Medical Imaging Flashcards
What are xrays?
electromagnetic waves with a wavelength in the range 10^-8 -10^-13m
Which has greater energy, an x ray photon or a visible light photon
x ray
How are x-rays produced?
fast moving electrons decelerated by the atoms of a metal and the kinetic energy of the electrons are transformed into x ray photons
Describe the process of the formation of x ray photons:
- electrons fired through high pd
- travel from cathode to anode (cooled by water)
- 1% of kinetic energy of electrons transformed into x ray photons
What are the 4 attenuation mechanisms?
simple scatter, photoelectric effect, compton scattering, pair production
How does the simple scatter work?
x ray photons scattered elastically by an electron
How does the photoelectric effect occur?
x ray photon disappears and removes an electron from the atom
How does compton scattering work?
the x ray photon is scattered by an electron, energy decreases, then ejected from the atom
How does pair production work?
x ray photon disappears to produce an electron positron pair
What is the relationship between intensity of x rays and thickness of materials?
intensity of parallel beams of x rays decreases exponentially with the thickness of the material
Why are x rays used for medical imaging?
the attenuation coefficient of bone is greater than that of soft tissue
What is a contrast material?
a material ingested/ injected into a patient to improve visibility of soft tissues on xrays
What is the main attenuation mechanism used in hospitals?
photoelectric effect
What needs to be proportional for the photoelectric effect to work?
atomic number cubed and attenuation coefficient
Why are barium and iodine used as contrast materials?
they have larger attenuation coefficients than soft tissue
In what medical situations is barium used?
showing the digestive system in xrays
In what medical situations is iodine used?
showing circulatory problems
What is a CAT scan?
Computerised Axial Tomography: records cross sectional area scans
How does a CAT scan work?
- patient lies horizontally in gantry ring
- fan shape of x rays to get cross sectional image
- detectors move down patient to produce 2D and 3D image
What isotope is used in the gamma camera?
technetium 99m
What is injected into a patient in order to show specific cells when using the gamma camera?
technetium 99m
What is a collimator?
long thin cylindrical tubes made from lead
How can gamma photons hit the scintillator?
travelling along the axis of the collimator
What is a scintillator?
material that allows gamma photons to hit it and produce thousands of photons of visible light
What are photomultiplier tubes?
electrical devices that produce electrical pulses whenever a photon of visible light is incident on it
What do the computer and display of a gamma camera do?
pulses used to pin origin of the gamma photon and show the concentration of Tc99m
What does PET stand for?
Positron Emission Tomography
How do PET scans work?
use fluorodeoxyglucose which has radioactive atoms of fluorine 18, injected and gathers in tissues with high rates of respiration
What is a disadvantage of PET scans?
expensive
What can PET scans do that other scans can’t?
show functions of organs on site
How do PET scans work?
- use fluorodeoxyglucose which has radioactive atoms and gathers in tissues with high rates of respiration
- computers determine the exact point of annihilation of FDG positron and electron from the arrival times of photons at detectors and the speed of the photons
- image of the FDG concentration levels displayed
What is an ultrasound?
any sound wave with a frequency greater than 20kHz
What is an ultrasound transducer?
device used to generate and receive ultrasound
What is the piezoelectric effect?
when materials produce emf, or when pd applied, causing material to expand or compress
What are common piezoelectric materials?
lead zirconate titanate and polyvinylidene fluoride
How are ultrasounds produced?
piezoelectric material vibrates when high frequency AC are applied on opposite ends, the vibration in the air produces US
How does an A scan work?
ultrasound partially reflected in boundaries between soft tissues
How does a B scan work?
US transducer moved across patients, brightness of dot corresponds to the intensity of the pulse
Define acoustic impedance:
product of its density and the speed of ultrasound in a material
What does coupling gel do?
reduces reflection of ultrasound on skin as has a similar impedance to that of of skin (impedance matching/ acoustic matching)
What is directly proportional to the speed of blood?
change in frequency
What happens to low energy xrays?
absorbed by the skin
