CT fails to topple diffusion-weighted imaging for use in stroke

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Proving newer isn’t necessarily better, diffusion-weighted imaging holds out over CT perfusion in determining lesion volume in ischemic stroke patients. The findings were published online Dec. 30 in Stroke.

Researchers wanted to determine if CT perfusion imaging could substitute for diffusion-weighted MR imaging (MRI) in either stroke treatment trials or individual patient triage. To that end, the retrospective study looked at CT and MRI available for 55 patients with CT and MRI scans taken a mean of an hour apart. Patients had ischemic stroke with anterior large vessel occlusion. They also had three-month modified Rankin Scale scores in their charts.

The timing between the two imaging modalities was important to provide minimum potential difference in progression of infarct and allow for maximum comparability.

According to Pamela W. Schaefer, MD, from the Department of Radiology at Massachusetts General Hospital in Boston, and colleagues, the images from CT and MRI correlated well, however blood volume or blood flow images provided by CT did not have clear enough results for clinical use. Using Bland-Altman analyses, Schaefer et al found that despite correlation, the variation between CT perfusion and diffusion-weighted lesion volume measurements was high. Their findings were similar to those in previous studies.

Both signal-to-noise ratios and contrast-to-noise ratios were higher for diffusion-weighted imaging than either CT cerebral blood volume or cerebral blood flow; signal-to-noise was eight times greater, while contrast-to-noise was more than four times higher than the comparative CT perfusion image values.

Schaefer et al also noted that in a simulated clinical study setting, the arms of studies that used CT blood volume or blood flow images instead of diffusion-weighted images would need between two to four times as many patients to achieve adequate power and determine efficacy. This, they noted, led to questions about justification of putting so many patients through a trial when the alternative is more reliable and required fewer patients.

“CTP [CT perfusion] should not replace DWI [diffusion-weighted imaging] in individual patient triage decisions or in clinical trials,” Schaefer et al wrote. They noted that large instrument error severely limits the usability of CT perfusion techniques in selecting patients for intra-arterial therapy.