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

Indirect Conversion

Introduction

thin-film transistor (TFT)

flat-panel amorphous silicon and amorphous selenium (a-Se)

were the first devices to move beyond the cassette into detectors that would reside in the table and wall stan

Introduction

What are flat-Panel detectors

the materials used for detecting the X-ray signal

Introduction

The sensor of the flat-panel detector are enclosed where

rigid protective housing

Active-Matrix Flat-Panel Imagers

Active-matrix flat-panel imagers (AMFPI) consist of

a flat-panel array with an X-ray absorption material.

Active-Matrix Flat-Panel Imagers

The two main types of x-ray absorption materials

photoconductors and scintillators.

Active-Matrix Flat-Panel Imagers

What is a photoconductor

are materials that absorb X-rays, resulting in an electrical charge

Active-Matrix Flat-Panel Imagers

What is a scntillator

phosphors that produce light when absorbing x-ray

Active-Matrix Flat-Panel Imagers

What is the function of AMFPI

measures the response of these materials to X-ray absorption and is a large area two-dimensional (2-D) array of pixels

-fastened to glass

What is the function of ( 2-D) array of pixels

device that measures X-ray absorption rather than counting or measuring the actual X-rays.

The choice of x-ray absorption material determines

whether the detector uses direct or indirect signal conversion

Active-Matrix Flat-Panel Imagers

What is the absorption material that is attached to the surface of the array of pixels

electrically(photoconductor) or

physically (phosphor material)

Direct Conversion

What happens in direct conversion

X-ray photons are absorbed by the coating material and immediately converted into an electrical signal

direct conversion

The flat-panel detector has a radiation-conversion material or ________________

photoconductor

direct conversion

The flat-panel detector has a radiation-conversion material or ________________ , is typically made of what material

(a-Se)

Amorphus selenium

direct conversion

Function of (a-Se)

material absorbs X-rays and converts them to electrons, which are stored in the TFT detectors

direct conversion

During conversion process of

(a-se) x-rays are converted to electrons... where are the electrons stored

In TFT detectors

direct conversion

What is TFT?

a photosensitive array made up of small pixels, also called a detector element (del) in these TFT arrays

direct conversion

TFT is a photosensitive array made up of small pixels called DEL's, Each of the pixels contains ____________

a photodiode

direct conversion

What is the function of the photodiode

that absorbs the electrons and generates electrical charge

direct conversion

Wht is the function of 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

1 million pixels can be read and converted to a composite digital image in less than 1 second

direct conversion

What are TFT switches

line of TFT switches each associated with a storage capacitor, allows the electrical charge information to discharge when the switches are closed

direct conversion

TFT switches allow electrical charge information to discharge when swicthes are closed... where is the information discharged to/onto

data columns and read out with dedicated electronics.

direct conversion

Specialized silicon integrated circuits are connected where

connected along the edges of the detector matrix

Direct conversion

Specialized silicon integrated circuits...... On one side, integrated circuits control the ____________

On the other side, low-noise, highsensitivity amplifiers perform___________(3 things)

line scanning sequence

the readout, amplification, and analog-to-digital conversion

Direct conversion

After the Specialized silicon integrated circuits finish its processes what happens next

High-speed digital electronics are then used to achieve fast image acquisition and processing

Indirect Conversion

Indirect conversion is similar to direct conversion how

In the use of TFT array

Indirect Conversion

Indirect Conversion is different from direct conversion how

Indirect is a 2 step process

Indirect Conversion

What is the 2 step process?

1. X-ray photons are converted to light

2. Light photons are converted to electrical signal

Jargon....

