Design of a next-generation conventional scintillator x-ray detector: Improved spatial resolution and fill factor

The most compelling advantage that photon counting detectors (PCDs) currently have over energy integrating detectors (EIDs) for CT applications may be their improved spatial resolution. However, PCDs are difficult to manufacture. Next-generation EIDs using conventional scintillator sensors could be easier to mass produce, but their spatial resolution is thought to be limited by fill factor considerations. Specifically, each EID pixel comprises a block of scintillator surrounded by inactive, optically reflective septa. The minimum width of optically reflective septa is ∼0.1 mm, and unless this is shrunk, decreases in pixel pitch inevitably cause an increasing fraction of dead space.

To propose a modified system geometry in which the detector pixels are tilted by a few degrees, such that rays that are incident on reflective septa would later encounter scintillator. Because reflective septa are nearly transparent to x-rays compared to scintillator, this geometry restores much of the lost fill factor.

Tilting can be achieved in several different ways. We discuss three implementation options: individual detector modules may be tilted, with gaps between modules that are seamless from the perspective of the x-ray source; the whole detector can be arranged to point towards a "false" focal spot that is offset slightly from the true focal spot; the scintillator wafer can structured differently, e.g. by sawing it at an angle. The pros and cons of each option are discussed. Tilting introduces blurring, but a smaller pixel size improves resolution, and the combination of both improves both resolution and fill factor. We used Monte Carlo simulations to compare a conventional detector with 1.0 mm, 0.75, or 0.5 mm pixel pitch to a tilted detector with 0.5 mm pixel pitch. We assumed negligible optical crosstalk and electronic noise, that the scintillator was 1.5 mm thick with attenuation similar to gadolinium oxysulfide, that the reflective septa was 0.1 mm wide at standard resolution or 0.08 mm at higher resolutions and composed of a low-attenuating polymer similar to acrylic infused with 20% titanium content. We also compared this to a simple model of a CdTe detector with 1.6 mm thickness, 0.3 mm pixel pitch, and Gaussian charge sharing.

Tilt increases the aperture of the pixel in an energy-dependent fashion. With a 1D ASG (85% fill factor) and at 120 kVp, the detection efficiencies (DQE(0)) of a conventional detector at 1.0 mm, conventional detector at 0.5 mm pixel pitch, CdTe detector, and tilted detector at 0.5 mm pixel pitch are 65%, 55%, 66%, and 64%, respectively. With a sparse 2D ASG (1.5 × 3.0 mm openings, 86% fill factor), the DQE(0) of a tilted detector with 0.75 mm is 69%. The DQE(0) of all detectors decreases at higher energy because of punch-through.

By tilting detector modules a few degrees, energy-integrating detectors can provide increases in spatial resolution without the expected losses in fill factor. This architecture could bring the resolution improvements associated with photon counting CT into the domain of energy-integrating detectors equipped with conventional scintillator sensors.