Determination Of Air Kerma Research Articles

Improved air kerma determination in the radiation field of the X-ray tube used in medical imaging systems, considering the type and thickness of the filter

In diagnostic radiology, the air kerma is an essential parameter. Radiologists consider the air kerma, when calculating organ doses and dangers to patients. The intensity of the radiation beam is represented by the air kerma, which is the value of energy wasted by a photon as it travels through air. Because of the heel effect in X-ray sources, air kerma varies throughout the field of medical imaging systems. One possible contributor to this discrepancy is the X-ray tube's voltage. In this study, an approach has been proposed for predicting the air kerma anywhere inside the field of X-ray beams utilized in medical diagnostic imaging systems. As a first step, a diagnostic imaging system was modelled using the Monte Carlo N-Particle platform. We used a tungsten target and aluminum and beryllium filters of varying thicknesses to recreate the X-ray tube. The air kerma has been measured in different parts of the conical X-ray beam that is working at 30, 50, 70, 90, 110, 130, and 150 kV. This gives enough data for training neural networks. The voltage of the X-ray tube, filter type, filter thickness, and the coordinates of each point used to calculate the air kerma were all inputs to the MLP neural network. The MLP architecture, known for its significant advancements in research and expanding applications, was trained to predict the quantity of air kerma as its output. Specifically, by considering X-ray tube filters of varying thicknesses, the trained MLP model demonstrated its capability to accurately predict the air kerma at every point within the X-ray field for a range of X-ray tube voltages typically used in medical diagnostic radiography (30–150 kV).

Integration of kerma-area product and cumulative air kerma determination into a skin dose tracking system for fluoroscopic imaging procedures.

The skin dose tracking system (DTS) that we developed provides a color-coded mapping of the cumulative skin dose distribution on a 3D graphic of the patient during fluoroscopic procedures in real time. The DTS has now been modified to also calculate the kerma area product (KAP) and cumulative air kerma (CAK) for fluoroscopic interventions using data obtained in real-time from the digital bus on a Toshiba Infinix system. KAP is the integral of air kerma over the beam area and is typically measured with a large-area transmission ionization chamber incorporated into the collimator assembly. In this software, KAP is automatically determined for each x-ray pulse as the product of the air kerma/ mAs from a calibration file for the given kVp and beam filtration times the mAs per pulse times the length and width of the beam times a field nonuniformity correction factor. Field nonuniformity is primarily the result of the heel effect and the correction factor was determined from the beam profile measured using radio-chromic film. Dividing the KAP by the beam area at the interventional reference point provides the area-averaged CAK. The KAP and CAK per x-ray pulse are summed after each pulse to obtain the total procedure values in real-time. The calculated KAP and CAK were compared to the values displayed by the fluoroscopy machine with excellent agreement. The DTS now is able to automatically calculate both KAP and CAK without the need for measurement by an add-on transmission ionization chamber.

Practical method for determination of air kerma by use of an ionization chamber toward construction of a secondary X-ray field to be used in clinical examination rooms.

We propose a new practical method for the construction of an accurate secondary X-ray field using medical diagnostic X-ray equipment. For accurate measurement of the air kerma of an X-ray field, it is important to reduce and evaluate the contamination rate of scattered X-rays. To determine the rate quantitatively, we performed the following studies. First, we developed a shield box in which an ionization chamber could be set at an inner of the box to prevent detection of the X-rays scattered from the air. In addition, we made collimator plates which were placed near the X-ray source for estimation of the contamination rate by scattered X-rays from the movable diaphragm which is a component of the X-ray equipment. Then, we measured the exposure dose while changing the collimator plates, which had diameters of 25-90mm(ϕ). The ideal value of the exposure dose was derived mathematically by extrapolation to 0mm(ϕ). Tube voltages ranged from 40 to 130kV. Under these irradiation conditions, we analyzed the contamination rate by the scattered X-rays. We found that the contamination rates were less than 1.7 and 2.3%, caused by air and the movable diaphragm, respectively. The extrapolated value of the exposure dose has been determined to have an uncertainty of 0.7%. The ionization chamber used in this study was calibrated with an accuracy of 5%. Using this kind of ionization chamber, we can construct a secondary X-ray field with an uncertainty of 5%.

