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 ^99mTc-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.

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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 primary hyperparathyroidism, one or more of the four parathyroid glands either develops a benign tumor called an adenoma or undergoes hypertrophy as a result of homeostatic dysregulation. The parathyroid gland take up Tc99m MIBI following an intravenous injection, and the patient's neck is imaged with a gamma camera to show the location of all glands. A second image is obtained after a washout time (approximately 2 hours), and mitochondria in the oxyphil cells of the abnormal glands retaining the Tc99m are seen with the gamma camera. This imaging method will detect 75 to 90 percent of abnormal parathyroid glands in primary hyperparathyroidism. An endocrine surgeon can then perform a directed parathyroidectomy (less invasive than traditional surgery) to remove the abnormal gland.

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 Breast-Specific Gamma Imaging (BSGI) has the highest specificity for breast cancer of any imaging test, and has a sensitivity of 66% based on positive biopsy compared to mammography and ultrasound with a 29% positive biopsy.

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. An article describing the procedure can be found here [1].

References

• Cardiolite.com
• Parathyroid.com
• Myoview.com
• Miraluma
• Dilon Breast-Specific Gama Imaging