Magnetic Resonance Magnetic Resonance ImagingImaging

Basic PrinciplesBasic Principles

V.G.WimalasenaV.G.WimalasenaPrincipalPrincipalSchool of RadiographySchool of Radiography

IntroductionIntroduction

Modern 3 Tesla MRI unit

Main magnet body

Patient Couch

RF Coil (for head)

Bore of the magnet

What is MRI?What is MRI?

Magnetic resonance imagingMagnetic resonance imaging ( (MRIMRI), or ), or nuclear magnetic resonance imagingnuclear magnetic resonance imaging ((NMRINMRI), is primarily a ), is primarily a Medical ImagingMedical Imaging technique most commonly used in technique most commonly used in radiologyradiology to visualize the structure and to visualize the structure and function of the body. function of the body.

It provides detailed images of the body in It provides detailed images of the body in any plane. any plane.

MRI provides much MRI provides much greater contrastgreater contrast between the different between the different soft tissuessoft tissues of the of the body than CT does, making it especially body than CT does, making it especially useful in useful in neurologicalneurological (brain), (brain), musculoskeletalmusculoskeletal, , cardiovascularcardiovascular, and , and oncological oncological (cancer) imaging. (cancer) imaging. Unlike CT, it uses Unlike CT, it uses no ionizing radiationno ionizing radiation, but , but uses a powerful uses a powerful magnetic fieldmagnetic field to align the to align the nuclear magnetization nuclear magnetization of (usually)of (usually) hydrogen atoms hydrogen atoms in water in the bodyin water in the body..

MRI Vs CTMRI Vs CT

Radiofrequency fields Radiofrequency fields are used toare used to systematically alter the alignment of the systematically alter the alignment of the nuclear magnetization of Hydrogennuclear magnetization of Hydrogen atoms, causing the hydrogen nuclei to atoms, causing the hydrogen nuclei to produce a produce a rotating magnetic fieldrotating magnetic field detectable by the scanner. detectable by the scanner. This signal can be manipulated by This signal can be manipulated by additional magnetic fields to build up additional magnetic fields to build up enough information to construct an image enough information to construct an image of the body.of the body.

Uses RF fieldsUses RF fields

MRI is a relatively new technology, which MRI is a relatively new technology, which has been in use for little more than 30 has been in use for little more than 30 years (compared with over 110 years for years (compared with over 110 years for X-ray radiography). ). The first MR Image was published in 1973 The first MR Image was published in 1973 and the first study performed on a human and the first study performed on a human took place on took place on July 3, 1977July 3, 1977..Magnetic resonance imaging was Magnetic resonance imaging was developed from knowledge gained in the developed from knowledge gained in the study of study of nuclear magnetic resonancenuclear magnetic resonance

HistoryHistory

Brief lay explanation of MRI physics Brief lay explanation of MRI physics

The body is mainly composed of water The body is mainly composed of water molecules which each contain molecules which each contain twotwo hydrogen nucleihydrogen nuclei or or protonsprotons..

When a person goes inside the powerful When a person goes inside the powerful magnetic fieldmagnetic field of the scanner these of the scanner these protons alignprotons align with the with the direction of the fielddirection of the field..

A second A second radiofrequencyradiofrequency electromagnetic electromagnetic field is then briefly field is then briefly turned onturned on causing the causing the protonsprotons to to absorbabsorb some of its energy. some of its energy.

When this When this fieldfield is is turned offturned off the protons the protons releaserelease this this energyenergy at a at a radiofrequencyradiofrequency which can be which can be detecteddetected by the scanner. by the scanner.

The The position of protonsposition of protons in the body can be in the body can be determineddetermined by applying by applying additionaladditional magnetic fieldsmagnetic fields during the scan which during the scan which allows an image of the body to be built up.allows an image of the body to be built up.

These are created by These are created by turningturning gradientsgradients coilscoils on and off which creates the on and off which creates the knocking sounds heard during an MR knocking sounds heard during an MR scan. scan.

Diseased tissue, such as tumors, can be Diseased tissue, such as tumors, can be detected because the protons in different detected because the protons in different tissues return to their equilibrium state at tissues return to their equilibrium state at different rates. different rates.

By changing the parameters on the By changing the parameters on the scanner this effect is used to create scanner this effect is used to create contrast between different types of body contrast between different types of body tissue.tissue.

Contrast agentsContrast agents may be injected may be injected intravenously to enhance the appearance intravenously to enhance the appearance of of blood vesselsblood vessels, , tumourstumours or or inflammationinflammation. .

Contrast agents may also be directly Contrast agents may also be directly injected into a joint, in the case of injected into a joint, in the case of arthrograms, MR images of joints. arthrograms, MR images of joints.

Use of contrast agentsUse of contrast agents

Unlike Unlike CTCT scanning scanning MRIMRI uses uses no ionizingno ionizing radiationradiation and is generally a and is generally a very safevery safe procedure. procedure.

