Medical applications of physics

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  • Created by: Abbykt123
  • Created on: 10-04-14 19:38

X-rays

X-rays form part of the electromagnetic spectrum. Their properties make them very useful in medicine. The detailed images they provide help to diagnose disease, while radiography uses high-energy X-rays to treat cancer. However, X-rays are hazardous so appropriate precautions must be taken to protect medical staff and patients from harmful effects.

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Properties of X-rays

X-rays are electromagnetic waves and part of the electromagnetic spectrum.

As with visible light and other electromagnetic waves, X-rays transfer energy and are transverse waves (their oscillations are perpendicular to the direction of energy transfer).

X-rays also have these properties:

  • they have a very short wavelength (about the same size as the diameter of an atom

  • they cause ionisation (adding or removing electrons in atoms and molecules)

  • they affect photographic film in the same way as visible light (turning it black)

  • they are absorbed (stopped) by metal and bone

  • they are transmitted by (pass through) healthy body tissue

These properties make X-rays very useful for medical diagnosis and treatment.

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Imaging using X-rays

Photographic film

X-rays are useful in applications such as medical imaging of bone fractures and dental problems. They are directed at the part of the body under investigation. They are transmitted through the body - except in areas where they are absorbed by dense structures like bone.

In older X-ray machines, white photographic film is placed behind the patient. X-rays pass through the patient’s body and into the photographic film. The film turns black where X-rays hit it. Bones absorb (stop) X-rays, so the film stays white where the bones are. Fractures show up as dark areas in the image of the bones on the film.

This method can be used for dental treatment, as decayed teeth will absorb X-rays less strongly than healthy teeth.

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CCDs

A CCD is a charge-coupled device. Modern X-ray machines use CCDs instead of photographic film. The images are formed electronically, allowing them to be recorded and stored more easily than the images from photographic film.

CT scans

Traditional X-ray imaging gives a two-dimensional (2D) view of the body from one angle. This can result in detail being obscured by other structures in the body. Computerised tomography (CT) scans involve taking a range of X-ray images from various positions.

These are processed by a computer to build a three-dimensional (3D) image. This image can be manipulated in order to see the structures within the body at different layers and from different points of view. This lets a doctor gain a much greater insight into what is wrong with a patient.

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Ionising effect of X-rays

The ionising properties of X-rays mean that they can damage the DNA in cells. The ionising effect of X-rays is more damaging to actively dividing cells in the body.

Low doses of X-rays may cause cancer - whereas high doses may kill cancerous cells.

Cancer treatment

Cancer is a disease in which cells divide uncontrollably because of changes in their DNA, forming tumours.

One method of treating cancer is to direct high energy X-rays at the tumours. This causes so much damage to the cancerous cells that they die. This treatment is called radiotherapy.

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Precautions when using X-rays

Patients are limited to the number of X-rays they are allowed to have so their bodies are not exposed to too much radiation. X-ray machines also produce relatively low energy X-rays, which reduce the risk of them damaging human tissue.

Hospital staff are also at risk from repeated exposure to low levels of X-rays. Shielded walls containing lead are built into all X-ray rooms to protect people outside the room. They have warning signs to show when the room is in use so that people do not enter.

Only trained specialist staff - called radiographers - are allowed to use X-ray machines. They routinely leave the room, or stand behind a screen containing lead, whenever X-ray machines are in use. In situations where radiographers cannot stand behind a screen, they wear lead aprons which act as a protective layer of clothing.

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Ultrasound

High frequency sound waves – ultrasound waves – have uses in industrial applications as well as in the field of medicine. Like X-rays, they can be used to diagnose disease, and they are also widely used in pre-natal scans to check the development of an unborn baby. Ultrasound is also useful in the medical treatment of kidney stones.

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Ultrasound

The range of human hearing is about 20 Hz to 20,000 Hz. Ultrasound waves have frequencies above about 20,000 Hz (which is 20 kHz). As this is above the normal hearing range for humans, we cannot hear ultrasound.

Ultrasound can be produced by some animals (such as bats and dolphins), as well as by some electronic devices.

