Digital Radiography Flashcards
Digital radiography uses what type of systems
- Direct detectors
2. Cassetteless system
What is the organizational approach to digital radiography
- Capture element
- Coupling element
- Collection element
Capture element
Where the x-ray is captured
Coupling element
That which transfers the x-ray generated signal to the collection element.
Coupling means attaching
Three types of collection elements
Photodiode
Charge-Coupled Device
Thin Film Transistor
2 light sensitive devices that collect photons
Photodiode
Charge-coupled device
Charge sensitive device that collects electrons
Thin film transistor
CCD are used in
Military use
Astronomy
Digital photography
What is a charge-coupled device
Silicone-based semi conductor that converts visible light photons into electrons.
A highly sensitive light sensitive solid-state device.
3 advantages of charge coupled device
Sensitivity—- ability of CCD to detect and respond to low levels of visible light which promotes lower dose.
dynamic range- ability of CCD to respond to a wide range of light intensity,
Size is very small so it is well-suited for DR applications
Charge couple devices have linear radiation response therefore
Image contrast is unrelated to x-ray exposure.
At low x-ray exposure CCD response is greater than filmscreen. Resulting in lower patient dose.
Describe indirect DR Cesium Iodide/CCD
Charge coupled devices can be tiled to receive the light from an area x-ray beam as it interacts with a scintillation phosphor such as cesium iodide.
What is the benefit of cesium iodide
It has a high photoelectric capture; so x-ray interaction is high and patient dose is lowered.
What are the steps from x-ray beam in cesium iodide CCD indirect DR
X-ray interacts with cesium iodine a scintillation phosphor which gives off light and interacts with fiber optic bundles and Is sent down to charge couple device array.
What is the challenge with cesium Iodide CCD indirect DR.
Creating a seamless Image at the end of each CCD in the tiled arrangement (The cross areas don’t capture image)
Solution is interpolation of pixel values at each tile interface (The lines get airbrushed out by interpolation)
Cesium Iodide/amorphous Silicon (indirect DR)
An indirect DR process by which x-rays are converted first to light then to an electric signal.
Cesium Iodide/amorphous Silicon (indirect DR)
Uses cesium iodide scintillation phosphor to?
Capture x-ray
Cesium iodide is coated over an active matrix array of amorphous silicon photodiodes.
Cesium Iodide/amorphous Silicon (indirect DR)
From X-ray
X-rays interact with cesium iodine and produce light.
The light interacts with the amorphous silicon to produce a signal.
The signal is stored in the thin film transistors until readout.
Cesium Iodide/amorphous Silicon (indirect DR)
Image receptor
Image receptor is made into individual pixels with each having a light sensitive face of cesium iodide with a capacitor and a thin film transistor embedded.
Challenge of Cesium Iodide/amorphous Silicon (indirect DR)
A large portion of the face of the pixel is covered by electronic components that are not sensitive to light.
20% does not contribute to the image.
Cesium Iodide/amorphous Silicon (indirect DR)
The percentage of pixel face sensitivity to x-ray is known as the
Fill factor
Currently this is 80%
Dilemma for indirect DR
As pixel size decreases the spatial resolution increases but at the expense of patient dose. This because as pixel size decreases, the fill factor decreases and x-ray intensity must be increased to maintain signal strength.
One solution for Dilemma of indirect DR
One solution is to use gadolinium Oxysulfide instead of Cesium Iodide which can be made thicker without compromising spatial resolution.
Direct Capture DR
Amorphous Selenium
No phosphor coating is used.
Direct Capture DR
From X-ray
X-rays interact directly with a thin layer of Amorphous Selenium, which produces a charged pair (by Direct Ionization of selenium) known as electron hole pairs (EHP). The EHP is the signal that charges the active matrix array of thin film transistors.
Charge is collected by a storage capacitor and remains there until read out by the switching action of the thin-film transistor.
A/D Convertor
Sent to computer
Advantage of direct capture DR
Spatial resolution is improved because there is no spreading of light.
Latent image is more accurate.
Disadvantage of direct capture DR
Increased patient does because there is no spread of light.
It takes more radiation to release enough electrons to adequately charge the capacitor.
What is the capture element and a coupling element in Direct capture DR
Amorphous Selenium
a-Se
What are the readout electronics of direct capture DR
Data line and gate switches
How many capacitors are in a flat panel detector
9 million
Digital fluoroscopy equipment includes
Computer
Video monitors
A complex operating Console
Digital fluoroscopy accommodate what process easily
Subtraction
What is rapidly replacing the TV camera tube in the fluoroscopy chain
Charge coupled devices, photosensitive silicon chips
PACS stands for
Picture archiving and communication system
PACS allows for what ? digital form
Display
Interpretation
And storage of medical images
Long term storage of images
Archive
Display
Work stations can display old and new images
PACS Communications
Archive, workstations and all modalities are networked together
PACS changed what two things
The type of storage space required for medical images.
Role of film librarian
5 advantages of PACS
- The problem of lost films is almost eliminated
- Fast access to prior studies
- Can do multiple modality exams on the same patient
- Can view the same exam from various workstations at the same time
- Workflow efficiencies
Disadvantages of PACS 4
- High initial cost
- Lower reliability than film based systems (downtime/disaster)
- Learning curve for radiologists and staff
- Potential decrease in diagnostic ability (file compression/archive)
Three principal components of PACS
- Display system
- The network
- The storage system
Teleradiography
The process of remote transmission and viewing of images to workstations in other areas of the hospital or to radiologist offsite.
PACS
Digitizer
A device that converts in analog radiograph to a digital image by the use of laserbeam technology
DICOM stands for
Digital imaging and communications in medicine
What type of digital detector do we use at St. Francis
Indirect
Amorphous silicon technology
What is the detector size and number of pixels, of digital detector do we use at St. Francis
The detector measures 41 x 41 cm
detector provides 4 million pixels of data.
Each pixel delivers up to 16 bits of data.
What is the role of cesium iodide in the indirect detector used at St. Francis
Since amorphous silicon cannot absorb x-rays directly, a scintillator material cesium iodine converts x-rays into visible light.
3 advantages of cesium iodide/a-Si indirect detector used at St. Francis
- Wide dynamic range: long scale contrast, improved contrast resolution.
- Less repeats
- No darkroom Needed
Where is the latent image stored in both indirect and direct DR
The matrix of uncharged and charged capacitors in the flat panel detectors
Advantage of indirect DR
Lower patient dose due to the use of phosphors which spread light
Disadvantage of the indirect method
Light scatters, degrading spatial resolution of the latent image, by exposing adjacent detectors.
The amount of charge stored in the capacitor is proportional to
The amount of xRay energy that exposed that detector
What reads the current as it passes through the TFT row by row
Gate switches and data lines.
The current is amplified and passed through the ? Then to a computer
A/D converter
The digital information is stored in the computer as
An array.
An indirect DR needs – mR exposure and direct needs –mE exposure
- 5
1. 0
DR image qualities are all related to
Pixel size
Why do we can press large files
Large files have more pixels to send; more traffic and slower transmission results.
10:1 compression means
Every 10 pixels we are only storing one pixel
Two types of software artifacts
Preprocessing includes dead pixels
Compression artifacts which actually represents a lack of data.
What is a histogram
A graph of frequency of occurrence versus digital value intervals
Interpolation is
The mathematical process of assigning a value to a dead pixel based on the recorded values of adjacent pixels.
Photodiode
A solid state device that converts light into an electric current.
Light photon collector.
