P3.1 Medical Physics

X-Rays are from the higher frequency range of the electromagnetic spectrum. This means they have higher energy, and are therefore 'ionising radiation'.

This means there is a risk of damage to living issue.

However, because X-Rays are absorbed in different amounts by different types of tissues, we can use them to bserve internal organs.

X-Rays are absorbed most by bones and teeth, and less by other tissues e.g. fat.

Basic X-Ray photograph only shows bones and teeth and metal implants; they do not distinguish other types of tissue very well.

Computer Aided Tomography(CT Scan) scans also use X-Rays. However, with move sophisticated detectors and analysis, they can provide much more detail and so distinguish between different types of tissue, e.g. muscle, fat, internal organs etc. They can also be used to provide 3D scans rather than 2D.

However, CT scans use a much higher dose of X-Rays. Darkens photographic film.

Ultrasound is a sound wave above human hearing i.e. above 20,000Hz.

Ultrasound is used because it is not ionising. However, Ultrasound doesn't produce as clear an image as X-Rays. Ultrasound also can't penetrate very far into tissue but the images produced as live.

Ultrasound waves are reflected when they reach a boundary between different tissues, i.e. where density changes.

The Ultrasound equipment measures two things:

• The time between the original sound wave and it's reflection
• The amount of sound reflected. This tells us how large the object/boundary is. And how much the density changed at the boundary.

We can also use ultrasound for crack detection.

Ultrasound can be used medically to treat issues such as kidney stones or gall stones. The sound waves are focussed on the kidney stones at much higher amplitudes than for scanning. The pressure from the waves break up the stone.

Refraction

Takes place when light (or any waves) crosses a boundary between 2 materials.

In order to calculate how much the ray bends, we need to know something about the material each side of the boundary.

In physics, we give each transparent material a refractive index. The larger the refractive index the greater it refracts light. Air has a refractive index of 1.

We calculate refractive index as:

At certain angle of incidence, a ray of light will be reflected rather than refracted. 

When light hits the boundary between glass/water and air it is usually refracted out but as the angle of incidence is increased an angle is reached where the light emerged along the boundary - the angle of refraction is 90°.

The angle of incidence in the transparent material when this happens is called the critical angle (only happens more to less dense).

The critical angle for glass is 42°.

For angles greater than the critical angle ALL the light is reflected back- this is known as TOTAL INTERNAL REFLECTION. Total internal reflection only happens when the light is travelling from more to less dense. We calculate the critical angle using:

Uses of total internal reflection:

• Optical Fibres

Converging Lens

Converging lenses bring the rays of light together as they pass through the lens.

Diverging Lens

Diverging lenses spread out the rays of light as they pass through the lens

The focal point is the point at which a lens will converge from a distant object, i.e. parallel rays of light. The focal length is measured from the centre of the lens to the focal point.

In order to create a clear image we can change the distance between lens and the sensor/film.

For a magnifiying glass, the object is inside the focal length and creates an upright, magnified, virtual image on the same side of the lens as the object.

Magnification

The magnification of a lens is given by:

Lens

... is a measure of how much a lens refracts light passing through it. This will depend on two factors:

• refractive index (bigger r.i. = bigger power)
• the curvature of the lens (larger curve = bigger power)

Power of a lens

Unit of lens power is the Dioptre.

In order to create a focused image of the outside world on the retina at the back of the eye, the ciliary muscle around the lens changes the lens' shape. The closer an object is to the eye the more the lens needs to refract the light so the lens thickens.

Without a lens - Eye lens does not refract light enough

With a lens - Converging lens used to assist eye

Near-Point - is the closest point at which the eye can produce a clear image on the retina

Without Lens - Eye refracts light too much

With Lens - Diverging lens used to assist eye

Far-Point - furthest point from the object at which the eye produces a clear image on the retina

As we age, the eye lens becomes less flexible so the near point gets further away (and the far point get nearer).