Miller, Daniel L.

Abstract The purpose of this work was to assess technical performance of a prototype high-ratio (r29), 80 line cm −1 grid for imaging conditions which mimic those for adult cardiovascular angiography. The standard equipment r15, 80 line cm −1 grid was used as a reference. Plastic Water ® LR phantoms with thickness in the range 20–44 cm were used to simulate adult patient attenuation and scatter. Grids were tested using x-ray field of view 20 and 25 cm and x-ray source to detector distance (SID) 107 and 120 cm. The primary transmission fraction ( T P ) was measured using both narrow beam geometry and a lead beam stop (BS) technique. Scatter transmission ( T S ) was measured with the lead BS technique. The quantum signal to noise ratio improvement factor ( K SNR ) was used to describe relative grid performance. The experimental conditions required revised theory to assess grid performance. Theory to account for the detector glare and underestimation of scatter intensity by the lead BS method was developed. Also, novel K SNR theory was developed to allow direct comparison of two grids operated at different SID. Mean T P was modestly lower for the r29 versus r15 grid (0.69 versus 0.75). When tested under equivalent scatter condition, T S of the r29 grid was approximately ½ that of the r15 grid (0.18 versus 0.34). K SNR of the r29 grid at SID 120 cm compared to the r15 grid at SID 107 cm increased linearly with phantom thickness (range 1.0 to ∼1.16). Findings of this work indicate that the r29 grid used at SID 120 cm is expected to provide improved image quality (or reduced patient radiation dose) when compared to the r15 grid used at SID 107 cm for adult cardiovascular patients and that the potential benefit of the r29 grid increases with patient thickness >20 cm.

PurposeWhile scatter from the patient is assumed to be the primary source of occupational radiation dose associated with fluoroscopically guided interventional procedures, the potential contribution of scatter from the x‐ray collimator assembly is unknown. The purpose of this work was to survey clinical x‐ray angiography systems to assess the potential contribution of collimator assembly scatter on occupational radiation dose.MethodsExperimental methods were designed to measure the relative contributions of scatter originating from within the collimator assembly of the x‐ray tube to total scatter, which included scatter from a patient‐simulating phantom. Measurements were acquired as a function of lateral distance from the x‐ray beam center using a posterior anterior (PA) projection and at a fixed location for variable right anterior oblique to left anterior oblique projections in the range −90º to 90º. For one system, the collimator assembly was partially disassembled to assess the scatter contribution of individual components. For two systems, 0.5 mm Pb was added to the inner surface of the collimator assembly cover and tested for efficacy to block collimator assembly scatter.ResultsConsidering all x‐ray systems and only the PA projection, collimator assembly scatter contributed 20–50% to total scatter. For x‐ray projection angles of −90º to 90º, the relative contribution of collimator assembly to total scatter was dependent on projection angle and ranged from 5% to 56%. X‐ray systems with kerma‐area product meters demonstrated higher collimator assembly scatter than those without. Considering all projection angles, the addition of 0.5 mm Pb to the inside of the collimator assembly cover reduced collimator assembly scatter from 28% to 16% of total scatter for both systems.ConclusionFindings from this work suggest that contemporary radiation safety practices and guidelines should be revised to account for scatter originating from the collimator assembly of angiographic x‐ray tubes.