Medical Physics
What is an X-Ray tube made up of?
- A Cathode
- An Anode
- A high voltage power supply
- A metal play
Give the role of the cathode in an x-ray tube
The cathode liberates e-'s from its surface via thermionic emission
Give the role of the anode in an x-ray tube
e-'s are emitted onto the anode (which is also known as a target metal). Made of tungsten and has a high melting point
Only 1% of e-'s KE are converted to x-rays where does the rest go.
It is converted to heat energy in the anode.
Why are the electrodes of the x-ray tube in a vaccum chamber
To prevent e-'s from colliding with air particles whilst travelling to the anode
Name 4 attenuation mechanisms
- Simple scattering
- Photoelectric effect
- Compton scattering
- Pair production
Explain the process of simple scattering
Where a low energy x-ray photon meets an e- in an atom causing it to be scattered without a change in energy
What is the photoelectric effect
Where an x-ray is absorbed by an inner shell e- causing it to be ejected from the atom as a photoelectron
What is compton scattering
Where an x-ray photon is deflected by an e- causing the wavelength of the photon to increase and the ejection of the e- from the atom at a high speed
What is pair production
Where an x-ray photon passes close to the nucleus of an atom causing the production of an electron-positron pair
Give the equation for the attenuation of X-Rays
I = I0e^-μx
Define 'contrast media'
A substance that is a good absorber of x-rays, which is used to image soft tissue
What causes a bigger attenuation coefficient
A larger atomic number
What two contrast media are used for organs and blood vessels
Iodine - for blood vessels
Barium sulfate - for the digestive system
Describe the process of a CAT scan
- The patient lies on an examination table that can slide in and out of large vertial ring
- There's an x-ray on one side and an array of detectors on the other
- The x-ray tube and detectors rotate around the gantry
- The x-ray tube produces a thin beam of x-rays, which is attenuated by different tissues
- Each time the tube does a 360 rotation, a 2d slice is formed
- These slices are turned into a 3d image by a computer software
Give advantages of CAT scans
- Images are more detailed
- Can distinguish between soft tissues of similar attenuation coefficients
Give disadvantages of CAT scans
- Takes longer than X-rays(10-30 mins)
- Patients receive a higher dose of radiation compared to an x-ray
- Patients have to remain as still as possible as slight movements can blur a slice
Define a 'Radioactive Tracer'
A radiactive substance that can be absorbed by tissue to see the function of an organ
Give 2 examples of radioactive isotopes in tracers
Technetium-99m
Flourine-18
What are the main componens in a gamma camera
- Collimnator
- Scintillator
- Photomultiplier
- Computer & display
What is the purpose of the collimator
To produce high resolution images. It absorbs gamma photons that aren't travelling parallel to it & only allows photons to the scintillator if they are parallel to the collimator
What is the purpose of the scintillator
Once photons reach the scintillator, e-s become excited & as they travel through the crystal, they excite more e-s. Once the e-s have de-excited, they emit energy in the form of light photons
What is the purpose of the photomultiplier
Once photons reach the photomultiplier, e-s are emitted from the photocathode. e-s accelerate through a dynodes which progressively have a higher pd. The energy gained by the e-s triggers the release of more e-s at each dynode, so a stronger signal is detected
Define 'Metastable'
Where the nucleus stays in a high energy state for long periods
Explain the process of a PET scan
- Flourine-18 is injected into the patient which emits positrons.
- The part of the body being studied is surrounded with gamma cameras
- Positron & e-s in the patient annihilate and emit a pair of gamma photons in opposite directions (conserves momentum)
- Each photon is detected at one of the opposite detectors & their arrival time is recorded
- The delay time between the gamma photons is used to find the location of annihilation (as speed of the photon is known)
Define 'Ultrasound'
A longitudinal wave which has a frequency above 20kHz
Why are high frequencies used in medical imaging
To detect smaller details in a scan (gives a higher resolution)
What is the piezoelectric effect
Where a pd is applied to a crystal causing it to expand and compress
How do transducers emit and receive ultrasound
Transmitting - A pd is applied to a piezoelectric crystal, which causes it to resonate and produce an intense ultrasound wave
Receiving - The piezoelectric crystal receives an ultrasound wave causing it to resonate and induce an EMF
Describe an A-Scan
Where a single transducer is used to determine the distance/depth of an internal structure
What does a B-Scan do & explain the process
Provides a 2D image by taking many scans in different locations.
- Transuducer is moved over patients skin
- Output on a computer is seen as a collection of dots
- Each dot shows the boundary between tissues
- The dots brightness is proportional to the intensity of the reflected wave
Define 'Acoustic Impedance' & give the formula
The product of the speed of sound in a substance and the density of a substance.
Z = ρc
Define the 'intensity reflection coefficient' & give the formula
The ratio of the intensity of the reflected soundwave to the incident soundwave
Ir/I0 = (Z2-Z1)²/(Z2+Z1)²
What is a coupling medium and why is it used
- A substance placed between the transducer and the body to reduce reflection
- This is so that more energy can be used to reflect internal components rather than wasting it on external components
Give 2 pieces of info an ultrasound gives about a boundary
- Depth of the boundary
- The amount of transmitted intesity received
Explain what doppler imaging is
A non-invasive technique used to measure bloodflow in an artery (blood has iron which is reflective)
Describe how doppler imaging works
- A transducer is placed at an angle to the arteries bloodflow.
- The ultrasound is then reflected by the bloodvessels
- Blood flowing towards the detector is reflected at a higher frequency, whereas blood flowing away from the detector is reflected at a lower frequency
- Δf = 2fvCosθ/c
Give advanteges of PET scans
Non-Invasive
Produces real-time images
Give disadvantages of PET scans
- It's expensive
- The patient is exposed to a small amount of activity