1994 Symposium on Radiation Measurements and Applications, 8th in a Series, Ann Arbor, Michigan. 16-19 May 1994.
In general, x-ray microtomography systems are limited by available x-ray flux and need long imaging times to obtain precise reconstructions with low quantum noise. We have used images recorded in high magnification to obtain volumetric (ie 3-dimensional) computed tomograms of small specimens (2 to 20 mm diameter). The system utilizes a cone beam reconstruction algorithm with image data obtained from a digital detector with high detection efficiency. Other groups have utilized x-ray synchrotron radiation emitted from high energy electron storage ring facilities to obtain computed tomograms. In this case, a high flux of parallel beam radiation is used with small detectors very high resolution.
We have analytically evaluated the quantum noise performance of a cone beam microtomography method wherein very small (2 mm) specimens are placed very close (8 mm) to a low power (50 Watt), very small (5 micrometer) point source of x-rays. With this geometry, the system resolution is dictated by the x-ray source dimensions and the flux is high due to source proximity. Electron Monte Carlo methods were used to predict x-ray emission strength and the thermal limit of a rotating anode tube was analytically estimated. The imaging time required for achieving a specified signal/noise was predicted using x-ray Monte Carlo calculations of detector efficiency. The performance using this cone beam, microfocus source method was then compared to the performance achieved with parallel beam, synchrotron systems.
While synchrotrons provide intense radiation beams which are highly parallel with very narrow spectral bandwidth, these attributes are not necessary for microtomography and the proposed microfocus source compares favorably with respect to the radiation flux penetrating the specimen. Our computer models predict a flux through the specimen of .5 x 10^10 photons/mm^2/s at 17 keV for our proposed rotating anode target (1 mA, 50 kV) as compared to .3-30 x 10^10 photons/mm^2/s at various synchrotrons. Furthermore, the parallel radiation beam in a synchrotron must be recorded with detectors having small element size which have low x-ray detection efficiency. The method proposed permits the use of detectors with very high detection efficiency. For these reasons, low power microfocus x-ray sources compare favorably to synchroton sources for microtomography applications.