Lungs SPECT image processing for volume and perfusion index estimation

In the evaluation of the lung status during pulmonary embolism and patient´s follow up, accurate volume estimation is important In this study a method for lungs´ volume determination by SPECT images and the definition of an index of perfusion scans homogeneity (LPI) are demonstrated. From lung perfusion SPECT data of 42 patients, volumes of reconstructed transverse slices were estimated by counting the voxels inside each slice. An edge detection program was used to mark the lung lobe´s edge and subtract background. The total lung lobe volume was derived by summing the slices´ volumes. Non uniform attenuation compensation was performed by quantitative filtered back-projection. A lung lobe segment volume analysis was also determined, in the same way, for the identification of lung section volumes with diminished radionuclide concentration and correlation of follow-up SPECT lung perfusion studies. Lung perfusion index (LPI ) is the ratio of lung segment volume over the whole organ volume and indicates the severity of the lung section perfusion situation. Comparison of LPI of repeat scans in follow up studies constitutes the pulmonary improvement factor. In order to evaluate the accuracy of these measurements, 5 cylindrical phantoms, [1000-1500] ml volume, containing Tc99m activity dispersed in simulated lung tissue (a mixture of styrofoam beds with water) were acquired under conditions similar to those of the patients´ study. Using the regression equation derived from the phantom study, volumes of the lungs´ lobes were computed. The method is simple and fast with a relative error up to 3% for length, 11% for area and 8% for volume determinations. The precision of the estimate varied, depending on the size and clinical status of lung lobe. We have developed a method, which assist to the accurate interpretation of perfusion scans by volume and semi quantitative lung perfusion index determination.