Image receptors Flashcards
Basic principles of Computed Radiography?
- “Cassette”-based
- Phosphor stores x-ray energy
- Read plate by scanning with laser
- Digital image produced on monitor
Label the PhotoStimulable Phosphor plate
Structure of PhotoStimulable Phosphor (PSP) Imaging Plate?
Protective layer – thin & transparent, protects against handling
Phosphor layer – (~0.2mm) Europium-activated Barium fluorohalides (bromide iodide), colour centre can be thought of as electron traps
Reflective Layer– sends light forward, ensures as much light as possible reaches detector
Conductive layer – removes static electricity and stray light
Support material – give strength
Light Shielding Layer – protects PSP from stray light which can fog (not as badly as film though)
Backing – soft polymer that protect back of cassette
How does the storing of the latent image work?
Attenuated beam incident on PSP plate Incident photons excite some electrons in Phosphor layer from the valence band to conduction band
Around 50% return to valence band emitting energy as light
The remainder are ‘trapped’ in an excited state within the crystallised molecular structure of the PSP plate
How is the phosphor plate read after a CR image is taken?
Laser light ‘reads’ PSP plate line by line Energy of laser gives ‘trapped’ electrons enough energy to become ‘untrapped’, emitting their energy as fluorescent light in the process.
The amount of light emitted will thus be proportional to the x-rays incident on the PSP
How does the storage plate reader produce an image from CR?
A laser is shone onto an oscillating mirror which creates analog signals across processor, the processor has light sensitive detectors that can record how much light is emitted for each part of the cassette
This is transferred into a digital signal by an ADC i.e. a pixel value (0-4096) to output an image made up of hundreds of lines.
Resolution of the system is dependent on the scanning frequency
This determines pixel size and image resolution
How is the plate, after a CR image, erased ready for the next image?
The plate is flooded with light
This ensures all electrons are ‘untrapped’ back in their rest state, ready for re-exposure
Image processing:
12 bit pixel value = …………grayscales
8 bit monitor = ………….grayscales
Human eye= ………….grayscales
How does the software decide how to present the pixels?
4096
256
80
It does this through use of a histogram, a graph of all pixel values Software must decide which pixel values to display across the grey scales available
What are the causes of unsharpness in CR plate and readers?
Plate made of phosphor crystals
Internal scatter of light
Similar problem with film-screen
Finite distance between plate and reader
Emitted light spreads out
What are the basic principles of Digital Radiography?
‘Cassetteless Imaging’
X-ray photons are converted directly into electronic signal
Explain how indirect digital imaging works?
- Uses a scintillator to turn x-ray photons into light (Caesium iodide)
- This fluoresces light which is recorded by light sensitive photodiode or CCD
- Photodiode: amorphous silicon, works the same way as DDR to produce charge read by TFT
- Charge Coupled Device: capacitors register light and store it as current of proportional strength. This current is digitised as a pixel value
Describe the make up of a flat panel detector and explain how direct digital imaging works?
A flat panel detector is made up of an array of detectors, each pixel is ~0.1mm2 giving the detector a resolution of ~5lp/mm. A Thin Film Transistor determines how each row is read to form the image.
When a photon comes out of the patient it causes an ionisation of the amorphous selenium layer (covering 80% of the detector) producing an electron.
As the detector has a small charge across it, the electron goes towards the positive end and is stored in a capacitor.
Charge stored in capacitor equal to the photons that caused the ionisations.
The charge stored can help determine a grey scale, if there is a high charge then a darker grey scale could be used.
Describe the scintillator layer used in indirect digital radiography?
- Phosphor layer acting as fluorescent screen
- Converts x-rays to light
- Commonly sodium activated caesium iodide
–High x-ray absorption and energy conversion
•Needle like crystal
–High resolution
Compare CR to DR
•Max kVp for CR is 90 kVp
–More sensitive to Compton Scatter so more kVp leaves images too flat or fogged for contrast
•Higher kVp used in DR
–Lowers patient dose
–Ensures adequate penetration
–Post processing can adapt for low contrast results
•High kvp’s generally lower patient dose (with mAs reduced) and give DR greater chance to display contrast required
Explain what Detective Quantum Efficiency (DQE) is?
A measure of how efficiently a detector uses the x-ray beam
DQE is the relationship between the density of useful quanta, and the density of x-ray quanta actually incident on the detector
The more x-rays used to form the image, the lower the relative noise level - i.e. better Signal to Noise Ratio (SNR).
For a given level of x-ray exposure a detector with higher DQE will give an image with better SNR (i.e. better image quality).
Alternative way of looking at it:
With a higher DQE you can obtain the same SNR (image quality) with a lower level of x-ray exposure.
What are the advantages of DR over CR?
–Cassette-less
–Instant image review
–Better resolution- finer detector sizes
–More sensitive at detecting photons (DQE)
•Reduces exposure required
–More sensitive to higher kVp
- Reduces dose
- Better penetration and image quality
What are the positives of digital radiography over film radiography?
–Wider exposure latitude reduces repeats
–Lower patient doses with DR (better DQE)
–Digital instant transfer of images hospital wide and also across hospitals
–Reduced ‘lost films’. Central archives of backed up images
–Teleradiology
–No film costs, reusable IR
–More post processing options
–Faster processing and quicker patient throughput
–Fewer x-ray rooms required
What are the negatives of DR compared to Film radiography?
