CT in RT Flashcards
Importance of CT in Treatment Planning
- Accurate determination of inhomogeneities/heterogeneities within the patient
- Provides 3D and 4D data for RT treatment planning calculations
- Creation of Digitally Reconstructed Radiographs
- Provide accurate structure identification, targets and OAR
Calculation Modes
Homogeneity Calculation Mode
- Considers all the tissue the same
Heterogeneity Calculation Mode
- Considers the different densities of tissues
Ways to account for inhomogeneities and heterogeneities in Calculation Modes
- Effective Depths
- CT numbers
Denisty Effect on Isodose Curves
- When passing through a less dense material/substance (e.g. air), the isodose curves are less effected than when passing through a substance with a greater density (e.g., bone)
Heterogeneities that may be present in a brain plan
- Sinuses
- Cavities
- Metal Artefacts
- Differing tissue densities (e.g., compact cranial bone)
Attenuation in CT
- Occurs as x-rays travel through the patient (absorption and scattering)
- Differences in attenuation are collected by the detectors
- This is what establishes the contrast between tissues on CT images
Cross-Sectional Images in CT
- Images collected are cross sectional
- An individual slice shows only the parts of the anatomy imaged at a particular level
- CT slice is further sectioned into elements called pixels
Why do inhomogeneities affect isodose curves?
- Attenuation of the beam is related to the electron density
What is Electron Density?
Number of electrons per cm^3
As most tissues have similar number of electrons per gram, the electron density is most dependent on the density of the tissue
What does Relative Electron Density do?
Compares the electron density of tissue to the density of water
Hounsfield Units
- Water is used as they reference material given a value of zero
- Tissues denser than water have positive values (e.g., +1000 = bone)
- Tissues less dense than water have negative values (e.g., -1000 = air)
CT Data importance Planning
When CT data is used for planning, calculations done in the ‘Pixel by Pixel’ mode will automatically account for density changes
Data is unique to each CT (due to pixel variation) -> therefore calibration is important)
Image Quality - Voxel
Represents a value on a regular grid in three dimensional space or object
Image Quality - Pixel
Each image is made up of a matrix of Picture Elements
Each pixel represents a voxel (2D representation of 3D object)
What is a CAT Scan
Computerised Axial Tomography
Benefits and Negatives of Differing the Slice Number
- Bigger space between the slices, could lead to missed pathology
- More slices taken, requires more storage to keep the data sets
How does the calculation of dose occur?
Is performed by using the CT numbers of the planning scan to account for differences in density
How is a TPS utilised?
A TPS will use a stored table to convert CT number to density
Table is established by scanning phantom materials of known density and correlating with the measured CT numbers
What is the purpose of a Tissue Characterisation CT Pahntom
Used to calibrate the CT unit by establishing the relationship between the electron density of various tissues and their corresponding CT number
CT Number conversion to relative electron densities for various kV
CT number conversion to relative electron density is different for differing KVs
HU and Relative Electron Densities
Bone is more compact in the skull, therefore there is a greater electron density
Seimens Definition AS Open 64/20
The CT scanner has the ability to do a 64 slice data set or 20 slice data set
Single Slice vs Multislice
Multislice
- Scan greater amount of the pateint by accumulating more slices
- Faster scanning time
Spiral/Helical Scanning
- Allows for volume scanning
- Increases scanning of larger volume in less time
- Includes new x-ray tube and detector technologies
Additional Radiation outside the Image Volume (Single vs Multi Slice)
To get a complete first slice of the scan, you begin radiation delivery earlier than where you intend to scan
Main Steps in Performing CT
- Register the Patient
- Select Protocol for body part
- Position Patient-head/feet first, zero couch selection of correct orientation is crucial
- Acquire scouts in the correct direction
- Select scan area and scan
- Acquire Data-confirm
- Computer builds images from data collected
- View images and check all data is sufficient PRIOR to the pateint leaving CT
- Perform any post processing and transfer of data
Patient Positioning and Immobilisation - Crucial for RT
- We need to immobilise the patient in a reproducible position that is suitable for the intended treatment technique
Constraints with regards to Patient Positioning
- The patient should be CT scanned with any stabilisation devices that will be used for their treatment
- Any equipment CT scanned must not produce arterfacts
- Both the patient and the equipment need to fit through the aperture of the CT scanner
Scout Scans
- Can decide where the superior and inferior borders of the scan will be
- Can decide how full the bladder is
- Provides reference for slice data acquisition and display
- Provides information for scan length, possible bladder and rectal filling
What Occurs during a Scout?
- Tube and detectors are fixed or stationary
- Couch moves at a low speed
- Low dose exam
- Low spatial and contrast resolution
- Cannot create a treatment plan on a scout scan due to the poor image and inability to recreate in 3D
Metal Artifacts
Due to the beam being absorbed almost entirely by the metal, this results in huge decrease in the amount of photons reaching the detector
Metal Artefact Reduction System (MARS)
- System which fills in the gaps
- Estimates what the density will be in the area
- Prefer not to use for large amounts of artefacts
Striations in an Image
Striations across the data set due to breathing
For anything in the chest and abdomen, we need to try and use some form of breathing control
Difference between CT Bore Size and FOV
CT Bore Size = Size of the Donut
FOV = What is able to be visualised of the patient
Importance of FOV
For treatment
- Need to encompass all of the patient and see the external surface
For Diagnostic
- Seeing external surface may not always be necessary
Most CT scanners have a FOV of 50-70cm diameter
Window Level (WL)
Determines the mid-range CT number
This is the centre CT number value displayed by the gray scale range
Window Width (WW)
Determines the range of CT numbers displayed by the gray scale
That is the upper and lower range of CT numbers to be viewed
CT numbers above the range are displayed as white and CT numbers below the range are displayed as black
Patient Dose is Dependent on Numerous Factors
- Higher mAs = more x-rays incident on the patient -> more dose
- Higher kV = better penetration gives lower image noise but causes a decrease in soft tissue contrast
- FOV - Size depends on what is going to be scanned
- Smalled FOV will have a greater image quality - Slice width or thickness
- Many more Parameters
kVp - Definition
kilovolts peak
- Affects the energy of the electrons from the cathode to anode
mA - Definition
milliamps
- Unit for measuring x-ray tube current (amount)
mAs - Definition
milliampere seconds
- mA x exposure time
