Scatter Correction Factors Research Articles

Complete sets of factors for absorption correction and air scattering subtraction in X-ray powder diffraction of loosely packed samples

Complete sets of correction factors for absorption and air scattering effects in X-ray powder diffraction are reported. Both symmetrical reflection and transmission techniques are considered and boundary conditions to be tested for any given value of scattering angles are listed. This may prove particularly useful on coding computer programs for correction of raw intensity data files. Effects of interstitial volume on correction for absorption and subtraction of air scattering were investigated. For samples of high interstitial volume, like loosely packed powders or aerogels, the contribution from air trapped inside the specimen is significant and leads to expressions of the air scattering correction factors different from those commonly reported in the literature.

Evaluation of projection pixel-dependent and pixel-independent scatter correction in SPECT

Dual energy window scatter subtraction methods in single photon emission computed tomography (SPECT) to compensate for the detection of Compton-scattered photons are based on the assumption that the number of scattered photons detected by each projection pixel in the primary (photopeak) energy window is proportional to the number of photons detected in a secondary (scatter) energy window. The reliability of this assumption is tested using Monte Carlo modeling of photon detection kernels for photon emission by Tc-99 m within a water-filled cylinder. The primary window scatter kernels and the secondary window kernels are correlated highly, and projection pixel-dependent and pixel-independent scatter correction factors are obtained from least squares fits of these kernels to each other. In a numerical experiment with noise-free projection data, average reconstructed intensities in regions of interest are nearly as well-estimated for both pixel-independent and pixel-dependent scatter-corrected projection data reconstructed using the approximate generalized matrix inverse (GMI) of the nonscatter photon detection matrix as for primary window projection data reconstructed with the approximate GMI of the photon matrix which includes scatter. >

Calibration of parallel-plate chambers: resolution of several problems by using Monte Carlo calculations.

For five commonly used parallel-plate ion chambers, Monte Carlo calculations are presented of the wall attenuation and scatter correction factors (Kwall = Awall-1 or katt-1) and the correction for nonhomogeneous composition of the chamber (Kcomp or related to km). The chambers are assumed to have 0.5 g/cm2 buildup caps made of the predominant material in each chamber. These correction factors are needed if air-kerma calibration factors are used to deduce the chamber's cavity-gas calibration factor, Ngas. The scatter from the material around the cavity more than compensates for the attenuation in the front wall and hence Kwall values are less than unity. The corrections for the nonhomogeneous composition can be significant. Thin collectors or insulators behind the collecting volume can have a major effect because electron backscattering depends strongly on material. The calculations agree well with experimental data and explain previous results that had not been understood. Calculations for different buildup caps predict trends in chamber in-air responses, which go in the opposite direction to the km values for homogeneous chambers of the same materials. It is argued that Pwall, the wall correction factor for photon-beam measurements in-phantom is, to first order, the same correction factor as Kcomp, calculated for a buildup cap of the same material as the phantom material and the initial assumption that the wall material is made of the same material. Hence, Pwall can also be significantly different from unity. Recommended values of kmkatt or the equivalent Ngas/Nx, and Pwall (in 60Co beams) are presented for the 5 chambers. These values treat the Exradin P11, Markus and Holt parallel-plate chambers as homogeneous in construction whereas the values for the NACP and Capintec PS-033 chambers demonstrate that the chambers are distinctly nonhomogeneous.

A pencil beam model for photon dose calculation.

A method for photon dose calculation in radio therapy planning using pencil beam energy deposition kernels is presented. It is designed to meet the requirements of an algorithm for 3-D treatment planning that is general enough to handle irregularly shaped radiation fields incident on a heterogeneous patient. It is point oriented and thus faster than a full 3-D convolution algorithm and uses the same physical data base to characterize a clinical beam as a full 3-D convolution algorithm. It is shown that photon therapy beams can be characterized with great accuracy from a combination of precalculated Monte Carlo energy deposition kernels and dose distributions measured in a water phantom. The data are used to derive analytical pencil beam kernels that are approximately partitionated into the dose from (i) primary released electrons and positrons, (ii) scattered, bremsstrahlung, and annihilation photons, (iii) contaminating photons, and (iv) charged particles from the collimator head. A semianalytical integration method, based on triangulation of the field, is developed for dose calculation using the analytical kernels. Dose is calculated in units normalized to the incident energy fluence which facilitates output factor calculation. For application in heterogeneous media, a scatter correction factor is derived using monodirectional convolution along the ray path. In homogeneous media results are compared with measurements and in heterogeneous media with Monte Carlo calculations and the Batho method.

