Magnetic Resonance Imaging System

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Magnetic ResonanceImaging System



Nuclear Magnetic Resonance (NMR )

Aims: � To understand the details of how NMR works.� To interpret NMR in context of human body.

MagneticNuclear Resonance

In the Nucleus InvolvesMagnets

In the Nucleus



El ectromagnetic spectrum

Near-Radio and Radio frequencies .This part of the EM spectrum used forthe first time in Medical Field. Currently used in Oceanography andSubmarines



Princip les of NMR Imaging Systems

Based on measurement of absorption of e lectromagneticradiation in the radio frequency region.

Atomic nuc lei, rather than outer e lectrons, are invo lved in theabsorption process. To cause nuc lei to deve lop the energystates required for absorption to occur, it is necessary to exposethe ana lyze to an intense magnetic fie ld of severa l thousandgauss.

Certain atomic nuc lei having odd no. of protons or neutrons orboth, have properties of spin and magnetic moment and, as aconsequence, exposure to a magnetic fie ld wi ll lead to sp littingof their energy leve ls.



Nuc lear Properties:

Assume nuc lei rotate about their axis. Thus, they have spin.angu lar momentum associated with partic le spin is a ha lf-integra l mu ltip le of h /2 T , where h is Planck s constant.

The maximum spin component for a particu lar nuc leus is itsspin quantum number I. A nuc leus has ( 2I + 1) discrete states.The angu lar momentum for these states are integra l va luesfrom I to I. In the absence of an externa l fie ld, these stateshave identica l energy.

The magnetic dipo le, Q , resu lting from nuc lear spin is orienteda long the axis of the spin and has a va lue characteristic foreach type of nuc leus.



Energy Leve ls in a Magnetic Fie ld:

In an externa l magnetic fie ld, a partic le possessing amagnetic Moment tends to become oriented such thatits magnetic Dipo le (hence, its spin axis) is para lle l tothe fie ld.

In the absence of a magnetic fie ld, the energies of themagnetic quantum states are identica l. Consequent ly,a large assemb lage of protons wi ll contain an identica l number of nuc lei with = +1/2 and m = -1 /2 . Whenp laced in a fie ld, the nuc lei tend to orient themse lvesso that the lower energy state ( m = +1/2 )predominates.



Effect of Externa l Magnet fie ld onMagnetic Momnet



Effect of Externa l Magnet fie ld onMagnetic Momnet



.

When a materia l is p laced in a magnetic fie ld Bo.

Some of the random ly oriented nuc lei experience an externa l

magnetic torque which tends to a lign the individua l para lle l oranti-para lle l magnetic moments to the direction of an app liedmagnetic fie ld.There is a s light excess of nuc lei a ligned para lle l with themagnetic fie ld and this gives the tissue a net magnetic momentM o.

With the magnetic moments being random ly oriented withrespect to one another, the components in the X- Y p lane cance l

one another out whi le the Z components a long the direction of the app lied magnetic fie ld add up to produce this magneticmoment Mo.



A ccording to the electromagnetic theory, any nucleus such as a hydrogenproton which possesses a magnetic moment attempts to align itself withthe magnetic field in which it is placed. This results in a precession (Fig.22.3) or wobbling of the magnetic moment about the applied magneticfield with a resonant angular frequency, (called the Larmor frequency)are determined by a constant (the magnetogyric ratio ) and thestrength of the applied magnetic field Bo. Each nuclide possesses acharacteristic value for but and Bo are related as follows:

= Bo



.

Another important phenomenon of NMR is that theapp lied externa l magnetic fie ld creates an energyabsorption state from a statistica l point of view .When a nuc leus with a magnetic moment is p laced in amagnetic fie ld ,the energy of the nuc leus is sp lit intolower (moment para

lle

lwith the fie

ld) and higher(anti-para lle l) energy leve ls.

