NUCLEAR MEDICINE

Principles of Nuclear Medicine

Nuclear medicine is a medical specialty that focuses on the use of radioactive materials called radiopharmaceuticals^* for diagnosis, therapy, and medical research. In contrast to radiologic procedures, which determine the presence of disease based on structural appearance, nuclear medicine studies determine the cause of a medical problem based on organ or tissue function (physiology).

In a nuclear medicine test, the radioactive material, or tracer, is generally introduced into the body by injection, swallowing, or inhalation. Different tracers are used to study different parts of the body. Tracers are selected that localize in specific organs or tissues. The amount of radioactive tracer material is selected carefully to provide the lowest amount of radiation exposure to the patient and still ensure a satisfactory examination or therapeutic goal. Radioactive tracers produce gamma-ray emissions from within the organ being studied. A special piece of equipment, known as a gamma or scintillation camera, is used to transform these emissions into images that provide information about the function (primarily) and anatomy of the organ or system being studied.

Nuclear medicine tests are performed by a team of specially educated professionals: a nuclear medicine physician, a specialist with extensive education in the basic and clinical science of medicine who is licensed to use radioactive materials; a nuclear medicine technologist who performs the tests and is educated in the theory and practice of nuclear medicine procedures; a physicist who is experienced in the technology of nuclear medicine and the care of the equipment, including computers; and a pharmacist or specially prepared technologist who is qualified to prepare the necessary radioactive pharmaceuticals.

Positron emission tomography (PET) is a noninvasive nuclear imaging technique that involves the administration of a positron-emitting radioactive molecule and subsequent imaging of the distribution and kinetics of the radioactive material as it moves into and out of tissues. PET imaging of the heart, brain, lungs, or other organs is possible if an appropriate radiopharmaceutical, also called a radiotracer or radiolabeled molecule, can be synthesized and administered to the patient.

Three important factors distinguish PET from all radiologic procedures and from other nuclear imaging procedures. First, the results of the data acquisition and analysis techniques yield an image related to a particular physiologic parameter such as blood flow or metabolism. The ensuing image is aptly called a functional or parametric image. Second, the images are created by the simultaneous detection of a pair of annihilation radiation photons that result from positron decay (Fig. 34-1). The third factor that distinguishes PET is the chemical and biologic form of the radiopharmaceutical. The radiotracer is specifically chosen for its similarity to naturally occurring biochemical constituents of the human body. Because extremely small amounts of the radiopharmaceutical are administered, equilibrium conditions within the body are not altered. If the radiopharmaceutical is a form of sugar, it behaves very much like the natural sugar used by the body. The kinetics or the movement of the radiotracer such as sugar within the body is followed by using the PET scanner to acquire many images that measure the distribution of the radioactivity concentration as a function of time. From this measurement, the local tissue metabolism of the sugar may be deduced by converting a temporal sequence of images into a single parametric image that indicates tissue glucose use or, more simply, tissue metabolism.

Fig. 34-1 A, PET relies on the simultaneous detection of a pair of annihilation radiations emitted from the body. B, In contrast, CT depends on the detection of x-rays transmitted through the body.

Historical Development

Dalton is considered the father of the modern theory of atoms and molecules. In 1803, Dalton, an English schoolteacher, stated that all atoms of a given element are chemically identical, are unchanged by chemical reaction, and combine in a ratio of simple numbers. Dalton measured atomic weights in reference to hydrogen, to which he assigned the value of 1 (the atomic number of this element).

The discovery of x-rays by Roentgen in 1895 was a great contribution to physics and the care of the sick. A few months later, another physicist, Becquerel, discovered naturally occurring radioactive substances. In 1898, Curie discovered two new elements in the uranium ore pitchblende. Curie named these trace elements polonium (after her homeland, Poland) and radium. Curie also coined the terms radioactive and radioactivity.

In 1923, de Hevesy, often called the “father of nuclear medicine,” developed the tracer principle. He coined the term “radioindicator” and extended his studies from inorganic to organic chemistry. The first radioindicators were naturally occurring substances such as radium and radon. The invention of the cyclotron by Lawrence in 1931 made it possible for de Hevesy to expand his studies to a broader spectrum of biologic processes by using ³²P (phosphorus-32), ²²Na (sodium-22), and other cyclotron-produced (man-made) radioactive tracers.

