Sean M. Hames

Microfocus X-ray sources for 3D microtomography

Abstract An analytic model for the performance of cone beam microtomography is described. The maximum power of a microfocus X-ray source is assumed to be approximately proportional to the focal spot size. Radiation flux penetrating the specimen is predicted by a semi-empirical relation which is valid for X-ray energies less than 20 keV. Good signal to noise ratio is predicted for bone specimens of 0.1 to 10 mm when scanned at the optimal energy. A flux of about 1 × 10 10 photons/mm 2 /s is identified for 0.2 mm specimens. Cone beam volumetric microtomography is found to compare favorably with synchrotron based methods.

A cone beam computed tomography system for true 3D imaging of specimens

A system for 3D cone beam computed tomography has been developed, consisting of a microfocus x-ray source and x-ray image intensifier coupled to a CCD camera. Full width at half maximum resolving power has been experimentally measured to be 70 microns when imaging 10 mm diameter objects. The 3D nature of the resulting image data can be used to visualize internal structure and compute parameters such as volume, surface area, and surface/volume orientation.

Quantum noise in digital X-ray image detectors with optically coupled scintillators

Digital X-ray imaging detectors designed for soft X-rays (1 to 50 keV) are significant for medical mammography, dental radiography, microradiography, and microtomography. Detector designs involve either direct absorption of X-rays in solid state devices or thin scintillator screens optically coupled to solid state sensors. Well designed scintillator systems produce 10 or more electrons per detected X-ray and, used with charge coupled devices (CCD), detect 100,000 X-rays per pixel before saturation. However, if the scintillator is directly coupled to the detector, radiation can penetrate to the semiconductor detector with a small number of events producing large charge and noise. The authors have investigated the degradation of image noise by these direct absorption events using numerical models for a laboratory detector system consisting of a 60 /spl mu/m CsI scintillator optically coupled to a scientific CCD. Monte Carlo methods were used to estimate the charge deposition signal and noise for both the CsI and the semiconductor. Without a fiber optic coupler, direct absorptions dominate the signal and increase the signal variance by a factor of about 30 at energies above 10 keV. With a 3 mm fiber optic coupler, no significant degradation is observed for input energies below 45 keV.

Direct measurement of resolution in volumetric imaging systems

The resolution of 2D imaging systems is frequently described by experimental estimates of the point, line, or edge spread function. It is shown that a response function across the normal to a boundary between two homogeneous volumes can provide a measure of resolution. The set of surface boundary voxels is determined by applying a simple threshold. The normal distance of every voxel to the surface is computed and an accumulator bin is incremented by the voxels gray level. This results in an aggregate surface response function, which is related to 3D point, line, and plane spread functions. This method can be applied to general boundary interfaces where precise surface normals are known, such as those on a sphere or plane. Results of applying this method to volumetric cone beam X-ray CT data are shown.<<ETX>>

Flexible laboratory system for 3D x-ray microtomography of 3-50 mm specimens

Point projection microradiography has established value for imaging large, wet, opaque, and intact specimens in 2D projection views. We have developed a 3D microtomography system by combining the principles of microradiography with computed tomography (CT). An extension of conventional CT methods is utilized to yield 3D data from 2D microradiographic projections. Use of 2D cone beam projections rather than 1D projections of a slice simplifies the specimen motion hardware, and reduces the amount of wasted radiation. Our imaging system consists of a microfocus x-ray source and x-ray image intensifier coupled to a CCD camera. The system is flexible in the size of specimens which can be imaged. Resolving power varies with specimen size from 4 lp/mm for 50 mm diameter objects to 40 lp/mm for 3 mm diameter objects. Image resolution is isotropic in three dimensions. The 3D nature of the resulting image data can be used to visualize internal structure and compute stereologic parameters such as volume, surface area, and surface/volume orientation. This instrument has been used to image bone specimens in studies of human vertebrae, human femoral necks, dog metacarpals, and rabbit tibias. Other applications include imaging small industrial parts, plastics, ceramics, composite materials, and geologic specimens.

Measurement of very small (1-10 micron) X-ray focal spot intensity distributions

A method using high-magnification, high-resolution, digital radiographs of a precise edge to obtain quantitative estimates of focal spot dimensions has been developed. The X-ray intensity distribution is determined from the edge response. The method has been used to assess the relationship between focal spot size and tube potential (kVp) for an X-ray source in which the electron beam is focused with an electromagnetic device.<<ETX>>

Monte Carlo calculation of X-ray spectra emitted by various anode materials at low voltages

Calculations of the absolute intensity and energy spectra of X-rays emitted by various targets have been performed for tube potentials in the 20-50 kV range using a single scattering Monte Carlo method. Monte Carlo modeling provides an accurate, practical method for obtaining X-ray spectra from arbitrary target materials for which experimental or semi-empirical results are not available. Our Monte Carlo program, SKEPTIC (Simulated Kilovolt Electron and Photon Transport In Condensed media), utilizes fast sampling schemes and employs extensive variance reduction techniques in order to reduce run time inefficiencies normally associated with single scattering electron transport simulations. Cross section data in the SKEPTIC physics model is taken from the partial wave elastic scattering formulation of Riley (1975) and the doubly differential bremsstrahlung description due to Pratt et al. (1977). Calculated spectra and intensities compare well with available experimental data.<<ETX>>

Measurement of noise and resolution in x-ray computed microtomograms

X-ray microcomputed tomograms with resolution of 50 microns or better have been obtained at several research centers by using either synchrotron or microfocus x-ray sources. Full 3D reconstructions have been obtained in these laboratories on specimens or small animals. In our laboratory we have studied embedded bone specimens by using x-ray cone beam microtomography-methods. For conventional, medical x-ray computed tomography systems, well accepted techniques and standards exist for characterizing the performance of an instrument. No comparable standards exist for computed microtomography and previous publications are vague with respect to the performance achieved. We report in this paper the experimental methods that we have developed to measure noise and resolution in our microtomography laboratory and the specific performance characteristics we have achieved.

Resolution and noise in X-ray microtomography of bone

The authors have previously used volumetric computed tomography for imaging bone specimens with approximately 50 micrometer resolution. Three dimensional measurement methods which can determine bone density to within one percent for detail as small as 5 micrometers would be useful for evaluating patterns of bone remodeling within trabelulae from biopsy specimens. Using theoretical modeling based on Monte Carlo photon/electron radiation transport calculations, the authors predict that this performance can be achieved using electron impact, microfocus X-ray tubes operating at 50 Watts with a total specimen scan time of 60 minutes for specimens having a 1.25 mm dimension and for reconstruction arrays of 256/spl times/256/spl times/256.<<ETX>>

Effect of direct x-ray interaction in the photodetector on image noise for a CCD/scintillator system

Digital x-ray imaging detectors for soft x-rays (1 to 50 keV) are significant for medical mammography, dental radiography, microradiography, x-ray diffraction, and x-ray telescope applications. For systems in which a scintillator is optically coupled to a photodetector, a small number of electrons are produced for each interaction in the scintillator (more than 10 e- in well designed systems). However, x-rays interacting directly in the photodetector produce a large charge for each detected event (about 4,700 e- for a 17 keV x-ray absorbed in silicon. These direct interactions can significantly increase image noise even when their interaction probability and contribution to image signal are low. In this paper we report on analytic investigations of the direct and indirect contributions to noise for a system consisting of a CsI scintillator, fiber optic coupler, and CCD photo-detector. The fiber optic coupler is shown to be essential to shield the CCD detector from direct interactions. A 3 mm thick fiber optic coupler is sufficient to eliminate most of the events that can noticeably increase noise.