MRI Technique Quickly Measures Carotid Flow


SAN FRANCISCO – Arterial spin labeling magnetic resonance imaging is much faster than and nearly as accurate as single-photon emission computed tomography for measuring blood flow in carotid stenosis, according to researchers from Kobe (Japan) University and Philips Healthcare Japan.

“It can be an alternative way to show stenosis with less invasiveness and cost,” said Dr. Yoshito Uchihashi, a neurosurgeon at the university who presented a poster on the topic at the annual meeting of the Congress of Neurological Surgeons.

Single-photon emission computed tomography (SPECT) is considered effective for measuring cerebral blood flow, but it requires the injection of a gamma-emitting radionuclide as a tracer into the patient’s bloodstream. A gamma camera detects the radiation from the radionuclide, and a computer uses a tomographic reconstruction to produce a three-dimensional image that shows blood circulation.

By contrast, in arterial spin labeling (ASL), a tracer is produced without injection by applying a 180-degree radiofrequency inversion pulse. This inverts the net magnetization of water molecules in the blood, effectively “labeling” them. As these molecules flow into areas of interest, their inverted spin reduces the total tissue magnetization, and “tag” images are made of these areas by MRI.

This image is compared with a control image that was made without labeling. The tag image is subtracted from the control image to create a perfusion image that shows the blood flow in each MRI slice.

To assess the effectiveness of ASL for carotid stenosis, Dr. Uchihashi and his colleagues used it to scan the cerebral blood flow of 20 healthy volunteers as a baseline. Then they repeated the imaging technique on 20 carotid stenosis patients who had undergone 123I-IMP-SPECT within the previous 3 days. The investigators used the techniques to predict the risk of hyperperfusion after carotid revascularization. They also assessed the patients’ vasoreactivity to acetazolamide using both imaging techniques. The scans of 12 surgically treated patients were made both before and after their operation.

To quantify their ASL results, they used quantitative STAR (signal targeting with alternating radiofrequency) labeling of arterial regions, a technique called QUASAR.

They found that ASL picked up a significant difference between the mean cerebral blood flow in the gray matter of the control volunteers and the carotid stenosis patients (P = .015).

Measurements of cerebral blood flow with the two imaging techniques were tightly correlated. ASL was equivalent to SPECT in detecting hypoperfusion, impaired vasoreactivity, and postoperative hyperfusion.

Dr. Uchihashi and his colleagues reported that the only weakness of ASL was that it tended to overestimate cerebral blood flow values, especially in regions with high perfusion.

On the other hand, ASL is much faster than SPECT, said Dr. Uchihashi. “The patient is only in bed for 6 minutes,” he said. By contrast, SPECT can take half an hour.

Dr. Uchihashi said he had nothing to disclose. One of his coauthors is an employee of Philips Healthcare Japan, the maker of the MRI scanner used in the study.