MT training: radiology

In vivo tests trace the amounts of radioactive substance within the body. They are given directly to a patient to evaluate the function of an organ or to image it. For example, in tracer studies a specific radionuclide is incorporated into a chemical substance and administered to a patient. The combination of the radionuclide and a drug or chemical is called a radiopharmaceutical (or labeled compound). Each radiopharmaceutical is designed to concentrate in a certain organ. The organ can then be imaged with the radiation given off by the radionuclide. A sensitive, external detection instrument, called a scintillation scanner, is used to determine the distribution and localization of the radiopharmaceutical in various organs, tissue, and fluids. The amount of radiopharmaceutical at a given location is proportional to the rate at which the gamma rays are emitted. Nuclear medicine studies depict the physiological behavior (how the organ works) rather than the specific anatomy of an organ.

The procedure of making an image to follow the distribution of radioactive substance in the body is called scanning, and the image produced is called a scan. Uptake refers to the rate of absorption of the radiopharmaceutical into an organ or tissue.

Radiopharmaceutical may be administered by many different route to obtain a scan of a specific organ in the body. For example, in case of a lung scan, the radio pharmaceutical can be given intravenously (perfusion studies, which rely on passage of the radioactive compound through the capillaries of the lungs) or by inhalation of xenon-133 (133Xe) gas (ventilation studies), which fills the air sacs (alveoli). The combination of these tests permits sensitive and specific diagnosis of clots in the lung (pulmonary emboli).

Other examples of diagnostic procedures that utilize radionuclides are

1. Blood and heart scan. Radio pharmaceutical (99mTc human serum albumin) is injected intravenously, and the blood flow is imaged as the tracer passes through the chambers of the heart and large vessels. Sequential images can be recorded on film to give a motion picture of the passage of blood through the heart. Diagnosis of various forms of heart disease is possible.

2. Bone scan. 99mTc is used to label phosphate substances and is intravenously injected. The phosphate compound is taken up preferentially by bone, and the skeleton can be imaged in 2 or 3 hours by use of a gamma camera. Waiting 2-3 hours allows much of the radiopharmaceutical to be excreted in urine and allows for better visualization of the skeleton. The scan is useful in demonstrating malignant metastases to the skeleton, which appear as areas of high uptake (“hot spots”) on the scan.

3. Brain scan. A radio pharmaceutical (99mTc pertechnetate) is injected intravenously, and about 2 hours later images of the brain are obtained via scanning. Normal scans would show no uptake of radio pharmaceutical in the brain because of the normal functioning of the blood-brain barrier (BBB) which prevents radio pharmaceutical from entering the brain from the blood. However, if the BBB is broken down by tumor or disease, radio pharmaceutical enters the brain and shows up on the brain scan. Brain scans are useful is detecting infarctions, abscesses, tumors, and hematomas.

4. Gallium scan. The radioisotope gallium-67 is injected and has an affinity for tumors and non-neoplastic lesions such as abscesses.

5. Liver and spleen scans. To visualize the liver and spleen, a radio pharmaceutical (99mTc and sulfur colloid) is injected intravenously, and images are taken with a gamma camera. Areas of tumor or abscess are shown as blank spots (regions of reduced uptake). Abnormalities such as cirrhosis, abscesses, tumor, hepatomegaly, and hepatitis can be detected by liver scanning, and splenomegaly due to tumor, cysts, abscess, or rupture and be diagnosed with spleen scanning.

6. Positron emission tomography (PET scan). This radio nuclide technique produces a cross-sectional (transverse) images for the distribution of radioactivity (through the emission of positrons) in a region of the body. It is similar to the CT scan, but radioisotopes are used instead of dye and x-rays. The radionuclides are incorporated (by intravenous injection) into the tissue to be scanned, and an image is made showing where the radio nuclide (such as carbon-11 [11 C] glucose, oxygen-15 [15 O] oxygen) is or is not being metabolized. For example, PET scanning has determined that schizophrenics do not metabolize glucose equally in all parts of the brain and that drug treatment can bring improvement to these regions. Thus areas of metabolic deficiency can be pinpointed by PET, making it helpful in diagnosing and treating other neurological disorder such as stroke, epilepsy, Alzheimer’s disease, and brain tumors, as well as abdominal disorders.

7. Radioactive iodine uptake by the thyroid gland. Radioactive iodine compound (131 I) is given orally by capsule (the amount of radioactive substance in the capsule is measured earlier). The amount of radioactivity taken up by the thyroid gland during the process of making thyroid hormone can be measured at 6 and 24 hours and compared with normal values. This measurement reflects the rate of hormone synthesis by the thyroid gland.

8. Single-proton emission computed tomography (SPECT). This technique involves an intravenous injection of radioactive tracer and the computer reconstruction of a three-dimensional image from the composite of many views.