Estimating a size‐specific dose for helical head CT examinations using Monte Carlo simulation methods

Abstract

Purpose Size‐specific dose estimates (SSDE) conversion factors have been determined by AAPM Report 204 to adjust CTDIvol to account for patient size but were limited to body CT examinations. The purpose of this work was to determine conversion factors that could be used for an SSDE for helical, head CT examinations for patients of different sizes. Methods Validated Monte Carlo (MC) simulation methods were used to estimate dose to the center of the scan volume from a routine, helical head examination for a group of patient models representing a range of ages and sizes. Ten GSF/ICRP voxelized phantom models and five pediatric voxelized patient models created from CT image data were used in this study. CT scans were simulated using a Siemens multidetector row CT equivalent source model. Scan parameters were taken from the AAPM Routine Head protocols for a fixed tube current (FTC), helical protocol, and scan lengths were adapted to the anatomy of each patient model. MC simulations were performed using mesh tallies to produce voxelized dose distributions for the entire scan volume of each model. Three tally regions were investigated: (1) a small 0.6 cc volume at the center of the scan volume, (2) 0.8–1.0 cm axial slab at the center of the scan volume, and (3) the entire scan volume. Mean dose to brain parenchyma for all three regions was calculated. Mean bone dose and a mass‐weighted average dose, consisting of brain parenchyma and bone, were also calculated for the slab in the central plane and the entire scan volume. All dose measures were then normalized by CTDIvol for the 16 cm phantom (CTDIvol,16). Conversion factors were determined by calculating the relationship between normalized doses and water equivalent diameter (Dw). Results CTDIvol,16‐normalized mean brain parenchyma dose values within the 0.6 cc volume, 0.8–1.0 cm central axial slab, and the entire scan volume, when parameterized by Dw, had an exponential relationship with a coefficient of determination (R2) of 0.86, 0.84, and 0.88, respectively. There was no statistically significant difference between the conversion factors resulting from these three different tally regions. Exponential relationships between CTDIvol,16‐normalized mean bone doses had R2 values of 0.83 and 0.87 for the central slab and for the entire scan volume, respectively. CTDIvol,16‐normalized mass‐weighted average doses had R2 values of 0.39 and 0.51 for the central slab and for the entire scan volume, respectively. Conclusions Conversion factors that describe the exponential relationship between CTDIvol,16‐normalized mean brain dose and a size metric (Dw) for helical head CT examinations have been reported for two different interpretations of the center of the scan volume. These dose descriptors have been extended to describe the dose to bone in the center of the scan volume as well as a mass‐weighted average dose to brain and bone. These may be used, when combined with other efforts, to develop an SSDE dose coefficients for routine, helical head CT examinations.

A Preliminary Study of Personalized Head CT Scan in Pediatric Patients

“…Organs-specific (brain, eye lenses, and salivary glands) radiation doses data from MSCT examinations were collected and downloaded from Radimetrics Enterprise Platform (REP, Bayer HealthCare, Whippany, New Jersey, USA) for analysis, which was based on Monte-Carlo-Simulation used to analyze the effective radiation doses of specific organs. 16 , 17 The total organ dose was first calculated for each slice using the CTDI voli at that slice and then was summed over all slices into the scan region:…”

“…A personalized non-enhanced head CT examination is required for pediatric patients in clinical practice; however, an effective tool or biomarker to guide the low-dose head CT protocol has not yet been established. 16 - 17 In this study, we used head circumference (HC) as an index to determine the tube current-time product of the head CT scan. The specific organs including the brain, eye lenses, and salivary glands, were employed to evaluate the radiation exposure.…”

Reference dataset for benchmarking fetal doses derived from Monte Carlo simulations of CT exams

“…The versatility afforded by most MC engines allows for a vast assortment of clinical situations to be performed as an attractive, reliable alternative to physical measurements involving patients (or cadavers). [7][8][9][10][11] Many MCbased methods are commercially available through dose management software packages and are widely used clinically. In addition, the reduction of cost of computational power and the increase in the availability of high-performance computing assures that the MC simulation approach will remain an important facet of CT dosimetry for the foreseeable future.…”

“…The transportation of ionizing radiation is simulated through a virtual representation of patient anatomy and CT scanner output. The versatility afforded by most MC engines allows for a vast assortment of clinical situations to be performed as an attractive, reliable alternative to physical measurements involving patients (or cadavers) 7–11 . Many MC‐based methods are commercially available through dose management software packages and are widely used clinically.…”

A Preliminary Study of Personalized Head CT Examination in Pediatric

“…Thus, reducing radiation doses from pediatrics' brain CT under optimal image quality has been a hot research topic [10,11]. Personalized brain CT examination protocol is required in clinical practice but an effective index factor to guide the brain CT protocol has not been established [12]. CTDI vol is not a direct measurement or estimate of patient absorbed radiation doses.…”