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Review for Rad Physics
50 questions, 21 over old material
Old material
How does x-ray interact with matter? Know this cold!
Chapter 13, p 149 - 160
X-ray beam photons interact with matter in 5 ways
1. Classical scatter – aka Thomson scatter – low energy
The net result of classical scattering is a change in direction of the x-ray without a change in
wavelength ( energy ). Involves low energy x-rays which contribute little to the radiograph. Classical
scatter occurs throughout th diagnostic range but is more prevalent at lower energies ( At 70 kVp 
3% classical scatter  contributes to small amount of film fog )
2. Compton effect - high energy, diagnostic range. Is the cause of all scatter. Takes away contrast 
results in longer gray scale. X-ray photon comes in, approaches the atom, … knocks outer shell
electron  recoil electron. Photon deviates and results in scatter. The photon is not used up in the
process. It interacts with the film and results in a low contrast film.
Occurs during the diagnostic range and begins to dominate above 180 kVp.
Almost all the scatter radiation that we encounter in diagnostic radiology comes from
Compton scattering. This scatter contributes no useful information to the image.
Characteristics of Compton scatter
Most likely to occur with loosely bound outer shell electrons
As x-ray energy increases the probability of penetration through tissue without interaction also
increases, there is increased probability of Compton relative to the photoelectric effect, there is
reduced probability of Compton scattering. As mass density of pt increases there is a
proportional increase in x-ray attenuation(partial absorbtion of energy) there is more Compton
scatter
3. Photoelectric effect - characteristic radiation – not many x-rays
1. electron – at the anode
2. photon – in the patient’s body; photon comes in, approaches the atom, K shell electron is
weaker than the photon energy  interaction probability is high: K shell electron is knocked out of
its shell, photon is used up, the electron that got kicked out is the photoelectron.
Result of photoelectric effect is 1 electron and 1 photon.
The general process of the photoelectric effect is ionization.
This occurs when an incident electron is totally absorbed during the ionization of an inner shell
electron. The incident photon disappears, the K shell electron = photoelectron – is ejected from the
atom with kinetic energy equal to the difference b/w the energy of the incident electron and the
binding energy of the k-shell electron.
A photoelectric interaction cannot occur unless the incident x-ray has energy equal to or greater than
the electron binding energy.
Most likely to occur with inner shell electrons
With tightly bound electrons
When x-ray energy is just higher than electron binding energy
As x-ray energy increases increased probability of penetration through tissue without interaction
Less probability of photoelectric effect relative to Compton scatter
Reduced absolute photoelectric effect
4. Pair production
Occurs only with high energy x-rays and gamma rays above 1.02 meV and predominates above 24
meV.
The incident X-ray interacts with the nuclear force field and is completely absorbed.
From this, 2 particles are produced:
1. One electron
2. One positron
This is bad if the patient is undergoing radiation therapy.
1
edited by Marie Paas
Rad Physics Tri 2
Test 2 material
July 1999
DISCLAIMER: Please be advised that the information contained within this text is extra study material and should NOT be
deemed as completely correct without mistakes. It is the responsibility of the user to verify the correctness of ALL information
contained within.
The positron is a positively charged electron, a form of antimatter. Therefore, it will only exist until it
meets another form of matter. When this reaction occurs, both particles will mutually destroy each
other in a process called Annihilation reaction. When this reaction occurs 2 photons are emitted in
opposite directions of each other with an energy of .51 meV each
This energy is the result of Einstein’s equation E=MC 2
If the energy of an electron is calculated in this equation, the result is .51 meV
Pair production rarely occurs in the diagnostic range, it is more common in therapeutic radiology
5. Photodisintegration
Occurs only with high energy X-rays and gamma rays above 10 meV. Some of these high energy
photons can penetrate the nuclear force field and strike the nucleus. When this happens, the photon
is completely absorbed and a particle ( nucleon ) from the nucleus is emitted. Rarely occurs in
diagnostic range
Summary
Photoelectric effect is most important for diagnostic radiology because it is responsible for
differential absorption and therefore subject contrast. Compton effect is important because it is
responsible for creating scatter radiation. Coherent or unmodified scatter could produce fog on a
film if no filtration is used to remove the low energy photons. Pair production and
photodisintegration are of no importance in diagnostic radiography.
Scatter increases film fog, decreases image contrast. Grids clean up scatter.
Binding energy
An electron from a higher shell drops down, has higher energy, needs to loose some, gives it off. The
difference in energy b/w the 2 shells is called binding energy. Energy which binds an electron to a given
shell. Binding energy is higher in shells closer to the nucleus. Since the nucleus of each element is
different (atomic number), each element will have its own characteristic binding energies for the various
shells
Potential energy:
Ability to do work due to position. Electrons farther out from the nucleus have more potential energy.
