Limitations to SV determination from APT images

The limitations related to the position-dependent point response function (PRF) of an applied potential tomography (APT) system are determined. The 3-D distribution of the PRF is measured. The thickness of the APT´s field of view (slice) and the spatial resolution are determined from these measurements. The effects of these parameters on the reconstructed image are discussed. The results show that the sensitive slice thickness is not uniform across the image plane and that the PRF is strongly dependent on position. Therefore, the appearance of localized resistivity changes in the images is a function of their position relative to the electrodes in 3-D space. The same amount of stroke volume (SV) at different spatial locations is therefore reconstructed differently. Hence, an accurate calculation of an index of cardiac volumes is complicated, and calibration is needed. A uniform sensitivity distribution across the image plane would result in more reliable volume indices. The FWHM resolution is position dependent and is a function of the conductivity change, being poorest at the center of the field.