Introduction to MRI

Introduction to MRI Learning objectives • Explain the fundamental physical principles behind MRI • Explain the origins of the information content in MRI NMR, MRI and Nomenclature • NMR = Nuclear Magnetic Resonance • MRI = Magnetic Resonance Imaging (a type of NMR) • MRI makes use of the property of Nuclear magnetic resonance (NMR) to image nuclei of atoms inside the body. NMR looks at the magnetic properties of atomic nuclei. o MRI is the observation of a spatial variation in an NMR observable • The ‘nuclear’ in NMR has nothing to do with nuclear reactors or biologically damaging radiation coming from radioactive materials Clinical advantages and disadvantages • Advantages: provides information otherwise unobtainable, generally does not require labelled probe molecules, non-invasive and does not require high energy radiation (no DNA damage), unique source of contrast (especially those involving molecular motion), excellent soft tissue contrast • Disadvantages: expensive, can make some feel claustrophobic, machines can be affected by movement (unsuitable for investigating problems such as mouth tumours where coughing or swallowing decreases image clarity), interferes with electronic devices e.g. pacemakers Simple spin physics • Most elements have at least on NMR sensitive isotope, including those that are biologically important (e.g. 1H, 7Li, 13C, 23Na, 31P…) – sensitivity of each isotope is however different o The overall sensitivity of a nucleus depends on the natural abundance of the nucleus and its inherent sensitivity o At present only 1H can provide a sufficient signal and thus is used to perform clinical MRIs o To perform MRI with isotopes with low abundance you need isotopically labelled samples e.g. 13C-lablled glucose • Without physically spinning, such nuclei possess an intrinsic (quantum mechanical) angular momentum called ‘spin’, as well as a magnetic moment – can be thought of as a magnetic dipole and behave like a microscopic bar magnet • A group of randomly orientated nuclei has zero net (vector sum) magnetisation Effects of an external magnetic field: • Applying a magnetic field (B0 in unit T – Tesla) changes spin physics due to alignment and precession o o Alignment: the spins align with the magnetic field Precession: What does an MRI detect? • Rotating a magnetic with respect to the coil (or vice versa) will generate a voltage which oscillates at the speed of rotation of the magnet • In NMR, the net nuclear spin magnetisation (M0) plays the role of the magnet and we place a coil around the spins to detect the oscillating voltage (at the Larmor frequency ) → origin of the NMR signal from which we get the MRI image • The spins do not emit radiowaves – we are detecting rotating magnetisation • As B0 increases, so does M0 → acquired signal will be stronger → why a strong magnet is needed o The strength of an MRI is referred to either by the field strength of the main magnet in tesla or by the 1H resonance frequency Precession (resonance) frequencies and magnetic fields • The stronger the magnetic field → greater the sensitivity • rf coil (placed around sample for perturbing spins + receiving signal) → in realm of radio frequency • The earth’s magnetic field is ~50 μT (1H frequency ~ 2kHz) • Most clinical whole body MRI scans are based on 1.5 and 3T magnets • Normally superconducting magnetics are used. Providing the magnetic is kept at extremely low temps, magnet is effectively permanent → cannot be simply switched on and off → not in use=on RF coils and filling factors • Signal-to-noise is better when the rf coil is just big enough (high filling factor) – being held tight also reduces motional artifacts An NMR/MRI experiment Synopsis: Each proton is at random motion → their magnetic properties cancel out each other → 0 magnetism generates from normal tissues. An MRI machine uses a powerful magnetic field to align the magnetisation → protons align with the magnetic field with majority along the field and minority opposite to it. • The Pulse Sequence encodes various NMR observables into the signal o Strong magnetic field gradients cause nuclei at different locations to process o 3D spatial information can be obtained by providing gradients in each direction → spatial differences in NMR observables provide the contrast • • NMR/MRI sequence: the ‘perturb’ step will always include radio frequency (rf) EM pulses (from RF coils) to excite the spins and may also include spatially modulated applied magnetic fields and delays o Extremely small energies are involved → weak signal → must use sum results of multiple scans with a Fourier Transformation to amplify the signal o The signal is sometimes called the free induction decay (FID) o After the rf excitation (after being perturbed), the spin system tries to regain thermal equilibrium through the process of spin relaxation f In an MRI, the processing magnetisation is picked up using the radiofrequency coil. Thus we get a decaying voltage with time, which is the NMR signal Sources of information • Type of molecule, environment, motion and position provide distinct signatures observed as changes in frequency and amplitude in NMR signals → these magnetic resonance observables give the contrast in MRI as well as the ability to do MRI NMR phenomena – an NMR/MRI experiment involves some combination of these factors Chemical Shift • Describes dependence of nuclear magnetic energy levels on the electronic environment in a molecule • Local electronic distributions affect resonance frequency since the electron shells shield the nucleus from B0 • Chemical shift effects are extremely small – in ppm (parts per million) of the base resonance frequency (0) • The very small differences in resonance frequency give chemical information Types of Molecular Motion • Different parts of a molecule may have different reorientational motion, but a single diffusion coefficient characterises the whole molecule 1. Molecules are always moving → translational diffusion 2. Molecule is tumbling Molecules in different tissues have different properties → water inside a brain tumour will have different movement properties from normal water.