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Myocardial perfusion imaging (MPI)
Perfusion imaging compares blood flow to the myocardium at stress and at rest: a defect present only at stress (reversible) is ischaemia, a fixed defect is scar — or an attenuation artefact.
Why it matters in the exam
The exam asks which stress to choose, how to prepare, and how to interpret a defect. Exercise is preferred when the patient can reach at least 85% of the predicted maximum heart rate. Pharmacological stress with a vasodilator (adenosine, regadenoson, dipyridamole) suits those who cannot exercise, and it is also the choice with LBBB or a permanent pacemaker, because exercise causes a false septal defect there. Dobutamine suits active bronchospasm or advanced heart block without a pacemaker. Before a vasodilator, caffeine is forbidden for 12–24 hours, because it blocks adenosine receptors; side effects are reversed with aminophylline.
Tracers: Tl-201 is a potassium analogue taken up through the sodium-potassium pump, with a first-pass extraction of about 85% and "redistribution" over hours — an advantage for viability, a drawback in its high radiation dose. Tc-99m sestamibi and tetrofosmin bind mitochondria with almost no redistribution, so two separate injections are needed for stress and rest; activity in the liver and bowel may interfere with the inferior wall. PET with Rb-82 or N-13 ammonia allows absolute measurement of blood flow and flow reserve, and so detects "balanced" three-vessel disease in which SPECT looks normal.
High-risk findings: extensive ischaemia (roughly more than 10% of the myocardium), transient ischaemic dilation of the left ventricle at stress (TID), post-stress dysfunction, and increased lung Tl-201 uptake. Artefacts: breast attenuation (anterior wall) and diaphragmatic attenuation (inferior wall, mainly in men); a fixed defect with normal wall motion on the gated study suggests artefact rather than scar. The decision on catheterisation rests on the extent of ischaemia, symptoms and medical therapy.
Key points
- Reversible defect — ischaemia; fixed — scar or artefact.
- LBBB or pacemaker — vasodilator, not exercise.
- No caffeine for 12–24 hours before a vasodilator.
- TID and extensive ischaemia — high risk.
- PET measures absolute flow and detects balanced disease.
Summary table
| Situation | Preferred stress | Reason |
|---|---|---|
| Can exercise, normal ECG | Exercise | Functional and prognostic information |
| LBBB or permanent pacemaker | Vasodilator | Exercise causes a false septal defect |
| Cannot exercise | Vasodilator | No caffeine for 12–24 hours |
| Active asthma or advanced block without pacemaker | Dobutamine | Vasodilator forbidden |
| Suspected balanced three-vessel disease | PET with flow measurement | SPECT may look normal |
A free exam question
איזה שיטת מאמץ מועדפת בנבדקים עם LBBB או קוצב קבוע ? (the question as examined, in Hebrew)
- א. מאמץ דינאמי בפרוטוקול MODIFIED BRUCE
- ב. מאמץ דינאמי בפרוטוקול BRUCE
- ג. מאמץ תרופתי עם דיפירידמול
- ד. מאמץ תרופתי עם דובוטמין
In the full version for subscribers
- A simple algorithm
- Memory aid
- Exam traps
- 5 review cards
- Enough for IMLE
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Sources
- ASNC Imaging Guidelines for SPECT Nuclear Cardiology Procedures: Stress, Protocols and Tracers, 2018 — Choice of stress, tracers and high-risk findings
- ESC Guidelines for the Management of Chronic Coronary Syndromes, 2024 — Functional imaging for ischaemia
- EANM Procedural Guidelines for Radionuclide Myocardial Perfusion Imaging with SPECT and SPECT/CT, 2015 — Patient preparation and artefacts
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.