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Journal of Medical Imaging

Optimization of beam quality for photon-counting spectral computed tomography in head imaging: simulation study
Author(s): Han Chen; Cheng Xu; Mats Persson; Mats Danielsson
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Paper Abstract

Head computed tomography (CT) plays an important role in the comprehensive evaluation of acute stroke. Photon-counting spectral detectors, as promising candidates for use in the next generation of x-ray CT systems, allow for assigning more weight to low-energy x-rays that generally contain more contrast information. Most importantly, the spectral information can be utilized to decompose the original set of energy-selective images into several basis function images that are inherently free of beam-hardening artifacts, a potential advantage for further improving the diagnosis accuracy. We are developing a photon-counting spectral detector for CT applications. The purpose of this work is to determine the optimal beam quality for material decomposition in two head imaging cases: nonenhanced imaging and K-edge imaging. A cylindrical brain tissue of 16-cm diameter, coated by a 6-mm-thick bone layer and 2-mm-thick skin layer, was used as a head phantom. The imaging target was a 5-mm-thick blood vessel centered in the head phantom. In K-edge imaging, two contrast agents, iodine and gadolinium, with the same concentration (5  mg/mL) were studied. Three parameters that affect beam quality were evaluated: kVp settings (50 to 130 kVp), filter materials (Z=13 to 83), and filter thicknesses [0 to 2 half-value layer (HVL)]. The image qualities resulting from the varying x-ray beams were compared in terms of two figures of merit (FOMs): squared signal-difference-to-noise ratio normalized by brain dose (SDNR2/BD) and that normalized by skin dose (SDNR2/SD). For nonenhanced imaging, the results show that the use of the 120-kVp spectrum filtered by 2 HVL copper (Z=29) provides the best performance in both FOMs. When iodine is used in K-edge imaging, the optimal filter is 2 HVL iodine (Z=53) and the optimal kVps are 60 kVp in terms of SDNR2/BD and 75 kVp in terms of SDNR2/SD. A tradeoff of 65 kVp was proposed to lower the potential risk of skin injuries if a relatively long exposure time is necessarily performed in the iodinated imaging. In the case of gadolinium imaging, both SD and BD can be minimized at 120 kVp filtered with 2 HVL thulium (Z=69). The results also indicate that with the same concentration and their respective optimal spectrum, the values of SDNR2/BD and SDNR2/SD in gadolinium imaging are, respectively, around 3 and 10 times larger than those in iodine imaging. However, since gadolinium is used in much lower concentrations than iodine in the clinic, iodine may be a preferable candidate for K-edge imaging.

Paper Details

Date Published: 6 November 2015
PDF: 17 pages
J. Med. Img. 2(4) 043504 doi: 10.1117/1.JMI.2.4.043504
Published in: Journal of Medical Imaging Volume 2, Issue 4
Show Author Affiliations
Han Chen, KTH Royal Institute of Technology (Sweden)
Cheng Xu, KTH Royal Institute of Technology (Sweden)
Mats Persson, KTH Royal Institute of Technology (Sweden)
Mats Danielsson, KTH Royal Institute of Technology (Sweden)

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