Computed Tomography Dose Index Measurements Research Articles

Development of In-House Head Computed Tomography Dose Index Phantoms Based on Polyester-Resin Materials

Introduction: Computed tomography dose index (CTDI) phantoms are used to optimize CT examinations in terms of image quality and the received dose. In this study, we aimed to develop cost-effective head CTDI phantoms from polyester-resin (PESR) materials as alternative phantoms. Material and Methods: The PESR was mixed with methyl ethyl ketone peroxide (MEKP) as a catalyst. The ratios of MEKP to PESR were 1:150, 1:200, 1:250, and 1:300, respectively. The phantom dimensions were designed similar to the standard CTDI phantom, i.e., length of 15 cm and diameter of 16 cm with five holes (diameter, 1.31 cm). The CTDI measurements using the PESR-MEKP phantoms were compared with the CTDI measurements using the standard polymethyl methacrylate (PMMA) phantom. Results: The results showed that the CTDI valuesof the PESR-MEKP phantoms were slightly higher (up to 6%) than the standard PMMA phantom. It was found that the CTDI measured by the PESR-MEKP phantom with a ratio of 1:300 had the least significant difference from the standard PMMA phantom; also, at this ratio, the phantom was the most homogeneous. Conclusion: The head CTDI phantoms based on PESR-MEKP materials were developed and evaluated in this study. It was found that the PR-MEKP phantom with a MEKP-to-PESR ratio of 1:300 was insignificantly different from the standard PMMA phantom. Also, the phantom was constructed easily at a more reasonable cost, compared to the standard phantom.

An improvement in automatic MTF measurement in CT images using an edge of the PMMA phantom

We have developed an improved method of modulation transfer function (MTF) measurement using an edge of the polymethyl methacrylate (PMMA) phantom which is normally positioned for computed tomography dose index (CTDI) measurement. In CTDI measurements, the peripheral holes of the PMMA phantom are set at the 3, 6, 9 and 12 o’clock positions. The upper edge of the phantom image has a hole, which precludes its use in calculating the MTF. To circumvent this, the phantom image was homogenized by replacing all pixel values inside the phantom with the median value of the phantom pixels. Subsequently, we were able to calculate the MTF by measuring the edge spread function (ESF) curve, calculate the line spread function (LSF) curve, and obtain the MTF curve by Fourier transform. The resulting MTF curve using our proposed method was then compared with the tail replacement method which had been proposed previously. Our method is superior to the tail replacement method which results in some inhomogeneity of the ESF curve in the part inside the phantom.

Technical Note: Using linear and polynomial approximations to correct IEC CTDI measurements for a wide-beam CT scanner.

To investigate a feasible correction to align the international electrotechnical commission (IEC) computed tomography dose index (CTDI) measurement with other approaches for an accurate measure of radiation output. Radiation dose measurements were performed in a GE 256-slice CT scanner using three methods. The first method used a 0.6cc Farmer chamber to measure peak dose and then to calculate dose length integral (DLI). The second method integrated dose profiles with a pencil chamber over 600mm for both body and head phantoms. Both methods achieved scatter equilibrium using a 600mm long body and head phantom. The third method followed IEC recommendations by adjusting traditional CTDI with beam width. We performed measurements using polymethyl methacrylate (PMMA) 32cm diameter body and 16cm diameter head phantoms, combining with various available bowtie filters and at different kV settings. Correction factors using linear or polynomial functions were developed based on these measurements. CTDI measurements using the DLI method and direct integration (DLP) method align with each other with an error of <6.7% for the body phantom, and 6.9% for head phantom respectively. The IEC method underestimates radiation dose for body and head phantoms relative to the DLI, with an error range from 8.9% to 19.4%, depending on the phantom and bowtie filter. A correction factor of 0.15 (15%) could be used for body and head phantom measurements with large body, head and pediatric head bowtie filters. While for body phantom with medium filter and head phantom with small body filter which are not routinely used for CTDI measurements, a correction factor of 0.30 (30%) could be used. The proposed correction factors are validated using various kV and filter combinations. Compared to a linear approximation, a polynomial correction is better at adjusting the IEC measurements, with an error of 5.2%. We found that the a1 coefficient of the polynomial correction is approximately equal to Aeq obtained from DLI measurements for all cases studied, with an average percent difference of 6.7%. Both linear and polynomial approximations can be used to correct the IEC measurements, aligning them with the direct integration of dose profiles or the point detector method of CT dosimetry on a 256 slice GE Revolution scanner. Using a polynomial correction may potentially bypass the need for an elongated phantom in the DLI method since the a1 coefficients are approximately equal to Aeq obtained from the DLI method.

