PET/MR shows promise for pulmonary nodule evaluation

The launch of PET/MR imaging has renewed interest in the role of MR and PET/MR in pulmonary nodule evaluation. The hybrid modality demonstrated high sensitivity in the detection of FDG-avid nodules and nodules 5 cm or larger, according to a study published online June 4 in Radiology.  

Although CT has served as the gold standard in the detection of lung nodules, its sensitivity in the discrimination of malignant from benign nodules is limited. Meanwhile, PET/MR has emerged and prompted interest in its potential utility, which pairs the high contrast and functional information of MR with metabolic PET activity. Hersh Chandarana, MD, from the department of radiology at New York University Langone Medical Center in New York City, and colleagues devised a study to evaluate MR, PET and PET/MR in the detection of lung nodules, using PET/CT as a reference standard.

The researchers focused on 32 patients who underwent PET/CT followed by PET/MR between August and October 2012. MR imaging utilized radial T1-weighted gradient-echo (radial volumetric interpolated breath-hold examination [VIBE]).

PET/CT depicted 69 nodules in the 32 patients, including 45 FDG-avid nodules in 13 patients. There was substantial agreement between PET/CT and PET/MR imaging for nodule detection (k = 0.97 for PET, k = 0.83 for PET/MR imaging).

PET/MR demonstrated higher sensitivity than PET/CT for all nodules at 61.6 percent and 70.3 percent, respectively. PET/MR also delivered greater sensitivity than PET/CT in the detection of FDG-avid nodules at 94.4 percent and 95.6 percent, respectively. “However, sensitivity for small non-FDG-avid nodules was lower with PET/MR imaging than with PET/CT.” The sensitivity of PET/MR in the detection of lesions smaller than 0.5 cm was 38 percent, and 22.9 percent for non-FDG-avid nodules.

Radial VIBE MR provided sensitivity of 62.3 percent for all nodules, 81.8 percent for nodules with a diameter of at least 0.5 cm and 80.0 percent for FDG-avid nodules.

Chandarana and colleagues acknowledged the diagnostic dilemma posed by non-FDG-avid nodules smaller than 0.5 cm in diameter. Although studies have indicated that these nodules may not be cancerous even in the presence of metastatic disease, the possibility of metastasis cannot be confidently excluded. “The clinical importance of these missed nodules is unclear and needs further investigation.”