Medical Imaging Exam 1

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Specific Ionization

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the average number of primary and secondary ion pairs produced per length of a charged particles path

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alpha particles

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helium nucleus; high energy; doesn’t travel far; large for particle

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59 Terms

1

Specific Ionization

the average number of primary and secondary ion pairs produced per length of a charged particles path

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2

alpha particles

helium nucleus; high energy; doesn’t travel far; large for particle

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3

water ionization / ionization begins where

10 eV

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4

Nucleons

neutrons and protons

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5

photoelectric effect dominates

in lower photonic energies (in x ray range)

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6

pair production

1.02 MeV, uncommon in medical imaging, creates a positron and electron pair, they annihilate. Energy above 1.02 MeV goes to KE

created from x ray interacting with electric field of nucleus

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7

Compton Scattering dominates

higher photonic energies

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8

Raleigh scattering in medical imaging

10% of interactions in mammography

5% of interactions in chest radiography

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9

Impact of Particle interactions in medical imaging

  • at lower energies, photoelectric dominates

  • at higher energies, compton.

  • Reighleigh is less than both, but still present.

  • pair production doesn’t lie in the medical imaging range

<ul><li><p>at lower energies, photoelectric dominates</p></li><li><p>at higher energies, compton.</p></li><li><p>Reighleigh is less than both, but still present. </p></li><li><p>pair production doesn’t lie in the medical imaging range</p></li></ul><p></p>
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10

Beam hardening

As radiation passes through a medium, the lower energies will be attenuated, so the beam that arrives at the other side of the medium will predominately be higher energy.

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11

Convolution

blurring processes in imaging. There is an equation.

Smoothing, average, add in more data, but don’t destroy relevant structures.

Use kernals for smoothing/changing functions

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12

Spatial resolution

how close can two objects be together before you can’t tell the difference between them

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13

range for x ray medical imaging (eV)

30 - 511 keV

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14

Beta particle

high speed electron or positron

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15

Auger Electron

an electron is emitted after another electron transitions to another electron shell, and it leaves with KE and no emitted X ray.

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16

Fluorescence / Characteristic X ray

emission of an x-ray after a particle interaction instead of KE

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17

secondary electrons/delta rays

electrons get knocked off, which can affect a neighbor, and another neighbor until it loses energyand creates secondary ionizations.

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18

soft tissue ionization

70% of energy deposition of energetic electrons is from ionization

less energetic electrons: ionization % goes up

at 40 eV, excitation and ionization equal out

ionization decreases until 10eV

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19

elastic collision

kinetic energy is conserved

electron knocks out a valence electron (loses little energy)

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20

inelastic collision

KE is not conserved

electron knocks out an inner shell electron (like K)

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21

Bremsstrahlung

inelastic

electron passing an atom nucleus is deflected

  • follows a curved path and accelerates (faster or slower) and emits x ray as a result

electrons

most intense when electron have high energies and when the material has a high atomic number

  • high N, high E

fraction of energy of electron has an equation, f = 0.0007ZE

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22

Resting energy of an electron

0.511 MeV

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23

Rayleigh Scattering

coherent/classical scattering

entire atom is excited (oscillation)

atom re-radiates at the same energy, excites another atom in a random direction

E in = E out

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24

Compton Scattering

Nonclassical/Inelastic scattering

most common in diagnostic x-ray

interaction between photon and outer electron

photon will come in, knock out the electron, some of the energy goes to KE, the rest goes to a scattered photon.

There are two equations. Please take a moment to look at them and remind yourself of them!

as photon energy increases, photons and electrons (scattered) move toward forward direction.

  • more likely to be detected

at higher incident energies, more energy will go to the electron

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25

Photoelectric Effect

all incident photon energy is transferred to an electron, which is ejected from the atom

this creates a vacancy that another electron will fill

this results in an Auger electron or a characteristic x ray

there is an equation for this related to atomic number and energy

there are no photons to degrade the image

for each electron shell, there is a jump in energy

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26

mass and linear attenuation coefficient

how likely x rays/ other EM are to make it through a medium. Coefficients depend on energy of incident photons and the medium. One takes into account the density.