Monte Carlo simulations of different CT X-ray energy spectra within CTDI phantom and the influence of its changes on radiochromic film measurements

PurposeIn this work we use Monte Carlo simulations to investigate change in Computed tomography (CT) X-ray energy spectra between exposures in air and within CT dose index (CTDI) phantom. While the results of these simulations will be relevant when measuring CTDI with any dosimeter, we apply the appropriate beam quality change correction for CTDI measurements using XR-QA2 model GafChromic™ film. MethodsDose profiles were measured with film strips, sandwiched between acrylic rods cut in half, placed within CTDI phantoms and scanned before and after irradiation with document scanner in reflective mode. Reference dosimetry system was calibrated in terms of air kerma in air, which was converted into absorbed dose using ratio of mass-energy absorption coefficients water-to-air for a given beam quality, following the AAPM TG-61 protocol. ResultsBeam qualities for all film positions within CTDI phantom show beam softening for HVLs above 6 mm Al and beam hardening for HVLs bellow 6 mm Al. Calculated CTDI values using HVL in air for all CTDI positions, and those calculated using the appropriate calibration curves based on beam quality correction show for Head CTDI phantom differences ranging from 0.3% to 2.1% and for Body CTDI phantom from 2.5% to 5.7%. ConclusionsWe describe method for CTDI measurements using radiochromic film dosimetry protocol corrected by the beam quality change within the phantom. Our results show differences in CTDI measurements of up to 5.7% when compared to using film calibration curves for beam quality in air.

Comparison of two different methods for CTDIw calculation in CBCT systems

Introduction: Dental Cone beam CT (CBCT) is an imaging modality which has emerged as a 3D imaging modality of choice for osseous structures in the maxillofacial region. With increased usage of CBCT imaging for diagnosis and treatment planning, it is imperative that stringent dose assessments must be performed for dose estimation to safeguard both the patients and operating staff. In CT scanners, CT dose index (CTDI) measurements are part of standardized quality assurance protocols that uses a 10-cm pencil ionization chamber to approximate dose output for a single axial scan. The weighted CTDI (CTDIw = 1 CTDIc + 2 CTDIp) is used for 3 3 CTDI calculations. The SEDENTEXCT project introduces a type of dose index calculation method for dental CBCT systems, that is calculating the dose measurements from the central and the peripheral positions to represent a dose index (Sedentex-DI2). In this study we utilize the same method to calculate CTDIw that is used in CT scanners and compare with the Sedentex-DI2 method as shown in: D =Dc+Dp to provide insights for developing a robust dose periphery 2 assessment in dental CBCT systems. Materials and Methods: The CTDI head phantom of PMMA with a diameter of 16 cm and 15 cm long and 10-cm cylindrical pencil-shaped air ionization chamber for CTDI measurements were used in this study. Two different CBCT machines scanned the phantom: CS9300 (Carestream Health, New York, NY), ProMax 3D mid (Planmeca Oy, Helsinki, Finland) and three FOVs was used in both of them. Scans in both of the systems were repeated another two times at the same exposure parameters to check the reproducibility of the measured dose. All dose measurements at off- axis FOVs were performed. Results: The CTDIw from SEDENTEXCT (DI2) method was 3.1245 mGy, 3.2296 mGy and 1.5447 mGy for (o8×5, o20×10 and of o4×5 cm2 FOVs, respectively) in ProMax 3D mid system. In CS9300 system these measurements were 3.104 mGy, 3.942 mGy and 6.648 Gy for FOVs of (o5×5, o10×5 and o10×10 cm2, respectively). But calculated CTDIw by standard CTDI method was 2.923 mGy, 3.308 mGy and 1.763 mGy for (o8×5, o20×10 and o4×5 cm2 FOVs, respectively) in ProMax 3D mid system and in CS9300 system these measurements were 2.969 mGy, 3.794 mGy and 6.502 Gy for FOVs of (o5×5, o10×5 and o10×10 cm2, respectively). Conclusion: The results show that the calculated CTDIw values were very close to each other in both the standard and Sedentex-DI2 methods and did not differ significantly. So, it seems that both methods can be used to measure the dose index in CBCT systems. However, it should be noted that the dose values in the peripheral holes indicate that there is no symmetric distribution for the dose in these systems, and therefore CTDI measurement methods cannot be applied to all different FOVs at different central and off-axis positions. We should introduce a valid dose index based on the dose distribution in each mode of the FOV for each system.

