Converting SDS to Percentage of Myocardial Ischemia
To convert a Summed Difference Score (SDS) to percentage of myocardial ischemia, divide the SDS by the maximum possible score (68 for a 17-segment model) and multiply by 100.
The Calculation Method
The conversion is straightforward using the standardized 17-segment left ventricular model 1:
- Formula: % Myocardial Ischemia = (SDS ÷ 68) × 100
This calculation is based on the 17-segment model where each segment can be scored from 0-4 (0=normal, 1=mild reduction, 2=moderate reduction, 3=severe reduction, 4=absent uptake) 2. The maximum possible SDS is 68 (17 segments × 4 points maximum difference per segment).
Understanding the Scoring System
The SDS represents the difference between stress and rest perfusion scores:
- SDS = Summed Stress Score (SSS) - Summed Rest Score (SRS)
- Each of the 17 myocardial segments is scored individually
- The difference scores are then summed across all segments 2
Clinical Interpretation Thresholds
While the percentage calculation provides a continuous measure, established clinical thresholds for SDS are 2, 3:
- SDS < 2: No significant ischemia (< 3% of myocardium)
- SDS 2-4: Mild ischemia (approximately 3-6% of myocardium)
- SDS 5-7: Moderate ischemia (approximately 7-10% of myocardium)
- SDS > 7: Severe ischemia (> 10% of myocardium)
A reversible defect involving ≥10% of the left ventricle (SDS ≥ 7) carries significant prognostic information 2.
Important Clinical Caveats
The relationship between SDS and actual percentage of ischemic myocardium is not perfectly linear because:
- Segment size varies anatomically (basal segments are larger than apical segments)
- Scoring is semi-quantitative and observer-dependent
- The 17-segment model assumes equal weighting of all segments, which doesn't reflect true myocardial mass distribution 1
Research demonstrates that even minimal ischemia (SDS 1-4, representing approximately 1-6% of myocardium) has prognostic significance, particularly in high-risk populations 4. However, for diagnostic purposes in elderly patients, an SDS threshold of ≥5 (approximately 7% of myocardium) provides better diagnostic accuracy for obstructive coronary disease than the traditional threshold of ≥2 3.
The conversion should always be reported alongside the imaging modality and quantification method used, as these affect the precision of the measurement 1.