What is generated during CT imaging and how?
- a three dimensional image
- from a large series of 2 dimensional x-ray images taken around a single axis of rotation
What must the x-ray tube be capable of?
Of high intensity x-ray output (usually 120kV) for long periods of time e.g. 30s,
needs to be consistently on with the same output for the entire duration of scanning
What else are some characteristics of the x-ray tube?
- high thermal capacity
- cooling, usually by oil via heat exchanger
- produces a fan beam
- filtration essential -> uniform x-ray beam
What collimators and what do they control?
- pre-patient collimators: control radiation dose to patient
- post-patient collimators: controls slice thickness
What characteristics should the x-ray detectors have?
- high detection efficiency - absorbing all the emergent x-ray beams energy to which they’re exposed to
- high conversion energy - enabling all the detected energy to contribute to image formation
- a wide dynamic range - so that all x-ray intensities, from the weakest to the strongest, are converted into proportional output signals
How are images acquired in CT imaging?
- projections are collected from all around the patient
- tube and detectors are rotated and measurements repeated
What is back projection?
Back projection in CT is used to reconstruct CT slices. It involves “smearing back” the projection across the image at the angle it was acquired, resulting in an image.
The more projections, the data that isn’t an object goes more in the background whereas the data that is comes out more.
However, this method can lead to a halo-shaped star around each image point, degrading contrast and blurring object boundaries.
So, filtered back projection is used.
Why do we use filtered back projection and how does it work?
- there is blurring of every single edge due to back projection process
- FBP removes parts of the projection data and can move real data too.
- If you have the original/raw data, you can apply any filter
- for the same data set, you can use different kernels/filters, it’s the same data, just reconstructed
BP and FBP?
- BP produces a smooth image that is an average of attenuation
- FBP removes the smooth appearance using a convolution filter + improves reconstruction
What is iterative reconstruction?
- reverses FBP and compares it to the original projection data
- because of the error in FBP, there will be a difference between the data sets
- creates a ‘candidate’ image and compares it to the current image
- improves the data until it matches the original projection data as closely as possible
- reduces noise in the image and dose (as mAs reduced)
What is a 2D matrix?
- an array of reconstructed results
- one picture element is called a pixel
- so the matrix is full of pixels (typically 512x512)
What is the relationship between field of view and spatial resolution?
- spatial res will increase with a smaller FOV
- as you decrease FOV, you make the pixel sizes smaller -> therefore higher spatial resolution (ability to differentiate two adjacent structures as being distinct from one another)
- optimise the FOV to be as small as possible, but includes everything you want to include.
What does each 2D pixel represent in a 3D image?
- each 2D pixel represents the average attenuation in a 3D voxel
- voxel = volume element
- takes into accounts Z/depth/slice thickness
What is the partial volume effect?
Occurs when tissues of widely different absorption are encompassed on the same CT voxel producing a beam attenuation proportional to the average value of these tissues.
Each voxel corresponds to a small volume within the patient
What is each pixel assigned? What does this mean and what is it compared to?
- each pixel is assigned a value - CT number
- average of all attenuation values in the voxel
- compared to the attenuation in water, quantify based on water
- anything less dense than water - negative Hounsfield unit
- anything more dense than water - positive Hounsfield unit
What are the Hounsfield units for: water, air, bone? What is the scale roughly?
- water = 0 HU
- air = -1000 HU
- bone = 500-1000 HU
- so scale is around -1000 to +1000
- each HU is assigned a shade of grey
What is windowing?
What is the window width (WW)? What does changing this do?
What is the window level (WL)? What does changing this do?
- windowing - applying grey levels to data
- WW - covers the HU of the tissues of interest, below WW = black, above WW = white
- narrower WW = higher contrast, wider WW = lower contrast
- WL - central HU of the numbers within WW
- higher WL = brighter image
what does helical CT imaging mean?
- the x-ray tube rotates continuously
- bed/table with patient on moves through the x-ray beam
- transmitted radiation takes the form of a helix
- information is acquired as a continuous volume of contiguous slices
- creates a 3D matrix of data points - true 3D imaging
How can large anatomical volumes be imaged?
During a single breath hold to reduce artefacts
What is the pitch?
What is the effect of increasing/decreasing it?
- the distance the table moves in one tube rotation divided by slice thickness
- increased table speed -> increased pitch
- increased pitch = less dose but poorer image quality
- decreased pitch = more dose but better image quality
What is a reconstruction interval?
In a spiral dataset, the interval that the images are reconstructed
Overlapping slices reduces the risk of missed lesions, e.g. reconstruction intervals are smaller than the slice thickness.
What is multi-slice CT?
- multiple rows or banks of detectors
- acquire multiple slices in one tube rotation
- increasing the volume imaged
What are CT scans planned with?
What does this do?
- planned on a ‘scout’.
- tube stays stationary and does an AP/lat scan
Then you plan: - slice thickness
- pitch
- reconstruction interval
- reconstruction algorithm
- filters
- contrast
what is multi-planar reformatting (MPR) for image reconstruction?
- 3D matrix means that the images can be reconstructed in 3 planes
- the images show reconstruction in 2 axis but the scanner is X, Y and Z
-
CT hardware and cross-sectional image viewing11
-
CT physics27
-
CT optimisation44
-
Brain Anatomy and Pathology59
-
Contrast Media / Pharmacokinetics47
-
MRI Principles30
-
MRI Safety31
-
Ultrasound Principles45
-
Principles of screening24
-
Biological effects of radiation17
-
Mammography - role, purpose & anatomy14
-
Mammography - image formation20
-
Nuclear Medicine22
-
Fluoroscopy41
-
Interventional Radiography22
