Magnetism Flashcards
Magnetism
- a physical phenomenon as a consequence of electric currents
- Magnetic fields appear when electric charges move.
Origin of the magnetic field
- 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
Magnets divided into three groups
Diamagnetics
Paramagnetics - important for medical diagnosis
Ferromagnetics
Diamagnetics
- 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
External magnetic field with strength B0 is applied to a diamagnetic substance. What happens?
- diamagnetic substance interacts with it and generates magnetic field with strength B’ with opposite direction
- diamagnetic is repulsed off from diamagnetic.
Paramagnetics
- 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
The behaviour of a group of protons in the presence of a magnetic field
- 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)
Precession
- 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
NMR
- The process of selective absorption of EM energy by nucleus with magnetic moments and consecutive irradiation of this energy back out is called NMR
Excitation of a proton
- 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
Requirements of NMR
- 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.
Recession frequency
- 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
NMR imaging methods
- 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
How is the MF produced in NMR
by electric coil
What determines the precession of nuclei being investigated in NMR
the direction of the MF produced by the electric coil
Magnetic resonance in nucleus (NMR)
- 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
NMR Imaging
- 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)
What does NMR imaging analyse?
- 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
T1
- 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
Resonance differences in water
How is contrast obtained in NMRI
- 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
Applications of NMR
Tumor diagnostics (T1)
МS (T1)
Cerebral haemodynamics (Gd)
Brain stroke (Gd)
NMR spectroscopy (chemical shift)
Oxygen levels
Cardiac function
Advantages of NMR
High contrast
Computed control over the image
Motion sensitivity (diffusion, blood flow)
Good resolution (<1 mm)
Noninvasive method
Disadvantages of NMR
Artefacts of movement
Contraindications (pacemakers, implants, etc)
Claustrophobia
T2
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
