Clinical value and interpretation

Clinical value of myocardial perfusion scintigraphy (MPS)

  • Myocardial perfusion scanning (MPS) is a commonly used test to assess myocardial ischaemia and viability

  • It is more commonly used than the alternatives: stress echocardiography (DSE) and cardiac MRI (CMR)

  • It is superior to the conventional ETT

  • Stress testing in MPS can be performed either physically (i.e. treadmill/bicycle) or pharmacologically (Adenosine, Dipyridamole or Dobutamine)

MPS can be a useful stress imaging technique in patients who:

  • cannot be exercised physically (i.e. arthritis, lung disease, age)

  • have ECG changes that make ETT interpretation difficult (i.e. bundle branch block)

  • have significant hypertension

  • are in atrial fibrillation often abruptly increasing heart rate, ST-changes due to Digoxin therapy)

  • are in renal, liver or heart failure (there is no contraindication)

  • E.g., a patient with a pacemaker in situ

    • in ETT, ECG not assessable due to broad QRS-complex following right ventricular pacing

    • in stress-echocardiography stress-induced hypokinesia is difficult to assess because of altered contraction due to right ventricular pacing

    • CMR not possible with pacemaker in situ

    • MPS can provide the answer although care in taken when interpreting the pacing related LBBB

Image interpretation

All images should be interpreted from the workstation viewscreen with careful inspection of the raw data to assess quality before reviewing the reconstructed slices.

Planar acquisition has mostly been taken over by SPECT, which allows left ventricular myocardial perfusion to be viewed as sets of orthogonal slices.

SPECT provides information about relative myocardial perfusion.

A 9, 17 or 20 segment grid is superimposed on a polar plot (reconstructed from short axis slices summarizing the SPECT acquisition data).

Interpretation of regional myocardial perfusion on stress and rest images

  • Severity: mild / moderate / severe

  • Extent: small / medium / large

  • Reversibility: fixed / partial reversibility / complete reversibility

    • Fixed defects (minimal counts at stress and rest) suggest scar

    • Reversible defects show improvement in counts at rest, suggest inducible ischaemia and therefore significant coronary stenosis

  • For quantitative analysis data can be compared to a database with a low pre-test-probability of coronary disease

Semi-quantitative scoring

Score Category Count Density
0 Normal Perfusion 70-100%
1 Mild Reduction 50-70%
2 Moderate Reduction 30-50%
3 Severe Reduction 10-30%
4 Absent Counts 0-10%

Semi-quantitation of defect extent, expressed as proportion of the overall left ventricular myocardium

Extent % of Myocardium Segments (of 17)
Small 5-10% 1-2
Medium 10-20% 2-3
Large 15-20% 2-3

Indirect markers of severe CAD

  • Exercise-induced increase in Tl-201 uptake in the lungs due to chronically increased left atrial pressure following left ventricular dysfunction

  • Transient ischaemic dilatation (TID), i.e. larger left ventricle on stress compared to rest (possible explanations include stunning, following vasodilator stress without true ischaemia, diffuse subendocardial ischaemia with false appearance of chamber dilatation)

  • In the presence of perfusion defects TID predicts severe CAD and a high risk of cardiac events

  • Reversible LV impairment after stress with a decrease in EF of more than 5 % may indicate severe CAD

Artefacts occur in any technique, must be recognised and interpreted in context

  • patient related artefacts

    • movement during image acquisition

    • anterior attenuation due to breast tissue (mostly women)

    • inferior attenuation due to elevated diaphragm in obese patients in horizontal position (mostly men)

    • attenuation by non-anatomical structures (breast implants, pacemaker/ICD)

    • cardiac variants (short septum, apical thinning)

  • ECG-related artefacts

    • significant variation in RR-interval (frequent ectopy, atrial fibrillation)

    • LBBB; septal relaxation becomes increasingly dyssynchronous as the heart rate increases which in turn reduces septal perfusion relative to other walls

    • Right ventricular pacing; see LBBB

      • Vasodilator stress with Adenosine or Dipyridamole can solve this problem as it usually does not impact on heart rate as much

  • Artefacts in DCM

    • often present with LBBB

    • due to LV dilation there is increased inferior attenuation

    • patchy myocardial fibrosis with fixed defects

Conditions sometimes creating problems with interpretation

  • LV hypertrophy, HCM, congenital heart disease

  • Balanced multi-vessel disease (seen in diabetics, elderly, previously documented severe three-vessel-disease)

    • MPS assesses relative myocardial perfusion, count densitiy is colour-coded and areas of low count density (abnormal) are compared to areas with high density (normal)

    • In significant stenoses of all coronary arteries, valid determination of regional ischaemia becomes difficult

    • Potentially significant stenoses might therefore be underestimated

    • TID is a prognostic discriminator and sometimes can be the only sign of balanced multi-vessel ischaemia

