Pulmonary embolism ventilation/perfusion scan


 * Associate Editors-in-Chief: Ujjwal Rastogi, MBBS [mailto:urastogi@perfuse.org]

Overview
Ventilation/perfusion scan (or V/Q scan or lung scintigraphy), show whether or not a lung area is being ventilated and perfused with blood (perfusion can be stopped due to obstruction caused by a clot). This type of examination is used less often because of the more widespread availability of CT technology, however, it may be useful in patients who have an allergy to iodinated contrastor in pregnancy due to lower radiation exposure than CT.

Principle
Technetium (Tc)-99m labelled macro-aggregated albumin particle, when injected, block small fractions of pulmonary capillaries and thus helps in assessment of lung perfusion at tissue levels via scintigraphy. If there is an occlusion of any branch of pulmonary artery, the particle will not reach the capillary, rendering the area "cold" on subsequent imaging.

Indications
This type of examination is used less often because of the more widespread availability of CT technology, however, it may be useful in patients
 * 1) Who have an allergy to iodinated contrast. To read more about contrast allergy, click here.
 * 2) In pregnancy due to lower radiation exposure than CT.
 * 3) Hospitals lacking CT facility or inexperienced medical staff.

Interpretation
The following table summarizes the possible outcome of V/Q scan:

Taking anatomical data into consideration, any defect that is located pleurally and have triangular/concave shape and in the anatomical distribution of a lung segment should be considered segmental in nature. It should also be noted that a defect of right lower lobe involving the medial basal segment may be undetectable in any view.

Supportive Trial Data
The PIOPED data suggested, that the normal perfusion scans are almost never associated with recurrent pulmonary embolism, even if anticoagulation is withheld. The other highlights of this study were:
 * High-probability scans, however only identified 41% of patients with PE.
 * In the setting of a high pre-test probability, a high-prob scan revealed PE in 95% of cases.
 * Unfortunately, 41% of all scans in PIOPED were interpreted as intermediate, and an additional 16% were interpreted as low-probability.
 * Upon angiography, however, 30 and 14% of these patients respectively were found to have PE.
 * Based on these numbers, there has been a huge movement to abolish the low-probability, and intermediate-probability categories, and have readings either be high-probability, normal, or non-diagnostic.
 * It should also be realized that the false-positive rate for high-prob scans was 14%, and that 72% of patients in PIOPED had a clinical – scan combination that required further investigation.


 * It has been suggested by some authors that in patients with an intermediate pre-test probability of PE, a venous ultrasound would provide the same justification for anticoagulation, as would a confirmed PE. Although, false positive results on venous ultrasound (3 percent patient reported in a study) would result in the use of anticoagulation in patients without PE.

Comparison with CT Pulmonary angiography

 * Spiral CT scanning is now a standard modality to non-invasively diagnose PE.
 * Initial studies reported sensitivities for diagnosing emboli to the segmental level (4th order branch) as high as 98%, however subsequent studies have found sensitivities to be lower.
 * The sensitivity is higher with more proximal clot.
 * Although smaller clot, in the subsegmental arteries, is certainly not as physiologically important as the larger, more proximal clot, they may be important predictors of future, larger clots.
 * The study consisting of 142 patients concluded that the sensitivity and specificity of CT angiography is higher than that of V/Q scan, as is the inter-observer agreement.
 * They recommend getting a CT angiography as the next test following an indeterminate V/Q scan, however caution that if the pre-test probability is ‘sufficiently high’, a standard angiogram should still be obtained after a negative CT angiography.
 * A cost-effective analysis using spiral CT angiography for the diagnosis of PE showed the following result.
 * The use of CT angiography in a diagnostic algorithm was by far and away a more 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 lung scan with 100 MBq of 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 CT
 * Less radiation exposure.
 * Diagnosis of choice in institution lacking CT facility or inexperienced staff.

Benefits of CTPA over V/Q Scan
 * Cost effective.
 * CTPA may additionally identify right heart dysfunction or alternative diagnoses.