A gamma camera for measurements of internal contamination after a radiological accident

Abstract

After a radiological accident with the release of large amounts of radionuclides into the environment, measurements of the external and internal exposure of the public will be urgently required. The aim of this study was to investigate the properties of a gamma camera with regard to measurements of internal contamination. The gamma camera was used as a detector without any imaging function. The lowest minimum detectable activity, 100 Bq 131I in thyroid measured over the neck and 400 Bq 137Cs in the whole body measured over the trunk (1 min measuring time), was found when using the lowest (50 keV-450 keV) energy interval among the three investigated (also 550-750 keV and 450-550 keV), in measurements without a collimator. The mechanical difficulties in handling a gamma camera without a collimator may also have to be considered. It is essential to pay attention to the influence of body size on both background and sensitivity. Provided proper calibrations are carried out and routines for measurements are established, a gamma camera is useful for fast identification and quantification of internal contamination with gamma-emitting radionuclides down to levels so low that in most situations they give a low contribution to the effective dose.

Medical gamma cameras in radiological emergency preparedness: determination of calibration factors and MDA for the GE Discovery NM/CT 670 Pro

“…The derived calibration factors and MDAs for 137 Cs and 60 Co showed good agreement with previous studies [6,[8][9][10], but it is important to bear in mind that different gamma camera models, energy windows, settings, phantoms and calibration geometries are used in different studies, which makes direct comparisons difficult. To the best of our knowledge, 152 Eu and 40 K have not been previously studied in this situation.…”

“…The detector heads can rotate 360 • around the patient table and are usually located 180 • relative to each other. Several studies have shown that the detection efficiency can be enhanced by removing the collimators normally used to enable visualization of the radionuclide distribution [6][7][8]. In this study, we were only interested in the quantification of radionuclides and, therefore, all measurements were performed without the collimators, leaving only a thin (0.5 cm) Plexiglas plate to protect the crystal from damage and contamination, the so-called intrinsic mode.…”

“…Although some studies have shown that a shorter distance between the detector and the phantom surface can increase the detection efficiency [8,14], the standard distance between the detector heads was used (68 cm, 64 cm between Plexiglass plates) for all phantom sizes to minimize the need for adjustments in a potentially stressful situation following an RN event. The calibration measurement time for each phantom size and radionuclide was 10 min, and the mean value of ten identical measurements was used.…”

“…Several studies have shown that, despite the above-mentioned drawbacks, the use of gamma cameras as a complement to WBCs for the estimation of the activity of the radionuclides that can be expected following an RN event, is feasible [6][7][8][9][10]. In the case of an RN event, the imaging capability is of minor interest compared to the ability to quantify the internal contamination.…”

“…The efficiency of gamma cameras can be increased by removing the collimator, which is placed in front of the detector to reduce scattered radiation from the subject in order to enhance the imaging capability of the camera. This allows a sufficiently low MDA to be measured during a reasonably short measuring time [6][7][8].…”