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.

FilamentThe 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 CupThe 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

ElectronsmA

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)

MilliamperageMilliamperage (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 x-ray 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 x-rays 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: