Method for transforming CT images for attenuation correction in PET/CT imaging

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.