ASL-MRI Promising for Alzheimer's Diagnosis

Megan Brooks

November 28, 2011

November 28, 2011 — Arterial spin-labeled (ASL) perfusion magnetic resonance imaging (MRI) can detect changes in brain function associated with Alzheimer's disease (AD) on par with 18-fluorodeoxyglucose (FDG) positron emission tomography (PET), according to 2 new studies.

The researchers suggest that there is considerable qualitative and quantitative similarity between the 2 techniques, and confirm that regional cerebral blood flow closely parallels regional cerebral glucose metabolism.

They add that ASL-MRI is potentially more suitable for screening and longitudinal disease tracking than FDG-PET because it is noninvasive, does not require radioisotopes or an injected contrast agent, and is easily incorporated into standard MRI routines. It's also cheaper and more widely available.

 

The 2 studies — one published online November 16 in Neurology and the other published online October 20 in Alzheimer's & Dementia — provide the first direct comparison of ASL-MRI and FDG-PET in patients with AD and age-matched control subjects.

Regional hypometabolism on FDG-PET is a well-established marker of AD. Measurement of regional cerebral blood flow using ASL-MRI has also recently shown diagnostic potential in AD. ASL-MRI uses magnetically labeled endogenous arterial blood water as a tracer to quantify regional cerebral blood flow.

In the 2 studies, cerebral blood flow and glucose metabolism were measured concurrently by injecting the PET tracer during the MRI study. John A. Detre, MD, professor of neurology and radiology at the University of Pennsylvania in Philadelphia, is the senior author of the 2 papers.

"Striking" Similarities on Visual Inspection

The Alzheimer's & Dementia paper reports the results of visual inspection of paired ASL-MRI and FDG-PET images from 13 patients with AD and 18 control subjects.

Both techniques revealed similar regional abnormalities in AD and comparable sensitivity and specificity for the detection of AD, the authors report. They say the similarities in the pattern of hypoperfusion on ASL-MRI and hypometabolism on FDG-PET was "striking," with classically affected regions, such as the temporal and parietal lobes, showing deficits with both techniques.

Interobserver agreement was better with FDG-PET (κ, 0.75; standard error. 0.12) than with ASL-MRI (κ, 0.51; standard error, 0.15), intermodality agreement was moderate to strong (κ, 0.45 to 0.61), and readers were more confident with FDG-PET.

The researchers also examined the potential clinical value of whole-brain ASL cerebral blood flow measurement, and compared this to FDG whole-brain standardized uptake value ratio, which provides a rough estimate of whole-brain metabolism.

This analysis, they report, showed "excellent diagnostic accuracy for both modalities," with area under receiver operating characteristic curves of 0.90 for FDG-PET (95% confidence interval [CI], 0.79 to 0.99) and 0.91 for ASL-MRI (95% CI, 0.80 to 1.00).

Overlapping Voxel-Level Data

The Neurology paper reports the results of a voxel-level comparison of ASL-MRI and FDG-PET images obtained from 15 patients with AD and 17 healthy control subjects.

"Our voxel-wise results indicated that ASL-MRI provides largely overlapping information with FDG-PET," Dr. Detre and colleagues report.

Both imaging techniques identified the typical AD pattern of compromised function in the bilateral parietal lobes and the posterior cingulate. "Conjunction analysis showed that the overlap between the 2 modalities in these areas was statistically significant," they note.

Both ASL-MRI and FDG-PET were able to distinguish neural networks associated with different neuropsychologic tests, "with good overlap" between techniques, and both showed "expected correlations with cognitive task performance," they report.

Ansaar T. Rai, MD, vice chair of radiology (clinical operations) and director of interventional neuroradiology at West Virginia University Health Sciences Center in Morgantown, who was asked by Medscape Medical News to comment on the study, called this "a very pertinent paper." He was not involved in the study.

"Even though it's a small sample, it's prospectively done; it's a real-time comparison between ASL and PET, and it shows very strong correlation between the 2 modalities," Dr. Rai said. "It's very interesting to see that cerebral blood flow and glucose metabolism [in the brain] correlate."

"This is a very good first step, but we have to see how it pans out," Dr. Rai added.

Awareness and Experience Growing

Dr. Detre told Medscape Medical News that "validation in larger numbers of patients and patients with milder disease is still needed."

"Our group is studying ASL-MRI in mild cognitive impairment and patients with other causes of dementia, and the large NIH-funded Alzheimer's Disease Neuroimaging Initiative will be obtaining ASL-MRI in 250 patients over the next 5 years," he explained.

Dr. Detre noted that all major MRI vendors provide the capability for ASL-MRI, although the software might have to be purchased in some cases. ASL-MRI is possible on standard 1.5 Tesla MRI scanners, but is much more sensitive on the higher field strength 3 Tesla MRI machines that are now available in most major medical centers, he noted.

"Radiologists are starting to use ASL for diagnosis, but awareness and experience are just starting to develop," said Dr. Detre.

"We do a fair bit of ASL, but our emphasis is on stroke. We have not done ASL for AD, but I'm going to talk to our MR physicist about it," Dr. Rai said.

The research was funded by grants from the National Institutes of Health and the Penn–AstraZeneca Alliance. Dr. Detre is an inventor on the University of Pennsylvania's patent for ASL-MRI, and receives institutional royalty sharing for its licensure. Dr. Rai has disclosed no relevant financial relationships.

Alzheimers Dement. Published online October 20, 2011. Abstract

Neurology. Published online November 16, 2011. Abstract

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