Improving Lesion Detection on Diffusion-Weighted Imaging in the Setting of Transient Global Amnesia

Improving Lesion Detection on Diffusion-Weighted Imaging in the Setting of Transient Global Amnesia

Transient global amnesia (TGA) is a sudden and temporary loss of memory with preservation of personal identity and mental status. It is rare and usually lasts for a short time.

The etiology is unknown, but proposed etiologies include thromboembolism, vertebral venous congestion, and vasoconstriction secondary to hyperventilation.  Recurrence is unusual.

Recent studies have demonstrated an association with 1- to 3-mm hyperintensities in the hippocampus in transient global amnesia. In general, raising the b-values used in diffusion-weighted imaging (DWI) will increase diffusion weighting, and cause increased noise.

Similarly, thinner sections may improve detection of small lesions at the expense of increased noise.

The objective of a recent study was to determine optimal diffusion-weighted imaging parameters for lesion detection in the setting of transient global amnesia.

The Study
16 patients (age range, 49 to 69 years) were studied over a 14-month period.

Methods
MRI was performed within 24 hours, and a second study was done at 3 days after onset of symptoms. MR imaging consisted of axial T1- and T2-weighted, fluid-attenuated inversion recovery, and gradient-echo, sagittal T1-weighted images, noncontrast head MR angiography (MRA), and contrast-enhanced neck MRA.

Initial DWI was performed with b-values of 1000 obtained at 3 mm and 5 mm and b-values of 2000 and 3000 at 3 mm.

The follow-up 72-hour study was performed with the diffusion images only. Sequences from all patients were interpreted randomly.

Results
13 of 16 patients (81%) had at least 1 hyperintense lesion on a DWI sequence. Seven patients had a single lesion, 2 had 2 lesions, 3 had 3 lesions, and 1 had 4 lesions.

There were 23 hippocampal lesions (10 in the head, 13 in the body) and 1 in the temporal lobe.

Lesion detection improved with thinner slices and b-values of 2000 or 3000.

On the initial studies, with b=1000, 5-mm images detected 3 lesions and 3-mm images detected 9 lesions.

The increase in b-value raised lesion detection to 13.

On the 3-day study, the high b-value sequence detected 24 lesions.

Assuming that the 3-day higher b-value lesions are 100% sensitive, lesion detection for the initial studies were 13%, 38%, 54% and 54% and 71%, 92%, 100%, and 100%, respectively, for the 3-day studies.

Conclusions
Approximately 81% of patients with transient global amnesia have at least 1 lesion on optimally performed diffusion-weight imaging.

In the setting of TGA, DWI is optimally performed with a b-value of 2000 and 3-mm section thickness. If no lesion is detected initially, then a repeat study in 3 days is recommended.

Reviewer’s Comments
This was a practical study that will benefit radiologists in evaluating TGA.

The authors point out that true analysis of sensitivity and specificity is not possible, other than comparing the sensitivity of the various protocols. There was no benefit in increasing the b-value from 2000 to 3000.

Author: Yaron Lebovitz, MD

Reference:
Weon YC, Kim JH, et al. Optimal Diffusion-Weighted Imaging Protocol for Lesion Detection in Transient Global Amnesia.  AJNR; 2008; 29 (August): 1324-1328