Introduction:
Radionuclide imaging is a nuclear medicine scan that provides images of the bone, organs, and other body parts using radiopharmaceuticals or radiotracers. Radioactive tracers or radiopharmaceuticals are individual atoms, marked molecules, hormones, or antibodies labeled with an isotope. Gamma-emitting radiotracers are commonly used in medical imaging. They are low ionizers and have a higher penetration rate; hence, they can be detected using cameras outside the body.
Technetium 99m is the most commonly used radiotracer in about 80 to 90 percent radionuclide imaging. Technetium 99m releases only gamma rays with an energy of about 140 keV that gamma cameras can easily detect in SPECT (single-photon emission computed tomography) scan.
What Are the Uses of Radionuclide Imaging?
A radionuclide scan is used for different imaging parts of the body such as bone, thyroid gland, liver, gallbladder, brain, lungs, and heart. Some of the radionuclide images are as follows:
1. V/Q Scan:
V/Q scan, or pulmonary ventilation (V) and perfusion (Q) scan, is a nuclear medicine imaging technique used to determine the airflow (ventilation) and blood flow (perfusion) to the lungs. V/Q scan is commonly used for the diagnosis of pulmonary embolism. 99Tc- DTPA (Diethylenetriamine pentaacetate) is the most widely used radiotracer in ventilation scans and 99mTc-MAA in perfusion scans.
2. Bone Scan:
A bone scan, also known as skeletal scintigraphy, is a diagnostic tool that uses nuclear medicine to detect bone diseases. Technetium-99m medronate (methylene diphosphonate [MDP]) is a commonly used radiotracer at the dose of 740 Mbq (20 mci) for adults. Sometimes, technetium-99m oxidronate (hydroxy methylene diphosphonate [HMDP]) and hydroxy methylene diphosphonate (HDP) are also used.
The bone scan is used in the diagnosis of bone diseases such as:
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Bone infection.
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Arthritis (swelling of the joints).
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Paget's disease (a bone disease that affects the normal bone recycling process).
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Fibrous dysplasia (a rare bone disease in which normal bone tissue is replaced by abnormal scar-like tissue).
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Osteomyelitis (swelling of the bone caused by infection).
3. Thyroid Scan:
The thyroid scan is a nuclear medicine imaging that uses radioactive materials to determine the structure and function of the thyroid gland. It is used to diagnose the thyroid disorders such as:
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Thyroiditis (swelling of the thyroid gland).
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Congenital thyroid disorders.
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Thyroid cysts.
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Hyperthyroidism (a disease characterized by excessive production of the thyroid hormone).
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Hypothyroidism (a disease characterized by decreased production of the thyroid hormone).
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Goiter (abnormally increased growth of the thyroid gland).
4. Heart Scan:
A heart scan, also called a myocardial perfusion scan, is used to detect the blood flow to the heart muscles and determine the pumping action of heart muscles. 99m- technetium and thallium - 201 is the most commonly used radiotracer in the myocardial perfusion scan.
5. Kidney Scan:
DMSA or dimercaptosuccinic acid scan is used to detect urinary tract infections, evaluate kidney function, and visualize scarring in kidneys. Technetium - 99m DMSA is the radiotracer used in kidney scans.
6. Lacrimal Scintigraphy:
Lacrimal scintigraphy, also called dacryoscintigraphy, is a nuclear medicine scan used to image the lacrimal apparatus in diagnosing obstruction of the lacrimal gland and nasolacrimal duct. Technetium-99m or pertechnetate is the commonly used radiopharmaceutical in this scan.
7. Lymphoscintigraphy:
Lymphoscintigraphy uses radiotracers in the diagnosis of lymphatic dysplasia and chylothorax. 99m labeled sulfur colloid is commonly used as a radiotracer in this scan.
8. PET (Positron Emission Tomography) Scan:
PET scan is used to detect normal and abnormal metabolic activity in the body. PET scan is helpful in the diagnosis of cancer, infection, heart diseases, and brain disorders. Fluorodeoxyglucose-18 (FGD-18) is the most commonly used radiotracer in PET scans.
How Do Patients Prepare for Radionuclide Imaging?
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Patients can normally eat and drink before their radionuclide scan. But gallbladder scan is exceptional in which patients are asked not to eat or drink a few hours before the scan.
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Patients may need to empty their bladder before the test as it may hide the view of pelvic bones.
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Sometimes patients are instructed to remove the metal objects or jewels from the examination site before the scan.
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The patients' medical conditions and regular medicines should be informed before the scan. For example, sometimes, patients are advised to stop barium or bismuth-containing medication, affecting the scan results.
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The patient should tell their health provider if she is pregnant or suspects that babies in the womb are more sensitive to radioactive tracers.
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Patients are advised to avoid taking foods containing iodine a week before the scan for the thyroid scan.
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Patients are instructed to avoid vigorous exercises, and deep-tissue massages for the PET (positron emission tomography) scan 48 hours before. Also, they should take a low-sugar, low-carbohydrate diet and non-caffeinated drinks.
How Is Radionuclide Imaging Performed?
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For radionuclide imaging, the doctors may inject a small amount of radioactive tracer into the patient's vein or ask them to swallow or inhale the tracers as gas.
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Then the radioactive tracers enter the bloodstream and are absorbed by the cells in the body. The patient might be asked to wait for about one hour for the radioactive tracers to be absorbed by the tissues.
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After that, patients are asked to rest quietly and not allowed to move, talk or do any other activities to minimize the brain stimulation before the scan.
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Patients are instructed to wear the hospital gown and lie flat on the imaging table during the scan. Their heads should be tipped backward for the thyroid scan and their necks extended. Children may feel discomfort from having to remain still during imaging. Family members or caretakers of the children may be allowed to stay in the room.
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After that, the gamma camera will take images from different angles. The gamma camera detects the emission of gamma rays from the radiotracer and produces images of the organs on the computer screen.
Conclusion:
SPECT scan is a single-photon imaging technique that uses gamma cameras with an energy of about 75 to 360 KeV. PET scan is a positron imaging technique that produces higher resolution images than SPECT scan. The radiotracers used for diagnostic imaging should have short half-lives of less than a day to prevent unnecessary radiation exposure to the patients. Radiotracers with long half-lives are used in medical treatments such as brachytherapy and gamma knife radiosurgery. The combination of CT (computed tomography) scan and gamma camera and CT scan, and PET scan in radionuclide imaging provides detailed information on the anatomic structures and metabolic process.
