Module 6.5 - Medical Imaging Flashcards
Simple scattering
Incident x-ray photon -> scattered x-ray photon with same energy
No change in energy
No absorption of the photon
Photoelectric effect
Incident x-ray photon -> photoelectron
Maximum kinetic energy of emitted photoelectrons = incident photon energy
Measuring energy of emitted electrons is a method of detecting/measuring x-ray energy
Compton scattering
Incident x-ray photon -> scattered x-ray photon + electron ejected
Electron ejected with small fraction of kinetic energy from incident photon, photon with remaining kinetic energy emitted
Mass-energy conserved
Photon and electron scattered in different directions due to conservation of momentum
If photon is deflected at a large angle, it has lost more energy so has a longer wavelength
Pair production
Incident x-ray photon -> positron + electron
Beam of high frequency x-rays causes photon to interact with nucleus
Photon vanishes, spontaneously producing a positron and electron (pair production occurs)
Define attenuation
A gradual decrease in intensity
2 methods of producing x-rays
1 - fast-moving electrons decelerate rapidly when interacting with a metal target; as an electron interacts with electric field around the nucleus an x-ray photon is emitted; due to conservation of energy the kinetic energy lost by the electron is equal to energy of emitted photon
2 - fast-moving electron ejects inner/local electron in an atom of target metal; electron from higher energy level moves down to replace it; energy released as x-ray photon of specific wavelength corresponding to difference in energy levels
How are electrons accelerated and then slowed down in the production of x-rays?
Accelerated - from cathode to anode, high p.d., evacuated chamber/vacuum (no collisions with air particles)
Slowed down - metal target
What do the characteristic ‘spikes’ or lines on the graph of intensity against photon energy or wavelength show?
Due to electrons in metal target gaining energy from incoming electrons and moving up energy levels
Electrons then drop back down due to electrostatic attraction to nucleus and emit wave of energy
Types of x-ray beam
Point source
Collimated
Intensity of x-rays from a point source
Intensity decreases with distance according to inverse square law
I = P / A
Intensity of x-rays from a collimated source
Intensity does not change over distance
Intensity decreases with distance when x-rays pass through a substance
Exponential relationship between intensity and distance through a material
Define the attenuation coefficient
Constant used to calculate how the intensity of x-rays decreases as they pass through a material
Explain contrast media
Bones absorb more x-rays than soft tissues as they have a higher attenuation coefficient, so used for traditional x-ray scans
To see detail of soft tissues which have similar attenuation coefficients, a contrast meal such as barium or iodine is used as they have a higher attenuation coefficient
What does CAT scan stand for?
Computerised axial tomography
Explain CAT scans
Process using multiple x-ray scans to produce images of ‘slices’ through the body in one plane in order to build up a 3D image
X-ray source and stationary detectors around patient
Patient may have contrast medium (e.g. barium meal)
Thin, fan-shaped x-ray beam produced from point source
X-ray source rotates around patient, producing slices of a cross-sectional image (due to thin beam)
More absorption by dense materials/materials with high acoustic impedance
Process continues in a spiral as patient moves through machine
Computer analyses data, 3D image formed
Advantages of CAT scan over x-ray image
Accurate image of position of internal organs without them being obscured by other structures
Sensitive to changes in density so better contrast for different soft tissues
Digital technology allows image to be rotated to view patient’s body from different angles without having to take more images which would expose them to more potentially harmful x-rays
Technology allows areas with density of bone or air to be removed/made transparent
Define tracer
A radioactive substance that is either injected or ingested by a patient
What must be considered about medical tracers?
Gamma sources must be used (alpha and beta would be absorbed by body, not pass through to detectors and damage body due to higher ionising power)
Half-life must be long enough to carry out investigation but not long enough to cause damage to cells (needs time to be brought from manufacturing site at hospital)
Must not be toxic to humans
Components of a gamma camera
Collimator
Scintillator
Photomultiplier tubes
Computer and display
Collimator
Collimates gamma photons
Series of hundreds of narrow parallel tubes in block of lead
Ensures all emerging rays are parallel
Lead absorbs non-parallel gamma rays (creates sharper image)
Scintillator
Large cylindrical crystal of sodium iodide
Sodium iodide is a fluorescent material and scintillates when it absorbs a gamma photon (emits many photons of visible light)
Connected to several photomultiplier tubes arranged in a hexagonal pattern
Photomultiplier tubes
Photon hits start of tube and electron is released via photoelectric effect
As that photoelectron travels down tube, it interacts with the material so multiple electrons are emitted
Current is created by each incident photon
Therefore effect of single photon is amplified
Each tube is connected to the computer
Computer and display
Computer receives electrical signals which it uses to build up an overall image shown on the display
Technetium-99m
Most common isotope used for tracers in gamma cameras
Can diagnose diseases in thyroid, liver, brain, kidneys, lungs, spleen, heart and circulatory system
Used to study blood flow
Half-life of 6 hours
Chemical properties allow small quantity to be incorporated into other molecules
Not toxic
Gamma emitter
What does PET scans stand for?
