CT Ionization Chamber Research Articles

On the length used for CT ionization chambers to determine CTDI

Objective. Computed tomography dose index (CTDI) calculations based on measurements made with CT ionization chambers require characterization of two chamber properties: radiation sensitivity and effective length. The sensitivity of a CT ionization chamber is currently determined in some countries by calibration in an x-ray field that irradiates the entire chamber. Determination of the effective length is left to the user, and this value is frequently assumed to be equivalent to the nominal length—typically 100 mm—stated by the manufacturer. This assumption undermines the intention and usefulness of CTDI calculation. Thus, a slit-based calibration, N KL, of the CT ionization chambers was proposed by collimating the x-ray beam to a well-defined aperture width. The aim of this work is to compare the two methods. Approach. Four different CT ionization chambers (Standard Imaging Exradin A101, Radcal 10x5-3CT, Victoreen 500-100, and Capintec PC-4P) are investigated in this work. Sensitivity profiles were measured for all four chambers and effective/rated chamber lengths were calculated. A novel Monte-Carlo based correction was proposed to account for the presence of the aperture. CTDI was calculated and compared for two calibration beams as well as for a commercial CT scanner using Exradin A101 and Radcal 10x5-3CT chambers. Main results. The nominal chamber length was found to deviate up to 21% compared to the effective length. Correction for the aperture depended on the aperture opening size. CTDI calculation results illustrate the potential 17% error in CTDI calculation that can be caused by assuming the effective chamber length is equivalent to the manufacturer’s stated nominal length. CTDI calculations with CT ionization chambers calibrated with an air-kerma length calibration method yield the smallest variation in the CTDI regardless of the chamber model. Significance. To avoid an erroneous CTDI, information regarding the chamber’s effective length must be included in the calibration or stated by the manufacturer. Alternatively, a slit-based calibration can be performed.

Radiation dose measurements in intraoral and panoramic dental radiography in the Southern Region of Saudi Arabia

Objective: This study aims to estimate patient radiation doses for intraoral and panoramic dental radiography in the southern region of Saudi Arabia. Material and Methods: For intraoral units, the radiation doses were measured in terms of incident air kerma, Ki, and the detector was placed at 50 cm from the focus point. For panoramic dental radiography, the radiation doses were measured in terms of air Kerma-area product, PKA, using a calibrated CT ionization chamber. Results: For intraoral, the values for the estimated incidence of air kerma, Ki, ranged from 1.25 to 4.61 mGy (average: 2.55 mGy). For panoramic, the values for estimated Kerma-area product, PKA, values ranged from 79.5 to 122.1 mGy.cm2 (average: 99 mGy.cm2) for adults and ranged from 43.3 to 80.2 mGy.cm2 (average: 58.5mGy.cm2) for paediatrics. Conclusion: The study’s outcomes were compared with previous studies from other countries, indicating similar or slightly higher values. These results could serve as baseline data for radiation protection optimization and the development of national diagnostic reference levels in Saudi Arabia.

Evaluation of radiation dose reduction during CT scans by using bismuth oxide and nano-barium sulfate shields

The purpose of the present study was to evaluate the radiation dose reduction and the image quality during CT scanning by using a new dose reduction fiber sheet (DRFS) with commercially available bismuth shields. These DRFS, were composed of nano-barium sulfate (BaSO4) filling the gaps left by the large bismuth oxide (Bi2O3) particles. The radiation dose was measured five times at a direction of 12 o’clock from the center of the polymethyl methacrylate (PMMA) head phantom by using a CT ionization chamber to calculate an average value. The image quality of measured CT transverse images of the PMMA head phantom depended on the X-ray tube voltage and the type of shielding. Two regions of interest in the CT transverse images were chosen, one from the right area and the other from the left area under the surface of the PMMA head phantom and at a distance of ion chamber holes located in a direction of 12 o’clock from the center of the PMMA head phantom. The results of this study showed that the new DRFS shields could reduce the dosages by 15.61%, 23.05%, and 22.71% at 90 kVp, 120 kVp, and 140 kVp, respectively, than with these of a conventional bismuth shield of the same thickness while maintaining image quality. In addition, the DRFSs produced were about 25% thinness than conventional bismuth. We conclude, therefore, that a DRFS can replace conventional bismuth as a new shield.

