If you’ve ever had an X-ray or an MRI, you’ve encountered medical physics. This is the branch of physics that deals with healthcare. It offers solutions and practical applications from physics and physics engineering in the medical field. Two areas that stand out are optics and imaging.
Medical physics covers many different areas, from technologies used to improve eyesight and hearing to help with the diagnosis of diseases (internal imaging techniques), the creation of tools to improve surgical methods (endoscopy), and disease treatment, such as radiotherapy.
Physics of the eye and ear
In any animal species, organs that help to sense the environment are the most important tools for survival. For humans, two of these sensory systems are hearing and vision.
Both mechanisms translate incoming signals into electrical impulses that are delivered to the brain. Evolution has made these systems so complex that they allow us to detect colours, the direction of sounds, and the intensity of both sound and light. The physical mechanisms that rule their functioning are important, and understanding them from the perspectives of physics and biology allows us to develop a range of healthcare solutions.
Physics of the eye
We can see the things around us because of light entering our eyes. This light undergoes a series of optical transformations to produce an image that we can see. Let us understand how it works.
- Light enters the eye through a clear dome called the cornea, which is a refractive medium that bends the entering light.
- The light then passes through the pupil, an aperture controlled by a muscle that expands and contracts to allow more or less light to pass.
- After passing the pupil, the light enters the lens, which focuses the light beam to the back of the eye with the help of the cornea.
- The image focused on the back of the eye is then projected onto the retina. This possesses a sensitive array of nerves that conduct the light signal to the brain.
Many of the processes in the eye are impacted by light, including focusing, lenses, and image processing. Thus, many applications from optics, a branch of physics that deals with light, have been used in corrections of eyesight problems.
Physics of the ear
We can hear the sounds around us because of the functioning of the ear. The ear can be split into three parts, each with its own set of functions. Looking at its structure, we find:
- The outer ear, which collects and transmits the sound to the middle ear.
- The middle ear, which converts the energy of a sound wave into internal vibrations of the middle ear’s bone structure. These vibrations are then converted into compressional waves in the inner ear.
- The inner ear, which transforms the energy of the compressional waves into nerve impulses that can be transferred to the brain.
See the image below for the different components of the ear.
Figure 1. Anatomy of the human ear. Source: Lars Chittka and Axel Brockmann, Wikimedia Commons (CC BY 2.5).
The role of physics in the medical field
Physics has a great impact, providing solutions and technology to help people in many areas of medicine and healthcare. Lenses and lens implants to correct vision and optical instruments to conduct surgeries via endoscopy use results and techniques from optics.
Areas such as hearing benefit from applications in acoustics and electronics. These help to develop mechanisms to translate audio to electrical impulses and to amplify them. Other applications of physics can be found in disease treatments, such as the use of radiation to kill cancer cells and shrink tumours. Other areas using applications of physics are electrocardiography and imaging.
Physics for cancer treatment
Nuclear physics has been one of the main tools used in the fight against cancer. Treatments use a beam of particles (electrons, photons, or protons) directed towards the tumour to aim to kill it and stop it from spreading further.
In some cases, radiation sources can be internal (inside your body), a treatment that is known as radiotherapy.
Figure 2. During radiotherapy, cancer cells are targeted with doses of radiation to damage their DNA and stop their growth. Source: Manuel R. Camacho, StudySmarter.
Cells carry instructions in their genome to produce daughter cells and replace tissue when it is damaged, or other cells have died.
Cancer cells carry damage in their genome that affects the instructions for growth and division (cancerogenesis). This leads to cancer cells multiplying without end and forming tumours.
Radiation is used to damage the DNA of these cells further. After some sessions, the DNA is so damaged that the cells cannot reproduce anymore. Radiotherapy is used in combination with other treatments, such as surgery and chemotherapy, to ensure high success in beating cancer.
Physics for hearing aids
Human hearing has a mechanism that translates sound first into mechanical vibrations and then into electrical impulses that are sent to the brain. As people age, parts of these mechanisms are lost or damaged.
Devices that can amplify incoming sounds have been developed to improve the hearing of anyone with hearing impairments. Using a microcontroller, these devices transform sounds into electrical signals and send them to the ear canal after they have been amplified.
Some hearing issues require specific and more direct solutions, which may include the implant of devices below the skin, such as:
- Bone-anchored hearing aids, which transmit the vibration through the inner ear.
- Cochlear implants, which send the electrical signal to the cochlea, the organ that translates the vibration into electrical signals.
- Auditory brainstem implants, which send the signal directly to the brain.
Physics of lenses and optics
Optics has been one of the main areas of medical physics, enabling us to produce devices such as telescopes and microscopes. Optics also helps us to produce lenses to correct vision. Defects in vision caused by a deformation of the eye curvature modify the image that is acquired by the brain. Glasses and lenses correct the light entering the eyes, thereby providing a solution for vision defects.
