Rika Baba MS, Presenter: Nothing to Disclose

Ken Ueda MS, Abstract Co-Author: Nothing to Disclose

Masakazu Okabe, Abstract Co-Author: Nothing to Disclose

Mariko Takahashi DDS, Abstract Co-Author: Nothing to Disclose

Haruhisa Nakano DDS, PhD, Abstract Co-Author: Nothing to Disclose

Koutaro Maki DDS, PhD, Abstract Co-Author: Nothing to Disclose

CT images using cone-beam X-ray and an area detector have non-uniformity caused by scattered X-rays. X-ray scattering decreases the accuracy of CT value and increases artifacts in the reconstructed images. Our aim was to develop an accurate and high-speed method to correct the scattered X-rays and acquire high-quality CT images.

We developed a new correction method for scattered X-rays. First, we formulated the approximate scatter component, S, in a measured transmittance image, as a function of the mean value, T, of the measured image, and the scatter parameter, k, which is defined as the ratio of scatter generation and attenuation coefficient. The k value is peculiar to imaging apparatus and geometry. It depends on the irradiation area and characteristics of the X-ray grid for scatter reduction and should be determined experimentally. Next, we measured transmittance images of our CT apparatus equipped with a flat panel detector, for various sizes of water phantoms and various collimator gaps, which restricted the cone-beam angle of the irradiated X-rays. Scatter component S was determined as a difference in the measured transmittance and that with a very narrow collimator gap. Finally, we acquired a k value for each collimator gap as the best fitting parameter for a T versus S plot. At the scatter correction stage, S was calculated using the k and T values.

Without the correction, pixel values of the reconstructed images obtained with the cone-beam CT decreased as the phantom size increased due to scattered X-rays. Non-uniformity appeared in the water phantom images: the pixel value at the central region decreased more than at peripheral regions. With the correction for a homogeneous water phantom, the pixel value was almost recovered. For an inhomogeneous head phantom, the typical absolute error value from the standard multi-slice CT value at the jawbone was improved from 27 to 8 percent by the correction.

The developed method is very effective for correcting non-uniformity due to scattered X-rays. It enables us to obtain high-quality reconstructed images with a more accurate CT value and few artifacts in cone-beam CT systems.

This scattered X-ray correction method improves the accuracy of the CT value in cone-beam CT using an area detector, and makes it possible to obtain high-quality CT images of a human head and abdomen.

Baba, R, Ueda, K, Okabe, M, Takahashi, M, Nakano, H, Maki, K, Scattered X-ray Correction Method for Cone-beam CT.  Radiological Society of North America 2008 Scientific Assembly and Annual Meeting, February 18 - February 20, 2008 ,Chicago IL. http://archive.rsna.org/2008/6011145.html