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Nuclear Medicine

Nuclear medicine is a subspecialty within the field of radiology. It comprises diagnostic examinations that result in images of body anatomy and function. The images are developed based on the detection of energy emitted from a radioactive substance given to the patient, either intravenously or by mouth. Generally, radiation to the patient is similar to that resulting from standard x-ray examinations.

Nuclear medicine images can assist the physician in diagnosing diseases. Tumors, infection and other disorders can be detected by evaluating organ function. Specifically, nuclear medicine can be used to:

• Analyze kidney function
• Image blood flow and function of the heart
• Scan lungs for respiratory and blood-flow problems
• Identify blockage of the gallbladder
• Evaluate bones for fracture, infection, arthritis or tumor
• Determine the presence or spread of cancer
• Identify bleeding into the bowel
• Locate the presence of infection
• Measure thyroid function to detect an overactive or underactive thyroid

Nuclear magnetic resonance, or NMR, can reveal the distribution of atoms in a sample of material. It can do the same in the body, generating images of internal structure without the use of X-rays. The medical need to see inside the human body from the outside has been met for many decades by recording the differential absorption of X-rays such as in an ordinary X-ray system. However, a major deficiency of the standard method of radiography is its inability to discriminate among overlapping structures.

This deficiency has been remedied in recent years by the development of X-ray computerized tomography, or CAT scanning (see above). Although CAT scanning has proved to be an extremely useful diagnostic tool, the information its images provide is basically physical--what the organ looks like. They tell little about the functional or physiological state of the internal organs. Moreover, a type of structure known as pathological lesions can go undetected in a CAT scan unless the lesions are large enough to change the size or shape of the organ. Beyond that X-rays, even in small doses, carry a finite risk of doing physiological harm.

A new technique for obtaining cross-sectional pictures through the human body without exposing the patient to ionizing radiation is nuclear magnetic resonance imaging. NMR imaging not only yields physical information comparable in many ways to the information supplied by a CAT scan but also promises to discriminate more sensitively between healthy and diseased tissue. This is founded on the well-established ability of NMR spectroscopy to elucidate the intricate structures of organic molecules and to provide insight into dynamic chemical processes. For several years biochemists have exploited NMR techniques to monitor metabolic reactions in animals and human beings. It is the recent development of methods for presenting NMR information in pictorial form that is now providing clinicians with a powerful diagnostic tool.

The experimental foundations of NMR spectroscopy were laid by scientists at Stanford University and Harvard University more than four decades ago; work for which they were awarded a Nobel prize in 1952. It had been known since the 1920's that many atomic nuclei have an angular momentum arising from their inherent property of rotation, or spin. Since nuclei are electrically charged, the spin causes a current which in turn generates a small magnetic field. Each nucleus of nonzero spin therefore has a magnetic moment, or dipole, associated with it. Only nuclei with an odd number of nucleons (protons or neutrons) exhibit a net spin and therefore lend themselves to NMR spectroscopy.

In general the magnetic dipoles of the nuclei with spin will be points in random directions. When they are placed in a magnetic field, however, they will orient themselves with the field's lines of force. For nuclei of the spin designated 1/2, such as protons (hydrogen nuclei), the only allowed orientations of the dipoles are parallel to the field or antiparallel to it (in the opposite direction). The two orientations have slightly different energies. In the case of protons the difference between the number of protons with spin "up" (parallel) and spin "down" (antiparallel) is very small: only about one part in 108, with a slight excess in spin up.

Nuclear magnetic resonance is inherently a three-dimensional phenomenon. The spatial resolution of a three-dimensional set of data is usually equal in all three dimensions. With three-dimensional data in hand, surfaces can be detected mathematically, enabling the clinician to determine the volume of organs or of pathologiacal lesions. In medical practice many factors must be considered when a particular imaging method is being chosen, particularly the time scale of involuntary movements of the tissue being studied. The head, for example, is a good subject for true three dimentional imaging because it can be held still for the duration of the scan. The heart, on the other hand, which beats incessantly, requires either a high-speed imaging method or one that can synchronize the data collected over a series of cardiac cycles.

Perhaps the greatest potential of all lies in the imaging of nuclei other than hydrogen, particularly the phosphorus nucleus. Phosphorus is a major constituent of the high-energy molecules adenosine triphosphate (ATP) and phosphocreatine, which mediate the transfer of energy in the living cell. From knowledge of such concentrations it is possible to infer the metabolic status of internal organs, and it many eventually be possible to add this capability to an imaging instrument. The future will undoubtedly see both an improvement in the quality of NMR images and a growing diversity of applications for nuclear magnetic resonance in clinical practice.

“We know that molecular imaging and nuclear medicine will lead to a much greater ability to characterize diseases, diagnose them at a very early stage, treat them effectively and monitor the effectiveness of such treatment,” said SNM President Peter S. Conti, M.D., Ph.D., who is professor of radiology, clinical pharmacy and biomedical engineering at the University of Southern California, Los Angeles, and director of the PET Imaging Science Center at USC’s Keck School of Medicine. “Molecular imaging and nuclear medicine research have a proven record of leading to improvements in the diagnosis and treatment of life-threatening cancer, heart and other diseases that affect millions each year,” noted Conti, speaking on behalf of the society, which has more than 16,000 physician, technologist and scientist members in 78 countries.

