Download Chapter 4 – atomic structure

Chapter 4 – atomic structure
Atom – smallest particle w/all the properties of an element
- electron, proton, neutron
- empty space
- electrically neutral in normal state
Ionization
– removal of an orbital electron from an atom
No outer shell can contain more than 8 electrons
Max electrons per shell = 2n2, n = shell number
Centripetal force keeps an electron in its orbit
Atomic mass number does not equal the precise mass of an atom
Isotopes
Same atomic number but different atomic mass
Isobar
Atomic nuclei with same atomic mass but different atomic numbers
Isotone
Atomic nuclei with same number of neutrons but different numbers of protons
Isomer
Atomic nuclei with same atomic number and same atomic mass number
Molecule
Combination of various elements
Compound
Any quantity of one type of molecule
Radioactivity
Emission of particles and energy in order to become stable
Radioactive Decay
Results in emission of alpha particles, beta particles, and gamma rays
Half-Life
Time required for a quantity of radioactivity to be reduced to ½ of its original
value
Radioactive Decay
Activity Remaining = Original Activity (0.5)n, n = number of half lives
Alpha Particle
Helium nucleus containing 2 protons and 2 neutrons
Beta Particle
Electron emitted from the nucleus of a radioactive atom
Chapter 5 – electromagnetic spectrum, radiologically important photons
X-ray photon is a quantum of electromagnetic radiation
EMR velocity is 3x108 m/s
Amplitude
½ the range from crest to valley over which the sine wave varies
Frequency
Number of wavelengths passing a point of observation per second
At a given velocity, wavelength and frequency are inversely proportional
Wave Equation
Wavelength = Velocity/Frequency λ = c/f
Electromagnetic Wave Equation
Velocity = Frequency x Wavelength c = f λ
Electromagnetic Wave Equation
f = c/ λ and λ = c/f
The electromagnetic spectrum includes the entire range of electromagnetic radiation
Diagnostic ultrasound is not part of the electromagnetic spectrum
The energy of a photon is directly proportional to its frequency
The only difference between x-ray and gamma ray is origin
Visible light is identified by wavelength, RF is identified by frequency, and x-rays are
identified by energy
Photons interact with matter most easily when the matter is approximately the same size
as the photon wavelength
X-rays behave like particles
Visible light behaves like a wave
Radiation attenuation is the reduction in intensity resulting from scatter and absorption
Inverse Square Law
I1/I2 = d22/d12
Radiation intensity is inversely related to the square of the distance from the source
The inverse square law is applicable to 7 times the longest dimension of the source
The x-ray photon is a discrete bundle of energy
Planck’s Quantum Equation
E = hf
Where E = photon energy, h = Planck’s constant, f = photon frequency in hertz
The energy of a photon is directly proportional to its frequency
Equivalent Planck Equation
E = hf, f = E/h, E = hc/ λ
Relativity
E = mc2
Where E = energy in joules, m = mass in kg, c = light in m/s
Chapter 8 – x-ray machine
Autotransformer
Has a single winding and is designed to supply a precise voltage to the filament
circuit and to the high voltage circuit of the e-ray imaging system
Autotransformer Law
Vs/Vp = Ns/Np
Where Vp = primary voltage, Vs = secondary voltage, Np = number of primary
windings, Ns = number of secondary windings
kVp determines the quality of the x-ray beam
Thermionic Emission
Release of electrons from a heated element
Product of x-ray tube current (mA) and exposure (s) is mAs, which is also electrostatic
charge ©
mAs times are used on falling-load and capacitor discharge imaging systems
High Voltage Generator
Contains 3 primary parts: high-voltage transformer, filament transformer, and
rectifiers
Rectification is the process of converting AC to DC
Voltage rectification is required to ensure that electrons flow from cathode to anode only
With 3-phase power, the voltage impressed across the x-ray tube is nearly constant, never
dropping to zero during exposure
Full-wave rectification or high-frequency voltage generation is used in almost all
stationary x-ray systems
During capacitor discharge, the voltage falls approximately 1kV/mAs
Less voltage ripple results in higher radiation quality an dquantity
Power = Current x Potential
Watts = Amperes x Volts
High voltage generator power (kW) = maximum x-ray tube current (mA) at 100 kVp and
100 ms
Power Rating (kW) = mA x kVp for high frequency and 3-phase
1000
Power Rating (kW) = 0.7 x mA x kVp for single phase
1000
Chapter 9 – x-ray machine
The protective housing guards against excessive radiation exposure and electrical shock
X-ray tubes are designed w/either a glass or metal enclosure
The cathode is the negative side of the x-ray tube and has two primary parts: a filament
and a focusing cup
Tungsten vaporization with deposition on the inside of the glass enclosure is the most