–Initial start up costs, training and resources
–Spatial Resolution better with film than some digital systems
–Loss of uniformity in Image presentation for reporters
–Erroneous post processing may mask pathology
–Comparison of images complicated by post processing
–Artefacts
BENEFITS OF FILM
MAY HAVE FINER CRYSTALS THAN DR ELEMENTS SO MAY HAVE FINER SPATIAL RESOLUTION (MAY STILL BE USED FOR MAMMO)
–Exposure Creep
–Lazy Radiography
–Less knowledge of manual exposures
•All can result in increased patient dose
What does DICOM stand for?
•Digital Imaging and Communications in Medicine
What does DICOM do?
- Ensures quality standards and uniformity across centres
- Patient and examination information embedded to ensure no mix ups
What does PACS stand for?
•Picture Archiving and Communications Systems
What is PACS?
- Intranet software for storing, retrieving and distributing digital medical imaging within and between radiology departments and hospitals
- Accompanied by Radiology Information Systems (RIS) used for appointments, record and dose keeping
Describe the typical software structure used to control information?
What is signal in an X-ray?
What is it dependant on?
•Useful information contributing to image
–Dependant on
- Exposure factors used and incident remnant beam
- Patient characteristics
- Receptor DQE
What is noise in an X-ray?
What are the sources of the noise?
•Random information not resembling image
–Sources
- Scattered Radiation
- Detectors
- Electronics
Describe the DYNAMIC RANGE graph comparing the range of DR exposures to Film.
Explain histogram analysis
- Software can trim lower scatter/noise/unexposed values
- Remaining histogram is compared to a stored ‘Look Up Table’, a reference histogram for each type of examination
- Therefore important to select the right examination!
- The image histogram is manipulated to try and match this look up table
- Look up table will also manipulate pixel values to apply the contrast, edge enhancement and noise reduction required
- ‘Raw data’ may be retrievable on workstations (not PACS) for manual manipulation of images
Explain windowing of an X-ray
- Window Level (WL): central digital number
- Window Width (WW): range of digital numbers
ww equals contrast: narrow window width accenuates density differences across a limited range of values = high contrast
A wide ww means lots of similar gray scales = low contrast
WL = brightness
What forms of post processing are there?
- Windowing
- Edge enhancement/smoothing
- Shuttering
- Image Orientation
- Markers/Annotations
- LUT changes
- CAUTION:
–Post Processing generally degrades image quality and is no substitute for good radiographic technique
–A well taken image will look good on any monitor (within reason) where as post processing changes can appear differently in different viewing conditions
What is radiographic contrast?
What influences the contrast?
- The difference between adjacent densities on an image
- Influenced by subject contrast but also:
–The image receptor used
–kVp
–Removal of scattered x-rays
(use of bucky/grid/air gap)
–Digital post processing
–Bit depth
•High Contrast not always preferred
What is ‘Contrast Resolution’?
•The ability to distinguish between objects of similar subject contrast
A high contrast has a low contrast resolution
What considerations need to be made when adjusting contrast in post processing?
- Digital Raw Image may contain more information albeit at low contrast
- Be careful manipulating contrast as it can hide detail
- Consider the clinical indications and what the referrer is looking for
What is spatial resolution?
- The minimum size of detail that can be observed in an image
- Usually measured in line pairs per mm.
What is spatial resolution dependant on?
–Crystal structure and processing system (CR)
–Detector size (DR)
–Matrix size of images and viewing monitors
–Projectional factors: Focal spot, image magnification, distortion
Where in the process of taking an X-ray is sharpness and resolution lost?
- At the Anode
- Diverging beam
- Exposure factors
- Patient motion
- At the image receptor
- At the viewing station
How is noise related to underexposure?
How does this affect the image quality?
- When the IR does not receive enough photons to read pixel values, Noise is introduced
- Signal to Noise ratio is decreased with underexposure, decreasing resolution of the image
- Quantum mottle, grainy appearance
How does overexposure affect the image produced?
- Saturation of the image
- A complete absence of noise
- Confirmed by Exposure Index
- Image “burn” if grossly overexposed
What does SNR stand for?
Signal to Noise Ratio
How are Exposure, Contrast and Resolution linked?
- Good exposure required for ideal contrast and resolution
- Poor contrast will affect image spatial resolution
- High contrast images make it easier to visualize fine detail ie improves resolution
Explain what is meant by Dose Creep?
For risk management it may be perceived to be better to overexpose than underexpose.
Under exposed has not received enough photons so information is not there.
Over exposed has too much information and can be saved.
This overtime leads to a creep of overexposing patients overtime
What is an EXI?
Exposure Index
•Numerical scale starting from 0, provided by the X-ray tube company, eg. siemens
What is a TEI?
•Target EXI: This is set for each examination as a suitable EI for the image (Eg: extremities 1000; chest 500, abdomen 250)
What are CR/DR artefacts?
Examples?
- Any unwanted information contributing to an image
- Avoidable and Unavoidable
- Examples:
–Ghost images (previous overexposure, may require two erasures)
–Dust particles or scratches on CR plate
–Quantum mottle
–Defective DR detectors (line error)
–Histogram analysis error (beware! May not require repeating)
–Mis-read Collimation
–Moire effect with grids
How should you view Digital images?
- Avoid glare/ poor lighting
- Ensure consistency across the department especially reporting areas
- Ensure highest possible image matrix on monitor and image
- QA of monitor settings:
–Spatial resolution
–Contrast resolution
–Luminescence
