JNM: Longer scan + TOF PET is best for lesion detection, localization

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A combination of longer scan time (of three minutes) and time-of-flight (TOF) PET imaging provides the “best performance” for imaging large patients or a low-uptake lesion in small or large patients, according to a study in the May issue of the Journal of Nuclear Medicine.

Suleman Surti, MD, from the department of radiology at the University of Pennsylvania in Philadelphia, and colleagues noted that phantom studies have shown improved lesion detection performance with TOF PET. Therefore, in this study, they evaluated the benefit of fully 3D TOF PET in clinical whole-body oncology using human observers to localize and detect lesions in realistic patient anatomic backgrounds.

With TOF imaging, the researchers surmised that they could achieve improved lesion detection and localization for clinically challenging tasks, with a bigger impact in large patients.

They chose 100 patient studies with normal 18F-FDG uptake, and imaged spheres (diameter, 10 mm) in air at variable locations in the scanner field of view corresponding to lung and liver locations within each patient. They corrected the sphere data for attenuation and merged it with patient data to produce fused list-mode data files with lesions added to normal-uptake scans. All list files were reconstructed with full corrections and with or without the TOF kernel using a list-mode iterative algorithm.

After this process, the images were presented to readers to localize and report the presence or absence of a lesion and their confidence level. The interpretation results were then analyzed to calculate the probability of correct localization and detection, and the area under the localized receiver operating characteristic (LROC) curve. The results were analyzed as a function of scan time per bed position, patient body mass index (BMI < 26 and BMI >26) and type of imaging (TOF and non-TOF).

Surti and colleagues reported that longer scan times led to an improved area under the LROC curve for all patient sizes. With TOF imaging, there was a bigger increase in the area under the LROC curve for larger patients (BMI >26).

Also, the researchers observed smaller differences in the area under the LROC curve for large and small patients when longer scan times were combined with TOF imaging.

When the study was originally conceived, the standard imaging protocol at the University of Pennsylvania PET Center required a three-minute scan per bed position. Currently, the protocol requires a scan time that varies between one and three minute per bed position based on the patient BMI. “This change in imaging protocol was based on a visual impression of image quality versus scan time and is consistent with the quantitative results derived in this study using human observers and the ALROC metric,” the authors wrote.

Based on their results, Surti and colleagues concluded that this three-minute imaging protocol also provides similar performance for all patient sizes for lesions in the same organ type with similar relative uptake, indicating an ability to provide a uniform clinical diagnosis in most oncologic lesion detection tasks.