**6.5 Pleural effusion**

Pleural effusions of significant size are usually well recognized on V/Q SPECT. On transverse sections, the patient being in a supine position, the lung is displaced in the anterior direction as the effusion occupies the posterior region. On sagittal slices, there is loss of posterior angle. The perfusion and ventilation are typically matched although a thin band of hyperventilation is common at the lung-effusion interface because of compressed lung tissue. It is not uncommon however that the perfusion is better preserved. In that case, there is failure of the vasoconstriction reflex combined with lung compression which cannot be overcome by deep inspiration. For confident interpretation of a non-embolic effusion, there should not be any mismatches elsewhere and there should not be any hint of a wedge-shaped perfusion defect underlying the effusion (mismatched or not) (figures 20 and 21).

Fig. 20. Pleural effusion. Left: transverse slices show upward displacement of lung on both ventilation and perfusion, creating a non-segmental defect. Note band of hyperventilation indicating partially compressed lung. Right: sagittal slices in another patient show relatively preserved perfusion but absent ventilation. In this case, the effusion does not permit lung expansion.

Ventilation Perfusion Single Photon Emission

a COPD patient was the culprit.

indeterminate study.

**7.3 Animal studies**

**7.2 Computer modeling**

Tomography (V/Q SPECT) in the Diagnosis of Pulmonary Embolism 159

Fig. 22. Reverse mismatch. Sagittal slices show absent ventilation to inferior lobe with relative preservation of perfusion. In such a case, there is a huge amount of un-oxygenated blood returned to the arterial circulation, causing hypoxemia. In this case, a mucous plug in

Also, with 3-D data, the assignment of a defect to a vascular or non-vascular origin is much easier which will result in both better specificity and a much lower rate of

The superiority of V/Q SPECT (sensitivity 97%) to planar imaging (sensitivity 77%) was tested in a computer model which clearly predicted much better accuracy with V/Q SPECT (Magnussen, Chicco et al. 1999). It is very interesting to note that this model accurately

The technique was also tested in a pig model with artificially produced small size emboli (Bajc, Bitzen et al. 2002). Again, the superiority of V/Q SPECT to planar imaging was clearly demonstrated, as sensitivity for V/Q SPECT was 91% while it was 64% for planar imaging for the same emboli. Specificity was also better for SPECT (87% vs 79%). It should be mentioned that a similar model had been employed earlier for validation of invasive pulmonary angiography and that the performance numbers in terms of

predicted the results of clinical studies which would appear several years later.

Fig. 21. Pleural effusion caused by embolism (sagittal and transverse slices). Note upward displacement of right lung by the effusion with an underlying. wedge shaped subsegmental mismatched defect.

#### **6.6 Reverse mismatches**

In most physiological and pathological conditions, ventilation and perfusion are matched. This is accomplished by the pulmonary vasoconstriction reflex which diverts blood away from poorly ventilated areas to prevent a right to left shunt equivalent. Indeed, perfusion of non-ventilated areas causes non-oxygenated blood to return to the arterial circulation and this has a profound impact on the arterial oxygen pressure (figure 22). Failure of the vasoconstriction reflex is not an uncommon finding on V/Q SPECT imaging. It is often seen in the context of pneumonia (established or pre-radiological), atelectasis or bronchial mucous plug in COPD patients. It is important to report this finding as it represents an obvious cause to the patient's oxygen saturation problems. Reverse mismatch in the context of emboli is extremely uncommon as it requires causation of a ventilation anomaly which persists with reperfusion, without any other mismatches elsewhere. It is theoretically possible in the sub acute phase**.** 
