**3.1 Criteria for acute PE according the european guidelines**

In accordance with the guidelines of the European Association of Nuclear Medicine (Bajc et al., 2009a, b), PE is reported if there is:

• V/P mismatch of at least one segment or two sub-segments that conforms to the pulmonary vascular anatomy.

No PE is reported if there is:


Quantitative Ventilation/Perfusion Tomography:

The Foremost Technique for Pulmonary Embolism Diagnosis 191

allowing estimation of the degree of total lung malfunction. A segmental reduction or a subsegmental total deficiency of function is attributed 1 point, a segmental total deficiency, 2 points. Each lung comprises according to our charts 9 segments, representing 18 points. Mismatch defects are expressed as mismatch points, which after division by 36 give the fraction of the lung that is embolised. Regions with ventilation or perfusion defects are

The study showed that patients with up to 40 % PE could be safely treated at home if ventilation abnormalities engaged less than 20 % of the lung. Since 2004, the Skåne University Hospital, Lund, has successfully treated more than 2000 patients with PE and

In PE, an embolus blocks the blood flow, causing a perfusion defect, while ventilation remains normal because there is no corresponding blockage in the airway. To characterize the pattern of perfusion defects is crucial. Perfusion defects due to blockage of a pulmonary artery should reflect the branching of pulmonary circulation and its classical segmental

On V/P SPECT images, it is relatively easy to identify segmental and sub-segmental patterns of perfusion defects. Figure 4a shows multiple perfusion defects in acute stage in a female patient with chest pains, who had fainted outside the hospital. Applying quantification on V/P SPECT images, extension of PE was estimated to be ca 60%. The patient was treated first with heparin for a week. However, Follow up showed limited regression (Fig 4b). As brain hemorrhage was excluded, thrombolysis was administered.

Figure 2 shows sagittal slices of right lung of a patient studied for acute breathlessness. A segmental perfusion defect was well delineated, in perfusion and V/Pquotient images (Fig 2a). Moreover, it was possible to see broncho-constriction in the posterior part of the lung (blue arrow). The extension of perfusion defect was estimated at 10% and ventilation defect 25%. Three days later nearly complete resolution of the embolus was observed, as well as

In planar images, the identification of a solitary segmental perfusion defect within middle lobe and lingula is often impossible or, at best, difficult. With tomographic images these

It is important to be aware that mismatch findings not having segmental character do not usually represent PE. Non segmental mismatch means that perfusion defects do not conform to segmental anatomy and are caused by other diseases. This is observed in patients with heart failure (Jögi et al., 2008), pneumonia, mediastinal adenopathy, post radiation therapy etc. Total absence of perfusion in one lung without any other region of mismatch is often caused by pathology other than PE, such as a central

Follow up is a frequently overlooked aspect of diagnostic strategies although it is essential both for clinical and scientific reasons. The follow up is necessary to assess the effect of treatment, especially to see the effect of anticoagulant therapy (Fig 4b) and in these cases to

be able to adjust therapy or, if necessary, continue with thrombolysis (Fig 4c).

totalled in order to estimate the reduction in overall lung function.

anatomy. A segmental defect is wedge shaped with its base on the pleura.

about 60 % of these patients were treated at home.

The following day perfusion was normalized (Fig 4c).

normalization of the ventilation (Fig 2b).

changes are well delineated.

tumour or abscess.

**3.3 Follow up** 

**3.2 Diagnosis of pulmonary embolism** 

• mismatch that does not have a lobar, segmental or sub-segmental pattern Non-diagnostic for PE is reported if there are:

• Multiple V/P abnormalities not typical of specific diseases.

