PET Scan Assessment of Myocardial Viability

(High-yield cardiology / imaging perspective)

1. Core Principle

PET assesses myocardial viability by comparing perfusion with metabolism.
Viable myocardium—especially hibernating myocardium—shows preserved glucose metabolism despite reduced blood flow.

Key concept:
Flow–metabolism mismatch = viable myocardium

2. Tracers Used

A. Perfusion Tracers

¹³N-Ammonia
⁸²Rb (Rubidium-82)
¹⁵O-Water

➡ Reflect regional myocardial blood flow

B. Metabolic Tracer

¹⁸F-FDG (Fluorodeoxyglucose)
➡ Reflects myocardial glucose uptake (cellular viability)

3. PET Patterns & Interpretation

Perfusion	FDG Uptake	Interpretation	Viability
↓	Preserved / ↑	Hibernating myocardium	✅ Viable
↓	↓	Scar / infarct	❌ Non-viable
Normal	Normal	Normal myocardium	✅
Normal	↓	Stunning / artifact / metabolic shift	Variable

🔑 Flow–Metabolism Mismatch

Low perfusion + high FDG uptake
Indicates viable myocardium that may recover after revascularization

🔑 Matched Defect

Low perfusion + low FDG uptake
Indicates irreversible myocardial scar

4. Pathophysiology Correlation

Hibernating Myocardium

Chronic ischemia
↓ Contractility
Shift from fatty acid → glucose metabolism
FDG uptake increases

Infarcted Myocardium

Myocyte loss
No glucose metabolism
FDG uptake absent

5. Patient Preparation (Crucial for Accuracy)

To enhance myocardial FDG uptake:

Fasting followed by:
- Oral glucose load or
- Insulin–glucose clamp
Target: Myocardial glucose utilization

⚠ Poor preparation → false-negative viability

6. Clinical Indications

PET viability testing is most useful in:

Ischemic cardiomyopathy
LVEF ≤35%
Patients being considered for:
- CABG
- High-risk PCI
When echo/SPECT results are equivocal

7. PET vs Other Viability Modalities

Modality	Viability Marker	Strength
PET	Glucose metabolism	Gold standard
Dobutamine Echo	Contractile reserve	Operator dependent
SPECT	Cell membrane integrity	Lower sensitivity
CMR (LGE)	Scar transmurality	Excellent spatial resolution

👉 PET has the highest sensitivity for detecting viable myocardium

8. Prognostic & Therapeutic Value

Presence of PET-viable myocardium:
- Predicts LV function recovery
- Associated with survival benefit after revascularization
Absence of viability:
- No benefit from revascularization
- Favor optimal medical therapy

9. Exam Pearls (NEET-SS / DM / DNB)

Gold standard for myocardial viability → FDG-PET
Hibernating myocardium → ↓ perfusion + ↑ FDG uptake
Matched defect → myocardial scar
PET assesses cellular viability, not just wall motion
FDG uptake requires insulin-mediated glucose transport

1. On FDG-PET, a myocardial segment shows severely reduced perfusion with intense FDG uptake. The dominant cellular mechanism responsible is:

A. Increased fatty-acid β-oxidation

B. Insulin-mediated GLUT-4 translocation

C. Mitochondrial uncoupling

D. Calcium-dependent apoptosis

Explanation: Hibernating myocardium shifts from fatty-acid to glucose metabolism with preserved insulin-dependent GLUT-4 activity.

2. Which PET finding most strongly predicts improvement in LVEF after CABG?

A. Matched perfusion–metabolism defect

B. Flow–metabolism mismatch >10% LV mass

C. Normal perfusion with reduced FDG uptake

D. Global FDG uptake reduction

Explanation: Extent of mismatch >10–15% LV predicts functional recovery and survival benefit.

3. FDG uptake is absent in a dysfunctional segment despite adequate glucose loading. Most likely explanation?

A. Transmural infarction

B. Stunning myocardium

C. Partial-thickness scar

D. Inadequate insulin effect

Explanation: Complete loss of FDG uptake with dysfunction indicates non-viable scar.

4. Which tracer pair defines the classical PET viability protocol?

A. Thallium-201 + FDG

B. N-13 ammonia + FDG

C. Tc-99m sestamibi + FDG

D. O-15 water + MIBG

Explanation: Perfusion (N-13 ammonia or Rb-82) + metabolism (FDG).

5. FDG-PET falsely underestimates viability most commonly in:

A. Anterior wall MI

B. Poor glycemic control / insulin resistance

C. Left bundle branch block

D. Right ventricular hypertrophy

Explanation: Inadequate insulin-mediated uptake leads to false-negative FDG studies.

6. PET demonstrates reduced perfusion, preserved FDG uptake, and absent contractile reserve on dobutamine echo. Best interpretation?

A. Non-viable scar

B. Hibernating myocardium

C. Stunning

D. Imaging artifact

Explanation: PET detects cellular viability even when contractile reserve is absent.

