CHARACTERIZATION OF A SMALL FIELD GAMMA CAMERA FOR THYROID SCINTISCANNING

Transcription

1 CHARACTERIZATION OF A SMALL FIELD GAMMA CAMERA FOR THYROID SCINTISCANNING E. Richetta Supervisor: M. I. Ferrero Co-examiner: G. Scielzo Examiner: G. Perno Aim Thyroid scintiscanning is a diagnostic methodology widespreadly used in Nuclear Medicine as morpho-functional examination. It is carried out with Iodine ( I 131, I 123) and Technetium (Tc 99m), to probe the presence of functional thyroid tissue or with damage tracer such as Thallium (Tl 201) and Gallium (Ga 67), to obtain diagnosticdifferential information, when cold nodules are present. The single head gamma camera MONOGAMMA, produced by L ACN, L Accesorio Nucleare, was installed at the Nuclear Medicine Department of the Mauriziano Umberto I Hospital of Turin in July The cooperation between the producer and the physics staff was developed to evaluate and verify the possibility to use this equipment for thyroid scintiscanning. Materials and Methods The prototype was designed to investigate small organs, such as the thyroid preeminently as shown in the following figure. The reduced dimensions of the detector (37 cm 34 cm 34 cm) and the presence of a motorised support allows vertical and horizontal movements. This characteristics and the low cost get this unit new and interesting if compared with the others available models, bigger and less practical o less easy to handle.

2 Movements of the gammacamera MONOGAMMA: vertical excursion and head rotation. In the upper detail, it is visible the rotation system. The acquisition system is not completly digital: the signal is digitized not at the output of the phtomultipliertube but after the Anger matrix. The acquisition s instruction is static with matrix dimension from 64*64 to 512*512. The detector is composed by NaI (Tl), 3/8 thick and the available collimators are parallel holes for Tc 99m and pinhole for I 131. The acquisition image format is its own and not DICOM standard. The acquisition and processing system is single and it is connected with a paper printer, while the printing on film support is not allowed. Aim of this study is to characterize the single head gamma camera MONOGAMMA to determine its performance and to plan an adequate quality control program in order to optimize the use of the equipment on the patients. In the first Chapters the gamma camera as well as its features are described. The quality assurance protocol is explained, referring to the national (D.Lgs 187/2000, CEI Norms and AIFM Guidelines) and international (NEMA Norms) norms. All the physics parameters to be measured are defined. In the first part of this work (July December 2003), the gamma camera was calibrated to acquire with the only Tc 99m. In this period measurements were carried out to determine the main characteristics of the equipment and to modify the software in order to realize the acquisition, the image processing and the quality control. For this purpose

3 the software was changed to allow the image analysis with ROI, counts profile and the uniformity and space resolution calculation and to create a patient management database. The image processing was also improved introducing filters and different colours and grey scale. In the second time (January - May 2004) the gamma camera was definitively calibrated and the acquisition with both Tc 99m and I 131 was possible. An acceptance test was carried out and following periodic measurements was performed to verify the stability and the repetibility of the different parameters. The results were compared to the tolerances of the adopted quality assurance protocol ad with the Minimum Acceptability Cryteria of the D. Lgs 187/2000. Firstly energetic resolution and peaking for both isotopes were measured, secondary intrinsic uniformity tests in different conditions (dependence from the head orientation, repeatability in 24 h) were performed. Sensitivity was measured for Tc 99m and the spatial resolution and pixel dimension were evaluated by means of capillary sources, with system acquisition. Particular attention was paid to performances referred determination of the saturation value. At the end the characteristics of the collimators were determined. The results of this measurements allowed the determination of reference values for the planning of the quality control program, as shown in Chapter 5. In the last Chapter measurements made on thyroid phantom and on patients are explained. Comparison between images obtained with the gamma camera and a thyroid scanner, taken as reference standard acquisition system, was also made. This allowed to determine the best operating condition to performe the thyroid scintiscanning with the gamma camera MONOGAMMA and point out some executive problems. Results and conclusions The characterization of the gamma camera showed that the equipment is time stable and fairly efficient. All the measured parameters respect the CMA required from the D.Lgs 187/2000. The energetic resolution was 18 % for the Tc 99m and 13 % for the I 131, while the peaking was always within 2 % for both isotopes. The intrinsic uniformity was within 10 %, even if integral values were high: 7,70 for UFOV and 7,62 for CFOV whereas

4 differential values are 1,99 and 1,88 respectively. The tolerance expected by the quality control protocol was respected, as shown in the following figure, where intrinsic uniformity measurements for CFOV are reported. The uniformity was independent from the head orientation. 12,0 Uniformità Integrale CFOV CFOV criterio minimo tolleranza Uniformità integrale 11,0 10,0 9,0 8,0 7,0 6, misure settimanali The mean value of planar sensitivity was equal to 168 cps/mbq for Tc 99m and in the constance measurements showed variation within 3 %. The saturation count rate (loss of 20 %%) was above 30 kcps, measured with two different techniques (two sources method and variation distance method): a quite low value, if compared with other modern gamma cameras. Spatial resolution was 8 mm in both direction, measured at 10 cm source-collimator distance. The dependence from this distance is showed in the graphic below and allowed to define the best acquisition distance within 10 cm. Pixel dimension vary from 0,5 mm for 64*64 matrix up to 4,0 mm for 512*512. Parallel hole collimator for Tc 99m is intact and uniform. In order to complete the quality control program the images reproduction system (monitor and printer) and the status of the pinhole collimator should also be evaluated.

5 Risoluzione spaziale vs distanza dal collimatore risoluzione spaziale (mm) 18,0 17,0 16,0 15,0 14,0 13,0 12,0 11,0 10,0 9,0 8,0 7,0 6,0 5,0 4,0 3, distanza dal collimatore (cm) Measurements carried out with thyroid phantom showed contradictory results needing more investigation in the course of this work. Comparison with thyroid scanner measurements put in evidence difficulty to detect cold nodules, particularly when patient detector distance is above 8-10 cm. Moreover the image printing on paper is not comparable with image displayed on the processing monitor, as shown in the following figure. To report only on monitor can be difficult in the organisation of the department work, because there is only one workstation. The possibility to print on film support and the DICOM format for images are hoped for the near future.

6 (A) (B) (C) (D) Measurements of thyroid phantom with Tc 99m carried out at 8 cm phantom-detector distance with the gamma camera MONOGAMMA, in colour scale (A), gray scale (B) and as visualized on the processing monitor (D). Image (C) show the same phantom acquired with a thyroid scanner: the not uptaking nodule is more evident and clearly visible. In the course of this work more acquisitions on patients with Tc 99m and I 131 will be performed, as well as examination of other pathology (i.e. linfonodo sentinella). The possibility to carry out uptake measurements will also be investigated, in comparison with traditional up take probe measurements. The quality control protocol, suggested in this work, will be supported and the respect of tolerance parameters will be monitored.