**8.2 Radiation dose**

160 Pulmonary Embolism

sensitivity and specificity were no better than that obtained with V/Q SPECT (Baile, King

New generation techniques to measure regional perfusion independently (by opposition to simply visualizing the artery and the thrombus) by means of perfusion thoracic CT (Wildberger, Klotz et al. 2005) or nuclear magnetic resonance (NMR) (Kluge, Gerriets et al. 2006) have correlated very well with V/Q SPECT measures of regional perfusion. Indeed, work with those techniques suggests that embolism is almost always totally occlusive and that inclusion of perfusion data enhances sensitivity over an approach based purely on

It is beyond the scope of this chapter to review individually all studies. However, even if proof of superiority of V/Q SPECT to planar imaging seems redundant, it has been proven clinically, with V/Q SPECT having an edge in sensitivity of more than 20% while still maintaining better specificity (Gutte, Mortensen et al. 2010; Bajc, Olsson et al. 2004). The negative predictive value of V/Q SPECT (the ability to reliably exclude PE) has been validated and is excellent, in the order of 98-99%, even in the presence of abnormal perfusion with a nonvascular pattern (Leblanc, Leveillee et al. 2007). Sensitivity is in the range of 96-99% and specificity hovers between 85% and 98%, depending on the study. The rate of non-diagnostic studies is 1-3%. Comparison to CTPA is unfortunately limited because of the lack of a large-scale prospective study comparing both techniques. A detailed discussion on each of the available studies can be found elsewhere (Leblanc and Paul 2010). However, all published studies have demonstrated that V/Q SPECT performs at least as well as CTPA for the diagnosis of pulmonary embolism (Reinartz, Wildberger et al. 2004; Suga, Yasuhiko et al. 2008; Gutte, Mortensen et al. 2009; Miles,

Although a large randomized prospective study comparing the two techniques is not available, the pooled published results suggest at this point that V/Q SPECT may have an edge in sensitivity while CTPA may have an edge on specificity. Better sensitivity of V/Q SPECT can be attributed essentially to sub-segmental embolism. Indeed, to visualize directly a thrombus in a sub-segmental vessel is difficult even with the latest CTPA technology. Also, existing literature suggest that inter-observer agreement is very low for sub-segmental embolism with CTPA (Ghanima, Nielssen et al. 2007). Since V/Q SPECT visualizes the resulting perfusion defect, it has a clear advantage. Indeed, even for a small sub-segmental defect implicating 25% of a segment, the pleural base of the defect will have at least 3 cm, a dimension easily resolved by the SPECT technique. On the other hand, since CTPA directly visualizes a filling defect, it is less prone to false positive studies since most (but not all) filling defects will represent embolus. This may not be true of sub-segmental emboli because the poor inter-observer agreement in this setting suggests limited specificity. Causes of false-positive mismatches on V/Q SPECT have

**8. V/Q SPECT and CTPA: Relative advantages and limitations**

**7.4 Validation with alternative perfusion techniques**

et al. 2000).

luminology.

**7.5 Clinical studies**

Rogers et al. 2009).

**8.1 Diagnostic performance** 

Estimated radiation dose is a complex subject and a detailed discussion is beyond the scope of this chapter and can be found elsewhere (Schembri, Miller et al. 2010). However, there is little doubt that when comparing state-of-the-art technology for both modalities, incurred radiation dose is much higher with CTPA and this is particularly true of the female breast. Depending on the exact protocol that is employed, total radiation dose for CTPA is in the range of 8-20 milliSievert (mSv) while it is 2,0 - 3,5 mSv for V/Q SPECT. The dose to the female breast varies between 10 and 70 mSv for CTPA (equivalent to 10-25 mammograms or 100-400 chest x-rays) while the corresponding breast dose for V/Q SPECT is less than 1,5 mSv.