common cause of tube failure
The x-ray tube current is adjusted by controlling the filament current
Thermionic emission at low kVp and high mA can be space-charge limited
The anode is the positive side of the x-ray tube; it conducts electricity and radiates heat
and contains the target
Higher tube currents and shorter exposure times are possible with rotating diode
The rotating anode is powered by an electromagnet induction motor
The focal spot is the actual x-ray source
The line focus principle results in an effective focal spot size much less than the actual
focal spot size
The smaller the anode angle, the larger is the heel effect
The hell effect results in smaller effective focal spot and less radiation intensity on the
anode side of the x-ray beam
Excessive heat results in reduced x-ray tube life
Maximum radiographic techniques should never be applied to a cold anode
The most frequent cause of abrupt tube failure is electron arcing from filament to
enclosure due to vaporized tungsten
Single Phase Heat Units
HU = kVp x mA x s
3 Phase/High Frequency Heat Units
HU = 1.4 x kVp x mA x s
Chapter 10 – x-ray production
Kinetic energy is energy of motion
Kinetic Energy
KE = ½ mv2
Where m = mass in kg, v = velocity in m/s, KE in joules
Approximately 99% of the KE of projectile electrons is converted to heat
Characteristic x-rays are emitted when an outer shell electron fills an inner shell void
Only the K-characteristic x-rays of tungsten are useful for imaging
This type of radiation is called characteristic because it is characteristic of the target
element
Bremsstrahlung x-rays are produced when a projectile electron is slowed by the electric
field of a target atom nucleus
In the diagnostic range, most x-rays are bremsstrahlung x-rays
A discrete spectrum contains only specific values
A continuous spectrum contains all possible values
Characteristic x-rays have precisely fixed (discrete) energies and form a discrete emission
spectrum
Bremsstrahlung x-rays have a range of energies and form a continuous emission spectrum
Maximum x-ray energy is associated with the minimum x-ray wavelength (λmin)
A change in mA or mAs results in a proportional change in the amplitude of the x-ray
emission spectrum at all energies
A change in voltage peak affects both the amplitude and the position of the x-ray
emission spectrum
A change in kVp has no effect on the position of the discrete x-ray emission spectrum
In the diagnostic range, a 15% increase in kVp is equivalent to doubling the mAs
The overall result of added filtration is an increase in the average energy of the x—ray
beam (higher quality, spectrum shift to the right) with an accompanying reduction in xray quantity (reduced spectrum amplitude)
Increasing target atomic number increases the efficiency of x-ray production and the
energy of characteristic and bremsstrahlung x-rays
Because of reduced ripple, operation with 3-phase power or high frequency is equivalent
to an approximately 12% increase in kVp or almost a doubling of mAs over single phase
power
Chapter 11 – x-ray emission, sectional (compensating) filtration
X-ray quantity is the number of x-rays in the useful beam
X-ray quantity and amperage
I1/I2 = mAs1/mAs2
Where I1 and I2 are the x-ray intensities at the mAs1 and mAs2, respectively
X-ray quantity is proportional to mAs
Remember that mAs is just a measure of the total number of electrons that travel from
cathode to anode to produce x-rays
mAs = mA x s
= mC/s x s
= mC
Where C (coulomb) is a measure of electrostatic charges and 1 C = 6.25x1018
electrons
X-ray quantity and kVp
I1/I2 = (kVp1)2/(kVp2)2
Where I1 and I2 are the x-ray intensities at kVp1 and kVp2, respectively
X-ray quantity is proportional to the kVp2
X-ray Quantity and Distance
I1/I2 = (d2/d1)2
Where I1 and I2 are the x-ray intensities at distances d1 and d2 respectively
X-ray quantity is inversely proportional to the square of the distance from the source
When SID (source-to-image receptor distance) is increased, mAs must be increased by
SID2 to maintain to maintain constant OD (average optical density
The Square Law
mAs1/mAs2 = SID12/ SID22
Where mAs1 = the technique at SID1 and mAs2 = the technique at SID2
Adding filtration to useful x-ray beam reduces patient dose
Penetrability is one description of the ability of an x-ray beam to pass through tissue
Attenuation is the reduction in x-ray intensity resulting from absorption and scattering
The HVL of an x-ray beam is the thickness of absorbing material necessary to reduce the
x-ray intensity to half of its original value
HVL is the best method for specifying x-ray quality
Steps to Determine the HVL (with figure 11-3)
1. Determine the x-ray beam intensity with no absorbing material in the beam
and then with different known thicknesses of absorber
2. Plot the ordered pairs of data
3. Determine the x-ray quantity equal to half the original quantity and locate this
value on the y-axis of the graph (A).
