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mit > X-ray and CT > Flashcards

X-ray and CT Flashcards

1
Q

For X-rays:
* Wave lengths:
* Energy range:
* Diagnostic X-ray energy range

A
  • Wave lengths: ca. 1 nm to smaller than 1 pm
  • Energy range: ca. 1 keV to several MeV
  • No hard wavelength/energy boundaries
  • Diagnostic X-ray energy range 20 – 150 keV
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2
Q

How can X-rays be generated?

A

X-ray tube: static target
X-ray tube: rotating anode
Synchrotron facilites
Inverse-Compton sources (MuCLS: Munich Compact Light Source)
Liquid metal jet sources

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3
Q

rotating anode adv-disadv

A

Extemely inefficient process (ca. 1 % X-rays), high heat load
* Rotating anode for higher power & better heat distribution
* Rotating anode tubes higher heat capacity due to improved material cooling

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4
Q

X-ray generation – two effects

A
  • Bremsstrahlung:
    Negative acceleration of an electron in the
    Coulomb field of the atom nucleus.
  • Characteristic emission of X-rays:
    Electron transitions in the inner shells lead to
    the emission of characteristic X-ray energies.
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5
Q

How can the X-ray spectrum be changed?

A
  • Changing acceleration voltage: shape plus intensity
  • Effect of changes of current: merely intensity
    *different filtration
    *dfferent application (eg. CT or mammography)
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6
Q

How do X-rays interact with matter?

A

-photoelectric absorption
-Rayleigh (elastic) scattering(wo/ loss of energy)
-compton (inelastic) scattering(w/ loss of energy)
-pair production

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7
Q

photoelectric effect info

A
  • Albert Einstein discovered (nobel prize 1921)
  • The incoming photon is absorbed completely by one of the
    electrons, which is ejected from its shell
  • This effect is strongly depending on the X-ray energy and the
    atomic number (≈ Z^4/E^3)
  • At lower X-ray energies and high atomic numbers this effect is the
    dominant effect in X-ray imaging
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8
Q

Compton scattering info

A
  • A.H. Compton discovered (nobel prize 1927)
  • Inelastic scattering
  • Fraction of the energy of a photon is transferred to the kinetic energy of a free electron
  • Scattered photon with lower energy
  • Proportional to the atomic number and only slightly dependent on
    energy (≈ Z/E0.2)
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9
Q

attenuation coefficient µ depends on

A

material type and energy of incoming photon

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10
Q

Lambert-Beer’s law

A

-exponential loss of intensity I0
-transmission of X-rays through a material decays exponentially

I = I0*exp(- µz)

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11
Q

How can X-rays be detected?

A

-film detectors (silver bromide-still used in industry)

-Photo-stimulated illumination

-charge coupled devices (CCDs) (Photo diode as in digital cameras
* Coupled with scintillators for X-rays
* Needs relatively large optics)

-scintillators
materials: gadolinium oxysulfide (Gadox), caesium iodine (CsI)

– flat-panel detectors (directly coupled to photodiode array)

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12
Q

Which parameters are relevant for X-ray detection?

A
  • Physical pixel size -> spatial resolution
  • Point spread function (PSF) -> spatial resolution
  • Efficiency (quantum efficiency), spectral response & read out time
  • Noise defined by dark current ->readout noise
  • Artifacts like pixel defects, afterglow, long dead time
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13
Q

Novel detector technique

A

– photon-counting detectors
* Fast readout
* No readout noise (dose reduction)
* Small pixel sizes: in CT 225 x 225 µm²
* Spectral separation
* Homogeneous signal response

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14
Q

Limitations of photon-counting detectors

A
  • Pile-up
  • K-edge fluorescence
  • Charge sharing
  • High flux
  • High energy tail (limits spectral separation)
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15
Q

technical components of a modern CT

A
  • Patient table
  • Gantry
    X-ray tube/generator
    Filters
    Collimator
    Detector
  • Computer
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16
Q

X-ray tube related components

A
  • Bow tie filter for homogeneous intensity
  • Collimator( field of view for dose reduction)
  • Additional filters (filtering of low energy photons for dose reduction)
17
Q

How does tomographic reconstruction work?

