well as the fidelity of the temporal information. The images were scored by an interventional radiologist in a blinded read and were compared with X‐ray digital subtraction angiography (DSA). It was found that with the right choice of parameters, modest improvements in both SNR and dynamic information can be achieved as compared to radial sliding‐window MRA. Magn Reson Med, 2009.

Comparison of Intraarterial MR Angiography at 3.0 T with X-ray Digital Subtraction Angiography for Detection of Renal Artery Stenosis in Swine

PURPOSE To compare the accuracy of catheter-directed intraarterial (IA) magnetic resonance (MR) angiography at 3.0 T with that of x-ray digital subtraction angiography (DSA) for the measurement of renal artery stenosis (RAS) in swine. MATERIALS AND METHODS Unilateral hemodynamically significant RAS (>50%) was induced surgically in six pigs with use of reverse cable ties. One to two weeks after surgery, each pig underwent x-ray DSA and MR angiography before and after percutaneous transluminal balloon angioplasty (PTA). X-ray DSA was performed before and after PTA of RAS by injection of iodinated contrast agent through a 5-F multiple-side hole angiographic catheter placed in the abdominal aorta under fluoroscopic guidance. MR angiography of RAS was performed before and after PTA of RAS on a 3.0-T clinical MR imager with use of gadolinium-based contrast agent. MR angiography and DSA images were analyzed with the full width at half maximum method. Percent stenosis measurements between x-ray DSA and MR angiography were compared with a paired t test and were correlated with linear regression and Bland Altman analysis (alpha = 0.05). RESULTS Six cases of RAS were induced and imaged successfully with DSA and MR angiography techniques before and after PTA. On x-ray DSA, median stenoses was 64% (95% CI 57%-80%) before PTA and 20% (95% CI 5%-32%) after PTA. Corresponding MR angiography median stenosis measurement was 69% (95% CI 58%-80%) before PTA and 26% (95% CI 16%-36%) after PTA. A paired t test comparison did not show a difference between DSA and MR angiography (P = .16). RAS measurements on MR angiography correlated closely (P < .01) with DSA measurements (r(2) = 0.92). CONCLUSION In swine, the accuracy of catheter-directed IA MR angiography with use of a clinical 3.0-T MR imaging unit for the measurement of RAS was similar to that of conventional x-ray DSA.

Time-resolved MRA using sliding window reconstruction for evaluation of renal arterial anatomy and perfusion

A. N. Keeling, R. K. Singh, C. Farrelly, H. Jeong, T. A. Cashen, J. Sheehan, J. C. Carr, and T. J. Carroll Dept of Cardiovascular Imaging, Northwestern Memorial Hospital, Chicago, Illinois, United States, Dept of Biomedical Engineering, Northwestern Memorial Hospital, Chicago, Illinois, United States, Dept of Biomedical Engineering, Northwestern Memorial Hospital, Chicago, Ilinois, United States, Dept of Cardiovascular Imaging, Dept of Biomedical Engineering, Northwestern Memorial Hospital, Chicago, Illinois, United States