Chapter 5: Thin-Film Transistor Flat Panel Array Image Acquisition

Thin-Film Transistor (TFT) flat panel amorphous silicon and amorphous selenium detectors

What were the first devices to move beyond the cassette into detectors that would reside in the table or wall stand, and were first introduced in 1995

Permanently enclosed inside a protective housing

With flat panel detectors, how are the materials used for detecting the X-ray signal and the sensors for recognizing the signal put in?

Flat Panel Detectors

What consists of a flat panel array with an X-ray absorption material (photoconductors and scintillators)?

The response of the photoconductors and scintillators to X-ray absorption

What does a flat panel detector measure?

2D / pixels / glass / substrate

Flat panel detectors is a large ____ array of _________ fastened to a thin _________ backing, or ______________

Amorphous Silicon Detector

What type of flat panel detector uses thin films of silicon integrated with arrays of photodiodes?

Photoconductors and Scintillators

What are the two main types of X-ray absorption materials in use today?

Photoconductors

What absorbs X-rays, resulting in an electrical charge?

Scintillator

What are phosphors that produce light when absorbing X-rays?

Indirect Conversion and Direct Conversion

What are the two major types of X-ray conversion?

Direct Conversion

What refers to X-ray photons absorbed by a coating material and immediately converted into an electrical signal?

Amorphous Selenium (a-Se)

In direct conversion, what is the radiation conversion material or photoconductor typically made of?

Absorbs x-rays and converts them to electrons, which are stored in the TFT detectors

In direct conversion, what does amorphous selenium (a-Se) do?

Thin-Film Transistor (TFT)

What is a photosensitive array made up of small pixels, which are also called detector elements (dels)?

A photodiode that absorbs the electrons and generates electrical charges

In the TFT array, what does each pixel contain?

The choice of the X-ray absorption material

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

Field Effect Transistor (FET) or Silicon TFT

What isolates each pixel element and reacts like a switch to send the electrical charges to the image processor?

1 million

More than how many pixels can be read and converted to composite a digital image in less than a second?

Indirect Conversion

What is a two-step process, where X-ray photons are converted to light, and then the light photons are converted to an electrical signal?

Gadolidium Oxysulfide (Gd2O2S) or thallium-doped Cesium Iodide (CsI[TI])

What are the scintillating phosphor materials used in indirect conversion radiography that rapidly absorb X-rays and produce light?

A photodetector such as hydrogenated amorphous silicon (a-Si:H)

In indirect conversion, after the X-ray photons are converted to light, what converts the light into an electrical charge?

scintillation

The phosphor layer is also known as the _________________ layer

Structured or Unstructured

What two things can the scintillation layer be?

Gadolinium Oxysulfide (Gd2O2S)

What detecting material is made from small crystals bound together in a turbid (powdered granules) layer along with a polyurethane material?

Unstructured

Is gadolinium oxysulfide structured or unstructured?

Allows light that was generated in the phosphor to escape

Because the crystalline layer in the gadolinium oxysulfide is filled with air pockets, what does this allow the light to do?

Reduces the efficiency of the phosphor and lowers spatial resolution

In gadolinium oxysulfide detecting material, since light is allowed to escape, what happens to the efficiency of the phosphor and the spatial resolution?

Cesium Iodide (CsI)

What detecting material is made by growing very thin crystalline needles perpendicular to the detector surface that work as light directing tubes?

Structured

Because of how cesium iodide crystals are grown, are they considered to be unstructured or structured?

A higher DQE or a greater detection of X-rays

Cesium iodide allows for a greater what?

The spatial resolution is higher

Since there is almost no light spread with cesium iodide detectors, what happens to spatial resolution?

It channels the light to the amorphous silicon photodiode array

After cesium iodide needles absorb the X-ray photons and convert their energy into light, what happens?

A sensing component and a switching component

What does each pixel in the TFT flat panel array contain?

A photodetector to detect the light

What does a phosphor/scintillator require?

Amplifies the light

What does a photodetector do?

Photodiode, phototransistor, or PMT

What is the same thing as a photodetector?

Storage Capacitor

What refers to the sensing element that is a capacitor and measures a charge if a photoconductor is selected?

