The basic principles of MRI and

1. The basic principles of MRI and Cardiac imaging by use of MRI History Bloch- and Purcell founded NMR (nuclear magnetic resonance) which lead to the medical use of it MRI (Damadian-1971) Mansfield- EPI 1987- used to perform realtime movie imaging of a single cardiac cycle Dumoulin- MRA- images of flowing blood without using IV contrast

2. Introduction The basic principles of magnetic resonance (MRI) form the foundation of understanding this complex subject. There are essentially two ways of explaining the fundamentals of MRI; classically and via quantum physics.

3. Atomic structure All things are made of atoms, including the human body. Atoms are very small. Half a million lined together are narrower than a human hair. They are organized in molecules, which are two or more atoms arranged together. The most abundant atom in the body is hydrogen. Found commonly in water and fat.

4. Atomic structure Black = electrons Purple = protons Green = neutrons Atoms consist of a central nucleus and orbiting electrons. The nucleus is very small, one millionth of a billionth of the total volume of an atom. It contains all the atom’s mass.

5. Atomic structure The atomic mass comes mainly from nucleons, subdivided into protons and neutrons. Mass number: S protons and neutrons. # of neutrons and protons in a nucleus is usually balanced, thus mass # is even. In some atoms, however, there more or fewer neutrons than protons. These atoms are called isotopes It is these atoms that are important in MRI

6. Atomic structure Electrons are particles that spin around the nucleus. # electrons = # of protons Application of external energy can knock electrons out. Causes deficit Causes elec. instability. Emission of energy called radioactivity

7. Motion in the atom Three types of motion are present within an atom. These are: Electrons spinning on their own axis Electrons orbiting the nucleus The nucleus itself spinning about its own nucleus The principles of MRI rely on the spinning of specific nuclei present in biological tissues.

8. Motion in the atom In nuclei that have an even mass number, i.e. # protons = # neutrons, half spin in one direction and half spin in the other. Nucleus has no net spin. However, in nucleus with odd mass #, spin directions are not equal and opposite, so the nucleus has net spin or angular momentum. These are know as MR active nuclei.

9. MR active nuclei MR active nuclei are characterized by their tendency to align their axis of rotation to an applied magnetic field. This occurs because they have angular momentum or spin and, as they contain positively charged protons, they posses electrical charge. The laws of electromagnetic induction refer to three individual forces – motion, magnetism and charge – and state that if two of these are present, then the third is automatically induced.

10. MR active nuclei MR active nuclei that have a net charge and are spinning (motion), automatically acquire a magnetic moment and can align with external magnetic field. The strength of the total magnetic moment is specific to every nucleus and determines the sensitivity to magnetic resonance.

11. The hydrogen nucleus The hydrogen nucleus is the MR active nucleus used in clinical MRI. It contains a single proton (atomic and mass number 1) It is used because it is most abundant in the human body and its solitary proton gives it a relatively large magnetic moment. Both of these characteristics enable utilization of the maximum amount of available magnetization in the body.

12. The hydrogen nucleus as a magnet The laws of electromagnetism state that a magnetic field is created when a charged particle moves. The hydrogen atom contains one positively charged proton that spins. Therefore it has a magnetic field induced around it, and acts as a small magnet. It has a north and south pole. Each of which is represented by a magnetic moment.

13. The hydrogen nucleus as a magnet In the absence of app. Magnetic field, magnet moments are randomly oriented When placed in a strong static external magnetic field, the magnet moments align w/ this field.

14. Alignment Quantum theory describes properties of electromagnetic radiation in terms of discrete quantities called quanta. Applying quantum theory to MRI, hydrogen nuclei posses energy in two discrete quantities term low and high. Low energy align their magnetic moments parallel to external field (spin-up). High energy align anti-parallel (spin-down).

15. Precession and the Larmor equation The influence of Bo produces an additional spin of the magnet moments of H around Bo. This secondary spin is called precession. It causes magnet moments to follow a circular path around Bo. Precessional path at a speed called magnet moments . The value of the magnet moments is governed by the Larmor equation.

16. Precession and the Larmor equation The Larmor equation states: ?o = Bo * ? Where: ?o is the precession frequency Bo is the magnetic field strength of the mgt. ? is the gyro-magnetic ratio. (MHz/T) ? of hydrogen is 42.57 MHz/T

17. Resonance Energy of the precessional freq. of H at all field strength corresponds to the radio frequency (RF) band of the electromagnetic spectrum. For resonance of H to occur, an (RF) pulse of exactly the Larmor frequency of H must be applied. Result are movement out of alignment away from Bo.

18. The MR signal As a result of resonance, in phase magnetization precesses at the Larmor freq. in the transverse plane. Faraday's laws of induction state that if a receiver coil or conductive loop is placed in the area of a moving magnetic field i.e. the magnetization precessing in a transverse plane, a voltage is induced in this receiver coil. The MR signal is produced when coherent magnetization cuts across the coil.

19. The free induction decay (FID) signal When the RF signal is switched off, NMV is again influenced by Bo and tries to realign with it. As the magnitude of transverse magnetization decreases, so does the magnitude of the voltage induced in the receiver coil. The induction of reduced signal is called the free induction decay (FID) signal.

20. Image formation -after the RF pulse is turned off, hydrogen protons begin to return to their original state=> release the stored energy slowly, signal picked up by body coils placed near the area being tested, and is sent to a computer as mathematical data which makes a Fourier transformation, which creates an image out of the data