Calculating Nuclear Cardiac Defect Size Using Summed Difference Score (SDS)
The Summed Difference Score (SDS) is calculated by subtracting the summed rest score (SRS) from the summed stress score (SSS), where each myocardial segment is scored on a 0-4 scale for perfusion defect severity across all segments. 1, 2
The Calculation Method
The SDS quantifies the reversibility of perfusion defects—essentially measuring the size of ischemic (stress-induced) myocardium:
Step 1: Score each myocardial segment on stress images using a 0-4 scale:
- 0 = normal uptake
- 1 = mildly reduced uptake
- 2 = moderately reduced uptake
- 3 = severely reduced uptake
- 4 = absent uptake
Step 2: Sum all segment scores from stress images = Summed Stress Score (SSS)
Step 3: Score the same segments on rest/redistribution images using the same 0-4 scale
Step 4: Sum all segment scores from rest images = Summed Rest Score (SRS)
Step 5: Calculate SDS = SSS - SRS
Segmentation Models
The calculation typically uses the 17-segment model as recommended by guidelines 1, though some studies reference 20-segment models 3. The 17-segment model is the preferred standard for reporting 1.
Clinical Interpretation of SDS Values
SDS specifically quantifies reversible ischemia (stress-induced perfusion defects that normalize at rest) 1, 2:
- SDS < 4: Normal or minimal reversible ischemia
- SDS 4-8: Mildly abnormal reversible defect
- SDS 9-13: Moderately abnormal reversible defect
- SDS > 13: Severely abnormal reversible defect 3
Guidelines emphasize that a reversible defect >10% of the left ventricle has prognostic information 1.
Critical Caveats
Software Variability
A major pitfall: Different software packages (Cedars QPS, Emory Cardiac Toolbox, 4D-MSPECT) can produce significantly different SDS values for the same patient 3. In one study, 48% of patients had disagreement in SDS classification between software packages, with 6 patients showing differences of more than one severity category 3. You must be aware of which software your institution uses and its specific reference values.
Reporting Requirements
According to European guidelines, SDS should be reported as part of the findings section when describing:
- Extent of defects: Quantified as percentage of LV or in summed scores 1
- Severity of defects: Quantified in summed stress/rest/difference scores 1
- Reversibility of defects: Specifically quantified using SDS 1
Relationship to Outcomes
The SDS has been validated for prognostic assessment. Studies demonstrate that changes in SDS correlate with clinical outcomes—for example, a decrease in SDS of -0.8 indicated improvement while an increase of +1.2 indicated worsening ischemia 2. The SDS provides independent prognostic value beyond visual assessment alone 4.