Step 1:X-ray photons striking the dielectric receptor are absorbed by a scintillation or phosphor material in the imaging plate that converts the incident X-ray photon energy to light

Step 2: A photosensitive array, made up of small (about 100 to 200 µm) pixels, converts the light into electrical charges. Each pixel contains a photodiode that absorbs the light from the scintillator and generates electrical charges. A FET or silicon TFT isolates each pixel element and reacts like a switch to send the electrical charges to the image processor. As with direct conversion, more than 1 million pixels can be read and converted to a composite digital image in less than 1 second

Indirect Conversion

Phosphor Based Scintillatiors- also known as

The scintillation Layer

Indirect Conversion

What are the two types of phosphor based scintillation materials

phosphor such as gadolinium oxysulfide (Gd2O2S), or thallium-doped cesium iodide (CsI[TI])

Indirect Conversion

What is the function of the gadolinium oxysulfide (Gd2O2S), or thallium-doped cesium iodide (CsI[TI]) scintillation layer

absorbs xrays and produces light.

Indirect Conversion

In what two ways can the two can the scintillation layer be composed

structured or unstructured

Indirect Conversion

Characteristics of a unstructured scintillation layer

produce more scattered light than structured layers, thereby decreasing the efficiency of the detector.

Indirect Conversion

Process of Conversion

1. Phosphor Based Scintilator (x-rays & produce light)

Next

2. Photodetector ( detects light then converts to electrical charge )

Indirect Conversion

What material is the photodetector made of

hydrogenated amorphous silicon (a-Si:H) photodiode array

Indirect Conversion

phosphor based scintillation material= gadolinium oxysulfide has a scintllation layer of small crystals bound together how

unstructured or turbid (powdered granules)

Indirect Conversion

phosphor based scintillation material(gadolinium oxysulfide)

has a crystalline layer filled with air pockets that allow light generated in the phosphor to escape laterally before it reaches the surface

What is the downside of this?

reduces the efficiency of the phosphor and lowering spatial resolution

Indirect Conversion

phosphor based scintillation material cesium iodide (CsI) scintillator.

The most common type of phosphor based scin material is

cesium iodide (CsI) scintillator.

Indirect Conversion

phosphor based scintillation material cesium iodide (CsI) scintillator.

The Scintillation Layer of Cesium Iodide is composed how

Because of how the CsI crystals are grown ( perpendicular to the detector surface; crystalline needles work as light-directing tubes) they are considered a structured scintillator

Indirect Conversion

phosphor based scintillation material cesium iodide (CsI) scintillator.

Is cesium iodide better than gado oxy and why

Greater DQE and there there is no light spread

Indirect Conversion

phosphor based scintillation material cesium iodide (CsI) scintillator.

No light spread=

higher spread resolution

Indirect Conversion

phosphor based scintillation material cesium iodide (CsI) scintillator.

Perpendicular crystaline needles absorb (x-rays & convert energy into light) the light is channeled to the

amorphous silicon photodiode array

Indirect Conversion

phosphor based scintillation material cesium iodide (CsI) scintillator.

Today because of the advances in materials and crystalline formation on the array, both CsI and Gd2O2S are being used in portable detectors.

In the past the crystaline structure was too delicate to be used outside of the protected enviornment of a fixed detector array

Flat-Panel Array Design and Performance

Design

The way an a-Si:H array functions is very complex; however, the design is fairly simple. Each pixel contains a

sensing component and a switching component

Flat-Panel Array Design and Performance

Design

What are the two types of switching components

1. 2 contact switching diode

2. 3 contact TFT

Flat-Panel Array Design and Performance

Design

The choice of switching components depends on

depends on the type of absorbing material and the vendor's choice of construction.

Flat-Panel Array Design and Performance

Design

A phosphor/scintillator requires a ______________ to detect light

photodiode or phototransistor

Flat-Panel Array Design and Performance

Design

If a photoconductor (a-Se) is selected, the sensing element is a __________

capacitor

Flat-Panel Array Design and Performance

Design

If a photoconductor (a-Se) is selected, the sensing element is capacitor that measure _________

charge

Flat-Panel Array Design and Performance

Design

To operate, the flat-panel detectors use about 2.5 km or 1.6 miles of electrical bias (a predetermined amount of voltage or current in an electrical circuit), control, and signal output lines laid out on the array surfaces so that each pixel is monitored and refreshed during the control and reading process. Each row of pixels is connected to the same horizontal control line, and each column is connected to the same vertical data line. The readout electronics are attached to the edges of the array, and the entire array and its components are enclosed in a protective housing.

