Fundamentals of X-ray Production
By
Professor Jarek Stelmark
X-rays were not developed; they were discovered, and quite by accident.
During the 1870s and 1880s, many university physics laboratories were
investigating the conduction of cathode rays, or electrons, through a large,
partially evacuated glass tube known as a Crookes tube.
On November 8, 1895, Roentgen was working in his physics laboratory at
Würzburg University in Germany. In late 1895, a Roentgen was working with a
cathode ray tube in his laboratory. He was working with tubes similar to our
fluorescent light bulbs. He evacuated the tube of all air, filled it with a special gas,
and passed a high electric voltage through it. When he did this, the tube would
produce a fluorescent glow.
Roentgen shielded the tube with heavy black paper, and found that a green
colored fluorescent light could be seen coming from a screen setting a few feet
away from the tube. He realized that he had produced a previously unknown
"invisible light," or ray, that was being emitted from the tube; a ray that was
capable of passing through the heavy paper covering the tube.
The discovery of x-rays is characterized by many amazing features, and this
causes it to rank high among the events in human history. First, the discovery
was accidental. Second, probably no fewer than a dozen contemporaries of
Roentgen had previously observed x-radiation, but none of these other
physicists had recognized its significance or investigated it. Third, Roentgen
followed his discovery with such scientific vigor that within little more than a
month, he had described x-radiation with nearly all the properties we recognize
today.
Crookes tubes generated the electrons needed to create x-rays by
ionization of the residual air in the tube, instead of a heated filament, so
they were partially but not completely evacuated.
The Crookes tube was improved by William Coolidge in 1913. The
Coolidge tube, also called hot cathode tube, is the most widely used.
It works with a very good quality vacuum. In the Coolidge tube, the
electrons are produced by thermionic effect from a tungsten filament
heated by an electric current. The filament is the cathode of the tube.
The high voltage potential is between the cathode and the anode, the
electrons are thus accelerated, and then hit the anode.
An x-ray tube is an electronic vacuum tube with components contained within
a glass or metal enclosure. The x-ray tube, however, is a special type of
vacuum tube that contains two electrodes: the cathode and the anode. It is
relatively large, perhaps 30 to 50 cm long and 20 cm in diameter. The glass
enclosure is made of Pyrex glass to enable it to withstand the tremendous heat
generated.
The cathode is the negative side of the x-ray tube; it has two primary parts: a
filament and a focusing cup.
Filament
The filament is a coil of wire similar to that in a kitchen toaster, except much
smaller. The filament is approximately 2 mm in diameter and 1 or 2 cm long. In
the kitchen toaster, an electric current is conducted through the coil, causing it
to glow and emit a large quantity of heat.
An x-ray tube filament emits electrons when it is heated. When the current
through the filament is sufficiently high, the outer-shell electrons of the filament
atoms are “boiled off” and ejected from the filament. This phenomenon is
known as thermionic emission.
Focusing Cup
The filament is embedded in a metal cup called the focusing cup (Because
all the electrons accelerated from cathode to anode are electrically negative,
the electron beam tends to spread out owing to electrostatic repulsion. Some
electrons can even miss the anode completely.
The focusing cup is negatively charged so that it electrostatically confines the
electron beam to a small area of the anode. The focusing cup forms them into a
cloud. This cloud is called a space charge The effectiveness of the focusing
cup is determined by its size and shape, its charge, the filament size and
shape, and the position of the filament in the focusing cup.
Most x-ray tubes have two filaments mounted in the cathode assemble “side by
side,” creating large and small focal spot size
Selection of one or the other focal spot is usually made with the mA station
selector on the operating console. Normally, either filament can be used with
the lower mA station—approximately 300 mA or less. At approximately 400 mA
and up, only the larger focal spot is allowed because the heat capacity of the
anode could be exceeded if the small focal spot were used.
The anode is the positive side of the x-ray tube; it conducts electricity and
radiates heat and contains the target.
The anode serves three functions in an x-ray tube.
• The anode is an electrical conductor. It receives electrons emitted by the
cathode
• The anode also provides mechanical support for the target.
