Scatter estimation by convolution filtering: application to digital subtraction angiography

This dissertation describes investigation of the effects of iodine beam hardening and scatter on the response linearity of a digital fluoroscopic x-ray imaging system to iodine concentration. A DSA (digital subtraction angiography) system was investigated. An upward shift in the average x-ray beam energy - beam hardening - caused by differential x-ray absorption by the iodine contrast was investigated both in the absence and presence of scatter. Scatter includes x-ray scatter within the patient and light and electron scatter within the image detector. In the scatter-free case, a computer model and experimental measurements showed good agreement. If C, the DSA image contrast - the image signal - was fit to a power law of the form C = A(pt)/sup Y/, where pt is the iodine areal density (product of iodine density and thickness in mg/cm2), a value of Y = 0.93 gave the best fit for the scatter free case. Results from similar experiments performed in the presence of scatter gave a value of Y = 0.72 as the best fit. Thus iodine beam hardening effects were found to be secondary to the effects of scatter in producing a nonlinear response of the DSA imaging system to varying iodine concentration