Quantitative Ventilation/Perfusion Tomography: the Foremost Technique for Pulmonary Embolism Diagnosis

The value of perfusion scintigraphy in the detection of pulmonary embolism (PE) was demonstrated as early as 1964 by Wagner et al. PE causes perfusion defects that conform to the anatomical distribution of the pulmonary vascular bed. Perfusion defects in acute PE are therefore of sub-segmental, segmental or lobar character. Ventilation is normally preserved in these areas and the observed wedge shaped mismatch between ventilation and perfusion is typical for PE. Planar ventilation/perfusion scintigraphy (V/P scan) was until the 1990s the method of choice for studying patients with suspected PE. However, the large Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED I), showed a high number of non-diagnostic examinations (65%) with V/P scan and the probabilistic interpretation criteria were confusing to the clinicians (Gray et al., 1993; The PIOPED Investigators, 1990). Planar imaging has become obsolete, particularly when the issue is identification and quantification of focal or regional aberration of organ function. The advantage of three dimensional tomography over planar imaging for PE detection had already been shown in 1983 in a study on dogs (Osborne et al., 1983). Furthermore, Magnussen et al. (1999) used a computerized model of PE to highlight the advantage of SPECT over planar imaging in the assessment of the size and location of perfusion defects. Using a dual head camera, Palmer et al. (2001), developed a fast and efficient method for ventilation/perfusion tomography (V/P SPECT ) for clinical practice with total acquisition time of only 20 minutes. Moreover, they developed an algorithm to calculate the quotient between ventilation and perfusion and to present it as V/Pquotient images. This facilitated PE diagnosis and the quantification of PE extension, which led to the use of the term quantitative V/P SPECT. Using a porcine model, Bajc et al. validated V/P SPECT for diagnosis of PE and confirmed the superior value of tomography over planar imaging with excellent interobserver agreement of defects down to the sub-segmental level (Bajc et al., 2002b). The objective of this chapter is to acquaint readers with the latest methodological approach of V/P SPECT in the diagnosis of PE, in accordance with the new guidelines of the European Association of Nuclear Medicine (Bajc et al., 2009a, b). In this chapter we also discuss the value of V/P SPECT in the follow up after acute PE and in the diagnosis of other cardiopulmonary diseases.