Evaluation of various attenuation corrections in lung SPECT in healthy subjects

The effect of increasinglymore sophisticated attenuation correction methods on image homogeneityhas been studied in seven healthysubjects. The subjects underwent computed tomography(CT), single photon emission computed tomography(SPECT) and transmission computed tomography(TCT) of the thorax region in the supine position. Densitymaps were obtained from the CT and TCT studies. Attenuation corrections were performed using five different methods: (1) uniform correction using onlythe body contour; (2) TCT based corrections using the average lung density; (3) TCT based corrections using the pixel density; (4) CT based corrections using average lung density; and (5) CT based corrections using the pixel density. The isolated attenuation effects were assessed on quotient images generated by the division of images obtained using various attenuation correction methods divided by the non-uniform attenuation correction based on CT pixel density(reference method). The homogeneitywas calculated as the coefficient of variation of the quotient images (CV_att), showing the isolated attenuation effects. Values of CV _att were on average 12.8% without attenuation correction, 10.7% with the uniform correction, 8.1% using TCT map using the average lung densityvalue and 4.8% using CT and average lung density corrections. There are considerable inhomogeneities in lung SPECT slices due to the attenuation effect. After attenuation correction the remaining inhomogeneityis considerable and cannot be explained bystatistical noise and camera non-uniformityalone.