7. Which statement best differentiates PET from LGE-CMR?

A. PET assesses cellular metabolism

B. PET defines scar transmurality

C. PET has superior spatial resolution

D. PET identifies microvascular obstruction

Explanation: PET = metabolism; CMR = structural scar.

8. Flow–metabolism mismatch exists because hibernating myocardium primarily:

A. Increases oxygen extraction

B. Switches substrate utilization

C. Develops fibrosis

D. Loses mitochondrial density

Explanation: Chronic ischemia → glucose-predominant metabolism.

9. Which PET pattern predicts no benefit from revascularization?

A. Matched perfusion–FDG defect

B. Reverse mismatch

C. Global hypoperfusion with preserved FDG

D. Partial mismatch

Explanation: Matched defects indicate irreversible scar.

10. FDG uptake in viable myocardium primarily reflects:

A. Wall stress

B. Integrity of sarcolemmal glucose transport

C. Myocyte hypertrophy

D. Coronary flow reserve

Explanation: FDG uptake requires intact cell membranes and GLUT transporters.

11. A PET scan shows globally reduced perfusion with preserved homogeneous FDG uptake in a patient with severe triple-vessel disease. This pattern most likely represents:

A. Extensive transmural infarction

B. Balanced ischemia with viable myocardium

C. PET attenuation artifact

D. Reverse mismatch pattern

Explanation: Balanced ischemia causes global perfusion reduction, but preserved FDG uptake confirms widespread viability.

12. Which PET finding best differentiates hibernating myocardium from stunned myocardium?

A. Reduced perfusion with preserved FDG uptake

B. Preserved perfusion with reduced FDG uptake

C. Matched perfusion–metabolism defect

D. Increased wall motion on dobutamine

Explanation: Stunning has normal perfusion; hibernation shows flow–metabolism mismatch.

13. Which condition most commonly produces a “reverse mismatch” (normal perfusion with reduced FDG uptake)?

A. Chronic hibernating myocardium

B. Diabetes with impaired insulin-mediated glucose uptake

C. Transmural myocardial infarction

D. Acute coronary occlusion

Explanation: Insulin resistance reduces FDG uptake despite adequate perfusion → reverse mismatch.

14. Which PET-derived parameter correlates best with long-term survival benefit after revascularization?

A. Absolute myocardial blood flow

B. Extent of flow–metabolism mismatch

C. SUVmax of FDG uptake

D. Global LV FDG uptake

Explanation: Percentage of mismatched myocardium predicts recovery and survival, not SUV alone.

15. FDG uptake on PET requires phosphorylation by hexokinase. This step primarily indicates:

A. Coronary flow reserve

B. Cellular metabolic integrity

C. Myocardial fibrosis burden

D. Mitochondrial oxidative capacity

Explanation: FDG trapping reflects intact intracellular metabolism—core definition of viability.

16. In ischemic cardiomyopathy, PET is superior to dobutamine echo because PET:

A. Has better spatial resolution

B. Detects viability without contractile reserve

C. Identifies subendocardial scar thickness

D. Directly measures wall stress

Explanation: PET identifies living myocytes even when contractile reserve is absent.

17. Which myocardial layer most commonly shows preserved FDG uptake in chronic ischemia?

A. Subepicardium

B. Subendocardium

C. Mid-myocardium only

D. Papillary muscles exclusively

Explanation: Subepicardial layers remain viable longer due to better collateral supply.

18. A dysfunctional myocardial segment shows mild FDG uptake with severely reduced perfusion. Best interpretation?

A. Transmural infarction

B. Partially viable myocardium

C. Imaging artifact

D. Acute stunning

Explanation: Reduced but present FDG uptake suggests partial thickness viability.

19. Which clinical scenario derives the LEAST benefit from PET viability assessment?

A. LVEF 25% with multivessel CAD

B. Acute STEMI within 48 hours

C. Chronic ischemic cardiomyopathy

D. High-risk CABG candidate

Explanation: PET viability is not indicated in acute MI settings.

20. PET evidence of non-viable myocardium should most strongly favor:

A. Optimal medical therapy

B. Surgical revascularization

C. High-dose inotropes

D. Preventive ICD implantation alone

Explanation: Revascularization does not improve outcomes when myocardium is non-viable.

21. Quantitative PET shows severely reduced resting myocardial blood flow with preserved FDG uptake. Coronary flow reserve is <1.5. This constellation best supports:

A. Stunning myocardium

B. Hibernating myocardium

C. Transmural scar

D. PET attenuation artifact

Explanation: Low flow + preserved metabolism + reduced CFR defines chronic hibernation.

22. Which factor most strongly limits the spatial resolution of PET viability imaging?

A. Positron range

B. Partial volume effect alone

C. Cardiac motion

D. Photon scatter only

Explanation: Positron travel before annihilation limits intrinsic PET resolution.

23. A segment shows moderate FDG uptake but no improvement in LVEF after complete revascularization. Most plausible explanation?

