What are the sensitivity and specificity of Angio CT (Computed Tomography) and Pulmonary Scintigraphy in diagnosing acute Pulmonary Embolism (PE)?

Sensitivity and Specificity of Angio CT and Pulmonary Scintigraphy in Acute Pulmonary Embolism

Multidetector CT angiography demonstrates sensitivity of 83-100% and specificity of 89-98% for acute pulmonary embolism, while V/Q scintigraphy shows sensitivity of 96-97.4% and specificity of 90-95% for residual/chronic PE. 1

CT Angiography Performance

Single-Detector CT

• Sensitivity: 57-91%
• Specificity: 84-100%
• Limitations:
  • Lower sensitivity for subsegmental emboli (37-93%)
  • Higher sensitivity for segmental and lobar emboli (53-100%)
  • Suboptimal negative likelihood ratios (0.09-0.46) 1

Multidetector CT (Current Standard)

• Sensitivity: 83-100%
• Specificity: 89-98%
• Improved performance compared to single-detector CT
• Still has potential for false negatives (negative likelihood ratios 0.02-0.41) 1
• Central and lobar emboli detection has excellent sensitivity (≥95%) 2
• Subsegmental emboli detection has limited sensitivity (~75%) 2

Technical Considerations for CT Angiography

• Contrast administration: 140-180 ml of contrast medium with 270-300 mg iodine/ml at 4-5 ml/s
• Requires patient to hold breath for a few seconds
• Image interpretation uses both soft tissue and pulmonary parenchymal windows 2
• Breathing artifacts can create pseudo-hypoattenuating areas that mimic clots 2

Ventilation/Perfusion (V/Q) Scintigraphy Performance

Conventional V/Q Scanning

• Has largely been replaced by CT angiography for acute PE evaluation 1
• Limitations:
  • High proportion of nondiagnostic studies
  • Inability to rule out alternative diagnoses 1
• For residual/chronic PE:
  • Sensitivity: 96-97.4%
  • Specificity: 90-95% 1

V/Q SPECT (Single Photon Emission Computed Tomography)

• Improved diagnostic performance over conventional V/Q scanning
• Decreases number of nondiagnostic studies
• High negative predictive value
• Addition of low-dose CT (V/Q SPECT/CT) further improves diagnostic accuracy 1
• Head-to-head comparison with CT angiography shows comparable accuracy 1

Clinical Implications and Recommendations

1. For Acute PE Diagnosis:

   • CT angiography is the preferred first-line imaging test due to:
     • Rapid acquisition
     • High diagnostic accuracy
     • Ability to visualize alternative diagnoses 1
   • Consider V/Q scan when:
     • CT is contraindicated (renal insufficiency, contrast allergy)
     • Patient cannot hold breath adequately

2. For Chronic/Residual PE Evaluation:

   • V/Q scanning remains the initial diagnostic test of choice
   • CT findings can be subtle in chronic PE and may be overlooked 1

3. Special Considerations:

   • CT angiography may miss approximately 15% of cases, particularly small subsegmental emboli 1
   • The clinical significance of isolated subsegmental PE remains debated 1
   • Caution is needed when clinical probability and imaging results are discordant 1

Pitfalls to Avoid

1. For CT Angiography:

   • Poor contrast timing leading to suboptimal vessel opacification
   • Motion artifacts from breathing
   • Misinterpreting perivascular tissue as thromboembolic material 2

2. For V/Q Scanning:

   • Interpreting abnormalities in patients with underlying lung disease
   • Nondiagnostic studies requiring additional imaging

3. General Diagnostic Approach:

   • Overreliance on imaging without considering pre-test probability
   • Failure to integrate clinical assessment with imaging results 1

By understanding the strengths and limitations of both modalities, clinicians can select the most appropriate imaging test based on the clinical scenario, patient characteristics, and available resources.