The fundamental point is that lobar, segmental or sub-segmental V/P mismatch serves as the basis for confirming a diagnosis of PE among patients with suspected PE. Howarth et al. state that: "at least 0.5 of a segment of ventilation/perfusion mismatch is considered diagnostic of PE" (Howarth et al., 2006). This simplified criterion gave the highest combined sensitivity and specificity, observer reproducibility and fewest indeterminate results. In PE, a mismatch has its base along the pleura and conforms to known sub-segmental and segmental vascular anatomy (Miniati et al., 1996). Applying these principles of interpretation, recent V/P SPECT studies of more than 3000 cases showed a negative predictive value of 97-99 %, a sensitivity of 96-99%, and a specificity of 91-98% for PE diagnosis (Bajc et al., 2008; Leblanc et al., 2007; Lemb & Pohlabeln, 2001; Miniati et al., 1996; Reinartz et al., 2004). The rate of non-diagnostic findings was 1-3%. V/P SPECT yields ventilation and perfusion images in exactly the same projections, facilitating recognition of mismatch. This is of particular importance in the middle lobe and lingula where mismatch may be overlooked if the lung is not accurately delineated by its ventilation images (Meignan, 2002).

Fig. 4. Patient with massive PE. A) Absent perfusion is seen in the right lung and subsegmental perfusion defects are seen in the left lung (arrows). **B)** After approximately one week of anticoagulant therapy. **C)** The day after thrombolysis.

Furthermore, V/P SPECT allows quantification of PE extension which is a prerequisite for individual treatment of PE. As suggested by Olsson et al., the number of segments and subsegments indicating PE are counted and expressed in % of the total lung parenchyma (Olsson et al., 2006). Moreover, areas with ventilation abnormalities were recognized

The fundamental point is that lobar, segmental or sub-segmental V/P mismatch serves as the basis for confirming a diagnosis of PE among patients with suspected PE. Howarth et al. state that: "at least 0.5 of a segment of ventilation/perfusion mismatch is considered diagnostic of PE" (Howarth et al., 2006). This simplified criterion gave the highest combined sensitivity and specificity, observer reproducibility and fewest indeterminate results. In PE, a mismatch has its base along the pleura and conforms to known sub-segmental and segmental vascular anatomy (Miniati et al., 1996). Applying these principles of interpretation, recent V/P SPECT studies of more than 3000 cases showed a negative predictive value of 97-99 %, a sensitivity of 96-99%, and a specificity of 91-98% for PE diagnosis (Bajc et al., 2008; Leblanc et al., 2007; Lemb & Pohlabeln, 2001; Miniati et al., 1996; Reinartz et al., 2004). The rate of non-diagnostic findings was 1-3%. V/P SPECT yields ventilation and perfusion images in exactly the same projections, facilitating recognition of mismatch. This is of particular importance in the middle lobe and lingula where mismatch may be overlooked if the lung is not accurately delineated by its ventilation images

Fig. 4. Patient with massive PE. A) Absent perfusion is seen in the right lung and subsegmental perfusion defects are seen in the left lung (arrows). **B)** After approximately one

Furthermore, V/P SPECT allows quantification of PE extension which is a prerequisite for individual treatment of PE. As suggested by Olsson et al., the number of segments and subsegments indicating PE are counted and expressed in % of the total lung parenchyma (Olsson et al., 2006). Moreover, areas with ventilation abnormalities were recognized

week of anticoagulant therapy. **C)** The day after thrombolysis.

• mismatch that does not have a lobar, segmental or sub-segmental pattern

• Multiple V/P abnormalities not typical of specific diseases.

Non-diagnostic for PE is reported if there are:

(Meignan, 2002).

allowing estimation of the degree of total lung malfunction. A segmental reduction or a subsegmental total deficiency of function is attributed 1 point, a segmental total deficiency, 2 points. Each lung comprises according to our charts 9 segments, representing 18 points. Mismatch defects are expressed as mismatch points, which after division by 36 give the fraction of the lung that is embolised. Regions with ventilation or perfusion defects are totalled in order to estimate the reduction in overall lung function.

The study showed that patients with up to 40 % PE could be safely treated at home if ventilation abnormalities engaged less than 20 % of the lung. Since 2004, the Skåne University Hospital, Lund, has successfully treated more than 2000 patients with PE and about 60 % of these patients were treated at home.