Dosimetric assessment of the exposure of radiotherapy patients due to cone-beam CT procedures.

Cone-beam computed tomography (CBCT) is widely used for pre-treatment verification and patient setup in image-guided radiation therapy (IGRT). CBCT imaging is employed daily and several times per patient, resulting in potentially high cumulative imaging doses to healthy tissues that surround exposed target organs. Computed tomography dose index (CTDI) is the parameter used by CBCT equipment as indication of the radiation output to patients. This study aimed to increase the knowledge on the relation between CBCT organ doses and weighted CTDI (CTDIW) for a thorax scanning protocol. A CBCT system was modelled using the Monte Carlo (MC) radiation transport program MCNPX2.7.0. Simulation results were validated against half-value layer (HVL), axial beam profile, patient skin dose (PSD) and CTDI measurements. For organ dose calculations, a male voxel phantom ("Golem") was implemented with the CBCT scanner computational model. After a successful MC model validation with measurements, a systematic comparison was performed between organ doses (and their distribution) and CTDI dosimetry concepts [CTDIW and cumulative dose quantities f100(150) and [Formula: see text]]. The results obtained show that CBCT organ doses vary between 1.2 ± 0.1mGy and 3.3 ± 0.2mGy for organs located within the primary beam. It was also verified that CTDIW allows prediction of absorbed doses to tissues at distances of about 5cm from the isocentre of the CBCT system, whereas f100(150) allows prediction of organ doses at distances of about 10cm from the isocentre, independently from its location. This study demonstrates that these dosimetric concepts are suitable methods that easily allow a good approximation of the additional CBCT imaging doses during a typical lung cancer IGRT treatment.

Parameter-based estimation of CT dose index and image quality using an in-house android™-based software

Compromise between radiation dose and image quality is essential in the use of CT imaging. CT dose index (CTDI) is currently the primary dosimetric formalisms in CT scan, while the low and high contrast resolutions are aspects indicating the image quality. This study was aimed to estimate CTDIvol and image quality measures through a range of exposure parameters variation. CTDI measurements were performed using PMMA (polymethyl methacrylate) phantom of 16 cm diameter, while the image quality test was conducted by using catphan ® 600. CTDI measurements were carried out according to IAEA TRS 457 protocol using axial scan mode, under varied parameters of tube voltage, collimation or slice thickness, and tube current. Image quality test was conducted accordingly under the same exposure parameters with CTDI measurements. An Android™ based software was also result of this study. The software was designed to estimate the value of CTDIvol with maximum difference compared to actual CTDIvol measurement of 8.97%. Image quality can also be estimated through CNR parameter with maximum difference to actual CNR measurement of 21.65%.

Use of image guided radiation therapy techniques and imaging dose measurement at Indian hospitals: A survey.

A national survey was conducted to obtain information about the use of image-guided radiotherapy (IGRT) techniques and IGRT dose measurement methods being followed at Indian radiotherapy centers. A questionnaire containing parameters relevant to use of IGRT was prepared to collect the information pertaining to (i) availability and type of IGRT delivery system, (ii) frequency of image acquisition protocol and utilization of these images for different purpose, and (iii) imaging dose measurement. The questionnaire was circulated to 75 hospitals in the country having IGRT facility, and responses of 51 centers were received. Survey results showed that among surveyed hospitals, 86% centers have IGRT facility, 78% centers have kilo voltage three-dimensional volumetric imaging. 75% of hospitals in our study do not perform computed tomography dose index measurements and 89% of centers do not perform patient dose measurements. Moreover, only 29% physicists believe IGRT dose is additional radiation burden to patient. This study has brought into focus the need to design a national protocol for IGRT dose measurement and development of indigenous tools to perform IGRT dose measurements.