4. Draw a horizontal line parallel with the x-ray from the point A in step 3 until
it intersects the curve (B).
5. From point B, drop a vertical line to the x-axis
6. On the x-axis, read the thickness of absorber required to reduce the x-ray
intensity to half of its original value point (C). This is the HVL.
Increasing the kVp peak increases the quality of an x-ray beam
Increasing filtration increases the quality of an x-ray beam
Added filtration results in increased HVL
Chapter 12 – x-ray interactions with matter, differential absorption
Coherent scattering is of little importance to diagnostic radiology
Compton Effect
Ei = Es + (Eb + EKE)
Where Ei = energy of the incident x-ray, Es = energy of the scattered x-ray, Eb =
electron binding energy, and EKE = kinetic energy of the electron
The probability of the Compton effect is inversely proportional to the energy (1/E) and
independent of atomic number
Compton scattering reduces contrast in an x-ray image
Photoelectric Effect
Ei = Eb + EKE
Where Ei = the energy of the incident x-ray, Eb = electron-binding energy, and
EKE = kinetic energy of the electron
The photoelectric effect is total x-ray absorption interaction
The probability of the photoelectric effect is inversely proportional to the third power of
the x-ray energy (1/E)3
The probability of photoelectric effect is directly proportional to the third power of the
atomic number of the absorbing material (Z3)
Pair production does not occur during x-ray imaging
Photodisintegration does not occur in diagnostic radiology
Differential absorption occurs because of Compton scattering, photoelectric effect, and xrays transmitted through the patient
Differential absorption increases as the kVp is reduced
To image small differences in soft tissue, one must use low kVp to get maximum
differential absorption
The interaction between x-rays and tissue is proportional to the mass density of the tissue
regardless of the type of interaction
Attenuation is the product of absorption and scattering
Chapter 13 – film construction, Kodak film
Image-forming x-rays are those that exit the patient and interact with the image receptor
The base of radiographic film is 150-300 micrometers thick, semi-rigid, lucent, and made
of polyester.
The latent image is the invisible change induced in the silver halide crystal
An ion is an atom that has either too many or too few electrons and therefore is
electrically charged
The result is the same whether the interaction involves visible light from an intensifying
screen or direct exposure by x-rays
Large-grain emulsions are more sensitive than small-grain emulsions
Crossover is the exposure of an emulsion by light from the opposite side of the
radiographic intensifying screen
Rare earth screens are made with rare earth elements, atomic numbers 57-71
Reciprocity Law
Exposure = Intensity x Time
= Constant Optical Density
The fog level for unprocessed film is approximately 0.2 mR (2 micro Gya)
Chapter 14 – radiographic processing
Developing is the stage of processing during which the latent image is converted to a
visible image
Fixing the silver halide that was not exposed to radiation is the process of clearing it from
the emulsion and hardening the emulsion to preserve the image
Wetting – (M = 15 sec, A = n/a)
Swells the emulsion to permit subsequent chemical penetration
Developing – (M = 5 min, A = 22 sec)
Produces a visible image from the latent image
Stop Bath – (M = 30 sec, A = n/a)
Terminates development and removes excess chemical from emulsion
Fixing – (M = 15 min, A = 22 sec)
Removes remaining silver halide from emulsion and hardens gelatin
Washing – (M = 20 min, A = 20 sec)
Removes excess chemicals
Drying – (M = 30 min, A = 26 sec)
Removes water and prepares radiograph for viewing
Synergism occurs when the action of two agents working together is greater than the sum
of the action of each agent working independently
Lack of sufficient glutaraldehyde may be the biggest cause of problems with automatic
processing
Archival quality refers to the permanence of the radiograph; the image thus does not
