Radiation background and beam quality in secondary X-ray imaging for human angiography with Au as contrast agent

Abstract A calculation is carried out to assess the capabilities of Secondary X-ray Imaging (SXI). Human angiography is used as an example. An originally well focused primary photon pencil beam is rastered through the target (the human heart) in two dimensions. The signal consists of fluorescent photons from a contrast agent, registered by a wide angle (of order π steradians) detector. The primary photons may be significantly more energetic than the signal photons. The last two features result in clearer images and a reduction of shadowing by obstructions inside the body. Sharp imaging is compatible with locally quantitative measurements, and with pixel-by-pixel elemental analysis. The detector need not be position sensitive. Most primary photons will be (predominantly Compton) scattered before they reach the target, the scattered photons form a broad halo around the unscattered primary beam, which remains sharp and well focused. To discriminate against scattered background, the photons have to pass through a position/momentum selector, a W–Hf absorber shield, and a time window. The calculation gives the approximate energy spectrum for the scattered photons generated, for the photons passing through the position/momentum selector, and for those arriving at the far side of the absorber shield, the last two evaluated for various time windows in the 1000– 167 ps range. The surviving background will cause relative image intensity fluctuations of the order of a percent. The primary beam intensity required for SXI is comparable or less than the intensity needed for the usual K-edge subtraction (KES) imaging process. However, for SXI the primary photon energy spread may be one or two orders higher than needed for KES, thus relaxing the requirements on the primary photon source. If an undulator is used, monochromatization may not be needed. That further reduces cost and demands on the photon source, which may be a small low energy electron ring. To realize the full potential of the method (beyond that stated above), would require a new type of detector.

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