THE LIMITATIONS OF POSTERIOR VIEW VENTILATION SCANNING IN THE DIAGNOSIS OF PULMONARY-EMBOLISM

In the diagnosis of pulmonary embolism some centres using Xe-133 for c omparison with multiple view Tc-99m perfusion images perform only sing le-breath posterior view ventilation scans. The purpose of this study was to test the reliability of the posterior view ventilation scan in the detection of lobar and segmental defects in ventilation. Occluding balloon catheters were placed in lobar and segmental bronchi during f ibreoptic bronchoscopy to produce defects of known anatomical location and size in normal volunteers. Subjects breathed Kr-81m/air during th e occlusions and images were acquired in the posterior, posterior/obli que and lateral projections. The posterior view images were classified by three experienced nuclear medicine physicians as normal or abnorma l. ff abnormal, the observers were asked to state which lobe or segmen t was involved. Segmental defects were missed in 28% of scan readings. Segmental defects were detected but incorrectly sited in 50% of readi ngs and correctly sited in only 22% of readings. The posterior view sc an with a defect involving the entire lingula was judged to be normal by all observers. Defects involving the right and left lower lobes wer e underestimated. We conclude that ventilation scanning techniques tha t assess the distribution of ventilation in the posterior view alone a re unreliable in the detection of segmental and lobar defects, and are likely to increase the false positive rate in the diagnosis of pulmon ary embolism.