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MRS432- CT > week 4, lec 1- image creation > Flashcards

week 4, lec 1- image creation Flashcards

1
Q

what is the image formation process?

A

CT gantry, patient and table, attenuation measurements and digital date –> CT computer –> CT numbers –> CT console –> PACS

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2
Q

what are the sequence of events after signals leave the detectors

A
  • CT detectors 
  • Pre-processing 
  • Reformatted raw data  - Convolution with filter
  • Image recon algorithm
  • Reconstructed images of CT numbers 
  • Image storage, display etc.
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3
Q

what is data acquisition

A

Taking ‘real life slices’ and converting them to numeric values

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4
Q

what is attenuation

A

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

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5
Q

what is the linear attenuation coefficient

A

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

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6
Q

what is the linear attenuation coefficient dependent on?

A

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)
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7
Q

what is fourier transform

A
  • A mathematical function that converts a signal in the spatial domain to a signal in the frequency domain
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8
Q

what is the spatial domain

A

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
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9
Q

what is the frequency domain

A

-In frequency domain, we deal with the rate at which the pixel values are changing in spatial domain

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10
Q

what is the reconstruction problem in CT

A

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
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11
Q

provide an overview of the principle behind CT

A
  • 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)
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12
Q

list 5 image reconstruction approaches

A
  • Simultaneous linear equations
  • Back projection
  • Filtered back projection
  • Fourier reconstruction
  • Iterative techniques (now-days)
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13
Q

what are 3 assumptions of heterogenous beam attenuation

A
  • Attenuation is not exponential
  • Quality and quantity of photons change
  • Lower energy photons absorbed, higher energy transmitted (photons that interact with detector)
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14
Q

what is the lambert-beer law

A

Describes what happens to the attenuation of the original intensity of photons

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15
Q

what is the objective of CT

A

To calculate the linear attenuation coefficient

This indicates the amount of attenuation that has occurred (between primary beam and beam that interacts with detectors)

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16
Q

what are CT numbers/ Hounsfield Units

A
  • 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
17
Q

what is the scan profile

A

A measure of the intensity (I) by each detector at a given angle of rotation

18
Q

what is the reconstruction process

A

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

19
Q

what is the process of filtered back projection

A
  • 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
20
Q

why is filtering done on the scan profiles

A
  • 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
21
Q

what is Iterative reconstruction

A
  • 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
22
Q

how does iterative reconstruction work

A
  • 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
23
Q

advantages of iterative reconstruction

A
  • 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
24
Q

assumptions about reconstruction from scan profiles in helical/ spiral CT

A
  • 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
25
Q

what is helical reconstruction

A
  • Will still use 1 rotation of data in the image reconstruction
  • 1 scan profile data will be at the exact location of the axial recon plane, other data will be a various distance from that plane
  • The closer the data is to the recon plane, the greater its importance in the image
  • The further the scan data is from the recon plane the less importance or weighting it will have
  • This is a process called interpolation.
26
Q

what is interpolation

A

-Interpolation is a method of calculating new data points within the range of known or existing data points

MRS432- CT (15 decks)

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