The effective dose of different scanning protocols using the Sirona GALILEOS(®) comfort CBCT scanner.

To determine the effective dose and CT dose index (CTDI) for a range of imaging protocols using the Sirona GALILEOS(®) Comfort CBCT scanner (Sirona Dental Systems GmbH, Bensheim, Germany). Calibrated optically stimulated luminescence dosemeters were placed at 26 sites in the head and neck of a modified RANDO(®) phantom (The Phantom Laboratory, Greenwich, NY). Effective dose was calculated for 12 different scanning protocols. CTDI measurements were also performed to determine the dose-length product (DLP) and the ratio of effective dose to DLP for each scanning protocol. The effective dose for a full maxillomandibular scan at 42 mAs was 102 ± 1 μSv and remained unchanged with varying contrast and resolution settings. This compares with 71 μSv for a maxillary scan and 76 μSv for a mandibular scan with identical milliampere-seconds (mAs) at high contrast and resolution settings. Changes to mAs and beam collimation have a significant influence on effective dose. Effective dose and DLP vary linearly with mAs. A collimated maxillary or mandibular scan decreases effective dose by approximately 29% and 24%, respectively, as compared with a full maxillomandibular scan. Changes to contrast and resolution settings have little influence on effective dose. This study provides data for setting individualized patient exposure protocols to minimize patient dose from ionizing radiation used for diagnostic or treatment planning tasks in dentistry.

Application of the optically stimulated luminescence (OSL) technique for mouse dosimetry in micro‐CT imaging

Micro-CT is considered to be a powerful tool to investigate various models of disease on anesthetized animals. In longitudinal studies, the radiation dose delivered by the micro-CT to the same animal is a major concern as it could potentially induce spurious effects in experimental results. Optically stimulated luminescence dosimeters (OSLDs) are a relatively new kind of detector used in radiation dosimetry for medical applications. The aim of this work was to assess the dose delivered by the CT component of a micro-SPECT (single-photon emission computed tomography)∕CT camera during a typical whole-body mouse study, using commercially available OSLDs based on Al2O3:C crystals. CTDI (computed tomography dose index) was measured in micro-CT with a properly calibrated pencil ionization chamber using a rat-like phantom (60 mm in diameter) and a mouse-like phantom (30 mm in diameter). OSLDs were checked for reproducibility and linearity in the range of doses delivered by the micro-CT. Dose measurements obtained with OSLDs were compared to those of the ionization chamber to correct for the radiation quality dependence of OSLDs in the low-kV range. Doses to tissue were then investigated in phantoms and cadavers. A 30 mm diameter phantom, specifically designed to insert OSLDs, was used to assess radiation dose over a typical whole-body mouse imaging study. Eighteen healthy female BALB∕c mice weighing 27.1 ± 0.8 g (1 SD) were euthanized for small animal measurements. OLSDs were placed externally or implanted internally in nine different locations by an experienced animal technician. Five commonly used micro-CT protocols were investigated. CTDI measurements were between 78.0 ± 2.1 and 110.7 ± 3.0 mGy for the rat-like phantom and between 169.3 ± 4.6 and 203.6 ± 5.5 mGy for the mouse-like phantom. On average, the displayed CTDI at the operator console was underestimated by 1.19 for the rat-like phantom and 2.36 for the mouse-like phantom. OSLDs exhibited a reproducibility of 2.4% and good linearity was found between 60 and 450 mGy. The energy scaling factor was calculated to be between 1.80 ± 0.16 and 1.86 ± 0.16, depending on protocol used. In phantoms, mean doses to tissue over a whole-body CT examination were ranging from 186.4 ± 7.6 to 234.9 ± 7.1 mGy. In mice, mean doses to tissue in the mouse trunk (thorax, abdomen, pelvis, and flanks) were between 213.0 ± 17.0 and 251.2 ± 13.4 mGy. Skin doses (3 OSLDs) were much higher with average doses between 350.6 ± 25.3 and 432.5 ± 34.1 mGy. The dose delivered during a topogram was found to be below 10 mGy. Use of the multimouse bed of the system gave a significantly 20%-40% lower dose per animal (p < 0.05). Absorbed doses in micro-CT were found to be relatively high. In micro-SPECT∕CT imaging, the micro-CT unit is mainly used to produce a localization frame. As a result, users should pay attention to adjustable CT parameters so as to minimize the radiation dose and avoid any adverse radiation effects which may interfere with biological parameters studied.

