DR construction: Definitions

Bias voltage

Causes the TFT "gates" to open when the voltage is changed from -5 to +10 volts.

Storage Capacitor

Component of the del that stores the electrical charge; known as the "heart" of the del.

Address driver

Controls the gate lines; is responsible for the order, or sequence, in which the dels are read out.

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.

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.

ADC

Analog-to-digital converter; receives the signal from the amplifier, converts the analog signal to digital data sent to the computer.

Glass substrate

Backing of the flat-panel detector active matrix array.

Amplifier

Boosts the electrical signal sent from the data lines, and then sends it through an ADC into the computer for processing.

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 positively-charged hole it leaves in the semiconductor molecule. The "hole" consists of the gap in the molecule where the electron is missing.

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.

Semiconductor

Detection surface of the del; will be primarily sensitive to x-rays or light, depending on the material used (selenium or siliconalight)

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 active-matrix array, consisting of the semiconductor detection surface, the TFT and the storage capacitor.

Gadolinium oxysulfide

Gd2O2S; another 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. This 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).

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.

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, whih 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.

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 criss-cross between the dels of the AMA.

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.

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.

Channel

The electrical charge stored within a del must be allowed to exit the capacitor and exit through the TFT gates when they open. The change in the bias voltage from -5 to +10 creates a channel of conductivity to open up along the semiconductor material of the del. This allows the stored electrical charge to flow out.

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.

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. This is due to the fact that the TFT and storage capacitor can only be miniaturized to a certain point. Therefore, if the del size is decreased, the semiconductor detection surface decreased, so it can absorb less x-rays or light. The fill factor will affect both the spatial and contrast resolution. A higher fill factor will increase both, a lower fill factor will decrease these image qualities.

TFT

Thin-film transistors; the electronic switching gate used in detector elements of direct-capture radiography (DR).

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