Rotating-Anode X-Ray Tube

A rotating-anode X-ray tube produces X-rays by directing fast-moving electrons at a metal target. The tube is evacuated so that the electrons can travel from the cathode to the anode without frequent collisions with gas molecules.

Heating the cathode releases electrons from its surface by thermionic emission. A high potential difference accelerates the electrons towards the anode, and they are focused onto a small region of its metal target. When the electrons interact with the target, some of their energy is emitted as X-ray photons.

Most of the electron energy becomes thermal energy in the target rather than X-rays. The anode rotates so that the electron beam strikes different parts of a circular track over time. This spreads the heating over a larger area, allowing a high tube loading while the region struck at any instant remains small. The tube can therefore retain a small effective focal spot: the apparent size of the X-ray-producing region as viewed from the direction of the beam.

Controls on X-ray output and energy

The tube current controls how many electrons reach the target each second. At fixed operating conditions, a higher tube current produces more X-ray photons per second.

The potential difference controls the maximum energy gained by each electron as it accelerates towards the target. The maximum possible photon energy is approximately equal to this energy:

Here $$e$$ is the magnitude of the electron charge and $$V$$ is the potential difference across the tube. Not every photon has this maximum energy: the tube produces a distribution of photon energies. Increasing the potential difference raises the maximum photon energy and also changes the X-ray output and its energy distribution.

Exposure time controls how long photons are produced. At a fixed output, a longer exposure produces a greater total number of photons.

Filtration removes photons from the beam, preferentially absorbing those with lower energies. The transmitted beam therefore has a different energy distribution, with a lower proportion of low-energy photons.

Continuous X-Ray Spectrum

Fast electrons striking a target pass through the electric fields of its nuclei. These fields deflect and decelerate the electrons. When an electron loses kinetic energy in this way, some of that energy can be emitted as an X-ray photon. This radiation is called bremsstrahlung, meaning braking radiation.

Different electrons can lose different amounts of energy in these interactions. They therefore produce photons with a range of energies and wavelengths, forming a continuous X-ray spectrum.

Maximum energy and short-wavelength cutoff

An electron accelerated from negligible initial kinetic energy through a potential difference $$V$$ gains kinetic energy $$eV$$, where $$e$$ is the magnitude of the electron’s charge. The greatest possible photon energy occurs when one electron transfers all of this energy to a single photon:

Here, $$h$$ is the Planck constant, $$c$$ is the speed of light, $$f_{\max}$$ is the maximum photon frequency and $$\lambda_{\min}$$ is the minimum wavelength.

A photon cannot receive more energy than the electron has available. Since higher-energy photons have shorter wavelengths, the spectrum has a short-wavelength cutoff at $$\lambda_{\min}$$: no photons are produced below this wavelength.

Increasing the accelerating potential difference gives the electrons more kinetic energy. This raises the maximum photon energy and decreases the minimum wavelength.

Characteristic X-Ray Spectrum

When a fast incoming electron strikes a target atom, it can remove an electron from an inner shell. This leaves a vacancy in the atom. An electron from a higher energy level can then move into the vacancy, releasing the lost energy as an X-ray photon.

The photon energy is therefore equal to the difference between the electron’s initial and final energy levels.

Characteristic X-ray lines

Atomic energy levels are discrete: an electron can occupy particular energies rather than any energy in between. Transitions between these levels therefore produce photons at particular energies. These appear as lines in the X-ray spectrum, superimposed on the continuous spectrum.

The energy levels depend on the target element, so the energies of the lines also depend on the element. The lines are called characteristic because their energies identify the element producing them.

Condition for producing a line

The incoming electron must have enough energy to remove an electron from the relevant inner shell. If it does not, that vacancy cannot form, so a transition into that vacancy cannot produce the corresponding characteristic X-ray line.

X-Ray Image Sharpness

Image sharpness describes how clearly an X-ray image shows boundaries and fine details. A sharp boundary appears well defined; a blurred boundary spreads over a wider region, making small details harder to distinguish.

