CT tube-voltage dependent attenuation correction scheme for PET/CT scanners

A scheme is presented for transforming Hounsfleld units (HU) measured by different CT scanners at different tube voltages to 511 keV linear attenuation values, needed for attenuation correction in PET data reconstruction. A Gammex 467 electron density CT phantom was measured using two Siemens Sensation 16 (slice) CF scanners, a GE Lightspeed CT and a Hitachi Sceptre-P3 CT at X-ray tube voltages (kVp) ranging from 80 kVp to 140 kVp. HLJ obtained for the various reference tissue substitutes in the phantom were compared with the known linear attenuation values at 511 keV. The transformation, appropriate for lungs, soft tissue, and bone, is implemented through the function 0.000095×(HU+1000) below a threshold of ∼70 HU and the function a×(HU+1000) + b above the threshold where a and b are fixed parameters that are approximately the same for the different scanners considered, but vary with kVp. To test the procedure three cylindrical inserts filled with solutions of K₂HPO₄ (to simulate bone tissue) were placed in a waterbath with an activity concentration equal to the known background. The concentrations of K₂HPO₄ in solution were 1.2 g/cc, 1.4 g/cc and 1.8 g/cc representing a range of anatomical bone densities, with HU ranging from about 300 to 1200. The use of the kVp dependent scaling compared to a fixed scaling procedure leads to a significant improvement in the reconstructed activity levels, resolving discrepancies of almost 40% for dense bone at 80 kVp. Results are also presented for patients studies involving multiple CT scans at different kVp settings, which should all lead to the same 511 keV linear attenuation values (y=x). A Linear fit to values obtained from the 120 and 80 kVp CT images using the kVp dependent scaling yielded y=0.995x with an R² of 0.99.