Digital imaging Flashcards
What is analogue image
Has a continuously varying density across it
A film image is a type of analogue image
Digital image
Has discrete pixels (picture elements) with numerical density values
Physical principles of digital radiography
In screen film radiography the film is the detector and storage medium
In digital radiography the detector only generates the image which is then stored on a digital medium
Digital imaging series
Generation
Processing
Archiving
Presentation
Types of digital radiography
Computed radiography (CR) Direct radiography (DR)
CR
uses imaging plates which need developing in a processor
DR
acquires an image directly without using a processor
Types of CR detectors
Indirect conversions > storage phosphor
Types of direct digital radiogtaphy
Indirect conversion:
- Scintillator tft
- Scintillator-CCD
- Image intensifier
Direct conversion:
- Photoconductor FPD
- Selenium-drum
Main digital image detector functions
Capture - absorbing x-rays
Coupling - linkage between the capture element and the converter
Conversion - change of x-ray energy into electrical signal which can be read, can be direct and indirect
Direct digital radiography (DDR)
X-ray energy is converted directly into electrical signal
Advantages of direct imaging
Saves a lot of time
No spread of light, straight conversion process
Give more attention to patient care because it is a quick process
Indirect imaging
X-ray is converted into light which is then converted to electrical signal
Advantages of indirect imaging
Light taken part in the process, the light can be amplified and increase sharpness
Process of digital radiography
Flat panel receptor fixed in an upright stand or x-ray table
Electrical signal goes straight down the wire to the computer
Image doesn’t need to be processed
Image sent to the workstation via electrical or wifi
Amorphous selenium DR system: Selenium drum detector system
X-rays>Photoconductor>analog-to-digital converter
Photoconductor converts x-ray energy to electrical charges
Amorphous selenium DR system: Flat panel detector system
x-rays>photoconductor>tft-arrays
Process of direct digital radiography
Photoconductor of amorphous selenium absorbs x-rays
Photoconductor generates electrical charges
Charge is held in a storage element on a two dimensional detector array
Process of Indirect DR
Each cell in the detector has a layer of scintillation crystals which produces light when hit by x-rays
amorphous silicon photodiode converts the light into an electrical chatge
the pattern of charge stored like in DDR
Types of CCD indirect radiography systems
Lens coupled CCD based systems
Slot scan CCD based system
Similarities of Direct and indirect DR
2D array of electrodes containing TFT
Computed radiography
CR images need to be processed
Image receptor is a plate in a portable cassete
The latent image on a plate is developed in a CR processor
Features of digital imaging
wide exposure latitude very low exposure gives image noise: quantum mottle wide dynamic range modern digital images have good res cr plates sensitive x-ray fogging
How can we tell if a digital image was over exposed
Clever automated processing means that an over exposed image looks fine
But all systems record an “exposure index or detector
dose index” (DDI) on each image
Radiation dose and CR/DR
Give less patient dose than x-ray films: saves repeats and lost images
Dose depends on technique and exposure factors
over exposures lead to high quality images and avoid image noise
Dose creep meaning
A term that is used to describe gradually putting exposures higher
Digital image quality
Contrast
spatial res
noise (Signal to noise ratio)
Digital image contrast
digital imaging amplifies the range of available contrast in body tissues
Effect of pixel size on image quality
small pixels give good res and “detail”
small pixels more vulnerable to image noise
Resolution in radiography
image resolution is limited by the ability of the system to resolve 2 small closely adjacent objects within the image
spatial res in the display
critical image quality factor seen as the detail and sharpness of the image on the monitor
What is image noise
Random fluctuation in signal due to imperfections in the image transfer process
Noise in image does not provide a good representation of body anatomy and can give salt and pepper appearance
Sources of noise
random electrical fluctuation in circuits quantum mottle background radiation and scatter pressure on digital plates detector faults
Quantom mottle
refers to the graininess of x-ray images
Detective quantum efficiency (DQE)
DQE refers to the efficiency of a detector in converting incident x-ray energy into an image signal
postprocessing
Windowing- adjusting contrast and greyscale Edge enhancement image reversal algorithm image stitching image filtering annotation cropping magnification
Windowing
Centring the window level we can just the central value of the density range of tissues that are seen
Window width adjusts the range of densities that are vibisble
