X-ray Detection systems (screen and digital) Flashcards
What is the difference between screen film and digital radiography?
Screen film:
plastic film held up to a light box (cannot be digitally manipulated).
Digital:
digital system with pixel values (can be manipulated) uses cassettes and scintillation
1. Computed radiography: make an image in a cassette that can be processed into a digital copy (analog storage and uses phosphorescence delay in converting energy into light)
2. Digital radiography: refers to way process and store the image
a. Indirect: uses scintillation and fluorescence (immediate conversion of x-rays into light). X-ray photons hit a scintillator layer, which then releases light photons that can hit an active matrix array that digitises the signal
b. Direct: doesn’t require scintillation – photons act directly on a photoconductor layer producing positive and negative charge. The negative charge is attracted to a charge capacitor that stores the latent image. It is then read out by TFT switches pixel by pixel.
CR: what is the definition? How does it store energy from the incident x-ray? What is the plate made up of?
Definition: make an image in a cassette that can be processed into a digital copy (analog storage and uses phosphorescence delay in converting energy into light)
-Contains a photostimulable phosphor plate to store energy form the incident x-ray plate requires light input to release the trapped energy (proportional to x-ray intensity).
-Plate usually made of barium fluorohalide dopped with europium, with surface coat and plate contained in light-tight cassette.
Roughly outline process for revealing an image in CR (and erasing the plate)
-X-ray photons absorbed into phosphor crystal: gives rise to a high energy photoelectron. This ionises atoms along it’s track and releases thousands of electrons. Electrons become trapped producing latent image.
-Uses phosphorescence (delayed light production) – commonly barium fluorohalide with divalent europium ions embedded in a polymer binder with the top surface protected by a layer of toughened plastic. Usually 0.3mm thick.
-Scanning takes about 30 seconds.
-CR reader: plate is removed form cassette and scanned with laser beam. Red laser light used (most phosphors release blue light) rotating mirror is used for scanning.
-Photomultiplier tubes measure the light intensity emitted from each scanned section (line by line) amplify the recorded signal and convert this to electronic signal using analogue-digital converter.
-Plate is erased by exposing it to a bright light source.
What image processing is done for CR? Detail some steps that occur before export to PACS
-Photostimulable phosphors have a very wide dynamic range (10 000:1)
-Initial image processing before export to PACS:
* Ignore signals outside collimation area
* Histogram analysis of the distribution of light intensities in the collimated area
* Application of a gradation curve to optimise readability.
How does CR spatial resolution compare to analogue film-screen radiography? Why is this? (resolution and effect of reading)
-CR has poorer spatial resolution than analogue film-screen radiography due to pixel size limitation and effect of laser reading
* Larger plates: 2500 x 3070 (chest and abdo imaging) – yielding in resolution of 3.5lp/mm vs film screen allows for much better resolution eg 8-12lp/mm
* Laser reading stage affects resolution as there is inherent scatter in the phosphor layer. A thinner phosphor layer decreases scatter and improves spatial resolution. Smaller phosphor layer also improves spatial resolution.
CR: what is a Detector dose indicator?
-Safety measurements to indicate the level of exposure of a radiograph. A radiograph that may have been overexposed will be automatically adjusted to look as though taken at the perfect exposure.
-ALARP
CR: what is type the relationship between contrast and dose? (Compared to screen radiography?)
is dose-dependent and is linear (not characteristic like in film screen radiography).
CR: outline some artefacts that can occur (7 in total) at the image acquisition stage
-Moire’ pattern: interference between grid and laser scan lines. Need to use grids with >60lines/cm
-Ghost artefact: imaging plate was not erased after previous use
-Fading of image: delay in acquisition and processing
-Light bulb effect: backscattered radiation
-Over and under exposure
-Cracks or focal radio-opacities: fault with imaging plate or dust.
-Linear radio-opaque or radiolucent lines: malfunctioning plate reader.
What do Moire’ and light bulb artefacts look like?
Digital radiography: difference between direct and indirect
Indirect systems
-Use a phosphor to absorb x-rays and release light photons which produce the image (eg layer CsI to convert x-rays into light before capturing the light photons via photodiodes in a TFT array or via tiled charge couple device (CCD) detectors.
Direct systems
-Amorphous selenium is used to allow the direct conversion of the x-ray photons to a charge captured by the TFT array.
DR Indirect (DRI): describe important features of scintillation layer - function, material. What is the effect on spatial resolution?
Scintilaltion layer:
-Scintillation layer converts photons into light.
-Uses caesium iodide to long tubular crystal structures where single x-ray photon is converted into light and funnelled down a small area.
