Medical Physics Flashcards
How are x-ray photons produced
When fast-moving electrons are decelerated by interaction with atoms of a metal
The kinetic energy of the electrons is transformed into X ray photons
What does an X-Ray consist of
An evacuated tube containing two electrodes - the tube is evacuated so that electrons pass through the tube without interacting with gas atoms
An external power supply is used to create a large p.d between these electrodes
The cathode is a heater - produces electrons by thermionic emissions
These electrons are accelerated towards the anode
The anode is made from a metal - known as the target metal - that has a high melting point
When are x-ray photons produced
When the electrons are decelerated by hitting the anode
What is the energy output of X-Rays
Less than 1% of the Kinetic Energy of the incident electrons
The remainder of the energy is transferred into thermal energy of the anode - oil is circulated to cool the anode or the anode is rotated to spread the heat over a large surface area
For what reason is the anode shaped
Anode is shaped so that the X-rays are emitted in the desired direction through a window
Why is the X-Ray tube lined with lead
To shield the radiographer from any X-rays emitted in other directions
What happens when one electron releases one X-ray photon
The maximum energy of a photon from an x-ray tube must equal the maximum kinetic energy of a single electron due to the conservation of energy
X ray photon energy and shortest wavelength calculations
hf = eV
hc/lambda = eV
lambda = hc/eV
What is the wavelength from an X-ray tube inversely proportional to
The accelerating potential difference
What does increasing the tube current do
Increase the intensity of the X-rays
What is the term attenuation used to describe
The decrease in the intensity of EM radiation as it passes through matter
What happens to the intensity of parallel beams of X-rays as they pass through matter
They will decrease
How many attenuation mechanisms are there by which X-ray photons interact with atoms and what do they do
4
Reduces the intensity of the collimated beam in the original direction of travel
What is Simple Scatter
Mechanism important for X-Ray photons with energy in the range 1-20 keV
X-ray photon interacts with an electron in the atom - but has less energy than the energy required to remove the electron - X-ray photon simply bounces off without any change to its energy
Insignificant for hospital radiography
Photoelectric Effect as an Attenuation Mechanism
Significant for X-ray photons with energy less than 100 keV
X-ray photon is absorbed by one of the electrons in the atom
Electron uses this energy to escape from the atom
Attenuation of X-rays by this type of mechanism is dominant when an X-ray image is taken
What supplies do X-ray machines typically use
30-100 kV
When is the Compton Scattering mechanism significant
Significant for X-ray photons with energy in the range 0.5-5.0 MeV
What is the Compton Scattering mechanism
Significant for X-ray photons with energy in the range of 0.5-5.0 MeV
Incoming X-ray photon interacts with an electron within the atom
Electron is ejected from the atom - but the X-ray photon does not disappear completely - instead is scattered with reduced energy
Both energy and momentum are conserved
What is the Pair production mechanism
Only occurs when X-ray photons have energy equal to or greater than 1.02 MeV
An X-ray photon interacts with the nucleus of an atom
It disappears and the electromagnetic energy of the photon is used to create an electron and its antiparticle - a positron
What does the transmitted intensity of X-rays depend on
The energy of the photons and the thickness and type of the substance
What happens to the intensity of a given substance and the energy of photons
The intensity falls exponentially with the thickness of the substance
The transmitted intensity is given by the equation I = (I0) x e^-(mu)x
I0 = Initial intensity before any absorption
x = Thickness of the substance
mu = attenuation coefficient / absorption coefficient (m^-1)
When are contrast mediums used
To improve the visibility of materials with low absorption coefficienst
What are the two most commonly used contrast mediums
Iodine and Barium
What is the predominant interaction mechanism for Barium and Iodine and why
Photoelectric Effect - they have a large atomic number and the attenuation coefficient is proportional to the cube of the atomic number
When is Iodine used as a contrast medium
In liquids
When is Barium Sulfate used as a contrast medium
To image digestive systems
