1. Radiation Protection and Physics of Diagnostic Radiology Flashcards
What is the only distinction between xrays and gamma rays?
- SOURCE: Xrays -> Electron shells; gamma rays -> unstable nucleus
What is the relationshiop between velocity of EM radiation, frequency and wavelength?
v (m/sec) = Frequency (/sec) x wavelength (m)
VELOCITY IS CONSTANT -> Speed of light = 3x10^8, therefore freq and wavelength inversely proportional
Formula for energy of EM radiation
Energy (electron Volt eV) = Plancks constant x (speed of light (m/s) / wavelength)
Plancks constant = proportionality constant between energy of photon and its wavlength)
6.6x10^-34
SO energy of EM inversely proportional to wavlength
What is the unit of energy for EM radiation? How is it defined?
Electron volt (eV)
The energy gained by one electron as it is accelerated through a potential difference of 1V
VERY SMALL -> xrays with as little as 15eV energy can form and have potential deleterious effecs…
What is an ion pair? List 5 deleterious consequences of ion pair formation
Electron (displaced from atom e.g. by xray) + positively charged atom
=> Ionisation of DNA can occur in patient
1) Mutation
2) Abortion / fetal abnormalities
3) Disease susceptibility / shortened life span
4) Carcinogenesis
5) Cataracts
Table of wavelengths of common EM radiation
What is the difference between radiation exposure, absorption, and dose equivalent? What are the units?
- Exposure (coulombs/kg (of air!) OR Roentgen): Amount of radiation in air, quantified by amount of electrical charge generated by ionisation of air.
- Absorbed dose (Gray OR Rad): Efficiency of radiation absorption differs between tissues / materials. 1 gray = amount radiation leading to absorption of 1 joule/Kg of tissue
- Dose equivalent (Sievert or Rem)= ABSORBED dose x weighting factor of radiation
What is the difference between deterministic and stochastic effects? GIve one example of each
- Deterministic: Threshold at which effects will occur. E.g cataracts
- Stochastic: Random, no dose threshold. E.g. radiation induced cancer -> Severity independent of dose.
What is the whole body Maximum Permissible Dose (MPD) set for radiation workers by the ICRP? And for foetuses / general public?
Rad worker: 20 millisievert (mSv) per year, averaged over 5 years provided no single year exceeds 50 millisievert
Foetus: <0.5mSv per month
Public: <1mSv per year
When considering ALARA, what 3 methods to limit radiation exposure should be considered?
- Distance: doubling distance reduces exposure by 4 (inverse square law)
- Time
- Shielding
Describe how film badges work
Plastic housing and clip
- Contain radiation sensitive film (aluminimum oxide or lithium fluoride crystals) -> Trap electrons energized by radiation. Electrons can be quantified.
Describe X ray tube function
- Tube filament = CATHODE: heats up, boiling off electrons into cloud. Number of electrons function of current through filament (mA)
- Metal target = ANODE: Cloud of electrons strike target when voltage differential across tube. kVp adjusts this differential -> Higher difference, higher velocity electrons, higher energy emitted Xrays
Describe collisional vs radiative electron interactions (IN TUBE)
- Collisional: Electron hits electron -> releases electron and characteristic X ray. Binding energy dictates release of electrons.
=> Only a small fraction
- Radiative: BREHMSTRAHLUNG, electron does not hit anything, brakes and bends its course around nuclesu, releasing energy as X-ray. Spectrum of Xrays result from this based on degree of energy loss. One electron may undergo several braking reactions. MOST XRAYS PRODUCED THIS WAY!
Most energetic xray in spectrum will have keV (kiloelectron volts) equivalent to preset kVp -> Very few xrays in spectrum will be produced to have max energy
What is the difference between kVp and keV?
kVp = voltage difference across tube (relates to features of electrical supply and transformer)
keV = Energy of electrons or photons. Can only travel at equal to or less than kVp equivalent (e.g 100 kVp, electrons in tube =/< 100 keV)
=> Very few electrons in tube will reach 100 keV due to fluctuations in voltage of filament. Spectrum of energies
What is the process of rectification?
- AC: 50% current +ve across tube, 50% -ve. When reversal of currrent, filametn positive, and electrons attracted to it -> Damaging.
REctification eliminates this, making the target always positive (details of rcess not provided
How effiicient are Xray tubes at producing x rays? What characteristics of the target are important for Xray tube functionality?
Not very! 90% energy converted to heat
TUNGSTEN target:
- High melting point (3422C)
- High atomic number (74) -> +ve, effective braking and more efficient production
- rotates to avoid pitting,
What is the focal spot? What is one advantage and one disadvantage of using a small spot?
Area of target struck by electrons -> Smaller, sharper image.
Can modify with angle of anode to create small EFFECTIVE spot size, while at the same time maintaining larger area of contract on target
Some tubes have TWO FILAMENTS -> smaller filament, smaller spot size
Small spot:
Adv: Better detail e.g. in small patient
Dis: Cant use high mA as risk of overheating / melting
What do mA, s and mAs refer to?
- mA = Amount of current across filament -> Number of electrons / number of xrays
- s = Duration of current application to filament AND voltage across tube
- mAs = PRODUCT of mA x s -> quantifies amount of radiation. Multiple combinations to create same mAs
What are the roles of the x ray generator?