Radioactive elements began to be produced in nuclear reactors developed by Fermi and colleagues in 1946. The nuclear reactor greatly extended the ability of the cyclotron to produce radioactive tracers. A key development was the introduction of the gamma camera by Anger in 1958. In the early 1960s, Edwards and Kuhl made the next advance in nuclear medicine with the development of a crude single photon emission computed tomography (SPECT) camera known as the MARK IV. With this new technology, it was possible to create three-dimensional images of organ function instead of the two-dimensional images created previously. It was not until the early 1980s, when computers became fast enough to acquire and process all of the information successfully, that SPECT imaging could become standard practice.

With the development of more suitable scintillators, such as sodium iodide (NaI), and more sophisticated nuclear counting electronics, positron coincidence localization became possible. Wrenn demonstrated the use of positron-emitting radioisotopes for the localization of brain tumors in 1951. Brownell further developed instrumentation for similar studies. The next major advance came in 1967, when Hounsfield demonstrated the clinical use of computed tomography (CT). The mathematics of PET image reconstruction is very similar to that used for CT reconstruction techniques. Instead of x-rays from a point source traversing the body and being detected by a single or multiple detectors as in CT, PET imaging uses two opposing detectors to count pairs of 511-KeV photons simultaneously that originate from a single positron-electron annihilation event.

From 1967-1974, significant developments occurred in computer technology, scintillator materials, and photomultiplier tube (PMT) design. In 1975, the first closed-ring transverse positron tomograph was built for PET imaging by Ter-Pogossian and Phelps.

Developments now continue on two fronts that have accelerated the use of PET. First, scientists are approaching the theoretical limits (1 to 2 mm) of PET scanner resolution by employing smaller, more efficient scintillators and PMTs. Microprocessors tune and adjust the entire ring of detectors that surround the patient. Each ring in the PET tomograph may contain 1000 detectors. The tomograph may be composed of 30 to 60 rings of detectors. The second major area of development is in the design of new radiopharmaceuticals. Agents are being developed to measure blood flow, metabolism, protein synthesis, lipid content, receptor binding, and many other physiologic parameters and processes.

During the mid-1980s, PET was used predominantly as a research tool; however, by the early 1990s, clinical PET centers had been established, and PET was routinely used for diagnostic procedures on the brain, heart, and tumors. In the middle to late 1990s, three-dimensional PET systems that eliminated the use of interdetector septa were developed. This development allowed the injected dose of the radiopharmaceutical to be reduced by approximately 6-fold to 10-fold.

One of the first organs to be examined by nuclear medicine studies using external radiation detectors was the thyroid. In the 1940s, investigators found that the rate of incorporation of radioactive iodine by the thyroid gland was greatly increased in hyperthyroidism (overproduction of thyroid hormones) and greatly decreased in hypothyroidism (underproduction of thyroid hormones). Over the years, tracers and instruments were developed to allow almost every major organ of the body to be studied by application of the tracer principle. Images subsequently were made of structures such as the liver, spleen, brain, and kidneys. At the present time, the emphasis of nuclear medicine studies is more on function and chemistry than anatomic structure. In PET, new image reconstruction methods have been developed to characterize better the distribution of annihilation photons from these three-dimensional systems.

Beginning in 2000, major nuclear medicine camera manufacturers developed combined PET and CT systems that can simultaneously acquire PET functional images and CT anatomic images. Both modalities are coregistered or exactly matched in size and position. The success of these camera systems led to the development of combined SPECT and CT systems as well. Significant benefits are expected for diagnosing metastatic disease because precise localization of tumor site and function can now be determined. Rapid enhancements and developments are anticipated with this technology over the next several years.

Comparison with Other Modalities

PET is predominantly used to measure human cellular, organ, or system function. A parameter that characterizes a particular aspect of human physiology is determined from the measurement of the radioactivity emitted by a radiopharmaceutical in a given volume of tissue. In contrast, conventional radiography measures the structure, size, and position of organs or human anatomy by determining x-ray transmission through a given volume of tissue. X-ray attenuation by structures interposed between the x-ray source and the radiographic image receptor provides the contrast necessary to visualize an organ. CT creates cross-sectional images by computer reconstruction of multiple x-ray transmissions (see Chapter 31). The characteristics of PET and other imaging modalities are compared in Table 34-1.

