PET MPI and Myocardial Blood Flow
Why PET?¶
PET MPI combines relative perfusion imaging with quantification of absolute myocardial blood flow.
Compared with SPECT, PET generally provides:
- Higher spatial and contrast resolution
- Routine attenuation correction
- Shorter acquisitions
- Lower radiation exposure
- Improved imaging in obesity and small hearts
- Quantification of myocardial blood flow and flow reserve
When available, ASNC now favors PET with myocardial blood-flow quantification for patients who otherwise meet criteria for MPI.
Perfusion Tracers¶
| Tracer | Half-life | Practical feature |
|---|---|---|
| Rubidium-82 | 76 seconds | Generator produced; rapid rest–stress imaging |
| N-13 ammonia | 10 minutes | Excellent images; requires nearby cyclotron |
| F-18 flurpiridaz | 110 minutes | Exercise-capable and distributable PET tracer |
Vasodilator stress is used for most PET MPI studies.
Interpretation¶
Interpret PET in layers:
- Raw data and PET/CT registration
- Relative stress and rest perfusion
- LV wall motion, volumes, and LVEF
- Rest and stress myocardial blood flow
- Myocardial flow reserve
- Coronary calcium and other CT findings
Myocardial Blood Flow¶
PET expresses flow in mL/min/g.
MFR integrates epicardial stenosis, diffuse atherosclerosis, and microvascular function.
| MFR | General interpretation |
|---|---|
| >2.0 | Usually preserved |
| 1.5–2.0 | Abnormal or indeterminate |
| <1.5 | Severely reduced; high risk |
Thresholds vary by tracer, software, hemodynamics, and laboratory protocol.
Integrating Perfusion and Flow¶
| Relative perfusion | MFR | Interpretation |
|---|---|---|
| Normal | Preserved | Low-risk study |
| Regional defect | Regionally reduced | Focal epicardial CAD |
| Normal | Globally reduced | Diffuse CAD, microvascular dysfunction, or inadequate stress |
| Abnormal | Globally reduced | Extensive or multivessel CAD; high risk |
Do not call microvascular disease too quickly
Reduced MFR with normal perfusion may reflect diffuse CAD, high resting flow, inadequate vasodilator response, caffeine exposure, technical error, or coronary microvascular dysfunction.
High-Risk Findings¶
- Large or multivessel perfusion defects
- Globally reduced stress MBF or MFR
- Stress-induced LV dysfunction
- Transient ischemic dilation
- Right ventricular uptake or dilation
- Extensive coronary calcium
- Ischemic ECG changes or hypotension
Common Pitfalls¶
- PET/CT attenuation-map misregistration
- Patient motion
- Caffeine or inadequate vasodilator response
- High resting MBF lowering the calculated MFR
- Infiltrated tracer injection
- Incorrect LV contours
- Treating a single numeric threshold as diagnostic without reviewing images
Other Cardiac PET Applications¶
- FDG PET for myocardial viability
- FDG PET for cardiac sarcoidosis
- FDG PET for cardiovascular infection
- Metabolic and inflammatory imaging
These require different patient preparation and should not be interpreted as routine perfusion studies.
Board Pearls¶
- PET measures absolute flow; SPECT usually does not.
- Normal relative perfusion does not guarantee normal flow.
- Reduced global MFR may reveal balanced multivessel disease.
- Always verify PET/CT registration before interpreting a defect.
- High resting flow can lower MFR despite adequate stress flow.
- PET flow results must be interpreted with perfusion, function, stress adequacy, and clinical context.