**2.5 Presentation of V/P SPECT**

188 Pulmonary Embolism

Technegas is a newer solid aerosol with extremely small carbon particles, 0.005-0.2 μm, labeled with 99mTc which are generated in a high temperature furnace. The small particle size implies that they are distributed in the lungs almost like a gas and are deposited in alveoli by diffusion (James et al., 1992). Technegas provides images which are equivalent to those with 81mKr. Technegas significantly reduces problems of central airway deposition and peripheral hotspots. Patients routinely admitted for V/P SPECT and a group of patients with known COPD were recently studied with both 99mTc-DTPA and Technegas showing superiority of the latter (Jögi et al., 2010). Unevenness of radiotracer deposition and degree of central deposition were significantly reduced with Technegas, particularly in the obstructive patients (Fig 3). In some patients, mismatched perfusion defects were only identified using Technegas because the significant peripheral unevenness of 99mTc-DTPA obscured mismatch. PE might have been overlooked in COPD patients using 99mTc-DTPA. In a few patients, 99mTc-DTPA yielded images of very poor quality. Technegas is therefore recommended as the superior radio-aerosol, particularly in patients with obstructive lung disease. A further advantage of Technegas is that relatively few breaths are sufficient to

Fig. 3. Comparison between 99mTc-DTPA and Technegas ventilation studies in a patient

Perfusion scintigraphy involves an intravenous injection of radio-labeled macroaggregates of albumin (MAA), sized 15-100 μm, which cause microembolization of pulmonary capillaries and pre-capillary arterioles in amounts reflecting regional perfusion. At least 60 000 particles are required to obtain a representative activity distribution (Heck & Duley, 1974). Routinely, about 400 000 particles are injected. As there are over 280 billion pulmonary capillaries and 300 million pre-capillary arterioles, only a very small fraction of the pulmonary bed will be obstructed. A preparation of 100 000- 200 000 particles is recommended for patients with known pulmonary hypertension or after a single lung transplant. Degradation of MAA results in its elimination from the lung

To perform V/P SPECT takes only one hour from referral to report (Bajc et al., 2004; Palmer et al., 2001). The ventilation study starts with inhalation of 25-30 megabecquerel (MBq) Technegas, usually 2-3 breaths. Immediately after ventilation SPECT, a dose of 100-120 MBq

99mTc-MAA is given intravenously for perfusion imaging.

achieve an adequate amount of activity in the lungs.

with COPD.

**2.2 Perfusion** 

within a few hours.

**2.3 Acquisition** 

V/P SPECT images are usually presented in frontal, sagittal and transversal projections, available in any modern system. The slices must be accurately aligned so that ventilation and perfusion slices match each other for correct comparison. Therefore, it is crucial to achieve this acquisition in one session with maintained body position. This is also a prerequisite for the calculation of V/Pquotient images, which greatly facilitates identification of ventilation/perfusion mismatches typical of PE as well as other patterns characteristic of other pulmonary diseases.

Volume rendered images, such as "Maximum Intensity Projection" are available with almost all SPECT systems, allowing rotating 3D views. This function is another valuable option, particularly for quantification and follow-up of PE patients.
