Chapter 5: TFT Flat-Panel Array Image Acquisition

Vocabulary flashcards summarizing key terms, materials, processes, performance metrics, artifacts, and emerging technology related to TFT flat-panel array image acquisition.

TFT Flat-Panel Detector

A flat-panel x-ray detector that uses thin-film transistors to capture and read out digital images, replacing traditional cassettes.

Active Matrix Flat-Panel Imager (AMFPI)

Detector consisting of a large 2-D pixel array on a glass substrate that measures x-ray absorption rather than counting individual photons.

Photoconductor

X-ray absorption material that converts incoming x-ray photons directly into an electrical charge (e.g., amorphous selenium).

Scintillator

Phosphor layer that converts x-ray photons to light, which is then changed to an electrical signal by photodiodes (e.g., CsI, Gd₂O₂S).

Direct Conversion

Single-step detection process in which x-ray photons are converted directly to electric charges within a photoconductor layer.

Indirect Conversion

Two-step detection process where x-rays are first changed to light in a scintillator, then the light is converted to electric charge by photodiodes.

Amorphous Selenium (a-Se)

500 µm-thick radiography photoconductor (200-250 µm for mammography) that directly converts x-rays to electrons in direct detectors.

Amorphous Silicon (a-Si:H) Photodiode

Hydrogenated amorphous silicon device that converts scintillator light to electrical charge in indirect detectors.

Detector Element (del) / Pixel

Small (≈100–200 µm) photosensitive unit within a TFT array that stores and transmits image charge.

Field-Effect Transistor (FET) / Silicon TFT

Switching component that isolates each pixel and routes stored charge to image processing electronics.

Storage Capacitor

Capacitor within each pixel (especially in a-Se systems) that holds the collected electrical charge until readout.

Structured Scintillator

Scintillation layer with ordered crystal geometry (e.g., needle-grown CsI) that channels light and yields higher spatial resolution.

Unstructured (Turbid) Scintillator

Random granular phosphor layer (e.g., Gd₂O₂S) that scatters light, lowering efficiency and spatial resolution but adds ruggedness.

Gd₂O₂S (Gadolinium Oxysulfide)

Turbid phosphor used in rugged, often portable detectors; allows lateral light escape and reduced resolution.

Cesium Iodide (CsI[TI])

Needle-like structured scintillator with high DQE and minimal light spread, most common in a-Si detectors.

Detective Quantum Efficiency (DQE)

Numeric measure of a detector’s ability to produce high-quality images at given radiation levels; flat panels have high DQE.

Signal-to-Noise Ratio (SNR)

Ratio comparing useful image signal to background noise; improved in a-Si:H flat-panel systems.

Flat-Panel Artifact

Image error caused by detector component defects, including dead pixels, calibration errors, or image lag.

Dead Pixel

Non-functioning pixel in the array; manufacturers keep defective pixels below ~0.1–0.2 % and correct them via interpolation.

Gain Calibration (Flat-Fielding)

Software procedure that creates a defect mask to equalize pixel gains and eliminate fixed pattern artifacts.

Offset (Dark Noise) Correction

Calibration removing inherent detector signal; if performed with residual charge present it can imprint inverse images.

Image Lag

Residual signal from a previous exposure that appears on subsequent images when the detector is read too quickly or overexposed.

Active Matrix Pixel Addressing

Method where horizontal control lines and vertical data lines sequentially activate pixels for readout.

Two-Contact Switching Diode

Simpler pixel switch design alternative to a TFT used in some flat-panel arrays.

Three-Contact TFT

Common pixel switch design providing precise control of charge readout in flat-panel detectors.

Spatial Resolution

Detector’s ability to distinguish small, closely spaced details; affected by scintillator structure and pixel size.

Tomosynthesis

Digital imaging technique acquiring rapid sequential frames (≈30 fps) and reconstructing them into sectional images, currently prominent in breast imaging.

Turbid Phosphor

Random, powdered phosphor layer (e.g., Gd₂O₂S) producing scattered light and lower resolution compared with structured needles.

Rugged Detector

Detector design (often using unstructured Gd₂O₂S) capable of portable use due to its physical durability.

Contrast Detail Phantom

Test object used to visually assess low-contrast performance of imaging detectors.