deteriorate with age but remains in its original state
Silver sulfide stain is the most common cause of poor archival quality
The shorter dimension of the film should always be against the side rail in order to
maintain the proper replenishment rate
Film transport time should not vary by more than plus or minus 2% of the time specified
by the manufacturer
Cleaning the tanks and the transport system should be a part of the routine maintenance
of any processor
Most processing faults leading to damp film are due to a depletion of glutaraldehyde, the
hardener in the developer
A finished radiograph that is damp easily picks up dust particles that could result in
artifacts
Chapter 15 – film screens, film, screens, spinning top test
The radiographic intensifying screen amplifies the image-forming x-rays that reach the
screen-film cassette
The phosphor converts the x-ray beam into light
Isotropic emission means radiation with equal intensity in all directions
Luminescence
Fluorescence – no lag, < 10-8 s
Phosphorescence – afterglow, >10-8 s
Intensification Factor
IF = Exposure required without screen
Exposure required with screen
Detective Quantum Efficiency
DQE = number of x-rays absorbed x 100
number of incident x-rays
Conversion Efficiency
CE = emitted light x 100
x-rays absorbed
Higher conversion efficiency results in increased noise
Generally, those conditions that increase the IF reduce special resolution
In mammography, the screen is positioned in contact with the emulsion on the side of the
film away from the x-ray source to reduce screen blur and improve spatial resolution
Screen-film compatibility is essential: use only those films for which the screens are
designed
Rare earth radiographic intensifying screens have the principal advantage of speed
The combination of improved CE and higher DQE results in the increased speed of rare
earth radiographic intensifying screens
Chapter 16 – beam restricting devices
Collimation reduces patient dose and improves contrast resolution
Approximately 1% of incident x-rays reach the image receptor
Scatter radiation increases as the field size of the x-ray beam increases
Compression of anatomy improves spatial and contrast resolution and lowers patient dose
Collimation reduces patient dose and improves contrast resolution
Under no circumstances should the x-ray beam exceed the size of the image receptor
Total Filtration
Total Filtration = Inherent Filtration + Added Filtration
Chapter 17 – grids, construction, performance
Reduced image contrast results from scattered x-rays
Grid Surface X-ray Absorption
% x-ray absorption =
width of grip strip
x 100
width of grid strip + width of interspace
Grid Ratio
Grid Ratio = h/D
High-ratio grids increase patient radiation dose
The use of high-frequency grids requires high radiographic technique and results in
higher patient radiation dose
Grid Frequency
Grid frequency = 10,000 micrometers/cm
(T + D) micrometers/line pair
The principal function of a grid is to improve image contrast
Contrast Improvement Factor
K = Radiographic Contrast with grid
Radiographic contrast without grid
The contrast improvement factor is higher for high-ratio grids
Bucky Factor
B=
incident remnant radiation
= patient dose with grid
Transmitted image – forming radiation patient dose without grid
As the bucky factor increases, radiographic technique and patient dose increase
proportionately
Selectivity
Sigma = primary radiation transmitted through grid
Scatter radiation transmitted through grid
Grid Characteristics
1. High-ratio grids have high contrast improvement factors
2. High-frequency grids have low contrast improvement factors
3. Heavy grids have high selectivity and high contrast improvement factors
Grid Cutoff
Distance to cutoff =
SID .
Grid Ratio
The main disadvantage of parallel and crossed grid is grid cutoff
High-ratio grids have less positioning latitude than low-ratio grids
In general, grid ratios up to 8:1 are satisfactory at tube potentials below 90 kVp. Grid
ratios above 8:1 are used with kVp exceeds 90 kVp.
