Speed of Light - 3.0 x 10 to the 8th
Energy & Wavelength are inversly related
Bremsstrhlung Spectrum - simple scatter, used for X-ray production
dependent on KE of e- & how close it approaches to nucleus
proportional to kVp squared
higher Z number of nucleus causes shift to higher spectrum
kVP - shifts peak to higher energy & increases max E of photons
also increases # of photons overall
mA increases # of e- with a proportional increase in photons
does not shift the energy peak or max E of photons
Characteristic Radiation - produced when an e- ejected by a collision
when higher shell e- drops to fill the space a photon produced w specific E
E is the difference between binding energies of ejected & droping electrons
Peaks are at a specific location for each target type
X-ray production - 15% of photon flux produced this way
photon emmited at specific energy difference between shells of electrons
NOT significant in tissue because e- binding no more than .5keV
Auger e- - photon E transferred to outer shell e- which is ejected
Rayleigh Scattering - Coherent Reactions, low E photon iteracts w all electrons
excited electrons then release low E photon, contributes to scatter
accounts for 5-10% of all interactions in radiology
Photoelectric Effect - low E photon interacts w an inner shell e-
All E transfered to e-, photon disappears, e- ejected
e- has E of photon minus binding energy
Predominate interaction in tissue imaging
Increase in occurance proportional to Z cubed of material
Decrease in occurance proportional to E cubed of photon
Characteristic photons may result when inner shell e- ejected
in tissue though the binding E is never higher than .5keV
K-edge - binding energy of the K shell e-
marked inc in absorbtion of photons at that energy (mass attenuation)
tungsten = 69keV, lead = 88keV
Compton Scattering - medium E photon interacting w an outer e-
Scatter w minimal loss of E, calculated by deflection from original path
direction of scatter predominately less than 90deg
lower E photons retain a larger percentage of their E with scattering
BUT, very high E photons can not be deflected greater than 90deg
never greater than .511MeV at 90deg, or .255MeV at 180deg
Dependent on Density which dictates the # of e- encountered
Independent of Z - the # of e- per gram is constant between elements
Pair Production - high E photon interacts w nucleus, photon disappears
electron & positron produced if photon E > 1.02Mev
any energy beyond this is divided between the two as KE
positron will interact w another e- once it reaches it's ground state
2 photons of 511keV produced, leave at 180deg to each other
probablility of interaction proportional to Z & energy of photon
Photodisitegration - very high E photon ejects part of nucleus
usually requires >7MeV
Tube Heat - most is radiated away directly, the rest is conducted
conduction to post portion of anode must be slow to protect bearings
Heat Units - kVp x mA x time, multiply by 1.35 for 3 phase machine
Maximum Tube Rating - kilowatts at maximum mA for 100keV for .1sec
Line Focus Principle - X-rays are produced at predominatly 90deg to e- beam
Area of focal spot on anode is larger than area photons are projected too
Effective focal spot size = actual size x sin of theta
theta = degree of anode angulation from verticle
small theta increases loadability but dec focal spot size & film coverage
Focal Spot - area that photons are projected on to is primary concern
smaller used for finer resoltion, large for higher production
mA - increase will cause "blooming"
increase area e- emit from due to repulsion at the cathode
kVp - increase will Decrease "blooming" size due to faster acceleration
electrons don't get time to repulse & spread out
Space Charge Limitation - electrons form a barrier when too many
Tube Current - limited by kVp, below 40kV very few can be pulled away
with kV>40 very small inc in tube current can have huge inc in e- production
Focus Cup - neg charged cup creates bias, presses electrons together
Focal Spot Size is limiting factor in Wattage used
.1mm spot handles 1kW, 1.2mm spot handles 100kWatts
Heel Effect - attenuation of photons by anode material
