Physics Outline Exam #4 - Ch 10 Slides

CR & DR detector design, response, capture, image acquisition, extraction and processing

Can CR systems be used with existing radiographic equipment?

Yes, but AEC must be recalibrated, and APR/technique charts must be adjusted

What are the primary components of a cassette-based CR system?

• Cassette

• Photostimulable Phosphor (PSP) plate

• Plate reader

• Computer workstation

What is a CR cassette made of?

• lightweight plastic

• lined with felt

• backed with aluminum

What are the layers of a Photostimulable Phosphor (PSP) plate?

• protective layer (yellow)

• phosphor layer (green)

• reflective layer (blue)

• conductive layer (light purple)

• color layer (purple)

PSP Plate (layer by layer): protective layer (yellow)

Thin plastic, protects phosphor layer

PSP Plate (layer by layer): phosphor layer (green)

• 85% Barium Fluorohalide halide family

• less than 1 mm thick

• europium activated

• Structured Phosphor:

  • columnar phosphor crystals with an active layer: looks like needles on end and packed together

• Turbid phosphor:

  • random distribution of phosphor crystals in active layers

PSP Plate (layer by layer): reflective layer (blue)

• reflects blue light

• reflects light released during the reading phase toward the photodetector

PSP Plate (layer by layer): conductive layer (light purple)

Reduces and conducts away static electricity; grounds

PSP Plate (layer by layer): color layer (purple)

Rigidity to the plate

Color layer of PSP (newer plates; not in all plates):

Absorbs stimulating light, reflects emitted light

Soft backing of PSP plate:

Protective; layer that provides cushioning and support for the color layer, enhancing durability.

What is the function of the phosphor layer in a PSP plate?

Absorbs x-rays and stores energy as trapped electrons, forming a latent image

What percentage of electrons are trapped during exposure?

50% of freed electrons are trapped in the conduction band

How long can trapped electrons remain in the plate?

For hours, though image quality deteriorates over time

The quantity and distribution of the liberated electrons is proportional to:

the x-ray exposure received in each particular area of the plate

During processing, the energy of trapped electrons are:

released by exposure to a laser

What happens when the PSP plate is exposed to x-rays?

phosphor atoms are ionized

Approximately half of the removed electrons are:

“trapped” in the conduction band

Step #1 in the general CR reader design: Driver Mechanism

moves the plate through the laser scanning process (in some the laser moves)

Step #2 in the general CR reader design: Optical System

made up of a laser, beam-shaping optics, light-collecting optics, and optical filters, to project and guide a precisely controlled laser

Step #3 in the general CR reader design: Photodetector

A photodetector senses the light released during scanning

Step #4 in the general CR reader design: Analog-to-digital converter (ADC)

converts light to an electronic signal

Step #5 in the general CR reader design:

Computer processes and displays the image

Step #6 in the general CR reader design:

Reader erases plate by grounding the metastable electrons

How is image-forming radiation handled in DR systems?

Image-forming radiation is captured and transferred to a computer from the detector array for viewing at the control panel

What are the two DR categories?

• Indirect capture

• Direct capture

What are the components of the first type of indirect capture system?

• Charge-coupled device (CCD)

• X-ray scintillator

• Optics

What is “tiling” in charged-coupled device (CCD) systems?

A process where several CCD detectors abut to create one larger detector—seams created are corrected by computer software

What is the function of the scintillator?

A cesium iodide phosphor plate that absorbs x-ray energy and emits visible light in response

What must be done to the cesium iodide scintillator?

It must be hermetically sealed to avoid moisture absorption and prevent rapid degradation

How is the scintillator connected to the CCD?

By using a fiber-optic bundle or optical lens system

What is the correct order of steps in Indirect 1 (CR) capture using a charged-couple device (CCD)?

1. X-rays absorbed by scintillator

2. Converted to light

3. Light energy transmitted to CCD

4. CCD converts to electronic signal

5. Electronic signal sent to computer for processing and display

What is Complementary Metal Oxide Semiconductor (CMOS) and how is it different from CCD?

