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medical physics Flashcards

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1
Q

What is the wavelength range of Xrays

A

10^-8 to 10^-13, typically 10^-10

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2
Q

outline the production of xrays

A
  • An xray tube consists of an evacuated tube containing two electrodes.
  • the vacuum is necessary so electrons don’t interact with the gas atoms
  • An external power supply of 50kV is connected
  • The cathode (negative) emits electrons via thermionic emission, usually a filament.
  • These electrons are accelerated towards the positive anode
  • The anode is the target metal usually made out of tungsten which has a high melting point
  • The electrons hit the target metal and decelerate very quickly
  • 1% of the KE energy of the accelerated electrons hitting the anode is converted into X-Rays.
  • The remaining 99% is thermal, which is why a coolant is needed, or a rotating anode
  • There is an x-ray transparent window which the x-rays are pointed through.
  • The rest of the machine is covered in lead to prevent damage to radiographers
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3
Q

Outline how to find the minimum wavelength of a emitted xray

A
  • KE of the accelerated electrons = eV
  • due to the conservation of energy the max energy of the emitted xray = the max KE of the inital electron due to the one on one interaction
  • max energy = max frequency = minimum λ
  • E=hf = hc/λ
  • λ = hc/eV where eV is the KE of the original particle
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4
Q

define attenuation

A

decrease in intensity of an electromagnetic radiation as it passes through matter

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5
Q

Outline the simple scatter model

A

incident x-ray photon interacts with an electron in the atom, but has less energy that required to remove the electron and so just bounces off with no change to its energy

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6
Q

Outline the photoelectric effect for xray absorption

A

incident x-ray photon is absorbed by one of the electrons in the atom, the electron uses this energy to escape the atom with the KE left over, normally seen in hospitals due to the incident x-ray photon energy

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7
Q

Outline Compton scattering

A

incident x-ray photon interacts with an electron in the atom, the electron is ejected from the atom and because of the high energy of the incident x-ray, it doesn’t disappear but also is ejected with lower energy

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8
Q

Outline pair production

A

incident x-ray interacts with the nucleus of the atom and disappears, ejecting an electron positron pair which conserve momentum

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9
Q

why are contrast medium used

A
  • to increase absorption coefficients
  • soft tissues have low absorption
  • used to model blood flow to spot clots
  • barium meal for digestive systems
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10
Q

What does CAT stand for

A

computerised axial tomography

  • axial meaning cross sections
  • tomography meaning slices
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11
Q

outline the function of a CAT scanner

A
  • patient lies om back
  • slides into a ring with detectors on one side and x-ray tubes on the other
  • xray tubes produce fan like beams of xrays
  • each different tissue absorbs different amounts of xrays and so detectors pick up different intensities
  • every 360 degree rotation a slice is made
  • these can be manipulated by computers to make a 3D image
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12
Q

outline the advantages of a CAT scan

A
  • creates a 3D image

- can distinguish between different tissues of similar attenuation coefficients

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13
Q

outline the disadvantages of CAT scans

A
  • have to stay still for a long time
  • relatively more expensive that a normal x-ray machine
  • x-rays are ionising and can be harmful in high exposure long time periods
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14
Q

important things to think about when choosing a medical tracer

A
  • high activity so only a small amount is needed
  • short half life so patient isn’t exposed to radiation a long time after the process
  • gamma emitter has it is less ionising and highly penetrative
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15
Q

What can we use to target cells in the brain

A

combine Tc-99 with sodium and oxygen to target cells in the brain

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16
Q

outline the process of a gamma camera

A
  • medical tracer is injected into the patient
  • travels to targeted cells
  • gamma photons are emitted toward the camera
  • firstly the collimator is a series of long thin lead tubes designed to eliminate all gamma photons incident not from the targeted area. i.e. coming out from an angle
  • parallel gamma photons pass down the collimator to the scintillator
  • this is made of sodium iodide and when a gamma photon hits it, produces many photons of visible light
  • about 1 in 10 gamma photons interacts with the scintillator
  • photomultiplier tubes converts these flashes into electrical pulses
  • these pulses are sent to a computer which constructs, using software a detailed image of the point and which the gamma photons left the body
17
Q

what is different in a gamma camera image to an x-ray image

A

gamma camera shows functions and processes of the internal structures, rather than anatomy

18
Q

name the isotope in the main medical tracer used in PET scans and outline why it is used

A

FDG with fluorine-18

  • positron emitter
  • with a short half life that decays into oxygen, a positron, a neutrino and a gamma photon
19
Q

what is the downside to using flourine-18

A

has to be produced on sight due to its half life

20
Q

how is flourine-18 produced

A

proton + oxygen-18 = flourine-18 + neutron

21
Q

what does FDG stand for

A

flourodeoxyglucose

22
Q

One advantage of using FDG

A
  • body treats it like glucose so can easily model the respiratory system and in tissues with high rates of respiration
23
Q

outline the process of PET scans

A
  • patient lies on a table and is surrounded by a ring of gamma cameras
  • the patient is injected with FDG
  • when the FDG emits a positron from beta+ decay the positron instantly annihilates with a nearby electron
  • this annihilation produces two gamma ray photons which travel in opposite directions to eachother to conserve momentum
  • The computer determines the point of annihilation by analyzing the time difference between the two gamma ray photons being received.
  • the two gamma photons are picked up by two detectors on exactly opposite sides of the patient
  • the computer is fed impulses created by the gamma cameras and makes an image using the time difference
  • this image has different colour and brightness
24
Q

Outline the advantages of a PET scanner

A
  • non invasive
  • can assess effectiveness of new drugs
  • can diagnose different types of cancers
25
Q

One disadvantage of PET scanners

A
  • Very expensive and only seen at larger hospitals due to price of machinery
26
Q

what is the nature of ultrasound

A

a longitudinal sound wave of frequency greater than 20 kHz

27
Q

benefits of ultrasound

A
  • non ionising
  • non invasive
  • quick
28
Q

what is the name of the effect that allows an ultrasound transducer to be both and emitter and receiver

A

piezoelectric effect

29
Q

outline the piezoelectric effect

A
  • crystals such as quartz produce an e.m.f when distorted i.e. compressed.
  • this is a reversible process, so if a p.d. is applied to the crystal, it distorts it.
30
Q

outline how the ultrasound transducer works

A
  • high frequency alternating p.d. is applied across the face of a crystal like quartz
  • this alternating p.d. compresses and stretches the crystal continuously
  • the frequency that it compresses and stretches at is matched to the natural frequency of the crystal
  • this makes it resonate creating an intense ultrasound signal
  • a damping substance surrounds the transducer the eliminate the initial vibrations so returning ultrasound pulses can be detected
  • the returning ultrasound pulses are incident on the crystal
  • this makes it vibrate
  • this creates an alternating e.m.f
  • this can be detected by electronic circuits and fed to a computer to create an image
31
Q

state the main difference between A and B scans

A

B scans are 2D and require the transducer to be moved, each angle the transducer is at produces a line of dots showing each boundary, therefore when moved around a 2D image is created.

32
Q

how to calculate acoustic impedance

A

Z = density x speed of sound in the material (Z=pc)

33
Q

Outline why a coupling gel is needed and what is does

A
  • when the transducer is placed over the skin there are tiny pockets of air as skin is not perfectly smooth
  • as the ultrasound passes from air to skin, because the acoustic impedance’s are very different, most ultrasound is reflected of the skin and so no ultrasound is actually transmitted into the skin
  • a coupling gel is applied to fill these gaps
  • the gel matches the acoustic impedance of skin allowing almost 100% transmission of the ultrasound into the skin
34
Q

outline doppler ultrasound imaging including method, good and bad points

A
  • non invasive
  • reveal blood clots
  • a transducer is held at an angle to the skin with coupling gel
  • it is placed over a blood vessel
  • the incident ultrasounds reflect of blood vessels causes a change in frequency of the received ultrasound
  • Δf = 2fvcosθ/c where v is the speed of the moving blood cells and c is the speed of ultrasound in the blood
  • f is the original frequency of ultrasound

Physics (21 decks)

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