LUNG COUNT RATE ESTIMATIONS USING A PORTABLE SCINTILLATION DETECTOR PRIOR TO V/PSPECT

Essay/Thesis: 30 hp Program and/or course: Medical Physicist Program Level: Second Cycle Semester/year: St/ 2015 Supervisor: Jakob Himmelman and Johanna Dalmo Examiner: Peter Bernhardt Report No: Keyword: V/PSPECT, portable scintillation detector, pulmonary embolism. Purpose: The aim of this study was to find a method using a portable scintillation detector to predict the tomographic image statistics after inhaling technegas prior to V/PSPECT, and to validate the method for clinical use. Background: V/PSPECT is a tomographic imaging (SPECT) method using Tc labeled carbon particles (technegas) and Tc-MAA, for examination of the lung ventilation and perfusion, respectively using a gamma camera. Technegas is inhaled and a sufficient count rate is important in the ventilation images. A too low count rate gives inadequate image quality and a too high count rate can lead to difficulties when constructing quotient images between ventilation and perfusion images. To verify an acceptable count rate the local clinical routine is to use the camera detector. This method increases e.g. the risk of contaminating the camera head because of its air cooling system. It would be advantageous preparing the patient in another room using a portable detector; saving camera time, lower the risk of contaminate the camera and to see the count rate in real time. Method: A portable scintillation detector (SSL Radhound with SS404 Al probe Southern Scientific UK, via Gamma Data AB) was used and its characteristics like rotational symmetry, laterally size of detection field, distance dependence and the best placement of the detector was investigated. The contribution from the technegas in patient administration set (PAS) is relatively high, and must be considered since only the count rate from the administrated technegas in the patient is of interest. Therefore an additional lead shield is placed on the detector. The positioning of the detector and the optimal height from the patient to the detector was investigated. The measurements were made with and without a PAS as well as with and without additional lead shielding to obtain a restricted field-of-view (FOV). To verify this method the number of counts in the diagnostic ventilation images was correlated to the portable detector count rate. Result: From measurements, the detector crystal was symmetric in response. The lateral size of the shielded FOV was a circle with a radius of 20 cm. From these investigations the method of choice was to use an additional lead shield, and place the detector 20 cm above the xiphoid process whit the patient lying on the examination bed in supine position. The correlation between the diagnostic ventilation images and the portable detector count rate implies that a count rate of approximately 14 kcps is favorable to achieve adequate statistic in the tomographic images, when the patient administration set was removed. To obtain the recommended tomographic image statistics within a 95 % confidence interval, the total number of counts was calculated to be 800-2500 kCts, and then the count rate from the portable detector should be 12-15 kcps.