Paul Kinahan PhD, Presenter: Nothing to Disclose

Adam Alessio PhD, Abstract Co-Author: Nothing to Disclose

Jeffrey A Fessler, Abstract Co-Author: Nothing to Disclose

Phillip M. Cheng MD, Abstract Co-Author: Nothing to Disclose

Hubert J. Vesselle MD, PhD, Abstract Co-Author: Nothing to Disclose

Thomas K. Lewellen PhD, Abstract Co-Author: Nothing to Disclose

The presence of materials such as contrast introduces biases in PET images due to the CT-based attenuation correction (CTAC) method implemented in PET/CT scanners. We propose methods for quantitative CTAC using low-dose dual energy CT (DECT).

We evaluated a trinary classification with simulations and measured DECT data. We simulated a test object based on a human abdominal liver scan with six areas of varying FDG uptake and with iodine-based contrast on corresponding locations in the CT image. The contrast-enhanced areas are (incorrectly) scaled for CTAC as bone, using current scanner technology, and the resulting biases in the reconstructed FDG PET images are evaluated as a function of increasing levels of contrast enhancement from 20 to 500 HU. CT measurements were made at 80, 100, 120, and 140 kVp of a 20 cm diam test phantom containing water and 5 cm diam cylinders of air, dilute iodine-based contrast agent and CaCl2 in solution (bone equivalent atomic number). The resulting CT numbers were evaluated at each tube voltage.

The simulations demonstrated that current methods introduce a bias in the FDG uptake measure that increases with increasing levels of contrast enhancement. The amount of bias is a complex function of both FDG and contrast enhancement, but ranged from maximums of 9% to 87% for contrast enhancements of 20 to 500 HU, as might occur in the latter case with focal accumulations of contrast. With correct material classification there was no bias other than PET partial volume (resolution) effects. For the measured phantom data there was essentially no change in CT number for air or water with tube voltage. For the CaCl2 solution the CT numbers increased from 546 to 897 HU, while the dilute contrast agent values increased from 856 to 1721 HU as the tube voltage was dropped from 140 to 80 kVp. Starting from a standard CT scan at 120 kVp, a second (very low-dose) CT scan at 80 kVp would show a difference of ~400 HU between contrast and CaCl2 (bone equivalent).

These results demonstrate that accurate CTAC is important with non-biological materials and that an accurate classification of bone vs. contrast, and thus accurate CTAC, is possible.

Kinahan, P, Alessio, A, Fessler, J, Cheng, P, Vesselle, H, Lewellen, T, Dual-energy CT for Quantitative Attenuation Correction in PET/CT Imaging.  Radiological Society of North America 2004 Scientific Assembly and Annual Meeting, November 28 - December 3, 2004 ,Chicago IL. http://archive.rsna.org/2004/4409889.html