Optimization of PET acquisition and reconstruction for head and neck PET/MRI

Objectives Head and Neck (HN) cancer staging may benefit from new hybrid technology such as PET/MRI. The dilemma between a sufficiently long PET acquisition for good quality 2 mm voxel size reconstructions with neurovascular MRI coil attenuation and a short acquisition time due to limited patient cooperation constitutes a challenge in clinical routine. Using an Ingenuity TF PET/MR we investigated the impact of reduced acquisition time on lesion detectability, SUV and SNR measurements.

Methods A retrospective reconstruction of listmode PET data (370MBq of F18-FDG injected, acquisition one hour after injection) was performed on 7 patients with histologically proven HN squamous-cell carcinoma (lesion size : 1 to 3 cm) and with neck dissection performed for cervical lymph node metastases (lesion size : 0.3 to 2.1 cm). The initial acquisition protocol was 6 min/bed in the HN area. Reconstructions were performed with 5, 4, 3, 2 and 1.5 min by clipping the listmode data. The relaxation parameters were adapted in order to keep a similar signal-to-noise ratio between reconstructions. Mean and maximum SUV as well as SNR were measured for each patient in the following areas: lung, cerebellum, cervical muscles, tumor site and lymph nodes.

Results SUVmean, SUVmax and SNR were not significantly different for all acquisition times except for the SNR of muscle which was significantly decreased for 1.5 min compared to 6 min. Lesions were noticeable on all the reconstructions. All positive lymph nodes on pathology reports detected on the 6min reconstruction (down to 6 mm) were still visible on the 2 min reconstruction but lymph nodes smaller than 7 mm were missed in 1.5 min reconstruction.

Conclusions Our preliminary data suggest that in HN squamous cell carcinoma it is possible to reduce the PET acquisition time to 2 min without affecting sensitivity and specificity for lesion detection.