if 100% midline, 75% near heel, 120% on cathode side
smallest sharpest focal spot is on anode side
Filtration - total filtration in a beam for 50kVp must be at least .5mm lead
for 50-70 need 1.5mm lead, for 70kVp need 2.5mm lead
3mm al will dec exposure by 80% using a 60kVp beam & same density
Grids - inc exposure but improve contrast by decreasing scatter
Grid Ratio = Hieght of grid / Distance between slats
Grid Cutoff - uniform if grid decentered or tilteed
nonuniform if grid flipped or off focal distance, too close worse
Screen - increase speed by factor of 34, decrease dose by the same
Flourescence - x-rays converted to light immediatly
luminescence - emission of light by crystals of inorganic salts when excited by
x-ray
Speed - increases as diameter of CaWo4 crystal increase
thicker screens increase speed, decrease sharpness
Intrinsic Efficiency - fxn of light generated/x-ray absorbed in %, high is fast
K-edge of material dictates absorbtion effeciency of diagnostic x-rays
Film - .1 to 1.0u grains of silver bromide transform E of photon
Latent Image Center - sensitivity area in silver halide crystal
3eV photon interacts w a bromide molecule liberating an e-
Ag+ ion then migrates to this site for each e-, often by the thousands
if > 3-4 present then grain is ready for development
silver crystal constructed w 5% postasium Iodide to stress the crystal
allows ions to migrate
Film Development - Amplifies latent image due to reduction of Ag-
those centers w Ag develop very rapidly
Hydroquinone Phenidone - reducing agent, hydrogen ions liberated
NaOh added as buffer solution
Sodium Sulfite - NaSo3 preservative, removes oxidation products
restrainers prevent development of unexposed grains
Temperature is critical to development characteristics
contrast has an optimum temp, highest at approx 92deg
speed & max poss development increase proportional to temp
Speed, Contrast, Fog & Max Density determined by development
Degree of Development determined by temp, time & chemical freshness
Film Fixing - nonreduced AgBr removed by Sodium Thiosulfate
gelatin hardener, stabalizer & buffer also
Silver Sulfide - results when film improperly washed, Brown color
Film Density - inc density by .3 = dec transmisssion by 50%
dec transmitted light by factor of 10 = -1 density
log of light emitted by box/light transmitted thru film
larger silver grains - more speed, less contrast - copy film, gamma of 1
medium grains - x-ray film, gamma of 2
small grains - print film, gamma of 3
Speed - the exposure at the casset to produce 1 + base fog
more speed means less photons to get same density
more speed due to inc conversion efficiency of screen cuases inc quantum mottle
this occurs because fewer x-rays produce the same amt of photons
they diffuse more, less sharp (rare earth screen)
more speed from increased stopping power of the screen will NOT inc noise
Noise - related to Speed
Speed = 1/exposure at an optical denstiy of 1
Characteristic Curve -
Illustrates relationship between film exposure & film density
Relative Exposure - film exposure related to a given exposure
subject determines these factors
Total Density - determined by factors in the film
Characteristic Curve Shape - shifted upward by
base + fog density - upward shift & blue tint added
shifted upward by Ag+ grains developed w/o being at latent center
Toe - exposure has not yet brought >3grains Ag+ to latent centers
Linear Portion - steepest portion determines film gamma
Average Gradient - calculation of slope at defined range
2 point spread of density above base fog
Shoulder - beyond here all silver is reduced
Processing time alters the curve
Radiographic Contrast - Subject contrast, film contrast, fog & scatter
change in density in adj areas of the radiograph
Subject Contrast - depends on thickness, density, atomic number & kVp
inc kVp will give low subject contrast but wide latitude
low kVp gives high subject contrast but low latitude, easily shifts of curve
Film Contrast - Processor & film factors, not effected by technique
Subject contrast is enhanced by film contrast if avg gradient >1
mAs - shifts position on curve, optimum in linear portion