SU‐E‐J‐19: An Intra‐Institutional Study of Cone‐Beam CT Dose for Image‐Guided Radiation Therapy

Purpose:To determine the variability of Cone‐Beam CT Dose Index (CB‐CTDI) across multiple on‐board imaging (OBI) systems within a single institution, and compare this to manufacturer provided data.Methods:The CB‐CTDI was measured on three Trilogy and three TrueBeam Varian OBI systems, for six different clinically used scan protocols. Measurements were taken using a 10 cm long CT ionization chamber in either a 16 cm (head‐simulating) or 32 cm (body‐simulating) diameter, acrylic, cylindrical, 15 cm long CTDI phantom. We assessed the variation in CB‐CTDI between the OBI systems and compared our measured values to the data provided by the manufacturer.Results:The standard error in the CB‐CTDI measured for all protocols was found to be within ±2% and ±5% of the mean for TrueBeam and Trilogy, respectively. For all head scan protocols, the measured TrueBeam values were lower than the manufacturer's reported values, with a maximum difference of 13.9% and an average difference of 11%. For the body scan protocols, the TrueBeam measured values were 3% and 13% greater than the manufacturer's reported values for two out of three protocols, and 38% lower than reported for the third protocol. In total, 7/18 CB‐CTDI measurements fell within the manufacturers specified range (±10%). Across all scans the Truebeam machines were found to have a lower CB‐CTDI than Trilogy, particularly the head scan protocols, which show decreases of up to 30% .Conclusion:The intra‐institutional variation of CB‐CTDI was found to be clinically acceptable at less than 5%. For the TrueBeam OBI system, over half of the measured scans failed to fall with in the manufactured quoted range of 10%, however, all measured values were within 15% of the manufacturer's reported values. For accurate assessment and reporting of imaging dose to radiotherapy patients, our results indicate a need for standardization in CB‐CTDI measurement technique.

Influence of difference in cross-sectional dose profile in a CTDI phantom on X-ray CT dose estimation: a Monte Carlo study

The longitudinal dose profile in a computed tomography dose index (CTDI) phantom had been studied by many researchers. The cross-sectional dose profile in the CTDI phantom, however, has not been studied. It is also important to understand the cross-sectional dose profile in the CTDI phantom for dose estimation in X-ray CT. In this study, the cross-sectional dose profile in the CTDI phantom was calculated by use of a Monte Carlo (MC) simulation method. A helical or a 320-detector-row cone-beam X-ray CT scanner was simulated. The cross-sectional dose profile in the CTDI phantom from surface to surface through the center point was calculated by MC simulation. The shape of the calculation region was a cylinder of 1-mm-diameter. The length of the cylinder was 23, 100, or 300mm to represent various CT ionization chamber lengths. Detailed analyses of the energy depositions demonstrated that the cross-sectional dose profile was different in measurement methods and phantom sizes. In this study, we also focused on the validation of the weighting factor used in weighted CTDI (CTDI w ). As it stands now, the weighting factor used in CTDI w is (1/3, 2/3) for the (central, peripheral) axes. Our results showed that an equal weighting factor, which is (1/2, 1/2) for the (central, peripheral) axes, is more suitable to estimate the average cross-sectional dose when X-ray CT dose estimation is performed.

