Comparison of hexagonal-hole and square-hole collimation by Monte Carlo simulation

For Monte Carlo simulations, fast calculation of the distance travelled by a photon in the septal material of parallel-hole collimators can be facilitated by using a square-hole design. Square holes can provide the same geometric efficiency, resolution and lead content (g/cm²) as a comparable hex-hole collimator by appropriate scaling of the hex-hole septal width and hole size. The authors´ objective was to study the validity of using square holes to estimate the hex-hole collimator response. The geometric, penetration and collimator scatter components were compared for low-energy (LE) and medium-energy (ME) designs, using square- and hex-holes and a 320 keV point source in air. The resolution (FWHM) was virtually identical for both hole shapes. The amount of penetration, and to a lesser extent collimator scatter, depended upon hole shape and pattern. However, for a 20% photopeak image, the error in the total square-hole estimate relative to the hex-hole was ~9% and ~3% for the LE and ME collimators, respectively. When the two-dimensional penetration artifact is not critical and a small error in the amount of penetration is acceptable (e.g., when using energy-appropriate collimators), square-holes can be substituted for hex-holes to decrease the time required for simulation