Understanding Nuclear Medicine: How It Works

"Nuclear medicine" is a radiology specialty that uses radioactive substances called "radiopharmaceuticals" to diagnose or treat diseases. Diagnostic procedures use very low radiation doses, comparable to general radiology, ensuring high safety and very low risk of side effects. Therapeutic use involves appropriate doses, with physicians always assessing the balance between benefits and patient safety beforehand.

Key features of nuclear medicine

A major advantage of nuclear medicine is its ability to assess the actual function of organs. When combined with other imaging tests, it provides a more comprehensive understanding of organ conditions, leading to more complete and accurate diagnoses. For example,

• A renal scan evaluates not only the size or shape of the kidneys but also how well each kidney functions.
• Myocardial perfusion scanning assesses blood flow to the heart muscle rather than the heart’s shape, evaluating whether blood supply is sufficient both at rest and under stress.

Therefore, nuclear medicine provides detailed diagnostic information and helps select the most appropriate treatment for each patient.

What are radiopharmaceuticals?

Radiopharmaceuticals are substances that combine very small amounts of radioactive material with chemicals, allowing them to target specific organs such as the kidneys, heart, bones, or thyroid.

Most diagnostic tests use radiopharmaceuticals that emit gamma or low-energy X-rays, typically administered in very small amounts. These are highly safe with low risk of side effects, and the body gradually eliminates them through natural excretory systems within a short time. The most commonly used radioactive substance is Technetium-99m (Tc-99m).

For PET/CT scans, radiopharmaceuticals emitting positrons are used in small, safe amounts. This increasingly popular test often employs Fluorine-18 Fluorodeoxyglucose (F-18 FDG), which reflects cellular sugar metabolism, especially in cancer and highly active cells. The body eliminates this substance through the kidneys and urine within a short time, with a low likelihood of side effects.

In therapeutic applications, radiopharmaceuticals aim to deliver radiation that destroys abnormal or unwanted cells, such as cancer cells or overactive thyroid cells. Substances emitting beta or alpha radiation are chosen for their suitable energy to target these cells.

Sufficient radiation doses cause cells to lose their ability to divide and lead to cell death. Treatment doses are higher than diagnostic doses to achieve effectiveness. Physicians carefully evaluate and balance treatment benefits against patient safety on a case-by-case basis before proceeding. The most commonly used therapeutic radiopharmaceutical is Iodine-131 (I-131), which plays a key role in treating thyroid diseases and certain thyroid cancers.

Nuclear medicine services

Diagnostic services

Nuclear medicine diagnostics focus on assessing organ function rather than structure, allowing early detection of abnormalities. Common tests include:

• Bone scans detect cancer spread to bones, bone infections, inflammation, or injuries not visible on standard X-rays.
• Renal scans evaluate the function of each kidney separately, detect urinary tract obstructions, and assist in planning urological surgeries.
• MUGA scans assess the left ventricle's contraction (Left Ventricular Ejection Fraction, LVEF), detect heart wall motion abnormalities, and monitor chemotherapy-related cardiac side effects.
• Myocardial perfusion scans assess blood flow to heart muscle to screen for coronary artery disease or evaluate the severity of ischemic heart conditions.
• Thyroid scans evaluate thyroid function, identify causes of hyperthyroidism, and characterize thyroid nodules.
• PET/CT scans use positron-emitting radiopharmaceuticals to evaluate cancer by locating tumors, detecting spread, and monitoring treatment response.
• Dual-energy X-ray Absorptiometry (DXA) measures bone mineral density to diagnose osteoporosis and assess fracture risk.

Therapeutic services

Nuclear medicine plays an important role in treating certain diseases using radiopharmaceuticals targeted to specific organs or tissues. It is often combined with primary treatments to enhance effectiveness, such as:

• Treating hyperthyroidism with radioactive iodine (I-131) which destroys overactive thyroid cells and controls excess thyroid hormone production.
• Post-surgical treatment of thyroid cancer to reduce recurrence risk by destroying residual cancer cells and metastases.
• Relieving pain from bone metastases by reducing pain and improving quality of life for patients.
• Treating certain cancers alongside main treatments, including neuroblastoma, prostate cancer, and neuroendocrine tumors.

Source: Associate Professor Dr. Kanungnij Thammanirat, Department of Nuclear Medicine, Radiology Department, Faculty of Medicine Ramathibodi Hospital, Mahidol University