Computed tomography Flashcards
What is computed tomography?
Medical X-ray Computed Tomography (CT) is an imaging procedure used to create detailed images of internal organs, bones, soft tissue and blood vessels of the human body.
How does CT work?
CT uses a narrow X-ray beam aimed at a patient and quickly rotated around the body producing signals that are processed by a computer to generate cross-sectional X-ray images, or “slices”, of the body
A number of slices can be stacked to form a three-dimensional image that can be visualized in a variety of ways that may aid in the identification and location of basic structures as well as possible tumours or abnormalities.
Why is CT a good method to detect cancer?
CT scanning is often the best method for detecting many different cancers since the images allow your doctor to confirm the presence of a tumour and determine its size and location.
CT scans can detect some conditions that conventional X-rays cannot because theycan show a 3D view of the section of the body being studied
Benefits of CT include more effective medical management by:
- determining when surgeries are necessary
- reducing the need for exploratory surgeries
- improving cancer diagnosis and treatment
- reducing the length of hospitalizations
- guiding treatment of common conditions such as injury, cardiac disease and stroke
- improving patient placement into appropriate areas of care, such as intensive care units
CT: what happens in an emergency room?
Patients can be scanned quickly so doctors can rapidly assess their condition.
Emergency surgery might be necessary to stop internal bleeding. CT images show the surgeons exactly where to operate. Without this information, the success of surgery is greatly compromised.
The risk of radiation exposure from CT is very small compared to the benefits of a well-planned surgery.
The principal advantages of CT are:
Rapid acquisition of images
A wealth of clear and specific information
A view of a large portion of the body
No other imaging procedure combines these advantages into a single session.
Limitations of Planar X-ray Imaging
Planar imaging is qualitative not quantitative
Each pixel’s value is related to the attenuation of the X-ray beam in the path from the source to the pixel possibly passing through several materials
The judgement of what the details in the image represent depends on prior knowledge (it’s a bone because we know what a bone should look like!)
Planar imaging projects complex 3D anatomy into a single 2D image
The 3D information is not recoverable
Structures may be:
Superimposed
Foreshortened
Magnified
Obscured
Subject contrast is reduced by scattered radiation
X-ray radiation is potentially harmful
Conventional radiography superimposes structures on the detector
Results in:
Inability to determine the depth of structures
Limited ability to resolve the shape of structures
Reduction in contrast
Could resolve depth by using orthogonal projections – limited added value and increase dose
In other situations, digital tomography may be useful
This is used to good effect in chest imaging and mammography (DBT - Digital Breast Tomosynthesis ) – I’ll cover this in our lecture on mammography
CT Scanner Basics
Gantry aperture (720 mm)
Microphone
Sagittal laser alignment
Patient guide lights
X-ray exposure indicator
Emergency stop buttons
Gantry control panels
External laser alignment
Patient couch
ECG gating monitor
Early CT
Computed Tomography was developed to acquire three-dimensional volumetric data using X-ray imaging
The original developments in CT were performed by Sir Godfrey Hounsfield and Allan M Cormack, whowere award the Nobel prize for medicine for their work in 1979.
The first commercial scanners were introduced by EMI in 1974.
The initial scanner designs were functional but very slow
5 minute/slice scan time
7 minutes to process the data
First scanners head only
Full body scanning introduced in 1975.
Reconstruction Methods
Filtered Back Projection (FBP)
Algebraic reconstruction technique (ART)
Simultaneous Algebraic reconstruction technique (SART)
Iterative Reconstruction (IR)
Fourier
Back Projection
Each successive profile is back projected into the image array and added to the accumulated values
Interpolation is required for profiles not orthogonal to the image array
CT Number
Reconstructed images are displayed using CT numbers sometimes referred to as Hounsfield Numbers.
These numbers provide a quantitatively meaningful values.
CT scanners produce a wide range of image values (Hounsfield Numbers) that can be difficult to display and cannot be simultaneously perceived by the human visual system.
In practice, only a limited range of values are displayed by selecting an appropriate “Window & Level”
Visualisation of CT data will be the subject of a future lecture.
CT Number: Equation
Reconstructed CT scans have voxel values calibrated in Hounsfield Units (H)
𝐻𝑈(𝑥,𝑦,𝑧)=1000×(𝜇(𝑥,𝑦,𝑧)−𝜇_𝑤𝑎𝑡𝑒𝑟)/𝜇_𝑤𝑎𝑡𝑒𝑟
Where
𝜇(𝑥,𝑦, 𝑧) is the average linear attenuation coefficient for a volume element voxel of tissue in the patient at location (x, y, z).
HU(x, y, z) represents the CT number at the same spatial coordinates
𝜇_𝑤𝑎𝑡𝑒𝑟 is the linear attenuation coefficient of water for the X-ray spectrum used.
Fanbeam Scanning
Collimators are used to reduce scatter that is introduced by the use of a fan beam.
Principle is similar to an anti-scatter grid
2D collimators are also used
Slice vs Volume Acquisition (Notes)
Depending on the application contiguous slices are not always acquired
For example, High Resolution Computed Tomography (HRCT) was a technique that was performed on SSCT scanners aimed at assessing generalized lung disease. Acquired 1-2 mm slices every 10–40 mm apart
Early scanners were not able to cover the whole chest in one breath-hold
The result is a few high-resolution images that are representative of the lungs in general
The technique was unsuitable for the assessment of lung cancer or other localised lung diseases
Modern scanners can complete high-resolution scans of the chest in one breath-hold.
If the clinical application requires the assessment of highly localized structures with complete anatomical coverage, that may require reformatting for display in multiple image planes, then an image volume with isotropic spatial resolution is preferred.
Multi-planar Reconstruction (MPR)
Example: Surgical planning.
