CT Number of Air Inside and Outside Objects

12/1/2011
This research is motivated by observations that the CT number of air in the trachea is greater than -1000. Presently, 12/1/2011, I believe that this deviation from air's nominal value of -1000 is caused by scatter. I submitted an abstract for the 2012 American Thoracic Society (ATS) meeting describing measurements of air trachea CT number of COPDGene cases. Calculations were done in 6/2009 using selected COPDGene cases. (5/25/2012 Here is the ATS Poster )

The trachea air results prompted investigations measurements of CT number in air holes of several phantoms. Ultimately, a new insert for the COPDGene Phantom was developed with an acrylic annulus. The CT number of air holes in the various phantoms including the new insert were presented during a QIBA Conference Call on the 3/10/2010 QIBA Conference Call. The table below are results extracted from presentation. The hole in Standard Insert is in the foam, the others surrounded by water density material.

The results presented on 3/10/2010 suggested that CT number in air hole depended on the size of hole. I also felt the scatter process was a local effect and caused by forward scatter photon. A "haze" of increased CT number around the water and acrylic rods suggest a local scatter process. There is a haze at the edge of the ring. The haze was observed in images from various scanners and at larger distance that expected from kernel tails. Ultimately, annuli of various inside diameters (trachea test objects) were fabricated.

The trachea test objects were scanned alone. The CT number in annuli holes were calculated.

12/13/2011 Compare Trachea Air INSP-EXP
The above analysis prompted a more careful comparison of the air trachea CT number of inspiration and expiration COPDGene case. In 6/2009, I had observed the air CT number difference between inspiration and expiration. I decided, for some reason, that the small difference in trachea cross-sectional area between inspiration and expiration did not have an effect. In the ATS abstract this difference is reported. By the time I submitted the ATS abstract, I felt that difference was caused by a change in the configuration of tissues around the trachea.

The trachea air CT number is increased for smaller trachea air cross-section. This result suggests that difference in trachea air CT number is in part caused by more tissue being closer to trachea region, consequently, using the air trachea CT number to correct for lung CT number variations associated with different CT scanner models and body mass index will have to account for the variation in trachea cross-sectional area.

Figure 1 from 2012 ATS Submission. This figure was from a presentation at a research meeting in 6/2009. Original Presention 6/17/2009