Lecture 1 Image Optimisation Flashcards
(20 cards)
Name the components of image quality
Contrast Resolution Noise Artefacts Blur
Name the physical components of image quality
Contrast
Resolution
Noise
Explain what is contrast
Difference in optical densities of an image. Few photons = reduced contrast
Different structures of image transmit different optical densities
Ie. without contrast we won’t be able to distinguish the heart from lungs
There are 3 types: subject, radiographic, subjective contrast
What is subject contrast?
Literally just the spread of optical densities because of the spatial distribution of attenuation coefficients of different body parts, contrast agent, and pathology
Attenuation coefficient relates to thickness and so the atomic number
Ie. fluid> ATomic number than air , therefore whiter on scans
Ie. in chest scans pm ribs traverse lungs and heart because ribs have greater atomic number and therefore attenuation coefficient
Similar/limited spread of attenuation coefficients of structures gives very bad contrast
What is radiography contrast?
Variation in beam intensities measured by the detector
Depends on subject contrast, scatter, exposure
Increase scatter reduces contrast by decreasing the differences in transmitted beam intensities (pouring water in bucket and the splashed outside )
Prevented by decreasing volume of tissues, use of grid, lead-backed cassette to prevent back-scatter, increase OID
Exposure over or under can be improved through post processing in the digital era
What is subjective contrast?
Your perspective of differences in optical densities
Ie. observer differences: contrast perception differs from each other
Ie. viewing condition: ambient lighting and display technology
Explain resolution
The Distinct nature of small structures in image
Ie. being able to distinguish between pneumonia and a collapsed lung (contrast doesn’t show distinct small features)
Determined by the imaging system, however, image reconstruction and display characteristics may modify the resolution of final image
There are 3 types; spatial, intensity, temporal resolution
What is spatial resolution?
Amount of pixels in image determines spatial resolution
Increased number of pixels/unit length, increases spatial resolution therefore clearer image
What is intensity resolution?
Quantifies smallest changes in brightness that is recognised
We talk about the intensity resolution in association with bit depth (number of bits of info stored in each pixel of image)
Ie. number of bits per pixel determines the number of Grey levels to be accommodated in an image (grey level =2^ bpp)
Increasing the bit/pixel = more range of intensities that can be represented, therefore, increase in intensity resolution
But as the image gets denser, needs more memory for storage
Compare the effect on image in regards to spatial and intensity resolution
Spatial resolution increases when pixel count increases, but bits stay same
But spatial resolution stays the same when bit depth increases and the pixel count remains the same, however, intensity resolution increases
What is temporal resolution?
Producing high spatial resolution in smallest time frame (millisecond)
Temporal resolution is determined by the duration of each cardiac phase (delta t)
Delta t = VPS x TR
VPS is set by the radiographer and TR is a fixed time repetition for a given set of imaging parameters
Images acquired in the time frame (interval) played back in condyle mood (motion video)
Ie. dynamic imaging, eeg, MEG, etc, have an increased temporal resolution
Explain noise
Graininess or mottled appearance
Increase in noise = decrease in visibility and resolvability of image features, therefore, decrease in contrast and resolution
Occurs due to random distribution of the X-ray photons on the detector and therefore the final image
So noise is measured by number of photons that hit the detectors as it is inversely related to the number of photons
What factors lead to the occurrence of noise in an image?
Tube current; mA increases = decrease noise and increase dose
Detector speed; fast detectors = increase noise because they require lesser photons to produce an image
Kvp: increase kvp = decrease noise, but bad subject contrast due to high scatter
What are the 2 quantities to describe noise?
Absolute and relative
Absolute: the absolute degree of photon fluctuations within the image - - - indicator of noise transfer characteristics of imaging technology
Relative: the extent of photon fluctuation relative to the signal present in the image
- it accounts for the impact of noise on how it impedes the visualisation of image features
Explain an artefact
Extraneous objects
Ie. imaging system artefacts: damaged intensifying screens within cassettes, static marks in films, processing artefacts, etc
Ie. radio opaque metallic materials overlying ROI (safety pins, jewellery, coins)
Explain image blur?
Image unsharpness of structures
Due to motion unsharpness, geometric unsharpness, and absorption unsharpness.
What is motion unsharpness?
Caused by
Patient; involuntary or voluntarily movement, inadequate immobilisation, long exposure time
Image receptor: Bucky/cassette holder not properly secured
X-ray tube: brakes not applied to the machine
What is geometric unsharpness?
Caused by distance between object and image receptor, therefore increase in OID=INCREase in magnification vice versa
Occurs because the view becomes wider and therefore blurry
Explain the effect of focal spot size on geometric unsharpness?
Use of fine focus will decrease geometric unsharpness
Rather than broad focal (eg. 1.2 mm2) use fine focal (eg. 0.6mm2)
What is absorption unsharpness?
Usually occurs due to the structure of the target object ie. particularly a spherical objects
In spherical objects, the centre absorbs the greatest and the photons fall off near the periphery therefore, will have ill defined borders
Reduced by increasing image contrast and digital enhancement
