Computed Tomography Flashcards
How is CT performed
Utilises an x-ray tube and a detector array rotating around the patient
Data collected is reconstructed into sectional (axial) images
Benefits of CT
- Fast
- Ideal for trauma and paediatrics
- Well tolerated
- Has replaced some conventional angiography
- helical datasets = high quality 3D imaging
- small attenuation differences visualised
- good demonstration of soft tissue
What are the Uses of CT
- No longer gold standard neuroimaging tool
- Widely used in initial investigation for many symptoms
- Complementary imaging with ultrasound
- Interventional procedures
What are the benefits of photon counting CT
- Allows smaller detector pixels -> improved spatial resolution
- Eliminates electronic noise -> lower radiation dose
- Allows spectral sensitivity-> Multi-energy information
- Lower energy quanta included -> improved image contrast
CT image production process
Data acquisition -> raw data (numbers) -> image reconstruction process
How to minimise dose when performing CT
minimise scan length
perform scout scan to confirm patient position/anatomy
Types of Contrast Agent Applications
Intravenous
Gastrointestinal
Intracavity
Iodine Based or Barium Based
What are the benefits of contrast agents
- Utilised to enhance the vasculature
- Demonstrates function and identifies pathological processes.
Consent regarding contrast agent use
- Informed consent to undergo contrast procedure
- Requires sufficient information to make informed choice
- Voluntary decision
Risks to Contrast Agent Use
Hypersensitivity to iodinated contrast media.
o Immediate
o Delayed
Acute Kidney injury – low risk in patients with eGFR > 45mL/min
What are algorithms and kernels
- Mathematical formulas which are applied to the raw data to create an image
Can be manipulated to alter the image:
o Edge enhancement -> beneficial for lung imaging
Window Widths and Levels
Influence the displayed appearance of the image
Do not alter the CT numbers of the image
- Same image may be photographed at a range of window
widths and levels (centres)
Approx window levels and window lengths for Cerebral Tissue
- ww = 100 - 150
- wl = 40 - 60
Approx window levels and window lengths for Bone
- ww = 2000 - 3000
- wl = 300 – 400
Approx window levels and window lengths for Lung Windows
ww = 1500
Wl = -600
Volume Rendering
3D technique that creates a semitransparent representation of imaged
structures – all voxels contribute to
the image
Roles of CT in oncology
- Diagnosis
- Staging - TNM
- Simulation for RT Planning
- Guidance for intervention
- Monitoring response to treatment
- Assessment during follow-up 3/12; 6/12; 12/12
tools to enhance visualisation of image
Exposure
Slice widths
Window widths
Use of contrast
Field of view
What is convolution filter
Mathematical approach to data representation in image (applied to raw data)
Manipulation of actual data
Multiplanar reconstruction
Image reconstruction into other planes
Can be at operator console or separate workstation
Maximum intensity projection
3D technique which displays the voxels of the highest value
Shaded Surface Display
3D technique that displays voxels on the edge of a structure
What is the purpose of protocol development
To accommodate and ensure consistency with patient factors (e.g., position, contrast)
For the application of effective post processing requirements
To limit and monitor radiation dose delivery
Strategies of reviewing CT images
Multiplanar reconstruction
- image reconstruction into other plans
Max intensity projection
- 3D technique which displays the voxels of the highest value
Shaded surface display
- 3D technique that displays the voxels on the edge of a structure
Volume rendering
- 3D technique that createsa semi transparent representation of imaged structures - all voxels contribute to the image
Impact of IV contrast on HU and how are they corrected in TPS
Increase attenuation of X rays therefore increased HU
This needs to be allocated normal tissue density as the patient will be treated without contrast
WITH CONTRAST -> density of structure increases
* following peripheral effusion in kidney -> continual increase with contrast
Effect of dose modulation on radiation dose to the patient
Optimise image quality whilst reducing patient dose
CAREDOSE4D is turned on and Dose length product decreases by half and so is Eff mAs
Impact of changing pitch
Decreasing pitch, increases scan time, decreases CTDvol (dose measure) and DLP as there is less image interpolation and more sliced being acquired
Increasing pitch decreases scan time and and increases CTD vol and DLP due to more interpolation
Benefit of CAREDOSE4D
It allows for dose modulation, and results in the following:
* decrease effective mAs and DLP
* decrease dose delivered to patient
Does the acquisition slice combination impact the projected radiation dose delivery? Why might the slice
combination be relevant?
Scan time and dose to patients increase
What is the benefit of producing a coronal series with a larger slice interval?
Allows for the image to follow the vessels and organs
Allows for a summary of images -> faster review time
How does attenuation occur in CT
Attenuation of the beam occurs as x-rays travel through the patient
Absorption and scattering occurs upon interaction with tissue
Differences in attenuation are collected by the detectors
This is what establishes the contrast between tissues on CT images
Main steps in performing CT
Register the patient
Select protocol for body part
Select patient orientation
Acquire scout scan
Select scan area and scan
Acquire data
Computer generates image from data collected
View images and check all data is sufficient prior to patient leaving CT
Perform any post processing and transfer of data
What is a scout scan
Low dose scan which be used to set scan parameters for higher dose scan
*verify patient position
*verify scan length
*verify patient anatomy
Provides low spatial and contrast resolution
Provide a definition for Hounsfield Units
Numbers used to define relative attenuation coefficients for each pixel of tissue in an image as compared to the attenuation coefficient of water
Provide a definition for Window Level
Determines the mid range CT number
This is the centre CT number value.
Provide a definition for Window Width
Determines the upper and lower range of CT numbers displayed by the gray scale
CT number above the range -> displayed as white
CT numbers below the range -> displayed as black
Impact of decreasing pitch
More mAs required for the same volume
Increase in image quality
Slower scan time
Role of CT in RT planning
Accurate determination of heterogeneities
Provides 3D and 4D information for treatment calcs
Creation of DRR’s
Accurate structure identification, target and OAR localisation
Conebeam CT -> IGRT
