Angular variation of scattering volume and its implications for Compton scattering tomography

If quantitative and qualitative precision is to be achieved in X-/γ-rays Compton scattering densitometry and tomography, the volume of interaction within which a useful signal is collected must be well defined and accurately estimated. This work presents a numerical study of the variation of scattering angles and collimator sizes. The effect of choosing back-scattering angles for measurement in both the densitometry and tomographic modes of this technique is studied. A symmetry is observed between the volume at forward and back scattering angles. A symmetry exists, however, within the scattering angle range 90±30°. It is proposed that the best volume/spatial resolution in Compton scattering densitometry/tomography is obtainable within the above mentioned scattering angle range.     

Sampling of angular distribution in Compton scattering

We develop an algorithm, based on the acceptance-rejection method, with a simple envelope rejection function for sampling the angular distribution in Compton scattering from Klein-Nishina differential scattering cross section. The algorithm enhances the sampling efficiency for high-energy photons increasingly with increasing photon energies, The sampling efficiency is 79.6, 89.5, 92.3, 95.9, 96.6% for photon energies 1, 5, 10, 50, 100 MeV respectively. We compare the speed of our algorithm with some methods currently used and find that our method is faster than the others in most cases.     

Development of high resolution focusing collimators intended for nondestructive testing by the Compton scattering tomography technique

An original method for building high resolution focusing collimators with a great number of channels is described. The method consists of filling a mould with tungsten powder and impregnating it with a fluid epoxy resin, by pumping through the powder. The fine strips, used as cores for the channels, are then pulled out. The collimator that was built had 49 fine slots, all converging in one little bar-shaped volume, to convey x-ray photons from this volume to the entrance window of a large scintillation crystal detector. The collimator exhibits a very good precision of the relative positions of slots, allowing the volume V to be very sharply shaped. Use of this collimator for nondestructive testing by the Compton scattering tomography (CST) technique allowed us to detect plane defects down to 0.1 mm, inside composite plates.     

A Compton scattering study of refractory niobium diborides

Isotropic Compton profile of NbB(2) using 20 Ci (137)Cs Compton spectrometer is compared with our theoretical profiles obtained from the density functional theory (DFT) within the first and the second order generalized gradient approximation (GGA) and the hybridization of Hartree-Fock and DFT. A good agreement between GGA based profiles and the experiment validates the applicability of second order GGA in momentum densities. Energy bands, density of states and real space analysis of the experimental profile show metallic character of NbB(2).     

Study of electron densities of normal and neoplastic human breast tissues by Compton scattering using synchrotron radiation

Electron densities of 33 samples of normal (adipose and fibroglangular) and neoplastic (benign and malignant) human breast tissues were determined through Compton scattering data using a monochromatic synchrotron radiation source and an energy dispersive detector. The area of Compton peaks was used to determine the electron densities of the samples. Adipose tissue exhibits the lowest values of electron density whereas malignant tissue the highest. The relationship with their histology was discussed. Comparison with previous results showed differences smaller than 4%.     

Monte Carlo simulation of multiple scattering in Compton spectroscopy.

The Compton scattered photons from a scattering medium can be analyzed in a Compton spectrometer and the primary energy spectrum derived using the Compton equation. Since the reconstruction algorithm assumes that only singly-scattered photons from the scatterer are detected, multiply-scattered photons introduce errors into the measurements unless they can be accurately corrected for. In this work, a Monte Carlo collision density estimator was used to determine the contribution from photons multiply-scattered from a Lucite scatterer in the form of a square rod. The ratio of multiply- to singly-scattered photons was determined for rod thicknesses from 0.5 to 5 mm and photons of energy 20 to 110 keV. The ratio in percent can be well approximated as a linear function of energy with slope 2.2. For thicknesses below 2 mm, the ratio is less than 5% at all energies.     

A Monte Carlo simulation of Compton scattering in positron emission tomography

A Monte Carlo technique was used to simulate some of the important physical processes involved in the coincidence detection of gamma rays by a positron emission tomograph. The major effect considered here is the detection of Compton scattered gamma rays as coincidence counts. The Compton scattering of gamma rays in a H2O filled phantom was simulated using the Klein-Nishina cross section. Results obtained in the form of profiles of activity were compared to a model which represents scatter as the convolution of the true signal with an exponential.     

Electron density of normal and pathological breast tissues using a compton scattering technique

Compton (incoherently) scattered photons which are directly proportional to the electron density of the scatterer, have been employed in characterising human breast tissues. Gamma ray photons scattered incoherently from normal and pathological breast tissue samples of nine breast cancer patients were measured using a high purity germanium detector and an americium (Am-241) source. The breast tissue samples were obtained from female patients undergoing mastectomy. The samples were examined in freeze dried form and the results were corrected for the reduction in the water content of each tissue type by use of the Mixture Rule. This study is aimed at providing electron density information in support of the introduction of new tissue substitute materials for mammography phantoms.     

Treatment of Compton scattering in maximum-likelihood, expectation-maximization reconstructions of SPECT images

This paper studies the extent to which lesion contrasts and signal-to-noise ratios in maximum-likelihood, expectation-maximization estimates of SPECT images can be improved by considering Compton scattering when calculating the photon detection probability matrix. Matrices are generated using a Monte Carlo code that realistically models a SPECT imaging system. For cold lesions having true contrasts of one, thirty-six combinations of three lesion sizes, three lesion locations, and four projection-data count levels are considered. Both hot and cold, 2-cm-radius lesions having various values of true contrast are studied at a few count levels and lesion locations. With scatter in the matrix, scatter effects are completely removed from the central regions of lesions whose radius is greater than the full width at half maximum of the spatial resolution. Resolution recovery is initially slower but eventually superior. Percent improvements in contrast are greater for lower-contrast cold lesions and higher-contrast hot lesions. In most cases, signal-to-noise ratios are comparable or better. Correct absolute activity levels are obtained.     

