6.5 Medical Imaging Flashcards
Describe the structure of an x ray tube and explain how it works
-Evacuated tube containing 2 electrodes
-Tube evacuated so electrons don’t collide with gas particles
-External p.d applied
-Cathode is a heater and produces electrons by thermionic emission
-Electrons accelerated towards anode
-Anode made from a metal with high m.p (tungsten)
How are x ray photons produced from x ray tubes
-Produced when electrons are decelerated by hitting the anode
-Energy output of photons is less than 1% of kinetic energy of the incident electrons
-Remaining energy is transferred into thermal energy
How can you reduce the heat on the anode
-Circulation of oil to cool anode
-Anode rotates to spread heat over larger surface area
Define attenuation
Decrease in intensity of an electromagnetic radiation as it passes through matter
What are the 4 types of attenuation mechanisms in order of lowest energy to highest
-Simple scattering (1-20keV)
-Photoelectric effect (30-100kV)
-Compton scattering (0.5-5MeV
-Pair production (energy >/= 1.02MeV)
Describe simple scattering
X ray photon is scattered elastically by an electron
Describe the photoelectric effect
X ray photon absorbed by one of the electrons and electron removed from the atom
Useful for x ray images in hospital
Describe Compton scattering
X ray photon scattered by electron, energy of photon reduced and electron removed from atom
Describe pair production
X ray photon interacts with nucleus of atom to produce an electron-positron pair
Electron and positron produced
What factors affect the intensity of transmitted x rays
-Thickness of material
-Photon energy
-Type of absorber
How are intensity and thickness of a substance linked
Intensity is inversely proportional to thickness
Why is a contrast medium used in medical imaging and give examples of some
Soft tissues have low absorption coefficients so contrast medium uses to improve visibility of their internal structures
Iodine and barium as they are harmless to humans
What is the relationship between attenuation coefficient and atomic number
Attenuation coefficient is directly proportional to the atomic number cubed
Why are iodine and barium used in medical imaging
They have high atomic number which means they are much more absorbent than soft tissues
Where in the body is iodine used for as a contrast medium
In liquids so in blood flow
Helps identify blockages and structure of organs such as the heart
What is the therapeutic use of x rays
Used to kill cancer cells
Describe the structure of a CAT scan and how it works
-X ray tube produces a thin fan shaped beam of x rays
-Detectors on opposite side of ring
-Intensity of beam transmitted through tissues recorded by detectors
-Injected with contrast medium
-Tube and detectors rotate around the ring
-Patient moves through the ring so x ray beam follows a spiral path
-Software manipulates 2D slices to create 3D image
Advantages and disadvantages of CAT scan
A :
-Produces 3D image
-Helps doctors asses shape,size and position of disorders such as tumours
-Can distinguish between soft tissues of similar attenuation coefficient
D :
-Expose patients to radiation doses equivalent to several years of background radiation which can be harmful
-Patient must sit very still for a long period of time
Which radiation is the least ionising
Gamma radiation
Radioisotopes chosen for medical imaging must have…
-Short half life to ensure high activity
Benefit of using radioisotope with short half life
Patient is not subjected to a high dosage and the radiation will not continue long after the procedure
What are some common radioisotopes used
Fluorine-18
Tc-99m
What is a radio pharmaceutical
A chemical which targets a specific organ or tumour
Radioisotope combined with elements which target specific tissues
What does a gamma camera do
Detects gamma photons from nuclei injected into a patient
Used to show functions and processes in the body
How is Tc-99m produced
Beta minus decay from Mo-99
Describe how a gamma camera works
-Gamma photons travel towards collimator
-Photons arriving at angle absorbed
-Photons travelling along the axis of the collimator reach the scintillator
-Gamma photon produces thousand of visible light photons
-Visible light photons travel into photomultiplier where each photon is converted into a pulse
-Output connected to computer
-Electrical signals processed to locate the gamma photon sites
-Software constructs an high quality image that shows the concentration of gamma photons within the body
