Exposure rate of patients leaving a nuclear medicine department after a SPECT MPI (Myocardial Perfusion Imaging) study.

Abstract

The aim of this study is to measure the rate of radiation emitted by patients that have undergone a SPECT MPI scan. According to the protocol of the MPI scan (stress-only or stress rest condition) the patient is intravenously administered the standardized radiopharmaceutical doses. The patient is still emitting radiation after the examination potentially exposing members of the public to radiation. The SPECT system used has CZT (Cadmium Zinc Telluride) detectors, which are generally more sensitive and accurate than photomultiplier detector SPECT systems. The data used are strictly based on Myocardial Perfusion Imaging examinations as the Onassis Cardiac Surgery Center specializes in Cardiology. Radiation exposure rates were measured after Stress Only and Stress/Rest examination protocols. The analysis of data showed that for distances of less than 3 meters from the patient’s body the Inverse Square Law is not exactly confirmed, and a corrected mathematical formula must be used, that represents the proper exposure rates. The problem with the inverse square law for short distances is that it is based on point source assumption. In this case the source is the patient’s body that the radiopharmaceutical circulates within. Analyzing the data, we notice that the experimental measurements are closer to the inverse square law at long distances. For the safety of the general public, the very low exposure rates of the Tc99m and its very short half-life of 6 hours, used in the MPI examinations should not worry anyone if the instructions for radiation protection are followed. In fact, these instructions are quite strict for the specific parameters of the MPI protocols.