Flat-Panel Array Design and Performance

Design

When the exposure is made, the sensing/storage component within the pixel contains the image information. The information is then read out line by line via the changing of the control line voltage so each pixel is connected to its corresponding data line at the bottom of the column

Flat-Panel Array Design and Performance

Design

Once the exposure is made

Sensing component within the pixel contain image info

Fig. 5.7A illustrates the detector in its initialized state, prepared and waiting for exposure. In Fig. 5.7B, the detector has been exposed and is awaiting reading of the array. Fig. 5.7C demonstrates the readout or release of signal of row 1. Notice that a voltage is applied across the control or switch line corresponding to row 1. In the final image of Fig. 5.7, row 1 has returned to an initialized state (the process of returning to this state varies per vendor and is beyond the scope of this textbook) after release of its signal and the readout process continues to row 2. Once the signal has been released, the energy is then transferred to the electronics that are attached to the edges of the array.

Detector Performance

How is detector performance measured

1. Numerical measurement of spatial resolution & DQE

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

Detector Performance

current performance measurement of signal to-noise ratio and DQE indicate that ____________________ perform better than other digital imaging systems. Remember, this is really about efficiency and not resolution

a-Si:H

What is detective quantum efficiency

is a measurement of the efficiency of an image receptor in the converting the x-ray exposure received to a quality radiographic image

What is the relationship between DQE and exposure

The higher the DQE, the lower the radiation exposure to produce a quality image, therby decreasing patient exposure

What is signal to noise ration (SNR)

A method of describing the strength of the radiation exposure

(signal) in the comparison to the amount of noise apparent in the digital image.

What is the relationship of SNR & the amount of noise

(Increasing the SNR means that the strength of the signal is high compared to the amount of noise)

Relationship between SNR & the amount of image quality

Increased SNR= improved quality of the digital image

Relationship between Increased Noise and visibility of anatomic detail

Increased Noise- Decreased visibility of anatomic details

What is contrast to noise ratio

A method of describing the contrast resolution compared with the amount of noise apparent in digital imaging

What is the relationship between CNR and the visibilty of anatomic details

Increasing CNR increases the visibility of anatomic details

The Digital Image

Regardless of the type of digital image receptor, the varying electrical signals are sent to the ADC for conversion of digital data

The digitized pixel intensifies are patterened in the computer form the image matrix

The image matrix is a digital composite of the varying x-ray intensities exiting the patient

Each pixel has a brightness level representing the attenuation of chracteristics of the volume if tissue images

Flat-Panel Artifacts

What are the 3 types of flat-panel artifacts

1.Dead Pixels

2.Incorrect Gain Cali

3.Image Lag & Incorrect Offset Orientation

Flat-Panel Artifacts

Dead Pixels

There is a possibility in imaging systems ______________ image information due to defects

misrepresenting/ losing

Flat-Panel Artifacts

Dead Pixels

Artifacts can be cause how?

by malfunctions

Flat-Panel Artifacts

Dead Pixels

As detector ages more dead pixels are located where

located on the edge

Flat-Panel Artifacts

Dead Pixels

What are some reasons or malfunctions that will result in defective pixels

Dust, scratches, static discharge, chemical corrosion, or interactions between materials can occur

Flat-Panel Artifacts

Dead Pixels

As the detector ages, the number of dead pixels increases but may not be detected if they are located

on the edges of the panel.

Flat-Panel Artifacts

Dead Pixels

Manufacturers make efforts to maintain a standard of less than approximately _______________

0.1% to 0.2% defective pixels

Flat-Panel Artifacts

Dead Pixels

What software is used to fill in dead pixels called

Interpolation method

Flat-Panel Artifacts

Incorrect Gain Calibration

Gain Calibration is used for what reason

used to correct flaws in the detector.

