Medical Physics Flashcards
How is an x-ray produced in a X-ray tube?
Overview
X-rays are produced when fast moving electrons are rapidly decelerated by interaction with atoms of a metal such as tungsten. The kinetic energy of the electrons is transformed in X-ray photons
Explain using the things involved in the x ray tube how an x- ray is formed? Describe features of an X-ray tube as well.
- An external power supply creates a high potential difference.
- the cathode is a heater which produces electrons by thermionic emission
- electrons are accelerated towards the anode, which is made from a target metal such as tungsten which has high melting point.
- X-ray photons are produced when electrons are decelerated by hitting the anode.(kinetic energy is converted into electromagnetic energy in the form of x-ray photons)
- the remainder of energy is transformed into thermal energy in the anode.
What does the term attenuation mean?
-attenuation is used to describe the decrease in the intensity of an electromagnetic radiation as it passes through matter.
How does attenuation work? Like what causes a decrease in intensity?
- X-ray photons interact with the atoms of the material they pass through.
- photons can be scattered or absorbed by the atoms which reduces the intensity of the x-rays.
What are the 4 attenuation mechanisms?
- simple scatter
- photoelectric effect
- Compton scattering
- pair production
Why is evacuated tube used in x-rays?
So that electrons can pass through the tube without interacting with gas atoms
What is simple scattering?
- 1-20 keV
- X-ray photons interacts with an electron in the atom, but has less energy than the energy required to remove the electron (work function )
- the x-ray photons bounces off without any change to its energy
Explain the photoelectric effect in terms on X-rays?
- Energy less than 100keV
- X-ray photons is absorbed by one electron in the atom. (One to one interaction)
- the electron uses this energy to escape from the atom
- this mechanism used mostly for hospital x-ray imaging
What is Compton scattering?
- 0.5-5 MeV
- the x-ray photons interacts with an electron inside the atom
- the electron is ejected from the atom but the x-ray photons does not disappear
- it scatters with reduce energy
- both energy and momentum are conserved
Pair production?
- energy equal to or greater than 1.02MeV
- X-ray photon interacts with the nucleus of the atom
- it disappears and all the EM energy of the photon is used to create an electron and a positron
What does higher value of attenuation coefficient mean?
- better absorber of x-rays the material is
Why is a contrast medium used?
To improve visibility of their internal structures
Why are iodine and barium often used as contrast mediums?
- they have a large atomic number
- the attenuation coefficient is proportional to the cube of the atomic number
Where is iodine often used as contrast medium?
-in liquids to view blood flow
Where is barium often used as a contrast medium?
-barium sulfate is often used to to image digestive systems
What are the therapeutic use of x-rays? (Linear accelerators)
- LINACs are used to create High-energy X-ray photons.
- these photons are used to kill of cancerous cells by Compton scattering and pair production
What are CAT scans?
- computerised axial tomography
- records a large number of X-ray images from different angles and assembles them into a 3D image using a computer
Describe how a CAT scan image is produced?
- X-ray source and the detector move around the body
- X-ray pass through the same section of the body,but from different directions and angles
- produces a thin slice image through the Body
- more X-rays are absorbed by dense material with high atomic number
- A contrast medium can be used
- it is better at differentiating other organs than a simple x-ray
- the patient is moved a small distance and the process is repeated
- a computer analysed the data a forms a 3D image.
Describe how the components of a CAT scanner are used to produce three-dimensional image of a patient?
- X-ray tube rotates around the patient/ x-ray beam passes through the patient at different angles
- a thin X-ray beam is used , so that a thin section of the patients body can be scanned
- images of slices/cross sections through the patient are taken
- the singles and data from detectors are used by the computer to produce a 3D image
Why are full body CT scan not offered to healthy patient?
-because it’s exposes them to ionising radiation which could cause cancer/damaged cells
What are some advantages of CAT scans?
- creates 3D image and therefore can assess shape,size and position of disorders
- can distinguish between soft tissues with similar attenuation coefficients
Disadvantages of CAT scans?
- expensive and longer to do than x-rays
- exposes the patient to much more radiation than an X-ray.
- patients must remain still during scanning process
What are medical tracers?
-a medical tracer is a radioactive substance which is injected into the patient or digested and is used for diagnosis or treatment of a patient
What are the properties of medical tracers?
- they are typically gamma sources (this way they can pass through tissue easily)
- their half life needs to be long enough to be detected but short enough so that they do not stay in the patient for a long period
- the activity must be large enough so that it can be detected from outside the body
- must be non-toxic
Where is Fluorine-18 used as a source?
In Positron Emission tomography (PET)
- it undergoes beta plus decay releasing a positron from a proton and forming a neutron in the nucleus.
- the position then annihilates with an electron in the patients body to form a pair of gamma photons which are detected to locate the F-18 source in the patient’s body.
