27.1 INTRODUCTION

Nuclear medicine is a diagnostic imaging modality that is used to obtain clinical information about most of the major tissues and organs of the body. Diagnostic information is obtained from the way the tissues and organs process radiolabeled compounds (radiopharmaceuticals). The radiopharmaceutical is typically administered to the patient though an intravenous injection. The radiopharmaceutical is carried throughout the body by the circulation where it localizes in tissues and organs. Images of these distributions are acquired with a scintillation camera. Ideally, the radiopharmaceutical would go only to abnormal areas. Unfortunately, this is never the case and the abnormal concentration of the radiotracer is often obscured by normal uptake of the radiopharmaceutical in the surrounding tissues. Images of higher contrast and better localization can be obtained with tomographic systems designed for nuclear medicine studies (SPECT systems). These are described in detail below.

The imaging of radiotracers in the body presents special challenges that are unique. The flux of gamma rays available for imaging is orders of magnitude less than that used in x-ray radiography or x-ray computed tomography (CT). In addition, the high energy of the gamma rays makes detection more difficult. As a result, the images produced in nuclear medicine studies are much noisier and have worse spatial resolution. In order to appreciate these problems and how they affect the design of nuclear medicine imaging devices, we will briefly review the physics of gamma ray interactions.1

The intensity of gamma...