The energy difference is such that a proton with specificfrequency (energy) is necessary to excite a nuc leus fromthe lower to the higher state . The excitation energy E is

given by Planck s equationE = hWhere h is Planck s constant divided by 2 .This energy is usua lly supp lied by an RF magnetic fie ld.





Free Induction Decay(FID)In NMR ,at room temperature, there are protons in a lowenergy state than in a high energy state.The excited protons tends to return or re lax to its low energystate with spontaneous decay and reemissions of energy at alater time ,in form of radio waves photons.This decay is exponentia l in nature & produces a Freeinduction decay (FID) signa l, that is the fundamenta l form of nuc lear signa l obtainab le from an NMR system.The amount of energy required to f lip protons from the para lle l to anti-para lle l orientation is proportiona l to magnetic fie ldstrength, stronger fie ld require more energy or higherfrequency.



ExcitationIf the materia l or tissue is subjected to another magnetic fie ld, say a

bar magnet p laced a long the Y-axis ,this wou ld cause netmagnetization to shift s light ly from the Z-axis .

An a lternative technique to accomp lish the same resu lt wou ld beapp ly an RF pu lse at the resonant frequency of the protons in tissue.

The ang le of rotation depends on the amp litude but primari ly on the

length of app lied radio-frequency pu lse.An RF pu lse of sufficient duration & power to rotate Mo through 90degree is referred as 90 RF pu lse.





.



.

Following are two re laxation mechanisms are associated with excitednuc lear spins:

1. Re laxation time T1 is referred to as spin- lattice re laxation process, itcharacterizes the time for the perturbed nuc lei to rea lignthemse lves with existing structure of materia l. Also known aslongitudina l re laxation time, as it is time constant for recovery of Zcomponent to its equi librium va lue Mo a long the direction of app lied magnetic fie ld.

2 . Re laxation time T 2 is referred to as spin-spin re laxation process, itcharacterizes the time for the perturbed in-phase spins to dephasewith respect to each other. A lso known as transverse re laxationtime, as it is time constant for decay of component of M in X-Y p lanewhich conventiona lly perpendicu lar to Z-axis or app lied magnetic

fie ld Bo.3. Transverse re laxation time(T 2) is faster than longitudina l

re laxation(T1) .



Fourier Transformation of the FIDIn NMR imaging systems, we are interested in sorting out whatfrequency component are present in spectrum & intensity of each

frequencies present .So we use Fourier transform which transforms time domain intofrequency domain



IMAGE RECONSTRUCTION TECHNIQUES

NMR produce medica l images based on four separatecharacteristics of tissue:

a) Based on the proton density of tissueb) Based on T1 re laxation time distribution of the tissue

c) Based on T 2 re laxation time distribution of the tissued) Based on NMR image sensitive to f low

But in actua l practice NMR images represent anama lgamation of a ll the four parameters.

Basica lly tota l imaging vo lume is divided into a grid matrixof rectangu lar ce lls ca lled voxe ls (vo lume e lements) a longthe spatia l coordinates.The scan is viewed on a cathode ray tube represented asa two dimensiona l image



Severa l schemes have been used for c lassification of various

NMR imaging methods .



Discrimination Based on Re laxation Rates

Relaxation process have a marked effect on magnitude of magnetization & on intensity(contrast ) of image.

This dependency of image intensity on re laxation rates used indiscrimination in bio logica l tissues.Experiments showed that rotation rates between tissues of important

organs depends large ly on their water content.For examp le in mice, brain, the brain has a water content of near ly80% whereas liver has between 66% to 70% has a direct effect onre laxation time T1.The sensitivity of NMR arises from the fact that a15% to 20% change in water concentration resu lts in a change of

about 200% in relaxation rate.



