Tc-99m sestamibi is a radiopharmaceutical used for parathyroid imaging and myocardial perfusion scintigraphy, known for its uptake in metabolically active tissues. For parathyroid scans, the adult dose is 740 MBq, with imaging performed at 15 minutes and 2 hours, while myocardial perfusion scintigraphy typically uses a 400 MBq dose at rest and 1.1 GBq during stress. The technique is vital for diagnosing conditions like coronary artery disease and parathyroid adenomas, with specific imaging protocols to assess both blood flow and tissue abnormalities.

1. Tc-99m sestamibi
Scans
Parathyroid and Heart
Dr Muhammad Qasim Khan

2. Tc-99m sestamibi

3. • Tc-99m sestamibi (sestamibi is a shortening of sesta-
methoxyisobutylisonitrile) is one of the technetium
radiopharmaceuticals.

4. Characteristics
• photon energy: 140 KeV
• physical half-life: 6 hours
• lipophilic cation
• normal distribution: thyroid, parathyroid, heart (myocardium)
• excretion: hepatobiliary and renal
• target organ: colon, kidneys, bladder, gallbladder
• brain uptake depends on blood-brain barrier (BBB) rupture;
correlates with cell proliferation indices (Ki67)

5. • Tc-99m-sestamibi is a radiopharmaceutical that is taken up by
cells with a high concentration of mitochondria i.e. highly
cellular/metabolically active tissues such as the myocardium of
the heart, but also abnormal tissues such as oxyphil cell
parathyroid adenomas, myeloma and breast cancer (the basis of
so-called scintimammography). This latter technique has a high
specificity for cancer but uses significant radiation dose.

6. Uses, doses and timings

7. Parathyroid scan
• adult dose: 740 MBq (20 mCi) IV
• timing: early scan at 15 min; delayed scan at 2 hours
When the radiopharmaceutical is used for parathyroid imaging,
immediate and delayed imaging of the neck and mediastinum is
performed; parathyroid adenomas are best demonstrated on
delayed imaging (1-2 hours). A thyroid scan using Tc-99m
pertechnetate follows. Parathyroid adenomas do not demonstrate
uptake on thyroid scans, whereas normal thyroid tissue does.

8. • Immediately caudal to the left thyroid lobe, there is punctate uptake in the early phase (15 min registration) that remains
active during the late phase (2 hours). The localization of this uptake matches the lesion identified on MRI confirming
parathyroid adenoma.
•

10. • Well defined soft tissue mass lesion is abutting the inferior pole of the left thyroid lobe. It shows
punctate uptake in the early phase (15 min registration) that remains active during the late phase
(2 hours) characteristic of parathyroid adenoma.

11. Myocardial perfusion scintigraphy
• adult dose: 400 MBq rest phase, 1.1 GBq stress phase
• timing: image 45-60 minutes following injection allowing for
clearance of tracer from the lungs and liver
• indications: investigation of chest pain, suspicion of coronary
artery disease, pre-surgical workup including prior to renal
transplantation

12. • Tc-99m sestamibi is the most common agent for performing
myocardial perfusion scintigraphy. Sestamibi is a lipophilic
cation, which diffuses from the blood into the myocardial cells.
The tracer is held intracellularly proximate to the mitochondria.
60% of sestamibi is extracted first pass from the coronary
blood flow and there is minimal myocardial clearance.
• Myocardial perfusion scintigraphy is performed with
both rest and stress phases, looking for areas of reversible
ischemia (exercise stress) or impaired myocardial flow reserve
(pharmacologic stress). Rest imaging is usually performed first,
with a stress phase conducted on the same day with a higher
dose injection or a subsequent day.

13. • An exercise stress test requires the patient to exercise on a
treadmill (or other equipment in some centers) to achieve 85%
of their maximum predicted heart rate e.g. following the "Bruce
protocol"/"modified Bruce protocol". Tc-99m sestamibi is
injected at 85% predicted maximum heart rate and the patient
continues to exercise for an additional 60 seconds while the
tracer is extracted by the myocardium. The patient is imaged 30-
60 minutes later. Beta-blockers should be avoided in the days
prior to the exercise stress test as they may prevent the patient
from reaching higher heart rates.

14. • A pharmacological stress test involves the infusion of a coronary
vasodilator such as adenosine, persantin or regadenoson. Each
drug has a unique infusion protocol. Caffeine is contraindicated
for 24 hours prior to persantin and adenosine studies.
Pharmacological stress testing examines coronary flow reserve,
with diseased coronary vessels having impaired physiological
response to the infused medication. Other territories of the
heart with normal flow will have a relatively higher amount of Tc-
99m tracer than those supplied by the diseased vessel.