Off-centre effect on dose reduction to anterior surfaces with organ-based tube-current modulation

Organ-based tube-current modulation (OB-TCM) is a new technique of radiation dose reduction to reduce superficial doses to eyes, thyroid, and breast. Several studies have investigated the dose reduction with OB-TCM. However, the superficial doses in case when bodies are located at off-centre positions, have not been reported. The purpose of this study was to evaluate the effect of shifting the patient's body off-centre on the reduction of dose to anterior surfaces of eyes and breast with OB-TCM. A 16-cm CT dose index phantom and a semi-anthropomorphic acrylic thorax phantom were used to represent a normal-sized adult. The effect of in-plane eye and breast shields in addition to OB-TCM on the radiation output was also studied in off-centre phantom locations using a CT ionization chamber. The superficial doses of phantoms at off-centre position of 50, 25, −25, −50, and −75 mm were measured using thermoluminescent dosimeter chips, and compared with superficial doses of phantoms located at the isocentre. The dose reduction trend is similar in the head and thoracic scanning regions, i.e. bismuth shielding alone offers less reduction than OB-TCM alone, and OB-TCM combined with bismuth shielding gives the best reduction. OB-TCM reduced by 26–28% of the anterior surface dose to the eyes, and increased by 25% the posterior surface dose. The free-in-air dose decreased as the chamber was placed away from the isocentre. OB-TCM could reduce by 22% the dose to the eyes, but gave the same dose to breast as automatic tube-current modulation. In phantoms placed at off-centre positions, OB-TCM increased by some 18% the dose to the eyes and doubled superficial dose to the breast in phantoms shifted downwards.

Dose indices in dental cone beam CT and correlation with dose–area product

In the 2011 project "Safety and efficacy of a new and emerging dental X-ray modality (SEDENTEXCT)", it was suggested that dose index (DI) and dose-area product (DAP) could be easily measured and used as diagnostic reference levels (DRLs), which would help in the management of radiation doses to patients in optimum exposure settings. Such indices could be directly related to effective dose. The purposes of this study, therefore, were to measure and calculate the DI and DAP in cone beam CT (CBCT) machines and to evaluate the correlation between the two. Dose measurements were performed on three-dimensional cone beam CT (3D-CBCT) machines [3D Accuitomo (J. Morita Mfg. Corp., Kyoto, Japan), Veraviewepocs (J. Morita Mfg. Corp.) and CS9300 (Carestream, New York, NY)] by exposing a cylindrical poly-methyl methacrylate (PMMA) phantom using a CT ionization chamber. These dose measurements were used for the calculation of Dose Indices 1 and 2, according to the methodology suggested by SEDENTEXCT. The DAP was measured using a DAP meter that was attached to the detector to cover the entire irradiated area. The DI1 ranged from 53.6 mR to 216.6 mR, the DI2 ranged from 77.1 mR to 325.0 mR and the DAP ranged from 101.1 mGy cm(2) to 457.9 mGy cm(2), depending on the machines and exposure settings. Index 2 had a better correlation with the DAP than Index 1. The DIs and DAP proposed by SEDENTEXCT may be useful for establishing DRLs for dental CBCT machines; however, further studies are necessary to determine which of these indices provide accurate dose estimates proportionally relating to the effective dose.

Characterization of a CT ionization chamber for radiation field mapping

A pencil-type ionization chamber, developed at Instituto de Pesquisas Energéticas e Nucleares (IPEN), was characterized with the objective to verify the possibility of its application in radiation field mapping procedures. The characterization tests were evaluated, and the results were satisfactory. The results obtained for the X radiation field mapping with the homemade chamber were compared with those of a PTW Farmer-type chamber (TN 30011-1). The maximum difference observed in this comparison was only 1.25%, showing good agreement.

