Influence of Air Gap and Antiscatter Grid on Pediatric Fluoroscopy and Angiography Image Quality
Abstract
Purpose
X-ray angiography systems are used to diagnose and treat pediatric congenital heart disease. Among the substantial variability in this patient population is that patient weight ranges from <1 kg premature infants to large adolescents. Given that the X-ray scatter to primary ratio decreases from large to small patients, the efficacy of an air gap or anti-scatter grid to mitigate scatter remains uncertain. The primary aim of this work was to assess the relative dose efficiency of an air gap versus a grid for pediatric patients and suggest a patient weight below which the anti-scatter grid should not be used.
Methods
Water-equivalent phantoms with thickness 6-24 cm were used with embedded test objects positioned at the X-ray system isocenter. The X-ray technique was manually set for each phantom thickness (0.4 mm focal spot, 56 kV, 3.4 ms, 0.3 mm Cu, phantom-specific mA). The air gap was modified by setting the X-ray source to detector distance (SID) to 107 and 120 cm. Test object SNR was measured with and without the OEM ratio 15:1 grid (r15). Analytical methods estimated pixel-specific snr2 contributions to test object SNR2, thus providing for the use of iodine disks and spatially complex pediatric implantable devices as test objects. Results were referenced to real patient effective thickness and weight to establish a patient weight below which the anti-scatter grid should not be used.
Results
For all phantom thicknesses, test object SNR was highest using a 120 cm air gap, non-grid technique and lowest when the anti-scatter grid was used (independent of SID). The 24 cm phantom thickness was consistent with effective thickness of ~60 kg patients.
Conclusion
For less than ~60 kg patients, the radiation dose efficiency of fluoroscopy and angiography imaging can be maximized by the use of a long air gap, non-grid imaging technique.