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Reflections

When ultrasound waves reach a boundary between two media (substances) with different densities, they are partly reflected back. The remainder of the ultrasound waves continue to pass through. A detector placed near the source of the ultrasound waves is able to detect the reflected waves. It can measure the time between an ultrasound wave leaving the source and it reaching the detector. The further away the boundary, the longer the time taken. The distance travelled by an ultrasound wave can be calculated using this equation:

s = v × t

where:

  • s = distance in metres, m

  • v = speed in metres per second, m/s

  • t = time in seconds, s

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An ultrasound machine can be used to detect cracks or flaws in materials such as metal. This is used in industry for quality control procedures to check manufactured objects, such as railway tracks and oil pipelines, for damage or defects.

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Medical uses of ultrasound

Medical imaging

The human body is composed of different tissues such as muscle and skin. Ultrasound directed at the body will be partly reflected at the boundary between these different tissues.

This principle is used in ultrasound scans. These are widely used in pre-natal scanning to check that a foetus is developing normally and to take measurements of its growth. Computers can combine many ultrasound reflection readings to produce a detailed image from them. Ultrasound imaging can also be used for diagnostic purposes in medicine. Specific organs, such as the liver, can be scanned to look for signs of disease.

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Removing kidney stones

Kidney stones are solid crystals formed from substances found in urine. They can sometimes build up into large stones inside the kidney. These can then pass into the ureter and cause a blockage, accompanied by severe pain.

High frequency ultrasound waves focused at a kidney stone cause it to vibrate, breaking it into small enough pieces to pass out in the urine.

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Comparing ultrasound and X-rays

Wavelength and frequency

Compared to ultrasound, X-rays have a much shorter wavelength (higher frequency). This means that X-ray photographs produce higher quality images than ultrasound scans. They show greater detail and clarity. This is vital in terms of detecting small bone fractures or abnormalities.

CT scans provide even higher quality images than normal X-ray photographs. The 3D images produced by CT scans allow analysis of different levels through the body without other structures obscuring the area of interest (as can happen with traditional X-ray imaging)

X-rays are ionising which means that X-rays damage living tissue and the DNA within cells. Rapidly dividing cells are particularly vulnerable to the effects of ionising radiation. This makes X-rays particularly damaging to a developing foetus. Ultrasound waves are not ionising, and so they are safe to use when performing a foetal scan.

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Lenses

Many things rely on lenses to work including projectors, microscopes, telescopes and even your eyes. Lenses use refraction to change the direction of light to form images. The type of image formed depends on the type of lens used, and how it is positioned relative to the object from which the rays came.

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Refraction

Refraction is the change of direction of light as it passes from one medium (substance) to another. The two media must have different densities, such as air and glass.

When light passes into a denser medium, it slows down. The rays bend towards the normal (the normal is at 90° to the boundary between the two media). This means that the angle of refraction is smaller than the angle of incidence.

When light passes into a less dense medium, it speeds up. The rays bend away from the normal. This means that the angle of refraction is greater than the angle of incidence.

There is one special case - refraction does not happen if rays of light cross the boundary at 90°. In this case, they carry straight on.

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Refractive index

The amount of refraction depends on:

  • the angle the light hits the boundary between the substances

  • the difference in relative densities, between the two media (which determines how much the speed of light changes)

The degree to which a material slows the speed of light (compared to its speed in a vacuum) is its refractive index. This can be calculated using the equation:

refractive index = sin i/sin r

Where:

i is the angle of incidence

r is the angle of refraction

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Lenses

A lens is a transparent block that causes light to refract to form an image. There are two types of lens:

  • converging

  • diverging

These have different effects on light and they form different types of image. The amount that they refract light depends on the refractive index of the material they are made from, and how curved the lenses are.

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Converging lenses

A converging lens is curved outwards on both sides. It is represented on diagrams by the symbol.

A diagrammatical interpretation of a convex lens which is a vertical line with arrow heads at each end pointing away from the vertical.

Rays from a single point on a distant object arrive at the lens parallel to one another. Converging lenses refract these parallel rays so that they are come together at a point called the principal focus (labelled F on a diagram).

These lenses focus the rays of light to produce a real image - an image that can be projected onto a screen. The focal length is the distance between the centre of the lens and the image.