• uses less power, cheaper, lower resolution/sensitivity

• each detector has its own amplifier, photodiode, and storage capacitor

• often used in dental, mammography, and 3D C-arm imaging

What components are used in indirect capture type two?

• Scintillator with cesium iodide or gadolinium oxysulfide as the phosphor

• Photodetectors

• Thin-film transistor (TFT) array

How is the panel structured in indirect capture (2)?

• The scintillator plate panel is configured into a network of pixels or detector elements (DELs).

• Pixels consist of a photodetector and a TFT.

Describe the components of the indirect (2) capture system: Scintillator

Phosphors: Cesium iodide or gadolinium oxysulfide (note: more light spread)

Describe the components of the indirect (2) capture system: Photodetector

• Amorphous silicon (a-Si) photodiode, a liquid that can be painted on a substrate

• Makes flat panel detectors possible

Describe the components of the indirect (2) capture system: Thin-film transistor (TFT) array

• Electronic components layered onto glass substrate

• Glass substrate: readout, charge collector, and light-sensitive elements

Indirect 2 (Digital Radiography - DR with Scintillator):

1. X-ray exposure to a scintillator (e.g., cesium iodide).

2. Scintillator emits light immediately.

3. Light is detected by a photodiode (typically amorphous silicon).

4. Photodiode converts light to electrical signal.

5. TFT (thin-film transistor) array or CMOS captures and digitizes the signal.

How does direct capture differ from indirect methods in DR?

—Indirect capture methods cause a loss of resolution.

—The direct capture method avoids this problem by not using a scintillator.

What components are used in the direct capture method?

—Photoconductor

—Thin-film transistor (TFT) array

What is the role of the Bias Electrode in the direct capture image formation process?

An electric field is applied

How does the photoconductor function in the direct capture image formation process?

Amorphous selenium (a-Se) absorbs X-ray energy, creating electric charges proportional to the X-ray energy

What happens to the electric charges in the TFT (Thin Film Transistor) during the direct capture image formation process?

1. Electric charges migrate to the TFT.

2. Charges are stored in storage capacitors.

3. Capacitors amplify and convert the signals to digital code through electronics.

How are signals transmitted in the direct capture image formation process?

The TFT reads and transmits signals, which are then sent to a computer workstation for processing and display

What is a histogram?

A graphic representation of the data set

What is histogram analysis?

Histogram analysis (term stated without additional description in this slide—used as a topic heading for what follows)

What is the "a priori" model in histogram analysis?

A priori model: Concept is one that can be acquired independent of experience

What is the neural analysis model?

Neural analysis model: Method of data analysis which imitates the way the human brain’s way of working

What happens as the electrons, liberated by the laser, return to their shells?

They release excess energy as light that is directed to the photodetector via a fiberoptic bundle or a solid, light-conducting material

What does the photodetector do?

The photodetector amplifies the light energy and converts it to an electronic signal

What happens to the electronic signal after amplification?

The signal is passed through an analog-to-digital converter (ADC) where it is digitized

What happens during digitization of the analog signal?

The analog signal is divided into a matrix

What determines the resolution of the digitized image?

The size of the matrix determines the resolution: the larger the matrix, the greater the number of pixels and the greater the resolution

What does scanning the plate result in?

The scanning of the plate results in a continuous signal being sent to the photodetector and onto the ADC for sampling and quantization

How is the image digitized?

The image is digitized by both location (spatial resolution) and intensity (grayscale) of each part of the signal

When is grayscale assigned in the digitization process?

Grayscale is assigned during the process of digitizing the image

What is bit depth?

Bit depth is the available grayscale

What determines the shade of gray each pixel displays?

The number of photons detected within a given pixel determines the shade of gray it displays

What does the computer of the CR reader do with the image data?

The computer of the CR reader puts the image data through a series of steps to create the displayed image

How many DR image extraction methods are there?

There are two indirect methods and one direct method

What is used in one indirect DR method?

