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Medical Physics > Magnetism > Flashcards

Magnetism Flashcards

1
Q

Magnetism

A
  • a physical phenomenon as a consequence of electric currents
  • Magnetic fields appear when electric charges move.
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2
Q

Origin of the magnetic field

A
  • Within the matter media, a magnetic field arises due to motion of electric charge of atoms – electrons and nucleons
  • Each atom generates magnetic field in small surrounding area, this magnetic field is result of the vector sum of:
  • orbital motion of electrons
  • its spinning rotation and nucleus magnetic momentum
  • each atom can be observed as analog of small magnet.
  • The atomic magnetic field sum determines magnetic field of the given object/matter and respectively its properties
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3
Q

Magnets divided into three groups

A

Diamagnetics

Paramagnetics - important for medical diagnosis

Ferromagnetics

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4
Q

Diamagnetics

A
  • substances which do not transmit magnetic field
  • decreases magnetic field penetrated within it
  • atomic magnetic fields are compensated and substance does not generate a magnetic field
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5
Q

External magnetic field with strength B0 is applied to a diamagnetic substance. What happens?

A
  • diamagnetic substance interacts with it and generates magnetic field with strength B’ with opposite direction
  • diamagnetic is repulsed off from diamagnetic.
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6
Q

Paramagnetics

A
  • The atoms of paramagnetic substances have magnetic moments and show behavior of magnet dipoles
  • But in absent of external influences magnetic moments are compensated due to the chaotic thermal motion
  • If an external magnetic field is applied the atomic magnetic vectors acquire the direction of this field and the resulting field is amplified
  • Paramagnetic properties of biological matter are of a great significance for the modern diagnostics methods
  • these materials are slightly attracted to the magnetic field
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7
Q

The behaviour of a group of protons in the presence of a magnetic field

A
  • Exposed to an external magnetic field, each spin or magnetic moment can acquire two different orientations:
  • “parallel” (spin up) (at a lower energy state)
  • “anti-parallel” (spin down) (at a higher energy state)
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8
Q

Precession

A
  • Under the action of an external magnetic field, protons start to precess
  • The intensity of precession (the angle of precession) depends on the magnitude of magnetic field applied
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9
Q

NMR

A
  • The process of selective absorption of EM energy by nucleus with magnetic moments and consecutive irradiation of this energy back out is called NMR
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10
Q

Excitation of a proton

A
  • The protons can interact with electromagnetic field in such manner that an energy of the field is to be absorbed
  • If initially a proton is put into precession by a magnetic field, the consecutive irradiation with a pulsed EMF causes excitation of the proton
  • This excitation state is characterized with alteration of the precession (switches to anti-parallel) and has short duration
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11
Q

Requirements of NMR

A
  • nucleus must have magnetic moment (it occurs in paramagnetic substances)
  • the presence of magnetic field with certain parameters
  • electromagnetic pulses with frequency depending on the magnetic field strength
  • EMF acts perpendicularly to magnetic field.
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12
Q

Recession frequency

A
  • under the action of constant magnetic field, B0 nucleus/proton starts to precess with a frequency v0
  • if EM pulse with the same frequency is applied the nucleus absorbs a maximal portion of EM energy, afterward this energy is emitted back out
  • The nucleus falls in resonance if the following equation is executed

RF = ν0 = γ B0

γ = gyromagnetic number

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13
Q

NMR imaging methods

A
  • In medicine the NMR is used as a diagnostic instrument
  • The method is called Magnetic Resonance Imaging and it is performed by means of device called MR Tomograph
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14
Q

How is the MF produced in NMR

A

by electric coil

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15
Q

What determines the precession of nuclei being investigated in NMR

A

the direction of the MF produced by the electric coil

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16
Q

Magnetic resonance in nucleus (NMR)

A
  • By means of another coil the EM pulses are generated to produce magnetic resonance in nucleus
  • Emitted quant energy of nucleus is detected by the same coil due to delay of emission
  • Emitted signal is named FID (free induction delay) and it carries an useful information for the object
17
Q

NMR Imaging

A
  • based on resonance detection of H nucleus
  • the hydrogen is the most common spread element in the human body
  • the single proton has uncompensated magnetic moment (H is paramagnetic)
18
Q

What does NMR imaging analyse?

A
  • P - proton density - gives small amount of info because there are variations in water conc. of diff. tissues
  • Time of relaxation - in state of megnetization, a short radio frequency pulse perturbs precession. after switching off the EM field, nucleus restores its initial state with magnetic vector directed towards B0
19
Q

T1

A
  • T1 - longitudinal time of relaxation (spin-lattice relaxation) - the time duration needed of protons to restore the max. amplitude of precession in longitudinal direction after switching off the EM pulse - indicator of the rate of megnetisation loss
20
Q

Resonance differences in water

A
21
Q

How is contrast obtained in NMRI

A
  • In NMRI the contrast is obtained by FID signal estimation
  • T1 fast restoring – dark pixel, slow restoring – bright pixel
  • T2 fast decreasing – bright pixel, slow decreasing – dark pixel
  • Solid matter – very short T1 and T2 (Bones ? )
  • Free water – very long T1 and T2
  • Cancer tissue - longer T1 than normal tissues
  • Fat – shorter T1 than muscles
22
Q

Applications of NMR

A

Tumor diagnostics (T1)

МS (T1)

Cerebral haemodynamics (Gd)

Brain stroke (Gd)

NMR spectroscopy (chemical shift)

Oxygen levels

Cardiac function

23
Q

Advantages of NMR

A

High contrast

Computed control over the image

Motion sensitivity (diffusion, blood flow)

Good resolution (<1 mm)

Noninvasive method

24
Q

Disadvantages of NMR

A

Artefacts of movement

Contraindications (pacemakers, implants, etc)

Claustrophobia

25
Q

T2

A

T2 – transversal time of relaxation:

The time duration required for the protons to restore the
equilibrium state losing their transversal magnetization after switching off the EM pulse. This is an indicator of the rate of magnetisation loss

Medical Physics (10 decks)

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