Carl A. Carlsson

Calculation of scattering cross sections for increased accuracy in diagnostic radiology. I. Energy broadening of Compton‐scattered photons

In this work, scattering cross sections differential with respect to both the scattering angle and the energy of the scattered photon are derived in the relativistic impulse approximation for the light elements H, Be, and Al, and photon energies between 30 and 200 keV. The energy broadening of the scattered photons reflects the momentum distribution of the target electrons. It increases with both increasing atomic number of the scatterer and with scattering angle. Even in light elements, the energy broadening is comparable with the intrinsic energy resolution of modern Ge spectrometers. In reconstructing primary photon energy spectra by means of a Ge spectrometer and Compton scattering techniques, i.e., by measuring the photons incoherently scattered at a given angle, the energy resolution is markedly impaired compared to direct measurements in the primary beam. This is usually explained as an effect of the nonzero acceptance angle of the detector. It is shown, however, that the fundamental energy broadening of the scattered photons is alone sufficient as an explanation. The Compton scattering technique is valuable in determining energy spectra in clinical situations. Aspects of its optimal performance are discussed. The commonly used scattering angle of 90 degrees seems adequate. At small scattering angles, the incoherent-scattering cross section is badly known due to electron-electron interactions and, for photon energies less than 100 keV, coherent scattering contributes appreciably to the total scattering even in media of low atomic number. In cases where coherent scattering dominates and where the energy degradation of the incoherently scattered photons is small compared to the energy resolution of the spectrometer, the reconstruction is simplified. The double-differential cross sections derived can be used to simplify calculations of the Compton component of the mass-energy absorption coefficient.

Energy imparted to the patient in diagnostic radiology: calculation of conversion factors for determining the energy imparted from measurements of the air collision kerma integrated over beam area

The energy imparted to the patient in diagnostic radiology, related to radiation risk in examinations of the trunk and head, can be deduced from a measurement of the air collision kerma (or exposure) of the incident primary photons integrated over beam area by using a thin, flat ionisation chamber covering the entire roentgen beam. Factors for converting the integral of the air collision kerma to energy imparted to the patient have been calculated using a Monte Carlo method. The patient is simulated by laterally infinite water slabs with thicknesses from 100-300 mm. Calculations are performed for monoenergetic photons (5-300 keV) and energy spectra commonly used in diagnostic radiology (40-130 kV acceleration potential differences and values of the half-value thickness of air collision kerma in aluminium from 0.9 to 9.9 mm). Correction factors which take into account the additional escape of scattered photons from the sides of a laterally finite water slab as a function of field size and focal distance are also given.

Imaging modalities in x-ray computerized tomography and in selected volume tomography.

This review of different principles used in x-ray computerized tomography (CT) starts with attenuation (transmission) CT. The pros and cons of different geometrical solutions, single-ray, fan-beam and cone-beam, are discussed. Attenuation CT measures the spatial distribution of the linear attenuation coefficient, mu. The contributions of different interaction processes to mu have also been used for CT. Fluorescence CT is based on measurements of the contribution, cZtauZ/rho, from an element Z with concentration cZ, to the linear attenuation coefficient. Diffraction CT measures the differential coherent cross section d sigma (theta)(coh)/d omega, Compton CT the incoherent scatter cross section sigma. The usefulness of these modalities is illustrated. CT methods based on secondary photons have a competitor in selected volume tomography. These two tomography methods are compared. A proposal to perform Compton profile tomography is also discussed, as is the promising method of phase-contrast x-ray CT.

Relations between effective dose equivalent and mean absorbed dose (energy imparted) to patients in diagnostic radiology.

Values of HE/D, the ratio of the effective dose equivalent HE to the mean absorbed dose D to the total body, have been derived as a function of beam quality (HVT) for a variety of Roentgen diagnostic examinations of the adult trunk and head, using tabulations from the literature. Problems in specifying beam quality are discussed. The energy epsilon imparted to the patient can, in Roentgen diagnostic examinations, be determined using a transmission chamber. Since, however, HB/epsilon depends more critically than HE/D on the mass M of the patient (D = epsilon/M), the latter quantity is preferred for risk assessment. HE/D takes values in the range 0.44-2.8 Sv Gy-1. This includes the value 1 Sv Gy-1 resulting from a uniform whole-body irradiation independent of M. The corresponding value of HE/epsilon = 1/M Sv J-1 if M is in kg. It is therefore recommended that clinical measurements of epsilon be supplemented by measurements of the patients mass so that D = epsilon/M can be estimated and tabulations of organ absorbed doses be supplemented with values of D so as to allow accurate derivation of the ratio HE/D. In optimising the technical parameters of a given examination using the transmission chamber, HE/D as a function of beam quality must be known.

Differences in reported backscatter factors for low-energy X-rays: a literature study

This work was ini tia ted by the large discrepancy in published values of the backscatter factor (BSF) as function of the half value layer (HVL) that exists in a new code of practice (IAEA, 1987) c ...