Second important part of machine (other than tube!)
- Roles:
Current to heat filament (VIA LOW VOLTAGE CIRCUIT)
Voltage differential across tube (VIA HIGH VOLTAGE CIRCUIT)
Timer (for filmaent heating and and voltage differential)
Rectification of voltage waveform (if required)
Nb: High frequency generators exist -> nearly constant voltage wave form. Increased efficiency of voltage / xray output (approx 2x more efficient). Compact, inexpensive and readily maintained.
List the 5 ways that photons can interact with matter
1) Coherent scattering
2) PE effect
3) Compton scattering
4) Pair production
5) Photodisintegration
Describe coherent scattering
- Photon interacts with object, changes course, BUT NOT ABSORBED AND DOESNT LOSE ENERGY
- Small proportion of xrays do this: approx 5%
- Disadvantageous:
Doesnt help with radiograph production (degrade quality)
May strike personnel
Describe photoelectric effect
- MOST IMPORTANT INTERACTION IN RADIOGRAPH PRODUCTION
- Photon ABSORBED COMPLETELY -> no scattered xrays
- Dislodges single electron (photoelectron) from inner shell -> this may then go on to ionise further within patient, and is evenually absorbed
- Characteristic Xrays -> low energy, absorbed locally and contribute to dose NOT IMAGE
- Probability of PE:
PROPORTIONAL to Z3 -> important for differential absorption / opacities!
DECREASES PROPORTIONAL to CUBE OF PHOTON ENERGY (1/E3)
How does photoelectric effect differ from collisional x-ray production?
- PE in patient, collisional in ANODE
- PE: PHOTON -> ELECTRON removal, and LOW ENERGY XRAY
- Collisional: HIGH ENERGY ELECTRON -> ELECTRON removal, and HIGH ENERGY XRAY (high binding energy of inner shell electrons in tungsten)
Describe Compton Scattering
- Photon strikes OUTER ELECTRON -> Ejected, and photon scattered at different angle / lower energy
- Electron: Compton electron / Recoil electron
- INDEPENDENT OF ATOMIC NUMBER, relates more to physical density (grams per cubic centimetre)
- Probability increases with photon energy, but decreases wen >1.02meV
- DISADVANTAGEOUS:
Differential tissue absorption diminished -> reduced contrast
Scatter - safety
Scatter - image degradation
Briefly describe how analog film screen radiography works
Photographic film with light sensitive (silver halide) emulsion -> Precipitation to silver deposits when exposed to xrays or visible light
Unexposed crystals removed during fixation
DEGREE OF BLACKNESS:
Amount of xrays striking film - attenuation and function of mAs
Energy of xrays = kVp
FFD: Closer, blacker
What is the inverse square law?
I1/I2 = (d2)2 / (d1)2
Where I = intesntiy (xrays / unit area); d is distance.
As FFD increases, intensity decreases as a function of THE SQUARE of the distance
Can also use to calculate required change in mAs value when intentionally altering FFD (e.g. shortened film distance required, what decrease in mAs needed?) Substitute mAs for I
How can patient motion be corrected for?
Shorter exposure time -> TRADE OFF increased mA to achieve required mAs for adequate film blackness
What effect does focal spot size have on image detail?
Small focal spot -> increased detail
TRADE OFF: Lower mAs must be used to avoid overheating filament
What is the term for blurring caused bye larger focal spot size?
Penumbra
List three contributors to amount of compton scatter
- Density of patient
- Volume of tissue irradiated
- Energy of xrays (kVp)
Why must the mAs be increased when a grid is used? By how much?
To account for attenuated xrays directly striking lead that would otherwise contribute to image.
2-3 x mAs typically
Typically how many lead strips are present in grids? And what grid ratio?
80-160 strips per inch
=> Number determines how conspicuous lines on a radiograph will be (assuming no bucky used). No effect on mAs
5:1-10:1
Describes HEIGHT of lead : width of gap -> Higher ratio more effective at attenuating -> requires more mAs
What is a focused grid?
A grid with increasingly tapered gaps towards periphery -> allows for beam divergence.
Must use at fixed FFDs
NB: Grids must be positioned with lines parallel to LONG axis of table, and with centre of beam within centre of plate -> otherwise losss of beam due to “lateral decentering”
What is the definition of distortion?
Unequal magnification of part of object being radiographed
What is the definition of radiographic contrast? What factors does it predominantly depend upon?
- Difference in film blackness between structures in the image -> distribution of gray shades in an image
- Depends on (technical factors):
Relationship between kVP and mAs
Film fogging (scatter)
- Depends on (inate factors):
Thickness
Density
Atomic number
Xray energy
Explain the role of mAs and kVp in radiographic contrast, including the concept of scale
- Innumerable combinations to produce satisfactory radiograph in terms of BLACKNESS, however contrast will depend on combo
- High mAs - low kVp -> Lower energy beam, more photoelectric (Z dependent) -> more contrast
=> Fewer greys, SHORT SCALE
- Low mas - high kVp -> higher energy beam, more compton (Z independent) -> less contrast
=> more greys, LONG SCALE