Geometric blur

X-rays originate from a small area of the source called the focal spot, rather than from a single point. Rays from different points within this spot pass around an object's edge and reach slightly different positions on the detector. The resulting overlapping shadows spread the edge, producing geometric blur.

Geometric blur can be reduced by:

• Using a smaller effective focal spot, so the rays originate from a smaller apparent area.
• Placing the object closer to the detector, so the slightly separated rays spread less before reaching it.
• Increasing the source-to-object distance, so the rays from different parts of the focal spot reach the object at more similar angles.

Movement and detector resolution

If the patient or imaging system moves relative to the other during an exposure, the image shifts while it is being recorded. The different positions overlap, causing movement blur. Keeping the patient still and using a short exposure reduce this blur.

The detector also has a limited spatial resolution: details that are too close together may not be recorded as separate features. Even when geometric and movement blur are small, the detector therefore limits how much fine detail the image can show.

X-Ray Image Contrast

Image contrast is the difference in recorded signal between regions of an image. It allows structures to be distinguished from their surroundings.

As an X-ray beam passes through the body, tissues attenuate it by absorbing or scattering some of its photons. If one region attenuates more of the beam than another, fewer photons emerge from that region. The detector records the resulting difference in transmitted intensity as an image-signal difference.

The contrast in the image therefore depends both on the difference in attenuation along the paths through the body and on the detector’s ability to represent the resulting difference in transmitted intensity.

X-ray energy and contrast

Attenuation depends on photon energy, but it does not change by the same amount for every tissue. Changing the tube potential changes the energies of the photons in the beam, so it can change the relative attenuation of tissues as well as the beam’s penetration.

A higher tube potential generally produces more penetrating X-rays and can reduce the attenuation difference between tissues. This can reduce image contrast. A lower tube potential can increase that difference, although the beam is then less penetrating.

Contrast media and scattered X-rays

A contrast medium is introduced into a selected region to increase the attenuation difference between that region and its surroundings. For example, a contrast medium in a blood vessel can make the vessel more distinguishable from nearby tissue.

Scattered X-rays can reach the detector from directions other than the original beam. They add unwanted signal that does not accurately represent attenuation along the intended paths, reducing the difference between image regions and therefore reducing contrast.

Restricting the beam reduces the volume of tissue irradiated, so less scattered radiation is produced. An anti-scatter grid placed before the detector absorbs much of the scattered radiation while allowing more of the directly transmitted radiation through. Both measures reduce the contribution of scatter to the image.

Patient Dose in X-Ray Imaging

X-rays transfer energy to the patient when they are absorbed or interact with tissue. Because X-rays are ionising, this energy can produce ions in the tissue. The patient’s absorbed dose describes the energy deposited per unit mass of tissue.

A diagnostically useful image requires enough X-rays to reach the detector. Reducing exposure can lower the patient’s dose, but if too few photons are detected, the image may not show the required detail clearly.

Limiting X-ray exposure

A shorter exposure time produces fewer X-ray photons. A lower tube current also produces fewer photons per unit time. Both can reduce patient dose, provided enough photons are detected to produce a useful image.

Restricting the X-ray beam to the region being examined avoids exposing tissue outside that region unnecessarily.

Photon energy, filtration and detection

Low-energy X-ray photons are readily absorbed by the patient, so many deposit energy without reaching the detector. Filtration removes these photons from the beam before it enters the patient. This reduces dose from photons that would contribute relatively little to the image.

Photon energy also affects how readily X-rays pass through the patient and reach the detector. Detection efficiency describes how effectively the detector uses the photons that reach it. A more efficient detector can produce a useful image with fewer incident photons, potentially allowing a lower exposure.

Dose control therefore means using the lowest exposure that still provides an image suitable for diagnosis, rather than reducing exposure without regard to image quality.