**8.6 Alternate diagnosis**

162 Pulmonary Embolism

VQ SPECT is extremely high, with failure occurring in less then 1% of cases in all studies. Using Technegas as a ventilation agent, it is possible to ventilate patients on mechanical

The presence of COPD is generally thought to decrease the usefulness of V/Q scanning. However, this stems from the use of a planar technique using an inferior ventilation agent (xenon-133) interpreted in probabilistic terms according to the PIOPED scheme. This is much less true with V/Q SPECT in which a superior ventilation agent is used (technegas) and determination of the nature of a vascular defect is much easier. In COPD, ventilation is often much more affected than perfusion and is it is generally possible to distinguish vascular from nonvascular type defects following the definition outlined above for PE. Also, in COPD, emboli will always follow vascular flow which, by definition, always corresponds to residual ventilated areas, thus permitting the identification of a mismatch. However, in cases of very severe COPD where there are very few residual normally ventilated regions, interpretation may be more difficult and theoretically, sensitivity may be decreased. There

There are no data suggesting that the performance of CTPA is altered by the presence of severe COPD. Therefore, in severe COPD, with a high pre-test likelihood of PE and with a

The presence of an abnormal lung x-ray causes special problems in nuclear imaging. Embolism detection is based on a mismatch between ventilation and perfusion. Ventilation is rarely possible in the presence of atelectasis, consolidations or marked infiltrate. Since embolism may create secondary atelectasis or lung infarcts, detection of PE on the basis of a mismatch may not be possible in that specific scenario. It must be stressed however that an x-ray anomaly does not preclude exclusion of PE by V/Q SPECT. Significant residual perfusion at the site of the chest x-ray anomaly or the presence of a nonvascular or nonpleural-based defect associated with the anomaly are reliable signs for the absence of PE. Also, PE is generally thought to be much more often multiple than single. Therefore, in most cases, you would expect identification of a mismatch away from the x-ray anomaly. Indeed, with the published data, using mismatching as the sole criteria for the presence of PE, the sensitivity of V/Q SPECT has been excellent. Nevertheless, there are no data on the incidence of solitary PE associated with a radiological anomaly. Therefore, if the anomaly has a vascular pattern on the perfusion study and the perfusion defect is complete, CTPA should be performed if the pre-test probability is significant. For lower pre-test probabilities, lower limb Doppler studies are probably sufficient. The presence of an x-ray anomaly is not thought to alter the capacity of CTPA to diagnose PE accurately. Altered sensitivity is unlikely since there are several studies confirming a high negative predictive value for CTPA. However, in the absence of a suitable gold standard, there is no data to evaluate the

The sensitivity of CTPA for the detection of chronic pulmonary embolism has been proven to be poor, with a sensitivity of probably not much more than 50%. The sensitivity of V/Q SPECT for the detection of chronic PE is excellent, probably on the

**8.4 Performance in difficult patients: COPD and abnormal x-ray** 

are however no studies to prove this point.

specificity of CTPA in that setting.

**8.5 Chronic PE and follow-up studies** 

V/Q SPECT difficult to interpret, CTPA may be warranted.

ventilation.

The capacity to provide an alternate explanation for the patient's symptoms is certainly one of CTPA's strong points. This subject has been recently reviewed extensively (Hall, Truitt et al. 2009). An alternate diagnosis for the symptoms (not previously known) can be expected in approximately 1/3 of patients undergoing CTPA for the exclusion of PE. However, a substantial number of those anomalies are also visualized on a chest x-ray and there is some concern that those that are not visualized on a standard x-ray may be of limited clinical consequence. However, for some alternate diagnosis (aortic dissection) CTPA is essential. Also, some tumours will obviously be visualized only on chest CT. As mentioned above, some diagnostic patterns other than PE can be recognized on V/Q SPECT. Early cardiac failure, identification of a large area of reverse mismatch and underestimation of the severity of COPD constitute the most frequent alternative explanations for the symptoms that are not apparent on a standard chest x-ray. Such alternate findings are frequent, occurring in nearly 40% of patients in one study (Bajc, Olsson et al. 2004). However, COPD in often known beforehand and the scope of potential diagnosis is narrower than with CTPA. It is to be noted that approximately 1/4 of patients undergoing CTPA will have incidental findings not related to the acute symptomatic episode that will require follow-up. Most of these findings are pulmonary nodules or thoracic adenopathy. The vast majority will prove benign on follow-up and thus, there is a potential for the generation of multiple follow-up studies with extra costs, extra radiation dose and significant concern for patients. Such incidental findings are inexistent for V/Q SPECT.