TABLE 34-1

Comparison of imaging modalities

^*Secondary function.

Radionuclides used for conventional nuclear medicine include ^99mTc (technetium), ¹²³I (iodine), ¹³¹I (iodine), ¹¹¹In (indium), ²⁰¹Tl (thallium), and ⁶⁷Ga (gallium). Labeled compounds with these high atomic weight radionuclides often do not mimic the physiologic properties of natural substances because of their size, mass, and distinctly different chemical properties. Compounds labeled with conventional nuclear medicine radionuclides are poor radioactive analogs for natural substances. Imaging studies with these agents are qualitative and emphasize nonbiochemical properties. The elements hydrogen, carbon, nitrogen, and oxygen are the predominant constituents of natural compounds found in the body. They have low atomic weight radioactive counterparts of ¹¹C (carbon), ¹³N (nitrogen), and ¹⁵O (oxygen). These positron-emitting radionuclides can directly replace their stable isotopes in substrates, metabolites, drugs, and other biologically active compounds without disrupting normal biochemical properties. In addition, ¹⁸F (fluorine) can replace hydrogen in many molecules, providing an even greater assortment of biologic analogs that are useful PET radiopharmaceuticals.

SPECT employs nuclear imaging techniques to determine tissue function. Because SPECT employs collimators and lower energy photons, it is less sensitive (by 10¹ to 10⁵) and less accurate than PET. Generally, PET resolution is better than SPECT resolution by a factor of 2 to 10. PET easily accounts for photon loss through attenuation by performing a transmission scan. This is difficult to achieve and not routinely done with SPECT imaging; however, newly designed SPECT instrumentation that couples a low-output x-ray CT to the gamma camera for the collection of attenuation information is now being used in selected sites to correct for gamma attenuation. Software approaches are also being investigated that assign known attenuation coefficients for specific tissues to segmented regions of images for analytic attenuation correction of SPECT data.

The differences between the various imaging modalities can be highlighted using a study of brain blood flow as an example. Without an intact circulatory system, an intravenously injected radiopharmaceutical cannot make its way into the brain for distribution throughout the brain’s capillary network ultimately diffusing into cells that are well perfused. For radiographic procedures such as CT, structures within the brain may be intact, but there may be impaired or limited blood flow to and through major vessels within the brain. Under these circumstances, the CT scan may appear almost normal despite reduced blood flow to the brain. If the circulatory system at the level of the capillaries is not intact, a PET scan can be performed, but no perfusion information is obtained because the radioactive water used to measure blood flow is not transported through the capillaries and diffused into the brain cells.

The image-enhancing contrast agents used in many radiographic studies may cause a toxic reaction. The x-ray dose to the patient in these radiographic studies is greater than the radiation dose in nuclear imaging studies. The radiopharmaceuticals used in PET studies are similar to the body’s own biochemical constituents and are administered in very small amounts. Biochemical compatibility of the tracers within the body minimizes the risks to the patient because the tracers are not toxic. Trace amounts minimize alteration of the body’s homeostasis.

An imaging technique that augments CT and PET is magnetic resonance imaging (MRI) (see Chapter 32). Images obtained with PET and MRI are shown in Fig. 34-2. MRI is used primarily to measure anatomy or morphology. In contrast to CT, which derives its greatest image contrast from varying tissue densities (bone from soft tissue), MRI better differentiates tissues by their proton content and the degree to which the protons are bound in lattice structures. The tightly bound protons of bone make it virtually transparent to MRI.

Fig. 34-2 Coregistered MRI and PET scans. Arrows indicate an abnormality on the anatomic image (A, MRI scan) and the functional image (B, PET scan). ¹⁸F-FDG PET image depicts hypometabolic area of seizure focus (arrow) in a patient with a diagnosis of epilepsy.

