The Eye Flashcards
What are X-rays?
Short wavelength electromagnetic waves
How is a radiograph made?
- X-rays are produced in an X-ray tube when fast moving electrons hit a target - They are directed at the patient and a lightproof cassette containing photographic film or a flat-panel detector is placed on the other side - X-rays pass through the body tissues but are absorbed by bones - The areas the X-rays reach become dark on the photographic film to create a negative image of the bones - Lead absorbers are placed where X-rays should not reach
How can a radiograph be made of an organ that contains soft tissue?
The soft tissue is filled with a contrast medium e.g. barium which easily absorbs X-rays to enable internal surfaces to be seen on the radiograph
What is a flat-panel detector?
A small screen containing a CCD with sensors covered by a layer of a substance that converts X-rays into light that then creates electronic signals in the sensors that are sent to a computer which displays the image
Why are X-rays and gamma rays dangerous?
- They ionise living cells - High doses can kill cells and small doses can mutate them - A film badge turns dark if exposed to radiation and is made of plastic
How does X-ray therapy work?
- X-rays are used to destroy cancerous tumours, with lead plates around other parts of the body - A gap between the plates allows X-rays to meet the tumour - X-rays for therapy are shorter in wavelength than those used for imaging
What is a CT scanner?
It produces a digital image of any cross-section through the body and can also be used to construct a three-dimensional image of an organ
How does a CT scanner work?
- The patient lies stationary on a bed that is in a ring of detectors - The X-ray tube automatically moved round the inside of the ring in small steps - The X-rays pass through the patient and reach the detector ring at each position - Electric signal from the detector are recorded by a computer until the tube has moved around the ring - The computer displays a digital image of the scanned area - Each detector receives X-rays that have travelled through different types of tissue - The detector signal depends on the different types of tissue along the X-ray path and how far the X-rays pass through each type of tissue
What is an A-scan and how does it work?
- Sight can be restored to a blind person if the lens is replaced with an artificial one but the surgeon needs to know how long the eye is so the lens will work properly - An A-scan is used and the transducer at the front of the eye sends ultrasound pulses into the eye which are reflected and detected by the transducer before they are displaced on an oscilloscope screen or on a computer monitor - We can measure the transit time of each pulse which is the time taken for the pulse to travel from the transmitter at the surface to and from the boundary that reflected it - distance travelled by the pulse = speed of ultrasound waves in body tissue x transit time - This must be halved to find the depth of the boundary below the surface because the pulse travels there and back
What are optical fibres and what are they used for?
Optical fibres are very thin glass fibres designed to transmit light or infrared radiation. We use them in medicine to see inside the body without cutting it open. Light rays can’t escape from the fibre because they are totally internally reflected each time they reach the fibre’s boundary
What does the refractive index equal in terms of the critical angle?
n=1/sin c
How does an endoscope work?
- An endoscope is used by a surgeon to see inside a body cavity like a stomach without cutting with body open - The endoscope is inserted through the patient’s throat and contains two bundles of optical fibres alongside each other - One of the bundles is used to shine a light over the cavity and the other to see the internal surfaces of the cavity - A tiny lens over the second bundle is used to form an image at the ends of the fibres in the bundle which can be seen directly or by using a camera at the other end of the fibre bundle - The endoscope can e used to observe a stomach ulcer or a bone fragment in a knee joint
How does laser eye surgery work?
- Laser light can be used as a source of energy in an endoscope to carry out some surgical procedures - It can cut away or burn away and destroy diseased tissue and seal off leaking blood vessels - This is possible because the energy can be focused on a very small area of a surface and the colour of the laser light can be matched to the type of tissue - Eye surgery on the retina can be carried out by applying the laser light through the pupil of the eye for a very short time - You should never look into or along a laser beam even after reflection because it will damage the retina and may cause permanent blindness, so special safety goggles should always be worn
How does a lens work?
By changing the direction of light passing through it
What is a converging (convex) lens?
- A converging (convex) lens makes parallel rays converse into a focus - The point where parallel rays are focused is called the principal focus (or focal point) - We use a converging lens as a magnifying glass and in a camera to form a clear image of a distant object
This is the eye.
Yes.