Measurements of absolute energy spectra for an industrial micro focal X-ray source under working conditions using a Compton scattering spectrometer

Knowledge of absolute photon energy spectrum is essential for image quality analysis and optimisation for any X-ray imaging method, for example, radiography and computerised tomography (CT). Conventional quantities such as half-value layer (HVL) and effective energy are easily calculated from energy spectra. These quantities are, however, of limited value and use for image quality analysis. For example, two energy spectra with the same effective energy but different distributions will not yield the same signal in energy-dependent (read 'most') detectors. Accurate absolute energy spectra are, unfortunately, hard to generalise, since they depend on the specific X-ray source characteristic, that is, target material, internal filtration, high-tension generator, working load etc. They are also laborious to measure, which makes them hard to obtain.In this work absolute energy spectra [1/(keV mAs sr)] for an industrial micro focal X-ray source have been measured under working conditions, using a Compton scattering spectrometer. The energy spectra were measured as a function of tube potential (30-190 kV for every 10 kV) at maximum tube charge (8 W, i.e., tube potential × tube current) for the smallest focus diameter (~5 μm). This is because the micro focal X-ray source in the application in mind is used mainly for high resolution CT, where its maximum fluence is required to shorten scanning times. Target material was tungsten. The spectra were measured for a highly focused fresh focal spot. Neither focal spot wear (age) nor defocusing of the focal spot was considered.The measured spectra were compared to simulated spectra for the same source supplied by the X-ray source manufacturer. It was found that the measured spectra have slightly different energy distributions with a lower mean energy even though their emitted numbers of photons were similar. The energy calibration, Δhν = 0.5 keV, was shown to be accurate compared to the energy resolution used.This work is a part of a larger project, where image quality dependence on X-ray equipment parameters has been studied. Even though the main interest has been in high resolution CT, much of the results and general discussions have wider applications. The full spectra data files are available on the Internet [5].     

Compton scattering spectrum as a source of information of normal and neoplastic breast tissues' composition

In this work we measured X-ray scatter spectra from normal and neoplastic breast tissues using photon energy of 17.44 keV and a scattering angle of 90°, in order to study the shape (FWHM) of the Compton peaks. The obtained results for FWHM were discussed in terms of composition and histological characteristics of each tissue type. The statistical analysis shows that the distribution of FWHM of normal adipose breast tissue clearly differs from all other investigated tissues. Comparison between experimental values of FWHM and effective atomic number revealed a strong correlation between them, showing that the FWHM values can be used to provide information about elemental composition of the tissues.     

Compton profiles and electronic structure of HgBr2 and HgI2

In this paper, we present the first-ever experimental Compton line shapes of HgBr₂ and HgI₂ using ¹³⁷Cs Compton spectrometer. To compare our experimental momentum densities, we have computed the Compton profiles using Hartree–Fock and density functional theory within linear combination of atomic orbitals. We have also computed the energy bands and density of states using the linear combination of atomic orbitals and full potential linearized augmented plane wave method. On the basis of equal-valence-electron-density profiles, it is seen that HgI₂ is more covalent than HgBr₂ which is in agreement with the valence charge densities. The experimental isotropic profiles are found to be relatively in better agreement with the Hartree–Fock data. We have also discussed the photoluminescence and detection properties of both the halides.     

A method for determination mass absorption coefficient of gamma rays by Compton scattering

A method was proposed for determination mass absorption coefficient of gamma rays for compounds, alloys and mixtures. It is based on simulating interaction processes of gamma rays with target elements having atomic numbers from Z=1 to Z=92 using the MCSHAPE software. Intensities of Compton scattered gamma rays at saturation thicknesses and at a scattering angle of 90° were calculated for incident gamma rays of different energies. The obtained results showed that the intensity of Compton scattered gamma rays at saturations and mass absorption coefficients can be described by mathematical formulas. These were used to determine mass absorption coefficients for compound, alloys and mixtures with the knowledge of their Compton scattered intensities. The method was tested by calculating mass absorption coefficients for some compounds, alloys and mixtures. There is a good agreement between obtained results and calculated ones using WinXom software. The advantages and limitations of the method were discussed.     

Low-level gamma spectrometry using beta coincidence and Compton suppression.

A low-level gamma-ray spectrometry system was developed using a Ge(Li) detector with 6% relative efficiency coupled to a 2pi beta plastic detector for coincidence selection and a massive NaI(Tl) detector for Compton suppression. The integral background count rate for (50-1500)keV was 0.5 s(-1)kg(-1) (Ge), using only beta coincidences. With Compton suppression, a value of 0.25 s(-1)kg(-1) (Ge) was obtained. Spectra with and without Compton suppression were studied for 60Co, 137Cs and 152Eu point sources. Considerations are made concerning the Compton suppression advantages in different situations.     

Theoretical and experimental analysis of titanium Compton profile using 137Cs

The objective of this study is to measure the Compton profile of a titanium coating on a glass substrate, comparing it with a profile computed by the Hillman and Skilman program based on the Hartree–Fock model. The experimental Compton profile was obtained using 661.7 keV gamma-rays from a ¹Ci ¹³⁷Cs source and a 3″×3″ NaI(Tl) detector. The experimental, theoretical and unfolded Compton profiles of titanium have been compared. The results illustrate the breadth of experimental Compton profile in comparison with the two others. This difference can be mainly attributed to the Doppler broadening effect, the final quantum state of the electron and partly by the approximations implemented in the theoretical model.