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The Physics of MRI

A brief presentation on The Physics of MRI by Nilotpal Das, proudly presented by Biomedicz (www.biomedicz.com)

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The Physics of MRI

  2. 2. WHAT IS MRI?  Magnetic resonance imaging (MRI) is a noninvasive medical imaging technique that helps diagnose medical conditions.  MRI uses a powerful magnetic field, radio frequency pulses and a computer to produce detailed pictures of organs, soft tissues, bone and virtually all other internal body structures.  An X-ray is very effective for showing doctors a broken bone, but if they want a look at a patient's soft tissue, including organs, ligaments and the circulatory system, then they'll likely want an MRI. And, another major advantage of MRI is its ability to image in any plane.
  4. 4. THE FIVE SIMPLE STEPS  Patient is placed in a magnet.  Radio wave is sent in  Radio wave is turned off  The patient emits a signal  The emitted signal from the patient is used for reconstruction of the image
  5. 5. THE PHYSICS OF MRI  Human body is made of cells, cells are made of various atoms, and all atoms contain protons in their nucleus.  Protons carry positive charge and possess spin.  We know, moving charge is electric current, and electric current induces magnetic field.  Thus, proton has its own magnetic field, and act as tiny bar magnets.
  6. 6. FIRST, THE PATIENT IS PLACED INSIDE THE MAGNET  When the protons are placed in an external magnetic field, they align themselves according to the external magnetic field like a compass needle aligns itself along the magnetic field of earth.  The protons may however align in two different ways: parallel or antiparallel to the external magnetic field.  These different types of alignments have different energy levels. Naturally the preferred state of alignment is the one which requires less energy.
  7. 7. LONGITUDINAL MAGNETIZATION  Proton pointing in opposite direction cancels each others magnetic effect in respective direction.  As there are more protons aligned parallel to the external magnetic filed, there is a net magnetic movement aligned with or longitudinal to the external magnetic field  In a strong external magnetic field a new magnetic vector is induced in the patient, who becomes a magnet himself.  This new magnetic vector is aligned with the external magnetic field, and thus called longitudinal magnetization.
  8. 8. PROTON PRECESSION  In the presence of an external magnetic field protons show a certain type of movement called precession, similar to the ‘wobbling’ movement of a spinning top just before it is about to stop.  It is important to know how fast proton precess. The number of times proton precess per second is called precession frequency. It Depends upon the strength of the magnetic field in which protons are placed.  The precession frequency is calculated using Larmor Equation:
  9. 9. NEXT, A RADIO WAVE IS SENT TO THE PATIENT  The purpose of this RF pulse is to disturb the protons which are peacefully precessing in alignment with the external magnetic field. For this we need a RF pulse that can exchange energy with the protons, a RF pulse with same frequency as the precessing protons.  Only when the RF pulse and the protons have the same frequency, can protons pick up some energy from the radio wave, a phenomenon called resonance (this is where the "resonance" in magnetic resonance come s from). We can calculate the frequency of the necessary RF pulse from Larmor Equation.
  10. 10. EFFECT OF RF PULSE  Some of the protons pick up the energy and move from lower energy level to higher energy level, that is, some of the protons that were previously pointing along the magnetic field align them against the magnetic field (antiparallel). This causes the longitudinal magnetization to decrease, as the number of excess protons aligned along the external magnetic field decreases
  11. 11. EFFECT OF RF PULSE  The RF pulse causes the protons to precess in sync. They now point in the same direction at the same time, thus their magnetic forces add up in the direction they are pointing. This results in a magnetic vector pointing to the side to which the protons precess, that is in the transverse direction (in the X-Y plane). This is called the transversal magnetization.
  12. 12. NEXT, THE RADIO WAVE IS TURNED OFF  Before RF pulse there was only longitudinal magnetization.  After the 90dgr RF pulse there is only transversal magnetization and this is spinning around  With time after the removal of RF pulse the transversal magnetization decreases and longitudinal magnetization increases in spiral motion
  13. 13. OBTAINING MRI SIGNAL FROM THE PATIENT  When an antenna is placed near the tissue, an electric current is set up in the antenna due to the spiralling movement of the magnetic vector from the transversal to longitudinal direction. Due to the spiralling movement, the magnetic vector gradually moves away from the antenna and thus the amplitude of the current induced reduces gradually. This is called FID signal, free induction decay.
  14. 14. COMPUTING AND DISPLAY  The received signal is then fed into a computer and, amazingly, a quarter of a second later an image appears on the screen.
  15. 15. REFERENCES  MRI Made Easy by Prof Dr Hans H. Schild; Published by Schering, AG.  Basic MRI Physics by Evert J. Blink.  http://science.howstuffworks.com/mri.htm  https://www.cis.rit.edu/htbooks/mri/inside.htm  http://en.wikipedia.org/wiki/Physics_of_magnetic_resonance_imagi ng