Estimation of mean lung attenuation for use in generating PET attenuation maps

In PET/MR the attenuation map required for PET reconstruction is obtained by segmenting MR images and assigning linear attenuation coefficients (LAC) to the segmented regions. Some tissues such as brain, muscle and fat have relatively uniform attenuation coefficients but the lung attenuation varies within and between patients. The variation in mean lung density was measured in a group of patients and found to correspond to LAC in the range from 0.02cm^-1 to 0.04cm^-1. The effect of this range of densities was measured on the PET reconstruction and was found to produce a substantial range of PET activities in the lung. It is proposed that a better estimate of lung density could be obtained from measuring the relative size of the lung. The feasibility of this is assessed by correlating lung volume with density in a group of patients.