Definition/Introduction

Radiology, as a specialty, is incredibly dependent on technology. As a result, radiologists and technologists need to understand the technology and the physical principles that create the advantages, limitations, and risks of the equipment used in practice. Understanding these principles provides an ability to improve image quality and select the most appropriate way to create a radiograph. A radiograph is the oldest and one of the most common modalities.[1] The general radiographic room contains a few basic components to produce X-ray beams: tube, tube housing, generator, beam filtration system, and collimator. 

Issues of Concern

The X-ray tube is the source of the X-ray beam. The basic components of the X-ray tube include a cathode, anode, rotor, envelope, tube port, cable sockets, and tube housing. The generator allows the radiology technologist to control three-technique factors: tube voltage applied across the X-ray tube, the tube current that flows through the X-ray tube, and the total exposure time during which the current flows. 

The cathode is the source of electrons. It is the negative electrode in an X-ray tube and, most commonly, made of a tungsten filament. The size of the filament used depends on the technique factors.  When energy is applied, the filament heats up; electrons build at the cathode, and, through a process called thermionic emission, these electrons are then released from the filament surface. The rate of electron release is determined by the current applied and the temperature of the filament. Electrons accelerate toward the positively charged anode.[2] The anode is where electrons decelerate, and the energy from deceleration is released in the form of heat and X-rays (photons).  The X-ray tube's output is emission-limited, and the filament current dictates the X-ray tube current. The tube current is proportional to the x-ray flux at any tube voltage applied. The emitted electrons are focused into a concentrated group accelerated toward the anode, striking a small area called the focal spot. The filament length and electron distribution determine the focal spot's size. 

The positively charged anode is the target of electrons released from the cathode. Most of the electrons that strike the anode deposit their kinetic energy, generated by the applied tube voltage and current, as heat. Only a small fraction go on to produce X-rays. As a result, a significant amount of heat is generated at the anode in the production of diagnostic images. Stationary anodes were used in the past. However, the small focal spot on a stationary anode limits the number of X-rays that can be produced without damaging the anode. Therefore, most X-ray machines today use a rotating anode. This allows for the spread of heat over a larger area, which allows for greater tube currents and exposure durations. The rotating anode is a disk mounted on a bearing supported rotor assembly. The rotor consists of a center iron cylinder with surrounding copper bars. The stator device is made of electromagnets that surround the rotor. When an alternating current passes through the electromagnets of the stator, it produces a rotating magnetic field. This field produces an electrical current in the rotor's copper bars, which, in turn, creates an opposing magnetic field to the one induced by the stator—the results in the rotation of the rotor device. Rotation speeds of up to 10,000 revolutions per minute can be produced.

The cathode, anode, rotor apparatus, and the other associated structures are collectively called the X-ray tube insert. They are all contained in a glass or metal enclosure and sealed under a high vacuum. This enclosure is known as the envelope. X-ray photons emitted from the focal spot scatter in all directions. The use of a tube port helps form a useful beam. 

The X-ray tube housing provides shielding and cooling of the X-ray tube insert. Typically, between the insert and the housing is a layer of oil that provides heat conduction and electrical insulation. A lead shield is also applied to the inside of the housing to attenuate X-rays that are not directed to the tube port. However, not all X-rays are blocked, and the fraction that penetrates the housing is known as leakage radiation. Each tube housing has a maximum tube potential that should not be exceeded during operation at the risk of an unacceptable amount of leakage radiation. 

Clinical Significance

As the X-rays come out of the tube port, the size and shape of the X-ray field can be adjusted by collimators. The collimator housing attaches to the tube port. A light source is placed at a virtual focal spot location and is reflected by a mirror angled at 45 degrees. Two pairs of parallel opposed lead shutters define the X-ray field and, when the collimator light is on, define the X-ray and light fields. 

As X-ray beams pass through materials, portions of the beam are attenuated, which results in a change in the shape of the produced spectrum. This is referred to as "filtration," and the change in spectra can be customized using different types and amounts of filter material. Inherent filtration occurs when X-ray attenuation is achieved by the anode material itself and the material placed over the exit window of the X-ray tube.[3] This reduces some of the low energy photons produced. Usually, additional filtration is used to reduce patient radiation exposure and improve image contrast. Common filter materials include aluminum and copper.[4] These materials will shift the average energy of the spectrum to higher values. By absorbing lower energy photons, it reduces radiation exposure by eliminating photons absorbed by the patient's soft tissue and not contributing to image production. Another common filter material is molybdenum.[5] Molybdenum removes a large portion of the high energy photons, which improves image contrast. Collimators are used in the X-ray tube housing to direct the beam to the area of focus. They are made of lead, which absorbs the photons and reduces radiation dose to the patient. These are different from X-ray filters in that collimators completely block photons instead of just blocking a part of the produced spectrum.[6]

The final piece of equipment used to produce X-rays is the tube generator. The X-ray generator provides an electrical current at a high voltage to the X-ray tube, resulting in the X-ray beam production.[1] A fundamental principle of electromagnetic induction is that a constantly applied current through a wire or coil produces a constant magnetic field. Variations in the current will produce variations in the magnetic field. Transformers use electromagnetic induction to change the voltage of an electrical power source. They will increase (step-up), decrease (step-down), or leave unchanged (isolate) the input voltage depending on the voltage required by the X-ray generator.

There are several X-ray generator designs. The most contemporary is the high-frequency inverter generator. It functions by converting the low-frequency, low-voltage input power to a high voltage output waveform with an inverter circuit. This allows for the application of a nearly constant voltage to the X-ray tube for efficient X-ray production. 

Factors that affect X-ray emission include the anode material, the X-ray tube voltage, X-ray tube current, beam filtration, and generator waveform. The anode target material affects the efficiency of X-ray radiation production. The output is roughly proportional to the atomic number of the anode material. Besides, the energies of the X-rays produced also depend on the target material. The tube voltage (kV) dictates the maximum energy of the produced photons. An increase in the voltage results in increased X-ray production efficiency, quantity, and quality of the resulting X-ray beam. X-ray beam intensity (or the number of photons in the beam) is directly proportional to the tube current and exposure time.[7] The only parameter to change the shape of the X-ray spectrum is the voltage.[2][8] Beam filtration will change the quantity and quality of the X-ray beam by preferentially filtering low-energy photons. For more filtered beams, the current required to achieve the desired X-ray intensity will be higher. Finally, the generator waveform affects the quality of the produced X-ray spectrum. For the same voltage, current, and exposure time, the output X-ray spectrum is of higher quality and quantity. 

Nursing, Allied Health, and Interprofessional Team Interventions

Understanding image production equipment and techniques are essential to understand the advantages, limitations, and quality control. A risk-benefit analysis must be performed before the use of imaging tests that utilize ionizing radiation. The risk of developing cancer from ionizing radiation depends on age, sex, target organ, and cumulative radiation dose.[9] 

These factors deserve consideration by an interprofessional team to ensure patient safety and improve outcomes. Imaging procedures should be performed when necessary and if benefits outweigh the risks. The ALARA principle, "as low as reasonably achievable," is used to emphasize safety and the importance of using the lowest radiation dose possible in imaging. Healthcare professionals, who are most at risk of radiation exposure, should also be trained in radiation safety to avoid excessive exposure.[10]