M. Louis Lauzon

Magnetic resonance imaging at 3.0 Tesla: challenges and advantages in clinical neurological imaging.

MR imaging at very high field (3.0 T) is a significant new clinical tool in the modern neuroradiological armamentarium. In this report, we summarize our 40-month experience in performing clinical neuroradiological examinations at 3.0 T and review the relevant technical issues. We report on these issues and, where appropriate, their solutions. Issues examined include: increased SNR, larger chemical shifts, additional problems associated with installation of these scanners, challenges in designing and obtaining appropriate clinical imaging coils, greater acoustic noise, increased power deposition, changes in relaxation rates and susceptibility effects, and issues surrounding the safety and compatibility of implanted devices. Some of the these technical factors are advantageous (eg, increased signal-to-noise ratio), some are detrimental (eg, installation, coil design and development, acoustic noise, power deposition, device compatibility, and safety), and a few have both benefits and disadvantages (eg, changes in relaxation, chemical shift, and susceptibility). Fortunately solutions have been developed or are currently under development, by us and by others, for nearly all of these challenges. A short series of 1.5 T and 3.0 T patient images are also presented to illustrate the potential diagnostic benefits of scanning at higher field strengths. In summary, by paying appropriate attention to the discussed technical issues, high-quality neuro-imaging of patients is possible at 3.0 T.

Cavitation After Acute Symptomatic Lacunar Stroke Depends on Time, Location, and MRI Sequence

Background and Purpose— Definitions for chronic lacunar infarcts vary. Recent retrospective studies suggest that many acute lacunar strokes do not develop a cavitated appearance. We determined the characteristics of acute lacunar infarcts on follow-up MRI in consecutive patients participating in prospective research studies. Methods— Patients with acute lacunar infarction on diffusion-weighted imaging were selected from 3 prospective cohort studies of minor stroke imaged within <24 hours of onset. Follow-up MRI was performed at 30 days (Vascular Imaging of Acute Stroke for Identifying Predictors of Clinical Outcome and Recurrent Ischemic Events [VISION] study, n=21) or 90 days (VISION-2 and CT and MRI in the Triage of TIA and Minor Cerebrovascular Events to Identify High Risk Patients [CATCH] studies, n=34). Evidence of cavitation on MRI was rated separately on fluid-attenuated inversion recovery, T1, and T2 sequences by 2 independent study physicians; discrepant readings were resolved by consensus. Results— Probable or definite cavitation on any sequence was more common at 90 days compared with 30 days (P⩽0.001 for all sequences). At 90 days, evidence of cavitation was seen on at least 1 sequence in 33 of 34 patients (97%). The T1-weighted sequence was most sensitive to the presence of cavitation (94% at 90 days). By contrast, the fluid-attenuated inversion recovery sequence frequently failed to show evidence of cavitation in the brain stem or thalamus (only 10 of 18 [56%] showed cavitation). Conclusions— MRI scanning at 90 days with T1-weighted imaging reveals evidence of cavitation in nearly all cases of acute lacunar infarction. By contrast, reliance on fluid-attenuated inversion recovery alone will miss many cavitated lesions in the thalamus and brain stem. These factors should be taken into account in the development of standardized criteria for lacunar infarction on MRI.

Atlas-Based Topographical Scoring for Magnetic Resonance Imaging of Acute Stroke

Background and Purpose— The Alberta Stroke Program Early CT Score (ASPECTS), a 10-point scale, is a clinical tool for assessment of early ischemic changes after stroke based on the location and extent of a visible stroke lesion. It has been extended for use with MR diffusion-weighted imaging. The purpose of this work was to automate a MR topographical score (MR-TS) using a digital atlas to develop an objective tool for large-scale analyses and possibly reduce interrater variability and slice orientation differences. Methods— We assessed 30 patients with acute ischemic stroke with a diffusion lesion who provided informed consent. Patients were imaged by CT and MRI within 24 hours of symptom onset. An MR-TS digital atlas was generated using the ASPECTS scoring sheet and anatomic MR data sets. Automated MR topographical scores (auto-MR-TS) were obtained based on the overlap of lesions on apparent diffusion coefficient maps with MR-TS atlas regions. Auto-MR-TS scores were then compared with scores derived manually (man-MR-TS) and with conventional CT ASPECTS scores. Results— Of the 30 patients, 29 were assessed with auto-MR-TS. Auto-MR-TS was significantly lower than CT ASPECTS (P<0.001), but with a median difference of only 1 point. There was no significant difference between the auto-MR-TS and the man-MR-TS with a median difference of 0 points; 86% of patient scores differed by ≤1 point. Conclusion— Auto-MR-TS provides a measure of stroke severity in an automated fashion and facilitates more objective, sensitive, and potentially more complex ASPECTS-based scoring.

A simulation-based analysis of the potential of compressed sensing for accelerating passive mr catheter visualization in endovascular therapy

Passive MRI is a promising approach to visualize catheters in guiding and monitoring endovascular intervention and may offer several clinical advantages over the current x‐ray fluoroscopy “gold standard.” Endovascular MRI has limitations, however, such as difficulty in visualizing catheters and insufficient temporal resolution. The multicycle projection dephaser method is a background signal suppression technique that improves the conspicuity of passive catheters by generating a sparse (i.e., catheter only) image. One approach to improve the temporal resolution is to undersample the k‐space and then apply nonlinear methods, such as compressed sensing, to reconstruct the MR images. This feasibility study investigates the potential synergies between multicycle projection dephaser and compressed sensing reconstruction for real‐time passive catheter tracking. The multicycle projection dephaser method efficiently suppressed the background signal, and compressed sensing allowed MR images to be reconstructed with superior catheter conspicuity and spatial resolution when compared to the more conventional zero‐filling reconstruction approach. Moreover, compressed sensing allowed the shortening of total acquisition time (by up to 32 times) by vastly undersampling the k‐space while simultaneously preserving spatial resolution and catheter conspicuity. Magn Reson Med, 2010.

SU‐E‐I‐129: Determining the Cramer‐Rao Lower Bound in Magnetic Resonance Imaging

Purpose: Maximizing the signal‐to‐noise ratio (SNR) and/or contrast‐to‐ noise ratio (CNR) is very frequently the primary goal when designing new imaging protocols or when choosing from among several magnetic resonance(MR)imaging methods that give similar physiological measurements. In both cases, minimization of the noise inherent in the reconstructed image becomes the key goal. By optimizing with the Cramer‐ Rao Lower Bound (CRLB), a best achievable case of variance can be found in the unbiased estimator sense and, ranges can be found were biased estimators improve upon the CRLB. Methods: We perform derivations of the MRimage channel in order to find the best case of the CRLB. Simulations with a digital brain phantom, using ideal parameters, are then performed and matched with the derivation. In addition, simulations with distortions from the B0 field, B1 field, gradient field and receiver coil sensitivity profiles were also performed to define the CRLB in the presence of machine imperfections. Results: From the derivation and simulation, we showed an increase in data variance of greater then 1000× when distortions from common machine imperfections are present. Using the derived CRLB value is not a suitable benchmark to compare biased estimators as the value is much lower then what is achieved practically. Conclusions: We have demonstrated in this work that improving upon the best case CRLB is not a reasonable goal, rather we should be focused upon achieving estimates that are 1000× times greater then the derived CRLB. This work allows us to find the bound where we may choose to switch from conventional FFT reconstruction to alternative methods, resulting in lower data variance.