Changes realized from extended bit-depth and metal artifact reduction in CT.

PURPOSE: High-Z material in computed tomography (CT) yields metal artifacts that degrade image quality and may cause substantial errors in dose calculation. This study couples a metal artifact reduction (MAR) algorithm with enhanced 16-bit depth (vs standard 12-bit) to quantify potential gains in image quality and dosimetry.
METHODS: Extended CT to electron density (CT-ED) curves were derived from a tissue characterization phantom with titanium and stainless steel inserts scanned at 90-140 kVp for 12- and 16-bit reconstructions. MAR was applied to sinogram data (Brilliance BigBore CT scanner, Philips Healthcare, v.3.5). Monte Carlo simulation (MC-SIM) was performed on a simulated double hip prostheses case (Cerrobend rods embedded in a pelvic phantom) using BEAMnrc∕Dosxyz (400,000,0000 histories, 6X, 10 × 10 cm(2) beam traversing Cerrobend rod). A phantom study was also conducted using a stainless steel rod embedded in solid water, and dosimetric verification was performed with Gafchromic film analysis (absolute difference and gamma analysis, 2% dose and 2 mm distance to agreement) for plans calculated with Anisotropic Analytic Algorithm (AAA, Eclipse v11.0) to elucidate changes between 12- and 16-bit data. Three patients (bony metastases to the femur and humerus, and a prostate cancer case) with metal implants were reconstructed using both bit depths, with dose calculated using AAA and derived CT-ED curves. Planar dose distributions were assessed via matrix analyses and using gamma criteria of 2%∕2 mm.
RESULTS: For 12-bit images, CT numbers for titanium and stainless steel saturated at 3071 Hounsfield units (HU), whereas for 16-bit depth, mean CT numbers were much larger (e.g., titanium and stainless steel yielded HU of 8066.5 ± 56.6 and 13,588.5 ± 198.8 for 16-bit uncorrected scans at 120 kVp, respectively). MC-SIM was well-matched between 12- and 16-bit images except downstream of the Cerrobend rod, where 16-bit dose was ∼6.4% greater than 12-bit. Absolute film dosimetry in a region downstream of a stainless steel rod revealed that 16-bit calculated dose, with and without MAR, agreed more closely with film results (1%-2% less than film) as compared to 12-bit reconstructions (5.6%-6.5% less than film measurements). Gamma analysis revealed that 16-bit dose calculations were better matched to film results than 12-bit (∼10% higher pass rates for 16-bit). Similar results were observed in two patient cases; the largest discrepancy was observed for a femur case where 12-bit doses, both with and without MAR correction, were 6-7 Gy lower (∼17%-20% of the prescription dose) as compared to 16-bit dose calculations. However, when beams are not directly traversing metal, such as a prostate cancer case with bilateral hip prostheses; the impact of 16-bit reconstruction was diminished.
CONCLUSIONS: These results suggest that it may be desirable to implement 16-bit MAR-corrected images for treatment planning purposes, which can provide a more accurate dosimetric approach coupled with improved visualization by suppression of CT artifacts.