Converting Energy to Medical Progress
Radiopharmaceutical Energy Reveals World of Human Biology
Nuclear medicine images are produced by the energy emitted from radiopharmaceuticals inside a patient's body with imaging systems ("scanners") that detect and process the energy signals. The special ability of radiopharmaceuticals to visualize human biology, both healthy and diseased, arises from their distribution through the body as "radiotracers." Nearly all radiopharmaceuticals (i.e., medically useful radiotracers) and imaging systems described here were discovered, designed, or developed by scientists supported by the BER Medical Sciences program during the past 50 years.
Biological Imaging: Of a Physiologic Process, Not Anatomy
Disease is a biological process, and nuclear medicine provides images of these biological processes.Most radiotracers interact with a biological process--such as bone mineral turnover, potassium transport in heart muscle, or glucose (sugar) metabolism in various organs or tumors--and emit low levels of radiation. Highly sensitive detector systems collect these energy signals, and computer programs reconstruct them into diagnostic images. Because it provides images of a biological process (physiology), nuclear medicine differs from other imaging techniques--such as x-rays, computed tomography (CT), magnetic resonance imaging (MRI), and ultrasound--which primarily visualize structure and shape (anatomy).
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| A single image from a brain scan (top left), a bone scan (bottom left), and a series of images from a heart scan (right) during exercise ("stress") and "rest." The brain scan shows reduced glucose metabolism in a pattern characteristic of Huntington's disease, evident years before the patient exhibited abnormal movements or other symptoms of this hereditary disease. The bone scan is from a patient with prostate cancer that has spread to the spine and other bones. The radiopharmaceutical, similar to the mineral in bone, accumulates at bone tumors (dark spots) because the diseased bone has faster mineral turnover. The heart scan, from a patient with coronary artery disease, shows where the heart muscle lacks adequate blood flow. The radiopharmaceutical, thallium-201, mimics potassium and accumulates more in regions of normal blood flow. | |
Radiopharmaceuticals: Equal Radionuclides Plus Carrier Molecules
Most radiopharmaceuticals have two components: a radionuclide and a carrier molecule. The radionuclide is an "excited" atom that emits energy so that the atom can convert to a more stable form. Common radionuclides used in nuclear medicine include technetium-99m, thallium-201, fluorine-18, indium-111, gallium-67, iodine-123, iodine-131, and xenon-133. Once a radiopharmaceutical is injected into a patient, the carrier molecule travels through the body until it interacts with its target cell, tissue, or organ system. Almost all the radionuclides, and many of the carrier molecules, used in nuclear medicine today were discovered or developed by BER scientists over the past 50 years.
Energy Signals: From the Inside Out
Like an x-ray image, a nuclear medicine scan depends on energy passing through the body toward a detection device. In nuclear medicine, radiopharmaceuticals placed in the body emit radiation from the inside out. Diagnostic nuclear medicine scans expose patients to levels of radiation comparable to what patients receive in routine x-ray procedures.
Imaging
Systems: Gamma Cameras Use Large Wafer-Like Detectors
Specialized imaging systems (e.g., gamma cameras or other scanners) stop gamma rays emitted from the patient. Fast, sophisticated computers map the energy signals into medically useful pictures that represent a biological process. The gamma camera was invented by a BER scientist in 1952.
PET and SPECT: Advanced Imaging Systems
Special imaging systems called "positron emission tomography" (or PET) and "single-photon emission computed tomography" (or SPECT) scanners produce 3-dimensional (tomographic) images. The scans look like multiple slices through the body. In SPECT, a gantry rotates one or more detectors around the body to acquire many image projections. PET scanners usually surround the body with a stationary ring of detectors. PET and SPECT were first conceived by BER scientists and developed over the 1950s, 1960s, and 1970s.
PET and SPECT: Advanced Radiopharmaceuticals
SPECT radiopharmaceuticals emit gamma rays, whereas PET radiopharmaceuticals emit another form of energy, positrons,which converts to gamma rays. These radiopharmaceuticals "interrogate" cells and molecules. They are "molecular probes" designed to provide answers about healthy, normal biology, the biological process of disease, and even the molecular errors that cause disease.
These PET scans (on the left) were obtained with fluorine-18 fluorodeoxyglucose (FDG, a form of sugar). F-18 FDG, the most common PET radiopharmaceutical used in medicine today, was developed by BER scientists in the 1970s.
Glucose (a sugar, the primary fuel for cells) is just one example of the thousands of molecules related to human biology that can serve as carrier molecules for radiopharmaceuticals. In the future, PET and SPECT radiopharmaceuticals may target gene function and expression to answer questions about the genetic causes of disease.
Whole-body PET scans from two patients. The left scan is normal; the right scan is from a patient with a lung tumor that spread from primary breast cancer. This scan shows increased F-18 FDG uptake in the tumor (arrow) because a growing tumor has a higher rate of sugar metabolism than the surrounding normal tissue.
Next: BER Medical Sciences
Tuesday, October 23, 2007