**8.3 Contraindications and technical success rate**

There are no contraindications to V/Q SPECT imaging. Some degree of prudence is required in cases of severe pulmonary hypertension (the number of particle injected should be limited) and reasonable efforts should be made to limit the dose during pregnancy, but any patient may undergo V/Q SPECT as long as he can tolerate a supine position for 20 min. Allergies are virtually nonexistent. There are no known deleterious effects on any organ system.

On the other hand, CTPA has specific contraindications. Allergies are relatively frequent and, depending on the severity, constitute an absolute or relative contraindication. If the decision is made to proceed with the study, patients must be prepared appropriately. Also, because of the injection of contrast, renal failure is a possible complication especially in patients with established underlying renal disease. In some subgroups, the risk of renal failure requiring dialysis is extremely high.

Performance in pregnancy has also been reviewed (Ridge, McDermott et al. 2009). The radiation dose to the foetus is very low for both techniques (< 1mSv) although it is significantly lower for CTPA. However, at this level, there is no increased risk for either technique. On the other hand, it is clear that the technical performance of CTPA in pregnancy is poor, with as many as one third of the studies being technically inadequate. This is probably due to increased pressure in the inferior vena cava during pregnancy with aspiration of large amounts of non-opacified blood during inspiration that interferes with optimal mixing of contrast coming from the superior vena cava. There are no technical limits to V/Q SPECT during pregnancy. Therefore, V/Q SPECT should be the preferred modality in this situation, especially considering the very high breast radiation dose given with CTPA in young patients with actively proliferating breasts.

High-quality imaging with CTPA requires accurate timing for contrast injection. In most studies, the technical failure rate hovers between 5 and 10%. The technical success rate for

Ventilation Perfusion Single Photon Emission

reperfusion.

et al. 2009).

for V/Q SPECT.

ratio.

**9. Future directions** 

**8.6 Alternate diagnosis** 

Tomography (V/Q SPECT) in the Diagnosis of Pulmonary Embolism 163

order of 95%(Tunariu, Gibbs et al. 2007). Specificity is in the same range. Therefore, chronic PE cannot be reliably excluded by CTPA and V/Q SPECT should always be performed if this diagnosis is suspected. It is to be noted that in chronic PE, mismatches may not have the typical vascular shape it usually has in acute PE because of partial

From the previous discussion, it is obvious that follow-up studies using CTPA may be of limited value. Conversely, the evolution of the perfusion defects on serial V/Q SPECT is very easy to follow and resolution or persistence of perfusion anomalies is easily identified. Follow-up studies are very important in PE, especially for the larger embolic processes which are at risk for later pulmonary hypertension. Also, since there is a risk of relapse, follow-up studies need to be done to have a baseline to evaluate this dynamic process. Therefore, even if the initial diagnosis has been made by CTPA, an initial V/Q SPECT study is very valuable. Also, it should be pointed out that it seems difficult to justify the higher radiation dose and contrast agent of CTPA solely for follow-up purposes. There are of course some signs that permit distinction by CTPA of acute and chronic embolism but differentiation between the two conditions is far from being always easy (Castaner, Gallardo

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

Technical enhancements which are under study at this point for V/Q SPECT include respiratory gating, SPECT-CT technique and quantitative evaluation of ventilation perfusion

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 ventilation.

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

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 are however no studies to prove this point.

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

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 specificity of CTPA in that setting.

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

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 order of 95%(Tunariu, Gibbs et al. 2007). Specificity is in the same range. Therefore, chronic PE cannot be reliably excluded by CTPA and V/Q SPECT should always be performed if this diagnosis is suspected. It is to be noted that in chronic PE, mismatches may not have the typical vascular shape it usually has in acute PE because of partial reperfusion.

From the previous discussion, it is obvious that follow-up studies using CTPA may be of limited value. Conversely, the evolution of the perfusion defects on serial V/Q SPECT is very easy to follow and resolution or persistence of perfusion anomalies is easily identified. Follow-up studies are very important in PE, especially for the larger embolic processes which are at risk for later pulmonary hypertension. Also, since there is a risk of relapse, follow-up studies need to be done to have a baseline to evaluate this dynamic process. Therefore, even if the initial diagnosis has been made by CTPA, an initial V/Q SPECT study is very valuable. Also, it should be pointed out that it seems difficult to justify the higher radiation dose and contrast agent of CTPA solely for follow-up purposes. There are of course some signs that permit distinction by CTPA of acute and chronic embolism but differentiation between the two conditions is far from being always easy (Castaner, Gallardo et al. 2009).
