**4. SPECT acquisition protocol**

The protocol can be tailored to a certain point to the preference of the different centers. The number of tomographic steps should be at least 64 while 128 are considered optimal. Higher radiation doses will permit either higher-quality images or faster acquisition times. If a lower dose range is preferred, a general-purpose collimator which has a higher sensitivity (but a lower resolution) can be used. With such a collimator, using a 64 X 64 matrix, acquisition time can be as low as 5 second per step with perfusion and 10 seconds per step in ventilation in a 128 step protocol with 20-25 MBq in ventilation and 100-120 MBq in perfusion (Palmer, Bitzen et al. 2001). Using a high-resolution collimator and a matrix of 128 X 128 will produce higher-quality images at the expense of a higher radiation dose and longer acquisition times. Depending on the number of steps, the activity will be more in the range of 35-40 MBq in ventilation and 180-200 MBq in perfusion and the time per step will be 15-20 seconds in ventilation and 7-10 seconds in perfusion. Reconstruction of the data should be iterative using OSEM (ordered subset expectation maximization). Eight subsets and two iterations are recommended. Using a higher number of subsets and iterations may produce sharper images but noise will also be increased. However, every center can optimize their protocol.

Ventilation Perfusion Single Photon Emission

partial mismatch.

caused by lung compression.

Tomography (V/Q SPECT) in the Diagnosis of Pulmonary Embolism 145

Fig. 2. Passive atelectasis (coronal and sagittal slice triangulation). Note non-segmental

Fig. 3. Sagittal SPECT slices showing non-embolic mismatch at posterior costophrenic angle

Image display should strive to match precisely each ventilation slice with the corresponding perfusion slice in all three planes (transverse, coronal, sagittal). This can be easily done either by not moving the patient or bed position between ventilation and perfusion or, alternatively, by using commercially available software which will co-register each set of images.
