Comprehensive Methodology for the Evaluation of Radiation Dose in X-Ray Computed Tomography

Dosimetry in kilovoltage cone beam computed tomography (CBCT) is a challenge due to the limitation of physical measurements. To address this, we used a Monte Carlo (MC) method to estimate the CT dose index (CTDI) and the dose length product (DLP) for a commercial CBCT system. As Dixon and Boone (1) showed that CTDI concept can be applicable to both CBCT and conventional CT, we evaluated weighted CT dose index (CTDIw) and DLP for a commercial CBCT system. Two extended CT phantoms were created in our BEAMnrc/EGSnrc MC system. Before the simulations, the beam collimation of a Varian On‐Board Imager (OBI) system was measured with radiochromic films (model: XR‐QA). The MC model of the OBI X‐ray tube, validated in a previous study, was used to acquire the phase space files of the full‐fan and half‐fan cone beams. Then, DOSXYZnrc user code simulated a total of 20 CBCT scans for the nominal beam widths from 1 cm to 10 cm. After the simulations, CBCT dose profiles at center and peripheral locations were extracted and integrated (dose profile integral, DPI) to calculate the CTDI per each beam width. The weighted cone‐beam CTDI false(CTDInormalw,normallfalse) was calculated from DPI values and mean CTDInormalw,normall.3emfalse(CTDIw,lfalse)¯ and DLP were derived. We also evaluated the differences of CTDIw values between MC simulations and point dose measurements using standard CT phantoms. In results, it was found that CTDInormalw,600¯ was 8.74±0.01 cGy for head and CTDInormalw,900¯ was 4.26±0.01 cGy for body scan. The DLP was found to be proportional to the beam collimation. We also found that the point dose measurements with standard CT phantoms can estimate the CTDI within 3% difference compared to the full integrated CTDI from the MC method. This study showed the usability of CTDI as a dose index and DLP as a total dose descriptor in CBCT scans.PACS number: 87.57.uq

“…The

{CTDI}_{normala, 600}

for head scan and

{CTDI}_{normala, 900}

{CTDI}_{normalw, normall}

{CTDI}_{normalw, normall}

values of various beam widths (1–10 cm nominal beam width).…”

Section: Methodsmentioning

confidence: 99%

Computed tomography dose index and dose length product for cone‐beam CT: Monte Carlo simulations of a commercial system

Kim

Song

Samei

et al. 2011

“…At present,

{CTDI}_{vol}

is still used to account for helical scanning parameters in clinical setting. The “dose equilibrium” proposed by AAPM is yet to be accepted to replace the traditional CTDI dosimetry approach (1,4) . In this study, it is shown that the excess percentage radiation doses decreased as a function of (increasing) nominal gantry rotation time.…”

Section: Discussionmentioning

confidence: 80%

“…Because of the continued growth in CT utilization in recent years, an increased emphasis has been placed on the importance of quality assurance/control and patient radiation dose assessment during CT examinations (9) . To design appropriate scanning protocols, it is important to understand not only the variable settings, but also the methodology to assess accurate radiation dose (1,4,10) …”

Section: Discussionmentioning

confidence: 99%

“…This is because the 100 mm pencil chamber dosimeter is simply too short to detect contributions from the primary and scattered radiation beyond the chamber range along z‐axis (3) . American Association of Physicists in Medicine (AAPM) published a report of comprehensive methodology for the evaluation of radiation dose in CT, and suggested measurement of “dose equilibrium” in place of CTDI which can be used for axial, helical, and cone‐beam scanning with or without table translation (4) …”

Section: Introductionmentioning

confidence: 99%

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Evaluation of gantry rotation overrun in axial CT scanning

Fukuda

Lin

Matsubara

et al. 2014

The purpose of this study was to develop and evaluate a simple method to assess gantry rotation overrun in a single axial CT scanning. The exposure time in the axial scanning was measured at selected nominal rotation times (400, 700, and 1000 ms) using a solid‐state detector, the RTI's CT dose profiler (CTDP). CTDP was placed at the isocenter and the radiation dose rate signal (profile) was recorded. Subsequently, the full width of this profile was determined as the exposure time (Taxial). Next, CTDP was positioned on the inner cover of the gantry with a sheet of lead (1 mm thick) placed on top of the detector. Gantry rotation time (Thelical) was determined by the time between two successive radiation peaks during continuous helical scanning. The gantry overrun time (Toverrun) is, thus, determined as Taxial‐Thelical. The exposure times in the axial scanning, Taxial, obtained with CTDP for nominal rotation times of 400, 700, and 1000 ms were 409.5, 709.6, and 1008.7 ms, respectively. On the other hand, the measured gantry rotation times, Thelical, were 400.0, 700.3, and 999.8 ms, respectively. Therefore, the overruns were 9.5, 9.3, and 8.9 ms for nominal rotation times of 400, 700, and 1000 ms, respectively. The evaluation of overrun in axial scanning can be accomplished with the measurements of both the exposure time in axial scanning and the gantry rotation time. It is also noteworthy that in this context, overrun implies overexposure in axial scanning, which is still used, particularly, in head CT examination.PACS number: 87.57.Q‐

“…However, in modern multislice CT scanners, the pencil chamber can underestimate the equilibrium dose and the dose line integral by 20%, as demonstrated when tested on body phantoms (8) . The pencil chamber method of determining

{CTDI}_{100}

is therefore not adequate for multislice scanners (9) such as CBCT.…”

Section: Introductionmentioning

confidence: 99%

A methodology for on‐board CBCT imaging dose using optically stimulated luminescence detectors

Mail

Yusuf

Alothmany

et al. 2016

Cone‐beam computed tomography CBCT systems are used in radiation therapy for patient alignment and positioning. The CBCT imaging procedure for patient setup adds substantial radiation dose to patient's normal tissue. This study presents a complete procedure for the CBCT dosimetry using the InLight optically‐stimulated‐luminescence (OSL) nanoDots. We report five dose parameters: the mean slice dose (normalDMSD); the cone beam dose index (CBDIW); the mean volume dose (normalDMVD); point‐dose profile, D(FOV); and the off‐field Dose. In addition, CBCT skin doses for seven pelvic tumor patients are reported. CBCT‐dose measurement was performed on a custom‐made cylindrical acrylic body phantom (50 cm length, 32 cm diameter). We machined 25 circular disks (2 cm thick) with grooves and holes to hold OSL‐nanoDots. OSLs that showed similar sensitivities were selected and calibrated against a Farmer‐type ionization‐chamber (0.6 CT) before being inserted into the grooves and holes. For the phantom scan, a standard CBCT‐imaging protocol (pelvic sites: 125 kVp, 80 mA and 25 ms) was used. Five dose parameters were quantified: normalDMSD, CBDIW, normalDMVD, D(FOV), and the off‐field dose. The normalDMSD for the central slice was 31.1±0.85 mGy, and CBDIW was 34.5±0.6 mGy at 16 cm FOV. The normalDMVD was 25.6±1.1 mGy. The off‐field dose was 10.5 mGy. For patients, the anterior and lateral skin doses attributable to CBCT imaging were 39.04±4.4 and 27.1±1.3 mGy, respectively.OSL nanoDots were convenient to use in measuring CBCT dose. The method of selecting the nanoDots greatly reduced uncertainty in the OSL measurements. Our detailed calibration procedure and CBCT dose measurements and calculations could prove useful in developing OSL routines for CBCT quality assessment, which in turn gives them the property of high spatial resolution, meaning that they have the potential for measurement of dose in regions of severe dose‐gradients.PACS number(s): 87.57.‐s, 87.57.Q, 87.57.uq