Slide 1. Xray circuit components.
1. Autotransformer: Operates on the principle of self induction. Found on the low
voltage side of the step up transformer. Alters the line voltage to prepare for transforming
it to thousands of volts
2. Major and minor kVp selector: Taps on the autotransformer select major kV
changes in increments of ten, or minor increments of one kV.
3. Line (voltage) compensator: As line voltage varies during the course of a day due to
changing demands on power, the line voltage compensator makes adjustments
accordingly
4. Timing circuit: Prior to being stepped up, the exposure is terminated by one of the
various timing devices: mechanical (archaic), electronic (common), mAs timer (falling
load), automatic exposure control (AEC which requires a backup time of no more than
150% of the anticipated time), and synchronous timers which have time stations in
increments of 60 Hz, such as 120/60, 60/60, 1/30, 1/20, 1/15, etc.
5. mA selector: Alters the incoming line voltage to prepare it for the step down
transformer
6. Rectifier bridge: Routes the stepped up voltage so AC current becomes pulsed DC
7. Step up transformer: Operates on mutual induction. Is the component that separates
the low and high voltage circuits. Steps up the voltage (and consequently steps down the
amperage) in accord to the turns ratio. Provides the high potential difference across the
cathode and anode.
8. Step down transformer: Operates on mutual induction. Is the major component of
the filament circuit. Steps down the voltage (and consequently steps up the amperage) in
accord to the turns ratio. Provides 3-5 amps which results in a maximum of 1.2 amps
(1200 mA) at the filament.
Slide 2
1. What type of rectifiers are illustrated in this diagram? Diodes, which are semiconducting, solid state materials that replaced the valve tube rectifiers of yesteryear.
2. What materials are they made of and what classification of a conducting material
are they? Silicon or gadolinium are common materials for semi-conductors.
3. Why are they necessary in an x-ray circuit? To “rectify” the AC into pulsed DC
4. What is a p-n junction? Positive/Negative. The interface of materials in a diode
where the characteristics of the materials change from providing electrons to having a
deficit of electrons, known as holes. Current flows in one direction only, enabling the
creation of the path that routes AC into DC. (In computers semi-conductor p-n junctions
perform a similar function, but are used as switching devices: on or off. Each of these
junctions is known as a bit, eight of which form a byte, a million of which form a
megabyte.)
5. If this component were not included in the circuit could the x-ray tube still
function. How? Yes. Because of the size of the anode and its ability to absorb electrons a
discharge from the anode to the cathode is unlikely. This is known as self rectification,
but is risky and was only used on early tubes.
6. What does the 1/60 s labeled on the sine waves refer to? Where does it come
from? What is this time in milliseconds? One since wave (one cycle of a generator)
occurs in 1/60 of a second. In the US there are 60 cycles per second, or 60 Hz. 1/60 is 17
ms (16.6).
Slide 3
1. At what degree are each of the red dots at? 90, 180, 270, 360 (or 0) and 90 again.
2. Where do these degrees come from, e.g., what created them? They represent the
relationship (angle) of the armature of the generator to the magnetic lines of force.
3. How does the three phase diagram differ from the 3 phase 6 pulse? The six pulse
diagram has been full wave rectified.
1. How many pulses per second are created by each of these sign waves? 60, 120,
360, & 720. High frequency is considered a constant potential (or close to it) though the
frequency reaches as high as 25,000 Hz (6000 ave).
2. What is the effect of generator efficiency on heat units (HU)? More efficient
generation of electricity creates more heat, and requires a correction factor to be included
in the HU formulation of kV x mA x S. The correction factor for 3 phase is 1.35. For high
frequency it is 1.41
3. What is the effect on the quality and quantity of the beam? Both are increased as
is evidenced by the emission spectra for generators.
4. The percentages listed are percentages of what? Ripple, which is the usable portion
of the waveform.
Slide 4
1. What is represented by A? Amplitude
2. What is represented by B? A wavelength
3. If B were repeated over an interval of time what would that be called? A period
4. What 4th component of a sine wave has not been represented? What is that
component a function of? Frequency, which is inverse to wavelength and is a function
of energy.
5. Which of these 4 components of the sine wave represents the energy of photons?
Frequency. Which represents the kV? Amplitude
1. What does this diagram illustrate? (what electrical component produces this
waveform) A DC capacitor discharge
2. What does this electrical component do, e.g., what is its function. Capacitors store
electrical energy as a static charge, enabling a large discharge when a path for current is
provided (the exposure switch is depressed). Pulsating AC is changed to continuous DC
in the process.
3. What kinds of equipment is it incorporated in? Primarily portable units
4. One type of equipment employs batteries. What kind are they? Also portable X-ray
machines. NiCd.
Slide 5
1. What does this diagram illustrate? How an AC waveform is changed into a high
frequency signal.
2. What equipment does this technology replace? All other generators (full wave,
three phase 6 and 12 pulse)
3. What are its principle advantages? Hardware is much smaller. Wave form is more
efficient.
4. Step 2 is similar to step 7, so why is step 3 necessary? Step 2 is low voltage. Step
seven is high voltage. Step 3 starts with a continuous DC waveform and transforms it
(chops it up in pieces) into a digital, high frequency signal
5. What happens between steps 3 and 4. What is the change measured in? The low
voltage digital waveform is transformed to high voltage, measured in kilovolts.
6. What happens to the current between steps 5 and 6? The high voltage digital
waveform is once again changed to continuous DC by capacitance smoothing.
Slide 6
1. What is the component labeled A? The rotor, as evidenced by its position inside the
glass envelope
2. What component not labeled goes with A? The stator, which is outside the glass
envelope
3. What is special about the glass enclosure at B? The window, which is thinner than
the surrounding glass to minimize interactions with the photons
4. What is the component C? What is the typical range of sizes, length and width
that C comes in? The cathode (negative electrode), which in the X-ray tube is a filament.
They are 7-15 mm long, 1-2 mm wide, and .1-.2 mm thick.
5. How does the focusing cup work? What influence does it have on C Surrounding
the filament, the focusing cup has an applied negative charge to force the electrons of the
space charge to a focused beam in order to prevent blooming of the electrons, which, if
not controlled by the focusing cup would enlarge the actual focal spot size.
6. How much electricity flows through C. What is it measured in? 3-5 amps
7. What names are given to D? Focus, focal spot, focal track, target, focal point.
Slide 7
1. What is the name of the principle this picture illustrates? The line focus principle
2. What two factors do the target angle influence? 1. Sharpness of detail. 2. Anode
cutoff. Heat loading is also correct due to the effect the angle has on the actual focus
size.
3. In regard to sharpness of radiographic detail is a larger or smaller angle better?
Smaller.
4. In regard to sharpness of radiographic detail is the focus of the electron beam,
the actual focal spot size, or the effective focal spot size most significant? The
effective focal spot size because it is the result of the other two.
5. What is the picture to the left a representation of? The focal spot
6. How was it made? With a pin hole camera and photographic enlarger.
7. Is it a perfect representation? No. There is an acceptable margin of error resultant of
computing the focal spot size, but good enough for acceptance testing.
8. What is the name of the specialized target to the left. A biangular focal track. When
the shallow angle is used with a small filament it produces the smallest focal spots, such a
the fractional focuses of less than 1mm.
Slide 8
1. What is the name of what this picture illustrates? The anode heel effect
2. Is it a beneficial or detrimental? Detrimental. A homogeneous distribution of the
quantity of photons in the beam is preferable.
2. How does it relate to the line focus principle? A shallow angle of the anode
provides a smaller effective focus and thus better detail, but, results in higher absorption
of photons on the heel end of the anode, thus causing the unequal distribution of photons.
3. What is its significance in regard to how the patient is positioned on the table,
e.g., how can it be used to advantage? When the patient is positioned in the
conventional manner, with the head to the technologist’s left, the patient’s head is under
the anode with the cathode closer to the foot end of the table. In this orientation a part
with tapering thickness should be positioned with the thinner end under the anode. The
anode heel effect is only significant when a 17” field length is used.
Slide 9
1. What part of the x-ray tube could be damaged, and what damage could be done
by not heeding the information on these four tube rating charts? Pitting, cracking, or
melting of the anode.
2. Why might there be four with a single X-ray tube? A separate chart is needed for a
large and small focal spot, as well as the low and high speed rotor, for these are variables
that effect heat loading.
3. Name a technique on each chart that will be permissible by a small margin. Find
where kVp intersects a time station. The mA lines to the left of the intersection are
permissible techniques that do not overload the tube. mA lines to the right engage the
overload protection circuit and disable exposure.
