Characterization of cross-compatibility of small animal insertable PET and MRI

The purpose of this study is cross-compatibility of PET and MRI was characterized to explore the optimal method overcoming possible interferences between them. MR phantom images were acquired by placing the PET components inside and outside RF-coil in 7-T MRI to examine the effect of the relative position of PET detector and RF-coil on MRI. Theoretical evaluation of shielding effectiveness (SE) of plate and mesh Cu shielding were calculated to characterize criteria for reducing mutual interference between PET and MRI. Experimental studies were performed that MR image quality as a function of the area and thickness of Cu plate was examined to characterize the effect of plate shielding method. Also, MR images as a function of the open area of Cu mesh were acquired. Moreover, it was proposed and evaluated to minimize the cross-interference that only crystal and photo-sensor are placed inside MRI bore and the PET signals were transmitted to the signal amplifier circuits using long cable for developing hybrid PET-MR imaging system. Significant artifacts were generated on MRI by inserting the PET module inside RF coil, but obvious degradation of the MR image quality was not observed by placing the PET module outside RF-coil. In theoretical evaluation, Cu plate shielding need to be thicker than 15~30 ?m and Cu mesh shielding need to be thicker than 0.7/1.5/ 5.5-mm for minimizing the mutual interference between PET and MRI when the hole sizes of mesh were 0.5/ 1/ 3-mm, respectively. Cu thickness did not affect the homogeneity but SNR of MR images changed from 150 to 110 when Cu thickness changed from 0 to 200 ?m. The SNR and homogeneity were considerably changed from 270 to 29 and from 88 to 69 when 160 cm
³
and 640 cm
³
Cu area were employed. The temperature of Cu was risen by ~ 1?C when large area shielding (640 cm
³
) was used. MR image quality was not improved by increasing open area in GRE sequence. Moreover, this study verified charge signal transmission method using long cable, and it was feasible to acquire artifacts-free PET-MR images without any shielding material by placing the amplifier outside MR bore. In summary, cross-interferences between two imaging modalities would be minimized by placing the PET module outside RF-coil and inside gradient coil of MRI. In Cu plate shielding, area is a potentially bigger risk factor than the thickness in deteriorating MRI SNR, homogeneity and stable temperature operation. Cu mesh shielding with <4% open area will maximize the cross-compatibility of PET and MRI. Charge signal transmission using long cable between PET detector and preamplifier allows maximizing the cross-compatibility of PET and MRI and this approach is preferable because it allows locating only PET detector inside MRI without any shielding material.