PART 2.2 - IMAGE BRIGHTNESS / CONTRAST
How are X-rays made?
Within paleoradiography it is important to know how X-rays are created as the operator will need to select the correct exposure factors according to the specimen under investigation. It is highly unlikely that you will ever need to perform maintenance on an X-ray tube, as this is dangerous and best left to a trained engineer. However, an awareness of the individual parts of the X-ray tube would be useful as improper use may cause damage.
Within radiography the basic x-ray exposure variables include:
Tube voltage (kV) - Measured in kilovoltage (thousand-volts)
Tube current (mAs) - Measured in milliampere-seconds (a thousandth of an amp per second)
Whether you are using a dental X-ray unit, computed tomography scanner or a clinical X-ray room all X-rays are produced using an X-ray tube similar to the one shown below:
Parts and purpose of the x-ray tube:
The X-ray tube uses incredible amounts of energy to produce X-rays, however the efficiency is very low and 99% of the energy is lost as heat. Modern X-ray tubes have a variety of ways to overcome this according to the levels of radiation required. As such, the X-ray tube used in dentist surgeries are different to those requiring higher and longer levels of X-ray exposure in computed tomography. Irrespective of this, the component parts have the same overall layout and purpose.
X-ray tube and housing assembly - X-ray tubes are either constructed from glass (older versions) or metal (newer versions) and contain a vacuum. The lack of atoms or molecules within the tube removes unwanted interactions. X-ray tubes are surrounded by protective housing such as lead to prevent the passage of unwanted X-ray radiation. Modern systems have sophisticated integration between these two component parts.
Cathode - This is the negatively charged portion of the X-ray tube which contains a filament (coil of wire). When an electrical current is passed through at high levels the electrons are 'boiled off' due to a phenomenon known as thermionic emission. The electron beam is attracted to the positively charged anode (see image above). One of the key variables within radiography is the tube current (mA) and is controlled by adjusting the filament current.
Anode - The anode is positively charged because it conducts electrons away, losing negative charge and thereby completing the electrical circuit. Anode designs can be both stationary (i.e. not moving) or rotating to increase thermal load, which allows greater X-ray production. An example of a rotating anode is seen in the image below. Both the rotating and stationary anode contain a target, where interactions cause X-ray emission.
Target - The target is where electrons hit the anode and X-rays are produced. It is typically made of tungsten due to a high atomic number, high melting point and efficient thermal conduction.
The creation of X-rays
The short explanation:
Electrons shoot across from the cathode to the anode and the energy transference creates X-rays.
Optional - Check out this YouTube explanation which also walks you through the component parts.
The long explanation:
It takes a lot of energy to free the electrons from the filament, but the end result is that the electrons have kinetic energy (the energy of motion) and are attracted at nearly half the speed of light to the anode. The electrons which achieve this state are called projectile electrons. The kilovoltage applied to the X-ray tube is proportional to the kinetic energy. Purists will say that we should use kVp - kilovoltage peak - as the voltage applied is not one discreet value but a range. For simplicity, we will refer to kV.
The projectile electrons interact with the heavy metal atoms of the Target and produce X-rays. Projectile electrons may interact with the electrons of the tungsten atoms or the atomic nucleus themselves. As stated previously, the emission of X-rays are low and the majority of kinetic energy is converted to thermal energy (heat) and electromagnetic radiation (infrared radiation). The small amount of X-rays that are produced can be termed as Characteristic Radiation and Bremsstrahlung Radiation.
Optional - View this YouTube video for an explanation of Characteristic and Bremsstrahlung radiation.
What happens when tube voltage (kV) is increased?
Increasing the tube voltage increases the kinetic energy of projectile electrons. The energy transference causes X-rays of similar energy, allowing them to penetrate through more matter and travel a greater distance. Incidentally, X-ray production is more efficient at higher energies, so there will be an increase in X-ray photons too.
What happens when tube current (mA) is increased?
Doubling the tube current doubles the number of electrons from the cathode, hence it also doubles the quantity of X-rays produced at the anode. Tube current is frequently presented as mAs, which demonstrates the current over a period of time (in seconds).
Increasing the duration (time) of tube current does not increase the quantity of X-rays. Within the clinical environment we may want to have very short X-ray exposure due to tricky patients (imagine an unruly child...) or to reduce motion of organs on the resultant radiograph. Extending the exposure time (i.e. a greater mAs) could be beneficial to the X-ray tube, as heating occurs over a longer time rather than in an intense moment.
Some of the paleoradiological research conducted by Ronald Beckett and Jerry Conlogue within their Mummy Roadshow was performed using a 1960's Picker X-ray generator. The X-ray unit had several limitations, one being a low tube current output (mAs). To overcome this they used a series of exposures over a long time, allowing the machine to cool down between. In doing so, the imaging was improved without damaging the X-ray tube.
Why is this important to paleoradiography?
The quality of an X-ray beam is important. An X-ray beam that contains multiple energy levels can be compared with a piano that played every tune at once no matter which key you press.
X-ray tubes are designed with filters to remove low energy (unwanted) X-rays.
We have the ability to control the quantity (mA) and energy/strength (kV) of X-rays being emitted from the X-ray tube and rely upon a high quality X-ray beam.
Having stronger X-rays (high kV) or more X-rays (high mA) directly influences the outcome of a radiograph.
