A measurement of the attenuation of radiation from F‐18 by a PET/MR scanner

Abstract The attenuation of 511 keV photons by the structure of a PET/MR scanner was measured prior to energizing the magnet. The exposure rate from a source of fluorine‐18 was measured in air and, with the source placed at the isocenter of the instrument, at various points outside of the scanner. In an arc from 45 to 135 degrees relative to the long axis of the scanner and at a distance of 1.5 m from the isocenter, the attenuation by the scanner is at least 5.6 half‐value layers from the MR component alone and at least 6.6 half‐value layers with the PET insert installed. This information could inform better design of the radiation shielding for PET/MR scanners.


| INTRODUCTION
During the planning for the installation of a PET/MR scanner (Signa, General Electric Healthcare, Chicago, IL), no information was available on the attenuation of radiation from PET radionuclides by the scanner itself. As a consequence, the facility was shielded conservatively against ionizing radiation by ignoring any attenuation by the instrument itself. Prior to energizing the magnet during the installation of the scanner, the attenuations by the MR portion alone and by the MR portion along with the PET insert of the radiation from a source of fluorine-18 were measured. Those measurements demonstrate that including the structure of the PET/MR scanner in a radiation shielding design could obviate an appreciable amount of structural shielding compared to that which is called for when treating the radioactive patient as a bare point source in air.

| MATERIALS AND METHODS
The attenuation of the PET/MR scanner was measured both before and after the installation of the PET insert. Both measurements were made prior to the ramping up of the magnet.  | 337 The measured half-value layers of the attenuation afforded by the MR component alone are shown in Fig. 1 and are tabulated in Table 1. These measurements have been previously reported in a preliminary form. 3 The measurements from the scanner after the PET insert and the patient handling system had been installed are shown in Fig. 2 and are tabulated in Table 2.
The estimated weekly dose distribution from the simulated work week in the absence of any shielding from the PET/MR itself is shown in Fig. 3. The estimated weekly dose distribution from the simulated work week including an approximation of the shielding afforded by the scanner is shown in Fig. 4.

| DISCUSSION AN D CONCLUSION
The gamma ray exposure constants of 0.573 and 0.656 mR-m 2 /mCi-  Structural shielding for PET is both heavy and expensive. When designing the installation described here, the weight of the shielding as well as that of the scanner itself was a significant concern to the structural engineers. Had this information been available then, less structural shielding would have been recommended and siting would have been easier.
3. Estimated weekly dose map from bare sources simulating a busy work week. The color coding of the dose map is in shades of blue for weekly doses below 20 uSv, which is the limit for the general public in the United States; shades of green for weekly doses between 20 and 100 uSv, which is 10% of the occupational dose limit in the United States and the ALARA 1 limit of the institution in which this scanner was installed; and shades of red for weekly doses of more than 100 uSv, which would exceed the ALARA 1 threshold.
F I G . 4. Estimated weekly dose map including attenuation by the PET/MR scanner. The red zone is entirely within the scanner room and the green zone does not extend far beyond the walls of the room. The gray areas close to the scanner are very low dose areas (i.e., a "cold" blue). When a fractional occupancy factor for members of the public is applied to the hallway at the top of the figure and to the rooms at the bottom of the figure, there is no need for any structural shielding in this particular example.