Sestamibi

Sestamibi is a radiopharmaceutical used in nuclear medicine imaging. It is also known as methoxyisobutylisonitrile or MIBI. The radioisotope attached to the sestamibi molecule is technetium-99m, forming 99Tcm-sestamibi (or Tc99m MIBI).

Its main use is for imaging the myocardium (heart muscle). It is also used in the work-up of primary hyperparathyroidism to identify parathyroid adenomas, for radioguided surgery of the parathyroid and in the work-up of possible breast malignancies.

Cardiac imaging
Technetium-99m sestamibi is a lipophilic cation which, when injected intravenously into a patient, distributes in the myocardium proportionally to the myocardial blood flow. As opposed to Thallium-201, MIBI does not undergo significant redistribution. Single photon emission computed tomography (SPECT) imaging of the heart is performed using a gamma camera to detect the gamma rays emitted by the technetium-99m sestamibi as it decays. Two sets of images are acquired. For one set, the Tc99m MIBI is injected whilst the patient is at rest and then the myocardium is imaged. In the second set, the patient is stressed either by exercising on a treadmill or pharmacologically. The Tc99m MIBI is injected at peak stress and then imaging is performed. The resulting two sets of images are compared with each other to distinguish ischaemic from infarcted areas of the myocardium. This imaging technique is also known as myocardial perfusion imaging (MPI).

Parathyroid imaging
In hyperparathyroidism, one or more of the four parathyroid glands have developed a benign tumor called an adenoma. The affected gland takes up Tc99m MIBI following an intravenous injection. The patient's neck is imaged with a gamma camera to show the tumor.

Breast imaging
Tc99m MIBI is also used in the evaluation of breast nodules. Malignant breast tissues concentrate MIBI to a much greater extent and more frequently than benign disease. As such, limited characterization of breast anomalies is possible. Scintimammography has the highest specificity for breast cancer of any imaging test, but its sensitivity is too low to allow for routine use.

Radioguided surgery of the parathyroids
Following the administration of Tc99m MIBI it collects in overactive parathyroid glands. During surgery, the surgeon can use a probe sensitive to gamma rays to locate the overactive parathyroid before removing it.