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Yr 13 Physics > Medical Physics > Flashcards

Medical Physics Flashcards

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1
Q

How is an x-ray produced in a X-ray tube?

Overview

A

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

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2
Q

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.

A
  • 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.
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3
Q

What does the term attenuation mean?

A

-attenuation is used to describe the decrease in the intensity of an electromagnetic radiation as it passes through matter.

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4
Q

How does attenuation work? Like what causes a decrease in intensity?

A
  • 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.
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5
Q

What are the 4 attenuation mechanisms?

A
  • simple scatter
  • photoelectric effect
  • Compton scattering
  • pair production
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6
Q

Why is evacuated tube used in x-rays?

A

So that electrons can pass through the tube without interacting with gas atoms

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7
Q

What is simple scattering?

A
  • 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
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8
Q

Explain the photoelectric effect in terms on X-rays?

A
  • 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
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9
Q

What is Compton scattering?

A
  • 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
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10
Q

Pair production?

A
  • 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
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11
Q

What does higher value of attenuation coefficient mean?

A
  • better absorber of x-rays the material is
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12
Q

Why is a contrast medium used?

A

To improve visibility of their internal structures

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13
Q

Why are iodine and barium often used as contrast mediums?

A
  • they have a large atomic number

- the attenuation coefficient is proportional to the cube of the atomic number

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14
Q

Where is iodine often used as contrast medium?

A

-in liquids to view blood flow

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15
Q

Where is barium often used as a contrast medium?

A

-barium sulfate is often used to to image digestive systems

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16
Q

What are the therapeutic use of x-rays? (Linear accelerators)

A
  • 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

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17
Q

What are CAT scans?

A
  • computerised axial tomography

- records a large number of X-ray images from different angles and assembles them into a 3D image using a computer

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18
Q

Describe how a CAT scan image is produced?

A
  • 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.
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19
Q

Describe how the components of a CAT scanner are used to produce three-dimensional image of a patient?

A
  • 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
20
Q

Why are full body CT scan not offered to healthy patient?

A

-because it’s exposes them to ionising radiation which could cause cancer/damaged cells

21
Q

What are some advantages of CAT scans?

A
  • creates 3D image and therefore can assess shape,size and position of disorders
  • can distinguish between soft tissues with similar attenuation coefficients
22
Q

Disadvantages of CAT scans?

A
  • 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
23
Q

What are medical tracers?

A

-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

24
Q

What are the properties of medical tracers?

A
  • 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
25
Q

Where is Fluorine-18 used as a source?

A

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.
26
Q

How does a gamma camera work?

A
  • 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
27
Q

What kind of image does a gamma camera produce?

A

An image that shows the function and processes of the body rather than its anatomy

28
Q

How does a photomultiplier tube work?

A
  • 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.
29
Q

Explain the basic principles of PET scanning?

A

It used medical tracers , produces slices through the body (3D), uses gamma radiation rather than X-rays.

30
Q

Explain in detail how PET scanning work and how the image is formed?

A
  • 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.
31
Q

Advantages of PET scans?

A
  • 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.
32
Q

Disadvantage of PET scans?

A

-very expensive

33
Q

What are ultrasound?

A
  • ultrasound are sound waves with a frequency above 20kHz.

- 1-15MHz are used in medical imaging.

34
Q

What is an ultrasound transducer?

A

-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.

35
Q

What is the piezoelectric effect?

A
  • 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.
36
Q

How does ultrasound inducer work?

A
  • 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.
37
Q

What is an A-scan?

A
  • 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.
38
Q

What is a B-scan?

A
  • 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.
39
Q

What is acoustic impedance?

A
  • the speed of the ultrasound in the material x Density of the material.
40
Q

What is a coupling gel and why is it used?

A
  • 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.
41
Q

What is the Doppler effect?

A

The Doppler effect is that change in frequency of a wave when it is reflected or produced by a moving source.

42
Q

Why is Doppler imaging used?

A

Used to evaluate blood flow in blood vessels and organs and can be used to reveal blood clot,narrowing of blood vessels.

43
Q

Describe how high frequency ultrasound can be used to determine the speed of blood through the arteries of a patient?

A
  • 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.
44
Q

Explain why the transducer is not placed at right angles to the surface of the patient skin in Doppler imaging?

A

-the Doppler shift in frequency depends on Cos X and if X=90 there would be no change in frequency.

45
Q

State what causes a large fraction of reflection of ultrasound at the boundary between substances?

A

When the acoustic impedances of the substances are very different

46
Q

One way in which F-18 nuclei is made.

A

Colliding O-18 with high speed protons

Yr 13 Physics (13 decks)

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