How can X-rays be used for therapy instead of imaging
Specialised X-ray machines called linacs (linear accelerators) are used to create high energy X-ray photons
These photons are used to kill of cancerous cells and do this through Compton Scattering and Pair Production
What does CAT stand for
Computerised Axial Tomography
Axial refers to the images taken in the axial plane
What does a CAT Scanner record
A large number of X-ray images from different angles and assembles them into a 3D image with the help of sophisticated software
How do the scanning process and analysis for a CAT scan work
Controlled by a graph
How does a CAT scan work
The patient lies on their back on a horizontal examination table that can slide in and out of a large vertical ring or gantry
Gantry houses an X-ray tube on one side and an array of electronic X-ray detectors on the opposite side
X-ray tube and the detectors opposite it rotate around within the gantry
X-ray tube produces a fan-shaped beam of X-rays that is typically only 1-10mm thick. Thin beam irradiates a thin slice of the patient, and the X-rays are attenuated by different amounts by different tissues. Intensity of the transmitted X-rays is recorded by the detectors - which send electrical signals to a computer
Each time the X-ray tube and detectors make a 360 degrees rotation - a 2D image or slice is acquired - by the time the X-ray tube has made one complete revolution the table has moved about 1cm through the ring
In the next revolution - the X-ray beam irradiates the next slice through the patients body - so the X-ray beam follows a spiral path during the 10-30 minute scan
Advantages and Disadvanatges of CAT scans
Advantages:
CAT scans can be used to create a 3D image which helps doctors when analysing
CAT scans can distinguish between soft tissues of similar attenuation coefficients
Disadvantages:
High exposure to radiation
Have to remain very still as any movement blurs the slice
What are radioactive isotopes used for in medicine
Diagnosis and therapy
With diagnosis doctors try to find out what is wrong with the patient
With therapy they attempt to cure the patient using ionising radiation
How an tumours be targeted with ionising radiation
When tumours can be targeted by gamma radiation or high energy X rays from outside the patient or through brachytherapy
Brachytherapy is when a radioactive source is implanted in or next to the tumour inside the patient
What is the gamma camera
A diagnostic tool where a detector of gamma photons emitted from radioactive nuclei injected into the patient
Why are gamma emitting sources ideal for medical imahing
Radioisotopes have to be placed inside the patient and their radiation detected from the outside
Gamma emitting sources are the least ionising and can be penetrate through the patient
Why must radioisotopes chosen for medical images have a short half lfie
Ensure high activity from the source so that only a small amount is required to form the image
Patient is not subjected to a high dosage of radiation that continues long after the procedure
What is Fluorine-18
Radioisotope used in PET scans that has a short half life and has to be produced on site at the hospital using a particle accelerator
What is Technetium-99m
An extremely versatile radioisotope that can be used to monitor the function of major organs
What is Technetium-99m
An extremely versatile radioisotope that can be used to monitor the function of major organs
m stands for metastable - the nucleus stays in a high energy state, with more energy that the stable nucleus for a longer period than expected
How is Technetirum-99m produced
Natural radioactive decay of molybdenum-99
How does molybdenum-99 decay
Beta minus emission with a half life of 67 hours
How does the Tc-99m isotope lose energy
Emits a gamma photon with energy 140 keV with a half life of about 6 hours
Stable Tc-00 remains - has a half life of 210,000 years
What is done to ensure the radioisotope reaches the correct organ or tumour
Radioisotope has to be chemically combined with elements that will target the desired tissues to make a radiopharmaceutical - also known as a medical tracer
How are irregularities identified in the function of the body
Through the concentration of the radiopharmaceuticals
What does a gamma camera detect
Gamma photons emitted from the medical tracer injected into the patient and an image is constructed indicating the concentration of the tracer within the patient’s body
How does a gamma camera detect
Gamma photons travel towards the collimator
