Cardiac PET

Rubidium-82 is produced from the decay of Strontium-82 through electron capture in a generator. It is used to access the blood vessels supplying the heart. Strontium-82 has a half-life of 25.5 days while Rubidium-82 has a half-life of 76 seconds. Heart muscles can take up Rubidium-82 efficiently through sodium–potassium pump. Compared with Technetium-99m, Rubidium-82 has higher uptake by the heart muscles. However, Rubidium-82 has lower uptake by heart muscles when compared to N-13 ammonia. But the positron energy emitted by Rubidium-82 is higher than N-13 ammonia and Fluorodeoxyglucose (18F). On the other hand, the positron range (the distance travelled by a positron from its production site until its annihilation with an electron) is longer when compared to other radiopharmaceuticals, causing reduced image resolution.^[2]

Myocardium has higher uptake for N-13 ammonia when compared to Rubidium-82, thus useful for myocardial perfusion imaging. However, its half-life is only 9.96 minutes. Therefore, on-site facilities such as cyclotron and radiochemistry synthesis facilities should be available. There may be patchy uptake if the subject has defects in lateral ventricular wall. N-13 ammonia may occasionally be degraded by liver, thus causing reduced visibility of the inferior wall of the heart. N-13 ammonia uptake by the lungs is minimal.^[2]

Oxygen-15 (O-15) water is considered the gold standard for non-invasive calculation of myocardial blood flow (MBF), and has high accuracy in detecting significant flow-limiting stenosis in coronary artery disease. O-15 water is metabolically inert, freely diffusable and has an extraction fraction from blood to tissue of close to 100%. Since O-15 water has no uptake in myocytes, static imaging is of no use and dynamic scanning with kinetic modeling is a requirement. Calculation of MBF can be based on both influx and efflux of O-15 water. This allows for calculation of the perfusable tissue fraction (PTF) which can be used to automatically correct for partial volume effects, otherwise commoly affecting the quantification. The half-life of O-15 is 122s which necessitates a nearby cyclotron and has previously limited widespread use of the technique. New advances with higher availability of small dedicated cyclotrons and development of bed-side production/injection systems has, however, lead to an increase of O-15 water utilization. While the short half-life has its drawbacks, it provides a low radiation dose and (as for Rubidium-82) allows for rest and stress imaging to be performed in the same session. A routine protocol can be performed in approximately 30 min. ^[3]

The most newely developed tracer for clincial use in myocardial perfusion imaging is Flurine-18 (F-18)-flurpiridaz. The tracer was approved by the U.S. Food and Drug Administration (FDA) in 2024. ^[4] Flurpiridaz is derived from the pesticide pydidaben and binds to the mithocondrial complex 1 in myocytes. F-18-flurpiridaz has high extraction fraction, low positron range, and considerably longer half-life (110 min) compared to the other PET-tracers used in myocardial perfusion imaging. The long half-life allows for shipment from production centers to imaging centers without a cyclotron or generator, and can thus potentially allow more widespread use of cardiac PET. ^[5]

• Patients with many risk factors e.g.
  high cholesterol, smoking habit, diabetes, obesity
  , high stress occupation, family history of heart attack
• Patients who are unable to exercise
• Patients suspected of
  heart attack
• Patients newly diagnosed with heart failure
• Patients with abnormal
  ECG or treadmill

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