Computed Tomography Flashcards
State the equation relating the incident and output x-ray intensity for an x-ray beam traversing a material of multiple different components.
ln(I_0/I) = int(mu).dx where I_0 is the input x-ray intensity, I the output x-ray intensity, int(mu).dx the integral of the attenuation coefficients and thicknesses of each component.
Explain the process of filtered back-projection image reconstruction.
- A large set of projection data is acquired.
- Each of these projections are back projected and combined to form an image of the object.
- To remove the 1/r blur created in this back projection process, each projection filtered prior to this process.
What is an advantage and disadvantage of iterative reconstruction over filtered back projection?
- Advantage: Has the potential to produce images with decreased noise and, therefore, lower dose is required.
- Disadvantage: Is computationally intensive.
Explain the process of iterative reconstruction.
- A large set of projection data is acquired.
- An initial guess is made at the image (often the filtered back projection).
- The forward projections of this initial guess are compared to the actual projections acquired.
- This initial guess at the raw data is then corrected to better match the actual projections.
- This process is iteratively repeated.
- Different algorithms can be used to determine the methods of comparison and correction. This is known as the reconstruction kernel.
How are Hounsfield units defined? What are the Hounsfield units of water, air and bone?
Hounsfield units are expressed relative to the linear attenuation coefficient of water by the following equation:
HU = ((mu_material - mu_water)/mu_water) x 1000.
HU_water = 0 as mu_material = mu_water.
HU_air = -1000 as mu_material = 0.
HU_bone = +300 to +2500 as mu_material is much larger due to the increased attenuation of bone.
How does a ‘first generation’ translate-rotate CT scanner work? How is a ‘second generation’ translate-rotate detector different? How is the now standard ‘third generation’ rotate-rotate scanner different?
- An X-ray tube and single detector perform successive translational acquisitions at multiple gantry angles.
- The ‘second generation’ scanner works by the same process but employs a bank of detectors. This means an increase amount of the patient is covered and, therefore, less translational acquisitions are required per gantry angle, decreasing the acquisition time required.
- In a ‘third generation’ scanner, the bank of detectors covers the whole patient, removing the need for multiple translational acquisitions per gantry angle. A fan shaped beam ensures the patient and all detectors are covered.
How does an electron beam CT scanner work? What are the advantages and disadvantages compared to conventional CT?
- Rotating scanner ring consists of target material.
- Electron gun directs electrons towards the rotating targets.
- X-rays are generated when the electrons interact with the targets.
- Advantage: No rotation of heavy tube required meaning the potential for faster rotation times (although this has been somewhat mitigated with modern technology).
- Disadvantage: Space constraint regarding the electron gun setup.
What level of filtration is typical of a modern CT scanner? Why is this less than it used to be?
- 6 mm Al.
- Ideally, a monoenergetic beam is required as CT essentially involves measuring attenuation and this is heavily dependent on energy. In older scanners, this meant heavy filtration was required. However, with modern computing power and reconstruction algorithms, this is no longer the case.
What is the reason for the bowtie filter? Why does a the bowtie filter need to be changed depending on the procedure?
- The bowtie shape results in increased attenuation of the beam towards the periphery. This reduces peripheral patient dose where there is less patient for the beam to pass through at the periphery. This, therefore, also ensures uniform noise across the patient cross section.
- The above points mean that the bowtie filter needs to be matched to the FOV (e.g. head or body).
What are the requirements of a CT detector? What type of material are modern detectors?
Requirements:
- Small to ensure good spatial resolution.
- High detection efficiency.
- Fast response with negligible afterglow.
- Wide dynamic range.
- Stable, noise-free response.
Modern detector are typically of solid state type.
What is the difference between single slice and multi slice CT scanning?
Single slice scanning involves a single detector element and a thin beam. Multi slice scanning uses multiple detector elements and a wider beam.
What is the difference between conventional axial/sequential CT scanning and modern helical/spiral scanning?
- In axial/sequential (also known as step-and-shoot) CT scanning, the couch moves the patient along the z-axis incrementally, pausing at each interval for a single rotation scan.
- In helical/spiral CT scanning, the couch constantly moves the patient while scanning. This makes for a quicker scanning process.
What is the nominal beam width in CT?
Nominal beam width = NT where N is the total number of slices (equivalent to the number of detectors in modern scanners) and T is the single slice thickness (equivalent to the detector thickness in modern scanners).
What is pitch in CT?
Pitch relates to how far the couch moves in relation to the rotation of the tube. It is defined as:
Pitch = Couch movement per rotation/Slice thickness OR
Pitch = Couch movement per rotation/Nominal beam width (this will mean a pitch of 1 results in tightly aligned spirals) OR
Helical pitch (often used by manufacturers) = Couch movement per rotation/Slice thickness (i.e. detector size).
What is a flying focal spot?
A two-position focal spot with rapid switching between each. This provides more projections through the patient and, therefore, more data to produce the final image. In this case, the beam width is half the nominal detector width.