Development and characterization of a graphite-walled ionization chamber as a reference dosimeter for 60Co beams

A graphite-walled ionization chamber with a sensitive volume of 6.4cm3 was developed at the Calibration Laboratory of IPEN (LCI) to determine the air kerma rate of a 60Co source. This new prototype was developed to be a simple chamber, without significant nongraphite components and with a simple set-up, which allows the determination of its various required correction factors by Monte Carlo simulations. This new ionization chamber was characterized according to the IEC 60731 standard, and all results were obtained within its limits. Furthermore, Monte Carlo simulations were undertaken to obtain the correction factors involved with the air kerma determination. The air kerma rate obtained with the graphite-walled ionization chamber was compared with that from the reference dosimeter at the LCI, a PTW ionization chamber (model TN30002). The results obtained showed good agreement within the statistical uncertainties.

Calculation of photon scattering and transmission correction factors for a free air ionization chamber at Nuclear Science and Technology Research Institute in Iran

In primary standard dosimetry laboratories, the air kerma strength of low energy brachytherapy sources 125I and 103Pd is measured by standard free air ionization chambers. At present, one of these free air ionization chambers is constructed in Radiation Application Research School, in Nuclear Science and Technology Research Institute in Iran. The transmission and scattering correction factors are needed for the absolute determination of air kerma strength.Transmission and scattering correction factors were calculated for a monoenergetic point isotropic source with energies from 5keV to 100keV for ionization chamber with 4cm and 6cm aperture radii by using the MCNP 4C code. The calculated scattering correction factors for 125I and 103Pd brachytherapy sources are 0.9987 and 0.9992, respectively and determined transmission correction is one for our free air ionization chamber of aperture radius 4cm.

SU‐EE‐A2‐04: Traceability to National Measurement Standards for Electronic Brachytherapy Calibrations

Purpose: To establish a primary air‐kerma standard for the Xoft miniature x‐ray brachytherapy source. Method and Materials: A new calibration facility has been designed at the National Institute of Standards and Technology (NIST) to directly realize air kerma from electronic brachytherapy sources operated up to 50 kV. The previously established primary x‐ray standard, the Lamperti free‐air ionization chamber, has been dedicated for this measurement capability. The spectrum emergent from the source is measured with a high‐purity germanium spectrometer. To account for possible anisotropy of emissions from the source, both the free‐air chamber and the spectrometer are rotated about the long axis of the source. Well‐chamber measurements are also performed to study the variability in the ratio of well‐chamber current to air‐kerma rate in anticipation of developing a method for transferring the NIST standard to secondary calibration laboratories and/or therapy clinics. Results: Comparisons of the air‐kerma rate realized by two NIST x‐ray primary standards, the Lamperti free‐air chamber (10 kV to 60 kV) and the Ritz free‐air chamber (20 kV to 100 kV), were in agreement to better than 1% as is typical for well‐established primary x‐ray standards. This comparison validated the use of the Lamperti chamber in establishing the measurement techniques required for the determination of air kerma. Conclusion: NIST has designed, constructed, and validated the experimental apparatus necessary for the measurement of air‐kerma from an x‐ray brachytherapy source using a primary standard. NIST acknowledges the support of Xoft, Inc., in this project.

Free-air ionization chambers

This paper reviews the current status of free-air ionization chamber standards for the determination of air kerma in low- and medium-energy x-rays. It describes the underlying definitions and concepts that permit the air kerma to be determined from a measurement of the charge produced in a free-air chamber. The most commonly used design, the parallel-plate free-air chamber, is discussed in detail, in particular with a view to optimization of the design parameters to minimize the overall uncertainty of the air-kerma determination. Various correction factors and their uncertainties are discussed, with an emphasis on the use of the Monte Carlo technique in the evaluation and optimization of certain corrections. Alternative free-air chamber designs are outlined. The results of international comparisons of free-air chamber standards are presented.