SU‐E‐J‐41: Measurements of CTDI100 and CTDIvol for MVCT Scans On a TomoTherapy System

Purpose: To measure the computed tomography dose index (CTDI) for MVCT scans on the TomoTherapy™ System. Methods: The CTDI100 and CTDIvol were measured using a Standard Imaging A17 ion chamber inserted into a standard CTDI phantom (32 cm diameter and 16 cm diameter acrylic cylinders for body or head CTDI measurements, respectively). Dose measurements were performed using both a ‘Fine’ pitch helical scan with a scan range of ±2 cm, as well as an equivalent axial scan generated using a calibration procedure. The two methods yielded identical results, indicating the scattered radiation is well contained within the A17 ion chamber's active length. The A17 ion chamber has a non‐standard active length for CTDI measurements of 11.0 cm. However, since the scattered dose is well contained within the A17's active length, the non‐standard length can be accounted for in the CTDI calculation. Finally, the CTDI100 calculation requires a choice of a nominal axial slice thickness that is not well defined for the TomoTherapy™ System since axial scans are not an available imaging mode. A 4 mm nominal slice thickness was chosen. For the CTDIvol calculations, this choice of nominal slice thickness results in a convenient pitch of 1, 2, and 3 for the Fine, Normal, and Coarse imaging modes, respectively. Results: On the 32 cm body phantom, the center CTDI 100 was 2.18 cGy and the average of the peripheral measurements was 2.72 cGy. The volume CTDI values are calculated to be CTDIvol (Fine) = 2.54 cGy, CTDIvol (Normal) = 1.27 cGy, and CTDIvol (Coarse) = 0.85 cGy. On the 16cm head phantom, the volume CTDI values are calculated to be CTDIvol (Fine) = 3.48 cGy, CTDIvol (Normal) = 1.74 cGy, and CTDIvol (Coarse) = 1.16 cGy. Conclusion: The CTDI values described here provide a reference measured on one representative TomoTherapy™ System.

SU-D-103-05: Radiochromic Film Based System for CTDI Measurements

Purpose: We demonstrate a new method for computed tomography dose index (CTDI) measurements performed on CT scanners using XR‐QA model radiochromic film dosimetry system. Method was tested by comparing measured to CT scanner listed CTDI values for fifteen different CT scanners at various locations. Methods: Reference dosimetry system was calibrated in terms of air kerma in air. Dose profiles were measured with XR‐QA2 radiochromic film strips, sandwiched between acrylic rods cut in half and placed within CTDI phantoms. Film strips were scanned before and after irradiation with Epson Perfection V500 scanner in reflective mode. Reflectance change was measured using red color channel from TIFF images of scanned films and it was converted into air kerma in air using calibration curves. Measured air kerma in air was subsequently converted into absorbed dose to water following AAPM TG‐61 protocol. We also investigated the impact of scan length by scanning CTDI phantom 5, 10, and 15 cm longitudinally. Results: Average doses along profiles from each film piece were used to calculate weighted CTDIvol. Average difference for the first 10 scanners between calculated and listed CTDIvol was 21.8% for Head and 10.4% for Body protocol. Our study also showed that as the scanning length was increasing, the difference between measured and tabulated CTDIvol decreased. For the last 5 CT scanners, the scanning length of 5‐10 cm was changed to 15 cm for both Head and Body scanning protocols, resulting in average difference between measured and tabulated CTDIvol values of 3.8% for Head and 9.4% for Body scanning protocol. Conclusion: In contrast to ionization chamber measurement, the proposed method requires only a single exposure, does not need stem effect correction, and provides an acceptable accuracy when compared to CT scanner listed CTDIvol values. It also measures dose profiles allowing measurements of DLP and peak doses.

