Page 339 - Biomedical Engineering and Design Handbook Volume 2, Applications

CHAPTER 11

NUCLEAR MEDICINE IMAGING

INSTRUMENTATION

Mark Madsen
University of Iowa, Iowa City, Iowa

11.1 INTRODUCTION 317 11.4 POSITRON EMISSION
11.2 CONVENTIONAL GAMMA RAY TOMOGRAPHY 332
IMAGING: SCINTILLATION CAMERA 318 11.5 SMALL ANIMAL IMAGING 343
11.3 SINGLE PHOTON EMISSION 11.6 SUMMARY 346
COMPUTED TOMOGRAPHY 326 REFERENCES 346

11.1 INTRODUCTION

Nuclear medicine is a diagnostic imaging modality that can be 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 radio-
pharmaceutical is typically administered to the patient through an intravenous injection. The radio-
pharmaceutical 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 radio-
pharmaceutical would only go to abnormal areas. Unfortunately, this is never the case and the
abnormal concentration of the radiotracer is often obscured by normal uptake of the radiopharma-
ceutical in the surrounding tissues. Images of higher contrast and better localization can be
obtained with tomographic systems designed for nuclear medicine studies: single photon emission
computed tomography (SPECT) and positron emission tomography (PET). These are described in
detail below.
Gamma rays are high-energy photons that are emitted as a consequence of radioactive decay.
In conventional nuclear medicine, the most commonly used radionuclide is 99m Tc which emits a
140-keV gamma ray. Other radionuclides used in conventional nuclear medicine are given in
Table 11.1. With conventional nuclear medicine imaging, the emitted gamma rays that reach the
detector are counted individually. This is often referred to as single photon detection. One partic-
ular type of radioactive decay, beta plus or positron emission, results in the emission of a positron
which is the antiparticle of the electron. The positron very quickly annihilates with an electron,
producing 2, approximately colinear 511-keV annihilation photons. The simultaneous detection of
the 2 annihilation photons by opposed detectors (coincidence detection) provides the basis for
PET imaging.
Although radionuclides are nearly ideal tracers, the imaging of radiotracers in the body pre-
sents 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 CT. In addition, the high energy of the

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