Dynamic iterative beam hardening correction (DIBHC) for an optimized assessment of cardiac perfusion in ECG-correlated CT

In cardiac perfusion examinations large concentrations of iodine in the ventricle cause beam hardening artifacts that lead to incorrect perfusion parameters. Beam hardening corrections are either implemented as simple precorrections which cannot account for higher order beam hardening effects, or as iterative approaches that are based on segmenting the original image into material distribution images. Conventional segmentation algorithms fail to clearly distinguish between iodine and bone. Our new algorithm, DIBHC, calculates the time-dependent iodine distribution by analyzing the voxel changes of a cardiac perfusion examination (typically N ¿ 30 ECG- correlated scans distributed over a total scan time T ¿ 20 s). These voxel dynamics are due to changes in contrast agent. This prior information allows to precisely distinguish between bone and iodine and is key to DIBHC where each iteration consists of a multi-material (soft tissue, bone, iodine) polychromatic forward projection, a raw data comparison and a filtered backprojection. Simulations with a semi-anthropomorphic dynamic phantom and clinical scans using a dual source CT scanner (2 × 128 slices, 100 kV, 160 mAs, 0.28 s) have been carried out. The uncorrected images suffer from beam hardening artifacts that appear as dark bands connecting large concentrations of iodine in the ventricle and bony structures. The CT-values of the affected tissue are typically underestimated by up to 20 HU. One iteration of DIBHC greatly reduces these artifacts yielding CT-value deviations of only 1 HU for the simulations and improvements of up to 56 HU for the measurements. DIBHC greatly reduces the beam hardening artifacts induced by the contrast agent dynamics (and those due to bone) now allowing for an improved calculation of perfusion parameters that are essential for quantifying myocardial perfusion.