MRI outperforms hybrids for bone tumor screening

August 1, 2006

Parallel imaging and a 32-channel coil help modality gain the upper hand over FDG-PET/CT

A preliminary trial at University Hospitals Munich has established MRI's ascendancy for 32 patients. Thirteen received whole-body MRI and FDG-PET/CT to diagnose suspected bone metastases from primary breast cancers. Six had a gastrointestinal tumor, and another six had cancers with an unknown primary origin. Other patients were diagnosed with lymphoma, hepatocellular carcinoma, sarcoma, or melanoma.

Whole-body MRI and FDG-PET/CT showed 91% and 78% accuracy, respectively, in identifying 127 bone lesions. MRI's sensitivity of 94% was significantly better than PET/CT's 78%. Although PET/CT was more specific (80% versus 76%), the difference was not significant, said chief investigator Dr. Gerwin P. Schmidt.

The superior performance of whole-body MRI was independent of lesion size, although MRI tended to detect small lesions that whole-body PET/CT missed. MRI was more accurate in eight out of nine anatomic regions. PET/CT performed slightly better in the lower spine, while the two modalities had problems finding small lesions in the upper shoulder and rib cage. Four metastases that were identified with whole-body MRI in the distal femur were not observed with PET/CT because of field-of-view limitations, Schmidt said.

A 32-channel radiofrequency coil array that provided 205 cm of coverage, combined with parallel imaging acceleration, boosted performance of the 1.5T scanner, Schmidt said. T1-weighted coronal imaging was performed in five stages that were then reconstructed to produce whole-body images. Sagittal T1-weighted and short-tau inversion recovery images were also acquired during abdominal and pelvic stages. The average room time for a scan was 45 minutes.

Whole-body PET/CT involved 200 MBq of FDG. Low-dose CT for attenuation correction preceded an emission scan with a 3D reconstruction algorithm; a diagnostic CT in late venous phase at 130 mA/130 kVp was also performed. Time in the room averaged 103 minutes.

Despite the results, Schmidt stopped short of recommending that whole-body MRI replace FDG-PET/CT for this application.

"The two techniques are complementary, but using both brings in the matter of cost," he said.