X-Ray Image Processing

X-rays are high-energy electromagnetic waves with short wavelengths and high frequencies. They develop from the electrons of an atom and become photons when they are energised from a higher energy level to a lower level, releasing energy in the process. This happens when fast electrons decelerate suddenly, and their large amount of kinetic energy transforms them into photons of electromagnetic radiation.

X-ray image processing: how are X-rays produced?

X-rays are produced using an X-ray tube, which you can see in the image below. It is a vacuum tube that can convert an electrical input into X-rays. Its vacuum chamber contains a cathode or filament, which is a negatively charged electrode, and a rotating anode, which is a positively charged electrode.

Here’s a simplified explanation of the process of producing X-rays:

1. The electrons are discharged from a hot filament at the cathode or negative terminal. This is also known as thermionic emission. Thermionic emission happens when large amounts of thermal energy are transferred to the electrons in the metals, and the electrons are discharged from the metal.
2. The beam of electrons is aimed towards the positively charged anode.
3. A voltage of around 200 kiloelectronvolts (keV) occurs between the two electrodes.
4. When the electrons arrive at the anode at high speed, they lose around 1% of their kinetic energy from the impact, which is emitted into X-ray photons. The energy is released by the outer shell electrons, which move to lower energy levels.
5. The remaining energy is converted into heat. The tungsten anode rotates at 3000 revolutions per minute for cooling to minimise overheating.

Schematic drawing of Crooke's X-ray tube, Wikimedia Commons

Calculating the energy of an X-ray

When an electron accelerates, it gains energy equal to one electronvolt (eV). We can calculate the maximum energy gained using the equation below. Note that e is the charge of one electron measured in coulombs (1.6 ⋅ 10^-9C), V is the voltage across the anode in volts, h is Plank’s constant in joules per second (6.63 ⋅ 10^-34J·s), f_max is the maximum frequency in Hertz (Hz), and c is the speed of light in metres per second (3 ⋅ 10⁸m/s).

We can find the maximum frequency and minimum wavelength of an X-ray by rearranging the above equations:

What is X-ray digital image processing?

Digital image processing is the process in digital X-ray images to enhance or suppress specific parts of an image in order to provide a clear diagnosis.

How are X-ray images formed?

As we mentioned above, X-rays are formed when highly energetic electrons are impacted on an anode and release energy in the form of photons. These photons are partially absorbed when they pass through materials. The amount of absorption depends on the type of material or substance.

A cassette, which holds a light-resistant film and an intensifying fluorescent screen, is placed behind the area of interest.

• When X-rays enter the body in soft tissues, like organs and muscles, these tissues cannot absorb the radiation, so the X-rays pass through the body, leaving the cassette behind. The patient is exposed to large amounts of radiation, and the film appears black in those spots as a result.
• When X-rays enter the body in hard tissues, like bones, these tissues absorb large amounts of radiation, which leaves the film exposed to much less radiation. As a result, the film appears to be white or grey.
• The intensifying screen fluoresces and emits a light that creates the image on the X-ray film.

Nowadays, the cassette is replaced by a computer. Digital radiography (DR) is a modern approach that instantly produces a digital radiographic image on a computer by using X-ray sensitive plates to obtain data during the exam. The data collected is instantaneously transferred to a computer without using a cassette. DR increases image quality and saves time.

The importance of X-ray attenuation

X-ray attenuation is when the net number of photons entering matter is reduced by absorption and scattering.

Here is the equation for calculating the intensity of X-rays transmitted through a substance relative to the initial beam intensity:

The intensity of reflected beam I and incident beam I₀ are measured in W/m² (watt per square metre), the absorption coefficient μ is measured in m^-1, and the distance travelled through a substance x is measured in m. The intensity decreases with the absorption distance. This is a problem as the quality of the image relies on reflected X-rays.

X-ray image quality factors

X-ray digital image processing is a process to obtain high-quality digital radiographic images in terms of maximising important details or suppressing unwanted details in the image as per the requirements needed for proper diagnosis. The most critical part of image processing is done when an X-ray machine is manufactured, but further processing is required. Some of these digital image processing factors are listed below.

• Contrast is the difference in the degree of darknessbetween structures. Greater contrast means the image will be clearer regarding the differences between tissues. Contrast is increased by using an efficient level of X-ray hardness, usually hard X-rays for bones and soft X-rays for tissue. Sometimes a patient is also given a contrast agent, which is a substance that temporarily changes the interaction of the patient’s body with X-rays.
• Sharpness measures the clearness of the structure’s edges of an X-ray image. Greater sharpness means that the edges of each structure can be seen with greater clarity. Sharpness is improved by using a narrow X-ray beam, decreasing the pixel size, or reducing X-ray scattering with a lead grid. Changing the X-ray’s energy in kV depending on the contrast is also required for imaging a specific body part. Each body part has different contrast abilities due to its composition. For example, lungs have high physical contrast due to air.
• Spatial enhancement is a digital processing technique that involves using spatial filters to highlight or minimise specific features of an image based on its spatial frequency. The spatial frequency measures the frequency of the various tones that appear in an image. Textured areas in an image (where the tones do not appear smooth over small areas) have high spatial frequencies, and areas with smooth tones and relatively slight tonal variation over small areas have low spatial frequencies.
• Sound reduction. Some of the X-ray radiation is intercepted by the X-ray machine itself or nearby objects. This is known as scattering and causes external noises to impact the X-ray image. Noise needs to be reduced so that the X-ray image is clear and does not contain irrelevant information.

X-ray image processing algorithms

Different algorithms are used to apply the image processing techniques mentioned above. It is an automatic procedure used to enhance the X-ray image quality. Some algorithms used to apply the digital image process techniques are listed below.

• Low-pass filters are used for spatial enhancement, which emphasises areas with low spatial frequencies. This is done by allowing low spatial frequencies to pass while blocking high spatial frequencies. Low-pass filters are used to smooth the appearance of an image.
• High-pass filters are used for spatial enhancement and work by emphasising areas with high spatial frequencies. This is done by allowing high spatial frequencies to pass while blocking low spatial frequencies. High-pass filters are used to sharpen fine details in an image.
• Directional or edge detection filters enhance linear features or features that are oriented in specific directions. These filters are used in geology for the detection of linear geologic structures.
• Linear contrast stretch is a filter that evaluates the maximum and minimum brightness in an image and expands this ratio to a greater area. Hence, there is plenty of maximum and minimum brightness areas. This highlights the contrast in the image by having more clear dark and light shadows.