Rad physics exam 3 study guide

Primary Beam

• radiation that has not passed through the patient or an object

Remnant beam

• radiation that has passed through the patient or object and carries the organized signal that will form the image

• on average the total intensity is less than 1% of the primary beam

• it includes primary, secondary, and scattered radiation

Attenuation

• this is the reduction in beam intesity as x-ray photons are absorbed or scattered, leading to interactions with matter, a reduction of the number of photons, and a loss of energy

What affects beam attenuation

• hardness of the beam (selected kVp and filtration)

• mass density of patient tissue

• atomic number of atoms in tissue

Differential absorbtion

• the different degrees of absorption in different tissues result in image contrast and formation of the x-ray image

Coherent scatter (classical or unmodified scattering)

• occurs when a photon interacts with an outer-shell electron or by the atom as a whole

  • involves lower energy photons with energy below 10keV

  • thompson and rayleigh scatter

  • no electrons ejected and no ionization that occurs

    • orbital electron is momentarily raise to a state of excitation

thompson scatter

• type of coherent scatter

• occurs with a single outer-shell electron

rayleigh scatter

• type of coheret scatter

• occurs with all electrons of the atom

Ei =

incident photon energy

Es =

scattered photon energy

Eb =

binder energy for electron shell

Eke =

kinetic energy imparted to ejected electron

compton scatter

• occurs when an incident x-ray photon interacts with a loosely bound outer shell electron, removes the electron from the shell, and proceeds in a different direction as a scattered photon

what does compton scatter result in

partial absorption of the x-ray photons energy

what does compton scatter produce

a recoil electron and a scattered photon

what does compton scatter primarily produce

• occupational exposure and image fog

what energy levels does compton scatter occur at

diagnostic energy levels

compton scatter incident photon energy equation

Ei = Es + Eb + Eke

Photoelectric effect

occurs when an incident photon knocks an inner shell electron out of orbit and totally absorbs all its energy

what does the photoelectric effect create

an ion pair

in the photoelectric effect what is ejected from the atom

a photoelectron

energy of incident photon equation for photoelectric effect

Ei = Eb + Eke

what does photoelectric effect most likely occur

when the x-ray photon has just slightly more energy than the binding energy of a K or K shell electron

what does photoelectric effect result in

ionization

what energy level does photoelectric effect occur at

diagnostic energy levels

in photoelectric effect as atomic number increases…

there is in increased probability of photoelectri interaction occuring

what is photoelectric effect responsible for producing

image contrast

what does photoelectric effect occur most in

bone and contrast media

structures that undergo a lot of photoelectric absoption are said to be

radiopaque

• metal

structures that undergo very little photoelectric absorption are called

radiolucent

• air

pair production

emits two particles with the same mass as an electron, a positron and a negatron

when does pair production occur

• occurs at photon energies of 1.02 MeV or greater when the incident photon comes close to the nuclear field and loses all of its energy to the nucleus

what happens in pair production when the positron combines with another electron

• an annihilation reaction occurs where matter is converted back into energy

when interaction is significant in radiation therapy and nuclear medicine

pair production

photodisintegration

• this is an interaction between an extremely high energy photon (above 10 MeV) an the nucleus itself (not just the force field) where all the energy is absorbed by the nucleus resulting in an emission of a nuclear fragment called a nucleon

what can nuclear fragment be in photondisintegration

neutron, proton, or alpha particle

purpose for radiation protection and safety

lesson the likelihood of radiation-induced cell damage

early effects of radiation damage

• radiation burns

• blood cell depression

• chromosome damage

• reproductive disfunction

• death

late effects of radiaiton damage

• cancer

• cataracts

• genetic defects

ALARA

as low as reasonably achievable

what does alara mean

principle describes the imrtance of minimizing radiation exposure while still obtaining necessary diagnostic information

what are the cardinal principles

• time

• distance

• shileding

time

reduce as much as possilbe

distance

maximize distance (inverse square law)

shielding

use lead barriers and protective apparel

occupational annual whole body dose limit?