SU‐E‐I‐72: Evaluation of CTDIw under the New Scan Modes of GE HD 750 Scanner

Purpose: To evaluate the weighted computed tomography dose index (CTDIw) under the high resolution mode of the state‐of‐the‐art GE Discovery HD750 scanner. The pencil ionization chamber measurements were cross‐validated with thermoluminescent dosimeters (TLDs).Methods: CT scans were performed on a GE Discovery CT 750 HD scanner under high resolution axial mode, using the scan parameters of 120 kVp, and 200 mA, 1 sec/rotation. Two beam collimations were used in this study: 40mm and 10mm. A Standard Imaging Exradin A101 CT ionization chamber (effective length 12.08 cm) was used to determine CTDIw. To calculate CTDIw using TLDs, the CT dose profiles at both central and peripheral positions in a CTDI head phantom were measured with arrays of TLD‐100 chips. With the measured dose profiles, integrations along the z‐direction within ± 50 mm were performed to calculate CTDI100 values. CTDIw with TLD was determined and compared with pencil chamber measurements.Results: In the studied scan modes, for 40 mm scan thickness, CTDIw was determined to be 34.37 mGy and 35.67 mGy (in terms of dose to air) from charge readings with pencil chamber and TLD‐measured CT dose profile, respectively. For 10 mm scan thickness, CTDIw was determined to be 41.97 mGy and 44.60 mGy with the pencil chamber and TLDs, respectively. Conclusions: The CTDIw determined by the TLD‐measured dose profile agreed well with pencil chamber measurements for both scan thicknesses studied. Future work will include measurements using additional scan thicknesses and under another new scan mode provided by the scanner: Gemstone Spectral Imaging (GSI) mode. Comparison with regular scan modes will be performed as well.

WE-A-201B-05: Dosimetric Characterization of a Dedicated Breast Computed Tomography Clinical Prototype System

Purpose: To determine the radiation dosimetry characteristics of a dedicated breast computed tomography (BCT) clinical prototype system. Method and Materials: The relevant parameters of the BCT system were replicated in a Monte Carlo simulation program based on the Geant4 toolkit. To model the system's x‐ray spectrum, empirical measurements of exposure under varying conditions and of the first half‐value layer were performed. To validate the Monte Carlo program, the exposures at the five insert positions of a standard head CT phantom while undergoing both a scout image and a full BCT scan were estimated, and the results compared to empirically measured values. The average glandular dose (AGD) was characterized for simulated homogeneous semi‐ellipsoidal breasts of varying sizes and glandular fractions. In all cases, the AGD was normalized to the exposure measured with a CTionization chamber in air located at the isocenter. Results: The modeled x‐ray spectrum was able to predict the empirically measured x‐ray exposures after varying thicknesses of aluminum to within 9.4%, while the exposure distribution at the inserts of the standard head CT phantom were estimated to within 11.5%. The normalized AGD to the homogeneous breasts varied from 2.51 mGy/R to 5.21 mGy/R, depending on breast size and glandular fraction. For an average breast, for which the system automatically sets the tube current at 100 mA, the AGD would be 7.55 mGy. Conclusion: The glandular dose from this BCT system was characterized for a range of breast sizes and compositions, and was found to result in similar values compared to typical multi‐view diagnostic mammographic examinations but higher values than a standard two‐view screening mammography examination. Further studies including the variation between mean glandular dose to homogeneous breasts and actual glandular dose to breasts with real tissue distribution are ongoing.

MO‐A‐M100J‐02: Performance Evaluation for New‐Technology CT Systems: 256‐Channel C

Medical physicists are challenged to assess image quality and patient dose for 256‐channel CT scanners. Whereas current concepts for measurement of image quality still remain valid for 256‐channel scanners, concepts for CT dosimetry will have to change fundamentally. Current methodology for CT dosimetry, e.g. assessment of CTDI with a 10 cm CT ionization chamber and standard CT dose phantoms, is not compatible with the x‐ray beam geometry of the new 256‐channel CT scanner. The beam width exceeds the length of the CT ionization chamber and, at the exit surface of the phantom; the beam width even exceeds the length of the body phantom. Several options to overcome dosimetric problems of cone beam CT scanners will be considered.Educational Objectives:1. Understand the design and operation concepts a new 256‐channel axial CT scanner.2. Understand concepts for specifying and measuring absorbed dose for a new 256‐channel axial CT scanner.3. Understand shortcomings in current concepts for assessment of absorbed dose and and patient dose for the new 256‐channel axial CT scanner.

