P3.1 Medical Applications of Physics Part One Flashcards
What are X-rays?
High frequency, short wavelength electromagnetic waves
How big is the wavelength of a X-ray?
Roughly the same size as the diameter of an atom
Are X-rays ionising radiation? What does this mean?
Yes, this means they can damage or even kill living cells
What happens when X-rays are directed at a human body?
They pass through body tissue, and absorbed by denser materials like bones and metal
How do X-rays affect photographic film? What does this mean they can be used for?
The same way as light, meaning they can be used to take photographs in which body parts, like bones, can be seen
What can X-ray photographs be used for?
To diagnose medical conditions such as bone fractures or dental problems
How are X-ray images formed nowadays?
Electronically using charge-coupled devices (CCDs)
What are CCDs?
Silicon chips that are divided up into a grid of millions of identical pixels
How do CCDs form an image from X-rays? What is quality of these images?
They detect X-rays and produce electronic signals which are used to form high resolution images (the same technology in digital cameras)
Can X-rays be used to take photos of tissue? Why? How?
Yes, tissue can absorb small amounts of X-ray radiation, so Computerised axial tomography can be used to produce images of soft and hard body tissue
What does CT scans mean?
Computerised axial tomography
What is the quality of the images produced from CT scans?
They produce high-resolution images
How does the dosage of X-rays change from CT scans and normal X-ray photographs?
CT scan use a high dose of X-rays in order to distinguish between the tiny variations in tissue density
How are CT scans carried out?
A patient is put inside a cylindrical scanner, and an X-rays beam is fired through the body from an X-ray tube to a detectors
Where does a CT scan take place?
In a cylindrical scanner
What happens to the X-ray tubes and detectors in a CT scan?
They are rotating during the scan
How are the images formed from a CT scan?
A computer interprets the signals from the detectors to form an image of a 2D slice through your body
How are 3D images formed from a CT scan?
Multiple 2D CT scans can be put together to make a 3D image
What does the high quality images produced from a CT scan allow doctors to do? (3)
Diagnose things such tumours, organ disease, blood flow issues and bone fractures
Why are CT scans sometimes avoided?
Because they involve a lot more exposure to X-rays than taking an X-ray photograph
What can X-rays be used for besides photographs?
Radiotherapy
What is radiotherapy?
The treatment of cancers with X-rays
How is radiotherapy carried out? (3 steps)
- X-rays are focused on the tumour using a wide beam
- This beams is rotated round 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
What are the safety measure when using X-rays? (2)
The radiographers carrying out the radiotherapy wear lead aprons and stand behind lead screens, and the X-ray exposure of the patient is always kept at a minimum
What are the results of prolonged exposure to ionising radiation? (3)
Cancer, radiation sickness and death
Why do radiographers wear lead aprons?
Because lead absorbs X-rays
What is ultrasound?
Sound waves of a higher frequency than the upper limit of human hearing
What is the range of human hearing?
20 to 20,000 Hz
What is partial reflection?
When a wave passes from one medium into another, some of the wave reflects off the boundary between the two media and some is transmitted (and refracted)
What is meant by ‘transmitted’ in waves?
Passes through
What does partial reflection allow ultrasound to be used as?
Ultrasound imaging
How does ultrasound imaging work?
A pulse of an ultrasound is pointed at an object, wherever there are boundaries some of the ultrasound will be reflected - knowing how fast the ultrasound is travelling, then the time it takes for the reflections to reach a detector can be used to calculate how far away the boundary is
What does the ultrasound detector receive?
Partially reflected ultrasound
What can you use to measure the time between reflected pulses in an ultrasound?
You can use oscilloscope traces from an ultrasound detector
How do you find the time between pulses in an oscilloscope?
You multiply the number of divisions (squares) by the timebase
What formula do you use to work out the distance between boundaries in an ultrasound?
s = v x t ==> distance in m = speed in m/s x time in s
What are kidney stones?
Hard masses that can block the urinary tract
How does an ultrasound remove kidney stones?
An ultrasound beam can be used to concentrate high-energy waves at the kidney stone, turning it into sand like particles
What is the main use of an ultrasound?
Pre-natal scanning
Why are ultrasounds used in pre-natal scanning’s?
Because ultrasound waves are non-ionising so they are safe and will not harm the baby
What are the problems with X-ray photographs and CT scans?
X-rays are ionising so they can cause cancer if you’re exposed to too high a dose - CT scans use even more X-ray radiation so are even more harmful
Why are children more at risk from the effects of radiation than adults?
Because children are still growing and so are much more sensitive to cell damage from radiation
What produces the best image quality out of ultrasounds, X-ray photographs and CT scans?
CT scans, followed by X-ray photographs and then ultrasounds
What is the image quality of ultrasounds?
They are typically fuzzy, making harder to diagnose some conditions using these images
What is the image quality of X-ray photographs?
They produce clearer images (than ultrasounds) of bones and metal, but not much else
What is the image quality of a CT scan?
They produce detailed, high resolution, images and can be used to diagnose complicated illnesses - 3D images can also be made from CT scans
What is refraction?
When waves change direction as they enter a different medium
What is the angle of incidence?
The angle the incident (incoming) ray makes with the normal
What is the angle of refraction?
The angle the refracted ray makes with the normal
Why does refraction occur?
Due to the change in density from one medium to another - which changes the speed of the waves
What happens when light enters a more dense medium?
It slows down, and bends towards the normal
What happens when light enters a less dense medium?
It speeds up, and bends away fro the normal
When does light not refract?
When a wave hits a boundary 90° it will not change direction
When light hits a medium, will all the light pass through the medium?
No some will be reflected - it all depends on the angle of incidence
What is the refractive index of a medium?
The ratio of the speed of light in a vacuum to the speed of light in that medium
What types of materials have refractive indexes?
Transparent materials
What is the formula to find the refractive index of a material?
refractive index (n) = sin i / sin r ==> ‘i’ is the angle of incidence, and ‘r’ is the angle of refraction
What are the two types of lenses called?
Converging (convex) and diverging (concave)
What is the shape of a converging lens?
It bulges outwards
What does a converging lens do to parallel rays of light?
It causes them to move together (converge) at the principle focus
What is the axis of a lens?
A line passing through the middle of the lens (horizontal)
Where is the principal focus of a converging lens?
Where all the rays hitting the lens parallel to the axis all meet
What is the distance from the centre of the lens to the principal focus called?
The focal length
Where is the principal focus found on a convex lens?
On each side of the lens
What happens when an incident ray travels parallel to the axis and refracts through the converging lens?
The ray passes through the principal focus on the other side
What happens when an incident ray travels directly through the centre of a converging lens?
It carries on in the same direction after passing through the lens
What happens when an incident ray passes through the principal focus before passing through a converging lens?
The ray refracts through the lens and travels parallel to the axis
What is the shape of a diverging lens?
It is concave - it caves inwards
What does a concave lens do to parallel rays of light?
It causes them to spread out (diverge)
What is the principal focus of a diverging lens?
The point where rays hitting the lens parallel to the axis appear to all come from
How do you find the principal axis of a diverging lens?
By tracing the rays back until they all appear to meet
What is the focal length of a diverging lens?
The same as converging lenses - the distance between the centre of the lens and the principal focus
What happens when an incident ray parallel to the axis refracts through a diverging lens?
It travels in line with the near-side principal focus (it appears to have come from the principal focus)
What happens when an incident ray passes directly through the middle of a diverging lens?
It carries on in the same direction
What happens when an incident ray is passing through a diverging lens towards the far-side principle focus?
It refracts through the lens and travels parallel to the axis