CT, MRI, and other anatomic imaging modalities provide complementary information to nuclear medicine imaging and PET. These imaging modalities benefit from image coregistration with CT and MRI by pinpointing physiologic function from precise anatomic locations. Greater emphasis is being placed on multimodality image coregistration between PET, CT, SPECT, and MRI for brain research and for tumor localization throughout the body (Fig. 34-3). All new PET imaging systems are fused with a CT scanner for attenuation and anatomic positioning information. Newer SPECT imaging systems incorporate CT technology for the same purposes.

Fig. 34-3 Combined SPECT/CT camera for a blending of imaging function and form. (Courtesy General Electric.)

Physical Principles of Nuclear Medicine

An understanding of radioactivity must precede an attempt to grasp the principles of nuclear medicine and how images are created using radioactive compounds. The term radiation is taken from the Latin word radii, which refers to the spokes of a wheel leading out from a central point. The term radioactivity is used to describe the radiation of energy in the form of high-speed alpha or beta particles or waves (gamma rays) from the nucleus of an atom.

BASIC NUCLEAR PHYSICS

The basic components of an atom include the nucleus, which is composed of varying numbers of protons and neutrons, and the orbiting electrons, which revolve around the nucleus in discrete energy levels. Protons have a positive electrical charge, electrons have a negative charge, and neutrons are electrically neutral. Protons and neutrons have masses nearly 2000 times the mass of the electron; the nucleus comprises most of the mass of an atom. The Bohr atomic model (Fig. 34-4) can describe this configuration. The total number of protons, neutrons, and electrons in an atom determines its characteristics, including its stability.

Fig. 34-4 Diagram of Bohr atom containing a single nucleus of protons (P) and neutrons (N) with surrounding orbital electrons of varying energy levels (e.g., K, L, M).

The term nuclide is used to describe an atomic species with a particular arrangement of protons and neutrons in the nucleus. Elements with the same number of protons but a different number of neutrons are referred to as isotopes. Isotopes have the same chemical properties as one another because the total number of protons and electrons is the same. They differ simply in the total number of neutrons contained in the nucleus. The neutron-to-proton ratio in the nucleus determines the stability of the atom. At certain ratios, atoms may be unstable, and a process known as spontaneous decay can occur as the atom attempts to regain stability. Energy is released in various ways during this decay, or return to ground state.

Radionuclides decay by the emission of alpha, beta, and gamma radiation. Most radionuclides reach ground state through various decay processes, including alpha, beta, or positron emission; electron capture; and several other methods. These decay methods determine the type of particles or gamma rays given off in the decay.

To explain this process better, investigators have created decay schemes to show the details of how a parent nuclide decays to its daughter or ground state (Fig. 34-5, A). Decay schemes are unique for each radionuclide and identify the type of decay, the energy associated with each process, the probability of a particular decay process, and the rate of change into the ground state element, commonly known as the half-life (T½) of the radionuclide.

Fig. 34-5 A, Decay scheme illustrating the method by which radioactive molybdenum (⁹⁹Mo) decays to radioactive technetium (^99mTc), one of the most commonly used radiopharmaceuticals in nuclear medicine. B, Graphic representation showing the rate of physical decay of a radionuclide. The y (vertical) axis represents the amount of radioactivity, and the x (horizontal) axis represents the time at which a specific amount of activity has decreased to one half of its initial value. Every radionuclide has an associated half-life that is representative of its rate of decay.

Radioactive decay is considered a purely random and spontaneous process that can be mathematically defined by complex equations and represented by average decay rates. The term half-life is used to describe the time it takes for a quantity of a particular radionuclide to decay to one half of its original activity. This radioactive decay is a measure of the physical time it takes to reach one half of the original number of atoms through spontaneous disintegration. The rate of decay has an exponential function, which can be plotted on a linear scale (see Fig. 34-5, B). If plotted on a semilogarithmic scale, the decay rate would be represented as a straight line. Radionuclide half-lives range from milliseconds to years. The half-lives of most radionuclides used in nuclear medicine range from several hours to several days.

