Medical Imaging Exam 1

Specific Ionization

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

alpha particles

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

water ionization / ionization begins where

10 eV

Nucleons

neutrons and protons

photoelectric effect dominates

in lower photonic energies (in x ray range)

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

Compton Scattering dominates

higher photonic energies

Raleigh scattering in medical imaging

10% of interactions in mammography

5% of interactions in chest radiography

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

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.

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

Spatial resolution

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

range for x ray medical imaging (eV)

30 - 511 keV

Beta particle

high speed electron or positron

Auger Electron

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

Fluorescence / Characteristic X ray

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

secondary electrons/delta rays

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

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

elastic collision

kinetic energy is conserved

electron knocks out a valence electron (loses little energy)

inelastic collision

KE is not conserved

electron knocks out an inner shell electron (like K)

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

Resting energy of an electron

0.511 MeV

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

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

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

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.

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

Tenth value layer (TVL)

ln(10)/mu = ___

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

Effective energy

more energy penetrates deeper

broad band, polyenergetic (range of x rays)

HLV through aluminum is___

Fluence

Photons/area

Flux

Photons / Area X Time

Energy Fluence

Fluence X Energy

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

Fission and Fusion

2 hydrogens fuse together, requires E

2 hydrogens are forced apart, releases E

Kerma

Kinetic Energy released in matter

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

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

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

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.

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

Line spread function

resolution analyze

a larger area than PSF

A line object is used

Edge Spread function

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

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

Phantom

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

Contrast resolution

There is an equation for this

Intrinsic: anatomic factors —> contribute most

Extrinsic: x ray energy, machine related things

Detector Contrast

X rays hit detector they are transformed into a signal

detector’s response affects contrast

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

Noise

precision of the image

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

Quantum Noise

Noise from randomness of photons

Anatomic Noise

Noise created by unwanted anatomical structures

Electronic Noise

Noise that involves electronics

Measurements involve flow of electrons.

• actual signal

• thermal sources and other electrical signals

• amplify signal may amplify noise

Structural Noise

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

Contrast Detail Diagrams

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

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

sensitivity

True positive / true positive + false negative

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

specificity

True Negative / True negative + False positive

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

PACS

Picture Archiving and Communication System

everything goes through this, where stuff is stored

Includes DICOM, HL7, EMR, and RIS

big firewall

Image compression

Lossy compression is bad —> can’t be reverted

reversible compression —> good

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

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