Flashcards in Fundamentals of CT Deck (29):
What are the basic principles of a CT scanner?
An x-ray tube produces a beam which passes through a patient. Underneath the patient is the detectors arranged in a circle and move around the patient. Irradiating the patient from a large variety of angles.
What is under the hood of a CT scanner? (no matter how old)
1. The x-ray tube
2. X-ray detectors
3. Cooling system
What are the essential imaging components of the X-ray Tube?
-Small focal spot
-High tube current
-Anode (high heat capacity)
-Tube needs sophisticated cooling system
Why is a small focal important for good images?
Smaller focal spot means that all x-rays appear to have come from the same point source.
A large focal spot means lots of x-rays come from slightly different angles creating a blurry image.
What are the essential imaging components of the collimation?
-Pre collimation determines the slice thickness
-Post collimation limits the amount of scatter reaching the detectors which would add 'Noise' to the image
What is an essential component of filtration?
As there is higher intensity at the edges of the fan beam compared to the centre. A bow-tie shaped filter is used. (bow tie smaller in the centre and thicker on the edges). This equalises the intensity across the fan beam creating a clearer image.
How does a gas detector (xenon) work?
Radiation ionises the xenon molecules and the electron produced are attracted towards a conductor to produce an electrical signal
How does a solid state detector (based on scintillation ) work?
Crystals absorb radiation and excess energy emitted as light.
Connected to a multiplier tube creating an electrical signal proportional to the amount of light which is in turn proportional to the amount of radiation.
(Lots of x-rays through crystals = Lots of light coming out)
The scintillator is backed by a photo-diode which changes the light to an electrical signal
Advantage of using a solid state detector compared to gas?
-Very fast (not depended on build up of ionisation)
-very small (just need a crystal and a photo diode)
-Smaller detectors = better image quality
-Lots of fans and detectors able to take multiple images
-Faster image= less patient movement
What does the CT 1st generation involve?
Single detector opposite the source exposed then rotated fractionally round to acquire data again
-Several minutes per slice
-require the patient to stay still for a long period of time
What does the CT 2nd involve?
-Multiple detectors (up to 30)
-Several projections per slice
-20 seconds per slice
-Gantry would take an image, rotate 30 degrees.
What does the CT 3rd generation involve?
-Large bank of detectors (usually 1000)
-Continuously rotating fan beam?
-1 second per slice
-Bowtie filter controls excessive variations in signal strength
What does the bowtie filter do?
-Attenuate peripheral aspect of fan beam
-overcome effects of beam hardening
-minimise skin dose
What does the CT 4th generation involve?
-Ring of fixed detectors surrounds the patient
-X-ray tube rotates inside detector ring through 360 degrees with a wide fan beam
-Limited by only being able to do 2 rotations before going in reverse to unwind the power cables
How was the power cable limitation of the 4th generation fixed?
Instead of power cables supplying power to the x -ray tube a slip ring is now used.
-Allows for continuous rotation
-Moves patient in Z direction on couch allowing for rapid gathering of data
How is image acquisition affected by the linear attenuation coefficient (u)?
As the x-ray beam passes through the patient, the change in output is completely dependent on the change in intensity.
(u) represents the extent to which the x-ray intensity is reduced by a material.
Bone (high LAC) = big reduction in intensity
Air (Low LAC)= Minimal Reduction of intensity
Each (u) is sampled at each detector to generate a projection.
How does Spiral CT work?
-Gantry rotates continuously in one direction on the slip ring
-At the same time the patient table moves at the same speed through the gantry
-faster CT means less chance of patient movement
What is spiral pitch?
The ratio of the patients movement through the gantry during one rotation relative to the beam collimation (slice thickness)
What happens if we have large pitch?
Pitch greater than 1mm creates interspaces and some portion of the body may be missed.
What does the 'Increment' refer to?
-The distance between the slices
How is image reconstruction achieved?
Through filtered back projection and adding information together.
A CT number is then assigned
What is filtered back projection?
As back projection produces blurred axial images. Filtering is used to improve the image quality.
What are CT numbers and how are they computed?
They are the pixel values assigned in the image.
-Computed by calculating the relative difference between the LAC of tissue and water
What is the CT number for water air and bone?
How do artefacts affect image quality?
-Degrade image quality by causing streaks, shading, rings and distortion.
What can cause streak artefacts?
Patient motion, organ motion, high density material, noise, mechanical failure
Rings on the image can be produced by 3rd generation scanners.
What is partial volume effect?
Assume a voxel comprises of a number of different substances. The calculated CT number represents an average of their properties.
Therefore material in any one voxel can affect values of surrounding voxels.
What does the partial volume effect result in?
-Difficult to differential between tissue boundaries
-slice thickness and spiral pitch affect PVE