Magnetic Resonance Imaging (MRI) - DESY

n n n n Magnetic Resonance Imaging(MRI)IntroductionThe ComponentsThe TechnologyPhysics behindMRMost slides taken pt-2086416

Introductionn What is MRI?n n n Magnetic resonance imaging (MRI) is a spectroscopicimaging technique used in medical settings toproduce images of the inside of the human body.MRI is based on the principles of nuclear magneticresonance (NMR), which is a spectroscopic techniqueused to obtain microscopic chemical and physical dataabout moleculesIn 1977 the first MRI exam was performed on ahuman being. It took 5 hours to produce one image.

Introductionn How Does it Work?n The magnetic resonance imaging is accomplishedthrough the absorption and emission of energy of theradio frequency (RF) range of the electromagneticspectrum.

n Why MRI ?n n n n Utilizes non ionizingradiation. (unlike x-rays).Ability to image in any plane.(unlike CT scans).Very low incidents of sideeffects.Ability to diagnose, visualize,and evaluate variousillnesses.The only better way to see theinsides of your body is to cutyou open!

The Components:n A magnet which produces a very powerful uniformmagnetic field.n Gradient Magnets which are much lower in strength.n Equipment to transmit radio frequency (RF).n A very powerful computer system, which translates thesignals transmitted by the coils.

The Magnetn The most important component of the MRIscanner is the magnet:n n The magnets currently used in scanners today are inthe .5-tesla to 2.0-tesla range (5,000 to 20,000gauss).Higher values are used for research.Earth magnetic field: 0.5-gauss

The Magnet (cont.)n n There are three types of magnets used in MRIsystems:n Resistive magnetsn Permanent magnetsn Super conducting magnets (the most commonlyused type in MRI scanners).In addition to the main magnet, the MRI machinealso contains three gradient magnets. These magnetshave a much lower magnetic field and are used tocreate a variable field.

The Technologyn How Does It All Work?n Spin:n n n The atoms that compose the human body have aproperty known as spin (a fundamental propertyof all atoms in nature like mass or charge).Spin can be thought of as a small magnetic fieldand can be given a or – sign and amathematical value of multiples of ½.Components of an atom such as protons,electrons and neutrons all have spin.

The Technology (cont.)n Spin (cont.):n n n Protons and neutronspins are known asnuclear spins.An unpaired componenthas a spin of ½ and twoparticles with oppositespins cancel oneanother.In NMR it is theunpaired nuclear spinsthat produce a signal ina magnetic field.

The Technology (cont.)n n Human body is mainly composed offat and water, which makes thehuman body composed of about 63%hydrogen.Why Are Protons Important to MRI?n positively chargedn spin about a central axisn a moving (spinning) charge creates amagnetic field.n the straight arrow (vector) indicates thedirection of the magnetic field.

The Technology (cont.)n n n When placed in a large magneticfield, hydrogen atoms have a strongtendency to align in the direction ofthe magnetic filedInside the bore of the scanner, themagnetic field runs down the centerof the tube in which the patient isplaced, so the hydrogen protons willline up in either the direction of thefeet or the head.The majority will cancel each other,but the net number of protons issufficient to produce an image.

The Technology (cont.)n Energy Absorption:n n The MRI machine applies radiofrequency (RF) pulse that is specificto hydrogen.The RF pulses are applied through acoil that is specific to the part of thebody being scanned.

The Technology (Cont.)Resonance (cont.)The gradient magnets are rapidly turned on and offwhich alters the main magnetic field.n The pulse directed to a specific area of the bodycauses the protons to absorb energy and spin indifferent direction, which is known as resonanceFrequency (Hz) of energy absorption depends on strength ofexternal magnetic field.

The Technology (cont.)Larmor Equationω γβ 2 π0For hydrogen at 1.5T:0γ 2.675x108 s 1Tβ 1.5Tω 63.864MHz00n n the resonance frequency, ω0, is referred to as the Larmorfrequencythis frequency is needed to excite transverse magnetization(precession)

The Technology (cont.)n Imaging:n n When the RF pulse is turned off the hydrogen protons slowlyreturn to their natural alignment within the magnetic fieldand release their excess stored energy. This is known asrelaxation. - two time-scales (see later)What happens to the released energy?n n n Released as heatORExchanged and absorbed by other protonsORReleased as Radio Waves.