SU‐FF‐I‐43: Air‐Kerma Length Calibration of CT Ionization Chambers

Purpose: To develop a slit‐based calibration method, for the quantity air‐kerma length, for CT ionization chambers which combines the radiation response and response length properties of the chamber. Method and Materials: CT ionization chamber response profiles are measured for 3 models, and the response length is calculated. Air‐kerma length calibration coefficients require a correction for radiation that is scattered from and transmitted through the aperture. The Monte Carlo transport code MCNP5 is used to generate this correction factor, Cst. A variable‐width aperture is used to test a previously published, linear‐regression based correction factor, Creg. Results: The response lengths of several CT ionization chambers are calculated based on the chamber's response profile. The response lengths vary between 9.9cm and 12.1cm, which results in errors of −20% to +1%. The Cst correction factor yields an air‐kerma length calibration coefficient that varies by 0.1% for aperture widths ranging from 5mm to 90mm. Creg yields air‐kerma length calibration coefficients that vary by 1% over the same range of slit width. Conclusion: The proper way to calibrate a CT ionization chamber is via air‐kerma length calibration, which combines the radiation response and response length in the calibration coefficient. Currently CT ionization chambers are calibrated with a large x‐ray field that exposes the entire collecting volume, which only characterizes the radiation response of the chamber. The response length is then assumed to be 10cm, which is not always true and can cause errors of up to 20% compared to using an air‐kerma length calibration coefficient.

Evaluation of the operational characteristics of a CT ionization chamber

The most common ionization chamber used in computed tomography dosimetry is the “pencil ionization chamber”. It is a special cylindrical dosimeter developed for attending computed tomography beams particularities. In this study, a Victoreen pencil ionization chamber was submitted to a set of tests for a detailed evaluation of its operational characteristics. Such as many kinds of detectors, especially field instruments, this ionization chamber had originally a preamplifier to keep it electrically more stable. In this study, the performance of the chamber was analyzed with the original preamplifier and after its removal, and the results were compared. The objective of the preamplifier removal was to enable connecting the chamber to other kinds of electrometers available in laboratories. The behavior of the pencil ionization chamber before and after the removal of the preamplifier was very similar, and the results obtained were always within the limits of international recommendations. The results obtained in both situations allow, if necessary, the preamplifier removal of the system without lack of precision in the measurements.

Radiation doses during CT fluoroscopy.

CT fluoroscopy (CTF) is a relatively new imaging modality that is particularly useful for performing complex biopsy procedures. Despite the obvious benefits, the potential exists to deliver considerable radiation doses to both the patients and medical staff. The purpose of our study was to quantify the radiation levels based upon typical clinical procedures. To assess the potential radiation risks, the patient radiation doses via the CT dose index (CTDI) method were measured during CTF for a GE Pro-Speed CT scanner using standardized head and body phantoms and a CT ionization chamber. The measurements were performed for a variety of kVp, mA, and slice thickness settings. To determine patient radiation doses, the CT kVp, mA, and total CTF scan times were recorded for various biopsy procedures. To determine the radiation doses to the hands of the radiologists, a radiation survey meter was used to measure the scattered radiation from standard phantoms. The effectiveness of various types of leaded gloves and shields were also determined. The measured CTDI values ranged from 20.4 cGy min(-1) to 63.1 cGy min(-1) of CTF. For a group of 78 patients, the clinically utilized imaging times varied from 13.0 to 407 s with an mean time of 96.6 s +/- 78.9 s (1 standard deviation). The scattered x-ray radiation at the position of the radiologists hands performing the biopsy procedures was measured to be 0.6 to 1.5 mGy min(-1). The thin leaded gloves provided a relatively minimal reduction in the scattered radiation to the hands between 11% and 44% dependent upon the kVp and the type of glove. However, floor mounted radiation shields reduced the scattered radiation levels to the body by 94% to 99%. In comparison to standard x-ray fluoroscopy, CTF employs much higher radiation dose rates due to the higher kVp, mA, and rotating geometry. It is important to minimize the radiation dose to patients and staff by limiting the imaging times, employing lower mA settings, and using appropriate radiation protection measures.