Pulmonary embolism ventilation/perfusion scan

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Editor(s)-In-Chief: The APEX Trial Investigators, C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief:

Overview

A ventilation/perfusion scan (otherwise known as V/Q scan or lung scintigraphy) is a study which shows whether an area of the lung is being ventilated with oxygen and perfused with blood. In the setting of a PE, perfusion can be obstructed due to the formation of a clot. The V/Q scan is less commonly used due to the more widespread availability of CT technology, however it may be useful in patients who have an allergy to iodinated contrast. It may also be useful in pregnant patients in an attempt to minimize radiation exposure.

Ventilation/Perfusion Scan

Principle

Technetium (Tc)-99m labeled macro-aggregated albumin particles, when injected, fill small fractions of the pulmonary capillaries. This helps in the assessment of lung perfusion at the tissue level via scintigraphy. When any branch of the pulmonary artery is occluded, the particles will not reach the capillaries rendering the area "cold" on imaging.

Indications

The utilization of V/Q scanning has declined since the advent of more widespread availability of CT technology, however it may be useful in particular subgroups of patients, such as:

1. Patients who have a known allergy to iodinated contrast. To read more about contrast allergy, click here.
2. In pregnant patients to minimize exposure to radiation.
3. For patients who are in a hospital lacking CT technology.

Interpretation

A normal or near normal V/Q scan excludes the diagnosis of PE. When the result of the V/Q scan is not normal or near normal, the results of the V/Q scan should be interpreted in combination with the pretest probability of PE. Shown below is a table that summarizes the possible outcomes of a V/Q scan.

V/Q Scan	Clinical Probability	Interpretation
Normal	Any probability	PE excluded
Low probability scan or non diagnostic	Low	PE excluded
High probability scan	High	PE confirmed
Any other combination of V/Q scan and clinical probability		Further tests are needed

Any defect that is located pleurally, has a triangular/concave shape, and is in the anatomical distribution of a lung segment should be considered segmental in nature. A defect of the right lower lobe involving the medial basal segment may be undetectable in any view.

Supportive Trial Data

Data from the PIOPED study suggests that normal perfusion scans are almost never associated with recurrent pulmonary embolism, even when anticoagulant therapy was withheld. Other findings from this study are as follows:

• High-probability scans only identified 41% of patients with PE.
• Amongst patients with a high pre-test probability, a high-probability scan revealed PE in 95% of cases.
• 41% of all scans in the PIOPED study were interpreted as intermediate probability, and 16% were interpreted as low-probability.
  • Upon angiography, 30% of intermediate probability, and 14% of low probability patients were found to have PE.
  • Based on these numbers, there has been a movement to abolish the low-probability and intermediate-probability categories. The idea is to re-categorize the results as high-probability, normal, or non-diagnostic.
• The false-positive rate for high-probability scans was 14%. 72% of patients in PIOPED had a clinical scenario and scan combination that warranted further investigation.
• It has been suggested by some authors that in patients with an intermediate pre-test probability of PE, a positive venous ultrasound would justify the initiation of anticoagulant therapy, as would a confirmed PE. If this indication was followed, the approximately 3% false positive rate of venous ultrasounds would result in the use of anticoagulation in some patients without a PE.

Comparison with CT Pulmonary angiography

• Spiral CT scanning is the standard modality to non-invasively diagnose a pulmonary embolism.
  • Initial studies reported sensitivities for diagnosing emboli to the segmental level (4th order branch) to be as high as 98%, however subsequent studies have found sensitivities to be lower.
  • The sensitivity is higher when the clot has a more proximal location.
  • Although smaller clots in the subsegmental arteries are not as physiologically relevant as the larger more proximal clots, they may serve as important predictors of future, larger clots.
  • A study consisting of 142 patients concluded that the sensitivity and specificity of CT angiography is higher than that of a V/Q scan.
    • Obtaining a CT angiography is recommended following an indeterminate V/Q scan. If the pre-test probability is ‘sufficiently high’, and CT angiography is negative, a standard CT angiogram should then be obtained.
• A cost-effective analysis using spiral CT angiography for the diagnosis of PE showed the following results.
  • The use of CT angiography in a diagnostic algorithm was the most cost-effective strategy.
  • If the sensitivity of CT angiography was < 85%, conventional angiography was associated with a lower mortality, but still remained a more expensive strategy.
• According to the International Commission on Radiological Protection (ICRP) the radiation exposure from a V/Q scan with Tc-99 m macroaggregate of albumi (MAA) is 1.1 mSv.
  • The radiation exposure from spiral CT is 2–6 mSv.
  • The radiation exposure from plain chest X-ray is approximately 0.05 mSv.

Benefits of V/Q Scan over CTPA

• Less radiation exposure.
• Diagnostic test of choice in an institution lacking a CT facility, or with inexperienced staff.

Benefits of CTPA over V/Q Scan

• Cost effective.
• CT may also identify right heart dysfunction, or provide an alternative diagnoses.