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27

Half Value Layer (HLV)

how much does it take for ½ of the beam to be gone

narrow beam geometry, we use broad - beam most of the time in imaging

__ = ln(2)/ mu

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28

Tenth value layer (TVL)

ln(10)/mu = ___

how much does it take for 1/10 of the beam to be gone?

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29

Effective energy

more energy penetrates deeper

broad band, polyenergetic (range of x rays)

HLV through aluminum is___

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30

Fluence

Photons/area

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31

Flux

Photons / Area X Time

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32

Energy Fluence

Fluence X Energy

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33

Strong Force

Force keeping protons and neutrons together in nucleus, ineffective over large distances

  • line of stability : too many or too few neutrons can screw it up

  • unstable nucleus becomes stable: radioactive decay

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34

Fission and Fusion

2 hydrogens fuse together, requires E

2 hydrogens are forced apart, releases E

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35

Kerma

Kinetic Energy released in matter

there is an equation related to this and mass energy transfer coefficient

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36

Mass energy transfer coefficient

mass attenuation x fraction of E photons transfer to charged particles as KE

higher energies have a lower coefficient of this

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37

Absorbed Dose

D = E/m

measured in rads or Gray or J/kg

mass energy adsorption coefficient dependent

mass energy adsorption could be smaller if electrons make bremsstrahlung

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38

Exposure

X = Q/M

___ in air is proportional to the dose in soft tissue (atomic numbers are similar)

W is here. It is related to dose and air and the equivalent dose.

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39

Point Spread Function

most basic measure of resolution and properties of an imaging system

based on a point

there will be some blurring

shift invariant

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40

Line spread function

resolution analyze

a larger area than PSF

A line object is used

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41

Edge Spread function

used with edges, there will be a certain spread. Turning the incident can help it be more accurate.

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42

Modular Transfer Function

a way to measure resolution as well

large and high frequencies —> amplitude will get smaller, will continue getting smaller with higher and higher f

at 10% of this, the spatial frequency can be determined.

Image will blur at a certain frequency of beam

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43

Phantom

Physical way of measuring resolution, has spacing on an object where the distance can help determine the resolution

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44

Contrast resolution

There is an equation for this

Intrinsic: anatomic factors —> contribute most

Extrinsic: x ray energy, machine related things

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45

Detector Contrast

X rays hit detector they are transformed into a signal

detector’s response affects contrast

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46

Display Contrast

Data collected has more bits than can be displayed. The higher resolution raw data must be turned into gray values for a monitor.

  • uses a Look Up Table (LUT)

contrast can be changed to highlight certain gray values

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47

Noise

precision of the image

  • can be reduced by number of photons increased or increase time of acquisition

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48

Quantum Noise

Noise from randomness of photons

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49

Anatomic Noise

Noise created by unwanted anatomical structures

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50

Electronic Noise

Noise that involves electronics

Measurements involve flow of electrons.

  • actual signal

  • thermal sources and other electrical signals

  • amplify signal may amplify noise

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51

Structural Noise

Noise from pixelated detectors with parallel channels, each of which has its own amplifier circuit — not perfectly tuned

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52

Contrast Detail Diagrams

compare different systems to see how they compare in terms of their contrast and detail.

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53

Reciever Operating Characteristic Curves (ROC)

Involves a truth table with normal, abnormal; true negative, false negative, true positive, false positive.

pick a true value and pick what is normal and abnormal around it

based on sensitivity and specificity of diagnosis

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54

sensitivity

True positive / true positive + false negative

how sensitive you are to to tell if there is a disease

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55

specificity

True Negative / True negative + False positive

more people may be called back that may not need it if this is reduced (False positives)

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56

PACS

Picture Archiving and Communication System

everything goes through this, where stuff is stored

Includes DICOM, HL7, EMR, and RIS

big firewall

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57

Image compression

Lossy compression is bad —> can’t be reverted

reversible compression —> good

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58

Grayscale Standard Display Function (GSDF)

calibrate luminance of displays according to predetermined values and such. Calibration is through a 3rd party software. Adjust luminance based on input pixel value

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59

Display Interface and Value of Interest (VOI) Look up Table (LUT) conversion

image is put through the look up table, a digital to analog converter will put it over to the display

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