Medical physics Flashcards
How are x-rays produced in an x-ray tube?
Electrons are accelerated by a high potential difference between cathode and anode to a high velocity - fast deceleration as x-ray hits very heavy metal - transfers kinetic energy to x-rays and heat.
What is compton scattering?
- Incident photon collides with electron around nucleus
- Electron is ejected and photon scattered, having lower energy.
- kinetic energy lost by incident photon is converted into a lower wavelength photon.
What is simple scattering?
When X-ray photon is scattered when colliding with electron, same energy before and after.
What is pair production?
X-ray photon interacts with nucleus of an atom and electron and positron are produced.
What is attenuation?
Reduction of intensity of a wave passing through medium.
Why are higher energy x-rays safer than lower energy x-rays?
They are less ionising.
How do image intensifiers improve the image quality of an x-ray image?
- X-ray photons are directed into a scintillator, emits light.
- Light directed to photocathode, photoelectric effect emits electrons.
- Electrons strike phosphorus screen, producing brighter image.
Why is a contrast material used?
- Contrast material absorb x-rays due to higher attenuation coefficient. Distinguishes outline of soft tissues.
How do CAT SCANS produce a 3D image? Advantages and disadvantages.
- X-ray source and detectors are moved in a spiral up and down the patient, creating thin cross section
- Denser material like bone absorb more photons, opposite for low density soft tissue.
- Levels of attenuation differ, attenuated via photoelectrons.
- Compute analyses the data and forms 3D image by identifying different levels of attenuation.
Advantages:
* 3D images
* less time consuming
Disadvantges:
* Ionising
* Expensive
How do Gamma camera’s produce an image?
- Gamma emitting isotope ingested
- Isotope enters blood stream and into blocked organ.
- Gamma passes through collimator, absorbs photons at an angle. Photons along axes of collimator pass through.
- Gamma rays hit scintillator, emits many visible light photons. Incident light photons hit photomultiplier which produces current via photoelectric effect.
- Computer creates image from electrical pulses.
- Organs have higher amount of gamma detection are blocked.
How do PET scans work?
- Beta plus emitting tracer injected into body.
- Organ specific tracer emits positron which annihilate electron within patiet.
- Produces gamma photons which travel n opposite directions.
- Patient is surrounded by ring of gamma detectors, arrival times indicates location
- 3D-image is created by computers connected to detectors.
- Takes 2-4 hours
What is the piezoelectric effect?
- Alternating voltage connected to quartz crystal causes quartz crystal to expand and contract at resonance frequency. This frequency is of ultrasound.
Why is ultrasound emmitted in pulses?
To be able to distinguish which reflection comes from which emission.
What is the frequency of medical ultrasounds?
1Mhz
How do A-scan ultrasounds work?
- Short pulse of ultrasound sent through body.
- Transducer receive reflected pulses, causes vertical spikes on CRO screen.
- X-axis shows time taken to detect the reflected waves and can be used to calculate distances.
