Detector, collimator and real-time reconstructor for a new scanning-beam digital x-ray (SBDX) prototype

Abstract

Scanning-beam digital x-ray (SBDX) is an inverse geometry fluoroscopy system for low dose cardiac imaging. The use of a narrow scanned x-ray beam in SBDX reduces detected x-ray scatter and improves dose efficiency, however the tight beam collimation also limits the maximum achievable x-ray fluence. To increase the fluence available for imaging, we have constructed a new SBDX prototype with a wider x-ray beam, larger-area detector, and new real-time image reconstructor. Imaging is performed with a scanning source that generates 40,328 narrow overlapping projections from 71 × 71 focal spot positions for every 1/15 s scan period. A high speed 2-mm thick CdTe photon counting detector was constructed with 320×160 elements and 10.6 cm × 5.3 cm area (full readout every 1.28 μs), providing an 86% increase in area over the previous SBDX prototype. A matching multihole collimator was fabricated from layers of tungsten, brass, and lead, and a multi-GPU reconstructor was assembled to reconstruct the stream of captured detector images into full field-of-view images in real time. Thirty-two tomosynthetic planes spaced by 5 mm plus a multiplane composite image are produced for each scan frame. Noise equivalent quanta on the new SBDX prototype measured 63%-71% higher than the previous prototype. X-ray scatter fraction was 3.9-7.8% when imaging 23.3-32.6 cm acrylic phantoms, versus 2.3-4.2% with the previous prototype. Coronary angiographic imaging at 15 frame/s was successfully performed on the new SBDX prototype, with live display of either a multiplane composite or single plane image.

Keywords: photon-counting detector; real-time tomosynthesis; scanning-beam digital x-ray; x-ray fluoroscopy.

Figure 1

SBDX prototype at the UW-Madison (A), and the operating principles of inverse geometry fluoroscopy (B). An electron beam is scanned over an array of positions on a large area target. A multihole collimator between the target and patient defines a series of narrow overlapping x-ray beamlets. The detector images captured in each scan frame are reconstructed at multiple planes using digital tomosynthesis, and a multiplane composite is formed for live display.

Figure 2

(A) shows the beamlet shift at plane z when the electron beam advances along a scan row. Tomographic angle (θ) is the maximum angular range of rays passing through a point (B).

Figure 3

Photon-counting detector array measuring 10.6 cm by 5.3 cm. The array has 320 × 160 elements, fabricated from 8 × 4 distinct ‘hybrid’ modules consisting of a 2 mm CdTe tile bonded to a readout ASIC. The 0.330 mm wide detector elements (orange) each contain 4 sub-elements (dashed lines). Veto logic links each sub-element (green) to four of its neighboring sub-elements (red).

Figure 4

Multihole collimator (A), showing close up of slots on the exit surface (inset). The collimator layers are shown for cross sections perpendicular (B) and parallel (C) to the detector long axis. (Red: tungsten, Green: brass, Dark blue: lead, Gray: aluminum, Light blue: water, Magenta: beryllium.) A single x-ray beamlet is shown in yellow.

Figure 5

Data flow in the reconstructor. Detector images acquired in a scan frame are distributed to GPUs, where a stack of tomosynthesis planes is reconstructed. A multiplane composite is generated from the plane stack for live display. Green blocks are PCIe switches. Right shows the plane stack reconstruction hardware (8 Nvidia K20 GPUs).

Figure 6

Noise equivalent quanta at the isocenter plane for the new and previous SBDX systems, at 120 kV, 12 kWp (A) and at 100 kV, 12 kWp (B). The 71 × 71 15 frame/s scan mode was used.

Figure 7

Simulation of the x-ray counts distribution within the detector area (A) for the new detector and collimator. A time-integrated detector image is shown in (B). The intensity variations across each tile arise from a gradient in the low energy discriminator threshold across the readout ASIC.

Figure 8

Multiplane composite image recorded during a 15 frame/s coronary angiogram (A) and one of the 32 tomosynthesis planes from the same frame period (B). Vessels that cross in the composite can be distinguished in individual planes (C), as shown for the three images corresponding to the white ROI.