How can the quality of images be improved
Using longer or thinner collimator tubes
What material is used in the scintillator
Sodium iodide
What is the difference between an x ray and gamma camera
X ray shows anatomy whereas gamma camera shows functions
How does a photomultiplier work
Visible light photon hits plate and electron emitted
Accelerated by dynode
Electron emission increases exponentially
Electrical pulse detected
How is fluorine-18 made
Made on site with a particle accelerator
Collide protons with oxygen-18
What is an advantage of using FDG
Body treats it as normal glucose so high concentration in areas of high respiration rate
Describe how a PET scanner works
-Patient lays on table surrounded by a ring of gamma detectors
-Each detector consists of sodium iodide scintillate and a photomultiplier tube and produces a voltage pulse for every gamma photon detected
-Diametrically opposed detectors connected to high speed computer
-Inject patient with FDG
-Positron produced when fluorine-18 decays annihilates an electron and produces a pair of back to back gamma photons
-Computer loops for a pair of gamma photons at diametrically opposed detectors
-Tiny differences in detection time, used to determine decay location
Advantages and disadvantages of PET scanner
A:
-Provide info on how parts of the body are functioning and reduces the need of surgery
D:
-Expensive
-Can only be located where FDG is produced on site
What is ultrasound
Any sound wave with frequency above 20kHz
What are benefits of ultrasound
Non incase
Non ionising
Quick
What is the frequency range of ultrasound used for medical imaging
1MHz - 15MHz
Ultrasound is ………. travelling through substances and ………… at boundaries. It is ………….. by small structures and apertures.
Refracted
Reflected
Diffracted
What is the wavelength of ultrasound in the body
Less than 1mm
What is an ultrasound transducer
Device to generate and detect ultrasound
Describe the piezoelectric effect
Some materials produce an emf when they are compressed, stretched, twisted or distorted
Explain how an ultrasound transducer works
-Apply a high alternating p.d across the faces of the quartz Crystal with same frequency as the natural frequency so the crystal resonates and produces an intense signal
-Transducer produces pulses of ultrasound typically around 5000 per second
-Same transducer detects returning pulses
-Reflected pulses makes crystal vibrate
-Piezoelectric effect creates an alternating emf
Describe how an A scan works
-Single transducer records along a straight line
-Each pulse partially reflected and partially transmitted at boundaries between separate tissues
-Reflected pulse detected at transducer will have less energy
-The pulse voltage at the ultrasound transducer displayed on an oscilloscope screen or computer screen
-Total distance travelled by pulse is 2L
-L can be calculated if average speed of ultrasound in the medium is known
Describe how a B scan works
-Transducer moved over patients skin
-Output of transducer connected to high speed computer
-For each position, a row of dots is produced on the screen, the dots correspond to the boundaries between 2 tissues
-Brightness of dots is proportional to intensity of the reflected ultrasound pulse
-Collection of dots corresponds to the different positions of the transducer on patient
-2D image produced
What happens to ultrasound at a boundary
A proportion of its intensity will be reflected and the remainder will be refracted
Define acoustic impedance and give SI units
The product of the density of a substance and the speed of the ultrasound in that substance
KgM-2S-1
Why is coupling gel used when using an ultrasound transducer
Without the gel there would be air pockets between the transducer and the skin. This would cause majority of the ultrasound to be reflected. The gel and skin have similar acoustic impedance so negligible reflection occurs
What is the Doppler effect in ultrasound
Uses the reflection of ultrasound from blood cells to help doctors evaluate blood flow through major arteries and veins. Blood flowing towards transducer will increase the frequency and blood flowing away will decrease the frequency
When will frequency of an ultrasound change
When it reflects off something moving. Frequency remains the same when it reflects off of something stationary
Why is the transducer never held at 90 degrees
What is the typical angle it is held at
cos90 = 0
No blood flowing in that direction
60 degrees
