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A Flat Panel Detector (FPD) is a modern digital X-ray detector used in digital radiography (DR), fluoroscopy, CT, and angiography systems. It directly converts X-rays into electrical signals, producing high-resolution digital images without the need for traditional film or image intensifiers.
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Structure of a direct-conversion flat-panel X-ray detector: a, X-ray detectors used in radiology instruments must be at least as large as the body part to be imaged, and require an integrated circuit of the same size. b, The circuit consists of a semiconductor sandwiched between two types of electrode: a bias electrode and an array of pixel electrodes. Detectors in which the semiconductor is methylammonium lead triiodide (MAPbI3) are much more sensitive to X-rays than currently used detectors.The absorption of a single X-ray quantum by MAPbI3 generates an electron 'track' consisting of thousands of electrons. The electric field created by the bias electrode projects and focuses the track onto the pixel electrodes. The resulting signal is then processed to generate an image.
Rowlands, J. Material change for X-ray detectors. Nature 550, 47–48 (2017). https://doi.org/10.1038/550047a
FPDs consist of:
Types:
| Type | Indirect Conversion FPDs | Direct Conversion FPDs |
|---|---|---|
| Principle | X-rays → light → electrical signal | X-rays → electrical signal directly |
| Component | • Scintillator (Cesium Iodide [CsI] or Gadolinium oxysulfide [Gd₂O₂S]) converts X-rays into visible light. | |
| • Photodiodes convert light into charge. | ||
| • TFT array digitizes and reads the signal. | • Uses amorphous selenium (a-Se) photoconductor. | |
| • X-rays create electron–hole pairs → directly collected by electrodes/TFT. | ||
| Advantages | High sensitivity, lower dose, commonly used in radiography & fluoroscopy | Higher spatial resolution (no light scatter) |
| Limitation | Light scatter slightly reduces spatial resolution | More expensive, sensitive to mechanical stress |
Cross-section of an indirect TFT detector using CsI structured phosphor shows the conversion of X-rays first into light, traveling through the structured phosphor to a photodiode etched on the TFT array, and the creation of a proportional charge stored in the local capacitor
Seibert, J.A. Flat-panel detectors: how much better are they?. Pediatr Radiol 36 (Suppl 2), 173–181 (2006). https://doi.org/10.1007/s00247-006-0208-0
Cross-section of a direct TFT detector using a thick a-Se semiconductor layer under high voltage shows the creation of ion pairs directly by X-ray absorption, the separation and collection of the opposite charges at the electrodes, and the storage of the charge in the local capacitors
Seibert, J.A. Flat-panel detectors: how much better are they?. Pediatr Radiol 36 (Suppl 2), 173–181 (2006). https://doi.org/10.1007/s00247-006-0208-0
| Digital radiography (DR) | Chest, skeletal, abdominal imaging. |
|---|---|
| Fluoroscopy | Gastrointestinal studies, interventional radiology |
| Angiography | High-quality vascular imaging with real-time guidance |
| Cone-beam CT | Dental, interventional procedures |
| Portable X-rays | Bedside ICU imaging |
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Offer improved image quality, reduced radiation dose, and better ergonomics.
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