X-Ray Production
A. Making the exposure
1. The x-ray machine is turned on, and a small amount
of current is sent to the filament to warm it and ready
it for much higher current
2. Radiographer takes equipment through warm-up
exposures to warm the filament further and warm the
anode
3. Radiographer chooses exposure factors on control
console for the exam to be performed with the assistance
of technique charts
a. Measurement
1. Part thickness should always be measured using
calipers
2. Caliper measurement is used to consult the
technique chart
b. Types of technique charts
1. Fixed kVp–variable mAs
a. Assumes optimal kVp for the part being
radiographed
b. Except for exceptionally large patients, kVp
never changes for a given projection
c. mAs is varied according to the part thickness
as measured with the calipers
d. Based on the assumption that thicker parts
absorb more rays and more rays must be
placed in the primary beam
2. Variable kVp–fixed mAs
a. kVp is varied according to part thickness as
measured with the calipers
b. Based on the assumption that thicker parts
require a beam with shorter wavelength rays
that are more penetrating
3. Variable technique (vary both mAs and kVp)
a. Provides for alteration of routine techniques
because of pathologic conditions, patient
age, ability to cooperate, body mass index,
casts, or contrast media
4. Electricity coming into the radiology department is
adjusted by the line voltage compensator in the x-ray
equipment to maintain it at a constant level
5. When making the exposure, the radiographer presses
the rotor switch and exposure switch in one continuous
motion
6. The induction motor begins spinning the anode; the
filament gets hotter
7. When the exposure switch is closed, the voltage
selected by the mA control flows from the autotransformer,
through the variable resistors, and into the
step-down transformer in the filament circuit
8. The filament heats considerably, boils off electrons
(thermionic emission), and creates a space charge or
electron cloud around the filament
9. At the same time, the alternating current and voltage
the radiographer selects by choosing taps off the
autotransformer are sent to the primary coils of the
high-voltage step-up transformer, where they are
boosted to kilovoltage levels
10. After leaving the secondary coils of the step-up transformer,
the voltage and alternating current are sent
through the rectifier, which changes the alternating
current to pulsating direct current
11. The kilovoltage creates a high potential difference
in the x-ray circuit, making the anode less negative
(relatively positive) and the cathode highly
negative
12. This high potential difference causes the electrons
to move at very high speed (approximately half the
speed of light) from the cathode to the anode
13. The collision of these projectile electrons with the
atoms of the target material causes a conversion of
the kinetic energy of the electrons (100%) to heat
(99.8%) and x-rays (0.2%)
14. Heat is produced when the projectile electrons strike
the outer shell electrons of the target material and
place them in an excited state, which causes them to
emit infrared radiation
15. The production of x-rays comes from two interactions
with the anode
B. Bremsstrahlung (brems) radiation
1. A projectile electron misses outer shell electrons in
the target and moves in close to the nucleus
2. Because the nucleus is positive and the electron is
negative, it is slowed or braked
3. The reduction in kinetic energy causes the electron to
slow and release energy as an x-ray photon
4. The resultant x-rays are called bremsstrahlung (braking)
x-rays because they are produced by the slowing
(braking) of projectile electrons
5. At diagnostic levels, most x-rays produced are from
brems interaction
C. Characteristic radiation
1. A projectile electron collides with an inner shell
electron of a target atom
2. It removes the inner shell electron from orbit and
ionizes the atom
3. A hole exists in the inner shell from the vacated electron
4. An electron from an outer shell falls in to fill the
hole
5. As the electron falls in, energy is given off in the
form of an x-ray photon
6. This creates a hole in its shell of origin, and an
electron from the next outer shell falls in to fill
this vacancy; this continues until the atom is stable
again
7. Each time an electron falls in to fill a hole, an x-ray
photon is given off
8. Each x-ray photon has a specific energy, equal to the
difference in the binding energies of the two shells
involved
9. Only x-rays produced at the K-shell are of sufficient
energy to be used in diagnostic radiography
10. Because the x-rays possess energy characteristic of
the specific binding energies of the atom involved,
they are called characteristic x-rays
11. Characteristic x-rays are produced at kVp levels
greater than 70 but only in small numbers
D. X-ray properties
1. Part of the electromagnetic radiation spectrum
2. Highly penetrating
3. Invisible
4. Travel at the speed of light (186,000 miles per
second)
5. Travel in straight lines as waves
a. Wavelength of diagnostic x-rays: 0.1 to 0.5 angstroms
(Å) (1 Å × 10−10 m, which is one 10-
billionth of a meter)
b. Wavelength is the distance from crest to crest or
trough to trough, or the distance covered by one
complete sine wave
c. Frequency: The number of waves passing a given
point per unit time
d. Wavelength and frequency are inversely proportional
to one another; as wavelength increases,
frequency decreases, and as wavelength decreases,
frequency increases
e. Short-wavelength rays are more penetrating;
long-wavelength rays are less penetrating
6. Invisible to the human eye
7. Have characteristics of waves and particles; travel in
bundles or packets of energy called photons
8. Exist in a wide range of wavelengths and energies
9. Can ionize matter and gases
10. Cause fluorescence of phosphors
11. Unable to be focused by a lens
12. Liberate a small amount of heat when passing
through matter
13. Electrically neutral
14. Affect photographic film
15. Cause biological and chemical changes through
excitation and ionization
16. Scatter and produce secondary radiation
E. X-ray beam characteristics
1. Because x-rays are produced by brems and characteristic
interactions at the anode, the resultant x-ray
beam contains many different energies
2. An x-ray beam containing many different energies is
called heterogeneous
3. The collection of all different energies (wavelengths)
of x-rays is called the x-ray emission spectrum
4. Discrete x-ray spectrum: Produced by characteristic
x-rays because energies involved are specific to the
target atom and are predictable
5. Continuous x-ray spectrum: Produced by brems radiation
because these energies all are different (from the
peak electron energy down to zero energy)
6. The maximum energy an x-ray photon can have corresponds
to the kVp that was used
7. Beam characteristics may be altered by using filtration
a. A filter is usually a sheet of aluminum placed in
the primary beam just as it exits the x-ray tube and
before it reaches the collimator
b. The oil and glass envelope of the x-ray tube offer
inherent filtration
c. Total beam filtration equals inherent filtration
plus added filtration
d. Total filtration must be at least 2.5-mm aluminum
equivalent
e. Filtration removes the low-energy
f. The result of removing soft rays from the beam is a
lower patient skin dose
g. Other types of filters that directly affect the radiographic
image may be used, known as compensating
filters (e.g., wedge, boomerang)
h. Half-value layer: Amount of filtration that reduces
the beam intensity by half
F. Heat units and their management
1. Heat units are a calculation of the total heat produced
during an x-ray exposure
2. Heat units are calculated using the following equations:
a. Single-phase, full-wave rectified equipment: kVp
× mAs
b. Three-phase, 6-pulse, full-wave rectified equipment:
kVp × mAs × 1.35 (Remember: This equipment
produces x-ray photons with 35% higher
average photon energy)
c. Three-phase, 12-pulse, full-wave rectified equipment:
kVp × mAs × 1.41 (Remember: This equipment
produces x-ray photons with 41% higher
average photon energy)
3. X-ray tubes and tube housing are constructed to
absorb certain levels of heat units
4. Most modern x-ray equipment automatically prevents
the user from making an exposure capable of
producing too much heat in the tube; this is indicated
on some control panels by the warning “technique
overload,” a red light, or a failure to get a green
light indicating that the anode is ready
5. Most modern x-ray equipment prevents overloading
of the tube during a series of exposures and does not
allow additional exposures until the tube has cooled
sufficiently
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