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Using Optimized Diffusion Images for Transient Ischemic Attacks

October 27, 2009
Written by: , Filed in: Emergency Radiology, Neuroradiology
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Background
Since the advent of DWI, we have come to understand that transient ischemic attacks (TIAs) are often associated with restricted diffusion. Although the clinical event is transient, cerebral infarctions do occur. Ideally, we would like to maximize the sensitivity of DWI in detecting these small recent infarcts.

Currently, most sites utilize very fast echo-planar imaging for diffusion sequences, often lasting <1 minute. This is necessary in the emergency setting of strokes, in which patients are often unable to cooperate fully.

The objective of a recent study was to optimize a DWI sequence for maximum sensitivity of detecting areas of restricted diffusion in the setting of TIAs.

The study has concluded that optimized diffusion-weighted MRI (DWI) is a longer scan with a higher NEX, nine diffusion directions, and thinner images.

Participants
36 patients referred for clinical suspicion of TIA.

Methodology
Each patient was scanned with two different DWI protocols. Both protocols utilized spin-echo, single-shot echo-planar imaging. The standard sequence used 6-mm thick contiguous sections with NEX of two or three diffusion directions, for a time of 51 seconds. The optimized version was done with NEX of 3, nine diffusion directions, and 3-mm sections with a gap of 0.3 mm. All other parameters were the same. The optimized version took six minutes.

Images were reviewed by two readers without clinical data. In patients in whom lesions were seen on both DWI sequences, the lesion intensity was compared with contralateral normal tissue to determine a signal intensity ratio (rSI) and ADC ratio (rADC).

Emergency Radiology The University of Washington School of Medicine, Harborview Medical Center, Department of Radiology * Features case-based presentations with extensive explanations – 15 hours of video * What, when, and how of imaging in relation to specific topics in emergency medicine * Stresses the interaction and communication required to be an effective part of the emergency medicine team * Provides guidance in diagnosis of polytrauma, commonly missed diagnoses, and when to change existing protocols Click here to read more or order: Emergency Radiology

Results of the Study
The median delay until MRI was performed was 34 hours, ranging from three hours to eight days. The readers agreed on all cases in interpreting the optimized DWI, but disagreed on some cases of the standard DWI, for a kappa value of 0.89. By consensus, 56 lesions were found on optimized images and 42 on standard DWI. Twenty patients had normal standard DWI; three of these had infarcts on the optimized scans.

Of the 16 patients with a positive standard DWI, optimized DWI demonstrated one to three additional lesions in each of seven patients, which were small, usually cortical, and within the same vascular territory. rSI was statistically higher on optimized views in patients imaged after 12 hours but not within 12 hours; rADC was not significantly different in any patients.

Conclusions
Optimized DWI improves the sensitivity of detecting small diffusion lesions in patients with TIA. It can also help in evaluating lesions of uncertain significance on standard DWI.

Reviewer’s Comments
This is a very useful, practical paper. Patients who have had a TIA are far more likely to be able to cooperate with a longer scan, and the study demonstrates that three additional patients (of 36) were classified as having a confirmed stroke as a result of this optimized technique at a cost of only five minutes, which is certainly a worthwhile tradeoff.

Author: Yaron Lebovitz, MD

Reference
Bertrand A, Oppenheim C, et al.
Comparison of Optimized and Standard Diffusion-Weighted Imaging at 1.5T for the Detection of Acute Lesions in Patients With Transient Ischemic Attack.
AJNR; 2008; 29 (February): 363-365.

Emergency Radiology The University of Washington School of Medicine, Harborview Medical Center, Department of Radiology * Features case-based presentations with extensive explanations – 15 hours of video * What, when, and how of imaging in relation to specific topics in emergency medicine * Stresses the interaction and communication required to be an effective part of the emergency medicine team * Provides guidance in diagnosis of polytrauma, commonly missed diagnoses, and when to change existing protocols Click here to read more or order: Emergency Radiology

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