High Contrast Images - Sharpeness limited, two small bones
Low Contrast Images - Noise limited, two soft tissue objects
Modulation Transfer Function - defines resolution & sharpness only
measures amt of info recorded/info available
max value = 1, minimum = 0
at MTF of .1 90% of info has been lost, at .75 25% is lost
Resolution - spatial freq in lines per mm at MTF of .1
high resolution w spatial freq of 10 lp/mm, CT only gets 2 lp/mm
Sharpness - average MTF from freq of 0-3 lines per mm
high sharpness if MTF is higher in the initial curve
Total MTF of a system is summation of all components, lowest value
Line Spread Fxn - measurement done w slit to quantify MTF
High MTF w film & no screen, Low MTF w thick screen
Focal Spot Size - one of the components with a defined MTF
especially improtant in MTF of a system during mag views
Latitude = 1/film contrast, inverse relation to contrast
high contrast film will have low latitude, lots of room on straight portion
low contrast film has high latitude, easily shifted onto heel or shoulder
chest films need a large latitude
mammo very narrow latitude, high gamma, steep curve
Magnification - focus film distance/focus object distance
Penumbra - makes the object look larger
penumbra reduced by dec focal spot size or dec object to film dist
Distortion - unequal magnification of different portions of the subject
created by decentering or tilting
Linear - best for blurring margins, parasite lines are the main artifact
Exposure Angle - defines slice thickness, don't confuse w tomographic angle
slice thickness = 1 / exposure angle, it is relative though w no true thickness
Wide Angle - Thin sections w very low subject contrast
Small high contrast objects like bones
problems due to high exposure & pt motion
Narrow Angle - Arc <10deg, Thick sections w high subject contrast
good for sharp images of entire organ, large w low contrast interfaces
problems due to incomplete blur out of focal plane, phantoms
Circular - creates uniform section thickness
problems created due to incomplete blur of margins, phantom images
no problems w parasite streaks
Pluridirectional Tomography - avoids tomographic motion in shape of object
less phantoms, well blurred margins, uniform section thickness
Problem is high dose & long exposure allowing pt motion
CT # = 1000 (u primary - u of water) / u of water
u is the linear attenutation coefficient, the most important factor in contrast
linear attenuation of air is 0, water 1.9, dense bone 3.2
kVp influences the CT # by lowering linear attenuation
linear attenuation increases as the beam passes thru the pt due to hardening
mA determines the dose/slice but has no effect on linear attenuation or contrast
Resolution = 1.5 x voxel size, .6 lp/mm average
Low Contrast Resolution - noise resolution, large voxel, small matrix, large FOV
discriminates large objects w similar density, noise limited resolution
Improve w smoothing algorythims & inc kVp
resolution can drop down to 10 lp/mm
High Contrast Resolution - Spatial resolution, small voxel, lg matrix, sm FOV
2 lp/mm best possible
Circumferential Resolution - worsens in an outward wedge shape
Radial Resolution - limited by the # of rays used to reconstruct the image
individual ray seen if one detector out on 3rd gen scanner
Dose - most in a given vol from the slice directly over, adjacent slices
contribute
Multiple Slice Average Dose - total dose recieved in central slice from all
combined
approx 30% higher than single slice dose
Resonant Frequency = 2 x the crystal thickness, independent of crystal type
waves generated by voltage potential placed across the pizeoelectric crystal
Q value = resonant frequency / bandwidth (v3 - v1)
describes the sharpness of the frequency response
flat curve = low Q, peaked curve = high Q
Velocity = frequency x wavelength, but velocity is constant 1540 m/s in tissue
Velocity is Independent of frequency or Intensity
Velocity is Dependent on wavelenth which is determined by media
Intensity - describes attenuation of a wave as it moves thru media in watts/m2
Independent of frequency and has NO affect on Velocity
Acoustical Pressure - the force of compression & rarifaction between waves
