P3 - Medical Applications Flashcards
Give two ways in which X-Rays are used in hospitals.
- To make images and CT scans.
- To destroy tumours at or near the body surface.
Why are X-rays dangerous?
X-rays damage living tissue when they pass through.
What is the absorption of X-rays like when passing through the body?
X-rays are absorbed more by bones and teeth (ie. dense materials) than softer tissues.
What is a CT scan?
CT scans distinguish between different types of soft tissue as well as between bone and soft tissue.
What are X-rays?
Electromagnetic waves with a short wavelength.
How are X-rays produced?
- A focused beam of electrons is made using a cathode.
- The electrons are fired at a high voltage to make them move quickly towards a positive metal plate.
- The high energy electrons excite the metal plate causing the release of an X-ray.
How are radiographs produced?
- X-rays are directed at the patient with a flat-panel detector on the other side.
- The X-rays pass through the designated area.
- X-rays pass through the soft tissue but are absorbed by bone.
- The parts of the detector hit by X-rays becomes darker producing a negative image.
How is a radiograph of an organ produced?
-The patient consumes a contrast medium such as barium which absorbs X-rays easily.
How are patients protected from the X-rays?
The parts not being scanned are covered by lead plates meaning X-rays only pass to the necessary area.
How are digital X-ray images produced?
- The flat-panel contains a CCD which is covered in a substance which converts X-rays into light.
- The light rays are converted into electrical signals which are sent to the computer.
How are X-rays dangerous?
- X-radiation ionises substances.
- High doses kill living cells/
- Low doses cause cell mutations and cancerous growth.
What is used to warn workers of X-rays?
Film badges show when the worker has been overexposed to radiation. A dosimeter.
How does a CT scanner work?
- The X-ray tube automatically moves around the ring.
- At each position, X-rays pass through the patient to the detector ring.
- Electronic signals from the detector are recorded by the computer so a 3D image can be produced.
Give three advantages of CT scanners.
- CT scanners can distinguish between bone and soft tissue.
- The image can distinguish between different types of soft tissue.
- CT scanners produce a 3D image.
Give two disadvantages of CT scanners.
- They give a high dose of ionising radiation.
- CT equipment is very expensive.
Give three advantages of X-ray machines.
- They can distinguish between bone and soft tissue.
- They give a lower dose of ionising radiation.
- They are relatively cheap.
What are ultrasound waves?
Sound waves above 20,000Hz.
What are ultrasound waves used for in medicine?
Ultrasonic scanning and destroying kidney stones.
Why can ultrasound waves be used to scan the body?
Ultrasound waves are partially reflected at a boundary between two different types of tissue.
Why is an ultrasound scan safer than X-ray?
An ultrasound scan is non-ionising.
What is a transducer?
The part of the ultrasound machine that produces and detects ultrasound waves.
What happens each pulse from the transducer?
- It is partially deflected at each boundary between tissues in its path.
- It returns to the transducer as a sequence of reflected pulses arriving at different times.
Give two advantages of using ultrasound.
- It is non-ionising.
- It can be used to scan organs and soft tissue.
What is an A-scan?
Measuring the length of an eyeball using ultrasound.
How does an A-scan happen?
- A transducer at the front of the eye sends ultrasound pulses into the eye.
- The reflected pulses are detected by a transducer and displayed on an oscilloscope.
How is the distance travelled by an ultrasound pulse.
speed of ultrasound waves in body tissue x transit time
How is the depth of a boundary below a surface calculated?
1/2 x speed of ultrasound wave x transit time
What is ultrasound therapy?
Breaking up kidney stones using an A-scan to determine their location.
What is the refractive index of a substance?
How much a substance can refract a light ray.
How is refractive index calculated?
n = sin(incidence) / sin(refraction)
What is the critical angle of a substance?
The angle of a light ray in a transparent substance that produces refraction along the boundary.
How is the critical angle related to the refractive index?
refractive index = 1 / sin(c) where c is the critical angle.
What is total internal reflection?
When the angle of incidence of a light ray is greater than the critical angle.
How are optical fibres used in medicine?
They are used to make endoscopes.
How do light and IR travel down an optical fibre?
At each boundary, the light is totally internally reflected.
