Non Ionising Imaging

What does non-ionizing imaging include?

Non-ionizing imaging is used in many different areas of medical physics for a variety of reasons. These areas include ultrasound imaging, magnetic resonance scanning, and fiber optics and endoscopy.

Ultrasound imaging

Ultrasound imaging (sonography) is a technique for seeing the interior of the body using high-frequency sound waves. Because ultrasound pictures are taken in real time, they can reveal internal organ movement as well as blood moving via blood vessels. When sound waves are transmitted into the body and then reflected back to a scanner, pictures are generated. Ultrasound imaging, unlike X-ray imaging, does not expose people to ionizing radiation.

A 2D fetal ultrasound.

In medical physics, ultrasound imaging can be used to identify the reasons for pain, swelling, and infection in the body's internal organs, as well as to inspect a baby in pregnant women or the brain and hips in infants. Abdominal scans often use a 7MHz frequency (f). On top of this, if you consider the fact that the speed of sound in tissue (vw) is approximately 1540m/s, then the wavelength (λ) of ultrasound would be:

\[\lambda = \frac{v_w}{f} = \frac{1540 m/s}{7 \cdot 10^6 Hz} = 0.22 [mm]\]

The general consensus is that you can successfully scan tissue to a depth of around 500 \(\lambda\). For 7 MHz, that is \(500 \cdot 0.22mm = 0.11m\).

There are no echoes when sound waves flow smoothly through uniform material. As a result, the ultrasound image on the screen is black, with no echoes. A wave is reflected back to the probe when sound waves strike tissue that absorbs or transmits the sound.

Depending on the strength of the reflection, the ultrasound picture is white or gray. Ultrasound, unlike x-rays or CAT scans, cannot identify tissue density. Instead, it looks for sonotransmission (the passage or reflection of sound).

On an ultrasound, very thick tissues, such as bone or kidney stones, rapidly reflect echoes and appear bright white. Echoes are also easily reflected in air, such as in the intestine. As a result, the bowel's border also looks white on an ultrasound. Therefore, things with vastly different densities (such as air and bone) may show on ultrasound as bright white.

Magnetic resonance scanning

Magnetic resonance imaging (MRI) is a radiological imaging method that creates images of the body's anatomy and physiological processes without using X-rays or ionizing radiation.

Strong magnetic fields, magnetic field gradients, and radio waves are used in MRI scanners to create pictures of the body's organs. The MRI equipment transmits magnetic and radio frequency waves into the patient's body to obtain a picture.

The atoms in the magnetic field produce energy that transmits a signal to a computer. The computer then converts the signal into an image using mathematical algorithms.

Example of an MRI result.

An MRI device picks up signals from the nuclei (centres) of hydrogen atoms in your body using a strong magnetic field and a burst of radio frequency waves. Because they contain few hydrogen atoms, air and hard bone do not produce an MRI signal. As a result, these regions appear in black.

The quantity of fat and water present in each tissue, as well as the machine settings utilized for the scan, affect the intensity of bone marrow, spinal fluid, blood, and soft tissues, which range from black to white.

A chest, abdominal, or pelvic MRI can be used to help diagnose or monitor therapy for a number of diseases.

Fibre optics and endoscopy

Endoscopy is a procedure that enables doctors to look into the body's pathways. The use of an endoscope to examine and check the interior of body organs, joints, or cavities is known as endoscopy.

An endoscope is a device that provides lighting and visibility into the interior of a joint using fibre optics and lens systems. Depending on the medical procedure, the part of the endoscope introduced into the body may be stiff or flexible. In endoscopy, the total internal reflection principle of optics is used. This is a phenomenon that happens at the border between two mediums, in which all light is reflected back into the first medium if the incidence angle in the first medium is larger than the critical angle.

This is how the principle works in endoscopy step by step:

1. One of the two main endoscope cables carries the light from a bright lamp into the body.
2. The light that reflects off of the internal body parts travels back through a separate fibre-optic cable, bouncing off the glass walls as it travels back.
4. The total internal reflection principle in this process works like this: due to the thinness of the fibres, light entering a fibre is more likely to contact the inner surface at an angle larger than the critical angle. It will, therefore, be completely reflected, making it all the way to the end of the endoscope where the operator is looking.

   Doctor using an endoscope.

   An endoscopy may be used to obtain tissue samples (biopsy) to screen for illnesses and disorders, including anaemia, bleeding, inflammation, diarrhoea, or cancers of the digestive system.