A. Technical PET error

B. Irreversible myocyte dedifferentiation

C. Acute stunning

D. Inadequate revascularization

Explanation: Some viable myocytes fail to regain contractility due to chronic structural remodeling.

24. Which PET finding most closely correlates with subendocardial viability despite a transmural perfusion defect?

A. Absent FDG uptake

B. Preserved FDG uptake limited to outer myocardial layers

C. Increased SUVmax globally

D. Normal coronary flow reserve

Explanation: Epicardial FDG uptake suggests partial-thickness viability.

25. FDG uptake is reduced globally with preserved perfusion in a fasting patient. Best corrective strategy?

A. Repeat scan without changes

B. Insulin–glucose clamp protocol

C. Use fatty-acid tracers

D. Switch to SPECT

Explanation: Insulin is essential to drive myocardial FDG uptake.

26. Which statement regarding PET viability and ICD decision-making is most accurate?

A. Viability does not replace LVEF criteria for ICD

B. Absence of viability mandates ICD

C. FDG uptake predicts sudden death risk

D. PET supersedes guideline ICD indications

Explanation: PET guides revascularization benefit, not primary ICD indication.

27. Which PET pattern most closely predicts reverse remodeling after CABG?

A. Matched defect <10%

B. Large flow–metabolism mismatch

C. Reverse mismatch

D. Normal perfusion only

Explanation: Extent of mismatch predicts LV reverse remodeling.

28. Which clinical condition most often causes underestimation of FDG uptake despite viable myocardium?

A. Diabetes mellitus

B. Hypertension

C. Hyperthyroidism

D. Anemia

Explanation: Insulin resistance impairs myocardial glucose uptake.

29. Which statement best explains why PET detects viability when dobutamine echo does not?

A. PET has higher temporal resolution

B. Metabolic integrity precedes contractile recovery

C. PET measures wall stress

D. PET detects fibrosis

Explanation: Viable myocytes may lack immediate contractile reserve.

30. Which PET-derived metric is most operator-independent?

A. Absolute myocardial blood flow

B. Visual FDG uptake scoring

C. Regional wall motion

D. Contractile reserve

Explanation: Quantitative flow measurement is reproducible and operator-independent.

31. Which myocardial region is most vulnerable to loss of FDG uptake in chronic ischemia?

A. Subendocardium

B. Subepicardium

C. Mid-wall exclusively

D. Papillary muscles

Explanation: Subendocardium has highest ischemic vulnerability.

32. Which statement regarding FDG SUV values is correct in viability assessment?

A. Absolute SUV cutoffs define viability

B. Relative uptake compared to normal myocardium is key

C. SUV correlates directly with CFR

D. Higher SUV always predicts recovery

Explanation: Relative FDG uptake matters more than absolute SUV.

33. PET shows preserved FDG uptake but extensive late gadolinium enhancement on CMR. Best interpretation?

A. PET false positive

B. Residual viable myocytes within scar matrix

C. Imaging mismatch error

D. Acute inflammation

Explanation: PET detects metabolic activity even within fibrotic tissue.

34. Which PET scenario predicts delayed, rather than immediate, functional recovery after revascularization?

A. Severe mismatch with long-standing dysfunction

B. Stunning myocardium

C. Acute ischemia

D. Normal perfusion

Explanation: Chronic hibernation recovers slowly after revascularization.

35. Which technical factor most affects FDG myocardial uptake variability?

A. Scanner crystal type

B. Patient metabolic preparation

C. Reconstruction algorithm

D. Attenuation correction method

Explanation: Metabolic state dominates FDG distribution.

36. Which PET finding argues strongly against benefit from surgical revascularization?

A. Large matched perfusion–metabolism defect

B. Partial mismatch

C. Preserved FDG uptake

D. Reduced CFR alone

Explanation: Matched defects indicate irreversible scar.

37. PET is preferred over SPECT for viability assessment primarily because PET:

A. Directly measures myocardial metabolism

B. Is cheaper

C. Has better availability

D. Detects wall motion

Explanation: PET assesses cellular viability, not just perfusion.

38. Which PET pattern is most likely to be misinterpreted without attenuation correction?

A. Inferior wall perfusion defect

B. Anterior wall mismatch

C. Global FDG uptake

D. Reverse mismatch

Explanation: Diaphragmatic attenuation commonly affects inferior wall.

39. Which statement best summarizes the prognostic value of PET viability?

A. Identifies patients who benefit from revascularization

B. Predicts arrhythmic death

C. Replaces angiography

D. Determines stent choice

Explanation: PET stratifies revascularization benefit, not rhythm risk.

40. The strongest indication for PET myocardial viability assessment is:

A. Severe LV dysfunction with multivessel CAD

B. Acute NSTEMI

C. Stable angina with normal EF

D. Post-PCI chest pain

Explanation: PET is most valuable in ischemic cardiomyopathy with severe LV dysfunction.

PET Scan Assessment of Myocardial Viability