Ionization – an electron goes to a higher state, looses the energy and drops back down. As it goes back
down, it gives off an electron – deionization.
Excitation – electron goes to a higher state.
Air is radiolucent
Basic principle of x-ray machines:
Electrons are boiled off, thermoionic emission. Amount of electrons is determined by the current going
through the filament – mAs.
The overall purpose of the X-ray machine is to convert potential energy to electromagnetic energy.
A high electric potential (high voltage) is created between the cathode and the anode of the X-ray tube
– the electrons are boiled off.
A high amperage filament circuit at the cathode provides free electrons
The high voltage causes the free electrons to accelerate and stream across the gap. KVp is the
potential difference b/w the cathode and the anode – potential energy
Thus the electric potential energy is converted to the kinetic energy of the electrons
When the electrons collide or interact with the atoms of the anode, some of the kinetic energy is
converted to electromagnetic energy in the form of X-rays
The X-ray tube must meet the following criteria:
1. Generate a source of electrons
2
edited by Marie Paas
Rad Physics Tri 2
Test 2 material
July 1999
DISCLAIMER: Please be advised that the information contained within this text is extra study material and should NOT be
deemed as completely correct without mistakes. It is the responsibility of the user to verify the correctness of ALL information
contained within.
This is provided by the high amperage filament circuit
2. Rapidly accelerate the electrons to the anode
Provided by the high voltage tube circuit
3. Must rapidly decelerate the electrons
Provided by the high atomic number atoms in the tungsten-alloy target in the anode
Bremstrahlung – most x-rays
Also called Brems, general, braking or white radiation, this accounts for 70-90% of all x-rays
produced. Also responsible for the heterogeneity of the X-ray beam. The higher the atomic number, the
higher the energy of the X-ray produced.
Characteristic Radiation
Accounts for 10-30% of all X-rays produced. The energy of the characteristic X-rays depends on the
material used as a target. Each orbit will produce its own specific energy of X-ray. The higher the atomic
number, the higher the energy.
Half-wave rectification
Represents a condition in which the voltage is not allowed to swing negatively during the negative
half of its cycle.
If a rectifier is placed in an AC circuit, only 1/2 the pulses will pass through the circuit
Electrons flow only during one-half of the input waveform cycle
By eliminating 1/2 the pulses, you lose 1/2 the energy of the original AC current
Since the resulting DC current rises and falls, it is called pulsating DC current
Yields 60 positive pulses per second (60 hz system) because AC is 120V rectified by ½
wave 60 hz
Full-wave rectification
page 101
In full wave rectified circuit the negative half cycle corresponding to the inverse voltage is reversed so that
a positive voltage is always directed across the x-ray tube.
In order to avoid losing 1/2 of the pulses, a rectifier bridge can be installed in the AC current
Requires a system of 4 rectifiers
This is better but there is still fluctuation ( peaks and valleys, kvp )
Ripple factor
This is the variation in the voltage across the X-ray tube expressed as a percentage of the maximum
value. A low ripple factor is desirable, high = undesirable
Example:
In a single wave rectified circuit the ripple factor is 100% because the voltage goes from zero to a
maximum value with each cycle. If you select the X-ray machine for 70 kVp, the resulting X-rays will
range from 0 to 70kVp
3 - Phase Machines (Triple phase )
A way to decrease the ripple factor is to use 3 separate AC current sources each of which is 120 degrees
out of phase to the other waves. With this type of wave pattern, the voltage never drops to zero and Xrays are constantly being produced. This is more efficient because, although still slightly pulsed, it is
almost direct current
Advantages of triple phase machines:
Short exposure times can be achieved
The X-rays produced have a higher average energy and more uniform
Special transformers are used with delta windings on the primary and star windings on the secondary
side.
Triple phase units produce more heat in the tube, so high speed anodes must be used
Produces very small ripple
3
edited by Marie Paas
Rad Physics Tri 2
Test 2 material
July 1999
DISCLAIMER: Please be advised that the information contained within this text is extra study material and should NOT be
deemed as completely correct without mistakes. It is the responsibility of the user to verify the correctness of ALL information
contained within.
High Frequency Machines
Uses a single source of alternating current which is sent to a microprocessor which changes the
frequency to 10,000 HZ and uses capacitors to store electricity to be discharged at a precise time.
Generate almost pure D.C current with almost no ripple
Heel effect – uneven distribution of beam … due to angle results in the primary beam being absorbed.
An uneven distribution of radiation from cathode side to anode side of the X-ray field.
Increase in radiation at the cathode end
Decrease in radiation at the anode end
There can be as large as a 40% difference between these two ends.
The effect is caused by absorption of radiation in the heel of the anode and by the angle of the
anode.
A smaller anode angle will increase the amount of the heel effect, because it will result in more
absorption of the beam.