Any photons arriving at an angle to the axis of the tubes are absorbed by the tubes - only those travelling along the axis of tubes reach the scintillator
A single gamma photon striking the scintillator produces thousands of photons of visible light
Photons of visible light travel through the light guide into the photomultiplier tubes
These tubes are arranged in a hexagonal pattern
A single photon of light entering a photomultiplier tube is converted into an electrical pulse
Outputs of all the photomultiplier tubes are connected to a computer
Software can process the electrical signals of the tubes very quickly to locate the impacts of the gamma photons on the scintillator
These impact positions are used to construct a high-quality image that shows the concentrations of the medical tracer within the patient’s body
What is a collimator
A honeycomb of long, thin tubes made from lead
What is often the scintillator material
Sodium iodide
What is the chance of a gamma photon interacting with the scinitillator
About 10%
How does a gamma camera differ from an X-ray imaging technique
It produces an image that shows the function and processes of the body rather than its anatomy
What does PET stand for
Positron emission tomography
What does PET stand for
Positron emission tomography
What does PET stand for
Positron emission tomography
What is used in PET scans and why
Fluorine-18
It is a positron emitter with a half-life of about 110 minutes
What does Fluorine-18 decay into
A nucleus of oxygen-18, a positron, a neutrino and a gamma photon
How can Fluorine-18 be made through collision
High speed protons collide with oxygen-18 nuclei and produce fluorine-18 nuclei and neutrons
What medical tracer do most PET scanners use
FDG (Fluorodeoxyglucose)
Advantages of using FDG
Similar to naturally occurring glucose but is tagged with a radioactive fluorine-18 atom in place of one oxygen atom - our bodies treat it like normal glucose and it accumulates in tissues with a high rate of respiration
What is used to detect activity from FDG
Using gamma detectors
What is another medical tracer used for PET scanning
Carbon monoxide using the carbon-11 isotope
How does carbon monoxide act as a medical tracer
Emits as a positron and has a half-life of about 20 minutes
Very good at clinging onto haemoglobin molecules in the red blood cells, so it can be transported through the body and the concentrations of carbon monoxide can be monitored in a PET scan
PET scanner process
The patient lies on a horizontal table and is surrounded by a ring of gamma detectors
Each detector consists of a photomultiplier tube and a sodium iodide scintillator and produces a voltage pulse or signal for every gamma photon incident at its scintillator
The patient is injected with FDG - the PET scanner detects the gamma photons emitted when the positrons from decaying fluorine-18 nuclei annihilate with electrons inside the patient
Gamma photons detected for the PET scan come from the annihilation of the positrons, not the gamma photons emitted by the decaying fluorine-18 nuclei
The annihilation of a positron and an electron produces two gamma photons travelling in opposite directions so momentum is conserved
The computer can determine the point of annihilation from the difference in the arrival times of these photons at the two diametrically opposite detectors and the speed of photons
The voltage signals from all the detectors are fed into the computer - which analyses and manipulates these signals to generate an image on a display screen in which different concentrations of the tracer show up as areas of different colours and brightness
Advantages of PET
Non invasive technique
Help diagnose different types of cancers, plan complex heart surgery, observe the function of the brain
Help doctors identify the ones of certain disorders of the brain
Help assess the effect of new medicines and drugs on organs
What is Ultrasound
Longitudinal sound waves with frequency greater than 20 kHz
Benefits of ultrasound
Form images of the internal structure ps of the body
Non ionising
Non invasive
Quick
Range of frequencies for Ultrasound used for medical imaging
1-15 MHz
What is the wavelength of ultrasound in the human body and what are the advantages
Less than 1mm - can be used to identify features as small as a few millimetres
What is an ultrasound transducer
Device used to generate and to receive ultrasound
How does an ultrasound transducer work
It changes electrical energy into sound and sound into electrical energy - by means of the piezoelectric effect
What is the piezoelectric effect
When crystals produce an EMF when they are deformed