4. What is the significance of the chart to the right. Be able to compute cooling
times. The anode cooling curve demonstrates the time after an exposure (or series of
exposures) necessary to cool the anode to enable another exposure. First calculate the
heat on the anode by the using the HU formula and generator correction factor (times the
number of exposures) and find that HU designation on the vertical axis of the graph.
Follow the curve down to the point where enough cooling has occurred that another
exposure (or series of exposures) can be made. Subtract the time of the first exposure
from the time of the second. With modern equipment heat overloading from standard
radiographic examinations is no longer a problem. It is only with arteriography and CT
equipment that overloading is still signficant.
Slide 10
1. What is this interaction: Anode heat
2. What part of the atom does it involve: Outer shell electrons only
3. What happens to the projectile electrons: After transfer of some kenetic energy,
which causes the outer shell electrons to vibrate and give off heat, the projectile electrons
continue on to have many more interactions before all their energy is absorbed. Then they
follow the circuit path to ground.
4. Are the target atoms ionized: No
Slide 11
1. What is this interaction: Characteristic
2. What part of the atom does it involve: Ionization can occur in any shell, so it can
involve K through O shell transitions of electrons. However, because only those which
transition into the K shell produce photons in the diagnostic range, we typically speak of
characteristic interactions in terms of K shell ionization events.
3. What happens to the projectile electrons: If they do not loose all of their energy in
the interaction they continue on to have many more interactions. Then they follow the
circuit path to ground.
4. Are the target atoms ionized? Yes
5. How efficient is the production of x-rays, how much waste? 1% X-ray, 99% heat.
6. Where does the energy of the x-rays come from? The kinetic energy of the
projectile electrons that struck the anode
7 what are K-rays, L-rays etc. When a transition is from any shell to the K shell it
produces a K X-ray. When a transition is from any shell to the L shell it produces a L Xray.
8. What is the energy of a K characteristic x-ray that came from the L shell, M
shell. 57 keV and 66 keV respectively.
9. What is the energy of a L characteristic x-ray that came from the M shell, N
shell. 9 keV and 11 keV respectively
10. What does this graph represent? Although we are only concerned with K X-rays
in radiology, this graph shows all the energy levels of K, L, M, and N rays that are
possible.
Slide 12
1. What is this interaction: what does the name mean? Bremssthrahlung or Brems,
which in German means braking radiation.
2. What part of the atom does it involve. The nucleus
3. What happens to the projectile electrons: If they do not loose all of their energy in
the interaction they continue on to have many more interactions.
4. Were the target atoms ionized? No
5. What is the maximum energy x-rays from this interaction can have? The keV
value that corresponds to the kVp value set on the operator’s console
6. Where does the energy of the x-rays come from? Kinetic energy the projectile
electrons that strike the target.
7. The x-ray beam comes exclusively from this interaction up to what energy? 69.5
keV, which is the binding energy of K shell tungsten atoms. (We know the range of K Xrays is actually 57 to 69.5 keV, but let’s not loose sleep over it).
8. At 100 kVp how much of the x-ray beam comes from this interaction? 80 to 90%
9. What happens to the projectile electron? If they do not loose all of their energy in
the interaction they continue on to have many more interactions. Then they follow the
circuit path to ground.
10. What is represented by “A” The Brems characteristic curve
11. What is represented by “B” The K shell edge, where the production of
characteristic radiation kicks in and creates a spike in the quantity of photons of that
value.
12. For a tungsten target what is the energy at “B” 69.5 keV
13. If it were not tungsten would it be the same? No, as is demonstrated on the
emission spectrum of various target materials.
Slide 13
1. What are these graphs of? e.g. what are they called collectively? They are called
emission spectra (or spectrum singularly) and they demonstrate how a change of factors
effect the quantity and quality of the X-ray beam
2. What do each of these graphs say about x-ray quantity? To answer this question
simply compare the amplitude of the curves of the two factors
3. What do each of these graphs say about x-ray quality? To answer this compare the
peak energy of the curves of the two factors, and the average energy of the beams as
demonstrated by a right shift in the peak of one of the curves.
4. What do each of these graphs say about the distribution of x-ray interactions
varied by the factors being graphed? See the “Emission Spectra” supplement to
chapter 7.
More questions? Email gary.zimmerman@oit.edu Answers will be returned by noon on
Monday.