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Diverging lenses

A diverging lens is curved inwards on both sides. It is represented on diagrams by the symbol

A diagrammatical interpretation of a diverging lens which is a vertical line with arrow heads at each end pointing towards the vertical.

Diverging lenses refract the parallel rays of light so that they spread apart from one another. This means that they form a virtual image - an image that cannot be projected onto a screen.

The diverging rays can only be seen by the eye, and appear as having come from a different point to where the object is. The point at which the rays appear to have come from is the principal focus (F). The focal length is the distance between the centre of the lens and the virtual image.

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Representing the formation of images by converging lenses

Ray diagrams are used to represent the action of converging lenses. They follow these conventions:

  1. A principal axis is drawn through the centre of the lens at 90° to it.

  2. The object is represented as an upright arrow (labelled ‘Object’) with its base on the principal axis.

  3. The ray diagrams show the rays of light from one point on the object – they do not show all of the rays of light from every part of the object. These rays are drawn from the tip of the object.

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4.   Not all rays from one point are shown. For simplicity, only three are shown:

   b) A ray passing in a straight line through the centre of the lens without being refracted.

    c) A ray passing through the principal focus in front of the lens, and being refracted parallel to the principal axis (this cannot be drawn if the object is closer to the lens than the principal focus)

5.   If a full lens is drawn, refraction is represented from the centre of the lens and not from each of the boundaries.

6.   The image forms where the rays meet. It is represented by another arrow (labelled ‘Image’). If the image is virtual (because the object is nearer the lens than the principal focus), the diverging rays must be traced back in straight lines until they meet.

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Ray diagrams allow us to work out the nature of the image that will be produced. The nature of images can be described as:

  • magnified or diminished (how big the image is compared to the object)

  • upright (the same way up as the object) or inverted (upside down compared to the object)

  • real or virtual

The nature of images from converging lenses

Where the object is compared to the principal focus of a determines the nature of the image produced.

Converging lenses can be investigated by changing the distance between the object and the lens. Using the apparatus below, the object can be moved closer or further away from the lens. The object distance is measured in focal lengths : F, 2F (twice the focal length), and so on. The image distance can also be measured in focal lengths.

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The table below summarises the nature of the images produced by a converging lens.

Distance of object Size Orientation    Type Position Further than 2F Diminished Inverted Real Between 2F and F 2F Same size Inverted Real 2F Between 2F and F Magnified Inverted Real Further than 2F F No image       because the emerging rays are parallel to the axis Closer than F Magnified Upright Virtual Same side as object

Notice that the image:

  • is inverted and real - unless the object is at F or closer

  • gets larger as the object gets closer

  • gets further away as the object gets closer - unless the object is at F or closer

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Magnifying glasses

If the object is placed between the principal focus (F) and the lens, a converging lens will produce a magnified, upright, virtual image. The image can only be seen by looking through the lens, and it appears on the same side of the lens as the object. The magnification by a magnifying glass can be worked out using the equation:

magnification = image height/ object height

An equation triangle showing image height over magnification and object height.

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We can state the following:

  • A magnification above 1 shows that the image is larger than the object.

  • A magnification of 1 shows that the image and object are the same size.

  • A magnification below 1 indicates that the image is smaller than the object.

Note that magnification is a number without a unit.

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The nature of images from diverging lenses; Diverging lenses always produce images that are virtual, upright and diminished (smaller) in size compared to the object. The image always appears to come from the same side of the lens as the object.Ray diagrams for diverging lenses follow similar conventions to those for converging lenses, but how the location of the image is determined is slightly different. Ray diagrams for diverging lenses follow similar conventions to those for converging lenses, but how the location of the image is determined is slightly different.

Three rays are drawn:

  1. A ray passing parallel to the principal axis and being refracted away from the principal axis.

  2. A ray directed at the principal focus behind the lens - but being refracted by the lens so that it runs parallel to the principal focus after leaving the lens.

  3. A ray passing in a straight line through the centre of lens without being refracted.

As the image is virtual the rays leaving the lens must be traced backwards in straight lines until they reach a point at which they cross. This is where the virtual image appears to come from.

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The eye

The eye is the sense organ that allows us to see. Some people develop vision defects as a result of being unable to focus light from objects onto the retina. These can be corrected using lenses placed in front of the eye, but an optometrist must determine the power of the lens required.