The shorter the focal film distance ( FFD ) from the tube to the film, the greater the heel effect will be. At
40” the heel effect is quite large.
The heel effect can be eliminated by the use of a wedge filter. Or place the head towards the anode (
thinner thoracics ) and the feet towards the cathode ( thicker thoracics ) to use the heel effect in our
favor.
Filtration – added vs inherent
Removes the soft or low energy X-rays from the X-ray beam which can reduce skin dose by 90%.
Consists of aluminum which is placed in between the X-ray tube window and the collimator.
Compensation filter is a type of added filter
Placed in the beam to modify the X-ray field distribution to compensate for wide variations in patient
thickness
NEW MATERIAL
Use common sense – cut out the fluff
Intensifying screens
How do they work, how are they rated, factors
Gurney Mott Hypothesis deals with the process of development of a latent image to a manifest image.
Takes place in the development tank.
Conduction Band Theory
An X-ray strikes an outer shell electron which is then raised to an excited energy state. This creates a
hole in the outer electron shell which is an unstable condition. The hole is filled when the excited electron
returns to its normal state. To do this it emits energy in the form of visible light
Crystal size
Spatial resolution
Speed
Why are screens used?
Phosphor layer
Is the active layer of the intensifying screen. Phosphor layer emits light during stimulation by X-rays,
converts the energy of the X-ray beam into visible light
Common Phosphors Used:
4
edited by Marie Paas
Rad Physics Tri 2
Test 2 material
July 1999
DISCLAIMER: Please be advised that the information contained within this text is extra study material and should NOT be
deemed as completely correct without mistakes. It is the responsibility of the user to verify the correctness of ALL information
contained within.
Calcium tungstate – not much in use any more
Zinc sulfide
Barium lead sulfate
Rare Earth phosphors – most often used now, but difficult to manufacture
Gadolinium – also contrast material, green
Lanthanum
Yttrium - blue
Properties of Intensifying Phosphors:
1. Should have a high atomic number, so that X-ray absorption is high
2. Should emit a large amount of light per absorption of X-ray photons, this is called conversion
efficiency
3. The spectral emission of the screen must match the sensitivity of the X-ray film, this is called spectral
matching
4. Should not be affected by heat, humidity, or other factors in order to keep them from spontaneously
discharging.
Luminescence
Light is emitted by the phosphor layer by a process called luminescence
Luminescence may occur by two different processes: NOTE Dr. Mestan made a mistake when
he talked about this during the review, he reversed it!
1. Fluorescence
Light is emitted within 10-8 seconds after being exposed to radiation
In this process, light is emitted promptly and the stops
Process similar to characteristic X-ray emission, BUT it involves outer shell electrons and is called the
conduction band theory
2. Phosphorescence
Light is emitted longer than 10-8 seconds after being X-rayed, which results in a delayed emission of
light causing an afterglow or lag. This is undesirable! ( This is used in Fluoroscopy and TV screens.
)The intensifying screens use phosphors that emit light by fluorescence, otherwise the film would turn
black from too much exposure
Spectral matching - film is sensitive to a specific color and is used with a screen with that color.
Duplixing
Developers
90 second
Also called fast access
Processes 300 film/hr with a 950F developer temperature
18 to 22 second developing time depending on brand
Most common processor currently on the market
The higher the temperature of the developer, the lower the contrast
Transport roller system
Series of rollers racks and drive chains powered by an electric motor, that transports the film through the
various solutions
This system is made up of 3 sub-systems
Roller sub-assembly - Two types of rollers
1. Transport rollers - 1” diameter, more numerous, mounted in pairs
2. Master rollers - 3” diameter, normally found at the bottom of each tank when the film must bend to turn
back upwards
Replenisher
Low volume practice – flood replenisher
5
edited by Marie Paas
Rad Physics Tri 2
Test 2 material
July 1999
DISCLAIMER: Please be advised that the information contained within this text is extra study material and should NOT be
deemed as completely correct without mistakes. It is the responsibility of the user to verify the correctness of ALL information
contained within.
High volume – automatic or continuous replenishment
The pH, temp, how long the film is in there is what makes the developer work.
Oxidation makes the developer old, after a certain period of time, the developer becomes exhausted or
used up.
This is governed by:
Rate of oxidation
Number of film processed
Accumulation of by-products (Bromides)
Know the properties of film
Sensitive to light, etc.
Silver reclamation, 2 types
1. Metallic replacement:
Uses steel wool or iron impregnated from inside a plastic container
The silver ions replace the iron and settle in the container in the form of silver sludge
Efficiency is about 80% - 85%, designed for low volume operation
2. Electrolytic
Uses electric current, yields high purity of silver
Brings in more money than metallic replacement, but cost is between $150 and $2000
6
edited by Marie Paas
Rad Physics Tri 2
Test 2 material
July 1999