Quantities and concepts used in radiation dosimetry

Radiation dosimetry is a pure physical science, as fostered by the elegant work of the International Commission on Radiation Units and Measurements (ICRU), in defining the basic quantities and units of dosimetry. Nevertheless, questions concerning the interpretation and application of some quantities still remain. The present work focuses on some of these questions and in particular deals with the quantity fluence which is frequently misunderstood. Radiation dosimetry is closely related to radiation transport theory, and the usefulness of the vectorial quantities used extensively in transport theory is pointed out. It is proposed that vectorial quantities be included in the radiometry considerations of the ICRU. This would contribute to clarifying the basic concepts of dosimetry and promoting its establishment as a physical science. Equations are given for calculating the absorbed dose in various conditions of radiation equilibrium, along with discussions of the quantities needed for their evaluations. These equations are relevant to the important field of cavity theory. Refinements of existing cavity theories, in particular those for photon and electron irradiations, can benefit from a deeper understanding of these equations and the various conditions of equilibrium in which they are valid.

The use of computed microtomography to monitor morphological changes in small animals.

RATIONALE AND OBJECTIVES We investigate the methodological aspects of computerised microtomography (Cm 1) for monitoring the development of osteoporosis in male Sprague-Dawley rats. METHODS 120 Rats were gastrectomized or sham operated. Femurs were prepared and tomograms with spatial resolutions of 5-500 mm were made. Bone diameters, bone areas and moments of inertia were determined from the tomograms. Optimal slice position and the need for spatial resolution for future in vivo applications were investigated. In order to minimise the absorbed dose to the specimen, a theoretical model for determination of optimal irradiation conditions is developed. RESULTS Gastrectomy caused dramatic changes in the bone architecture. The main features were vaccuolisation of the bone and reduced amounts of compact bone. While the outer diameters of tubular bones were largely unaffected, their inner diameters were greatly increased following gastrectomy. Relative bone area and moment of inertia were greatly reduced. Optimal photon energy was 12 keV. CONCLUSIONS It is possible to monitor gastrectomy-evoked changes in bone morphology at various sites in rats with computerised microtomography. The changes are suggestive of osteoporosis.

Shaping X-ray spectra with filters in X-ray diagnostics

The influence on image contrast, tube load and patient mean absorbed dose of different ways of shaping diagnostic X-ray spectra by placing filters in the beam is derived for two radiographic models (abdominal screen-film radiography and intra-oral, dental radiography) using a computational model. The filters are compared at either equal tube load (keeping tube potential constant) or equal contrast (adjusting the tube potential with the different filters), but always at equal energy imparted per unit area to the image receptor. Compared at equal tube load and relative to standard aluminium filtration, reductions in the mean absorbed dose in the patient of 15–25% can be achieved using filters of Cu, Ti, W and Au (increasing the tube load by 30–40% compared with standard aluminium filtration). However, contrast is also reduced by 7%. Compared at equal contrast, the dose reductions are smaller, about 10%. Filters of copper are generally recommended, as are filters of aluminium. The use of bandpass filters (K-edge filters) should be restricted to examinations where the need for substantial variation in tube potential from patient to patient is small. The benefit of using thicker filters than those commonly used today (increasing tube load by factors of 1.4–2.0 compared with no added filter) is small as the dose reduction is most rapid for small initial values of added filters, and the increase in tube load increases steadily with increasing filter thickness.

Generalised use of contrast degradation and contrast improvement factors in diagnostic radiology. Application to vanishing contrast

Although field area and object thickness are important parameters in comparisons of techniques for optimal reduction of scattered radiation to the image, they are in practice seldom varied. For this reason, we suggest that contrast degradation (CDF) and contrast improvement (CIF) factors be more frequently used and appropriately defined to make the dependence of CDF and CIF on field area (collimation) and object thickness (compression) explicit. Definitions are formulated and the results of experiments and Monte Carlo calculations (comprising effects of collimation, compression, air gap, antiscatter grid, detector thickness) cited to illustrate their usefulness. Currently used expressions for CIF (derived assuming monoenergetic radiation) lack a factor to account for the change in primary contrast caused by the antiscatter method when this affects the energy distribution of the transmitted primary photons (grids and compression) or the fractions of photon energy imparted to the detector (when comparing different detectors). Values of this factor are calculated for some cases. Also, the appropriate choice of physical quantity to be used in the formulae for CDF and CIF is discussed. The energy imparted to the detector is advocated since this is directly related to the detector signals forming the image on, e.g. the x-ray film.

Methodologic aspects of computed microtomography to monitor the development of osteoporosis in gastrectomized rats

RATIONALE AND OBJECTIVES We investigated the methodologic development of computed microtomography (CMT) for monitoring the development of osteoporosis in male Sprague-Dawley rats. METHODS Eight rats were gastrectomized and eight rats were sham operated. Femurs, tibias, and tails were prepared, and CMT scans with spatial resolutions of 5-500 microns were made. Bone diameters, bone areas, and moments of inertia were determined from the CMT scans. Optimal slice position and the need for spatial resolution and energy optimization for future in vivo applications were investigated. RESULTS Gastrectomy caused dramatic changes in the bone architecture of the tibia and the femur. The main features were vacuolization of the bone and reduced amounts of compact bone. Although the outer diameters of tubular bones (femur and tibia) were largely unaffected, their inner diameters were greatly increased following gastrectomy. Relative bone area and moment of inertia were greatly reduced. The optimal photon energy was 12 keV. CONCLUSION It is possible to monitor gastrectomy-evoked changes in bone morphology at various sites in rats using CMT scanning. The changes are suggestive of osteoporosis. By optimizing the energy spectrum and spatial resolution, as well as choosing the proper slice position, it should be possible to keep absorbed doses low enough to avoid acute radiation injury in repeated in vivo measurements.