Is the piezoelectric effect a reversible process
Yes as when an external p.d is applied across the opposite faces if the crystal - the electric field can either compress or stretch the crystal
How is ultrasound generated
A high frequency alternating p.d is applied across opposite faces of a crystal - this repeatedly compresses and expands the crystal
The frequency is chosen to be the same as natural frequency of oscillation of the crystal
The crystal resonates and produces an intense ultrasound signal
What does an ultrasound transducer emit
Pulses of ultrasound - typically 5000 pulses every second
How can a transducer be used to detect ultrasound
Any ultrasound incident on the crystal will make it vibrate - so the crystal is compressed and expanded by tiny amounts
This vibration generates an alternating e.m.f across the ends of the crystal - this can be detected by electronic circuits
What do modern ultrasound transducers use
Lead zirconate titanate or polyvinylidene fluoride
What is the simplest type of ultrasound scan called
An A-scan
How does an A-scan work
A single transducer is used to record along a straight line through the patient
The transducer sends ultrasound pulses into the body of a patient
Each pulse of ultrasound will be partly reflected and partly transmitted at the boundary between any two different tissues
Reflected or echo pulse will be received at the transducer
It wiki gave less energy due to energy loss
The pulsed voltage at the ultrasound transducer is displayed on an oscilloscope screen or computer screen as a voltage against time plot
What is the time interval in an A-scan
Ultrasound pulse to travel from the front of the transducer to the retina and then back to the transducer
What is the total distance travelled by the ultrasound pulse
2L
Where L is the distance between the transducer and the retina
How do B-scans work
The transducer is moved over the patients skin
Output of the transducer is connected to a high speed computer
For each position of the transducer- the computer produces a row of dots on the signal screen - each dot corresponds to the boundary between two tissues
The brightness of the dot is proportional to the intensity of the reflected ultrasound pulse
Collection of dots produces correspond to the different positions of the transducer over the patient - making a 2D image of a section through the patient
What happens at boundary
When a uniform beam of ultrasound is incident at a boundary between two substances - a proportion of its intensity will be reflected and the remainder will be refracted
The fraction of the ultrasound intensity reflected at the boundary depends on the acoustic impedance of both media
What is acoustic impedance of a substance defined as
The product of the density and the speed of ultrasound in that substance
Units for acoustic impedance
kg m^-2 s^-1
What does reflected intensity depend on
The acoustic impedances
What is the formula for the ratio of reflected intensity given that the angle of incidence is 0 degrees
(Z2 - Z1)^2 / (Z2 + Z1)^2 = Ir / Io
Ir/ Io is also known as the intensity reflection coefficient
What is a coupling gel
A substance with acoustic impedance similar to that of the skin is smeared onto the skin and the transducer
The gel fills air gaps between the transducer and the skin and ensures that almost all the ultrasound enters the patients body
When is the term impedance matching / acoustic matching used
When two substances have similar causes of acoustic impedance
What is Doppler ultrasound
The reflection of ultrasound from iron-rich blood cells to help doctors to evaluate blood flow through major arteries and veins
What can Doppler ultrasound reveal
Blood clots, the narrowing of walls due to the accumulation of fatty deposits, the amount of blood flow to a transplanted kidney or liver
How does Doppler ultrasound work
The ultrasound transducer is pressed lightly over the skin above the blood vessel
The transducer sends pulses of ultrasound and receives the reflected pulses from inside the patient
Ultrasound reflected off tissues will return with the same frequency and wavelength - but that reflected off the many moving blood cells will have a changed frequency
Frequency is increased when the blood is moving towards the transducer and decreased when the blood is receding from the transducer
The frequency shift is directly proportional to the speed of approach or recession of the blood
Formula for change in observed ultrasound frequency
Change in frequency = (2 x observed frequency x speed of moving blood cells x cos(theta)) / (speed of the ultrasound in blood)