Diagnosis & prognosis of IHD

Assessment in suspected CAD

  • Perfusion defects can be attributed to specific myocardial territories

  • The sensitivities to detect single and multi-vessel disease are high

  • In patients with low risk clinical features the absence of inducible ischaemia predicts a low risk - <1% annual cardiac event rate; by comparison ETT is a poor discriminator

  • Higher risk patients unable to exercise undergoing pharmacological stress still have a low cardiac event rate if the MPS is normal

Assessment of acute chest pain

  • Perfusion defects are detectable until several hours following the resolution of CP

  • Imaging can be delayed without redistribution (using Tc-99m tracers)

  • Patients with normal resting MPS can be safely discharged (negative predictive value for ACS >99%)

  • Resting SPECT has similar sensitivity to serial Troponin testing, but is superior to initial Troponin alone

  • Patients with abnormal resting MPS need further invasive investigation

  • Patients with normal MPS have a rate < 1% of developing MI within the next 30 days

  • Patients with an abnormal MPS have a rate of 10% of developing MI within 30 days

Assessment in known CAD

  • MPS is more precise in localising a significant stenosis in coronary arteries than the pattern of ST-depression in ETT

  • Perfusion defects can help make a decision given any number of angiographic stenoses to target for revascularization

  • A normal MPS in patients with known CAD is associated with a low risk for adverse cardiac events

  • MPS is of prognostic significance in terms of revascularization if inducible ischaemia of more than 10% of the left ventricular myocardium is present

  • Inducible ischaemia of less than 10% of the left ventricular myocardium should be treated medically, as this management is associated with a lower cardiac event rate

Prognostic value in patients with chest pain

  • MPS delivers additional information towards risk stratification in patients with known or suspected CAD

  • The major cardiac event rate in patients with chest pain and normal MPS is less than 1% per year (comparable to asymptomatic population)

  • The major cardiac event rate in patients with chest pain and abnormal MPS is about 7.4% per year

Special groups (women, diabetes, elderly)

MPS in patients with diabetes

  • MPS is a good alternative to ETT in diabetic patients who are frequently impaired in their exercise capacity due to raised BMI, polyneuropathy and claudication

  • The major adverse cardiac event rate in diabetic patients with normal MPS is <3%, and 27% with abnormal MPS

  • MPS delivers additional information for risk stratification over clinical data

  • Any given perfusion deficit in a diabetic is associated with a higher risk compared to the non-diabetic population

  • Any given perfusion deficit in a type I diabetic is associated with a higher risk compared to a type II diabetic

MPS in elderly patients

  • Pharmacological MPS has been shown to be safe and feasible in elderly patients (>65 years) whose exercise capacity is often reduced

  • A combination of pharmacological & dynamic stress often helps to improve image quality and overall diagnostic accuracy

MPS in women

  • High proportion of false positive ETT

    • often atypical symptoms

    • lower pre-test-probability of IHD

    • lower exercise capacity

    • less specific ST-changes

  • Prognostic value and cost effectiveness of MPS in women as good as in men

  • Gated SPECT in particular very useful at distinguishing artefacts from genuine defects

Heart failure/hibernation

Definition:
“Assessment of myocardial viability is defined as relatively preserved perfusion ≥50% of normal tracer uptake, or reversible inducible ischaemia in a dysfunctional segment”

Viability assessment in patients with severely impaired LV systolic function and CAD helps planning for revascularisation.

Role of MPS in the interpretation of LV dysfunction:

  • Intact micro-circulation and myocardial cells take up tracer

  • Allows for exclusion of significant CAD as underlying cause of LV dysfunction if no perfusion defects

  • Assessment of hibernating myocardium (dysfunctional at rest but with tracer uptake and relatively reduced tracer uptake during stress) and the likely value of revascularization in patients with known CAD

  • Functional improvement occurred in 80% of those segments after revascularization compared to 30% of segments with viability and dysfunction but without reversibility

  • Myocardial segments whose count density is at least 50% of maximal counts are considered to contain sufficient viable myocardium to justify revascularization

  • Segments with count density of 30 – 50% are likely to contain a mixture of viable myocardium and scar but functional recovery is unlikely

  • A left ventricle, in which 25% or more of the myocardium remains viable, is likely to improve with revascularization .

Pre-operative risk assessment

  • Aging population are undergoing even more complex surgery

  • Non-cardiac surgery: absence of inducible ischaemia predicts low risk of postoperative cardiac death or non-fatal myocardial infarction

  • NPV of a normal study is very good, however the PPV of an abnormal study is <10%

  • Current guidelines essentially only recommend MPS for high risk aortic and vascular surgery patients

MPS in patients before/after revascularization

  • High sensitivity and specificity to detect significant coronary stenosis after revascularization

  • Identification of hibernating myocardium which is dysfunctional but retains the potential to recover following revascularization

  • Assessment of functional significance of anatomical stenosis and need for revascularisation also after angiography

  • Perfusion defects due to microvascular dysfunction after successful PCI are common but do not represent inducible ischaemia

Therefore assessment for ischaemia early after PCI (2 months) should not be done via MPS