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Pediatric Nuclear Medicine

Pediatric nuclear medicine refers to these types of examinations in babies, young children and teenagers.Pediatric nuclear medicine is used in the diagnostic workup of many childhood disorders that are congenital (present at birth) or acquired later. It helps in the evaluation of different organ systems, including the kidneys, liver, heart, lungs and bones. Examples of how nuclear medicine may be used in children include the diagnosis of urinary blockage in the kidney, infections and trauma in the bones, gastrointestinal bleeding, and various tumors and their sites of spread in the body.

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Cardiac Nuclear Medicine

Cardiac nuclear medicine refers to these diagnostic tests that are used to examine the anatomy and function of the heart.

Cardiac nuclear medicine tests are indicated for individuals with unexplained chest pain or chest pain brought on by exercise (called angina) to permit the early detection of heart disease.

The most common cardiac nuclear medicine procedure, called myocardial perfusion scanning, enables the visualization of blood-flow patterns to the heart walls. The test is important for evaluating the presence and extent of suspected or known coronary artery disease (blockages) as well as the results of previous injury to the heart from a heart attack, called a myocardial infarction. It also can be done to evaluate the results of bypass surgery or other percutaneous revascularization procedures designed to restore the blood supply to the heart.

Heart-wall movement and overall heart function can be evaluated with cardiac gating, a technique that synchronizes the images of the heart with different parts of the cardiac cycle (contracting or relaxing) as determined by an electrocardiogram (ECG), which records the electrical currents that activate the heart muscle and cause it to pump.

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Positron Emission Tomography (PET)

Positron emission tomography, also called PET imaging or a PET scan, is a diagnostic examination that involves the acquisition of physiologic images based on the detection of radiation from the emission of positrons. Positrons are tiny particles emitted from a radioactive substance administered to the patient. The subsequent images of the human body developed with this technique are used to evaluate a variety of diseases.

PET scans are used most often to detect cancer and to examine the effects of cancer therapy by characterizing biochemical changes in the cancer. These scans can be performed on the whole body. PET scans of the heart can be used to determine blood flow to the heart muscle and help evaluate signs of coronary artery disease. PET scans of the heart can also be used to determine if areas of the heart that show decreased function are alive rather than scarred as a result of a prior heart attack, called a myocardial infarction. Combined with a myocardial perfusion study, PET scans allow differentiation of nonfunctioning heart muscle from heart muscle that would benefit from a procedure, such as angioplasty or coronary artery bypass surgery, which would reestablish adequate blood flow and improve heart function. PET scans of the brain are used to evaluate patients who have memory disorders of an undetermined cause, suspected or proven brain tumors or seizure disorders that are not responsive to medical therapy and are therefore candidates for surgery.

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Lymphoscintigraphy

Lymphoscintigraphy provides a view of the workings of the lymphatic system, which is a network of small channels, like arteries and veins, that transport the fluid and cells of the immune system through the lymph nodes and throughout the body. This fluid, called lymph, normally flows slowly from the periphery toward the center of the body and into the general circulation. If lymphatic flow is blocked, the areas of drainage that are affected can become swollen.

A scintigram is a type of picture that uses a radiopharmaceutical (a radioactive drug), which is injected or taken orally, that makes the lymphatic system visible to specialized cameras. The study is performed in the Nuclear Medicine section of the hospital, where the radiopharmaceuticals are prepared and the pictures are taken. Lymphoscintigraphy can be helpful for localizing points of blockage and is also important for identifying abnormal lymph nodes and planning a biopsy or surgery for suspicious areas. Generally, the radiation dose is similar to that of a standard x-ray examination.

Lymphoscintigraphy can assist the physician in diagnosing diseases. It can help detect tumors, infection and other disorders such as the following:

• Lymphoscintigraphy can help diagnose lymphedema, a condition in which lymphatic fluid accumulates in soft tissues and may lead to inflammation and obstruction. This nuclear medicine test has all but replaced lymphangiography, a diagnostic x-ray procedure that used an oil-based contrast material that required surgical incisions on both feet to expose and inject the lymphatics directly.
• Lymph flow in an arm or leg may be evaluated with lymphoscintigraphy by injecting radioactive material into a web space between the fingers or toes and recording images for 60 minutes. Local anesthesia is not necessary.
• When planning surgery for a breast tumor, it is helpful to assess the lymphatic drainage beforehand to identify the sentinel lymph node (the first lymph node that receives lymph drainage from the tumor site) for excisional biopsy. A radiopharmaceutical is injected either just beneath the skin around the areola (nipple); at two to four sites around the tumor; beneath the skin above the tumor; or into the tumor itself on the day of surgery. Imaging usually is completed within 30 minutes, but may take up to one to two hours. Lymphoscintigraphy of the breast is very safe. Side effects are infrequent, and morbidity is much reduced compared with axillary lymph node dissection, which formerly was the routine staging procedure for patients with breast cancer and no obvious spread (metastasis).
• Malignant melanoma is an aggressive form of skin cancer that may spread rapidly to distant body sites. Lymphoscintigraphy may be performed preoperatively in order to identify the sentinel lymph node. A tumor-negative sentinel lymph node is strong evidence that there has not been spread of the tumor. This is important for staging the disease and planning treatment management.

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