The Solid-State X-Ray Image Intensifier (SSXII): An EMCCD-Based X-Ray Detector

Abstract

The solid-state x-ray image intensifier (SSXII) is an EMCCD-based x-ray detector designed to satisfy an increasing need for high-resolution real-time images, while offering significant improvements over current flat panel detectors (FPDs) and x-ray image intensifiers (XIIs). FPDs are replacing XIIs because they reduce/eliminate veiling glare, pincushion or s-shaped distortions and are physically flat. However, FPDs suffer from excessive lag and ghosting and their performance has been disappointing for low-exposure-per-frame procedures due to excessive instrumentation-noise. XIIs and FPDs both have limited resolution capabilities of ~3 cycles/mm. To overcome these limitations a prototype SSXII module has been developed, consisting of a 1k x 1k, 8 mum pixel EMCCD with a fiber-optic input window, which views a 350 mum thick CsI(Tl) phosphor via a 4:1 magnifying fiber-optic-taper (FOT). Arrays of such modules will provide a larger field-of-view. Detector MTF, DQE, and instrumentation-noise equivalent exposure (INEE) were measured to evaluate the SSXIIs performance using a standard x-ray spectrum (IEC RQA5), allowing for comparison with current state-of-the-art detectors. The MTF was 0.20 at 3 cycles/mm, comparable to standard detectors, and better than 0.05 up to 7 cycles/mm, well beyond current capabilities. DQE curves indicate no degradation from high-angiographic to low-fluoroscopic exposures (< 2% deviation in overall DQE from 1.3 mR to 2.7 muR), demonstrating negligible instrumentation-noise, even with low input signal intensities. An INEE of < 0.2 muR was measured for the highest-resolution mode (32 mum effective pixel size). Comparison images between detector technologies qualitatively demonstrate these improved imaging capabilities provided by the SSXII.

Figure 1

Figure 1(a). Schematic of the solid-state x-ray image intensifier (SSXII) module which includes a CsI(Tl) scintillating plate, a fiber-optic taper (FOT) and an EMCCD camera with fiber-optic input window. Note: the FOT holder has been omitted so that the inner components can be seen. Figure 1(b). A photograph of the assembled SSXII detector module with the power supply unit attached.

Figure 2

Radiograph taken with the SSXII of the neurovascular device phantom consisting of : A) 150 μm diameter contrast filled polyethylene tubing, B) asymmetric stent with platinum markers, C) stent crimped on a 1 mm diameter balloon tipped catheter, D) 200 μm diameter contrast filled polyethylene tubing, E) stretched platinum coil and F) stainless-steel mesh patch.

Figure 3

MTF of the SSXII with (solid line) and without (dotted line) a 4:1 FOT. For comparison, the MTF_{No Taper} was corrected for the difference in effective pixel size to allow comparison between the two curves. Resolution well beyond 6 cycles/mm is demonstrated in both curves.

Figure 4

The difference between the two MTF curves in fig. 3 is determined as a function of spatial frequency by dividing the two, showing additional modulation resulting from the fiber-optic taper.

Figure 5

The measured DQE of the SSXII. Calculated results from a parallel cascade model demonstrate good agreement.

Figure 6

DQE of the SSXII on a semi-log plot for various exposures, indicating there is no drop as a result of instrumentation noise inclusion, even at low exposures.

Figure 7

Measured variance versus exposure data and the corresponding linear regression are used to measure an instrumentation-noise equivalent exposure (INEE) of 0.17 μR

Figure 8

MTF comparison between the SSXII and two FPDs.

Figure 9

DQE comparison between the SSXII and a FPD demonstrating substantial improvements of the SSXII at low exposures.

Figure 10

Images of an expanded stent with platinum markers taken with a FPD, an XII and the SSXII with 194, 120, and 32 μm pixel sizes respectively, demonstrating the improved resolution of the SSXII where individual stent struts are clearly visible.

Figure 11

Images taken of a neurovascular device phantom with a FPD, an XII and the SSXII with 194, 120, and 32 μm pixel sizes respectively, further demonstrating the improved resolution of the SSXII.