Computed Tomography: Main principles, CT Scanner Design Flashcards
How is the human body viewed in CT?
Built up of a finite number of discrete slices and volume elements (voxels)
In CT, what is the x/y plane?
Transverse plane
In CT, what is the z axis?
Orientated perpendicular to the scan and image plane
Approximately parallel to the body’s longitudinal axis
What is S?
Slice thickness
What has to be measured to calculate the attenuation value P along each ray from the X-ray source to detector?
- Intensity I of x-rays attenuated by object
2. Primary intensity Io of X-rays
What is the simplest case given in CT?
Measurement of a homogenous object with monochromatic radiation
What does the intensity of X-rays fall off exponentially with?
Absorber thickness d
What is the attenuation P defined as?
Natural logarithm of the radio of primary intensity Io to attenuated intensity I
For Inhomogenous object, what does the total attenuation resulting from each ray path interval depend on?
Local value of the attenuation coefficient μi
What does CT consist of?
Measuring many such line integrals exactly
How can a two-dimensional distribution of an object characteristic be determined?
Exactly if an infinite number of line integrals are given
What is sufficient to compute an image to a good approximation?
A finite number of measurements of the distribution of attenuation coefficient
What is used in real CT scanners?
Polychromatic X-ray radiation
What does Linear attenuation coeffucuent depend strongly on?
X-ray energy E
What are examples of dependence on energy may yield?
- Beam hardening effects that lead to image artefacts
How is an object measured in CT?
A sufficiently high number of attenuation integrals or projection values have to be recorded
Necessary to carry out measurements in all directions
What is the simplest measurement set up?
- A radiation source with adequate collimation emits a pencil beam
- The intensity, attenuated by the object, is registered by the detector placed outside
- For a given angular position, this set up of radiation source and detector is moved linearly (transition) and the intensity is measured
How is an attenuation profiled created which is termed as a projection?
Determining the ratios of primary intensity recorded in periphery and the attenuated intensities recorded behind the object and taking their logarihms
How are projections measured?
Successively for successive angular positions
Where is the complete set of projections transferred to?
Data processing unit
How is a CT image computed?
Back-projection is usually utilized
How does back-projection work?
Each projection value is added to all the picture elements in the computer memory along the direction in which it was measured
What does each detail in the object and that is represented in the attenuation profile contribute to?
Pixel value at the desired image point
Entire imge as well
When will an unsharp image result from?
Considering only 2 or 3 projections
How do you avoid the unsharpening of each projection?
Each projection has to be convoluted before back projection with a mathematical function, the convolution kernel
What does the convolution kernel constitute?
A pointwise multiplication of the convolutional kernel and attenuation profile and addition of resulting values
What does convolution represent?
A high-pass filtering procedure which generates overshoots and undershoots at object boundaries
What does the convolution additionally offer the possibility to do?
Influence image characteristics by the choice and design of the convolution kernel
- from soft or smoothing to sharp or edge enhancing
What does CT measure and compute?
spatial distribution of linear attenuation coefficient
How is the computed attenuation coeficient displayed as?
CT number relative to the attenution of water
What are CT numbers measured in?
Hounsfield units (HU)
What is windowing?
Values above the chosen window will be displayed as white
Values below the window as black
When is a narrow window chosen?
Display of very small attenuation differences as given in the brain
What is an increase in CT number assigned to?
Increased density and/or an increase in effective atomic number
What does Dual-energy CT make use of?
the energy dependence in μ due to the materials’ atomic numbers
two scans with different spectra are carried out and the attenuation values and the differences in attenuation for the two spectra have to be evaluated
What is the aim of Dual energy CT?
Generate material-selective images and to determine material density as accurately as possible
What do modern CT scanners mainly use?
Fan-shaped X-ray beam
What are X-ray tubes capable of producing?
Tube currents up to 800mA and with significantly improved cooling systems
Why is having an improved cooling system important?
Long continuous exposures can be made on a regular basis during the day
Gantry tilt
The gantry has a capability to tilt, most CT scanners allow a tilt of the scan plane up to ±30% with respect to the axis of rotation. This allows the selection of scan planes directly through the structures of interest which is important for critical regions, such as the base of the skull or the lumbar spine.
What do real X-ray tubes have?
Extended focus sizes
for example 0.8mm x 1.2mm
What does Collimators in front of the detector serve?
to minimize signal contributions from scattered radiation
What does the bow-tie filters reduce?
the demands on the dynamic range of the detector system and scattered radiation intensities arising from peripheral object zones
What does the detector transform?
Incident X-ray intensity into a corresponding electrical sinal
What can double the number of simultaneously measurable slices?
Flying focal spot
