Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

methods for measuring the camera performance. All manufacturers of scintillationcameras publish NEMA performance measurements of their cameras, which areused by prospective purchasers in comparing cameras and for writing purchasespecifications. Before the advent of NEMA standards, manufacturers measured theperformance of scintillation cameras in a variety of ways, making it difficult to comparethe manufacturers' specifications for different cameras objectively. The measurementmethods specified by NEMA require specialized equipment and are notintended to be performed by the nuclear medicine department. However, it is possibleto perform simplified versions of the NEMA tests to determine whether anewly purchased camera meets the published specifications. Unfortunately, theNEMA protocols omit testing of a number of important camera performance parameters.Ideally, a nuclear medicine projection image would be a two-dimensional projectionof the three-dimensional activity distribution in the patient. If this were the case,the number of counts in each point in the image would be proportional to the averageactivity concentration along a straight line through the corresponding anatomyof the patient. There are three main reasons why nuclear medicine images are notideal projection images-attenuation of photons in the patient, inclusion of Comptonscattered photons in the image, and the degradation of spatial resolution withdistance from the collimator.Attenuation in the patient by Compton scattering and the photoelectric effectprevents some photons that would otherwise pass through the collimator holesfrom contributing to the image. The amount of attenuation is mainly determinedby the path length through tissue and the densities of the tissues between a locationin the patient and the corresponding point on the camera face. Thus, photons fromstructures deeper in the patient are much more heavily attenuated than photonsfrom structures closer to the camera face. Attenuation is more severe for lowerenergy photons, such as the 68- to 80-keV characteristic x-rays emitted by TI-201(Il ""0.19 cm- I ), than for higher energy photons, such as the, 140-keV gamma raysfrom Tc-99m (Il = 0.15 cm- I ). Nonuniform attenuation, especially in thoracic andcardiac nuclear imaging, presents a particular problem in image interpretation.The vast majority of the interactions with soft tissue of x- and gamma rays ofthe energies used for nuclear medicine imaging are by Compton scattering. Somephotons that have scattered in the patient pass through the collimator holes and aredetected. As in diagnostic radiology, the relative number of scattered photons isgreater when imaging thicker parts of the patient, such as the abdomen, and themain effect of counts in the image from scattered photons is a loss of contrast. Aswas mentioned earlier in this chapter, the number of scattered photons contributingto the image is reduced by pulse height discrimination. However, setting anenergy window of sufficient width to encompass most of the photopeak permits aconsiderable amount of the scatter to contribute to image formation.Peaking a scintillation camera means to adjust its energy discrimination windows tocenter them on the photopeak or photopeaks of the desired radionudide. There are