Radiology: Flat-Panel Image Acquisition

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Questions and Answers

What determines whether the detector is direct conversion or indirect conversion?

The thickness of the x-ray absorption material

The number of pixels in the array

The type of TFT array used

The type of x-ray absorption material (correct)

What is the function of the storage capacitor in a direct conversion detector?

To store the electrical charges (correct)

To generate electrical charges

To release the electrical charges

To absorb x-rays

What is the purpose of the photoconductor in a direct conversion detector?

To absorb light

To store electrical charges

To absorb x-rays and convert them to electrons (correct)

To generate electrical signals

What is the typical material used as the radiation-conversion material in direct conversion detectors?

Amorphous selenium Signup and view all the answers

What is the primary function of the sensing element in a photoconductor?

To measure charge Signup and view all the answers

What happens to the signal after it is released from a pixel?

It is transferred to the electronics attached to the edges of the array Signup and view all the answers

What is the purpose of gain calibration in a flat-panel detector?

To correct flaws in the detector Signup and view all the answers

What causes dead pixels in a flat-panel detector?

Dust, scratches, static discharge, chemical corrosion, and interactions between materials Signup and view all the answers

What is the purpose of the readout process in a flat-panel detector?

To read out the image information line by line Signup and view all the answers

What could be one result of incorrect offset correction in a flat-panel detector?

Residual signal from previous exposure remains visible Signup and view all the answers

What is used to identify and isolate dead pixels in a flat-panel detector?

Gain calibration Signup and view all the answers

What is the term for the faint image of the previous exposure that may be visible when an image is taken prior to the detector releasing all of the signal?

Image lag Signup and view all the answers

What is the purpose of collimating as closely as allowable to reduce the possibility of image lag?

To reduce the amount of unattenuated beam Signup and view all the answers

What is the result of performing an offset correction prior to the residual signal leaving the pixels?

A negative image of the residual signal is created Signup and view all the answers

What is the name of the imaging technique that uses a fulcrum to move the bucky and tubehead in concert with one another?

Tomography Signup and view all the answers

What is the primary function of a field-effect transistor (FET) in a digital radiography detector?

To isolate each pixel element and react like a switch to send the electrical charges to the image processor Signup and view all the answers

What is the purpose of the scintillation layer in indirect conversion?

To absorb x-ray photons and produce light Signup and view all the answers

Which of the following materials is known for its ruggedness and is often used in portable detectors?

Gadolinium oxysulfide (Gd2O2S) Signup and view all the answers

What is the advantage of using cesium iodide (CsI) scintillators over gadolinium oxysulfide (Gd2O2S) scintillators?

Higher DQE and higher spatial resolution Signup and view all the answers

What is the function of the crystalline needles in cesium iodide (CsI) scintillators?

To channel light to the amorphous silicon photodiode array Signup and view all the answers

What is the purpose of the photodiode or phototransistor in a digital radiography detector?

To detect the light produced by the phosphor/scintillator Signup and view all the answers

What is the difference between structured and unstructured scintillation layers?

Structured layers have a higher DQE, whereas unstructured layers have a lower DQE Signup and view all the answers

What limited the possibility of tomosynthesis in the past?

Inability of digital detectors to acquire images rapidly in succession Signup and view all the answers

What is the main advantage of direct conversion detectors in TFT flat-panel technology?

They have a higher spatial resolution Signup and view all the answers

What is the primary limitation of indirect conversion detectors in TFT flat-panel technology?

They have a lower spatial resolution Signup and view all the answers

What is the main application of tomosynthesis currently?

Breast imaging Signup and view all the answers

What is the main difference between direct and indirect conversion detectors?

The method of converting x-rays to an electrical signal Signup and view all the answers

What is the potential consequence of incomplete transfer of the signal generated in the detector?

Artifacts, especially with short acquisition or rapid succession acquisitions Signup and view all the answers

Study Notes

TFT Flat-Panel Array Image Acquisition

TFT flat-panel amorphous silicon and amorphous selenium detectors were introduced in 1995.
There are two major types of x-ray conversion: direct and indirect.

Direct Conversion

X-ray photons are absorbed by the coating material (amorphous selenium) and immediately converted into an electrical signal.
The flat panel detector has a radiation-conversion material or photoconductor, typically made of amorphous selenium (a-Se) with a thickness of:
- 500µm for radiography
- 200-250µm for mammography
The material absorbs x-rays and converts them to electrons, which are stored in the TFT detectors.
The thin-film transistor (TFT) is a photosensitive array made up of small (about 100 to 200 μm) pixels, which are also called detector elements (del).
Each pixel contains a photoconductor that absorbs the electrons and generates electrical charges.
The charges are then sent to the storage capacitor to await the releasing of the charge.
A field-effect transistor (FET) or silicon TFT isolates each pixel element and reacts like a switch to send the electrical charges to the image processor.

Indirect Conversion

Similar to direct detectors, indirect conversion uses TFT technology, but with a two-step process:
1. X-ray photons are converted to light
2. The light photons are converted to an electrical signal
A phosphor (such as gadolinium oxysulphide or thallium doped cesium iodide) rapidly absorbs x-rays and produces light.
The scintillation layer can be either structured or unstructured, with unstructured layers producing more scattered light, thereby decreasing the efficiency of the detector.
The light is then converted into an electric charge by a photodetector (such as hydrogenated amorphous silicon photodiode arrays).

Gd2O2S Detectors

Gd2O2S detecting material is made from small crystals bound together in an unstructured or turbid layer along with a polyurethane material.
The crystalline layer is filled with air pockets, allowing light to escape, but reducing the efficiency of the phosphor and lowering spatial resolution.
Gd2O2S was used primarily for rugged applications, such as portable detectors.

CsI Detectors

Cesium iodide crystals are grown, making them a structured scintillator.
The scintillator is made by growing very thin crystalline needles (5 µm wide) perpendicular to the detector surface, which work as light-directing tubes.
The needles absorb the x-ray photons and convert their energy into light, channeling it to the amorphous silicon photodiode array.
CsI detectors have higher detection of x-rays (higher DQE) and higher spatial resolution than Gd2O2S scintillators.

Flat Panel Array Design and Performance

The flat panel array consists of a sensing component and a switching component.
There are two types of switching components: a 2-contact switching diode or a 3-contact thin film transistor (TFT).
The sensing component depends on the type of absorbing material and the vendor's choice of construction.
The array operates with about 2.5 km of electrical bias, control, and signal output lines.
Each pixel is monitored and refreshed during the control and reading process.

Artifacts

Dead pixels can be caused by malfunctions in the detector, and can be corrected by the manufacturer's software.
Incorrect gain calibration can cause artifacts, and should be performed according to the equipment manufacturer's guidelines.
Image lag can occur due to rapid succession of images or over exposure, and can be corrected by increasing the amount of time between exposures and using the detector's software.

Emerging Technology

Tomosynthesis is an emerging technology that uses advances in selenium detector and amorphous-silicon TFT arrays to acquire images at a rate of 30 frames per second.
The detector is able to acquire images rapidly in succession, allowing for reconstruction of images similar to a stack of CT images.
Tomosynthesis is currently being used in breast imaging, particularly for patients with dense breast tissue.

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Description

Learn about the principles of TFT flat-panel image acquisition, including indirect and direct x-ray conversion, active matrix flat-panel imagers, and x-ray absorption materials.