Comparison of hybrid ⁶⁸Ga-PSMA PET/MRI and ⁶⁸Ga-PSMA PET/CT in the evaluation of lymph node and bone metastases of prostate cancer

To evaluate the reproducibility of the combination of hybrid PET/MRI and the ⁶⁸Ga-PSMA-11 tracer in depicting lymph node (LN) and bone metastases of prostate cancer (PC) in comparison with that of PET/CT. A retrospective analysis of 26 patients who were subjected to ⁶⁸Ga-PSMA PET/CT_low-dose (1 h after injection) followed by PET/MRI (3 h after injection) was performed. MRI sequences included T1-w native, T1-w contrast-enhanced, T2-w fat-saturated and diffusion-weighted sequences (DWI_b800). Discordant PET-positive and morphological findings were evaluated. Standardized uptake values (SUV) of PET-positive LNs and bone lesions were quantified and their morphological size and conspicuity determined. Comparing the PET components, the proportion of discordant PSMA-positive suspicious findings was very low (98.5 % of 64 LNs concordant, 100 % of 28 bone lesions concordant). Two PET-positive bone metastases could not be confirmed morphologically using CT_low-dose, but could be confirmed using MRI. In 12 of 20 patients, 47 PET-positive LNs (71.9 %) were smaller than 1 cm in short axis diameter. There were significant linear correlations between PET/MRI SUVs and PET/CT SUVs in the 64 LN metastases (p < 0.0001) and in the 28 osseous metastases (p < 0.0001) for SUV_mean and SUV_max, respectively. The LN SUVs were significantly higher on PET/MRI than on PET/CT (p_SUVmax < 0.0001; p_SUVmean < 0.0001) but there was no significant difference between the bone lesion SUVs (p_SUVmax = 0.495; p_SUVmean = 0.381). Visibility of LNs was significantly higher on MRI using the T1-w contrast-enhanced fat-saturated sequence (p = 0.013), the T2-w fat-saturated sequence (p < 0.0001) and the DWI sequence (p < 0.0001) compared with CT_low-dose. For bone lesions, only the overall conspicuity was higher on MRI compared with CT_low-dose (p < 0.006). Nodal and osseous metastases of PC are accurately and reliably depicted by hybrid PET/MRI using ⁶⁸Ga-PSMA-11 with very low discordance compared with PET/CT including PET-positive LNs of normal size. The correlation between PET/MRI SUVs and PET/CT SUVs was linear in LN and bone metastases but was significantly lower in control (non-metastatic) tissue. (orig.)