Shielding Considerations and Requirements for a PET/CT Scanner Facility

Ning Yue, Ph.D., DABR

Chief of Physics, Professor of Radiation Oncology

The Cancer Institute of New Jersey

UMDNJ-Robert Wood Johnson Medical School

New Brunswick, New Jersey

Positron Emission Tomography (PET) is being used more and more not only as an imaging modality for diagnosis but also for the purpose of delineation of target volume in radiation therapy. The physical principle of the PET tomography is to coincidentally detect the two photons that are produced during a positron-electron annihilation process. The photons such produced possess an energy of 511 keV, which high level distinguishes a PET/CT scanner from conventional CT scanner as far as radiation protection is concerned and has to be taken into consideration in the shielding design of a PET/CT scanner facility.

Unlike conventional CT scanner x-ray sources, the radiotracers used in PET imaging have relatively short half-lives and a constant dose rate cannot be assumed in the PET dose calculations. Furthermore, since most of the annihilation incidents take place inside patient body, a significant amount of radiation is absorbed by the patient body itself. Therefore, in general, the radiation dose at a point of interest over a time t should be computed as:

(1)

　

where L is effective dose equivalent rate, A(0) is the radiotracer strength at time 0, d is the distance between the patient and the point of interest, T_1/2 is the half-life of the radionuclide, and is patient body attenuation factor.

In a typical PET/CT scanner facility, in addition to PET/CT scanner room, there is usually a patient uptake room where patient is kept in quiet and resting state for appropriate amount of time (in the range of 30-90 minutes). The radiation dose contributions from both the scanner room and the uptake room should be included, and equation 1 should then be expressed as:

(2)

　

　

where t_u is uptake time, t_s is scanning time, and t= t_u + t_s, d_u is the distance from the point of interest to the patient in the uptake room, and d_s is the distance from the point of interest to the patient in imaging position. It is also assumed that the contribution after the imaging is negligible.

The photons from the transmission component of PET/CT scanner are usually absorbed by the detectors and patients, and are considered to be negligible. The radiation contribution from the CT component can be computed as for a conventional CT scanner. The rest of the shielding calculations are similar to the conventional one except that the attenuation coefficients of shielding materials are different for the PET component and the CT component.

Though ¹⁸F is the most commonly used radionuclide in PET applications, different types of radionuclides have also been considered or used as radiotracers. Those radionuclides have different half-lives, which can be as short as 76 second (⁸²Rb), and can be as long as 4.2 days (¹²⁴I). Some of the radionuclides can even emit photons with energies much higher than 511 keV (e.g., 1693 keV photons emitted from ¹²⁴I). Depending on the type of radionuclide to be used, these characteristics need to be taken into account in the PET facility shielding design.