**9. Future directions**

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 ratio.

Ventilation Perfusion Single Photon Emission

typical of established COPD.

indicating reverse mismatch.

**10. Role of V/Q SPECT in PE**

Tomography (V/Q SPECT) in the Diagnosis of Pulmonary Embolism 165

Fig. 24. V/Q quotient of severe COPD, coronal (left) and sagittal (right) slices. Black

Fig. 25. V/Q quotient of pneumonia, coronal slice. Note wedge shaped area in black

Given the high sensitivity associated with a low indeterminate rate, absence of contraindications and low radiation dose, V/Q SPECT seems ideally suited to be the initial

indicates reverse mismatched areas, green indicates partial mismatches while blue indicates normally matched areas. Widespread areas of non-matched perfusion and ventilation are

Respiratory gating takes advantage of the fact that image acquisition can be timed electronically with a device that identifies the patient's respiratory movements. This has the potential to create images of better quality, especially for the lung regions closer to the diaphragm where movement during acquisition causes some blurring. Better images have been confirmed in at least one study. Since the majority of emboli are in the lower lung fields, this enhancement may be significant for some patients (Suga, Yasuhiko et al. 2008).

In recent years, hybrid machines, called SPECT-CT, that combine a nuclear camera with a standard CT have become available. Those machines permit acquisition of physiological nuclear medicine data and anatomical CT images that can be perfectly registered. Therefore, physiological anomalies (in the case of lung imaging, ventilation or perfusion anomalies) can be mapped directly on the anatomical images. The potential of this technique is mainly to enhance the specificity of V/Q SPECT, as potential causes of falsepositive imaging can be readily identified with this technique (Roach, Gradinscak et al. 2010). There is however the drawback of the higher radiation dose although most protocols will use a low-dose CT.

Quantitative evaluation of the ventilation and perfusion ratios has been the subject of physiological evaluation in normal patients but also in different pathological settings including embolism (Palmer, Bitzen et al. 2001). This type of evaluation has the potential to enhance reading accuracy in embolism, accurately quantify the volume of lung affected by the embolic process and to pinpoint patterns which may be indicative of other types of lung pathology (Suga, Kawakami et al.) (figures 23 to 25). Basically, relative matching of perfusion is illustrated by a color display to show areas of normal matching, mismatches (altered perfusion with preserved ventilation) and reverse mismatches (altered ventilation with preserved perfusion).

Fig. 23. V/Q quotient of embolism, coronal (left) and sagittal (right) slices. Red indicates the area of complete mismatch, while blue shows normally matched regions.

Respiratory gating takes advantage of the fact that image acquisition can be timed electronically with a device that identifies the patient's respiratory movements. This has the potential to create images of better quality, especially for the lung regions closer to the diaphragm where movement during acquisition causes some blurring. Better images have been confirmed in at least one study. Since the majority of emboli are in the lower lung fields, this enhancement may be significant for some patients (Suga, Yasuhiko et

In recent years, hybrid machines, called SPECT-CT, that combine a nuclear camera with a standard CT have become available. Those machines permit acquisition of physiological nuclear medicine data and anatomical CT images that can be perfectly registered. Therefore, physiological anomalies (in the case of lung imaging, ventilation or perfusion anomalies) can be mapped directly on the anatomical images. The potential of this technique is mainly to enhance the specificity of V/Q SPECT, as potential causes of falsepositive imaging can be readily identified with this technique (Roach, Gradinscak et al. 2010). There is however the drawback of the higher radiation dose although most

Quantitative evaluation of the ventilation and perfusion ratios has been the subject of physiological evaluation in normal patients but also in different pathological settings including embolism (Palmer, Bitzen et al. 2001). This type of evaluation has the potential to enhance reading accuracy in embolism, accurately quantify the volume of lung affected by the embolic process and to pinpoint patterns which may be indicative of other types of lung pathology (Suga, Kawakami et al.) (figures 23 to 25). Basically, relative matching of perfusion is illustrated by a color display to show areas of normal matching, mismatches (altered perfusion with preserved ventilation) and reverse mismatches (altered ventilation

Fig. 23. V/Q quotient of embolism, coronal (left) and sagittal (right) slices. Red indicates the

area of complete mismatch, while blue shows normally matched regions.

al. 2008).

protocols will use a low-dose CT.

with preserved perfusion).

Fig. 24. V/Q quotient of severe COPD, coronal (left) and sagittal (right) slices. Black indicates reverse mismatched areas, green indicates partial mismatches while blue indicates normally matched areas. Widespread areas of non-matched perfusion and ventilation are typical of established COPD.

Fig. 25. V/Q quotient of pneumonia, coronal slice. Note wedge shaped area in black indicating reverse mismatch.
