FUNDAMENTALS OF MAGNETIC RESONANCE INSTRUMENTATION
Only the very basic principles will be covered here. The main components of all MR imaging systems are: the main magnet, gradient coils, radio frequency transmitter and receiver coils and computers.
The main magnet is used to produce a strong, uniform external magnetic field that is powerful enough to induce measurable tissue magnetisation. Magnetic field strength is measured in Tesla, with most clinical magnets ranging from 0.3 – 1.5T (15,000 Gauss). By comparison earth’s magnetic field is very small, measuring about one-half the Guass. Smaller electromagnetic coils, called "shim coils" are used to fine tune the static magnetic field and make it more uniform that the center of the main magnet (where images are acquired).
Gradient coils are electromagneticcoils with magnetic field strengths that are only a small fraction of the main external field. Gradient coils are used to vary the magnetic field at the center of the main magnet in a deliberate, predictable way along each of three perpendicular directions (x, y and z). Gradient coils are switched on and off very rapidly during image acquisition and are used to provide spatial localisation.
Radio frequency (RF) coils are of two types; transmit and receiver coils. The transmit RF coils are used to excite nuclei by putting precisely timed RF pulses into the patient to excite nuclei within a specific tissue slice. Receiver coils measure MR signal output from tissue and vary in size from large whole-body coils to small surface receiver coils. The latter are closely applied to the anatomic region of interest. Compared to whole-body receiver coils, surface coils have an advantageous signal-to-noise ratio. Surface coils can provide exquisitely detailed images of a limited anatomic area.
Computers are used for data storing, processing and image display.
Fundamentals of Nuclear Magnetic Resonance
Like many forms of spectroscopy NMR utilises electromagnetic radiation to probe the fundamental properties of matter. In NMR the radiation employed is in the radio-frequency (RF) portion of the electromagnetic spectrum and the property studied is the interaction of this radiation with the nuclear magnetic moment. An understanding of the principles of nuclear magnetic resonance imaging is founded on the principles of quantum and classical mechanics.
An object with a mass, m being about an axis with velocity of v, has an angular momentum La.
La
Where La, r and v are vectors. A spinning object also possesses an angular momentum whose vector points in the direction of the thumb when the right-hand rule is applied. Spinning is a natural phenomenon for electrons and nuclear particles. Besides orbiting about the nuclear axis, electrons, protons and neutrons spin about their own axes. As a result of these motions there are two angular momenta associated with each electron and nuclear particle: orbital angular momentum and spin angular momentum. The magnitude of the angular momentum of a subatomic particle observed about an arbitrary Z-axis is limited to the values: Laz = ml (h/2p
) Where h is the Plank constant equal to 6.6 X 10-34 joule-sec, and ml is the magnetic quantum number. The combination of all nuclear angular momenta in a nucleus generates a single number I, called the spin quantum number and determined by the spin of unpaired neutrons or protons. For a given nucleus the value of I must be 0,
Due to their charge properties all nucleons including neutrons have a magnetic dipole moment. It is well known that electrons and protons possess the same charge but different sign. It is less known, however, that although the net charge is zero, neutrons have an asymmetrical charge distribution in the particle. A unit called the nuclear magneton m mN is used to express the magnitude of the magnetic dipole moment of the nucleus.
1 m mN = 5.05 X 10-27 J tesla
The magnetic dipole moments of the proton and the neutron are respectively m mP = 2.79 m mN and m mn = -1.91 m mN where the + or – sign indicates whether the magnetic dipole moment is in or opposite to the direction of angular momentum.
The nuclear magnetic moment (m m) is related to the nuclear angular momentum through the expression
m m = g Laz
where g , termed the gyro-magnetic ratio.
For protons in an external magnetic field, the individual nuclear magnetic moments align themselves in one of two energy states. The lower energy state in which the magnetic moment is aligned parallel to the magnetic field correspond to ml = +1/2, and is the ground state. Like-wise nuclei with their magnetic moment aligned anti-parallel to the applied magnetic field reside in a higher energy state where ml = -1/2. The relative population difference between the two energy levels is governed by the Boltzmann distribution.
At equilibrium the populations of the energy states described in the quantum mechanical model are stable. For every transition from the ground state to the higher energy state there is a transition occurring from the higher energy state to the ground state. Thus, the net exchange of energy between the spin system and the outside world is zero. If the spin system is irradiated with electromagnetic irradiation of frequency fo where
D E = hfo = g (h/2p )Bo
transition will be induced between energy levels.
A small sample of material is placed in the static magnetic field, Bo. The RF transmitter generates the B1 magnetic fields perpendicular to the static field through a coil surrounding the sample. The B1 field tips the nuclear magnetisation vector into the XY plane. After the RF field turned off, the precessing magnetisation vector induces a voltage in the same coil, which is amplified and displayed on the oscilloscope.
Prior to the advent of the pulsed techniques the NMR signal was detected by either sweeping the RF frequency or varying the static or dc magnetic filed to generate the resonance condition. The signal obtained from these continuous-wave techniques is a spectrum where the amplitude of the signal is plotted as a function of frequency. Continuous-wave techniques have been almost entirely replaced by current pulsed-NMR technology. The major disadvantage of continuous-wave methodology is the inefficiency in observing only one frequency spectrum at a time. With the advent of pulsed-NMR techniques the entire frequency spectrum is excited simultaneously with a hard RF pulse and the resulting FID contains all of the information available in the continuous-wave spectrum. Furthermore by adding subsequent FID’s the NMR signal-to-nose ratio can be improved by factor of Ö N where N is the number of accumulated scans or FID’s. This occurs because the NMR signal, which is coherent with time, increases by a factor of N as subsequent FID’s are added.