Grid Selection Factors
1. Patient dose increases with increasing grid ratio
2. High-ratio grids are usually used for high-kVp examinations
3. Patient dose at high kVp is less than that at low kVp
One disadvantage of the air-gap technique is image magnification with associated focal
spot blur
Chapter 18 – radiographic exposure and technique
KVp controls radiographic contrast
Ampere
1A = 1 C/s = 6.3 x 1018 electrons per second
With a constant exposure time, mA controls x-ray quantity and therefore patient dose
X-ray quality remains fixed with a change in mA
Short exposure time reduces motion blur
mAs
Milliamperes (mA) x exposure time (s)
mAs controls OD
Equivalent Exposures of equal mAs
mAs = mA x time
mA (1st exposure) x time (2nd exposure) = mA (1st exposure) x time (2nd exposure)
mA (1st exposure) = time (2nd exposure)
mA (2nd exposure) time (first exposure)
Total Projectile Electrons
mA = mC/s, therefore mAs = mC/s x s = mC
mAs is one measure of electrostatic charge
Only the x-ray quantity is affected by changes in mAs
Distance has no effect on radiation quality
The Direct Square Law
mAs versus SID
mAs (2nd exposure) = (SID)2 (2nd exposure)
mAs (1st exposure) = (SID)2 (1st exposure)
Distance (SID) affects OD
Changing the focal spot for a given kVp/mAs setting does not change x-ray quantity or
quality
Half-wave rectification results in the same radiation quality as that for full-wave
rectification, but the radiation quantity is halved
The radiation quality does not change when going from half-wave to full-wave
rectification; however, the radiation quantity doubles
Three-phase power results in higher x-ray quantity and quality
High-frequency generation results in even higher x-ray quantity and quality
Chapter 19 – radiographic quality
Spatial resolution improves as screen blur decreases, motion blur decreases, and
geometric blur decreases
Radiographic noise is the random fluctuations in the OD of the image
The use of high mAs, low kVp settings, and slower image receptors reduces quantum
mottle
Radiographic Quality Rules
1. Fast image receptors have high noise and low spatial resolution and contrast
resolution
2. High spatial resolution and contrast resolution require low noise and slow
image receptors
3. Low noise accompanies slow image receptors with high spatial resolution and
contrast resolution
An increase in LRE of 0.3 results from doubling the radiation exposure
Optical Density
OD = log10 Io
It
Higher fog density reduces the contrast of the radiographic image
Base plus fog OD has a range of approximately 0.1 to 0.3
The reciprocity law states that the OD on a radiograph is proportional only to the total
energy imparted to the radiographic film
Film contrast is related to the slope of the straight-line portion of the characteristic curve
Image Receptor Contrast
Average Gradient = OD2 – OD1
LRE2 – LRE1
Where OD2 is the optical density of 2.0 plus base and fog densities, OD1 is the
optical density of 0.25 plus base and fog densities, and LRE2 and LRE1 are the
LREs associated with OD2 and OD1 respectively
Image Receptor Speed
Speed =
1
.
Exposure in roentgens to produce an OD of 1.0 plus base + fog
Speed vs mAs
New image receptor speed = old mAs
Old image receptor speed new mAs
Latitude and contrast are inversely proportional
Factors affecting the finished radiograph
The concentration of processing chemicals
The degree of chemistry agitation during development
The development time
The development temperature
Geometric Factors
Magnification
Distortion
Focal-spot blur
Magnification Factor
MF = Image Size/object size
= SID/ SOD
Object Size = Image Size x (SOD/SID)
Minimizing Magnification
Large SID
Small OID
Unequal magnification of different portions of the same object is called shape distortion
Distortion depends on
Object thickness, position, and shape
Thick objects are more distorted than thin objects
If the object plane and image plane are not parallel, distortion occurs
Focal spot blur occurs because the focal spot is not a point
Focal spot blur is the most important factor in determining spatial resolution
Focal Spot Blur
SOD/OID = Effective focal spot/Focal spot blur
The focal spot blur is small on the anode side and large on the cathode side
Radiographic Contrast
RC = film contrast x subject contrast
kVp is the most important influence on subject contrast
Patient motion is usually the cause of motion blur
Principles to be considered when planning a particular examination
1. Use of intensifying screens decreases patient dose by a factor of at least 20
2. As the speed of the image receptor increases, radiographic noise increases and
spatial resolution is decreased
3. Low-contrast imaging procedures have a wider latitude, margin of error, in
producing an acceptable radiograph
Keep exposure time as short as possible
The primary control of radiographic contrast is kVp
The primary control of OD is mAs