Computed Tomography Flashcards
Describe attenuation.
The radio density of a material measured by how much stopping power it has on radiation through absorption and reflection
How is attenuation measured?
-In hounsfield units
-By the different radiodensities of the different components of the body
What are linear coefficients and how are they used in CT?
Values that are converted to Hounsfield units and produce a greyscale image. They are used to describe how much of the X-ray beam is attenuated by a material
List the 5 components of a CT scanner and their functions.
-Gantry: houses the X-ray tube, detectors and generators
-X-ray Tube: rotates 360 degrees in the axial plane around the patient’s body
-Filters: improve efficiency of the X-ray beam
-Collimators: reduce patient dose and improve image quality
-Detectors: arranged in rotating rows in a continuous arch directly opposite the X-ray tube
2nd generation CT scanners…
…have multiple detectors and a fon-shaped beam which makes it faster than a 1st generation scanner.
What 4 things should a CT detector have?
-Dynamic range to allow the detector to respond to a wide range of beams
-A fast response
-Robust stability
-High efficiency so it can capture, absorb and convert X-ray photons into electrical signals
What is the difference between 3rd gen and 4th gen CT scanners?
A 3rd-gen CT scanner has a rotating X-ray tube linked to an arch of detectors that can only rotate 180 degrees to each side of the patient.
Whereas 4th-gen CT scanners have detectors all around the gantry that allow the X-ray beam to rotate 360 degrees
What do 1st generation CT scanners consist of?
A detector and a pencil-like X-ray beam that takes 5 minutes to capture a slice and 20 minutes to analyse data
List the 3 features of the scanner that dissipate heat quickly.
-High-speed rotors
-Direct oil cooling of the anode
-Focal spot cooling algorithms
What is the equation for attenuation?
tissue m - water m
______________________
water m x 100
Windowing is..
…the process of changing CT numbers to manipulate the CT image greyscale by adjusting WW & WL
Describe the functions of WW and WL.
-WW: controls the image contrast and highlighting
-WL: controls the image density
Define the purpose of a CT detector.
To convert photons into electrical signals called analog data which is then converted to digital information for computer reconstruction.
State the difference between Conventional data acquisition and Volume data acquisition.
Volume data acquisition is where the X-ray tube and detectors continuously rotate around the patient. Whereas conventional data acquisition the table moves slice by slice.
Name 5 features of a multiple detector.
-Faster scanning times
-Takes multiple slices
-Optimises scanning times after contrast injection
-Reduced radiation times
-Acquire thick and thin slices over large volumes
What are the features of a spiral CT scanner?
-Rotating slip rings which allow electricity to be conducted by brushes touching them
-Conduction that permits continuous X-ray tube and detector rotation without rotating the table
What is spatial resolution?
The ability to tell apart structures that differ in density
True or false?
Contrast resolution is the scanners ability to tell apart smaller differences in attenuation to tell apart smaller tissues.
True.
Decreasing the FOV…
…increases spatial resolution and decreases SNR
What happens when the mAs is decreased?
Spatial resolution increases, and SNR and radiation dose are decreased
What does SNR compare?
The levels of desired signals on an image to the level of the background noise.
True or false?
Increasing the matrix increases spatial resolution and radiation dose but decreases SNR.
False, increasing the matrix has no effect on the radiation dose.
When the pitch is increased…
…the SNR, spatial resolution and radiation dose decreases
SNR and radiation dose are increased when…
…kVp is increased and spatial resolution is decreased.
Why is windowing important?
-A higher WL & WW results in a darker greyscale and viewing of dense bones
-Manipulates greyscale for more detail
-Lowering the WW & WL whitens the greyscale which allows for soft tissue viewing
Are motion artifacts patient-based or physics-based?
Patient-based, it is photon starvation artifacts that are physics-based
How can photon starvation artifacts be reduced?
-mA modulation
-Adaptive filtration
-Iterative reconstruction
List 4 causes of patient-based motion artifacts.
-Patient
-Cardiac
-Bowel motion
-Respiratory
Describe how ring artifacts occur.
A faulty detector, during its rotation around the patient, can produce a ring artifact of signal loss
Name 5 ways radiation dose to the patient can be reduced.
-Increase the pitch
-Use automatic mA modulation
-Ensure examination is justified beforehand
-Reduce kVp
-Use short scan time
-Set right exposure factors to avoid repeat exams
-Ensure the patient is correctly positioned