The most important determinant of Intensity
Half-Power Thickness - thickness which reduces intensity by -3dB
Grayscale Display - echo displayed as shade of gray relative to intensity
echo amplitude resolution used to skew towards weak diffuse echoes
Attenuation - removal of energy by absorption, scattering & reflection
mechanical wave converted to heat, Proportional to frequency
energy loss in beam = sum loss of each layer
tissue = 1, water = 0, bone = 20, lung = 40, air = 12
lower temperature & inc viscosity lower particle movement, inc attenuation
Acoustic Impedence = tissue density x velocity of sound (in units of Rayl's)
Reflection Coefficient - fraction reflected at an interface when angle is 0
< 1% at a given interface is ideal
reflection is due to interface & NOT attenuation
Refraction - bending of a wave as it crosses obliquely between 2 media
wave bends because two sides of wave move at different velocities
responsible for creating artifacts & phantoms
Geometric Spreading - determined by frequency & shape of transducer
Inc frequency causes lengthening of fresnel zone, narrowing of fraunhofer angle
also affected by the shape of the transducer face
NOT assoc w the inverse square law
Axial Resolution - frequency dependent
2 objects must be >1/2 the spatial pulse length
Lateral Resolution - dependent on beam width
improved by focused transducers at its focal zone, where beam is narrowest
Doppler Effect - shift in frequency created by moving interfaces
dependent on frequency of transducer\
Gradient Recalled Echo Images - partial flip angle followed by 180
effective trans relaxation (T2*) instead of T2
Susceptibility differences cause irreversible dephaseing, shortening T2*
Trabecula in marrow form inhomogeneous susceptibilities
loss of marrow signal results, not seen in diaphyseal region
Hemorrhage - Susceptibility from Iron products also causes signal loss
STIR - Short Tau Inversion Recovery, starts w an initial 180
a 90 is given when fat protons at their "null point", approx 350msec
a second 180 then rephases the protons
Increased marrow to lesion conspicuity results, esp in fatty marrow
Excercise increases signal in muscle on these images
FSE - Fast Aquisition Spin Echo, produces images w long TR in short time
k-space filling is altered by aquiring trains of 2-16 echoes after one 90 pulse
the TE becomes a midline value instead of a fixed value
middle lines of k-space are assoc w the greatest signal & highest contrast
Fatty marrow sustains a higher signal intensity than conventional Spin Echo
increased difficulty distinguishing fatty from red marrow
improved sensitivity using fat suppresion techniques
Fat Suppresion Techniques - use chemical shift between fat & water protons
they resonate at slightly diff frequencies due to enviroment differences
Fat is suppresed by using low intensity RF centered on fat resonance
fat magnetization is rotated mult times in the direction of the applied RF
This nulls the Z component of fat magnetization & blocks fat signal
routine spin echo or FSE sequence can then be performed on wate
Chemical Shift Artifact - occurs in freq encoding direction
effect from interface of fat & water
Electron Capture - unstable proton captures e- to turn to neutron
k shell e- taken w fixed portion of E taken away as neutrino
l shell e- drops in producing characteristic X-ray
Auger e- occasionally produced instead of X-ray
E from inward migrating e- ejects outer shell e-
also called internal photoelectric effect
predominates when Z less than 30
Beta Minus Decay - neutron changed to proton
e- & neutrino emitted, Z number increases by one
E of Beta particle varies from 0-max transition E
average E must be determined experimentally
Gamma emitted due to residual nuclear Energy
Beta Positive Decay - rarly occurs above N of 55
unstable proton yeilds e+ and turns to a nuetron
Positron matter-antimatter anihilation rxn w an e-
annihilation photons result w 511keV each
Parent Daughter Equilibrium -
Equilibrium occurs when the T1/2 of the daughter in < than parent
daughter APPEARS to have same T1/2 as parent by detected activity
Secular Equilibrium - occurs when T1/2 of parent is >>> than daughter