Measurements of the dose delivered during CT exams using AAPM Task Group Report No. 111

The computed tomography dose index (CTDI) measured with a 10 cm long pencil ionization chamber placed in a 14 cm long PMMA phantom is typically used to evaluate the doses delivered during CT procedure. For the new generation of CT scanners, the efficiency of this methodology is low because it excludes the contribution of radiation scattered beyond the 100 mm range of integration along z. The AAPM TG111 Report proposes a new measurement modality using a small volume ionization chamber positioned in a phantom long enough to establish dose equilibrium at the location of the chamber. In this work, the AAPM report was implemented. The minimum scanning length needed to obtain cumulative dose equilibrium was evaluated. The equilibrium dose was determined and compared to CTDI values informed by the CT scanner, and the dose values were confirmed with TLD measurements. The difference between doses measured with TLD and with the ionization chamber (IC) was below 1% and the repeatability of the measurements' setup was 0.4%. The measurements showed that the scanning lengths needed to reach the cumulated dose equilibrium were 450 mm and 380 mm for the central and peripheral axes, respectively, which justifies the phantom length. For the studied clinical protocols, the doses measured were about 30% higher than those informed by the CT scanner. For the new generation of CT systems with wider longitudinal detector size or cone‐beam technology, the current CTDI measurements may no longer be adequate, and the informed CTDI tends to undervalue the dose delivered. It is therefore important to evaluate CT radiation doses following the AAPM TG111 methodology.PACS number: 87.57.qp, 87.53.Bn

TU-D-217A-01: CTDI and Patient Dose: A European Perspective.

1. Understand that CTDI is a technical concept for scanner acceptance and constancy testing 2. Learn about concepts for patient- and scanner-specific patient dose estimates 3. Learn about the concept of diagnostic reference levels and its strengths and weaknesses Research sponsored by Siemens Healthcare and by CT Imaging GmbH, both in Erlangen, Germany.

Experimental assessment of the influence of beam hardening filters on image quality and patient dose in volumetric 64-slice X-ray CT scanners

Beam hardening filters have long been employed in X-ray Computed Tomography (CT) to preferentially absorb soft and low-energy X-rays having no or little contribution to image formation, thus allowing the reduction of patient dose and beam hardening artefacts. In this work, we studied the influence of additional copper (Cu) and aluminium (Al) flat filters on patient dose and image quality and seek an optimum filter thickness for the GE LightSpeed VCT 64-slice CT scanner using experimental phantom measurements. Different thicknesses of Cu and Al filters (0.5–1.6mm Cu, 0.5–4mm Al) were installed on the scanner’s collimator. A planar phantom consisting of 13 slabs of Cu having different thicknesses was designed and scanned to assess the impact of beam filtration on contrast in the intensity domain (CT detector’s output). To assess image contrast and image noise, a cylindrical phantom consisting of a polyethylene cylinder having 16 holes filled with different concentrations of K2HPO4 solution mimicking different tissue types was used. The GE performance and the standard head CT dose index (CTDI) phantoms were also used to assess image resolution characterized by the modulation transfer function (MTF) and patient dose defined by the weighted CTDI. A 100mm pencil ionization chamber was used for CTDI measurement. Finally, an optimum filter thickness was determined from an objective figure of merit (FOM) metric. The results show that the contrast is somewhat compromised with filter thickness in both the planar and cylindrical phantoms. The contrast of the K2HPO4 solutions in the cylindrical phantom was degraded by up to 10% for a 0.68mm Cu filter and 6% for a 4.14mm Al filter. It was shown that additional filters increase image noise which impaired the detectability of low density K2HPO4 solutions. It was found that with a 0.48mm Cu filter the 50% MTF value is shifted by about 0.77lp/cm compared to the case where the filter is not used. An added Cu filter with approximately 0.5mm thickness accounts for 50% reduction in radiation-absorbed dose as measured by the weighted CTDI. The FOM results indicate that with an additional filter of 0.5mm Cu or minimum 4mm Al, a good compromise between image quality and patient dose is achieved for CT images acquired at tube voltages of 120 and 140kVp. The results seem to indicate that an optimum filter for high kVp acquisitions, routinely used in cardiovascular imaging, should be 0.5mm copper or 4mm aluminium minimum.

