G485 Medical Physics - Nuclear Medicine

Medical Physics is split into two sections: 

1. X-rays & Ultrasound 

2. Nuclear Medicine 

This mind map is for Nuclear Medicine

PLEASE ZOOM IN WHEN USING AS THERE IS A LOT OF INFORMATION AND IT IS VERY CRAMPED WHEN ZOOMED OUT :)

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  • Created by: abbiemca
  • Created on: 20-04-16 22:11
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  • Medical Physics (II) Nuclear Medicine
    • The Gamma Camera
      • The gamma camera is a system which detects gamma-rays and converts them into an image!
      • Four main components: lead collimator tubes, the scintillator (crystal), the photomultipliertubes and the computer
        • Scintillator
          • As relatively few gamma rays pass through the collimator tubes, the scintillator converts each gamma ray photon into many visible light photons
            • Increases the signal with which to work with
        • Photomultiplier Tubes
          • Each visible light photon is converted into one electron
            • Each electron is accelerated towards a dynode and 2-3 electrons are produced
              • Continuouslyfired towards dynodes until there are lots of electrons present
        • Computer
          • Detects the electrical signal from photomultiplier tubes and converts the signal into an image
            • Each photomultiplier tube corresponds to one pixel
      • The quality of the image can be improved by using narrower or longer collimators, or a longer scanning time
    • Medical Tracers
      • A medical tracer is a way of determining the structure of the body using a radioactive source
        • Patient is then placed inside a gamma camera
      • Most commonly used tracer is Technetium-99m (metastable)
        • Metastable means the isotope remains in an excited state for some time after its production
          • Decays into its ground state by gamma emission
        • Other commonly used tracers are iodine-131 and fluorine-18
      • Explain what is meant by a medical tracer
        • A radioactive substance is ingested or injected into the patient
          • and the resulting radiation is detected (by gamma cameras) to reveal the structure of the body
    • Positron Emission Tomography (PET)
      • tomography is the product of 3D images of the internal structure of a body by observing how waves react at boundaries in the body
      • A positron is the antiparticle of the electron
      • Fluorine-18 is commonly used in medical imaging. It decays by beta+ decay and emits a positron
        • The positron and electron annihilate each other to form 2 identical gamma rays at 180 degrees to each other
    • Magnetic Resonance Imagine (MRI)
      • Main components of an MRI scanner: large superconducting magnet, RF coils, gradient coils and a computer
        • Superconducting Magnet
          • Needed to produce external magnetic field to align protons
        • RF Coils
          • One coil to transmit RF pulses into body. Another to detect RF signal emitted from relaxing protons
        • Gradient coils
          • Produce an additional magnetic field that varies across patient's body. Results in Larmor frequency of nuclei will be slightly different for reach part of the body
            • Only a small volume of the body is the right field value for resonance and the computer can locate the source of the RF signal
        • Computer
          • Controls gradient coils and RF pulses, which stores and analyses the received date producing and displaying an image
      • Advantages:non-ionising and non invasive. Better contrast between soft tissues
      • Disadvantages: patients with metallic objects cannot be scanned. Patent has to remain still for a long time. Unsuitable for claustrophobic patents due to confined space
      • Operation of MRI scanner
      • MRI vs. PET
        • Positron Emission Tomography (PET)
          • tomography is the product of 3D images of the internal structure of a body by observing how waves react at boundaries in the body
          • A positron is the antiparticle of the electron
          • Fluorine-18 is commonly used in medical imaging. It decays by beta+ decay and emits a positron
            • The positron and electron annihilate each other to form 2 identical gamma rays at 180 degrees to each other
        • PET uses radioactive substances, MRI does not
        • PET reveals brain function so can diagnoses diseases such as Alzheimer's
        • MRI shows tissue variation
  • Make sure that the gamma rays are all travelling in the same direction!
    • Lead Collimator Tubes
      • Four main components: lead collimator tubes, the scintillator (crystal), the photomultipliertubes and the computer
        • Scintillator
          • As relatively few gamma rays pass through the collimator tubes, the scintillator converts each gamma ray photon into many visible light photons
            • Increases the signal with which to work with
        • Photomultiplier Tubes
          • Each visible light photon is converted into one electron
            • Each electron is accelerated towards a dynode and 2-3 electrons are produced
              • Continuouslyfired towards dynodes until there are lots of electrons present
        • Computer
          • Detects the electrical signal from photomultiplier tubes and converts the signal into an image
            • Each photomultiplier tube corresponds to one pixel
    • This ensures a clear image
  • Alpha  and beta are unsuitable as they are very damaging to cells (a lot of energy absorbed by only a few cells
    • gamma mostly leaves the body with much less interaction
    • Must be a gamma emitting source
      • Medical Tracers
        • A medical tracer is a way of determining the structure of the body using a radioactive source
          • Patient is then placed inside a gamma camera
        • Most commonly used tracer is Technetium-99m (metastable)
          • Metastable means the isotope remains in an excited state for some time after its production
            • Decays into its ground state by gamma emission
          • Other commonly used tracers are iodine-131 and fluorine-18
        • Explain what is meant by a medical tracer
          • A radioactive substance is ingested or injected into the patient
            • and the resulting radiation is detected (by gamma cameras) to reveal the structure of the body
  • The half life of a medical tracer is a few hours
    • Any longer is unsuitable because there would be a low decay constant and so the activity would be small and hard to detect
    • Any shorter means the source would have decayed between ingesting and detecting
  • Basic principles of a PET scan
    • Brain/body is surrounded by gamma cameras
      • Positrons from F-18 nuclei annihilate electrons
        • This annihilation produces 2 identical gamma ray photons traveling in opposite directions
          • Decay time between the 2 photons is used to determine location of annihilation (which are areas of increased activity)
            • Computer is connected to gamma cameras and an image is formed using the electrical signals from the cameras
  • Positrons from F-18 nuclei annihilate electrons
    • This annihilation produces 2 identical gamma ray photons traveling in opposite directions
      • Decay time between the 2 photons is used to determine location of annihilation (which are areas of increased activity)
        • Computer is connected to gamma cameras and an image is formed using the electrical signals from the cameras
  • Superconducting Magnet
    • Needed to produce external magnetic field to align protons
  • Protons precess around the magnetic field which is provided with  the strong electromagnet
    • The frequency of precession is  the Larmor frequency
      • Transmitting coils provide pulses of radio waves f frequency that is equal to Larmor frequency
        • Protons absorb energy from the radio waves, resonate and enter into a higher energy state (they become excited)
          • When the protons return back to their original energy state (relaxation), they emit photons of radio frequency
            • The relaxation time is the average time taken for the protons to return back to their original energy state
              • relaxation time spends on tissue type
                • a computer processes all of the signals from receiving coils, and suing computer software, produces a 3D image
    • Protons have spin and so they behave like tiny magnets
      • Operation of MRI scanner
  • Protons absorb energy from the radio waves, resonate and enter into a higher energy state (they become excited)
    • When the protons return back to their original energy state (relaxation), they emit photons of radio frequency
      • The relaxation time is the average time taken for the protons to return back to their original energy state
        • relaxation time spends on tissue type
          • a computer processes all of the signals from receiving coils, and suing computer software, produces a 3D image

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