**7. Validation of V/Q SPECT**

#### **7.1 Theoretical basis**

SPECT has proven advantageous in just about every field in nuclear medicine. It is now standard procedure either as the sole examination (myocardial perfusion imaging and brain imaging) or a very useful adjunct to just about every other organ imaging (bone, gallium, white blood cell, liver, hemangioma, somatostatin). The theoretical advantages in lung imaging are the same as they are in every other organ: suppression of overlapping structures, better imaging contrast and 3-D representation of the data. Better depiction of sub-segmental and even some segmental defects is expected, resulting in better sensitivity.

Fig. 21. Pleural effusion caused by embolism (sagittal and transverse slices). Note upward

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

SPECT has proven advantageous in just about every field in nuclear medicine. It is now standard procedure either as the sole examination (myocardial perfusion imaging and brain imaging) or a very useful adjunct to just about every other organ imaging (bone, gallium, white blood cell, liver, hemangioma, somatostatin). The theoretical advantages in lung imaging are the same as they are in every other organ: suppression of overlapping structures, better imaging contrast and 3-D representation of the data. Better depiction of sub-segmental and even some segmental defects is expected, resulting in better sensitivity.

displacement of right lung by the effusion with an underlying. wedge shaped sub-

segmental mismatched defect.

possible in the sub acute phase**.** 

**7. Validation of V/Q SPECT**

**7.1 Theoretical basis** 

**6.6 Reverse mismatches** 

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 a COPD patient was the culprit.

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 indeterminate study.
