Georg Matscheko

Compton spectroscopy in the diagnostic x-ray energy range. II. Effects of scattering material and energy resolution.

The overall performance of a Compton spectrometer and, in particular, its energy resolution are investigated both experimentally and theoretically for different scattering materials. Using low-Z (less than or equal to 8) scatterers of moderate sizes (scatterer diameter d less than or equal to 5 mm), there are negligible disturbances due to coherent and/or multiple scattering at 90 degrees scattering angle and photon energies above 20 keV. Two factors contribute to decreasing the energy resolution compared with that in direct measurements: (i) the velocity distribution of the electrons in the scatterer and (ii) the scattering geometry. Of these, (i) is dominant for photon energies less than or equal to 100 keV. The optimal scattering material is a metal of as low Z as possible, i.e. beryllium. However, polyethylene and lucite are normally sufficiently good scatterers. The scattering geometry may become the dominating factor decreasing energy resolution at high photon energies hv greater than or equal to 150 keV.

Compton spectroscopy in the diagnostic X-ray energy range: I. Spectrometer design

The optimal design of a Compton spectrometer for measuring photon energy spectra from x-ray tubes in a clinical laboratory is analysed. The demands are: (i) coherent and multiple scattering distort the measurements and must be avoided; (ii) the measuring time should be as short as possible to avoid unnecessary wear on the x-ray tube; and (iii) the impairment in energy resolution due to the scattering geometry should be kept minimal. A scattering angle of 90 degrees is advocated. Scatterers (of low-atomic-number material) in the shape of long circular rods (0.5-4 mm diameter, 20-40 mm long) are preferable to scattering foils. Use of a short focus-scatterer distance (approximately 200 mm) is to be preferred compared to using a large detector area (greater than or equal to 4 mm diameter) in order to establish a sufficiently high count rate in the detector. Short focal distances and a 90 degrees scattering angle are advantages in measuring energy spectra in the gantry of CT machines where the available space is limited. To limit the geometrical energy broadening to less than 1 keV, the spread in scattering angles of registered photons must not exceed 1-2 degrees for incident photon energies of 100-150 keV.

Measurement of absolute energy spectra from a clinical CT machine under working conditions using a Compton spectrometer

Absolute measurements of photon energy spectra (keV-1 sr-1 mA-1 s-1) from a clinical CT machine have been performed under normal working conditions (140 mA tube current) using a Compton spectrometer. The inaccuracy of the measured spectra is estimated to be +/- 6%, determined by uncertainties in dead-time corrections and in the parameters of the geometrical set-up. Absorbed doses measured in thermoluminescent LiF dosimeters agree within this uncertainty with calculated ones derived from measured spectra (80 kVp, 120 kVp and 140 kVp) and tabulated mass energy absorption coefficients for LiF. Comparison with tabulated energy spectra from the literature clearly shows the effect of the extremely small anode angle (7 degrees) in the CT machine (15 degrees and 17 degrees for the tabulated energy spectra).

A compton scattering spectrometer for determining X-ray photon energy spectra

With the use of more sophisticated diagnostic technologies it is becoming increasingly important to know the energy spectra of the primary photons from clinical x-ray tubes. At the high fluence rates used under working conditions, it is necessary to greatly reduce the number of photons to the detector per unit time in order to avoid pulse pile-up. The Compton scattering method is very suitable for this reduction and hence it has been further developed in this work in the primary-photon energy range 20-200 keV. The movement of the electrons in the scattering target causes an energy broadening of the Compton scattered photons. This broadening results in a decreased energy resolution, which is particularly seen as a smearing out of the characteristic x-ray peaks of the anode material. Comparison between the spectrum obtained using a Compton spectrometer and unfolded with our reconstruction and the spectrum measured directly in the primary beam shows very good agreement even though relatively simple reconstruction algorithms have been used.

Prospects for microcomputerized-tomography using synchrotron radiation

A microversion of a computerized tomograph (CT) is described, in which the object is subjected to a successive series of translations with rotation by a small angle in between. The spatial resolution is determined by collimators and translation step lengths and is today, with clinical X-ray tube, of the order of 100 μm. The use of synchrotron radiation instead of X-ray tubes offers the advantages of much higher fluence rates, which can be used to diminish the exposure times from days to minutes or to increase the spatial resolution from 100 μm to about 1 μm. The possibility to receive monoenergetic photons of selectable energy makes it possible to avoid spectral hardening image artifacts, as well as to optimize the information sampling with regard to average absorbed dose or exposure time. Selectable photon energies are valuable also for tomochemistry applications.

Measurements of high energy photon spectra of synchrotron radiation from a storage ring

Abstract A new method for measuring the photon fluences and polarization degree, in different points, from high energy synchrotron radiation has been developed. Radiation from a beam of photons was scattered under right angles in two independent directions and the scattered radiation was measured and deconvoluted in order to obtain information about the original spectra. The results agree very well with theoretical calculations using experimental parameters for the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL), USA.