-Improves spatial resolution but creates a much less intense light signal than gadolinium oxysulfide.
DRI: what is the role of the active matrix? What is it made of?
Active matrix
-Made of layer of hydrogenated amorphous silicon and forms the readout electronic.
-Each pixel has: photodiode (amplifies signal from incident light photons), charge capacitor (stores signal of latent image), thin film transistor (TFT switch)
DRI: What is a fill factor? Equation? What is the relationship between pixel size and fill factor
-TFT and charge storage capacitor take up a small area of each pixel, prevent formation of image in this area.
-Fill factor = sensitive area/overall area
*Decreasing pixel size improve resolution but circuitry size stays the same t/f efficiency of the array changes.
DRI: describe image formation in indirect DR using TFT
- CsI:TI absorbs x-ray photons and releases light photons
- These light photons are then absorbed in the photodiodes and the charge stored in the charge storage capacitor at each pixel location
- The latent image is read out sequentially to a bank of charge sensitive amplifier (TFT switches)
- The resulting voltage signal is then digitised and transferred to the system computer where the DR image is built up
DRI: what other system is there (other than TFT?) Describe the MOA briefly
Charged coupled device (CCD) chip
-Take light and convert to a digital signal.
MOA CCD Indirect DR system:
-Scintillation layer converts x-ray photons into light photons.
-Coupling layer couples light photons to CCD chip.
-Light photons are converted into electrons at CCD chip.
-As electrons are released they stay in dexels (separated by voltage gates) until these are read sequentially by row to digitise the signal into a pixel value.
-The more electrons available: the darker the image appears.
DRI: how do TFT and CCD MOAs differ?
-Differs from CCD as in CCD the charged electrons are released through voltage gates in TFT light photons are converted to electron current via photodiode layer this is then diverted to the capacitor which can be released when the TFT array is switched on.
Digital Radiography Direct (DRD): what are the differences with indirect? What components does it share with indirect? What is the most common photoconductor?
No scintillation required: converts x-ray energy directly into an electronic signal.
Shares the DEL component with the indirect TFT array.
This directly converts x-ray photon energy into free electrical charge carriers (electrons and holes) i.e. the “middle-men” or light photons, are cut out. The most commonly used photoconductor is amorphous selenium (a-Se).
DRD: outline how the image is formed
- X-ray photon absorbed by a-Se photoconductor
- Electrical charge carriers (negative electrons and positive holes) are created in the a-Se
- A surface electrode at positive potential attracts and discards all the electrons
- The positive charges are drawn to the charge storage capacitor forming the latent image
- The latent image is then read out sequentially by gating each row of TFT switches (each TFT corresponds to one pixel) in turn to read the charge pattern and transfer to a bank of charge sensitive amplifiers
- The resulting voltage signal is then digitised and transferred to the system computer where the DR image is built up
- Post-processing
DRD: describe 5 artefacts in DRD (detector drops, backscatter, image saturation, ghost image and irregular shading)
-Detector drops: white or black artefacts after individual pixels/electronics are damaged.
-Backscatter: backscattered radiation from detector electronics is visible on image
-Image saturation: loss of information when the processing algorithm dynamic range is exceeded.
-Ghost image: previous image not cleared
-Irregular shading across the field: non uniform variations of the sensitivity or grain of the x-ray absorption layer
DRD image processing - correction. What is gain calibration?
Gain calibration: uses previously acquired mask image comprising an image acquired with a uniform x-ray beam and subtracting this gain mask image from the patient’s image
DRD image processing - correction. What is pixel calibration?
Pixel-calibration: defects in pixel array can be corrected by interpolating the data values of neighbouring pixels which are functioning correctly using a reference map
DRD image processing: what is auto-ranging?
Automated analysis of the image histogram: excludes very high and low values which would otherwise adversely affect the image contrast and brightness. The remainder of the histogram is normalised to maximise image display. The data needs to be matched to the display device.
DRD image processing: outline the steps of auto-ranging
- Identification of relevant image field
- Generation of a histogram of the data representing the number of pixels at each grey-scale value
- Analysis of the histogram to exclude ranges of data which contain no clinical information (very high and low values)
- Selected grey-scale range normalised to match the display image
Windowing: what does changing window width do? What does having a large window width do to the range of shades displayed and how does this affect contrast?
-Window width: alters the image contrast. The larger the width, the larger the range of shades displayed and t/f the smaller the difference in contrast between each shade (eg lung and bone window) – results in a smaller difference (ie contrast) in the grey value between the represented Hounsfield units.
