Design of synchrotron light source and its beamline dedicated to dual-energy x-ray computed tomography.

A synchrotron light source dedicated to medical applications has been designed at National Institute of Radiological Sciences. The storage ring, with circumference of 80 m, is designed for acceleration of 2.3 GeV and a stored current of 420 mA. It is equipped with two multipole wigglers to produce sufficient photon flux in a hard x-ray region required for medical applications. The purposes of the synchrotron light source are clinical performance of medical diagnoses clinically and research and development relating with medical applications. One of the most interesting applications for us is dual-energy x-ray computed tomography (CT). It gives the information about electron density of human tissue. The information plays an important role in advancing heavy-ion radiotherapy of cancers. Electron density can be derived from attenuation coefficients measured by different energy x rays. In this paper, a practical method of the dual-energy x-ray CT with synchrotron radiation is proposed with the theoretical consideration. The primitive experiment using monochromatic x rays emitted from radioisotopes proved the procedure of analysis mentioned here effective to derive electron densities from linear attenuation coefficients for two x rays of a different energy. The beamline dedicated to dual-energy x-ray CT is also proposed. It has a multipole wiggler as a light source and it mainly consists of a dual crystal monochromator and a rotating filter for attenuating photon flux of x rays and two-dimensional detector.

Effect of scattered radiation on image noise in cone beam CT.

Cone beam CT has a capability for the 3-dimensional imaging of large volumes with isotropic resolution, and has a potentiality for 4-dimensional imaging (dynamic volume imaging), because cone beam CT acquires data of a large volume with one rotation of an x-ray tube-detector pair. However, one of the potential drawbacks of cone beam CT is a larger amount of scattered x-rays, which may enhance the noise in reconstructed images, and thus affect the low-contrast detectablity. Our aim in this work was to estimate the scatter fractions and effects of scatter on image noise, and to seek methods of improving image quality in cone beam CT. First we derived a relationship between the noise in a reconstructed image and in an x-ray intensity measurement. Then we estimated the scatter to primary ratios in x-ray measurements using a Monte-Carlo simulation. From these we estimated the image noise under relevant clinical conditions. The results showed that the scattered radiation made a substantial contribution to the image noise. However, focused collimators could improve it by decreasing the scattered radiation drastically while keeping the primary radiation at nearly the same level. A conventional grid also improved the image noise, though the improvement was less than that of focused collimators.