The Technology (cont.)n Measuring the MR Signal:n n n the moving proton vector induces a signalin the RF antennaThe signal is picked up by a coil and sent tothe computer system.the received signal is sinusoidal in natureThe computer receives mathematical data,which is converted through the use of aFourier transform into an image.

original long.alignmenttransverseprecession

The Image

Physics of MRIIt is an interplay ofn Magnetismn Resonance

n Fig: 1. A) The top spinning in the earth's gravity. Thegravity tries to pull it down but it stays upright due toits fast rotation. B) A charge spinning in the magneticfield Bo.

n n A) The protons spinning in the nature, without anexternal strong field. The directions of spins arerandom and cancel out each other. The netmagnetization is nearly 0.B) In the presence of a large external magnetic fieldBo the spins align themselves either against (highenergy state) or along (low energy state). There is aslight abundance of spins aligned in the low energystate.

Longitudinal magnetizationmore spins with lower energy,i.e. parallel to external field

A) The compass needle (a small magnet) aligns itself with a N/S-S/N direction when placed in a large magnetic field.B) When another strong magnet is brought near the alignedcompass needle the magnetic fields of all three magnetsinteract in such a way that the mobile, weakest magnet (thecompass needle) realigns itself away from its originalorientation.C) When the perturbing magnetic field is removed suddenly thecompass needle magnet realigns itself with the large externalmagnet field, but before realigning, it wobbles around thepoint of stability and gradually comes to rest.

n Fig: 4. The spin of a proton can be represented by avector B with a direction and magnitude. Its relation tothe direction of the external magnetic field Bo isrepresented by an angle.

n n n n A) The spin of a proton aligned to Bo in the Z-axis.B) An external perturbing magnetic field, B1, isapplied which knocks the vector out of its axis, whichnow is aligned at a new angle with respect to Bo.C) As the perturbing field B1 is removed the vectorgradually starts returning back to its original state andD) begins to wobble

n n A) The falling water rotates a wheel to which a magnet isattached. When this magnet rotates it induces an alternatingcurrent in a coil of wire which can be detected.B) A magnetic field (spin of a proton) rotating near a coil of MRantenna induces a similar current in the loop which can bedetected.

ResonanceThe gradient coils.n A) the body placed in the core ofthe magnet with B0 aligned to itslong axis.n B) the gradient coil oriented inthe Z-axis (along the long axis ofthe body) which gradually andlinearly increases from left toright.n C) At the center of the gradientfield, the frequency is equal tothat of B0, but at a distance Δxthe field changes by a factor ofΔB0.

Resonancen The resonance equation shows that the resonancefrequency ν of a spin is proportional to themagnetic field, Bo, it is experiencing.n ν γ BoWhere γ is the gyromagnetic ratio. [the ratio ofthe magnetic moment of a spinning chargedparticle to its angular momentum]

.,Two time-scales: T1 and T2n n n n n T1 is the spin-lattice relaxation time-scale for thelongitudinal magnetization to come back to itsinitial valueT2 is the spin-spin relaxation timefor the transverse magnetization to decrease tozeroT1-weighted imaging: higher spatial resolutionT2-weighted imaging: higher tissue contrasttypically both time-scales are used

Recap: What Does the ImageRepresent?n n For every unit volume of tissue, there is anumber of cells, these cells contain watermolecules, each water molecule contain oneoxygen and two hydrogen atoms.Each hydrogen atom contains one proton inits nucleus. Different tissues thus producedifferent images based on the amount of theirhydrogen atoms producing a signal

Referencesn n n n n Ballinger, Ray. Basics of MRI. htm Retrieved: 7/7/03Buckwalter, Ken, M.D. Magnetic Resonance es/mri/iu lectures/mri homepage.htm Retrieved: 7/6/2003Gould, Todd, RT, MR, ARRT. How MRI m Retrieved:7/5/2003Hornak, Joseph, PhD. The Basics of MRI. .htmlNagasaki School of Medicine, Department of Radiology.Basics of 20of%20the%20FOOT/MRI-CDNUH/nf-basic1.html Retrieved 7/7/03.

spins are known as nuclear spins. ! An unpaired component has a spin of ½ and two particles with opposite spins cancel one another. ! In NMR it is the unpaired nuclear spins that produce a signal in a magnetic field. Th