chapter 5 -- TFT flat-panel array image acquisition

TFT

thin-film transistor

• flat-panel amorphous silicon and amorphous selenium detectors were introduced in 1995

• first devices to move beyond the cassette into detector that would reside in the table or wall stand

  • also can be used portably as a wireless device

• the materials used for detecting the x-ray signal and the sensors for recognizing that signal are permanently enclosed inside a protective housing

AMFPI

active matrix flat-panel imagers

• consist of a flat-panel array with an x-ray absorption material associated with TFT technology

  • pixels (detector element-Del) are arranged in matrix

  • two types of x-ray absorption materials in use today:

    • photoconductors

    • scintillators

• measures the response of these materials to x-ray absorption

• the absorption material is attached to the surface of this array either electrically (photoconductor) or physically (scintillating phosphor material)

• two major types of x-ray conversion:

  • indirect conversion

  • direct conversion

• the x-ray absorption material determines whether the detector is direct or indirect conversion

photoconductors

absorb x-rays resulting in an electrical charge

• direct conversion type

scintillators

phosphors that produce light when absorbing x-rays (need photodiode for converting light into electrical signal

• indirect conversion type

direct conversion

• 1 step: from x-ray to latent image

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

• the flat panel detector has a photoconductor, typically made of amorphous selenium

  • (a-Se)

  • absorbs x-rays, converts it into electrons

• this material absorbs x-rays and converts them to electrons (electrical signal), and then stored in TFT array — latent image

• the TFT is a photosensitive array made up of small pixels

  • these pixels are also called a detector element (del) in TFT arrays

  • each pixel includes light (x-ray) sensitive material and related electronic devices

    • converts x-ray into electrical charges

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

• TFT isolates each pixel element

  • reacts like a switch to send the electrical charges to the image processor

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

  • the information is discharged onto the data columns and it read out with dedicated electronics

• specialized circuits are connected along the edges of the detector matric

  • on one side, integrated circuits control the line scanning sequence

  • on the other, low-noise, high-sensitivity amplifiers perform the readout, amplification, and analog-to-digital conversion

indirect conversion

• a two-step process:

  • x-ray photons are converted to light, and then the light photons are converted to an electrical signal

• a phosphor such as Gd₂O₂S or thallium-doped cesium iodide (CsI[TI]) rapidly absorbs x-rays and produces light

  • the phosphor layer is known as the scintillation layer

• the scintillation layer can either be structure or unstructured

• the light is converted into an electrical charge by the photodiode, i.e. hydrogenated amorphous silicon (a-Si:H) photodiode array

• then electrical signal is digitalized by ADC and sent to computer for processing

Gd₂O₂S detectors

• made from small crystals bound together in an unstructured or turbid (powered granules) layer along with a polyurethane material

• the crystalline layer is filled with air pockets that allow light that was generated in the phosphor to escape

  • allows the light to escape laterally before it reaches the surface

    • light spreading, reducing the efficiency of the phosphor and lowering spatial resolution

    • unstructured layers produce more scatter light, decreasing detector efficiency

CsI detectors

• the most popular type of amorphous silicon detector uses a CsI scintillator

  • scintillator is grown into very thin crystalline needles (5 micrometer wide) perpendicular to the detector surface that works as light-directing tubes, much like fiber optics

• considered a structure scintillator

• this allows for greater detection of x-rays

  • better DQE performance than Gd₂O₂S scintillator

  • there is almost no light spread, the spatial resolution is higher

• from x-ray to electrical signal = two-steps

flat-panel array design and performance

• each pixel contains a sensing component and a switching component

  • TFT: acts as a switch of stored charges

  • Capacitor: for electrical charge storage

  • fill factor

  • pixel pitch

• for it to operate, there are about 2.5 km or 1.6 miles of electrical bias (a predetermined amount of voltage or current in the 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

• when the exposure is made, the sensing and storage component within the pixel contains the image information

• the information is read out line by line by changing the control line voltage

  • each pixel is connected to its corresponding data line at the bottom of the column

  • the process of returning to the state varies per vendor

• after release of its signal, the readout process continues to row two

• after the signal has been released, the energy is then transferred to the electronics that are attached to the edges of the array

• currently flat-panel a-Si:H detectors perform better than other digital imaging systems

• DQE, spatial resolution, contrast resolution

fill factor

defined as the ratio of light-sensitive areas of a pixel (or detector element) to the total area of the pixel

• it is in percentage

  • higher = better performance (DQE)

pixel pitch

the physical distance between two adjacent pixels from center to center

• the smaller the distance, the smaller the pixel pitch, the higher the spatial resolution

flat-panel array detector performance

detector performance measures:

• numerical measurement of spatial resolution and DQE

• observation of low contrast objects in a contrast detail phantom

detective quantum efficiency (DQE)

the measurement of the efficiency of a detector in converting incident x-ray energy into an image signal

spatial resolution

the ability of an imaging system to differentiate two adjacent structures as being distinct from one another

• refers to an imaging system’s ability to distinguish between small details or objects that are close together

• it’s a measure of how sharply an imaging system can represent fine structures within an image

contrast resolution (low contrast resolution)

observation of low contrast objects in a contrast detail phantom

• the ability of any imaging modality to distinguish between differences in image intensity

flat-panel artifacts

• dead pixels

• incorrect gain calibration

• image lag and incorrect offset correction

• because manufactures of the pixels is so complex, it is inevitable that the array structure will sustain some damage

  • dust, scratches, static discharge, chemical corrosion, and interactions between materials can occur, resulting in some defective pixels

dead pixels

• some pixels may be malfunctioning or dead (not functioning at all)

• as the detector ages, the number of dead pixels increases

  • these may not be detected if they are located on the edges of the panel

• manufacturers build software programs into their systems to identify and isolate dead pixels

  • the software “fills in” the dead pixels with information from surrounding pixels

incorrect gain calibration

• gain calibration is used to correct flaws in the detector

  • if an area of the detector has a large amount of dead pixels or if there is in area of the detector that has poor connection between the conversion layer and the a-Si/TFT array, a noticeable artifact can be seen on the screen

• to remove the potential of having the artifact interfere with diagnosis, a process known as gain calibration or flat fielding occurs

• this process will remove these densities on the image by creating a mask of those defects. When the subsequent image is taken, the software will use the mask to remove unwanted densities and leave only the diagnostic information

• 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

• these faint marks could lead to a misdiagnosis

image lag

• some flat-panel systems have the ability to take images faster than the detector can accommodate

  • if an image is taken before the detector releases all previous signal, a faint image of the previous exposure may be visible (image lag)

• when the detector is unable to eliminate all of the signals from the previous image, lag can occur

  • caused by rapid image succession and overexposure or an area with too little beam attenuation

• to reduce the possibility of image lag:

  • increase the delay between exposures

  • collimate closely to reduce the unattenuated beam

  • set appropriate technical factors

  • if multiple examinations are being done, acquire the image requiring the highest technique factors last

• can also be corrected with the detector’s software using the dark noise or offset correction

offset correction

• refers to the background electronic signal generated by the digital detector system in the complete absence of x-ray radiation

• determines the amount of signal inherent in the detector

  • if offset correction is performed before residual signal leaves the pixels, that information will be stored as inherent and could cause a negative image of the residual signal

• always follow the equipment manufacturer’s guidelines when performing any detector calibration