NUCLEAR PHARMACY

The radionuclides used in nuclear medicine are produced in reactors, or particle accelerators. Naturally occurring radionuclides have very long half-lives (i.e., thousands of years). These natural radionuclides are unsuitable for nuclear medicine imaging because of limited availability and the high absorbed dose the patient would receive. The radionuclides for nuclear medicine are produced in a particle accelerator through nuclear reactions created between a specific target chemical and high-speed charged particles. The number of protons in the target nuclei is changed when the nuclei are bombarded by the high-speed charged particles, and a new element or radionuclide is produced. Radionuclides can be created in nuclear reactors either by inserting a target element into the reactor core where it is irradiated or by separating and collecting the fission products.

The most commonly used radionuclide in nuclear medicine is ^99mTc, which is produced in a generator system. This system makes available desirable short-lived radionuclides—the daughters—which are formed by the decay of relatively longer lived radionuclides—the parents. The generator system uses ⁹⁹Mo (molybdenum-99) as the parent. ⁹⁹Mo has a half-life of 66.7 hours and decays (86%) to a daughter product known as metastable ^99mTc. Because ^99mTc and ⁹⁹Mo are chemically different, they can easily be separated through an ion-exchange column. ^99mTc exhibits nearly ideal characteristics for use in nuclear medicine examinations, including a relatively short physical half-life of 6.04 hours and a high-yield (98.6%), 140-keV, low-energy, gamma photon (see Fig. 34-5 A,).

Because radiopharmaceuticals are administered to patients, they need to be sterile and pyrogen-free. They also need to undergo all of the quality control measures required of conventional drugs. A radiopharmaceutical generally has two components: a radionuclide and a pharmaceutical. The pharmaceutical is chosen on the basis of its preferential localization or participation in the physiologic function of a given organ. A radionuclide is tagged to a pharmaceutical. After the radiopharmaceutical is administered, the target organ is localized, and the radiation emitted from it can be detected by imaging instruments, or gamma cameras.

The following characteristics are desirable in an imaging radiopharmaceutical:

• Ease of production and ready availability

• Low cost

• Lowest possible radiation dose to the patient

• Primary photon energy between 100 keV and 400 keV

• Physical half-life greater than the time required to prepare the material for injection

• Effective half-life longer than the examination time

• Suitable chemical forms for rapid localization

• Different uptake in the structure to be detected than in the surrounding tissue

• Low toxicity in the chemical form administered to the patient

• Stability or near-stability

^99mTc can be bound to biologically active compounds or drugs to create a radiopharmaceutical that localizes in a specific organ system or structure when the radionuclide is administered intravenously or orally. A commonly used radiopharmaceutical is ^99mTc tagged to a macroaggregated albumin (MAA). After intravenous injection, this substance follows the pathway of blood flow to the lungs, where it is distributed throughout and trapped in the small pulmonary capillaries (Fig. 34-6). Blood clots along the pathway prevent this radiopharmaceutical from distributing in the area beyond the clot. As a result, the image shows a void or clear area, often described as photopenia or a cold spot. More than 30 different radiopharmaceuticals are used in nuclear medicine (Table 34-2).

TABLE 34-2

Radiopharmaceuticals used in nuclear medicine

DMSA, dimercaptosuccinic acid; DTPA, diethylenetriamine pentaacetic acid; HMPAO, hexamethylpropyleneamine oxime; MAG3, mertiatide.

Fig. 34-6 Normal perfusion lung scan using 3 mCi of ^99mTc tagged to a macroaggregated albumin (^99mTc MAA) on a large field-of-view gamma camera approximately 5 minutes after injection of the radiopharmaceutical. LPO, left posterior oblique; LT, left; RPO, right posterior oblique; RT, right. (Courtesy Siemens Medical Systems, Iselin, NJ.)

Radiopharmaceutical doses vary depending on the radionuclide used, the examination to be performed, and the size of the patient. The measure of radioactivity is expressed as either the becquerel (Bq), which corresponds to the decay rate, expressed as 1 disintegration per second (dps), or as the curie (Ci), which equals 3.73 × 10¹⁰ dps, relative to the number of decaying atoms in 1 g of radium.