Positron emission scanning
Explain PET scanning
Fluorodeoxyglucose (fluorine-18 added onto glucose) is injected into the patient
Beta-plus decay occurs so a positron is emitted from the nucleus
It is attracted to an electron, resulting in electron-positron annihilation which produces two gamma rays travelling in opposite directions (due to conservation of momentum)
These are detected by a ring detector which can pinpoint where the gamma rays came from
Glucose is used in all respiring cells so fluorodeoxyglucose is used in cells/tissues that respire more (e.g. heart or cancerous cells)
This allows a 3D image to be built up, highlighting any areas with high activity
Diagnostic purposes of PET scans
Study blood flow and metabolism
Detect problems with nervous system
Find changes in brain that may lead to epilepsy
Study and analyse certain cancers
Determine how advanced a cancer is or to help choose the best treatment for it
Compare PET scans and CAT scans
PET is more expensive
PET can take 2-4 hrs, CAT can take 30 mins
In PET dose of tracer is equal to exposure of standard x-ray at a hospital, in CAT doses are equal to 5 yrs of exposure to background radiation and have been linked to developments of cancers
PET show biological function of part of body, CAT shows detailed tissue and bone structure images
PET uses gamma, CAT uses x-rays
PET uses FDG, CAT uses iodine and barium
Define ultrasound
Longitudinal wave with a frequency greater than 20kHz
What is the piezoelectric effect?
The change in volume of a material when a p.d. is applied across its opposite faces
The production of an induced e.m.f. when certain crystals are placed under stress
Explain how the piezoelectric effect is used with ultrasound
Piezoelectric crystal used in ultrasound transducer which emits and detects ultrasound waves (as waves are sent out in pulses so if it’s constantly emitting the echoes wont be detected)
Ultrasound is produced at a frequency of around 1MHz
Crystal contracts when an alternating p.d. is applied across it
Crystal will oscillate at same frequency as alternating supply
A high frequency alternating p.d. will result in more oscillations and ultrasound waves will be emitted
Reverse process to detect ultrasound
Advantages of ultrasound scanning
No known dangers as ultrasound is not ionising radiation
Can obtain real-time images of soft tissue (e.g. heart)
Machines are inexpensive and portable
Examples of ultrasound scans
Prenatal scans - determine due date, structural abnormalities, baby’s sex
Echocardiogram - monitors heart function, real-time images
Uses of radiation in medicine
Diagnosis (PET, x-rays, CAT, tracers)
Treatment (radiotherapy, palliative care, fluoroscopy)
Sterilisation
Define piezoelectricity
The phenomenon of turning mechanical energy into electrical energy and visa versa
A-scan
Amplitude scan
Short pulse of ultrasound sent into body at same time as electron beam travels across screen of cathode ray oscilloscope (CRO)
Transducer receives reflected pulses causing vertical spikes on CRO screen
X-axis shows time taken for echo to be detected by transducer, can be used to work out depth/thickness of tissue
No photo but measurements can be taken to determine dimensions
B-scan
Brightness scan
Real time 2D/3D image built up from many echoes recorded from several transducers in an array or a transducer moved to different angles/positions around the patient
Greater amplitude of reflected pulse, brighter dot
Range of brightnesses shown where different bone, liquid and soft tissue reflect different proportions of ultrasound beam
Define acoustic impedance
Density of material x speed of sound through material
Define impedance matching
Reduction in intensity of reflected ultrasound at the boundary between two substances, achieved when the two substances have similar or identical acoustic impedances
Explain impedance matching
Gel must be used to couple transducer to skin due to very high value of the fraction of incident ultrasound waves that are reflected when ultrasound passes between two very different materials
Gel has similar acoustic impedance to skin
Explain how the Doppler effect can be used in medicine
For a moving object (blood) the reflected pulse will have a different frequency to the one that was emitted
Greater change of frequency mean greater speed
Lower frequency/positive difference means blood is flowing away from source
Higher frequency/negative difference means blood is flowing towards source
Can be used to measure speed of moving object (blood)
