Chapter 2: Introduction to Nuclear Medicine

WHAT IS NUCLEAR MEDICINE?

Nuclear medicine uses radioactive compounds called radiopharmaceuticals or radiotracers that interrogate physiologic or pathologic processes at a molecular level and provide targeted therapy for a variety of diseases. Nuclear medicine studies are clinically used to assess most organ systems with almost 100 different types of studies or therapies performed in the United States. Many more radiotracers play a crucial role in research.

When used as an imaging agent to evaluate organ function, metabolism, or membrane receptor characteristics, the amount of radiotracer administered is in the picomolar or nanomolar range, which avoids disturbing the process under evaluation while still yielding data that are quantifiable and comparable to normative standards. As functional deficits arise before morphological changes in many diseases, nuclear medicine studies can detect disease in early stages when curative or more effective palliative treatment choices may be an option. This chapter reviews the fundamentals of imaging generation for gamma and positron-emitting radiopharmaceuticals and common nuclear medicine procedures based on the organ system.

IMAGE GENERATION

Radiopharmaceuticals contain radioactive atoms that are unstable and produce ionizing radiation when they decay. High-energy rays produced by the decay interact with special light-producing crystals in nuclear medicine cameras to create the images. The types of imaging rays from radioisotope decay can be broadly placed in two categories: gamma rays or positron emission.

When gamma ray emitters such as technetium-99m decay, rays are emitted of a specific energy typically reported in kiloelectron volts (keV). For example, the workhorse of gamma imaging, technetium-99m, decays to produce a single 140 keV ray for imaging. Some radioisotopes give off several imaging rays, and gamma cameras are able to distinguish between the specific energy rays allowing for simultaneous imaging of two or more radiotracers.

Gamma cameras fundamentally are composed of 4 main parts: the collimator, a crystal, photomultiplier tubes, and electronics for processing the data (Fig. 2.1). A collimator is a sheet of lead with holes designed to reduce scatter when placed between the patient and the crystal. After interacting with the gamma rays that passed through the collimator, the sodium iodide doped with thallium crystal NaI(Tl) emits faint light, which is then detected by the photomultiplier tubes and processed into viewable nuclear images or scintigrams. The two-dimensional images obtained are termed “planar” (Fig. 2.2). Because the gamma rays are coming from within the patient as opposed to from an external source as in conventional imaging, the change in the position of the camera alters the image dramatically as the tissue attenuation changes. Figure 2.3 depicts the steps necessary to obtain images of a patient injected with a radiopharmaceutical intravenously. Imaging protocols vary depending on the type of radiopharmaceutical utilized, route of administration, and clinical question.