Image Optimization and Quality Flashcards
what are the three types of resolution
Detail
Contrast
Temporal
what determines contrast resolution
- Scan converter
- # bits/pixels
- post processing curves
- monitor settings
- Ambient light
- Acoustic impedance mismatch
What is contrast resolution
Ability to distinguish structures based on variations of brightness
Restriction for contrast resolution
Number of bits per pixel in the display
Two limiting factors of contrast
Dynamic range of human eye and non-linear compression necessary o map the enormous signal dynamic range to the smaller dynamic range
more bits and pixels is useful for what
Color 9can distinguish more shades of colour then gray)
Low contrast resolution
ability to discern features with subtle differences in signal level and depends on the magnitude of the signal differences as well as the size of the object
Displayed contrast translates to what
Pixel calculates to brightness levels on the monitor via gray scale mapping
Temporal resolution
Ability to distinguish dynamics or changes over time
what is the principal limit to temporal resolution
Frame rate
every factor that lowers frame rate lowers what
Temporal resolution
what factors can lower frame rate
Depth of imaging Sector size Multiple foci Persistence Number of acoustic lines
monitor display rate can degrade what
Temporal resolution
Techniques that _____can affect temporal resolution
Average
what are two examples of averaging techniques that can affect temporal resolution
Compound Imaging
Persistence
How to improve frame rate
- Narroe the imaging sector
- Decreasing depth which decrease PRP
- Decreasing line density (fewer lines)
- Turning of multi-focus which decreases the number of pulses needed per line
detail resolution
lateral, axial and elevational
what is detail resolution dependent on
Transducer characteristics
lateral resolution
Resolve two structure side-by-side (lateral dimension)
lateral resolution is the…..
minimum reflector separation in the direction perpendicular o the beam direction that can produce two separate echoes when the beam is scanned across the reflectors
Lateral resolution is determined by
Beam width and scan line density
lateral is resolution is best where
Focal zone of the beam
best lateral resolution is where
Narrowest beam laterally
elevational resolution
Determined by the beam width in the elevational plane
elevation resolution changes with….
Depth
where is elevational resolution best
Narrowest point of the beam
Elevational resolution=
Elevation beamwidth
Axial resolution
minimum reflector separation along the direction of sound travel
what is axial resolution determined by
Spatial pulse length
what do broadband transducers do?
Operate at high frequencies and produce short duration pulses
to improve axial resolution
Must reduce the SPL
Axial resolution equation
SPL/2
Signal
Any phenomenon desired to be measured
Noise
Any unwanted signals
Noise floor
The amplitude level below which no signals are visible because of the presence of noise
Signal to noise ratio
The amplitude of the signal divided by the amplitude of the noise
SNR determines what
Sign quality
higher SNR
Better imaging situation and excluding artifact, more trustworthy data
Overall desired situation (SNR)
High signal
low noise
Apparent versus true SNR
True SNR does not chnage with gains while the apparent SNR does
-Increasing amplification will increase noise and signal by the same amount
If gain is too low
Can make signal appear weak even if there is good SNR
Noise: electronic noises
The electronics used to amplify the tiny returning echoes add random signals
This added energy is created by random excitations of electrons within the electronics
The amplitude of these signals are small, and are seen as white speckle on the image or in the Spectrum
In colour, electronic noise shows up as random colour pixels
Noise: electrical interfernce
It is possible for transducers or ultrasound machines to receive energy emanating from other electrical devices
This energy often shows up as a bright ‘flashlight’ down the middle of an image or as a bright horizontal or zigzagging lines in spectral Doppler referred to as Doppler tones
Noise: clutter
Large returning echoes from structures that obliterate weaker signals
Examples include large specular reflections in imaging and valves in Doppler
Noise: Haze
There are many types of haze. It can created by poor transducer to skin contact or by beam aberration from tissue characteristics
All cardiovascular sonographers have seen haze associated with imaging through the lungs
Preprocessing
Signal conditioning that occurs in real time and cannot be removed from an image once acquired
Some examples are receiver gain, receive focusing, and receiver compression
all receiver functions are
Preprocessing
Post porcessing
Any processing which can be changed after the data is acquired such as data compression, colorization and reject. Post processing can be performed on frozen data as well as live imaging
Dynamic range
refers to the ratio of the maximum to the minimum of any quantity
Largest to the smallest echoes from a patient
