week 4, lec 1- image creation Flashcards
what is the image formation process?
CT gantry, patient and table, attenuation measurements and digital date –> CT computer –> CT numbers –> CT console –> PACS
what are the sequence of events after signals leave the detectors
- CT detectors
- Pre-processing
- Reformatted raw data - Convolution with filter
- Image recon algorithm
- Reconstructed images of CT numbers
- Image storage, display etc.
what is data acquisition
Taking ‘real life slices’ and converting them to numeric values
what is attenuation
reduction in intensity of x-ray beam as it passes through an object (the patient)
The remaining x-rays are transmitted and reach the detectors
what is the linear attenuation coefficient
Describes how easily a volume of material can be penetrated by a beam
e.g. the fraction of a beam of x-rays or gamma rays that is absorbed or scattered
what is the linear attenuation coefficient dependent on?
the x-ray beam average energy (Eave)
- Change Eave and µ changes
- To get a true measure of µ, the x-ray beam must be monochromatic (1 energy for all photons)
what is fourier transform
- A mathematical function that converts a signal in the spatial domain to a signal in the frequency domain
what is the spatial domain
Spatial domain, we directly deal with the image matrix.
- X-ray beam passes through patient, image profile denoted by f(x) obtained
- Use of these coefficients allows assists in reconstruction of CT image
what is the frequency domain
-In frequency domain, we deal with the rate at which the pixel values are changing in spatial domain
what is the reconstruction problem in CT
There is an object of unknown x-ray attenuation characteristics
- Beam is NOT homogenous
- Need to determine the individual linear attenuation coefficient (µ) for each voxel
- Attenuated beam exits the subject it comes into contact with a series of electronic detectors
- Raw data needs to be converted into a numeric map
- X-ray tube and detectors rotate through 360 degrees
- At each angle (typically <1 degrees, so more than 360 angles)
- The x-ray beam passes through the patient and is attenuated by the patient’s body
provide an overview of the principle behind CT
- Data is collected by the detectors (forms the slice profile)
- The x-ray beam passes through the patient and is attenuated by the patient’s body
- Intensity values are converted to digital values
- Slice profiles typically are “filtered”
- Detectors gather attenuated x-rays which are then amplified and converted into digital signal as RAW data (aka projections)
list 5 image reconstruction approaches
- Simultaneous linear equations
- Back projection
- Filtered back projection
- Fourier reconstruction
- Iterative techniques (now-days)
what are 3 assumptions of heterogenous beam attenuation
- Attenuation is not exponential
- Quality and quantity of photons change
- Lower energy photons absorbed, higher energy transmitted (photons that interact with detector)
what is the lambert-beer law
Describes what happens to the attenuation of the original intensity of photons
what is the objective of CT
To calculate the linear attenuation coefficient
This indicates the amount of attenuation that has occurred (between primary beam and beam that interacts with detectors)
what are CT numbers/ Hounsfield Units
- CT numbers or Hounsfield units are used instead of linear attenuation coefficients (because thy are solid numbers; no decimals)
- CT number = Digital image
- CT number is a number related to the linear attenuation co-efficient of the object
what is the scan profile
A measure of the intensity (I) by each detector at a given angle of rotation
what is the reconstruction process
converts scan profile data into a matrix of µ’s, then change the µ to CT no’s / HU. Reconstruction is a mathematical process using a variety of methods
what is the process of filtered back projection
- Changing the filter yields a trade-off between noise and sharpness of the image
- It is an analytic reconstruction algorithm designed to overcome the limitations of conventional back-projection; it applies a convolution filter to remove blurring
- It utilizes simultaneous equations of ray sums taken at differing angles of a sine wave to compute the values of attenuation coefficients
why is filtering done on the scan profiles
- Process is linear: if it works for a point object (at any location), then it will work for any image
- results in a sharper reconstructed image
what is Iterative reconstruction
- IR does offer this ability to have high spatial and high contrast resolution and can do so at a lower dose
- Iterative reconstruction is an alternative reconstruction algorithm that can remove the noise from low-dose images using a variety of mathematic models
- Reduces rad exposure while improving image quality
how does iterative reconstruction work
- ASIR uses matrix algebra to continually process and compared to assumed ideal models over and over to improve the image voxel-by-voxel until they converge to a final value
- The software builds an image and then revises it with scores of reiterations to enhance image quality
- Radiation exposure from multi detector is high
- The data set is processed in a continuous loop where calculations are performed to create the different images
advantages of iterative reconstruction
- IR software can help reduce streaking, metal and blooming artefacts
- Increase the contrast-to-noise ratio in bariatric imaging (higher radiation doses) by better filtering noise
- Increase iodine opacification to help reduce contrast dosage
assumptions about reconstruction from scan profiles in helical/ spiral CT
- Additional maths is needed for fan beam reconstruction.
- Helical (or spiral) CT has the patient move while the tube and detectors are rotating
- Pitch is a parameter used to describe the “stretch” in the Z direction of the helical path