50 mSv

cumulative dose limit

age x 10mSv

what are some exapmles of protective equipment

• lead aprons

• thyroid shields

• lead gloves

• lead glasses

what is the thickness of lead aprons

• front: minimum 0.5 mm Pb Eq

• back: minimum 0.25mm Pb Eq

what is the thickness of lead gloves

minimum of 0.25 mm Pb Eq

what do you use to include only antomy of interest

beam limitation/collimation

what do you select to produce a diagnostic image

• highest kVp

• lowest mAs

filters

remove lower energy photons

where do you place the filters

in between the tube and the patient

how much Al Eq is required

2.5 mm Al Eq total filtration required when at or above 70 kVp

what grid do you use

the grid with the lowest possilbe grid ration

what does the grid do

absorb scatter and some of primary beam

where do you put the grid

between patient and image receptor

is their shilelding done ot protect patients?

no routine shielding

what is done to protect patients

minimize repeats

where is a dosimeter worn

• collar level outside apron

what are the 4 types of dosimeters

• film badge

• thermoluminescent dosimeters (TLD)

• Optically stimulated luminescence dosimeters (OSL)

• pocket dosimeters

what are the types of protective barriers

• primary

• secondary

what is primary protective barrier

protects against direct exposure to primary beam

what are requirements for primary protective barriers?

• must withstand up to 100 MeV

• must be 1.5 mm Pb Eq

what is secondary protective barrier

protects against exposure form scatter and leakage radiation

what are requirements for secondary protective barrier

must be 0.8 mm Pb Eq

what is located behind the secondary protective barrier

control booth

how tall do lead wall barriers need to be

7 ft in height

what is a controlled area

• occupied by radiation workers

• must keep exposure rate below 100mR/week

what is an uncontrolled area

• anyone can occupy

• must keep exposure rate below 10mR/week

what is exposure

a measure of intensity of the beam

• measure in C/kg

what is absorbed dose

• amount of energy deposited through ionization by the raiation source ni a tissue or other absorbing material per unit mass

• measured in Gy or mGy

what is equivalent dose

• modification of the absorbed dose that takes into consideration the damage caused by a particular type of radiation and is calculated by multiplying the absorbed dose by the weighting factor

• measure in Sv

effective dose

• modification of the absorbed dose that takes into consideration relative radiation sensitivities of different itssue types

• measured in Sv

air kerma

• kinetic energy released by the ionizations per unit mass of air

• measured in Gy or mGy

what is the unit of measure for radionuclides

millicuries (mCi)

what does the U.S Nuclear Regulatory Commision (NRC) do

• regulate nuclear materials and their by porducts

• they make laws regarding dose limits

what does the U.S Food and Drug Administration (FDA) do

• regulate radiopharmaceuticals and the performance and safety of x ray equipment

• make laws regarding inspection and maintenance of equipment and regulate supplies as well

what are agreement states

states have an agency incharge of enforcing the NRC and FDA regulations

what does this in missouri (the agreement state)

• the missouri radiation control program with the department of health and senior services (DHSS)

whole body occupational exposure limit to any organ

50 rem/yr (500m Sv)

occpuational dose limit for whole body

5 rem/yr (50 mSv)

lens of eye occupational dosel limit

15 rem/yr (150 mSv)

skin of whole body occupational dose limit

50 rem/yr (500 mSv)

extremities occupational dose limit

50 rem/yr (500mSv) to each extremity

embryo/fetus/pregnancy occupational dose limit

• 0.5 rem or 500 mrem/pregnancy

• (5mSv) or 0.05 rem (0.5 mSv)/month

general public infrequent exposure dose limit

• 0.5 rem (5 mSv) per year

general public continuous exposure dose limit

• 0.1 rem (1 mSv) per year

• students fall under the continuous exposure limits for the general public

Direct DR

x rays → electrical signal → digital image

what does direct DR do

converts x rays directly into an electrical signal

what does direct DR use

amorphous selenium photoconductor

indirect DR

x rays → light → electrcal signal → digital image

what does indirect DR do

converts x rays to light and then to electricity

what does indirect DR use

a scintillator (CsI or GOS) and amorphous silicon photodiode

TFT array

• can be used in both direct and indirect DR systems

• the control the readout of the detector elements (DELs)

pixel

• a 2D picture element

• each cell in matrix

• size determined by sampling frequency of DEL size

what is a voxel

volume of anatomy represented by a pixel or voxel element

matrix

each x ray image is displayed as a matrix of intensities

what is matrix size determined by

• receptor size or field of view

• pixel size