Method for transforming CT images for attenuation correction in PET/CT imaging
Abstract
A tube-voltage-dependent scheme is presented for transforming Hounsfield units (HU) measured by different computed tomography (CT) scanners at different x-ray tube voltages (kVp) to linear attenuation values for attenuation correction in positron emission tomography (PET) data reconstruction. A Gammex 467 electron density CT phantom was imaged using a Siemens Sensation 16-slice CT, a Siemens Emotion 6-slice CT, a GE Lightspeed 16-slice CT, a Hitachi CXR 4-slice CT, and a Toshiba Aquilion 16-slice CT at kVp ranging from 80 to
. All of these CT scanners are also available in combination with a PET scanner as a PET/CT tomograph. HU obtained for various reference tissue substitutes in the phantom were compared with the known linear attenuation values at
. The transformation, appropriate for lung, soft tissue, and bone, yields the function
below a threshold of
and
above the threshold, where
and
are fixed parameters that depend on the kVp setting. The use of the kVp-dependent scaling procedure leads to a significant improvement in reconstructed PET activity levels in phantom measurements, resolving errors of almost 40% otherwise seen for the case of dense bone phantoms at
. Results are also presented for patient studies involving multiple CT scans at different kVp settings, which should all lead to the same
linear attenuation values. A linear fit to values obtained from
CT images using the kVp-dependent scaling plotted as a function of the corresponding values obtained from
CT images yielded
with an
value of 0.999, indicating that the same values are obtained to a high degree of accuracy.