Radiation Safety in Nuclear Medicine

The radiation protection requirements in nuclear medicine differ from the general radiation safety measures used for diagnostic radiography. The radionuclides employed in nuclear medicine are in liquid, solid, or gaseous form. Because of the nature of radioactive decay, these radionuclides continuously emit radiation after administration (in contrast to diagnostic x-rays, which can be turned on and off mechanically). Special precautions are required.

Generally, the quantities of radioactive tracers used in nuclear medicine present no significant hazard. Nonetheless, care must be taken to reduce unnecessary exposure. The high concentrations or activities of the radionuclides used in a nuclear pharmacy necessitate the establishment of a designated preparation area that contains isolated ventilation, protective lead or glass shielding for vials and syringes, absorbent material, and gloves. The handling and administration of diagnostic doses to patients warrants the use of gloves and a lead syringe shield, which is especially effective for reduction of exposure to hands and fingers during patient injection, at all times (Fig. 34-7). Any radioactive material that is spilled continues to emit radiation and must be cleaned up and contained immediately. Because radioactive material that contacts the skin can be absorbed and may not be easily washed off, it is very important to wear protective gloves when handling radiopharmaceuticals.

Fig. 34-7 A, Area in a radiopharmacy in which doses of radiopharmaceuticals are prepared in a clean and protected environment. B, Nuclear medicine technologist administering a radiopharmaceutical intravenously using appropriate radiation safety precautions, including gloves and a syringe shield.

Technologists and nuclear pharmacists are required to wear appropriate radiation monitoring (dosimetry) devices, such as film badges and thermoluminescent dosimetry (TLD) rings, to monitor radiation exposure to the body and hands. The ALARA (as low as reasonably achievable) program applies to all nuclear medicine personnel.

Instrumentation in Nuclear Medicine

MODERN-DAY GAMMA CAMERA

The term scintillate means to emit light photons. Becquerel discovered that ionizing radiation caused certain materials to glow. A scintillation detector is a sensitive element used to detect ionizing radiation by observing the emission of light photons induced in a material. When a light-sensitive device is affixed to this material, the flash of light can be changed into small electrical impulses. The electrical impulses are amplified so that they may be sorted and counted to determine the amount and nature of radiation striking the scintillating materials. Scintillation detectors were used in the development of the first-generation nuclear medicine scanner, the rectilinear scanner, which was built in 1950.

Scanners have evolved into complex imaging systems known today as gamma cameras (because they detect gamma rays). These cameras are still scintillation detectors that use a thallium-activated sodium iodide crystal to detect and transform radioactive emissions into light photons. Through a complex process, these light photons are amplified, and their locations are electronically recorded to produce an image that is displayed as a hard copy or on computer output systems. Scintillation cameras with single or multiple crystals are used today. The gamma camera has many components that work together to produce an image (Fig. 34-8).

Fig. 34-8 Typical gamma camera system, which includes complex computers and electronic mechanical components for acquiring, processing, displaying, and analyzing nuclear medicine images.

COMPUTERS

Computers have become an integral part of the nuclear medicine imaging system. Computer systems are used to acquire and process data from gamma cameras. They allow data to be collected over a specific time frame or to a specified number of counts; the data can be analyzed to determine functional changes occurring over time (Fig. 34-9, A and B). A common example is the renal study, in which the radiopharmaceutical that is administered is cleared by normally functioning kidneys in about 20 minutes. The computer can collect images of the kidney during this period and analyze the images to determine how effectively the kidneys clear the radiopharmaceutical (see Fig. 34-9, C to E). The computer also allows the operator to enhance a particular structure by adjusting the contrast and brightness of the image.

Fig. 34-9 A, Posterior renal blood flow in an adult patient using 10 mCi of ^99mTc with DTPA imaged at 3 seconds per frame. The image in the lower right corner is a blood-pool image taken immediately after the initial flow sequence. Together the images show normal renal blood flow to both kidneys. B, Normal, sequential dynamic 20-minute ^99mTc with mertiatide (MAG3) images. C, Renal arterial perfusion curves showing minor renal blood flow asymmetry. D, Renal cortical analysis curves showing rapid uptake and prompt parenchymal clearance. E, Quantitative renal cortical analysis indices showing normal values.

