New CT perfusion scanning techniques that are safer and faster than older CT protocols could offer a practical imaging alternative to MR perfusion for stroke patients, according to two studies presented at the 2007 RSNA meeting. Findings also emphasize a new trend for widening the window for thrombolysis.
A number of clinical trials are trying to extend the window for management of acute stroke with clot-busting tPA beyond the three-hour accepted standard. MR is the imaging modality of choice for enrolling patients in these studies. But CT has recently proven to be just as reliable and may be more practical, according to Dr. Pamela Schaefer, associate director of neuroradiology at Massachusetts General Hospital.
Schaefer and colleagues assessed 45 patients with acute stroke of the middle cerebral artery who underwent CT perfusion and MR perfusion within nine hours of symptom onset. Study inclusion criteria specified that CTP and MRP scans had to be performed within three hours of each other.
The investigators compared quantitative CT and MR lesion measurements and sought to establish whether a difference between these two parameters could influence patient selection. They found a significant correlation between CTP and MRP measurements of the stroke's core and penumbra that independently could grant inclusion of a patient into a trial. They also found that most discrepancies were the result of inadequate CTP coverage.
CT and MR perfusion measurements matched the trial inclusion criteria in 84.4% of cases. Mean transit time, cerebral blood flow, and diffusion/perfusion mismatch lesion volumes from CT and MR exams correlated significantly (p < 0.001). The modalities showed disagreement in seven cases. MRI excluded five patients that CT did not. CT, in turn, excluded two patients from trial selection that MR results approved for inclusion.
"Inaccurate CT coverage will be minimized as more centers purchase 64-slice scanners, which allow approximately 8 cm of coverage with current double contrast bolus or 'shuttle mode' scanning techniques," Schaefer said.
In another study, researchers in the U.K. released results from a new CT protocol that acquired CT perfusion and angiography data simultaneously over an 8-cm block in 30 stroke patients. The technique yielded information of diagnostic quality with a lower radiation dose and less contrast than the sequential 4-cm block method currently in use, according to coauthor Michael Bourne, BMBS, who presented the study. Studies took less than 15 minutes.
"We are trying to move away from this issue of the time window to a physiological management of stroke. If they have viable brain, that's what we are trying to get to," Bourne said.