The determination of phantom and collimator scatter components of the output of megavoltage photon beams: measurement of the collimator scatter part with a beam-coaxial narrow cylindrical phantom

The separation of the total scatter correction factor S c, p in a collimator scatter component, S c, and a phantom scatter component, S p, has proven to be an useful concept in megavoltage photon beam dose calculations in situations which differ from the standard treatment geometry. A clinically applicable method to determine S c is described. Measurements are carried out with an ionization chamber, placed at a depth beyond the range of contaminant electrons, in a narrow cylindrical polystyrene phantom with a diameter of 4 cm of which the axis coincides with the beam axis. S c, p is measured in a full-scatter phantom and S p can be derived from S c, p and S c. In order to obtain a reliable separation, i.e. excluding the influence of contaminant electrons, measurements of S c, p have been carried out at depths of 5 cm for photon beams with a quality index (QI) up to and including 0.75 and a depth of 10 cm with QI larger than 0.75. These depths are in accordance with recommendations given in recent dosimetry protocols. The consistency of the method was checked by comparing calculated and measured values of S c, p for a set of blocked fields for a range of photon beam energies from 60Co up to 25 MV showing a maximum deviation of 2%. The method can easily be implemented in existing procedures for the calculation of the number of monitor units to deliver a specified dose to a target volume.

Multiple scattering corrections for lidar detection of obscured objects

The importance of multiple scattering for lidar detection of a spherical object obscured by an aerosol is assessed using Monte Carlo radiative transfer calculations. Multiple scattering correction factors are significant and depend on the location and size of the object, and the field of view and time resolution of the detector.

Measurements of dose distributions in small beams of 6 MV X-rays

Dose distributions produced by small circular beams of 6 MV X-rays have been measured using ionisation chambers of small active volume. Specific quantities measured include tissue maximum ratios (TMR), total scatter correction factors (St), collimator scatter correction factors (Sc) and off-axis ratios (OAR). Field sizes ranged from 12.5 to 30 mm diameter, and were defined by machined auxiliary collimators with the movable jaws set for a 4 cm*4 cm field size. Due to the lack of complete lateral electronic equilibrium for these small fields, the accuracy of the measurements was also investigated. This was accomplished by studying dose response as a function of detector size. Uncertainties of 2.5% were observed for the central axis dose in the 12.5 mm field when measuring with an ionisation chamber with a diameter of 3.5 mm. The total scatter correction factor exhibits a strong field size dependence for fields below 20 mm diameter, while the collimator scatter correction factor is constant and is defined by the setting of the movable jaws. Off-axis ratio measurements show larger dose gradients at the beam edges than those achieved with conventional collimator systems. Corrected profiles measured with an ionisation chamber are compared with measurements made with photographic film and LiF thermoluminescent dosemeters.

Dosimetry of 24-MV x rays from a linear accelerator.

Dosimetric characteristics of a 24-MV photon beam produced by a Varian Clinac 2500 linear accelerator are presented. Particular attention is paid to measurements and applications relating to depth of maximum dose and scatter correction factors. New experimental methods were adopted to investigate scatter factors for field sizes ranging from 5 X 5 to 40 X 40 cm. For the largest field investigated, a 3% phantom scatter factor relative to a 10 X 10 cm field was determined. From the study, clinically useful scatter-phantom ratios were generated. The study also demonstrated that the estimated correction factors for scatter in a medium caused negligible changes (approximately equal to 0.1%) on the percent depth-dose values derived from the measured tissue-phantom ratio (TPR) data. The paper also quantitatively compares results obtained for dosimetric parameters like scatter output, tissue-phantom ratio, and percent depth-dose with those obtained from similar machines used for radiotherapy. Measured data relating to these parameters are expressed by taking a reference depth of 8 g/cm2 in a medium for normalization purposes and its importance discussed. Additional measurements presented in this work include beam quality and collimator effect on dose rate. Phantoms of various sizes and materials and a variety of detectors were used throughout the investigation.

Dose measurements and calculations of small radiation fields for 9-MV x rays.

Measurements of dose distribution for square fields with sizes ranging from 1 X 1 to 30 X 30 cm for a 9-MV x-ray beam from a Neptune 10 linear accelerator, manufactured by CGR, are reported. Special attention was paid to field sizes smaller than 4 X 4 cm, used in radiosurgery techniques. To express the dose-monitor units relationship, total, collimator, and phantom scatter correction factors were obtained by experimental measurements. A strong dependence of these factors on the smallest field sizes (less than 4 X 4 cm) was shown. Measurements of the maximum depth dose dmax, plotted as a function of field size, showed a maximum at about 5 X 5 cm, in good agreement with previous results. dmax was also measured for the smallest fields, demonstrating that the contaminating electron component of the x-ray beam was not responsible for the dmax shift. Analysis of the penumbra width of cross dose distributions, as a function of field sizes, allowed us to postulate that the dmax shift could be due to the phantom scattered photons, which in turn were generated by the collimator scattered photons. Newly derived tissue-maximum ratio and scatter-maximum ratio data were used for dose profile calculations of 2 X 2, 4 X 4, and 10 X 10 cm field sizes. The agreement between experimental and calculated data was found to be +/- 2% within the geometrical edges of the fields and +/- 6% outside of them. A dose profile from the isocenter of a 2 X 2 cm square field moving through a 360 degree rotation arc was obtained and compared with that from the center of a 125I shielded source, as measured by Ling. Advantages and problems relating to the use of x-ray beams from linear accelerators in radiosurgery are discussed.

The density scaling theorem applied to lateral electronic equilibrium.

