Making the case for precision: PET for atherosclerosis

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FDG PET imaging of inflammation has proven to be very useful when evaluating atherosclerosis, but researchers have not yet come to a consensus about the proper protocol for this up-and-coming molecular imaging technique.

Pauline Huet, PhD, from the Paris Sud University, Orsay, France, and a team of researchers have taken one step closer to standardizing the method. The key, according to a meta-analysis of 49 previous studies published Feb. 28 in the Journal of Nuclear Medicine, is using PET systems that provide superior spatial resolution and advanced partial volume corrections, and reporting every last detail in order to maximize the usefulness of the research.

To say there was variability in reporting is an understatement. The researchers found 46 methods of lesion quantification, 51 reconstruction protocols and 53 different acquisition methods in just 49 articles.

There were several confounding factors in previous reporting that made it very difficult to accurately assess many, if not most, of the studies. A major factor was researchers neglecting to document spatial resolutions and voxel sizes.

“Key parameters such as the spatial resolution and the voxel size were not even mentioned in almost 70 percent of the articles,” noted Huet and colleagues. “This lack of appropriate reporting prevented from [sic] a sound comparison of the results given that spatial resolution and voxel size highly impact the severity of partial volume effect  and tissue fraction effect that in turn strongly biases the measurements in small lesions such as plaques. Last, protocols highly varied in the metrics used to assess the severity of the disease. This high variability reveals a lack of consensus regarding which parameter should be measured and how this should be done.”

There was very wide variability in the acquisition protocol used to assess arterial plaques. The investigators clued into the fact that, in 15 out of the 49 cases, the original PET scan was acquired to evaluate the patient for cancer and then later reassessed for atherosclerosis. The use of chemotherapy and other treatments could also throw off measurements used to evaluate entirely different diseases.

Variability also was found in relation to the reconstruction protocols used. Half of the studies applying a reconstruction algorithm used iterative reconstruction, but one third did not report which algorithm was used.

“Again, these differences might be due to the fact that part of the studies were initially designed for tumor imaging, so that images were not necessarily reconstructed using a protocol suitable for accurate quantification in subcentimeter lesions,” the researchers wrote.

About 89 percent of studies indicated the standard uptake value (SUV) and target to background ratio, but variability remained in the way these parameters were measured.  Precision measurements and definitions were given in only 10 percent of the studies and a dismal 6 percent of the studies applied metrics that reveal the scope of atherosclerosis by providing contrasting measurements from different regions. All sorts of SUV metrics were found to be lackluster for assessing inflammation in atherosclerosis without extremely precise partial volume corrections.

What appears clear is that this promising molecular imaging technique requires great care and precision, not only in evaluating patients with atherosclerosis, but also in reporting data for future study. FDG imaging of atherosclerosis may yet prove to be a good candidate for PET/MR imaging.

“PET-MR protocols might substantially ease the implementation of partial volume effects by facilitating the delineation of plaque component,” Huet and colleagues concluded.