Flat-Panel Artifacts

Incorrect Gain Calibration

What are the two ways a noticeable artifacts can be seen

If an area of the detector has a large number of dead pixels or if there is an area of the detector that has poor connections between the conversion layer and the a-Si/TFT array

Flat-Panel Artifacts

Incorrect Gain Calibration

To remove the potential of having that artifact interfere with diagnosis, a process known as ____________ or ____________

gain calibration or flat fielding is used

This process will remove these densities on the image by creating a mask of those defects

Flat-Panel Artifacts

Incorrect Gain Calibration

When a subsequent image is taken, the software will use the 1._____________ to remove the unwanted densities and leave only the 2._______________.

mask

diagnostic information

Flat-Panel Artifacts

Incorrect Gain Calibration

Gain calibration should be performed under the same or similar circumstances in which clinical images will be taken. The calibration should be done according to the equipment manufacturer's guidelines.

Flat-Panel Artifacts

Incorrect Gain Calibration

In the automatic exposure control (AEC) cells appear as slightly positive squares in the image. The gain calibration was performed with the detector outside of the Bucky and then a subsequent image was taken with the detector in the Bucky. This could happen if anything is left on the detector during a gain calibration. These faint marks could lead to a misdiagnosis.

Flat-Panel Artifacts

Image Lag and Incorrect Offset Correction

What is Image Lag

flat-panel systems can take images faster than the detector can accommodate. If an image is taken prior to the detector releasing all of the signal from the previous image, a faint image of the previous exposure may be visible

Flat-Panel Artifacts

Image Lag and Incorrect Offset Correction

How does image lag occur

Almost a double exposure of sorts.. when the detector is unable to get rid of all signal from previous image

Flat-Panel Artifacts

Image Lag and Incorrect Offset Correction

Reason Lag may occur? (2)

-Read succession of image is only one reason

-Lag might occur if overexposure or an area with little beam attenuation such as a techs lead marker

Flat-Panel Artifacts

Image Lag and Incorrect Offset Correction

Techniques to reduce Lag(3)

-Increase the amount of time between exposures giving the detector sufficient time to release all of the residual image

-Reduce the amount of attenuated beam by collimating as clearly as allowable

-Set appropriate technical factors for each body part being examined. If multiple examinations are being done, acquire the image requiring the highest technical factors last.

Flat-Panel Artifacts

Image Lag and Incorrect Offset Correction

Image Lag can also be corrected

How?

can be corrected with the detector's software using the dark noise or offset correction.

Flat-Panel Artifacts

Image Lag and Incorrect Offset Correction

Offset correction

If an offset correction is performed prior to residual signal leaving the pixels, that information will be stored as inherent and could cause a negative image of the residual signal.

Flat-Panel Artifacts

Image Lag and Incorrect Offset Correction

For example, in an offset correction was performed after an examination was done with a right-side marker in an area of unattenuated radiation. Residual signal remained in that area and a subsequent offset correction was performed. The R is now viewed inversely because that signal created a mask during the offset correction. In Fig. 5.10 it is clear that the signal is still very strong and this image was taken 10 minutes after the initial exposure.

For example, in an offset correction was performed after an examination was done with a right-side marker in an area of unattenuated radiation. Residual signal remained in that area and a subsequent offset correction was performed. The R is now viewed inversely because that signal created a mask during the offset correction. In Fig. 5.10 it is clear that the signal is still very strong and this image was taken 10 minutes after the initial exposure.

Emerging Technology

Back in the days tomo was used

Emerging Technology

Describe how tomo machines are constructed.

Tomograms were acquired by attaching a fulcrum to the Bucky and tubehead. The Bucky and tubehead would move in concert with one another and the tube would arc across the patient

Emerging Technology

fulcrum point determines what

determine the area in the body imaged.

This same concept is now being used in digital imaging and it is called tomosynthesis

Emerging Technology

Tomosynthesis is possible with what type of advances

advances in the selenium detector and amorphous silicon TFT arrays,

Emerging technology

During tomosynthesis how fast can images be acquired

rate of 30 frames per second.

Emerging Technology

After the acquisition of entire set of frames required for the part

the frames are sent to a postprocessing workstation for reconstruction

The images are then sent to a viewing station as a stack of images similar to a stack of computed tomography (CT) images

Emerging Technology

When is tomosyn used

in breast imaging for patients with dense breast & has been used on musculoskeletal imaging, with limited use