The NMR Spectrometer



NMR Machine



BASIC NMR COMPONENTS

The basic components of an NMR imaging system are asfo llows :

1) A magnet, which provides a strong uniform, steady, magnetfie ld Bo;

2) A n RF transmitter, which de livers radio-frequency magneticfie ld to the samp le;

3) A gradient system, which produces time-varying magneticfie lds of contro lled spatia l nonuniformity ;

4) A detecti o n system, which yie lds the output signa l;

5) A n imager system, inc luding the computer, whichreconstructs and disp lays the images.







The three aspects of NMR imaging which cou ld cause potentia l hea lth hazard are:

1. H eating due t o the RF p ow er: Katinis (198 2) reports that a temperatureincrease produced in the head of NMR imaging wou ld be about O.3°C.This does not seem like ly to pose a prob lem.

2 . S tatic magnetic field: Although no significant effects of the static fie ld

with the leve l used in NMR are known, Pastakia (1978) mentions aboutthe possib le side effects of e lectro-magnetic fie lds. There cou ld be aslight decrease in cognitive ski lls, mitotic de lay in s lime mou lds, de layedwound hea ling and e levated serum trig lycerides.

3. Electric current inducti o n due t o rapid change in magnetic field: It isbe lieved that osci llating magnetic fie ld gradients may induce e lectriccurrents strong enough to cause ventricu lar fibri llation. However, nodamage due to NMR from exposures has been reported (Marx 1980). Itis suggested that fie lds shou ld not vary at a rate faster than 3 tes la / s.



ADVANTAGES OF NMR IMAGING SYSTEM1) The NMR image provides substantia l contrast between so ft tissues that

are near ly identica l in existing techniques. NMR images that disp lay T1and T 2 properties of tissue provide tremendous contrasts betweenvari o us s o ft tissues, contrasts approaching 150% are possib le in T1 andT2 images, whi le contrasts of on ly a few percent are possib le betweenso ft tissues with X-rays .

2) Cross-sectiona l images with any orientation are possib le in NMRimaging systems.

3) The a lternative contrast mechanisms of NMR provide promisingpossibi lities of new diagnostics for patho logies that are difficu lt orimpossib le with present techniques.

4) NMR imaging parameters are affected by chemica l bonding and,therefore, offer potentia l f o r physio logica l imaging.

5) NMR uses no ionizing radiation and has minima l, if any, hazards f o r operators of the machines and f o r patients.

6) Un like CT, NMR imaging requires no moving parts, gantries orsophisticated crysta l detectors. The system scans by superimposinge lectrica lly contro lled magnetic fie lds. Consequent ly, scans in any pre-determined orientation are possib le.

7) With the new techniques being deve loped, NMR permits imaging of entire three dimensiona l vo lumes simu ltaneous ly instead of s lice byslice, emp loyed in other imaging systems.



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Nonmedica l App lications of NMRIPo lymeric Materia ls

Manufacturing defects voids, occ luded so lvents or partic les, cracks, channe lsPhase structure mixing of b lends, composites, fi llers, p lasticizers, foamsReactions po lymerization, thermoset curing, adhesives, cross linkingDiffusion swe lling agents, so lvents, gasesEnvironmenta l effects aging, wear, mechanica l or other stress, oxidation

heat distributionInorganic Materia ls

Ceramics cracks, voids, binder distributionOil we ll cores physica l defects, f luid quantification, distribution and mixing,

diffusion, f lowMeta ls hydrogen distribution, current f low

Crystals defectsChemistry and Chemica l Engineering

Reactions in So lution spatia l heterogeneity of reactants, products, or conditions,osci llations, kinetics

So lid-State Reactions anisotropy of reaction, kineticsChromatography co lumn packing, band spread, e lutionFlow f low dynamics, turbu lence, mu ltiphaseBioreactors design

Agricu lture and FoodWood Harvesting knots, defects, diseasesPlant Bio logy water f low and diffusion, diseasesSoil water content and distributionGrain water distribution and transportAgricu ltura l Products damage, ripening, diseases, insectsProcessed Foods baking, cooking, storage, spoi lage



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Magnetic Resonance Imaging System

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