A Sestamibi scan is a specialized nuclear medicine imaging procedure used to visualize the function of specific tissues. This test involves injecting a small, safe amount of a radioactive material, known as a radiotracer, into a vein. A gamma camera detects the energy emitted by the tracer as it circulates and is absorbed by target organs. The resulting images provide functional information about the tissues, which goes beyond the anatomical detail offered by standard X-rays or CT scans.
How the Sestamibi Compound Works
The Sestamibi compound, formally known as Technetium-99m Sestamibi, is the active agent in this diagnostic test. The “sesta” part of the name refers to the six methoxyisobutylisonitrile (MIBI) ligands bound to the radioactive isotope, Technetium-99m (\(\text{}^{99\text{m}}\text{Tc}\)). This radiotracer is a lipophilic cation, meaning it is fat-soluble and positively charged, allowing it to pass easily through cell membranes.
Once inside a cell, the Sestamibi compound accumulates primarily within the mitochondria. Because of this mechanism, the tracer is preferentially taken up by cells that are highly metabolically active and possess a large number of mitochondria. This accumulation makes the compound useful for imaging hyperactive parathyroid glands and the muscle tissue of the heart. The \(\text{}^{99\text{m}}\text{Tc}\) isotope emits gamma rays, which the gamma camera detects to map the tracer’s location and concentration.
Key Applications in Diagnosis
The Sestamibi scan is utilized for two main diagnostic purposes based on the tracer’s unique accumulation properties. One application is parathyroid imaging, which focuses on the four small glands in the neck that regulate calcium levels. The scan is ordered for hyperparathyroidism, a condition often caused by an overactive parathyroid adenoma (a non-cancerous tumor).
In parathyroid imaging, the overactive adenoma retains the Sestamibi tracer longer than the surrounding, normal thyroid tissue, which rapidly clears the compound. This difference in clearance rate, known as a “washout” effect, allows a physician to pinpoint the location of the abnormal gland on delayed images. Localizing the adenoma before surgery is a benefit, as it allows surgeons to perform a minimally invasive procedure, removing only the problematic gland.
The second main use is cardiac imaging, specifically Myocardial Perfusion Imaging (MPI), often called a nuclear stress test. This procedure assesses blood flow to the heart muscle (myocardium) to detect coronary artery disease. The scan is performed in two parts: once when the heart is at rest and again after physical exercise or pharmacological stress.
Comparing the rest and stress images helps physicians distinguish between permanently damaged heart tissue (a fixed defect) and tissue receiving insufficient blood flow only during exertion (a reversible defect, or ischemia). Areas with reduced blood flow show less tracer uptake, creating a visible defect. This functional assessment provides information about the extent and severity of coronary artery blockages.
Patient Preparation and Procedure Steps
Preparation for the Sestamibi scan varies depending on whether the cardiac or parathyroid application is performed. For a parathyroid scan, preparation is minimal, requiring the removal of neck jewelry or metal objects that could interfere with the images. Patients are typically allowed to eat and drink normally, but must inform staff if they are pregnant or breastfeeding due to the radiotracer.
For the cardiac stress test, preparation is more rigorous, especially regarding dietary and medication restrictions. Patients must avoid all caffeinated products, including coffee, tea, colas, and chocolate, for 24 hours before the test. Caffeine can interfere with pharmacological stress agents, potentially leading to inaccurate results. Patients are also instructed to fast for several hours and may need to temporarily stop certain heart medications, such as beta-blockers, as directed by their physician.
The procedure begins with the intravenous injection of the Sestamibi tracer, usually into a vein in the arm. For a parathyroid scan, initial images are captured shortly after injection, followed by delayed images 90 minutes to three hours later to observe the tracer’s washout from the thyroid. For a cardiac scan, rest images are taken first, followed by a waiting period and a second injection of the tracer at the peak of physical or chemical stress. Imaging follows the second injection to capture the heart’s function under stress conditions.
Interpreting the Scan and Safety
The images captured by the gamma camera are interpreted by a nuclear medicine specialist or a radiologist. For a parathyroid scan, an area retaining the radiotracer on delayed images, appearing as a “hot spot,” suggests the presence of an overactive adenoma. If no abnormal retention is observed, the scan is considered negative for a localizing adenoma.
In cardiac imaging, results are analyzed by comparing the rest and stress images. A region showing reduced tracer uptake on both images indicates a fixed defect, suggesting a prior heart attack and permanent tissue damage. Conversely, an area with reduced uptake during stress but normal uptake at rest indicates a reversible defect, pointing to a blockage that limits blood flow only under increased demand.
Patient safety is a primary consideration, and the radiation exposure from the Sestamibi scan is low, comparable to other common diagnostic imaging tests. The radioactive material, Technetium-99m, has a short physical half-life of about six hours, meaning it decays quickly. The body naturally clears the tracer over time, with much of it excreted through the urine and feces within 48 hours. The procedure is generally non-invasive, with most patients only experiencing minor discomfort from the initial intravenous line insertion.