Computerization of the nuclear pharmacy operation also has become an important means of record keeping and quality control. Radioactive dosages and dose volumes can be calculated more quickly by computer than by hand. The nuclear pharmacy computer system may be used to provide reminders and keep records as required by the Nuclear Regulatory Commission (NRC), the U.S. Food and Drug Administration (FDA), and individual state regulatory agencies. Computers also assist in the scheduling of patients, based on dose availability and department policies.

Computers are necessary to acquire and process SPECT images (see next section). SPECT uses a scintillation camera that moves around the patient to obtain images from multiple angles for tomographic image reconstruction. SPECT studies are complex and, similar to MRI studies, require a great deal of computer processing to create images in transaxial, sagittal, or coronal planes. Rotating three-dimensional images can also be generated from SPECT data (Fig. 34-10, A).

Fig. 34-10 A, Three-dimensional SPECT brain study using 20 mCi of ^99mTc ECD showing a patient with a left frontal lobe brain infarct (top). Baseline and Diamox challenge transaxial, coronal, and sagittal images of the same patient, showing the left frontal lobe brain infarct (bottom). B, Three-dimensional SPECT liver study using 8 mCi of ^99mTc sulfur colloid. A mass is seen on the three-dimensional image (left) and transaxial images (right).

Computer networks are an integral part of the way a department communicates information within and among institutions. In a network, several or many computers are connected so that they all have access to the same files, programs, and printers. Networking allows the movement of image-based and text-based data to any computer or printer in the network. Networking improves the efficiency of a nuclear medicine department. A computer network can serve as a vital component, reducing the time expended on menial tasks while allowing retrieval and transfer of information. Consolidation of all reporting functions in one area eliminates the need for the nuclear medicine physician to travel between departments to read studies. Centralized archiving, printing, and retrieval of most image-based and non–image-based data have increased the efficiency of data analysis, reduced the cost of image hard copy, and permitted more sophisticated analysis of image data than would routinely be possible.

Electronically stored records can decrease reporting turnaround time, physical image storage requirements, and use of personnel for record maintenance and retrieval. Long-term computerized records can also form the basis for statistical analysis to improve testing methods and predict disease courses. Most institutions now use some form of picture archiving and communication systems (PACS) to organize all of the imaging that is done. PACS are the foundation of a digital department, allowing for easy transfer, retrieval, and archiving of all imaging done in the nuclear medicine department.

QUANTITATIVE ANALYSIS

Many nuclear medicine procedures require some form of quantitative analysis to provide physicians with numeric results based on and depicting organ function. Specialized software allows computers to collect, process, and analyze functional information obtained from nuclear medicine imaging systems. Cardiac ejection fraction is a common quantitation study (Fig. 34-11). In this dynamic study of the heart’s contractions and expansions, the computer accurately determines the ejection fraction, or the amount of blood pumped out of the left ventricle with each contraction.

Fig. 34-11 Gated first-pass cardiac study and quantitative results, including cardiac ejection fraction, of a normal patient. A, Anterior image of the left ventricle at end-diastole (relaxed phase), with a region of interest drawn around the left ventricle. B, Same view showing end-systole (contracted phase). C, Curve representing the volume change in the left ventricle of the heart before, during, and after contraction. This volume change is referred to as the ejection fraction (EF); normal value is approximately 62%.

Imaging Methods

A wide variety of diagnostic imaging examinations are performed in nuclear medicine. These examinations can be described on the basis of the imaging method used: static, whole-body, dynamic, SPECT, and PET.

STATIC IMAGING

Static imaging is the acquisition of a single image of a particular structure. This image can be thought of as a “snapshot” of the radiopharmaceutical distribution within a part of the body. Examples of static images include lung scans, spot bone scan images, and thyroid images. Static images are usually obtained in various orientations around a particular structure to show all aspects of that structure. Anterior, posterior, and oblique images are often obtained.

In static imaging, low radiopharmaceutical activity levels are used to minimize radiation exposure to the patients. Because of these low activity levels, images must be acquired for a preset time or a minimum number of counts or radioactive emissions. This time frame may vary from a few seconds to several minutes to acquire 100,000 to more than 1 million counts. Generally, it takes 30 seconds to 5 minutes to obtain a sufficient number of counts to produce a satisfactory image.