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The eye 2

The eye

You should be able to label and describe the functions of the main parts of the eye.

The component parts of the eye (Suspenory ligament, Conjunctiva, Iris, Pupil, Cornea, Ciliary muscle, Lens, Retina, Fovea and Optic nerve) (http://www.bbc.co.uk/schools/gcsebitesize/science/images/triple_science/044_bitesize_gcse_tsphysics_medical_eyestructure_464.gif)

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  • cornea; Tough, transparent covering over the front part of the eye. Convex in shape, Refracts light as it enters the eye (by a fixed amount).
  • iris; Coloured part of the eye that contains muscles. These relax or contract to adjust the size of the pupil, Controls how much light enters the pupil.
  • pupil; Hole in the middle of the iris, Allows light to pass through as it enters the eye.
  • Lens; Transparent, bi-convex, flexible disc behind the iris. It is attached to the ciliary muscles by the suspensory ligaments,Refracts light to focus it onto the retina. The amount of refraction can be adjusted by altering the thickness and curvature of the lens.
  • Ciliary muscles; Muscles connected to the lens by suspensory ligaments, Adjust the shape of the lens to make it more or less curved, so as to increase or decrease the refraction of light.
  • Suspensory ligemants; Connect the ciliary muscles to the lens and hold the lens in place,Slacken or stretch as the ciliary muscles contract or relax, to adjust the thickness and curvature of the lens.
  • Retina; The lining of the back of eye containing two types of light receptor cells. Rods are sensitive to dim light and black and white. Cones are sensitive to colour, Contains the light receptors, which trigger electrical impulses to be sent to the brain when light is detected.
  • Optic nerve; carries impulses from the retina to the brain.
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the eye 4

The eye can alter the shape and curvature of the lens to adjust the degree of refraction. This is called accommodation. It allows light to be focused onto the retina from near or distant objects.

Accommodation is achieved by the contraction or relaxation of the ciliary muscles, which slacken or stretch the suspensory ligaments.

The table summarises how accommodation works.

Object

Ciliary muscles

Suspensory ligaments

Muscle tension on the lens

Lens shape

Near

Contracted

Slackened

Low

Fat, more curved

Distant

Relaxed

Stretched

High

Thin, less curved

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Range of vision

The near point is the closest an object can be from the eye without the object appearing blurred. If an object is brought closer to the eye than the near point, the lens cannot become sufficiently curved to refract the diverging rays to focus them onto the retina. For someone with normal vision, the near point is 25 cm.

  • Someone with long sight can see distant objects clearly, but their point is further away than 25 cm. This means they cannot focus properly on objects.

The far point is the furthest an object can be from the eye without it appearing blurred. The eye of someone without vision defects is able to clearly focus on distant objects at infinity. This is because the light rays arrive parallel to each other.

  • Someone with short sight can see near objects clearly, but their far point is closer than infinity. This means they cannot focus properly on distant objects.

The distance between the near point and the far point is called the range of vision.

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  • Comparison between the eye and a camera

Cameras are devices that focus light onto a photosensitive surface using a converging lens. They have some similarities to the eye.Like the eye, the image produced by a camera is diminished, inverted and real.

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  • Lens; To focus light onto the photosensitive surface at the back of the camera. This can either be photographic film or a CCD (charge-coupled device), Lens - which focuses light onto the retina.
  • Focusing screw; Allows the user to adjust the focus for nearer or more distant objects, Ciliary muscles - which stretch or slacken the suspensory ligaments to adjust the shape of the lens.
  • Apeture; Allows the user to adjust the amount of light entering the camera in different light conditions, Iris - which adjusts the amount of light entering the eye through the pupil.
  • Shutter; Allows the user to adjust the length of time that light enters the camera, which controls the amount of light to which the photosensitive surface is exposed, Not applicable (although we do have eyelids).
  • Photosensitive surface; Detects and records the light which is focused onto it. This can be on photographic film, but digital cameras use CCDs which convert light into electrical signals which can be stored, Retina - which detects light and converts it into electrical impulses which are sent to the brain.

