Backgrounder: Nuclear Medicine – A Window into the Body
Imagine being able to see inside the body and actually observe how it works–blood through the veins, thoughts lighting up the brain, organs performing their complex tasks. Picture being able to spot abnormalities –tumors, obstructions, infections –that are missed by other diagnostic tools. And finally, think of being able to treat some of those disorders by sending special radiopharmaceutical to the precise location of the problem inside the body.
All this is possible through Nuclear Medicine, a highly sophisticated medical discipline that uses radioactive substances to chart the inner workings of the body. Other medical imaging techniques such as X-ray, ultrasound, magnetic resonance imaging (MR) and computed tomography (CT ) offer detailed pictures of anatomical structure but not function. The imaging techniques of Nuclear Medicine show internal functioning, such as blood flow, organ activation or increased cellular activity , revealing both how a healthy organ works and how a diseased one differs from it.
For this reason, Nuclear Medicine is making great strides in pure research as well as in diagnosis, treatment monitoring and, in a few cases, the treatment itself. Today, Nuclear Medicine is at the frontier of discovering and understanding complex physiologic processes of the brain, heart and other organ systems, revealing more each year about the body’s metabolism. While almost 40 years old, Nuclear Medicine is just coming into its own, using state-of-the-art tools and techniques. Some consider Nuclear Medicine the wave of tomorrow in both diagnosis and treatment.
By its very nature, Nuclear Medicine is a multi-disciplinary field, dependent on contributions from physics and chemistry as well as medicine. It has an enormous impact on every field of medicine, especially cardiology (heart), neurology (brain and nervous system), oncology (cancer), orthopedics (bone), endocrinology (hormonal system), gastroenterology, (digestive system), hematology (blood}, nephrology (kidney}, and pulmonary (lung}.
HOW NUCLEAR MEDICINE WORKS
During a standard Nuclear Medicine procedure, a patient ingests, inhales or is injected with a small amount of a radiotracer – a chemical compound attached to one of several radioactive substances, known as radionuclides. Each radiotracer travels through the blood stream and is taken up by body tissues or organs in different concentrations, so specific areas or organs can be targeted and studied. For example, radioactive iodine is picked up by the thyroid in greater concentrations than by other parts of the body.
Small amounts of gamma rays, similar to flashing light, are detected from the targeted organ by a machine called a gamma camera. The gamma camera works on the same principle as a regular camera, but picks up gamma rays instead of light rays.
Since changes in functions often occur in a diseased organ before changes in structure, radionuclide scanning is valuable diagnostic tool because is can detect these changes earlier than other techniques can. For example, an early developing tumor may cause increased blood flow and cellular activity, which would allow a greater amount of radionuclide to be deposited at that site. Thus, the tumor would show up in Nuclear Medicine tests earlier than structural changes would be visible in X-rays.
Nuclear Medicine imaging can also be used for early evaluation of a treatment, since small improvements in function can be detected. If no improvement can be seen, this suggests a need for a change in therapy.
Because radionuclides have short half-lives and deliver only a very low dose of radiation, Nuclear Medicine imaging is an extremely safe procedure, exposing the patient to a smaller amount of radiation than a routine X-ray procedure. Some of the most commonly used medical radionuclides are technetium-99m, iodine-123, thallium-201, gallium-67, indium-111, xenon-133 and fluorine-18.
Approximately 12 million Nuclear Medicine procedures are performed annually on patients in the United States. Another 100 million Nuclear Medicine procedures are performed on clinical laboratory samples or in biochemical research each year.
MORE SOPHISTICATED NUCLEAR MEDICINE IMAGING
* SINGLE-PHOTON EMISSION COMPUTED TOMOGRAPHY (SPECT) – Taking the standard Nuclear Medicine scan a step further, the SPECT procedure uses a special technique –a tomography –in which the gamma camera rotates around the patient, enabling it to focus on one plane at a time. The net result is a series of many clear images, each at a different parallel plane, as if the organ cut evenly into many thin slices, and each slice were individually photographed. The computer records all these images and can put them together in any configuration desired.
Metaphorically, picture the organ as a loaf of bread. The SPECT procedure visually breaks down the loaf into, say, 15 slices. Aided by a computer, the technologist then looks at the loaf of bread (or organ) sliced horizontally, vertically, diagonally, thick or thin. If there a green mold (or tumor) were growing inside the loaf, its exact location could be pinpointed at two inches down and three inches over on the fourth slice of bread. A regular Nuclear Medicine scan, on the other hand, provides one image of all these slices stacked together.
The ability to examine both the whole loaf and the individual slices offers physicians a much more exact tool for diagnosis and treatment. For example, using SPECT, a doctor often can differentiate a patient suffering from incurable Alzheimer’s disease from other treatable disorders such as depression, stroke or brain tumor. The only definitive way to diagnose Alzheimer’s disease is still through brain biopsy or autopsy but the SPECT brain scan often gives strong support to the clinically suspected diagnosis.
SPECT brain imaging also offers great promise in detecting impending strokes in patients who may only be exhibiting mild pre-stroke symptoms. Recent studies utilized SPECT to investigate biochemical changes in the brain caused by alcohol and substance abuse. One important study discovered that drug-induced brain damage is reversible if drugs are eliminated –a compelling reason for addicts to seek help.
POSITRON EMISSION TOMOGRAPHY (PET) – In PET, a cyclotron is used to produce positron-emitting radionuclides such as carbon, fluorine, nitrogen and oxygen –all naturally-occurring elements in the body. For example, fluorine-18 can be attached to a biochemical substance such as glucose (a type of sugar) to form a radiotracer that can be immediately metabolized by the body. The big advantage that PET offers over other imaging procedures is this ability to demonstrate actual biochemical activity as it is happening.
After inhalation or infusion, these radiotracers travel through the subject’s bloodstream to the area being studied, where they undergo a biochemical reaction as part of body’s normal metabolism and emit signals displayed as an image on a video screen according to the amount of biochemical activity in each area.
By learning the patterns of biochemical activity in normal brain function, for example, Nuclear Medicine physicians and scientists can detect abnormal brain patterns that occur in various medical disorders. One surgeon used a PET scan to help him localize and remove the abnormal area in the brain of a child with severe epilepsy. The child is now seizure-free, experiencing only minor disabilities.
PET can also be used after a heart attack to decide if a by-pass operation would be successful or if a more-drastic heart transplant is required. A patient in Phoenix who was scheduled for a heart transplant learned through PET that a by-pass was possible. Five weeks after a five-vessel by-pass operation, he walked out of the hospital, saving himself the possibility of organ rejection, infection and other complications. On the other hand, if a surgeon performs a by-pass operation only to discover that the heart is beyond repair, he must reschedule the patient for a heart transplant. Through PET, the correct surgical procedure often can be scheduled.