Estimation of the weighted CTDI∞for multislice CT examinations

The aim of this study was to examine the variations of CT dose index (CTDI) efficiencies, ε(CTDI(100))=CTDI(100)/CTDI(∞), with bowtie filters and CT scanner types. This was an extension of our previous study [Li, Zhang, and Liu, Phys. Med. Biol. 56, 5789-5803 (2011)]. A validated Monte Carlo program was used to calculate ε(CTDI(100)) on a Siemens Somatom Definition scanner. The ε(CTDI(100)) dependencies on tube voltages and beam widths were tested in previous studies. The influences of different bowtie filters and CT scanner types were examined in this work. The authors tested the variations of ε(CTDI(100)) with bowtie filters on the Siemens Definition scanner. The authors also analyzed the published CTDI measurements of four independent studies on five scanners of four models from three manufacturers. On the Siemens Definition scanner, the difference in ε(CTDI(W)) between using the head and body bowtie filters was 2.5% (maximum) in the CT scans of the 32-cm phantom, and 1.7% (maximum) in the CT scans of the 16-cm phantom. Compared with CTDI(W), the weighted CTDI(∞) increased by 30.5% (on average) in the 32-cm phantom, and by 20.0% (on average) in the 16-cm phantom. These results were approximately the same for 80-140 kV and 1-40 mm beam widths (4.2% maximum deviation). The differences in ε(CTDI(100)) between the simulations and the direct measurements of four previous studies were 1.3%-5.0% at the center∕periphery of the 16-cm∕32-cm phantom (on average). Compared with CTDI(vol), the equilibrium dose for large scan lengths is 30.5% higher in the 32-cm phantom, and is 20.0% higher in the 16-cm phantom. The relative increases are practically independent of tube voltages (80-140 kV), beam widths (up to 4 cm), and the CT scanners covered in this study.

Dosimetry concepts for scanner quality assurance and tissue dose assessment in micro-CT

At present, no established methods exist for dosimetry in micro computed tomography (micro-CT). The purpose of this study was therefore to investigate practical concepts for both dosimetric scanner quality assurance and tissue dose assessment for micro-CT. The computed tomography dose index (CTDI) was adapted to micro-CT and measurements of the CTDI both free in air and in the center of cylindrical polymethyl methacrylate (PMMA) phantoms of 20 and 32 mm diameter were performed in a 6 month interval with a 100 mm pencil ionization chamber calibrated for low tube voltages. For tissue dose assessment, z-profile measurements using thermoluminescence dosimeters (TLDs) were performed and both profile and CTDI measurements were compared to Monte Carlo (MC) dose calculations to validate an existing MC tool for use in micro-CT. The consistency of MC calculations and TLD measurements was further investigated in two mice cadavers. CTDI was found to be a reproducible quantity for constancy tests on the micro-CT system under study, showing a linear dependence on tube voltage and being by definition proportional to mAs setting and z-collimation. The CTDI measured free in air showed larger systematic deviations after the 6 month interval compared to the CTDI measured in PMMA phantoms. MC calculations were found to match CTDI measurements within 3% when using x-ray spectra measured at our micro-CT installation and better than 10% when using x-ray spectra calculated from semi-empirical models. Visual inspection revealed good agreement for all z-profiles. The consistency of MC calculations and TLD measurements in mice was found to be better than 10% with a mean deviation of 4.5%. Our results show the CTDI implemented for micro-CT to be a promising candidate for dosimetric quality assurance measurements as it linearly reflects changes in tube voltage, mAs setting, and collimation used during the scan, encouraging further studies on a variety of systems. For tissue dose assessment, MC calculations offer an accurate and fast alternative to TLD measurements allowing for dose calculations specific to any geometry and scan protocol.