What are the purposes of the two bundles of optical fibres in an endoscope?
- Shine light in the cavity.
- See internal surfaces of the cavity.
What shape is a converging lens?
Convex
What is a converging lens?
A lens which focuses parallel rays to the principal focus.
What shape is a diverging lens?
Concave
What is a diverging lens?
A lens which makes parallel rays spread out as if they came from a principal focus point.
What is a real image?
An image formed by a converging lens if the object is further away from the principal focus.
What is a virtual image?
An image formed by a diverging lens and, if the object is nearer to the lens than the principal focus, a converging lens
How is magnification calculated?
image height / object height
What are converging lenses used for?
- Magnifying glass
- Camera
What are diverging lenses used for?
Correcting short sight.
What is the focal length of a lens?
The distance from the centre of the lens to the principal focus.
What is the principal axis of a lens?
The straight line that passes along the normal.
What happens to a ray parallel to the principal axis having passed through a converging lens?
The ray is refracted through the principal focus.
What happens to a ray travelling through the centre of a converging lens when the lens surfaces are parallel?
The ray travels through without change of direction.
What happens to a ray which passes through the principal focus before a converging lens?
The ray is refracted to be parallel with the principal axis.
What happens to an image if the object distance is greater than twice the focal length?
The image is smaller.
When taking a picture of a distant object, how far must the film be from the lens?
The distance from the lens to the film must be equal to the focal length.
When taking a picture of a nearer object, how far must the film be from the lens?
The nearer an object, the greater the distance from the lens to the film.
What is the cornea?
The tough transparent layer of the eye through which light passes.
What is the purpose of the cornea?
- Protect the eye.
- Focus light onto the retina.
What is the retina?
A layer of light-sensitive cells at the back of the eye.
What controls the amount of light entering the eye?
The iris controls the size of the pupil.
What is the iris?
The coloured ring of muscle controls light intake.
What is the purpose of the eye lens?
To focus light onto the retina.
What is the vitreous humour?
The transparent jelly-like substance in the eye.
What is the purpose of the vitreous humour?
To support the back of the eye.
What is the purpose of the optic nerve?
To carry nerve impulses from the retina to the brain.
What is the purpose of the ciliary muscles?
Change the thickness of the eye lens.
What attaches the ciliary muscles to the eye lens?
Suspensory ligaments.
What is the aqueous humour?
The transparent watery liquid at the front of the eye.
What is the purpose of the aqueous humour?
To support the front of the eye.
what is the range of human vision?
25cm to infinity.
How is lens power defined?
1 / focal length in metres
What is the unit of power of a lens?
Dioptre
What value of power does a converging lens give?
A positive value.
What value of power does a diverging lens give?
A negative value.
What is short sight?
An eye that can only focus on close objects.
Where is the uncorrected image for short sight formed?
In front of the retina.
What can cause short sight?
- The eyeball is too long.
- The eye lens is too powerful (ie. not thin enough).
How is short sight corrected?
Placing a diverging lens of suitable focal length in front of the eye.
What is long sight?
An eye that cannot focus of nearby objects.
Where is the uncorrected image for long sight formed?
Behind the retina.
What causes long sight?
The eye lens cannot be made thick enough to focus an image.
How is long sight corrected?
Placing a converging lens of suitable focal length in front of the eye.
What type of lens does an eye have?
Variable focus converging lens.
What type of lens does a camera have?
Fixed focus converging lens.
How is the focus of the eye adjusted?
Ciliary muscles alter the lens thickness.
How is the focus of a camera adjusted?
Adjustment of lens position.
What is the image formed by an eye?
- Real
- Inverted
- Mag. less than 1
What is the image formed by a camera?
- Real
- Inverted
- Mag. less than 1
How is an image detected in an eye?
Light sensitive cells on the retina.
How is an image detected in a camera?
- Photographic film
- CCD sensor
How is brightness controlled by an eye?
The iris controls the width of the pupil.
How is brightness controlled by a camera?
Adjustment of aperture.
What does the focal length of a lens depend on?
- The refractive index of the material.
- The curvature of the two surfaces.
How does manufacturing of a lens differ with refractive index?
The greater the refractive index of the material, the flatter and thinner the lens can be.
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