The reliability of computed tomography (CT) values and dimensional measurements of the oropharyngeal region using cone beam CT: comparison with multidetector CT.

Objectives: To evaluate the reliability of cone beam CT (CBCT) values and dimensional measurements of oropharyngeal air spaces as compared with those of multidetector row CT (MDCT).
Methods: A soft tissue equivalent phantom with different sized holes was used. The phantom was scanned using CBCT and MDCT. In addition, a volunteer was examined using both methods for clinical comparison. The CT data were retrieved to a personal computer and treated using image processing software for three-dimensional rendering and dimensional measurements. The CT values of air, water and soft tissues were measured experimentally and the CT values of air, fat and muscle were measured in a clinical case. The diameter of each hole drilled in the phantom was measured on CBCT and MDCT images using the software tool and digimatic callipers.
Results: The CBCT values were quite different from the Hounsfield units obtained with a typical MDCT system. The standard deviations were almost ten times larger with CBCT. In vivo assessment showed that the CBCT values for fat had a wide range that partially overlapped the values for muscle. The measurement of distances greater than 4 mm was consistent for all methods. The difference for holes was less than 0.2 mm.
Conclusions: The phantom study showed that CBCT provides limited quantitative CT values for each pixel on sliced images for differentiating air, water and soft tissues. However, the measurement of air spaces with CBCT was quite accurate.