X-rays are a type of energy that comes from an atom's electrons. When these electrons move from a higher energy level to a lower one, they turn into photons. This process releases energy and creates high-energy waves with short wavelengths and high frequencies called X-rays. These waves are used in X-ray image processing to create detailed images of the human body. The images are produced when fast-moving electrons suddenly slow down and transform into photons of electromagnetic radiation. This allows doctors to see inside the body and diagnose medical conditions.

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

X-rays are produced using an X-ray tube, which 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. The process of producing X-rays involves thermionic emission, which emission of electrons from a heated metal (cathode). The electrons are discharged from a hot filament at the cathode or negative terminal, and are then accelerated by a high voltage and aimed towards the positively charged anode. When the electrons arrive at the anode, they lose around 1% of their kinetic energy from the impact, which is emitted into X-ray photons. The remaining energy is converted into heat, and the tungsten anode rotates at 3000 revolutions per minute for cooling to minimise overheating.

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Calculating the energy of an X-ray

To calculate the maximum energy gained by an electron when it accelerates, we can use the following equation:

E = eV

Where E is the maximum energy gained by an electron in electronvolts (eV), and V is the voltage across the anode in volts.

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

E = hf

Where h is Plank’s constant in joules per second (6.63 ⋅ 10-34J·s), and f is the frequency in Hertz (Hz).

The maximum frequency can be expressed as:

fmax = (eV)/h

And the minimum wavelength can be expressed as:

λmin = c/fmax

Where c is the speed of light in metres per second (3 ⋅ 108m/s). These equations are useful in X-ray image processing and can help determine the quality of the X-ray image produced.

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

Attenuation of X-rays is when the net number of photons entering matter is reduced by absorption and scattering. This occurs when X-rays travel through a material, and the intensity of the beam is reduced due to the absorption of X-rays. The equation for calculating the intensity of X-rays transmitted through a substance relative to the initial beam intensity is I=I0.e- (μ/ρ).ρl, where I and 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 mass attenuation coefficient is commonly measured in cm2/g, and the mass extinction coefficient is an old term for this quantity.

X-ray image quality factors

X-ray digital image processing is a crucial process to obtain high-quality digital radiographic images for accurate diagnosis. The process involves maximising important details and suppressing unwanted details in the image as per the requirements. The most crucial part of the image processing is done during X-ray machine manufacturing, but further processing is required to obtain high-quality images. Some of the digital image processing factors include contrast, sharpness, spatial enhancement, and sound reduction.

Contrast is the difference in the degree of darkness between structures, and it is increased by using an efficient level of X-ray hardness, usually hard X-rays for bones and soft X-rays for tissue. Sharpness is the clearness of the structure's using narrow X beam, decreasing pixel size or reducing X-ray scattering with a lead grid. Changing-ray's 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.

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. Sound reduction is also important to reduce irrelevant information in the X-ray image caused by scattering. These factors are crucial to obtain high-quality images for proper diagnosis.

X-ray image processing algorithms

Different algorithms are used to apply various digital image processing techniques in X-ray image processing. Low-pass and high-pass filters are used for spatial enhancement, emphasising areas with low and high spatial frequencies, respectively. Directional or edge detection filters enhance linear features or features oriented in specific directions. Linear contrast stretch is a that evaluates the brightness in an image and expands this ratio to a greater area, highlighting the contrast in the image by having more clear dark and light shadows. These algorithms are used to apply various digital image processing techniques, including contrast, sharpness, spatial enhancement, and sound reduction, to enhance or suppress specific parts of the X-ray image for a clear diagnosis.

X-Ray Image Processing

What is X-ray imaging for image processing?

X-ray digital image processing is a process that is used to obtain high-quality digital radiographic images in terms of maximising important details or suppressing unwanted details in the image.

How is an X-ray image formed?

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 film that is light-resistant and an intensifying fluorescent screen, is placed behind the area of interest. 

What is digital image processing radiography?

Digital image processing radiography is the process of enhancing digital radiography images so that the quality of the image is better.

How do you process a digital X-ray?

You can process a digital X-ray by changing the contrast, spatial enhancement, and sharpness and by reducing sound.

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