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

Marika Bajc and Jonas Jögi *Department of Clinical Physiology Lund University and Skåne University Hospital, Lund Sweden* 

#### **1. Introduction**

184 Pulmonary Embolism

Zhang, L. J., Yang, G. F., Zhao, Y. E., Zhou, C. S., & Lu, G M. (2009). Detection of pulmonary

embolism using dual-energy computed tomography and correlation with cardiovascular measurements: a preliminary study. *Acta Radiol* Vol. 50 (8): 892-901.

> 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.

Quantitative Ventilation/Perfusion Tomography:

and perfusion is seen already after three days.

and it is too expensive for general use.

The Foremost Technique for Pulmonary Embolism Diagnosis 187

Fig. 2. A patient with acute PE. Sagittal ventilation (V), perfusion (P) and V/P quotient (V/Pq) images of the right lung. **A)** At the initial examination, segmental perfusion defects are seen (arrow). The V/P mismatch is clearly delineated on V/P quotient images which improves visualization. Reduced ventilation is observed in posterior parts of the lung, where perfusion is preserved (blue arrows). **B)** Normalization of ventilation (blue arrows

concentration will reflect alveolar ventilation. Ventilation is performed during continuous breathing of this gas. 81mKr has higher gamma energy than 99m-Technetium (99mTc) (191 compared to 140 keV) allowing simultaneous imaging of ventilation and perfusion. 81mKr is diluted from a rubidium generator that has a half life of 4.6 h. Its availability is limited

Routinely in clinical practice, inhalation of a radio-aerosol is used for ventilation scintigraphy. Aerosol particles are liquid or solid. The size of the particles is of critical importance. Particles larger than 2 μm are deposited in large airways. Smaller particles are deposited by sedimentation and diffusion in small airways and alveoli. Particles smaller than 1 μm, are mainly deposited in alveoli by diffusion. Aerosol deposition is modified by flow pattern. High flow rates at forced breathing patterns and turbulent flow enhances particle deposition in airways and increases the likelihood of hot spots formation on

Diethylenetriaminepentaacetic acid labeled with technetium, 99mTc-DTPA, is the most common agent used for ventilation scintigraphy. It is soluble in water and the size of the molecule is 492 Dalton. The average size of particles after nebulization is at best 1.3 to 1.8 μm. Due to the water solubility, particle size tends to increase during inhalation and to agglutinate in cases of bronchial obstruction where there are turbulent flows; this leads to the creation of hot spots. Because of the water solubility, 99mTc-DTPA particles also diffuse through the alveolo-capillary membrane to the blood. In a healthy patient, clearance of 99mTc-DTPA occurs with a half life of about 70 minutes. Increased clearance, leading to a shorter half life is observed where there is alveolar inflammation for any reason, such as alveolitis of an allergic or toxic nature and even in smokers. Clearance of 99mTc-DTPA can

ventilation images, particularly in Chronic Obstructive Pulmonary Disease (COPD).

for diagnostic purposes be measured at a routinely performed V/P SPECT.

Efficient and effective diagnostics for PE and other diseases should meet the following basic requirements:


Fig. 1. V/P SPECT images of a patient with normal ventilation (V) and perfusion (P). Techn = Technegas, MAA = Macroaggregated albumin