daughter activity quickly equals parent, no apparent dec in parent activity
occurs in most radiopharmaceutical generators
Transeint Equilibrium - when parent T1/2 approx 10x > than daughter
Daughter activity grows & slightly exceeds parent
Parent has detectable dec in activity
Tc generator uses this kind of equilibrium
Dose Recieved by Population
Average Background - 125mRem/yr, >50% of total
Diagnostic Radiography - 30mRem/yr, highest man made source
Radiation Worker - 300mRem/yr, only .2mRem contribution to whole population
Maximum Permisable Dose - MPD, 5rem for radiation worker, 170mrem population
pregnant women - 500mrem during gest, 1-10Rem abortion considered, >10 do it
Becquerel - one disitegration per sec, 1 Curie = 3.7 x 10 to the tenth Bq
Rad - unit of absorbed dose, quantity of radiation which heats 1kg by 100ergs
Gray = 100Rad, quantity which heats 1kg by 1joule
Linear Transfer Energy - Rate of E transfer from an ionizing radiation to target
NO threshold dose, damage caused is a linear function
Gamma rays have very low LET
Alpha rays have very high LET
Quality Factor - Q, scale of LET based on photons & e- as base of 1
slow neutrons = 5.0, all other neutrons & alpha particles = 20.0
Rems = Rad x Q 1 seivert = 100 Rems
Oxygen Radicals - esp hydroxyl radical are primary cause of damage
they travel approx 5 angstroms
hydrated aqueous e- travel 30-50 angstoms
Cystein & all sulfhydryls protect against oxygen radicals
Law of Bergonie - radiosensitivity of a cell varies directly w rate of mitosis
varies indirectly w degree of differentiation
Radiaton Sensitivity
4 most sensitive tissues in human are marrow, breast, thyroid & lung
Lymphocytes most sensitive of hemopoietic tissue, RBC's least
Bone marrow reserve capacity most important factor in LD50 of an animal
Breast - 6-8 cancers per million Rads per year
Juvenile Thyroid - 7rads to thyroid in pt <5yrs old causes 3x inc risk of Ca
The younger the pt the higher the risk
Fetus takes up Iodine at 13wks
Lung - Radon the main culprit, difficult to quantify exposure & effect
Lens of Eye - early radiation cataracts ar UNIQUE
Pinpoint & posterior, later they are difficult to distinguish
Embryo - most sensitve at 18-45 days during organogenesis
brain most sensitive at 8-15wks
cancer risk increases by 40% w pelvic X-ray
Radiation Genetics - No unique mutations, only inc risk of mutation
direct damage to DNA minimal from ionizing radiation
2 in 1million chance of mutation per gene per rad
most are recessive, only manifest in F1 generation
events are very rare even at high dose
Mutations vary logrythmicaly w DNA per haploid genome
Barley most effected, bacteria least
90% of breaks repair w a half-life of 90min, by 6h sig repairs completed
measured w lymphocytes cultured in vitro, look for breaks
changes seen in A-bomb survivors 40yrs later, STABLE genetic î
Stochastic Effects - events who's occurance is proportional to dose
Non-stochastic effects - erythema, sterility, vessel damage,cataracts
Doubling Dose - dose which will double spontaneous mutation rate
bad pregnancy - 69rems, sex chromosome aneuploidy - 250rems
average 150rems
mouse nads - 20-30rads high dose rate, 90-100rads in low dose rate
Genetically Significant Dose - GSD, a measure of what is sig in population
higher in ovary than testi except in hip & lower ext films
Radiation Therapy - dependent on radiation sensitivity of cell types
Fractional Doses - allow time for less sensitive tissue to repair (6hrs)
Dq - factor which determines shoulder of curve
low for cancer tissue, high for N, melanoma is exception, very high
Oxygenation - any tissue higher than 30mmHg of O2 sensitized
hypoxic cells in periphery of tumor cell resistant
Oxygen Enhancement Ratio - OER, ratio calculated in proportion to Nitrogen
heating above 41deg makes cells radiosensitive
Cells most sensitve in M & G2 mitotic phase, stable in S phase
Relative Biological Effectiveness - a comparison to standard of 250kV X-ray
RBE of high LET radiation increases until 100kev/um then decreases
varies according to ability of a cell to repair itself
increases in tissues that have a broad intial response curve