Development of an MR-compatible gamma probe for combined MR/RI guided surgery

We have developed and tested an MR-compatible gamma probe to simultaneously obtain anatomical information and functional information during surgery. The probe consists of a probe head, an optical fibre bundle and a photo-multiplier tube (PMT). The NaI(Tl) scintillator contained in the probe head is connected to a 7 m optical fibre bundle that transfers the scintillation photons produced in the NaI(Tl) to an area of low magnetic field or out of the MR-scanner's magnetic shielded room. Although the light loss due to the optical fibre bundle was more than 90%, the photo-peak of the gamma for Co-57 (122 ke V) could be observed. The point spread function was 4.5 mm full width at half maximum (FWHM) at 5 mm from the collimator surface for 122 ke V gamma photons. Furthermore, there was no sensitivity change outside the MR-scanner, inside the MR-scanner without scanning and inside the MR-scanner with scanning. The probe produced a small artefact on the phantom image of the MR-scanner due to the susceptibility difference of the alloy used for the collimator and the shield. However, the artefact was only limited to area surrounding the probe. These results indicate that the developed MR-compatible gamma probe will make it possible to realize the combined MR/RI guided surgery that provides surgeons with anatomical and RI distribution information simultaneously.