Assessment of Image Quality of a PET/CT scanner for a Standarized Image situation Using a NEMA Body Phantom. “The impact of Different Image Reconstruction Parameters on Image quality”
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Radiologists and medical practitioners are working daily with images from integrated Positron Emission Tomography/ Computed Tomography (PET/CT) scanners in order to detect potentially lethal diseases. It is thus very important to ensure that these images have adequate image quality. For the staff responsible of quality assurance of the applied scanner, it is important to ensure that the reconstruction procedures and image protocols in use enable acquisition of image with a high quality with respect to resolution and contrast, while the data sets are containing as little noise as possible. The goal of the quality assurance work will be to continuously make sure that, data acquisition settings and especially the reconstruction procedure that is utilized for routine and daily clinical purposes, enables lesions or cancer cells and diseases to be detected. This master thesis project aims at evaluating a reconstruction algorithm (iterative reconstruction) and some key parameters applied in image reconstruction. These parameters include selected filters (Gaussian, median, Hann and Butterworth filter), selected full width at half maximum values (FWHM: 3, 5, and 7 mm) and image matrix sizes (128 x 128 and 168 x 168 pixels respectively), in order to provide information on how these key parameters will affect image quality. The National Electrical Manufacturers Association (NEMA) International Electrotechnical Commission (IEC) Body Phantom Set was used in this work. It consists of a lid with six fillable spheres (with internal diameters 37, 28, 22, 17, 13 and 10 mm respectively), lung insert, body phantom (which represent the background volume) and a test phantom. The work in this thesis project has been carried out using the radiopharmaceutical tracer an F-18 FDG, fluotodeoxyglucose, produced with a cyclotron, a General Electric’s PETtrace 6 cyclotron, at the Center for Nuclear Medicine/PET at Haukeland University Hospital in Bergen, Norway. The applied radiopharmaceutical F-18 FDG was produced in a 2.5 ml target volume at the cyclotron. After the production, this volume was delivered from the cyclotron into a 20 ml sealed cylindrical glass already containing 17.5 ml of non-radioactive water. The activity level in this new solution with 20 ml F-18 FDG and water was measured in a dose calibrator (ISOMED 2010TM). The solution was diluted further, in an iterative process, a number of times in order to acquire the necessary activity concentrations for both the selected hot spheres and the background volume. The aim was to obtain activity concentrations for sphere-to-background ratios of either 4:1 or 8:1. The sphere-to-background ratio in this work is the ratio between the radioactivity level in four small spheres (with diameters 22, 17, 13 and 10 mm respectively, and having a total volume of 9.8 ml for all the 4 spheres) and the radioactivity level in the main body of the applied phantom; the so-called background volume (9708 ml). The two bigger spheres (28 and 37 mm) were filled with non-radioactive water in order to represent areas without radioactivity, i.e. “cold spheres”. When the spheres and volumes under study were filled with the desired level of activity and the activity level was measured, the spheres were positioned into the applied body phantom and the phantom was sealed to avoid spillage. The prepared NEMA IEC body phantom was placed on the table of a Siemens Biograph 40 PET/CT scanner in a predetermined reproducible position and scanned using a standard clinical whole body PET/CT protocol. The acquired images were reconstructed. Three repetitive studies were done for each concentration ratio. For each experiment performed, the sphere-to-background ratios were either 4:1 or 8:1. A selection of different standardized reconstruction parameters and different image corrections were applied. This was done in order to study what impact changes of the reconstruction parameters will have on the image quality. The image quality being defined by a quantification of the measured relative contrast in the images studied. The procedures followed while performing the PET/CT were in compliance with the recommended procedure presented in the NEMA NU2 – 2007 manual (from the manufacturer of the NEMA IEC body phantom described above). The reconstructed images were analyzed manually on a PET/CT workstation and also analyzed automatically with python programming software specially developed for the purpose of this work. The image quality results obtained from analyzes of the reconstructed images when different reconstruction parameters were used, were thereafter compared to the standardized protocol for reconstruction of PET/CT images. Lastly, the results have been compared with other similar work on the same subject by Helmar Bergmann et al (2005).