CT Resolution & Noise Flashcards
What is image quality determined by?
Resolution
Contrast
Noise
What are the two types of resolution in CT scanning?
Transaxial resolution (axial across the patient)
Z-sensitivity (along the length of the patient in the Z direction)
What does the resolution depend on?
It depends on the pixel voxel and matrix size
What is resolution affected by?
It is affected by the size of individual detectors that collimates the narrowness of the x-ray beam and the number of angles at which the circulatory tube is energised
What ability does spatial resolution in CT have?
It has the ability to distinguish between objects or structures that differ in density
Why is high spatial resolution important?
It is important as it allows for the discrimination between structures that are localised within very close proximity to each other
What affects sptial resolution?
Everything an e.g. is :-
- Field of view
As the field of view increases, so does the pixel size, resulting in a decrease in spatial resolution
What type of field view is person operated?
The scanned filed of view is a fixed size and the display filed of view is under person control
What is the relationship between the voxel size and spatial resolution?
The smaller the voxel size the hight the spatial resolution, a larger focal spot size will decrease the resolution
What is the blooming effect?
It is when the focal spot size enlarges as a current increases due to the repulsion of adjacent electrons within the anode
What is needed in order for a sharp image to be produced?
To produce sharp images the focal spots need to be small but they also need to be able to withstand the great heat loading without melting the anode
When is a small focal spot used?
A small focal spot is used when looking at very fine details and small slices for recons of bony detail
When is a larger focal spot used?
A larger focal spot is used when looking for a really short exposure time
How does magnification affect spatial resolution?
If magnification is increased the resolution is decreased
How does patient movement affect spatial resolution?
It will decrease the resolution and creates an artefact
How does pitch affect spatial resolution?
The higher the pitch is the faster the movement in the scanner, it reduces the resolution
What is a kernal?
A kernal is a convolution algorithm, it is a process used to modify the frequency contents of a projection data prior to that projection during the image reconstruction
What does kernals affect?
Kernals affect the appearance of images of structures by sharpening the image
What results in filtered back projection?
The process of deconvolution coupled with back projection
How does slice thickness affect spatial resolution?
The larger the slice thickness the lower the spatial resolution
How does detector size affect spatial resolution?
An increase in detector size will decrease spatial resolution
What is resolution?
Resolution is the measure of how apart two objects must be, before they can be seen as separate details on an image
What are the properties of Transaxial resolution?
Flying focal spot
Focus detector distance (FDD)
Focus isocentre distance (FID)
What is noise?
Noise is an unwanted change in the values of the voxels and appears as a homogenous image (grainy)
How is noise measured in CT?
Noise is measured via a signal to noise ratio (SNR)
The higher the ratio the less noise is present in the image
It is comparing the level of desired signal or the number of photons to the level of the background noise, the number of voxels deviating from the normal
How does noise affect a cross sectional image?
Noise in a cross sectional image will equal to a decrease in image quality and will be harder to get contrast resolution
What affects noise?
- mAs - 2x = 40% increases the signal noise ratio
- Increasing the dose of scan will decrease the amount of noise hence improves the contrast resolution but in turn increases patient dose
- Slice thickness, the thicker the slice the more photons are available which means better signal to noise ratio
- Size of patient
How is QA managed in CT?
QA is a perspex scanning tool used to check the quality of the machine, the check-ups can happen daily and or weekly depending on the manufacturer
What affects contrast in CT?
Noise
Tube current
Inherent tissue properties
Kilovoltage beam
Use of contrast media
