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MR211 Unit 3- CR/DR

TermDefinition
Flat panel detector Two-dimensional (2-D) flat-panel array of detector elements with an x-ray absorption material, fastened to a thin glass backing, or substrate.
Del Detector element; an individual hardware cell in a DR image receptor, capable of producing a single electronic readout from incoming photon energy (either light or x-ray). Component of the active-matrix array, consisting of the semiconductor detection surface, the TFT, and the storage capacitor.
TFT Thin-film transistors; the electronic switching gate used in detector elements (dels) of direct-capture radiography (DR) TFT flat-panel detectors.
Storage capacitor Component of the del that stores the electrical charge; known as the “heart” of the del.
Fill factor The percentage of a del’s area dedicated to photon absorption. The larger the del, the higher the fill factor; the smaller the del, the lower the fill factor (because the TFT and storage capacitor can only be miniaturized to a certain point. If del size is decreased, the semiconductor detection surface decreases, so it can absorb less x-rays or light. The fill factor will affect both the spatial & contrast resolution. A higher fill factor increases both; a lower fill factor decreases these image qualities.
Semiconductor Detection surface of the del; will be primarily sensitive to x-rays or light, depending on the material used (selenium is sensitive to x-rays, silicon is sensitive to light).
a-Se Amorphous selenium; del semiconductor material used in direct conversion DR. Absorbs incoming x-rays and converts them to electrical charge. The amorphous (non-crystalline) form of selenium is used because it allows for a finely controlled thickness of the absorption layer to be coated onto the detector.
a-Si Amorphous silicon; Del semiconductor material used in indirect conversion DR. Absorbs incoming x-rays and converts them to light. The amorphous (non-crystalline) form of silicon is used because it allows for a finely controlled thickness of the absorption layer to be coated onto the detector.
Cesium iodide CsI; one type of phosphor used in the scintillation layer of indirect conversion DR detectors. Considered a structured phosphor, crystals are grown in a needle-like shape. The shape of this phosphor allows for greater x-ray detection (higher DQE) and minimal light spread (higher spatial resolution) when compared to gadolinium oxysulfide. Originally too delicate for portable detector usage, but due to advances in material construction, now commonly used in these types of detectors.
Gadolinium oxysulfide Gd2O2S; type of phosphor used in the scintillation layer of indirect conversion DR detectors. Considered an unstructured or turbid phosphor, crystals are arranged as powdered granules bonded to a polyurethane material. Configuration allows for the formation of air pockets, which creates more light spread (lower spatial resolution) and reduced efficiency (lower DQE) when compared to cesium iodide. Initially used primarily in portable detectors due to its ruggedness (from the unstructured phosphor layer).
Photodiode Solid-state electronic device that converts light or x-ray energy into electrical current. By using amorphous silicon as the semiconductor surface in indirect conversion DR, the del becomes a photodiode, which converts the light to an electrical charge, which is then stored in the capacitor.
Electron-hole pair Created during exposure; x-rays or light (depending on the type of conversion) ionize molecules of selenium or silicon, freeing up electrons. Each of these ionizing events creates the electron-hole pair, which consists of the freed electron and a positively charged hole it leaves in the semiconductor molecule. The “hole” consists of the gap in the molecule where the electron is missing.
Top electrode The freed electrons (from the electron-hole pairs discussed above) are attracted to the positive charge of the top electrode, which is an extremely thin conductor layer found in the del. The freed electrons drift up to the top electrode because of this positive charge.
Del electrode Negatively charged electrode within the del; the positively charged “holes” (see above) drift downward in the del because of their attraction to the del electrode. Creates a net positive charge at the base of the semiconductor that is stored in the del’s storage capacitor.
Glass substrate Backing of the flat-panel detector active-matrix array. Supports the components of the AMA.
AMA Active-matrix array; panel of electronic detector elements laid out in rows and columns (matrix), used to convert incoming light or x-ray photons into an electrical signal.
Data lines Network of wires within the AMA; when the charge flows out from each individual del, the data lines send that charge to the amplifier. Data lines and gate lines crisscross between the dels of the AMA.
Gate lines Network of wires within the AMA; controlled by the address driver. As bias voltage is applied to the gate lines, it causes the TFT “gates” to open in sequence. From there the charge is dumped into a corresponding data line, which sends the charge along to the amplifier. So the gate lines work to open the TFT switches, and the data lines receive the charge once they do.
Address driver Controls the gate lines; is responsible for the order, or sequence, in which the dels are read out.
Bias voltage Causes the TFT “gates” to open when the voltage is changed from -5 to +10 volts.
TFT “gates” TFT acts as a switch, or gate. When the TFT gate is closed, the electrical charge of that del remains in the storage capacitor. When a bias voltage is applied to the gate line of that del’s row, the TFT gate then will open to allow the charge to flow through the data line to the amplifier.
Amplifier Boosts the electrical signal sent from the data lines, and then sends it through an ADC into the computer for processing.
ADC Analog-to-digital converter; receives the signal from the amplifier, converts the analog signal to digital data sent to the computer.
Protective Layer IP layer of very thin, tough, clear plastic for protection of the phosphor layer
Phosphor Layer IP layer of photostimulable phosphor that “traps” electrons during exposure
Reflective Layer IP layer that sends light in a forward direction when released in the cassette reader
Conductive Layer IP layer that absorbs and reduces static electricity, which may degrade image quality
Support Layer IP layer of semi-rigid material that holds the other components of the IP together and provides mechanical strength
Light-shielding Layer IP layer that prevents light from erasing data on the plate or leaking through the backing, which would decrease spatial resolution
Backing Layer IP layer that is made of a soft polymer that protects the back of the cassette
Barcode In cassette-based CR imaging systems, this allows the technologist to match the image information with the patient-identifying barcode on the exam request
Stimulated Phosphorescence In CR systems, this delayed emission of light occurs due to restimulation by the CR reader’s laser scanning the imaging plate (aka photostimulable luminescence or PSL)
F-centers In the CR imaging plate, these are metastable defects in the pure crystal structure of a barium fluorohalide compound when it’s doped with europium; act like “electronic holes” that can trap electrons freed from x-ray ionization of the crystal
Raster Pattern Scanning from left to right and then moving down line-by-line; the CR reader’s laser moves in this type of pattern when scanning the IP
Fast Scan Direction Direction in which laser beam scans across the IP in the CR reader
Slow Scan (Subscan) Direction Direction of IP movement through the CR reader
Light Channeling Guide Bundle of optic fibers in the CR reader that picks up light emitted from the IP, and channels it to the photomultiplier tube (PMT)
Photomultiplier Tube (PMT) The CR reader component that amplifies and converts the emitted light from the IP into an electrical signal. Contains a photocathode for the conversion of light photons to electrons. The electrical signal is then sent to the ADC for digitization.
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