How does a gamma camera work?
- a gamma camera detects gamma photons from the medical tracer
- gamma photons travel towards a collimator(long thin tubes made of lead) , any photons arriving at an angle are absorbed by the lead so only those travelling along the axis of the tubes enter the scintillator.
- the scintillator is made of sodium iodide, a single gamma photon striking the scintillator produces thousands of photons of visible light
- the photons of visible light travel through the light guide into photomultiplier tubes.
- photomultiplier tubes are arranged in hexagonal pattern, a single photon of light is converted into a voltage, and the photomultiplier tubes are all connected to a computer, which can process the electrical signals and produce an image of the concentration of the medical tracer within the patient’s body
What kind of image does a gamma camera produce?
An image that shows the function and processes of the body rather than its anatomy
How does a photomultiplier tube work?
- a single photon of visible light hits a photocathode, producing a photoelectron
- the electron is accelerated to an electrode, and the impact of the electron with the electrode produces 4 secondary electrons, which are then accelerated towards another electrode
- the number of electrons increases exponentially and produces a voltage.
Explain the basic principles of PET scanning?
It used medical tracers , produces slices through the body (3D), uses gamma radiation rather than X-rays.
Explain in detail how PET scanning work and how the image is formed?
- the body is surrounded by a ring of gamma detectors
- patient injected with FDG - that contains F-18
- the positron from the (F-18 nuclei) annihilate electrons
- the annihilation of a positron and an electron produces two identical gamma photons travelling in opposite directions
- the delay time between these two photons is used to determine the location of the annihilation/F-18.
- complete connected to detectors and an image is formed by the computer using electrical signals from the detectors.
Advantages of PET scans?
- Non-invasive technique
- can be used to diagnose different types of cancers and to help plan complicated heart surgeries.
- also observe functions of the brain.
Disadvantage of PET scans?
-very expensive
What are ultrasound?
- ultrasound are sound waves with a frequency above 20kHz.
- 1-15MHz are used in medical imaging.
What is an ultrasound transducer?
-is a device used to both generate and to receive ultrasound. It changes electrical energy into sound and sound into electrical energy via the piezoelectric effect.
What is the piezoelectric effect?
- crystals (e.g.quartz) produce an emf when compressed or distorted or become distorted when an emf is applied.
- reversible process
- the application of a p.d across a crystal causes an expansion/contraction.
How does ultrasound inducer work?
- High frequency alternating potential difference is applied across opposite faces of the crystal
- this compresses and expands the crystal.
- if the frequency equals the natural frequency of the crystal, it causes it to resonate and emit pulses of ultrasound.
- a transducer is also used to detect ultrasound.
- any ultrasound incident on the transducer will cause it to vibrate and hence produce an alternating emf, which can be detected.
What is an A-scan?
- simplest type of ultrasound scan.
- records a straight line through patient .
- single inducer
- can determine thickness of bone, or distance between lean and retina
- each pulse of ultrasound will be partially reflected and partly transmitted at the boundary between any two tissues.
- the reflected pulse will be received by the transducer and will have less energy than the original pulse.
- the pulsed voltage is displayed on an oscilloscope or computer as voltage against time.
What is a B-scan?
- 2D image
- transducer is moved over the skin.
- the output of the transducer is connected to a high speed computer which produces a row of dots on a digital screen, where each dot corresponds to a boundary between two tissues.
- the brightness of the dot is proportional to the intensity of the reflected ultrasound.
What is acoustic impedance?
- the speed of the ultrasound in the material x Density of the material.
What is a coupling gel and why is it used?
- when an ultrasound transducer is placed on the skin,air pockets will form,which would result in a huge amount of reflection (due to skin air boundary).
- coupling gel with an acoustic impedance similar to skin is smeared in the skin and transducer, filing the air gaps so that minimum ultrasound is reflected.
What is the Doppler effect?
The Doppler effect is that change in frequency of a wave when it is reflected or produced by a moving source.
Why is Doppler imaging used?
Used to evaluate blood flow in blood vessels and organs and can be used to reveal blood clot,narrowing of blood vessels.
Describe how high frequency ultrasound can be used to determine the speed of blood through the arteries of a patient?
- ultrasound transducer emits and detects ultrasound
- the transducer is placed at an angle (to the artery)
- ultrasound is reflected by the blood
- change in frequency/wavelength of the reflected ultrasound is related to the speed of blood.
Explain why the transducer is not placed at right angles to the surface of the patient skin in Doppler imaging?
-the Doppler shift in frequency depends on Cos X and if X=90 there would be no change in frequency.
State what causes a large fraction of reflection of ultrasound at the boundary between substances?
When the acoustic impedances of the substances are very different
One way in which F-18 nuclei is made.
Colliding O-18 with high speed protons
