Journal of X-Ray Science and Technology - Volume 8, issue 1

Authors: Hammersberg, Peter

Article Type: Other

Citation: Journal of X-Ray Science and Technology, vol. 8, no. 1, pp. 1-3, 1998

Authors: Hammersberg, Peter | Stenström, Mats | Hedtjärn, Håkan | Mångård, Måns

Article Type: Research Article

Abstract: 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 (~5µm). 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, δhv = 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. Show more

Keywords: Micro focus X-ray source, X-ray spectrum, Compton scattering spectrometer

Citation: Journal of X-Ray Science and Technology, vol. 8, no. 1, pp. 5-18, 1998

Authors: Hammersberg, Peter | Stenström, Mats | Hedtjärn, Håkan | Mångård, Måns

Article Type: Research Article

Abstract: All irradiated objects in the vicinity of an imaging set-up increase the amount of scattered radiation. It is a well-known fact that if this radiation is allowed to impinge on the image collector it naturally degrades the image quality. It also causes problems with equipment characterisation, such as X-ray energy spectra measurements. However, it may not be a well-known fact that when a small radiation safety enclosure is used - common for industrial applications - this …type of scattered radiation is astonishing large. In this work a quantitative and qualitative analysis has been carried out in an attempt to explain the origin of the problem, why it occurs and how it may be treated. The scattered radiation increases rapidly when the fractions of X-ray photons in the primary X-ray spectrum above the K-edges of the enclosure wall material increase. High-energy photons are scattered and/or will generate characteristic radiation in the enclosure wall material instead of being absorbed. For lead, which is commonly used as shielding and wall material, this noise becomes significant when tube potentials over 90--100kV are used, since the K-edges for lead are 72-74keV (Kα) and 85-87keV (Kβ). Below tube potentials of 100kV, the noise from scattered radiation is insignificant, even if the primary beam is wide enough to hit the enclosure walls. Above 100kV it increases rapidly and for this application the number of scattered photons was 50% In this particular case this problem was accentuated above tube potentials of 100kV since the steel housing of the micro focal X-ray source used was penetrated by high-energy photons in unwanted directions. Collimation of the X-ray beam just in front of the X-ray source output window to narrow the solid angle of the primary X-ray beam to avoid direct radiation of the enclosure walls was not enough to suppress this scattered noise. A wider X-ray shield, covering the entire front of the X-ray source had to be applied. Other solutions to decrease scattered radiation would be the following: Increased internal shielding of the X-ray source is the first choice if the same possibilities for geometrical magnification of the imaged object are to be maintained. Increased distances to the enclosure walls and other objects inside, that is, to use larger enclosures or even separate X-ray chambers would also decrease scattered radiation. To line the enclosure with a series of X-ray shielding materials with K-edges at lower and lower energies is another alternative. The lowest K-edge material should be placed closest to the detector followed by the second lowest K-edge material etc., to effectively absorb backscatter and characteristic radiation from the enclosure walls. This would, however, be a rather expensive and complicated solution. Show more

Keywords: X-ray radiography, computerised tomography, image noise, scattered radiation, leak radiation

Citation: Journal of X-Ray Science and Technology, vol. 8, no. 1, pp. 19-29, 1998

Authors: Mångård, Måns | Hammersberg, Peter | Sandborg, Michael

Article Type: Research Article

Abstract: A model will be proposed for predicting the expected value and variance of the measured signal-level in collected radiographic images obtained with an image-intensifier-based X-ray radiography system. The model parameters are determined from both theoretical and experimental data and incorporate all parameters that can be varied by the system operator, except CCD-camera readout rate. The proposed model predicts the expected value and variance of the grey-level in the output image with high accuracy. It is also …shown that it is very important to compensate for the inhomogeneous pixel sensitivity when comparing the variance of the signal-level in a pixel from sequentially collected images with the variance determined in a single image. Show more

Keywords: Image intensifier, X-ray radiography, modelling

Citation: Journal of X-Ray Science and Technology, vol. 8, no. 1, pp. 31-50, 1998

Authors: Mångård, Måns | Hammersberg, Peter

Article Type: Research Article

Abstract: It is of interest to know the minimum discernible detail size when performing non-destructive testing with high-resolution computerised tomography. But it is difficult to empirically find optimal parameter settings that maximise detectability for each individual imaging task. In this work a method to determine the optimal performance for a high-resolution computerised tomography system has been developed. It reveals the detectability limit for specific contrasting details in terms of imaged object diameter when the …signal-to-noise ratio, SNRδS,CT, between the contrasting detail and its surroundings has been maximised using optimal data collection parameter settings. This work includes modelling and verification of the total unsharpness of the CT-system in terms of modulation transfer-function, MTF. Together with the limiting perception factor and maximised SNRδproj in the CT projection data, the detectability limits for any specific contrasting detail are determined as a function of imaged object material and geometry. Maximised SNRδproj was obtained by optimising the parameters: X-ray source tube potential, X-ray filter, exposure time and optical aperture. The importance of using optimal settings when performing CT-investigations, especially when investigating objects with large diameters, is demonstrated. It was found that it is possible to predict the detectability for any specific contrasting detail. It was also discovered that the optimal settings for a given object diameter and material are dependent on the detail material. In some cases an improvement of the detail signal-to-noise-ratio was obtained by using a combination of X-ray filter materials. It was also found that SNRδproj is very sensitive to the thickness of denser X-ray filter materials (higher atomic number and density). Show more

Keywords: Computerised tomography, CT, detectability, optimal performance

Citation: Journal of X-Ray Science and Technology, vol. 8, no. 1, pp. 51-73, 1998

Authors: Hammersberg, Peter | Mångård, Måns

Article Type: Research Article

Abstract: Conventional computerised tomography systems (CT) are usually equipped with polyenergetic X-ray sources, which prevents accurate density measurements because of the general CT-image artefact called beam hardening (BH). BH results in false gradients of the linear attenuation coefficient in the CT cross section images, indicating a non-existent density or composition gradient in the imaged object. A number of methods have been proposed to correct for, or limit the effect of, beam hardening. One of these is called …linearisation of the CT-data, in which the polyenergetic CT-data are transformed to monoenergetic CT-data. This requires knowledge of the CT-data as a function of object thickness. Data points to derive this function are usually measured using a set of samples of different object material thicknesses at the imaging parameter settings used and fitted with a polynomial. However, the sample preparation makes this method tedious to use. In this work a simulation method has been developed, which can accurately simulate the polyenergetic CT-data for any arbitrary object material and thickness if a priori information of the object material density and composition exists. The simulation method requires detailed knowledge of the imaging system, that is, X-ray energy spectra, detector response and information transfer from detector to digitised data. Besides developing the simulation tool, it has been shown that one of the major difficulties with this BH-correction method is to accurately determine the curvature of the function representing the polyenergetic CT-data. Earlier proposed endorsements to fit a second-degree polynomial to the polyenergetic CT-data are not sufficient to describe its curvature, at least a polynomial of degree eight or higher is required. Here cubic-spine interpolation is used, which avoids the problem. Show more

Keywords: Computerised tomography, artefacts, beam hardening

Citation: Journal of X-Ray Science and Technology, vol. 8, no. 1, pp. 75-93, 1998