A material that absorbs X-rays and emits an electrical signal

What is a photoconductor?

indirect / direct

A photodetector is used in ___________ capture and a photoconductor is used in _____________ capture

The sensing/storage component

When an exposure is made to a flat panel detector, what is within the pixel that contains the image information?

Read out line by line by changing the control line voltage

In a flat-panel detector, how is the information within each pixel read out?

Initilization

What prepares the flat panel detector for use?

In an off position

In the initialized state of a flat panel detector, what is the position of all the switches?

Nyquist Theorem

What states that when sampling a signal, the sampling frequency must be greater than twice the bandwidth of the input signal, so the reconstruction of the image will be nearly perfect?

Nyquist Frequency

What is the highest spatial frequency that a digital detector can record?

Aliasing

If the spatial frequency is greater than the Nyquist frequency or if the sampling occurs less than twice per cycle, what occurs?

A wrap around image that appears as two superimposed images slightly out of alignment

What type of image is produced when aliasing occurs?

Moiré Effect

What artifacts are produced when aliasing occurs?

If the grid frequency is near the Nyquist frequency of the detector array

When can aliasing also occur?

1. Numerical measurement of spatial resolution and DQE

2. Observation of low-contrast objects in a contrast detail phantom

What are the two ways to measure a flat panel detector?

Fill Factor

What is an important feature of the pixel in the TFT array that is defined as the ratio of the sensing area of the pixel to the area of the pixel itself?

1. TFT

2. Capacitor

3. Sensing area

In a pixel of the TFT array, what are its three main components?

From the layer above it

In a pixel of the TFT array, where does the sensing area receive its data from?

The sensing area of the pixels

In a pixel of the TFT array, what is the fill factor equal to?

20% of the pixel is the area occupied by detector electronics and 80% represents the sensing area

What does a fill factor of 80% mean?

Spatial resolution and contrast resolution (signal-to-noise) ratio

In a pixel of the TFT array, what two things does the fill factor affect?

Provide better spatial resolution and contrast resolution

In a pixel of the TFT array, detectors with high fill factors or large sensing areas will provide what?

Flat-Fielding

What corrections are made to purposely even out the overall signal or brightness across the entire area of the imaging field?

The anode heel effect

What does flat-fielding correct for?

The anode heel effect

What are flat-panel detectors particularly sensitive to?

Dust, scratches, static discharge, chemical corrosion, and interactions between materials

What are some things that can occur and result in some defective pixels?

Dead Pixels

What refers to pixels that may be malfunctioning or not functioning at all?

The number of dead pixels increases and they may not be detected if they are located on the edges of the panel

As the detector ages, what happens to the number of dead pixels, and why may they not be detected?

0.1% / 0.2%

Manufacturers make efforts to maintain a standard of less than approximately ____% to ____% defective pixels and build software programs into their systems to identify and isolate dead pixels

Interpolation Method

What is the software made by manufacturers to "fill in" the dead pixels with information using the surrounding pixels as a guide?

Gain Calibration

What is used to correct an area of the detector that has a large number of dead pixels or an area of the detector that has poor connections between the conversion layer and a-Si/TFT array?

Image Lag

If an image is taken prior to the detector releasing all of the signal from the previous image, and a faint image of the previous exposure is visible, what is this called?

1. A rapid succession of images

2. Overexposure or an area with little beam attenuation

What are the two reasons image lag can occur?

1. Increase the amount of time between exposures

2. Collimate as close as possible to reduce the amount of unattenuated beam

3. Set appropriate technical factors for each body part

What are the three ways to reduce the possibility of image lag?

Offset Correction

What determines the amount of signal inherent in the detector and removes the inherent signal that remains?

Offset Correction

What can image lag be corrected with?

Always follow the equipment manufacturer's guidelines

Before performing any detector calibrations, what should be followed?

Dose Area Product (DAP)

What is the quantity that not only reflects the patient's radiation dose but also the amount of tissue irradiated?

mGy/cm^2

What unit is the dose area product expressed in?

Collimated beam size or field size

What is DAP dependent on?

A meter embedded in the collimator

What is DAP measured by?