Figure 3. Sight disorders such as myopia are caused by the deformation of the eye. Source: Manuel R. Camacho, StudySmarter.
In myopia, the image focuses before it reaches the back of the eye, making it blurry. Glasses help to refocus the image before it enters your eye, thereby correcting this defect.
In many cases, myopia can be also be treated by implanting lenses inside the first layer of the eye. These intraocular lenses can also help with cataracts, replacing the natural lenses of the eye when they become cloudy.
Physics of electrocardiography
A test called an electrocardiogram allows doctors to measure a patient’s heartbeat. This technique is known as electrocardiography.
The electric potentials in the heart muscle produce a measurable electrical trace, an electrocardiograph or ECG. The analysis of the signal delivered can show the heart’s performance and give valuable information on any strange behaviour.
Physics of non-ionising imaging
This includes the study of ultrasound imaging and magnetic resonance imaging (MRI), techniques that use low energy photons or sound to produce images of internal components of the body.
Ultrasound imaging
This works by transferring sound waves to the body and detecting their reflection waves. We cannot detect these ultrasound waves, which have a sound range of 20kHz and above that is beyond our hearing capacity.
With ultrasounds, doctors can get detailed images in real-time at a relatively low cost without causing any harm to the patient. Ultrasound imaging allows medics to diagnose a variety of medical conditions, including heart valve disorders, possible tumours, and other organ abnormalities.
Magnetic resonance scanning
Magnetic resonance imaging, commonly known as ‘MRI scans’, is used for producing a detailed image of a cross-section of a patient’s body. It utilises strong magnetic fields and radio waves to produce images by inducing rapid small changes into the body’s hydrogen atoms.
MRI devices allow doctors to obtain detailed images of the inner body without using high energy radiation.
Physics of Ionising imaging
Imaging techniques also use high-power radiation, utilising small doses of electromagnetic radiation to recreate images of the body as x-rays or gamma rays for tracing. Other processes of imaging can also use the emission of beta particles.
In general, these techniques can produce very accurate images of internal parts of the body. However, exposure to them should be limited to ensure the security of the patients.
X-ray imaging
X-rays are waves with short wavelengths and high intensities. They are used in medicine to create black and white images of the inside of the body. The image colours and tones are a product of the absorption of the x-ray photons.
X-rays have high energy and thus penetrate matter more easily, but their absorption and penetration depend on the density of the material.
X-rays were discovered by the German scientist Wilhelm Röntgen.
On the images, bones, which absorb the x-rays, appear in white. Tissues, which absorb fewer x-rays, are displayed in grey, while air, which absorbs only a very small number of x-rays, shows as black.
The technique of creating, processing, and interpreting x-rays is called radiography. X-rays are routinely used for checking fractures, but they can also spot pneumonia and even breast cancer. More recently, x-rays have been used to check for possible signs of COVID-19 complications.
Radionuclide imaging
A radionuclide is a nucleotide that is radioactive in nature. It is an isotope of an element with an unstable nucleus. Its decay results in the emission of subatomic particles or electromagnetic radiation.
A radionuclide scan is a type of imaging that employs a small amount of an isotope called a tracer to identify cancer, injuries, infections, and other conditions. The tracer is either injected into a vein or eaten. Once within the body, the tracer travels through circulation to the organ of interest, such as the thyroid, heart, or bones. The radiation is then detected and interpreted by a camera or sensor.
The tracer emits gamma rays, which are comparable to x-rays. A gamma camera detects these gamma rays, which are then processed by a computer to create a picture of the target organ. Potential problem areas emit stronger gamma rays, which show up on the scan as bright patches. PET scans, gallium scans, and bone scans are all examples of radionuclide scans.
Except for a little prick in the case of an injection, a radionuclide scan is painless. The isotope must travel to the target organ, which may take several hours. The patient can normally leave the testing facility during this period and return for the scan, which can take anywhere from one to five hours.
Medical Physics - Key takeaways
- Medical physics deals with the physics of healthcare and how physics produces applications used in medicine.
- The eye and the ear are two sensory systems that convert signals, light and sound respectively, into electrical impulses that are then delivered to the brain.
- Optics and acoustics, both branches of physics, have a great impact on eyesight and hearing healthcare.
- The analysis of the sensory systems and how they work has helped to create devices and lenses that correct problems with hearing and vision.
- Cancer treatment uses nuclear physics to combat cancer cells.
- Medical imaging and monitoring use ionising and non-ionising techniques to monitor and deliver information about internal parts of the body, helping doctors gather more precise information about their patients.
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