A
  • Line integral at each detector position
  • Several projection from different angles
  • Reconstruction
18
Q

General reconstruction approaches:

A

Analytical reconstruction:
* Brute force
* Simple backprojection
* Fourier based methods:
- Fourier-slice theorem
- Filtered backprojection (FBP)

Iterative reconstruction:
* Hybrid model iterative reconstruction
- Iterative filtering
* Model-based iterative reconstruction
- Algebraic reconstruction method (ART)
- Statistical iterative reconstruction (SIR)

19
Q

Fourier slice theorem

A

1D Fourier transform of line integral equals cut through 2D Fourier transform
of object under same angle

20
Q

Fourier based methods – 2D approach

A
  • First idea:
    1. Take 1D Fourier transform of projections from Fourier slice theorem
    2. Sample 2D frequency space f(u,v)
    3. Multiply with 2D filter kernel (𝜔, 𝜃)
    4. Take 2D inverse Fourier transform

 Works in theory, but:
* Prone to distortions
* High spatial resolution high dose & slow

21
Q

Filtered backprojection (FBP)

A
  • Limitation to 1D Fourier transform
  • Include backprojection
  • Essential steps in filtered backprojection (FBP)
    1. Fast Fourier transform of line integral p
    2. Multiplication with filter function k
    3. Inverse Fast Fourier transform
    4. Backprojection over all angles
22
Q

How many projections are needed?

A
  • Nyquist sampling criterion has to be fulfilled:
    number of projections = 𝝅 /2 times sample width (=detector width)
23
Q

Beam geometry

A

Parallel beam
* 180 degrees
* Simple reconstruction algorithm

Fan beam
* 180 degrees plus fan angle
* Managable reconstruction algorithm

Cone beam
* Approximation
* FDK algorithm

24
Q

Iterative reconstruction

A

Hybrid techniques
* Iterative filtration on projections or reconstructed images

Model-based (projection and backprojection)
* Algebraic reconstruction technique (ART)
* Statistical iterative reconstruction (SIR)

Currently combination of iterative reconstruction with deep learning
* Sparse sampling for further dose saving

25
Q

Iterative CT reconstruction – biggest impact

A

Dose reduction

26
Q

How is CT data represented?

A

CT number – Hounsfield units
CT number relates attenuation coefficient µ(x,y,z) to water

27
Q

How can we define image quality and dose ?

A
  • Contrast, noise, dose, & spatial resolution
28
Q

Major clinical scan parameters defining image quality

A
  • Tube rotation time
  • Tube current and exposure time
  • Tube voltage (80 – 150 kVp)
  • Slice thickness & increment
  • Reconstruction kernel / algorithm
  • Patient size-dependent techniques
29
Q

double tube current

A

double number of photons and thus dose

30
Q

Tube voltage change

A
  • Defines contrast
  • Change spectrum shape and
    intensity
  • Increase tube voltage from 120 kV
    to 140 kV ->double number of photons and dose / non-linear increase
31
Q

Reduce slice thickness to half

A

 half the number of photons
 dose needs to be doubled for same image quality

32
Q

What is dual-energy CT?

A
  • Attenuation coefficient energy dependent
  • Energy dependency can be used for material/tissue differentiation
33
Q

Photon-counting CT

A

Advantages:
* high spatial resolution (250 x 250 µm²)
* always on
* standardized output
* multimaterial decomposition with K-edges

34
Q

CT Artifacts

A
  • Scattering
  • Ring artifacts (Hot/dead/miscalibrated detector pixel)
  • Motion artifact
  • Reconstruction artifacts
  • Metal artifacts(implants, correction by posrprocessing algorithms)
  • Beam hardening(X-ray spectrum changes with transition of object)
  • Undersampling & missing projections
35
Q
A

mit (9 decks)

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