Computed Tomography Flashcards
(39 cards)
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.
What is the effect of increasing pitch on dose?
Assuming an equal nominal beam width, increasing the pitch will increase the couch speed per rotation. This will spread out the incident spiral of radiation and mean a lower radiation dose to the patient.
What is over-beaming in CT? How is this quantified numerically and monitored?
- Over-beaming arises from the penumbra of the x-ray beam caused by the finite size of the x-ray source. In CT, this radiation surpasses the edges of the detector bank and is, therefore, wasted (it contributes to patient dose but not the image).
- It is defined by the z-axis geometric efficiency:
z-axis geometric efficiency = Area under dose profile within active detectors/Area under total dose profile. This value must be displayed on the scanner if it is below 70%. It is also checked for all slice width combinations during commissioning.
What is over-ranging/over-scanning in CT? What can increase the level of over-ranging/over-scanning?
- Over-ranging refers to the fact that the actual irradiated scan range is larger than that nominally selected. This is due to the fact that image reconstruction algorithms for helical scans are based on interpolation.
- Increased pitch or a wider detector bank can increase the level of over-ranging/over-scanning.
What is automatic tube current modulation (ATCM)? How do ATCM algorithms vary by manufacturer?
- This is akin to an AEC in general radiography.
- Modulation in tube current is based on the attenuation characteristics of the patient. This information is usually obtained from the scanogram. Each rotation can also be used to determine attenuation characteristics and, therefore, required tube current for the next rotation
- The variations in tube current will depend on the reference value in the set protocol. Different algorithms are applied to control ATCM depending on manufacturer e.g.:
- Constant image noise will mean tube current is increased linearly with attenuation.
- A more variable image noise depending on patient size can be used to ensure larger patients are not overexposed.
What is a scanogram/topogram/scout view? What are they used for?
- Initial scan prior to the ‘main’ scan.
- No tube rotation is involved. The couch moves through the scanner while (typically) AP/PA and lateral views are acquired.
- Low dose compared to the ‘main’ CT scan.
- This helps ensure correct patient positioning.
- It also guides the automatic tube current modulation and automatic tube voltage modulation (if available) processes.
How does automatic tube voltage variation work?
- A kV appropriate to the patient is selected from scanogram data.
- This kV is maintained for the whole scan rather than varying like automatic tube current modulation.
- This is less common and only some scanners have this technique available.
What is superficial organ shielding? What is an issue associated with this?
- Superficial organs (e.g. breasts, eyes etc.) can be protected by reducing mA for certain angles of rotation.
- Using superficial organ shielding will likely mean automatic tube current modulation can not be performed at the same time.
Why might a larger beam width and faster rotation time be beneficial?
- It will speed up the scanning process reducing the chance of any motion artefacts e.g. the full heart could be scanned in one rotation.
- The faster scanning process will also increase patient throughput.
How is CTDI_air measured? What is the equation for CTDI_air? What corrections are required to for a CTDI_100 value?
- A pencil CT ionisation chamber which encompasses the whole beam (i.e. is partially irradiated) is used to measure the dose from a single slice rotation.
- CTDI_air is then calculated using the following equation:
CTDI_air = (1/s).int(D(x))dx where D(x) is the dose profile across the slice and s is the nominal slice width. - To determine a CTDI_100 value, a correction is required to account for the fact that the chamber is only partially irradiated. The correction factor is L/nt where L is the length of the chamber (100 mm), n is the number of slices and T is the slice width.
