What does an atypical attenuation artifact on a stress (exercise) test mean?

Understanding Atypical Attenuation Artifacts on Stress Tests

Atypical attenuation artifacts on stress tests represent non-pathological findings caused by soft tissue interference with imaging that can mimic perfusion defects but do not represent true myocardial ischemia or infarction. 1

What Are Attenuation Artifacts?

Attenuation artifacts occur when photons emitted from the myocardium are absorbed by overlying tissues before reaching the camera detector during SPECT (Single-Photon Emission Computed Tomography) imaging. These artifacts:

• Create apparent perfusion defects that may be mistaken for true myocardial ischemia
• Commonly appear as fixed defects (present in both stress and rest images)
• Occur in predictable patterns based on body habitus and positioning
• Reduce test specificity and may lead to unnecessary additional testing

Common Patterns of Attenuation Artifacts

1. Gender-specific patterns:

   • Women: Anterior wall and anterolateral defects due to breast tissue attenuation
   • Men: Inferior wall defects due to diaphragmatic attenuation

2. Atypical patterns:

   • Exaggerated diaphragmatic attenuation in women
   • Anterior wall defects in men with significant pectoral musculature or gynecomastia
   • Variable patterns based on patient positioning (especially in upright vs. supine imaging)
   • Artifacts that don't follow familiar patterns due to SPECT system geometry 1

How to Identify Attenical Attenuation Artifacts

Several techniques help differentiate attenuation artifacts from true perfusion defects:

1. Attenuation Correction Methods

• CT-based attenuation correction: Creates patient-specific anatomical attenuation maps
• Radionuclide transmission scan: Alternative method for attenuation mapping
• Two-position imaging: Acquiring images with the body in different positions (supine/prone or upright/supine)
• Combined corrections: Best results when attenuation correction is combined with scatter and collimator resolution correction 1

2. Gated SPECT Analysis

• Wall motion and thickening assessment: True perfusion defects typically show abnormal wall motion and thickening, while attenuation artifacts show normal function
• High diagnostic value: Studies show 96-97% of fixed defects with clinical MI have abnormal function, while 64-77% of fixed defects without clinical MI have normal function 2, 3
• Improves specificity: Reclassifying fixed defects with normal function as artifacts reduces unexplained fixed defects from 14-29% to 3-10% 2, 3

3. Specific Markers for Attenuation

• Apex/anterior wall ratio: A ratio >1 (higher counts at apex than anterior wall) is not physiological and suggests breast attenuation artifact in women 4
• Location patterns: 91% of fixed defects with normal function occur in women with anterior defects (48%) or men with inferior defects (43%) 3

Clinical Implications and Management

1. Diagnostic accuracy:

   • Attenuation correction significantly increases definitive interpretations (from 37% to 84%)
   • Reduces need for rest imaging (from 77% to 43%) 5

2. Radiation exposure:

   • Proper identification of attenuation artifacts can eliminate unnecessary rest imaging
   • Prone imaging can reduce radiation exposure by obviating rest studies when defects disappear in prone position 6

3. Clinical decision-making:

   • Correctly identifying attenuation artifacts prevents unnecessary coronary angiography
   • Prone imaging correctly identified attenuation artifacts in 74% of inferior wall and 63% of anterior wall defects 6

Best Practices for Managing Attenuation Artifacts

1. Use attenuation correction when available:

   • CT-based attenuation correction is the ideal approach 1, 7
   • Consider SPECT/CT when upgrading nuclear instrumentation 1

2. When attenuation correction hardware is unavailable:

   • Implement two-position imaging (supine/prone or upright/supine) 1, 7
   • Use gated SPECT to assess wall motion and thickening 2, 3

3. Interpretation strategies:

   • Compare attenuation-corrected and non-corrected images
   • Integrate functional data from gated SPECT
   • Consider typical patterns of attenuation based on gender and body habitus
   • Be aware that "recalibration" of the interpretive eye is required with high-sensitivity SPECT systems 1

Pitfalls and Caveats

• Attenuation patterns may not follow familiar gender-specific patterns
• Position of the patient during imaging affects attenuation patterns
• Interpreters need experience with specific SPECT systems and reconstruction algorithms
• Attenuation correction performs best when combined with other corrections (scatter, resolution recovery)
• Inconclusive stress imaging due to attenuation artifacts may require additional testing 1

By correctly identifying atypical attenuation artifacts, clinicians can avoid unnecessary additional testing, reduce radiation exposure, and improve diagnostic accuracy in cardiac stress testing.