• The anode also must be a good thermal dissipator. When the projectile
electrons from the cathode interact with the anode, more than 99% of their
kinetic energy is converted into heat. This heat must be dissipated quickly
Target
The target is the area of the anode struck by the electrons from the cathode. In
stationary anode tubes, the target consists of a tungsten alloy embedded in the
copper anode. In rotating anode tubes, the entire rotating disc is the target
In stationary anode tubes, the target consists of a tungsten alloy embedded in
the copper anode
The rotating anode x-ray tube allows the electron beam to interact with a
much larger target area; therefore, the heating of the anode is not confined
to one small spot, as in a stationary anode tube. Rotational speeds of the
anodes ranges from 3,000 -4,000 rpms for the general purpose x-ray tubes
and 10,000 to 12, 000 rpms for the special applications x-ray tubes (ex. CT.)
The rotating anode is powered by an electromagnetic induction motor.
An induction motor consists of two principal parts separated from each other
by the glass or metal enclosure. The part outside the enclosure, called
the stator, consists of a series of electromagnets equally spaced around the
neck of the tube. Inside the enclosure is a shaft made of bars of copper and
soft iron fabricated into one mass. This part is called the rotor.
The three things needed to produce x-rays are now present:
• The large potential difference to give kinetic energy to the filament
electrons (provided by the kVp setting)
• A quantity of electrons provided by mAs
• A place for interaction (the target of the anode).
The electron cloud is attracted to the anode target because of the huge potential
difference. In fact, these filament electrons will reach speeds of about half the
speed of light in the short 1 to 3 cm between the focusing cup and anode target
when 70 -80 kVp is utilized.
As the electron cloud flows from cathode to anode, it is a continuation of the flow of
electricity through the x-ray circuit.
As they penetrate the target surface these filament electrons interact with the
atoms of the target (Tungsten). These electrons are suddenly decelerated. Their
loss of kinetic energy resultys in the production of mostly heat and small amount
of x-rays.
Potential difference kVp
Electrons
mA
X-ray photons
The primary exposure technique factors the radiographer selects on the
control panel are milliamperage, time of exposure, and kilovoltage peak
(kVp). Depending on the type of control panel, milliamperage and
exposure time may be selected separately or combined as one factor,
milliamperage/second (mAs)
Milliamperage
Milliamperage (mA) is a measure of the rate of current flow across the x-ray
tube, that is, the number of electrons flowing from filament to target each second.
The number of available electrons is determined by the filament heat.
When filament heat is increased, more electrons are available each second to
cross the tube. Thus, increasing the filament heat increases the mA in the xray tube circuit. When more electrons strike the target, more x-rays are
produced, so mA controls the volume of x-ray production.
High mA settings produce more x-rays per second, and low mA settings
produce x-rays at a slower rate. Stated differently, mA controls the intensity of
the x-ray beam, determining the number of photons that will strike the film
during 1 second. If the mA is doubled, the x-rays are doubled, and if the mA is
halved, the x-rays are reduced by 50%.
Exposure time refers to the length of time that the x-rays are turned on. It is
the duration of the x-ray exposure. Exposure time is measured in units of
seconds (sec). Most x-ray exposure times are less than 1 second, and
therefore milliseconds (msec) are used: 1 millisecond equals 0.001 second.
A timer in the x-ray circuit terminates the exposure after a preset length of
time. The quantity of x-rays produced is directly proportional to the exposure
time. For example, it is apparent that if the exposure time is doubled, twice
as many x-rays will be produced.
When the kVp is increased at the control panel, a larger potential difference
occurs in the x-ray tube, giving more electrons the kinetic energy to produce xrays and increasing the kinetic energy overall. The result is more photons
(quantity) and higher energy photons (quality).
In radiography it is often useful to know the total quantity of an exposure. The
quantity of x-ray photons in an exposure cannot be determined by either the mA
or the exposure time alone. Although mA determines the rate of x-ray
production, it does not indicate the total quantity, because it does not indicate
how long the exposure lasts. Exposure time does not indicate the total quantity
either, because it does not measure the rate of x-ray production. To determine
the quantity of radiation involved in an exposure, both mA and time must be
considered.
The unit used to indicate the quantity of exposure is milliampere-seconds,
abbreviated mAs. This unit is the product of mA and exposure time: