medical physics Flashcards
how are x-rays prod
electrons are emitted from a metal filament (cathode) through thermionic emission. ( when it is heated)
the electrons are then accelerated by the high p.d between the cathode and the target metal which is usually tungsten (anode)
the x-ray tube is a vacuum so that electrons can pass through with out interacting with particles
when the electrons strike the target, the electrons decelerate and some of the kinetic energy is transformed to xray photons .
wha is the range of x-ray wavelengths
10^-8 to 10^-13 m
what is thermionic emission
heating a cathode to release electrons
what are the 4 attenuation mechanisms and give a description of each
simple scatter- low energy photons bounce off an atom with no change in momentum
photoelectric effect- A photon is completely absorbed by a metal ejecting an electron from the surface
compton scatter- the xray photon interacts with an electron within an atom , and the electron is ejected and the photon is scattered with reduced energy
pair production- a photon interacts with a nucleus of an atom producing an electron positron pair.
define attenuation
the decrease in intensity of electromagnetic radiation as it passes through matter.
give 3 factors which effect attenuation of an xray
the specific material being xrayed
the photon energy
the thickness o the material
define the attenuation coefficient
a measure of the amount of xray photons absorbed by a substance per metre
how can we find the attenuation coefficient using count rate and the geiger muller tube experiment
C=C0 x e^-ux
carry out experiment and find BG radiation
C0 = the count rate of radiation source without a material present in between
plot graph of lnC against x
x= thickness of material
y intercept = lnC0
gradient = attenuation coefficient u
describe how the components of a CAT scanner can produce a high quality image of the internal structure of a patient
the xray tube and the detectors rotate around the patient
a thin fan-shaped beam of xrays is emitted
the intensity of the transmitted xrays is recorded by the detectors
after each 360 degree rotation , an image of a thin cross-section of the body is produced showing the soft tissues
the tube continues to spiral down the body building up a 3d image
as the computer uses the signal from the detectors to produce a 3D image
why would bone and muscle have a better xray image contrast than fat and muscle
because bone and muscle have a greater difference in attenuation coefficient contrast therefore better contrast
an xray scan of the hearts and its blood vessels shows very poor contrast explain what can be done to improve this
a contrast medium / iodine can be injected into the blood
this has a higher attenuation contrast
how can we calculate the max wavelength of the xrays for the pair production proces
max wavelength= min E
the min energy required for pair production is given by E=mc^2
m= 2me
then find wavelength after finding E
Describe the shape of an intensity of x-rays against the wavelength graph
The spectrum is produced by electron striking the target of an x-ray tube. There are narrow peaks at specific wavelengths which are produced by the bombarding electrons which removes the electrons close to the nuclei. These gaps are then filled with electrons dropping from a higher energy state and photons are released at this wavelength.
Explain what would happen to the intensity of x-rays against wavelength graph if the accelerating potential difference is increased in an x-ray tube
When the accelerating potential difference is increased the kinetic energy of the electrons increase therefore the maximum kinetic energy of the x-ray photos also increase however the shortest wavelength will decrease therefore the graph would shift to the left and have higher peaks for the electrons removing electrons close to the nuclei however these peaks will be in the same position the wavelength of the so-called K lines only depend on the material of the target
Compare CAT scans and an x-ray tube
An x-ray scan is quicker and cheaper compared to a CAT scan
However CAT scans can produce a 3-D image
The x-rays can be harmful and CAT scan are prolonged therefore the patient is exposed to x-rays for longer
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
They are typically gamma source
Describe the property of medical traces require
They are typically gamma sources so this way they can pass through tissue
The source is half-life must be long enough to be detected but short enough so that they do not remain in the patient’s body
it’s activity must be large enough so that it can be detected outside of the body
And it must be non-toxic
What are the two medical traces/gamma sources used normally?
Technician 99m, and it is used to monitor the major organs such as the heart kidney brain
Fluorine 18, which is used in PET scan
Describe how a gamma camera works
Gamma photons are passed due a collumbator which consists of thin lead tubes that only allows gamma photons that are travelling parallel to each other any travelling at an angle will be absorbed by this thin lead therefore the galleries are said to be columnated
They then passed through a scintillator which is a silicone iodide crystal every gammaray which strikes the Cator will produce multiple visible light photons on
These then passed through a photomultiplier tube where a single photo on a visible light is converted into an electrical signal such as a voltage
The electric electrical signals are then used to create an image of the concentration of the gum and hence the tracer
Explain a PET scan
This is when a patient is surrounded by a ring of gamma detectors
And a tracer that emits a Poron as it decay through beta plus radiation is injected into the patient
The positron annihilate with an electron in the body
These produces two Gamaret photons which travel in the opposite direction due to the conservation of momentum
Then the delay time of the two photos is used to locate the location of the annihilation
Then a computer is connected cutie detector in order to form an image of the annihilation
Explain the advantages and disadvantages of a PET scan
Advantage
it is a non-evasive technique and can produce real time images
Dis
It is very expensive and a radioactive source is used Therefore, the patient is exposed to a small amount of activity
What does the photo multiplier in a gamma camera do?
It takes an input of a single photo of visible light from the creator and uses this to produce up to 1,000,000 electrons to produce an electrical signal
How can the gamma camera produce a higher quality image?
This can be improved by using longer thinner culminated or by scanning for a longer time period
Explain how the structure of a photomultiplier tube uses a single photo to produce up to 1 millions of electrons
A single photo on a visible light strikes the cathode the photon then releases an electron and the electron is accelerated towards the first dynode
it strike dynode causing the release of several more electrons. These electrons are then accelerated towards the dynode and each of these electrons releases even more electrons. This will continue until the electron strike the anode at the end of the tube where a voltage is detected.
PET scanners are expensive because they require nearby or on site particle accelerator that produce fluorine 18 discuss the ethical issues this raises in the treatment of patient
Doctors would have to make difficult decisions about who can and cannot have a PET scan
Some patients will miss out on PET scans because of their location as not all patients will have access to these scans
Explain how medical traces are used to diagnose the condition of organs
Teresa is injected into the body
The tracer is absorbed by the organ
And a beta detector/gamma camera is used to detect radiation from the body
