There are reference values for different lung volumes based on age, weight, BMI, BSA, etc., e.g., Garcia-Rio F et al (2009), doi:10.1164/rccm.200901-0127oc. Are there equivalent methods that would allow CT-based estimates of reference values? For example, a CT would allow determining the gas volume of a patient at an end-expiratory breath-hold. But, that volume is different from FRC the patient would have if healthy and in an upright position. It would be ideal if the CT could be used for an anatomical measurement that would allow estimating the reference volume, e.g., FRC for that patient and the deviation of gas volume in the CT from the reference value.
That is an interesting question - can I ask what your end goal would be with that information? We have measured lung volumes with our 19F MRI based approaches, and have measured lung function parameters in CF patients. However, issues such as effort, supine vs. standing, etc. will all cause discrepancies in those measures of course.
Yes, you describe the problem very accurately: the gas volume at the time of imaging is not necessarily at a well-defined status, e.g., the functional residual capacity (FRC) at passive equilibrium between alveolar and atmospheric pressure. Additionally, body position affects regional end-expiratory gas volume in the lungs. So, I'm wondering if the FRC (in a standard upright position) as a measure of 'lung size' is strongly correlated with an anatomical feature related to lung size, perhaps, an anatomical parameter of the rips or the sternum so that this parameter could provide a reliable reference for normalization.
My guess is that this hasn't been done, but I have not looked. So, you may need to do it yourself - one option would be to take a lot of CTs that are taken at a known inspiration and start making measurements? Clinically, only the inspiration/expiration CTs are usually standardized, but local practice may differ. Unfortunately you probably need a few hundred CTs (sex / age / performance status) to get at an accuracy that is hopefully better than the reference values you already mentioned. Happy to discuss offline if you need any other suggestions.
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