One indirect method uses a cesium iodide phosphor plate as the scintillator coupled to a CCD by a fiberoptic bundle or optical lenses

What happens to the x-ray energy in this method?

The x-ray energy is absorbed by the scintillator and converted to light

What happens to the light produced by the scintillator?

The light is transmitted to the CCD and an electronic signal is created

What happens to the electronic signal?

This analog signal is passed through an ADC where it is digitized

What does the second indirect method use as a scintillator?

The second indirect method uses either a cesium iodide or a gadolinium oxysulfide scintillator coupled to a photodetector and a TFT

How is the panel structured in this method?

The panel is configured as a network of pixels with each pixel containing a photodetector and TFT

What happens to the x-rays in this system?

The x-rays are absorbed by the scintillator and converted to light

What happens to the light in this system?

The light is absorbed by the photodetectors and converted to an electronic signal that is collected by the detector elements (DELs) and then digitized by an ADC

What components are used in the direct method?

The direct method uses an amorphous selenium photoconductor and a TFT array

What is applied before exposure in this method?

Before exposure, an electric field is applied via the bias electrode across the surface of the amorphous selenium layer

What happens during x-ray exposure in the direct method?

During exposure, x-rays are absorbed by the amorphous selenium and electric charges are created in proportion to received x-ray exposure

What happens to the electric charges?

The charges are stored in storage capacitors attached to TFTs where they are amplified and converted to digital code

What happens after the image is captured in all three DR systems?

From this point all three DR systems go through the same basic image-forming steps previously described

What is the first step in this shared image processing sequence?

A histogram is created and analyzed

What happens after the histogram is created?

The exposure field is recognized, and the histogram analysis occurs

What processing step follows histogram analysis?

Automatic rescaling takes place

What contributes to the image in these DR systems?

Only the detector pixels that were exposed contribute to the image

To which type of workstation do display workstation guidelines apply?

Display workstation guidelines apply to primary display workstations

What is the required minimum luminance level?

Maximum luminance levels of at least 171 cd/m²

What contrast response requirements must be met?

Contrast response requirements that meet American Association of Physicists in Medicine (AAPM) Task Group 18 requirements

What is the minimum luminance resolution required?

A minimum of 8-bit luminance resolution

What type of glare must be minimized in display systems?

Minimal veiling glare

What environmental factors should be minimized at the display station?

Minimizing reflections from, and levels of, ambient light sources

What are postprocessing functions?

Postprocessing functions are computer software operations available to the radiographer and radiologist that allow manual manipulation of the displayed image

What does the windowing-leveling function allow?

The windowing-leveling function allows the radiographer to expand any region of the grayscale to one that can be seen and differentiated

What caution is associated with using postprocessing functions?

Caution: Overuse of these functions can negatively alter the digital image data set

What does Detective Quantum Efficiency (DQE) express?

An expression of the radiation exposure level that is required to produce an optimal image

What does DQE measure?

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

What does a higher DQE indicate about radiation exposure and image quality?

The higher the DQE of a system, the lower the radiation exposure required to produce a quality image, thereby decreasing patient exposure

What does DQE “predict”?

DQE “predicts” patient dose.

What are digital receptors more sensitive to?

Digital receptors are much more sensitive to scatter radiation and low-energy radiation in general

What effect does increasing kVp have on scatter radiation?

Increasing kVp increases the opportunities for scatter production

What are the two ways to effectively control scatter radiation’s effect on the image?

Collimation and grid use

What is PACS (MIMPS)?

An electronic network for communication between the image acquisition modalities, display stations, and storage

What is Digital Imaging and Communication in Medicine (DICOM)?

A common language used by different systems for communication

New Classifications from the U.S. Food and Drug Administration has changed PACS to MIMPS, or medical image management processing systems. T/F

True

A PACS system consists of the following:

—Acquisition (imaging modalities)

—Display (viewing and workstations)

—Storage (archive server)

What is the role of MIMPS?

To allow for the display and storage of medical image

What does teleradiology allow in relation to MIMPS?

Through teleradiology, image files can be accessed throughout the facility or even by clients outside the facility