- 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

Computed Tomography Flashcards by Casey Smith

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

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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

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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

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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

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CT image production process

Data acquisition -> raw data (numbers) -> image reconstruction process

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How to minimise dose when performing CT

minimise scan length
perform scout scan to confirm patient position/anatomy

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Types of Contrast Agent Applications

Intravenous
Gastrointestinal
Intracavity

Iodine Based or Barium Based

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What are the benefits of contrast agents

Utilised to enhance the vasculature
Demonstrates function and identifies pathological processes.

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Consent regarding contrast agent use

Informed consent to undergo contrast procedure
Requires sufficient information to make informed choice
Voluntary decision

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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

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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

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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)

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Approx window levels and window lengths for Cerebral Tissue

ww = 100 - 150
wl = 40 - 60

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Approx window levels and window lengths for Bone

ww = 2000 - 3000
wl = 300 – 400

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Approx window levels and window lengths for Lung Windows

ww = 1500
Wl = -600

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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