Medical Applications of Physics

Xrays are electromagnetic waves and are part of the electromagnetic spectrum. As with other electromagnetic waves, Xrays transfer energy and are transverse waves.

The properties of Xrays are- 

• They have a very short wavelength 
• They cause ionisation (adding or removing electrons in atoms or molecules)
• They affect photographic film in the same way as visable light (turning it black)
• They are absorbed (stopped) by metal and bone 
• They are transmitted (pass through) by healthy tissue

Photographic film-

Xrays are useful in medical imaging of bone fractures and dental problems. 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 onto the film and then it turns black where the xrays hit it so the film stays white where the bones are. Fractures show up as dark areas in the image of the bones.

CCD'S-

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.

The ionising properties of x-rays means they can damage the DNA cells. Low doses of xrays may cause cancer- whereas high doses kill cancer cells.

Cancer Treatment-

• To treat cancer.....  
  • The xrays are focused on the tumour using a wide beam.
  • The beam is rotated around the patient with the tumour at the centre
  • This minimises the exposure of normal cells to radiation and so reduces the chances of damaging the rest of the body
• This causes so much damage to the cancer cells that they die - This is radiotherapy..

Precautions when using x-rays-

• Patients are limited to a number of xrays they are allowed to have 
• Shielded walls containing led 
• Only specially trained staff (radiographers) are allowed to perform xrays 
• Wear lead aprons 

CCD'S-

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.

• The range of human hearing is 20Hz to 20,000Hz
• Ultrasound waves have a frequency of above 20,000Hz so humans cant hear ultrasound
• Ultrasound can be produced by some animals (bats and dolphins) and electronic devices

Reflections:

Ultrasound waves are partially reflected when they meet a boundry between two different media with different densitys

A detector placed near the source of the ultrasound waves is able to detect the reflected waves. The time taken for the reflections to reach a detector can be used to determine how far away such a boundary is.

The distance travelled by ultrasound can be worked out like....

s= v x t            s= distance in meters    v= speed in metres per second    t=time (seconds)

(REMEMBER IT ALWAYS TRAVELS THERE AND BACK SO YOU HAVE TO DIVIDE IT BY 2)

1 µs = 0.000001

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.

They are used widely in pre-natal scanning to check that a foetus is devloping normally. Computers can combine many ultrasound reflection readings to produce a detailed imagine for them. 

Removing kidney stones-

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

Is it safe?

• Ultrasound waves are non-ionising and safe
• Xrays are ionising, they can cause cancer and are not safe to use on developing babies
• CT scans use a lot more xray radiation so the patient is exposed to more ionising radiation. CT scans arent taken unless they are really needed.

Image Quality

• Ultrasound images are typically fuzzy - harder to diagnose some conditions
• Xrays produce clear imagines of bones at metal but nothign else
• CT scans produce detailed images and can be used to diagnose complicated illnesses because of the high resolution images 

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. The change in density changes the speed of the waves (slow down)

• The normal line is an imaginary line that is 90 degrees to the surface and where the light hits it.
• The angle of incidence in the angle in which the light ray hits the prism compared to the normal line.  
• The angle of refraction is the angle at which the light ray inside the prism compared to the normal line.

refractive index = ^{sin i}/_{sin r}

A lens is a transparent block that causes light to refract to form an image.

The two types of lenses are - Converging and Diverging.

Converging Lens-

The converging lens has two parralel light rays, when they hit the convex lens they change direciton and the light rays will refract and then the two rays will meet at the principal vocus.

The focal length is the distance between the principal vocus and the lens.

Formation of an image-

When you form an image from an object the lines wont meet, so to find out where they would meet if you carried it on, you carry it on on the other side. where the two lines meet is the image. - The image will always be bigger than the object (therefore is how a magnifying glass works)

• The characteristics of the magnifying glass image are - Virtual, Upright and Magnifyed 
• magnification= image height divided by object height 

When the paralllel light rays hit the lens the light rays spread apart *they diverge)

You find the principal vocus by imagining the light rays carry on behind the diverging lens.

THE OBJECT ON RAY DIAGRAMS ARE ALWAYS SHOWN AS AN ARROW.

The imaged formed is where the rays meet. (Watch mysciencegcse)

Ray diagrams help us to work out the nature of the image produced...

• Magnified or dimished (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 

so.... If the image is smaller than the object it would be diminished, If it pointed in the same direction as the object it would be upright and it if was on the same side of the lens as the object it would be virtual.

• Cornea - Refracts light as it enters the eye 
• Iris - Controls how much light enters the pupil 
• Pupil - Allows light to pass through as it enters the eye 
• Lens - Refracts light to focus it onto the retina - The amount of refraction is changed by altering thickness and curvature of the lens 
• Ciliary Muscles - Adjust shape of lens to make it more or less curved 
• Suspensory ligaments - Slackens or stretches as the ciliary muscles contract or relax to adjust lens thickness and curvature 
• Retina - Contains light receptors which trigger electrical impulses to be sent to brain when light is detected

Accomodation - The eye can alter the shape and curvature of the lens to adjust the degree of refraction.

If the object was NEAR the ciliary muscles would contract and the suspensory ligaments would slacken, the muscle tension on the lens would be low and the lens would be fat and more curved

If the object was FAR the ciliary muslces would be relaxed, the suspensory ligaments would be stretched, muslce tension on the lens would be high and the lens would be thin and less curved

The near point is the closest an object can be from the eye without the image being blurred. A normal persons near point is about 25cm 

• Someone with long sight can see distant objects clearly but their point is further than 25cm 

The far point is the furthest an object can be from the eye without the image being blurred. A person with normal vision can focus on distant objects at infinity 

• Someone with short sight can see near objects clearly but their far point is closer than infinity

Cameras are devices that focus light onto a photosensitive surface using aconverging lens. They have some similarities to the eye. Same as the eye the camera image is diminshed, inverted and real.

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.

Short Sight can be caused by....

• The eyeball being elongated - The distance between the lens and the retina is too great
• The lens being thick and curved - the light is focused infront of the retina.

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

Long sight is caused by:

• The eyeball being too short - The distance between the lens and retina is too small
• A loss of elasticity in the lens - It cannot become fat enough to vocus (often age-related)

As a result, the lens focuses light behind the retina instead of onto it.

Long-sightedness can be corrected by putting a converging lens infront of the eye.

The power of a lens = 1 divided by the focal length (UNIT FOR POWER = DIOPTRES = D

                                                                          UNIT FOR FOCAL LENGTH = METRES = M)

Convex lens - Answer will always be positive 

Concave lens - Answer will always nbe negative 

Focal Length 

The focal length can be effected by:

• The curvature of the lens - The curvature can change if you have a thicker lens  (Thicker lens = shorter principle focus)
• Refractive Index - Different materials that have different refractive index (low refractive index/high)
• Also, you could have a surface with a higher refractive index but make it have the same focal length as one with a lower refractive index by making it thinner (useful for if you want to have thinner glass)

• Lasers have a low divergence - spreads out very little
• Can be  used for cutting, caustering and burning 
• Caustering = destory damaged tissue and stop bleeding in surgery 

Total Internal Reflection

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 (reflection would occur) This is known as total internal reflection. 

Critical angle = where the reflected ray disapears 

Refractive index = 1 divided by sin c 

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. In endoscopes doctors send light down into peoples bodys to be able to see