Changes to the VNIIM air kerma primary standard

The result of extensive Monte Carlo calculations and a re-evaluation of the volume of the primary standard has led to an increase in the determination of air kerma at the VNIIM by 9.6 × 10−3. The contributions to this change are summarized in this short note to announce that the VNIIM primary standard changed with effect from 1 November 2007.

Evidence for using Monte Carlo calculated wall attenuation and scatter correction factors for three styles of graphite-walled ion chamber

The basic equation for establishing a 60Co air-kerma standard based on a cavity ionization chamber includes a wall correction term that corrects for the attenuation and scatter of photons in the chamber wall. For over a decade, the validity of the wall correction terms determined by extrapolation methods (KwKcep) has been strongly challenged by Monte Carlo (MC) calculation methods (Kwall). Using the linear extrapolation method with experimental data, KwKcep was determined in this study for three different styles of primary-standard-grade graphite ionization chamber: cylindrical, spherical and plane-parallel. For measurements taken with the same 60Co source, the air-kerma rates for these three chambers, determined using extrapolated KwKcep values, differed by up to 2%. The MC code ‘EGSnrc’ was used to calculate the values of Kwall for these three chambers. Use of the calculated Kwall values gave air-kerma rates that agreed within 0.3%. The accuracy of this code was affirmed by its reliability in modelling the complex structure of the response curve obtained by rotation of the non-rotationally symmetric plane-parallel chamber. These results demonstrate that the linear extrapolation technique leads to errors in the determination of air-kerma.

Fluence non-uniformity effects in air kerma determination around brachytherapy sources

In ionization chamber dosimetry close to brachytherapy sources, the non-uniform photon fluence in the vicinity of the air cavity of the ionization chamber must be corrected for. In this paper, experimentally determined relative non-uniformity correction factors are determined for a PTW23331 chamber and a number of Farmer-type chambers. It is concluded that the `anisotropic' theory of Bielajew agrees better with the experimental results than the `isotropic' theory of Kondo and Randolph. The experiments show that neither the material choice nor the radius of the central electrode have any significant effect on the non-uniformity correction factors. Experiments on the wall-material dependence in the non-uniformity correction factor, which is predicted by the anisotropic theory, were inconclusive. Parameters for the Farmer-type chambers for calculation of theoretical non-uniformity correction factors are given. Data to derive anisotropic fluence non-uniformity correction factors are given in tabular form. An error in the Kondo and Randolph equations is reported and the corrected equations presented.

The IPEMB code of practice for the determination of absorbed dose for x-rays below 300 kV generating potential (0.035 mm Al - 4 mm Cu HVL; 10 - 300 kV generating potential)

This new code of practice for the determination of absorbed dose for x-rays below 300 kV has recently been approved by the IPEMB and introduces the following changes to the previous codes: (i) The determination of absorbed dose is based on the air kerma determination (exposure measurement) method. (ii) An air kerma calibration factor for the ionization chamber is used. (iii) The use of the F (rad/röentgen) conversion factor is abandoned and replaced by the ratio of the mass - energy absorption coefficients of water and air for converting absorbed dose to air to absorbed dose to water. New values for ratios of these coefficients are recommended. Perturbation and other correction factors are incorporated in the equations. (iv) New backscatter factors are recommended. (v) Three separate energy ranges are defined, with specific procedures for each range. These ranges are: (a) 0.5 to 4 mm Cu HVL; for this range calibration at 2 cm depth in water with a thimble ion chamber is recommended. (b) 1.0 to 8.0 mm Al HVL; for this range calibration in air with a cylindrical ion chamber and the use of tabulated values of the backscatter factor are recommended. (c) 0.035 to 1.0 mm Al HVL; for this range calibration on the surface of a phantom with a parallel-plate ionization chamber is recommended.