WHOLE-BODY IMAGING

Whole-body imaging uses a specially designed moving detector system to produce an image of the entire body or a large body section. In this type of imaging, the gamma camera collects data as it passes over the body. Earlier detector systems were smaller and required two or three incremental passes to encompass the entire width of the body.

Nearly all camera systems used for whole-body imaging incorporate a dual-head design for simultaneous anterior and posterior acquisition. Whole-body imaging systems are used primarily for whole-body bone or whole-body tumor imaging and other clinical and research applications (Fig. 34-12).

Fig. 34-12 Whole-body scan performed using 25 mCi ^99mTc HDP in a 25-year-old man. Study was normal. A, Anterior and posterior whole-body view in linear gray scale. B, Anterior and posterior whole-body view in square-root gray scale, to enhance soft tissue. (Courtesy General Electric.)

DYNAMIC IMAGING

Dynamic images display the distribution of a particular radiopharmaceutical over a specific period. A dynamic or “flow” study of a particular structure is generally used to evaluate blood perfusion to the tissue; this can be thought of as a sequential or time-lapse image. Images may be acquired and displayed in time sequences of 1/10 second to longer than 10 minutes per image. Dynamic imaging is commonly used for first-pass cardiac studies, hepatobiliary studies, and renal studies.

SPECT IMAGING

SPECT produces images similar to the images obtained by CT or MRI in that a computer creates thin slices through a particular organ. This imaging technique has proved very beneficial for delineating small lesions within tissues, and it can be used on virtually any structure or organ. Improved clinical results with SPECT are due to improved target-to-background ratios. Planar images record and show all radioactivity emitting from the patient above and below the region of interest (ROI), causing degradation of the image. In contrast, SPECT eliminates the unnecessary information.

With SPECT, one to three gamma detectors may be used to produce tomographic images (Fig. 34-13). Tomographic systems are designed to allow the detector heads to rotate 360 degrees around a patient’s body to collect “projection” image data. The image data are reconstructed by a computer in several formats, including transaxial, sagittal, coronal, planar, and three-dimensional representations. The computer-generated images allow for the display of thin slices through different planes of an organ or structure, helping to identify small abnormalities.

Fig. 34-13 SPECT camera systems. A, Single-head system. B, Dual-headed system. C, Triple-headed system. (A, Courtesy General Electric. B, Courtesy Siemens Medical Systems, Inc. C, Courtesy Marconi Medical Systems.)

The most common uses of SPECT include cardiac perfusion, brain, liver (see Fig. 34-10, B), tumor, and bone studies. An example of a SPECT study is the myocardial perfusion thallium study, which is used to identify perfusion defects in the left ventricular wall. ²⁰¹Tl is injected intravenously while the patient is being physically stressed on a treadmill or is being infused with a vasodilator. The radiopharmaceutical distributes in the heart muscle in the same fashion as blood flowing to the tissue. An initial set of images is acquired immediately after the stress test. A second set is obtained several hours later when the patient is rested (when the ²⁰¹Tl has redistributed to viable tissue) to determine whether any blood perfusion defects that were seen on the initial images have resolved. By comparing the two image sets, the physician may be able to tell whether the patient has damaged heart tissue resulting from a myocardial infarction or myocardial ischemia (Fig. 34-14).

Fig. 34-14 ²⁰¹Tl myocardial perfusion study comparing stress and redistribution (resting) images in various planes of the heart (short axis and long axis). Perfusion defect is identified in stress images but not seen in redistribution (rest) images. This finding is indicative of ischemia.

COMBINED SPECT AND COMPUTED TOMOGRAPHY IMAGING

A blending of imaging function and form is available. By merging the functional imaging of SPECT with the anatomic landmarks of CT, more powerful diagnostic information is obtainable (Fig. 34-15). This combination has a significant impact on diagnosing and staging malignant disease and on identifying and localizing metastases. This new technology can be used for anatomic localization and attenuation correction. According to manufacturers, statistics show that adding CT (for attenuation correction and anatomic definition) changes the patient course of treatment 25% to 30% from what would have been done when using the functional image alone.