Longer PET Acquisition Time Increases FDG-Avid Lymph Node Detection in Rectal Cancer

Longer PET acquisition times during PET/MRI for rectal cancer increases the number of FDG-avid lymph nodes detected without increasing scan time, according to a study published in the American Journal of Roentgenology.

Researchers from the University of California, San Francisco, and the San Francisco VA Medical Center, sought to determine if extended PET acquisition times in the pelvis during PET/MRI increase detection rates of potentially metastatic lymph nodes in patients with rectal cancer.

Twenty-two patients with biopsy-proven rectal cancer participated in the study. All underwent imaging via simultaneous 3-T time-of-flight PET/MRI; seven underwent two separate PET/MRI examinations. This provided a total of 29 studies. Each examination included both a whole-body PET/MRI and a dedicated pelvic PET/MRI with both 3- and 15-minute PET acquisitions for the pelvis. Three radiologists interpreted each examination with PET only, MRI only, then combined PET and MRI examinations, using all available images. Additionally, the 3- and 15-minute PET acquisitions of the pelvis were reviewed separately by a single radiologist.

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The results showed that 94 lymph nodes were identified as abnormal on PET, all with MRI anatomic correlates. Of these:

• 37 (39.4%) were seen only on the dedicated 15-minute acquisition
• 57 (60.6%) nodes measured 5 mm or less, including 29 (30.9%) seen only on the 15-minute acquisition
• 31 (33.0%) nodes measured 5.1–10 mm, including eight (25.8%) seen only on the 15-minute acquisition

Of the 17 subjects imaged for initial staging, 11 (64.7%) were upstaged as a result of the increased PET acquisition time (10 from N1 to N2 and one from N0 to N1).

The researchers concluded that longer PET acquisition times during PET/MRI for rectal cancer increased the number of FDG-avid lymph nodes detected without increasing scan time.