One key difference between a camera and the eye is that a camera does not focus light onto the photosensitive surface by adjusting the shape of the lens. Instead, the focusing screws move the lens forwards or backwards in order to focus the image onto the photosensitive surface.

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Correcting vision defects

Short sight

Someone with short sight can see near objects clearly, but their far point is closer than infinity. This means they cannot focus properly on distant objects.

Short sight is caused by one of the following:

  • The eyeball being elongated - so that the distance between the lens and the retina is too great.

  • The lens being too thick and curved - so that light is focused in front of the retina.

Short-sightedness can be corrected by placing a diverging lens in front of the eye

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Long sight

Someone with long sight can see distant objects clearly, but their near point is further away than 25 cm. This means they cannot focus properly on near objects.

Long sight is caused by one of the following:

  • The eyeball being too short - so the distance between the lens and retina is too small.

  • A loss of elasticity in the lens - meaning it cannot become fat enough to focus (which is often age-related).

As a result, the lens focuses light behind the retina instead of onto it. Long-sightedness is corrected by putting a converging lens in front of the eye

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Power of lenses

When correcting long or short sightedness, an optometrist must choose a lens that refracts the light sufficiently for light to be focused onto the retina. The degree to which a lens refracts the light is the power of the lens, measured in dioptres, D. Lens power can be calculated using the following equation

P = 1/f

where: P = lens power in dioptres, D

f = focal length in metres, m

The focal length, f, of a lens is determined by two factors:

  • the refractive index of the material that the lens is made from the curvature of the two surfaces of the lens (how thick or fat the lens is) (Note that the power of a diverging lens (used to correct short-sight) is negative, while the power of a converging lens (used to correct long-sight) is positive.)

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People who have short or long sightedness require a certain lens power, with a certain focal length, to correct their vision.

For a given focal length, the greater the refractive index, the flatter the lens can be. This means that modern lenses made from materials with a high refractive index are thinner, making them lighter for people to wear.

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Other applications of light

The properties of light mean that it has been used in many modern-day applications at home, in industry, and in the field of medicine. Lasers are used in applications ranging from cutting materials to laser eye surgery. Optical fibres can carry large amounts of information for use in telecommunications, and they allow detailed images from endoscopes.

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Other applications of light 2

Lasers

Lasers can be used as an energy source for many applications:

  • cutting through materials (such as metal)

  • burning (eg laser engraving)

  • cauterising

Cauterising

In medicine, lasers are used to destroy damaged tissue or to stop bleeding, eg after surgery. This is known as cauterising.Lasers are used in eye surgery to repair damaged retinas and to correct vision defects. A laser can be used to precisely cut the cornea, altering its shape to correct a person’s vision.

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Other applications of light 3

Total internal reflection

Light going from a dense medium, such as glass, into a less dense medium, such as air, speeds up at the boundary. This causes light rays to bend when they pass from glass to air at an angle other than 90º. This is refraction. However, not all of the light refracts - and a small amount reflects back into the glass.

As the angle of incidence increases, so does the angle of refraction. Beyond a certain angle, called the critical angle, all the waves reflect back into the glass and no refraction occurs. This is known as total internal reflection.

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Other applications of light 4

Optical fibres

An optical fibre is a thin rod of high-quality glass. Light getting in at one end undergoes repeated total internal reflection - even when the fibre is bent - and emerges at the other end.Optical fibres have become very important in high-speed communications, such as cable TV and high-speed broadband services.Information, in the form of pulses of light, is sent down bundles of optical fibres. Fibre optic cables are able to carry more signals than traditional copper cable telephone lines.

Endoscopes

Endoscopes also use optical fibres. A doctor can insert a bundle of optical fibres into the body. Some carry light into the body, and some carry light reflected off internal body surfaces back out. This allows the doctor to see the inside of the body clearly – and help them diagnose diseases like cancer, or see what they are doing during keyhole surgery.

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Other applications of Light 5

Total internal reflection and the critical angle – Higher tier

The critical angle is the angle above which total internal reflection occurs. It varies depending on the refractive index of the material - the lower the refractive index, the higher the critical angle

The critical angle of a material can be calculated using this equation:

refractive index = 1/sin c

where c is the critical angle in degrees, °

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