Some theoretical considerations concerning the application of the density scaling theorem to secondary electrons from high-energy x-ray beams are discussed. It is concluded that the theorem is expected to be applicable with acceptable accuracy for 10-MV x-ray beams in low-density materials. Measurements reported by different groups of workers are discussed and shown to be in general agreement with the theorem, with one exception. It is shown that the theorem can be applied in combination with phantom scattering correction factors to calculate the dose in homogeneous bodies which differ from water in their density. The method of calculation is similar to the equivalent tissue-air ratio method, but takes account of the effects of loss of lateral electronic equilibrium.

Revision of tissue-maximum ratio and scatter-maximum ratio concepts for cobalt 60 and higher energy x-ray beams.

The concept of tissue-maximum ratios (TMR) as a basis of dose computations has posed problems when applied to a whole range of clinically used megavoltage beams. Another problem is the definition of scatter-maximum ratio (SMR) which assumes a value of zero at the depth of maximum dose. This paper describes new methods of measuring collimator and phantom scatter correction factors. The definitions of TMR and SMR are modified in order to compute phantom scatter at any depth, including depth of maximum dose. The revised concept is basic and general enough that it can be applied to x-ray beams of any energy fields of any shape and isocentric as well as non-isocentric modes of treatment.

Variations in depth-dose data between open and wedge fields for 4-MV X-rays.

Central-axis depth-dose data for 4-MV x rays, including tissue-maximum ratios, were measured for wedge fields. Comparison with corresponding open-field data revealed differences in magnitude which increased with depth, field size, and wedge thickness. However, phantom scatter correction factors for the wedge fields differed less than 1% from corresponding open-field factors. The differences in central-axis per cent depth doses between the two types of fields indicate beam hardening by the wedge filter. This study also implies that the derivation of tissue-maximum ratios from central-axis per cent depth is as valid for wedge as for open fields.

Calibration of an Airborne Gamma Ray Spectrometer over Mary Kathleen Uranium Mine

Airborne gamma ray spectrometer test flights were made over the Mary Kathleen Uranium Ltd orebody with a large crystal assembly and modern digital recording system. The decrease in response with height is shown theoretically to be a combination of an exponential air absorption factor and a geometrical factor. The results of these tests indicate that for this specific detecting system and gamma ray source, the overall attenuation effect with height becomes exponential for the total count and thorium channels with apparent attenuation coefficients of 7.4 ´ 10?3 and 7.6 ´ 10?3 m?1 , respectively. The uranium channel count rate shows a departure from an exponential relationship; this departure can be allowed for by a height dependent component in the stripping (Compton scatter) correction factor. Ground surveys with a calibrated portable spectrometer enable abundance conversion factors to be derived for the airborne system.

IV 不均等物質による線量分布の変化(放射線治療時における組織内線量分布)(第27回総会)

The presence of tissues of different composition or density, such as bone, lung and air cavity, influences the dose distribution. The distortion of dose distribution depends on the quality of thd radiation, the size of the field, and the size and composition of heterogeneities. For cobalt 60 gamma rays and 4 MV x rays, corrections are negligible for the reduction of the dose due to the interposition of bone and due to the lack of dose build-up beyond an air cavity in cavity in clinical practice. In this parer, the methods of calculation of the dose distribution in the treatment of thoracic lesions with cobalt 60 gamma rays and 4.3 MV x rays used in the Cancer Institute Hospital are described. (1) From transverse tomographs and radiographic transit dose measurements, it is possible to determine the thickness and density of the lung and the water equivalent thickness of body to be traversed by each pencil of the beam considered. (2) When the density and thickness of lung in the pass of the beam are determnied, radiation dosage can readily be calculated at any point in the thoracic section by the following correction method. The correction factor (that is, actual/water phantom dose) can be expressed as [numerical formual] where S is the scatter correction factor, μe is the effective absorption coefficient in water, l is the passed thickness of the air-filled lung, and ρ is the average density of the air-filled lung. In general, the scatter correction factor S may be expressed as [numerical formual] where t is the depth in water-equivalent tissue beyond the lung. When t =0,k=C and when t>0,k is unity. B and C are given by B=0.138-0.243ρ+0.105ρ^2 C=0.975+0.016ρ-0.086ρ^2 The effective absorption coefficient can be expressed as μe=0.0656-0.011 logA where A is the field size at the surface. (3) For a mediastinal lesion treated with any angle of rotation and for a tumor locatea in the lung treated with rotation greater than 240 degrees, the effect of lung on the overall treatment plan may be neglected for clinical purposes, provided the absolute dose at the center of rotation is corrected for the presence of lung tissue.

Low-angle light-scattering instrument for DNA solutions.

AbstractA light‐scattering instrument capable of measurements on native DNA at angles as low as 10° is described. The major features of the instrument, which make it capable of low‐angle measurements, are the use of an intense light source in which the incident beam is monitored directly and the use of a long, rectangular sample cell in which the scattered light can be measured at low angles with no interference from the incident beam. Methods for calibrating the instrument and for determining scattering correction factors are described. Procedures for the preparation and use of various calibration standards are given.