Changes realized from extended bit‐depth and metal artifact reduction in CT
Authors: Glide‐Hurst C. , Chen D. , Zhong H. , Chetty I. J. 
Published: May 2013

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 cm2 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.