Lung ventilation/perfusion SPECT for diagnosing pulmonary embolism. Is the required minimal P/V count rate ratio fulfilled in routine situation? Results from a multicentre survey.

UNLABELLED

V/P-SPECT is a sensitive and specific procedure for the detection or exclusion of pulmonary embolism. It is important to conform to the guidelines in order to obtain reliable results. The sequence usually starts with the ventilation, followed by the perfusion scan. According to the current guidelines the count rate of the second study should be at least 3 times that of the first study. It is not always easy to meet this requirement, particularly since the deposed quantity of ventilation activity is not well known. The aim was therefore to analyse data from a multicentre survey to determine whether this essential precondition was strictly met.

METHODS

In a multicentre survey 48 tertiary hospitals were asked to submit projections of all V/P-SPECT studies performed in January 2014. In total, 286 complete data sets from 16 institutions could be evaluated. First, the count rates of the first projections from the V-studies were subtracted from those of the P-studies. The resulting count rates in the first SPECT projections were then divided to calculate the activity ratios between the pure perfusion and ventilation scans (P/V-ratio at least 3 according to the guidelines).

RESULTS

The range of the P/V ratio was 0.57-78.71, the mean P/V ratio was 6.94 ± 9.56. For 71 of the 286 external V/P studies (about 25%) the P/V ratio was < 3, in 23 studies (about 8%) the ratio failed to even reach the factor 2.

CONCLUSIONS

An activity ratio of 3 between the perfusion and ventilation scan was not reached in about 25% of the 286 V/P studies (in around 8% the P/V ratio was <2), so that V/P studies were performed inadequately in a considerable number of procedures. Controlling the count rate increase during the perfusion tracer application (e. g. by handheld monitor) is therefore essential to avoid insufficient data.