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CT detector construction & efficiency

A modern solid-state CT detector element is shown: incident x-rays enter a ceramic scintillator (e.g. gadolinium oxysulphide) that converts x-rays to visible light, which passes to a photodiode producing an electrical signal; reflective septa optically isolate each element and create dead space that lowers fill factor and efficiency. Its detection efficiency exceeds 90 percent. This is contrasted with an older pressurised xenon gas ionisation detector (about 10 to 20 atmospheres, deep along the beam) whose efficiency is only about 50 to 60 percent. A multi-row MDCT array of small elements separated by septa is shown, where binning adjacent elements along the patient axis gives a thicker slice with higher SNR but lower z-resolution. The material requirements are high detection efficiency, fast response with low afterglow, and wide dynamic range.

CT detectors: solid-state scintillator + photodiode vs older xenon gas, and the MDCT array A modern solid-state CT detector element is shown: incident x-rays enter a ceramic scintillator (e.g. gadolinium oxysulphide) that converts x-rays to visible light, which passes to a photodiode producing an electrical signal; reflective septa optically isolate each element and create dead space that lowers fill factor and efficiency. Its detection efficiency exceeds 90 percent. This is contrasted with an older pressurised xenon gas ionisation detector (about 10 to 20 atmospheres, deep along the beam) whose efficiency is only about 50 to 60 percent. A multi-row MDCT array of small elements separated by septa is shown, where binning adjacent elements along the patient axis gives a thicker slice with higher SNR but lower z-resolution. The material requirements are high detection efficiency, fast response with low afterglow, and wide dynamic range. Modern solid-state detector element incident x-ray scintillator ceramic (GOS / CdWO₄) x-rays → light light → photodiode photodiode → electrical signal signal reflective septa optically isolate each element → dead space lowers fill factor & efficiency detection efficiency > 90% Older xenon gas ionisation detector incident x-ray pressurised xenon gas ~10–20 atm deep along beam x-rays → ion pairs → ionisation current current low-density gas → must be pressurised & deep to absorb x-rays detection efficiency ~50–60% multi-row array (MDCT) channels (in-plane) z binned → 1 slice grid lines = reflective septa between elements small elements → finer sampling → better spatial resolution bin adjacent elements (z) → thicker slice ↑ SNR, ↓ z-axis resolution detector material requirements • high detection efficiency • fast response / low afterglow • wide dynamic range (avoid saturation & noise)
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