Principles of Digital Radiography with Large-Area, Electronically Readable Detectors: A Review of the Basics
Harrell G. Chotas, MS1,
James T. Dobbins, III, PhD1,2 and
Carl E. Ravin, MD1
1 Departments of Radiology, Digital Imaging Research Division (H.G.C., J.T.D., C.E.R.)
2 Biomedical Engineering (J.T.D.), Duke University Medical Center, Box 3302, Rm 139, Bryan Research Bldg, Research Dr, Durham, NC 27710.
View larger version (27K):
[in a new window]
|
Figure 1. Direct-readout electronic x-ray detectors use either a direct technique or an indirect technique for converting x rays into an electric charge. Direct-conversion detectors have an x-ray photoconductor, such as amorphous selenium, that converts x-ray photons into an electric charge directly, with no intermediate stage. Indirect-conversion devices have a scintillator that first converts x rays into visible light. That light is then converted into an electric charge by using an amorphous silicon photodiode array or a CCD. Thin-film transistor (TFT) arrays may be used in both direct- and indirect-conversion detectors.
|
|
View larger version (25K):
[in a new window]
|
Figure 2. Direct-conversion thin-film transistor detectors use a uniform layer of evaporated amorphous selenium to convert x rays into electron-hole pairs. Electric fields that are established within the selenium by means of bias voltages channel the charge to the nearest collector, which preserves spatial resolution. Very high fill factors are achievable with appropriate electrode design.
|
|
View larger version (21K):
[in a new window]
|
Figure 3. In indirect-conversion detectors, the x-ray scintillator can be structured or unstructured. Structured scintillators, which typically are crystalline cesium iodide, reduce the spread of visible light; this improves spatial resolution and permits the use of thicker scintillator materials for improved quantum detection.
|
|
Copyright © 1999 by the Radiological Society of North America.