Largest to the smallest signal the human eye can detect
signal dynamic range is larger/smaller then display dynamic range
Larger
The display dynamic range
Exceeds the visual dynamic range of the human eye
compression
Is the general term for any technique which maps a larger dynamic range into a smaller dynamic range
Due to the limit of the human eye, reflected signals must be compressed to map the enormous signal dynamic into significantly smaller dynamic range
there are a ______amount of compression schemes
infinite
Compression is known as
lossy compression scheme
because the dynamic range of signals returning from the body are so much greater then dynamic range of the human eye
Non-linear (log) compression must be used
dynamic range of the human eye is less than 36dB
equivalent of 65 shades of grey
Potential issue with ompression
Inability to distinguish mass from tissue
back end compression
Under user control
why is some compression done in the back end (under user control)
to help eliminate the loss of information associated with compression
Tissue colorization
dynamic rnage of the eye is extended
intended to improve visualization when significant dynamic range must be preserved
colour maps help make a distinction for what
low-level echoes
what is the role of the scan converter
Converting A-Mode lines to B-Mode lines
Organizing the successive lines of data into a formatted image
Must keep track of which lines of data should be presented at what location on the screen
Averaging based techniques
improvement in SNR
Averaging techniques use what
Constructive and destructive interference
Signal coherence
signal does not change from one instance to another (but noise does)
Using averaging techniques……
Two signals will be added together while noise will be less as it is not a constant signal
Spatial compound imaging
Multiple images are created over time then averaged together to create one image
These multiple images are formed at various angles and then average together to form a frame
Spatial compound imaging uses what
Various angles
what does a varying steered angle between frames result in
Many specular reflector based artifacts are reduced/eliminated by compound imaging
what are two benefits of spatial compound imaging
Improved SNR
Reduction in specular reflectors
frame rate and temporal resolution: spatial compound imaging
uses 3-9 images
- buffering technique to limit degradation
- Any process that uses averaging creates a degradation in temporal resolution, short duration events have the potential to be averaged out
Image persistance
Like compound imaging, the goal of persistence is to reduce noise and improve SNR
Uses an algorithm
what are differences between image persistence and spatial compound imaging
The angle is not changed between frames
A weighted average is applied to each frame so that newer frames “count” for more than older frames
Spatial averaging
looks at small local regions within the same frame
how does spatial averaging reduce noise
By adjusting pixels according to an algorithm that looks at the values of the nearest neighbors
spatial averaging gives image what look
Smooth
Frequency compounding/Averaging
The reduction in speckle artifact and noise by dividing reflected signals into sub bands of limited frequencies, making their own images, then combining those images to make a composite image
frequency compounding/averaging helps to reduce what
Speckle artifact
what is adaptive frame averaging designed to do
Increase the SNR
what does adaptive frame averaging allow for
short duration events to be evident without the penalty for lag for the fast moving structures
Adaptive Processing
Adjust system parameters to optimize the image based on algorithms
Adaptive processing is known as
Auto-optimization
Confocal imaging
Each transmit focal zone is formed with a different center frequency
What does confocal imaging take advantage of
broad transmit bandwidth is split into multiple frequency ranges
what does Confocal imaging do
Improves SNR (lateral resolution)
Confocal imaging does what
Multiple small transmit bandwidths with different center frequencies are used to collect signals along each line of sight
Multiple transmit foci improves what
Lateral reoslution
however, multiple transmit foci reduces what
temporal resolution because there are multiple foci and therefore the frame rate is slower
what is banding noise
Due to imperfect fusing of the multiple transmitted lines
Common with multiple foci
Panoramic Imaging
extend the field of view by acquiring lines of data while the sonographer scans the transducer across the patient. Correlation techniques are used to decrease the effect of movement on the quality of the image. (Think of it like stitching the image together)
Zoom
Technique to accomdate the desire to visulize regions of images in a larger format
what are the two common approaches to zoomed image
Acoustic zoom (write) Non-Acoustic (read)
Non acoustic
Simply magnifies existing pizels without increasing line desnity
Acoustic zoom
retransmits acoustic lines potentially using different line density to achieve better rolution
