Topic I: Medical Physics (Elective)

Medical Physics applies the physics of light, sound and radiation to the human body and to medicine. You will see how the eye and ear act as natural optical and acoustic instruments, how their defects are corrected, and how ultrasound, X-rays, CT scanning and radiotherapy are used to look inside the body and to treat disease.

The eye and image formation

The eye works like a camera. Light enters through the transparent cornea, which does most of the focusing, passes through the pupil (whose size is controlled by the iris to regulate light), and is fine-focused by the flexible lens onto the light-sensitive retina at the back. The cornea and lens together form a real, inverted, diminished image on the retina, which the brain interprets the right way up. The ciliary muscles change the shape of the lens to focus on objects at different distances, a process called accommodation: the lens becomes more curved (more powerful) for near objects and flatter for distant ones. The retina contains rods for dim-light and black-and-white vision and cones for colour vision.

Defects of vision and their correction

Common defects arise when the image does not form on the retina. In short sight (myopia) distant objects focus in front of the retina, often because the eyeball is too long or the lens too powerful, so the person sees near objects clearly but distant ones blurred; it is corrected with a diverging (concave) lens. In long sight (hypermetropia) near objects would focus behind the retina because the eyeball is too short or the lens too weak, so close work is blurred; it is corrected with a converging (convex) lens. The power of a lens is measured in dioptres, equal to 1 over the focal length in metres, and a converging lens has positive power while a diverging lens has negative power.

The ear and hearing

The ear converts sound waves into nerve signals. The outer ear funnels sound down the ear canal to the eardrum, which vibrates. These vibrations pass through three small bones in the middle ear that amplify them and transmit them to the oval window of the cochlea in the inner ear. The fluid-filled cochlea contains tiny hair cells that respond to vibrations of different frequencies and generate electrical signals sent to the brain along the auditory nerve. The healthy human ear responds to frequencies from about 20 hertz to 20000 hertz, and is most sensitive in the range of speech. Loudness is related to the intensity of the sound, and prolonged exposure to loud sound can permanently damage the hair cells and cause hearing loss.

Sound intensity and hearing loss

The intensity of a sound is the power it carries per unit area, and the human ear responds to an enormous range of intensities, so loudness is measured on the logarithmic decibel scale. An increase of 10 decibels corresponds to a tenfold increase in intensity but is perceived as roughly a doubling of loudness. Quiet conversation is around 60 decibels, busy traffic around 80, and sounds above about 85 decibels for long periods risk damaging hearing. Hearing loss can be conductive, where sound fails to reach the inner ear because of a blockage or damaged middle-ear bones, or sensorineural, where the hair cells or auditory nerve are damaged. Hearing aids amplify sound to help compensate for some losses.

Ultrasound and imaging

Ultrasound is sound with a frequency above 20000 hertz, well beyond human hearing. In medical imaging a transducer sends pulses of ultrasound into the body and detects the echoes reflected from boundaries between different tissues. The time for an echo to return gives the depth of the boundary, and the strength of the echo indicates the type of tissue, building up an image. Because it uses no ionising radiation, ultrasound is safe enough to monitor a developing fetus in pregnancy and to examine soft tissues, the heart and blood flow. A gel is used between the transducer and skin to exclude air, which would otherwise reflect almost all the ultrasound and prevent it entering the body.

X-rays and CT scanning

X-rays are high-frequency, high-energy electromagnetic waves that can pass through soft tissue but are strongly absorbed by denser material such as bone. In a plain X-ray, the radiation passing through the body exposes a detector, and dense structures cast shadows, producing an image that reveals broken bones and dental problems. A CT (computed tomography) scan takes many X-ray images from different angles around the body and uses a computer to combine them into detailed cross-sectional or three-dimensional images, giving far more information than a single X-ray. Because X-rays are ionising and can damage cells, the dose is kept as low as practicable, lead shielding protects staff, and exposure of patients is carefully justified and limited.

Radiotherapy and nuclear medicine

The same ability of ionising radiation to damage cells is turned to advantage in radiotherapy, which uses high-energy gamma rays, X-rays or particle beams to destroy cancer cells. Because radiation harms healthy tissue too, beams are aimed precisely and directed from several angles that all cross at the tumour, so the tumour receives a high dose while surrounding tissue receives much less. In nuclear medicine a radioactive tracer with a short half-life is introduced into the body and its gamma emissions are detected to image how organs function, for example the thyroid or the heart. Throughout, the benefit of diagnosis or treatment is weighed against the risk from the radiation dose, and safety measures keep exposure to a minimum.