Moving segments region of interest attenuator for x-ray fluoroscopy

Region of interest (ROI) imaging has previously been proposed as a means of reducing x-ray fluoroscopy radiation dose. Previous ROI attenuators made of partially attenuating metal plates change beam quality, which may lead to uncertainty in image restoration procedures. The design and construction of a prototype moving segments ROI attenuator (MS-ROI), which maintains beam quality across the whole field of view is described. The x-ray beam intensity is halved by 36 lead segments which are rapidly rotated between the x-ray tube and patient, with a central hole projecting a circular ROI at full intensity. Image processing techniques with automatic detection of the ROI boundary were used to homogenize image brightness across the whole image. Images restored using these techniques were judged to be visually acceptable, with a good match between pixel values inside and outside the ROI. Image contrast within the ROI was improved by 18% due to reduced scatter and veiling glare from the periphery. The introduction of the MS-ROI attenuator also results in a 48% increase in statistical noise in the area outside the ROI, with no significant change in object contrast. The patient entrance dose measured using the dose area product (DAP) method was reduced by 53.4% under manual exposure control, with the dose to operators reduced by 48.4% under automatic brightness control. Further work is needed to determine whether the attenuator can be used with pulsed fluoroscopy, and to reduce vibrational effects on the ROI boundary. The MS-ROI attenuator provides a more constant ratio of central-to-peripheral image intensity, and maintains uniform beam quality and image contrast across the whole image in comparison to simple metal plate attenuators.