Automated Framework for Digital Radiation Dose Index Reporting From CT Dose Reports

Radiation exposure from CT studies has increased over the past 30 years in the United States and now constitutes approximately 50% of the radiation dose index administered in the health care setting. Tracking CT dose index (CTDI) is cumbersome because it relies on a manufacturer-generated screen capture, which contains the estimated dose index exposure for the patient. The radiation dose index information is not digital but, rather, is "burned" into the image (i.e., not in numeric form, not as part of the image header or elsewhere associated with the study), making it difficult to automatically share these data with other information systems. The purpose of the dose index reporting application (DIRA) we developed for CT is to extract the radiation dose index information from the CTDI reports to eventually perform automated quality control, promote radiation safety awareness, and provide a longitudinal record of patient-specific health care-related radiation exposure. A random selection of 518 CTDI reports were processed by the DIRA and the dose index information was extracted. CTDI reports using a standard DICOM C-STORE to the DIRA allow an automated process to compile radiation dose index and patient information in a Web-based framework using a structured query language (SQL) database. Our initial tests showed that the DIRA accurately extracted dose index information from 518 of 518 CTDI reports (100%). Because the extracted CTDI descriptor-dose-length product-is based on standard CTDI measurements obtained using fixed-size cylindric polymethylmethacrylate phantoms, preliminary studies have been performed to correct for patient size by applying correction factors derived from CTDI measurements using a range of phantom sizes from 6 to 32 cm in diameter. Our system provides a way to automatically track CTDI on existing CT scanners and does not rely on the DICOM SR Dose Index Report standard, which is available on only the newest CT scanners. A modular and vendor-independent DIRA system can be integrated with any existing CT scanner. This system greatly facilitates digital dose index reporting and makes it possible to provide a longitudinal record of the health care radiation exposure estimate in an individual patient's health record.

Patient-size-dependent radiation dose optimisation technique for abdominal CT examinations

Since patient doses from computed tomography (CT) are relatively high, risk-benefit analysis requires dose to patients and image quality be optimised. The aim of this study was to develop a patient-dependent optimisation technique that uses patient diameter to select a combination of CT scanning parameters that minimise dose delivered to patients undergoing abdominal CT examinations. The study was performed using cylindrical phantoms of diameters ranging from 16 to 40 cm in order to establish the relationship between image degradation, CT scanning techniques, patient dose and patient size from two CT scanners. These relationships were established by scanning the phantoms using standard scanning technique followed by selected combinations of scanning parameters. The image noises through phantom images were determined using region of interest software available in both scanners. The energy depositions to the X-ray detector through phantoms were determined from measurements of CT dose index in air corrected for attenuation of the phantom materials. The results demonstrate that exposure settings (milliampere seconds) could be reduced by up to 82 % for smaller phantom relative to standard milliampere seconds, while detector signal could be reduced by up to 93 % for smaller phantom relative to energy depositions required when scanned using standard scanning protocols. It was further revealed that the use of the object-specific scanning parameters on studies performed with phantom of different diameters could reduce the incident radiation to small size object by up to 86 % to obtain the same image quality required for standard adult object. In view of the earlier mentioned fact, substantial dose saving from small-sized adults and children patients undergoing abdomen CT examinations could be achieved through optimal adjustment of CT scanning technique based on the patient transverse diameter.

Pseudohelical Scan for the Dose Profile Measurements of 160-mm-Wide Cone-Beam MDCT

Our objective was to determine what the reasonable total phantom length should be for the measurements and determination of length equilibrium, specifically for the Aquilion ONE cone-beam MDCT system. Radiation dose measurements of a 160-mm-wide cone-beam-MDCT scanner and its radiation dose profile require a different approach than the traditional or conventional method using thermoluminescent dosimeters or small ionization chambers, which have been suggested by some investigators. In order to obtain the radiation dose profile of a cone-beam MDCT, two key elements must be addressed: proper instrumentation for the detection of radiation beam and inclusion of the tails of the radiation dose profile. In this study, a small (2 x 2 x 0.3 mm) solid-state detector was used to measure the dose profile, which required the introduction of a stepping motor to pull the detector through the phantom. Inclusion of the tails of the radiation dose profile meant more than one standard CT dose index phantom would be required to encompass the dose profile tails as much as practically possible. In fact, at minimum, a total of five standard CT dose index (CTDI) phantoms would be required to ensure the entire dose profile is included and detected. In the case of Toshiba Aquilion ONE MDCT with the maximum beam width of 160 mm, the phantom length that is required for the radiation dose profile measurement should be at least 750 mm, or 5 standard CTDI body phantoms. Current CTDI measurements utilizing 150 mm or 350 mm phantom lengths significantly underestimate the total dose of wide cone-beam MDCT. The measurement method outlined in this study amounts to an introduction of a new CT dose profile measurement using a pseudohelical scan.