# 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)