Chapter 7 Physics of radiography
It is essential that any practitioner operating within the realms of an imaging department and using ionising radiation has a sound knowledge base. In order to comprehend the various factors affecting the production of diagnostic images, there is a requirement to demonstrate an awareness of the fundamental definitions of classical physics and how these terms may be applied to radiography. An understanding of atomic structure, the electromagnetic spectrum, electricity, magnetism and the inverse square law are also essential principles that can be applied to radiography.
Radiography involves the safe use of ionising radiation and the production of quality images. The process by which images are produced involves the conversion of energy from one form to another and the use of various specialised materials, such as the X-ray tube. The law of conservation of energy states that energy cannot be created or destroyed but merely changes from one form to another. Understanding this fundamental law is central to the basic knowledge base of a practitioner. Within the scope of basic physics there are numerous SI units and their definitions, and the practitioner must be aware of these base units.
This is used to measure the size of an object or, in radiography, the distance between different aspects of the imaging system. The SI unit of length is the metre (m) and in radiography one of the most important length measurements is the distance between the X-ray tube focal spot and the imaging receptor. With the advent of digital imaging this distance has become known as the source–image distance (SID), but it is also still widely known as the focus–film distance (FFD). In terms of assessing image quality, the practitioner may wish to use a smaller unit of length, the millimetre (mm), for areas such as focal spot size calculations and image unsharpness. It is important to remember to convert all parameters into SI units before undertaking any calculations.
Matter is the fundamental material of which everything in the universe is made. Matter is composed of particles known as atoms and molecules. The SI unit of mass is the kilogram (kg). Different materials contain varying amounts of these particles and therefore vary in overall density, and this determines the characteristics of the overall exposure settings used by the practitioner. For example, lead is heavier than wood because it has more densely packed atoms.
This is the movement of electrons flowing per unit time within a conductive material, such as copper. The SI unit of electric current is the ampere (A) and in diagnostic radiography this determines the quantity of electrons produced by the filament and hence the overall number of X-ray photons in the beam. The term milliamp (mA) is used in diagnostic radiography to express the tube current and is one thousandth of an ampere (10−3 A). The amount of electric charge flowing through an X-ray tube during an exposure is the sum of tube current (mA) and duration of the exposure time in seconds (s). This is expressed as mA s and in order to obtain a short exposure time (e.g. 0.03 s for a chest radiograph) the electric current (mA) needs to be suitably higher.
Derived SI units result from a combination of the base units and some of them are used frequently in radiography. Practitioners are required to name them and define their values (Table 7.1). Some of these derived SI units are outlined below.
|Term and SI unit||Definition||Application to radiography|
|Energy (joule; J)||The ability to do work||Production of X-rays|
|Mass (kilogram; kg)||A measure of the number of atoms and molecules in a body||Important when determining the radiation dose to a patient|
|Gray (joules per kilogram; Gy)||The energy imparted to a body by ionising radiation||Unit of absorbed radiation dose measurement|
|Sievert (joules per kilogram, Sv)||The energy imparted to a body by ionising radiation multiplied by the quality factor||Unit of radiation dose equivalent, which takes biological factors into account|
|Power (joules per second)||The rate of doing work||Output of X-ray generator|
|Electric current (ampere; A)||The movement of electrons flowing per unit time||Quantity of electrons flowing per unit time|
|Electric charge (coulomb; C)||1 ampere flowing per second||Quantity of electrons flowing per second|
|Electrical potential (volt; V)||The force which moves electrons within a conductive material||Potential difference across an X-ray tube, acceleration of electrons and quality of X-ray beam|
|Frequency (hertz; Hz)||The number of cycles per second||Electromagnetic radiation|
This may be a difficult concept to understand but fundamentally energy is the ability to do work. This may be demonstrated in radiography as potentialenergy (PE), which is applied to the negative (cathode) and positive (anode) ends of an X-ray tube, subsequently causing the flow of electrons across the vacuum environment. This is kinetic energy (KE) and is subsequently converted into X-ray energy (photons) when the electrons interact with the anode material (tungsten atoms).
The potential difference between the cathode and anode of an X-ray tube is measured in kilovolts (kV). This determines the acceleration of electrons across the X-ray tube and hence the quality (penetrating power) of the X-ray beam.
This is the rate of doing work and is measured in joules per second (J s−1) or watts (W). This unit is referred to in terms of the power output of diagnostic imaging equipment generators. For example a typical general X-ray room will have 50 kW generator to supply electric power to the X-ray equipment. However, a mobile X-ray unit may only have a 35 kW generator because the requirement to produce higher exposure values is less.
The nucleus contains a number of particles called protons and neutrons, together termed nucleons. Each nucleon is nearly 2000 times the mass of an electron. This means the mass of an atom is concentrated in its nucleus, around which the much lighter electrons orbit. If a nucleus were scaled up to the centre spot of a football pitch, the electrons would start orbiting around the perimeter of the pitch in their various orbits stretching out for several miles. This analogy demonstrates why X-rays may pass through a body of material unattenuated, as the X-ray photons may simply pass ‘between’ the electron orbits and totally miss the nucleus of an atom.