But Patients with some But Patients with some metal implantsmetal implants, , cochlear implantscochlear implants, and , and cardiaccardiac pacemakers pacemakers are are prevented fromprevented from having an MRI scan due having an MRI scan due to effects of the strong magnetic field and to effects of the strong magnetic field and powerful radiofrequency pulses.powerful radiofrequency pulses.

Safety precautionSafety precaution

Uses of MRIUses of MRI

MRI is MRI is used to imageused to image every partevery part of the body, of the body,

But is particularly useful in But is particularly useful in – neurological conditions, neurological conditions, – disorders of the muscles and joints, disorders of the muscles and joints, – for evaluating tumors and for evaluating tumors and – showing abnormalities in the heart and blood showing abnormalities in the heart and blood

vessels. vessels.

System componentsSystem components

Magnet coils

Shim coils

Gradient coils

RF coils

Magnet bore

Magnet power supply

Shim power supply

Gradient amplifiers

RF transmitter

Host computer

Operator consol

Image processor

Image disk

RF receiver Digitizer

Explaining Basic principlesExplaining Basic principles

This is an Integration of Two ways of This is an Integration of Two ways of explaining. i. eexplaining. i. e

ClassicallyClassically

Via quantum physicsVia quantum physics

It describesIt describes Properties of atomsProperties of atoms Their interaction with magnetic fieldsTheir interaction with magnetic fields

Atomic structureAtomic structure

Central nucleus & Central nucleus & orbiting electronsorbiting electrons

NucleusNucleus– NucleonsNucleons

(Protons & neutrons)(Protons & neutrons)

Atomic numberAtomic number

Mass numberMass number

Electrically stableElectrically stable

Motion within the atomMotion within the atom

There are three There are three types of motion types of motion within an atomwithin an atom

1.1. Electrons spinning Electrons spinning on their own axison their own axis

2.2. Electrons orbiting Electrons orbiting the nucleusthe nucleus

3.3. The nucleus The nucleus spinning about its spinning about its own axisown axis

The principles of MRI The principles of MRI rely on the rely on the spinning spinning motion of specificmotion of specific nucleinuclei present in present in biological tissues biological tissues

These are called (These are called (MR MR active nuclei)active nuclei)

MR active nuclei ?MR active nuclei ?

MR active nuclei are Characterized by MR active nuclei are Characterized by their their tendency to align their axis of rotationtendency to align their axis of rotation to an applied magnetic fieldto an applied magnetic field

Due to the laws of electromagnetic Due to the laws of electromagnetic induction, induction, nuclei that have a net chargenuclei that have a net charge and are spinning and are spinning acquire a magnetic acquire a magnetic momentmoment and are and are able to align with an able to align with an external magnetic fieldexternal magnetic field

MR active nuclei MR active nuclei continued..continued..

The nuclei with The nuclei with odd odd mass numbersmass numbers undergoes this undergoes this interactioninteraction

The result of this The result of this interaction is interaction is angular angular momentum or spinmomentum or spin

Important Examples

HydrogenHydrogen 11

CarbonCarbon 1313

Nitrogen Nitrogen 1515

OxygenOxygen 1717

FluorineFluorine 1919

SodiumSodium 2323

PhosphorusPhosphorus 3131

The magnetic moment alignmentThe magnetic moment alignment

The alignment of the magnetic moment is The alignment of the magnetic moment is measured as the measured as the total of the nucleartotal of the nuclear magnetic momentsmagnetic moments and is expressed as a and is expressed as a vector sumvector sum

The The strengthstrength of the of the total magnetic momenttotal magnetic moment is is specific to every nucleusspecific to every nucleus and and determines the determines the sensitivitysensitivity to magnetic to magnetic resonanceresonance

The hydrogen nucleusThe hydrogen nucleus

The hydrogen nucleus is the MR active The hydrogen nucleus is the MR active nucleus used in clinical MRInucleus used in clinical MRI

Very abundant in the bodyVery abundant in the body

Solitary proton gives a Solitary proton gives a relatively large relatively large magnetic momentmagnetic moment

The hydrogen nucleus as a magnetThe hydrogen nucleus as a magnet

The nucleus contains The nucleus contains one positively one positively charged proton that charged proton that spinsspins

The spin of the proton The spin of the proton induces a magnetic induces a magnetic field around it and field around it and acts as a small acts as a small magnetmagnet

S

NN

S

The magnetic vectorThe magnetic vector

The magnetic moment of each nucleus The magnetic moment of each nucleus has vector properties.has vector properties.

i.e. it has i.e. it has sizesize and and directiondirection and is denoted and is denoted by an arrowby an arrow

size

direction

Alignment of the magnetic Alignment of the magnetic momentsmoments

In the absence of an applied magnetic In the absence of an applied magnetic field the magnetic moments are randomly field the magnetic moments are randomly orientedoriented

When placed in a strong external magnetic When placed in a strong external magnetic field the magnetic moments of the field the magnetic moments of the hydrogen nuclei align with this magnetic hydrogen nuclei align with this magnetic field , field , parallelparallel or or anti-parallelanti-parallel (as shown in (as shown in next slide)next slide)

Alignment of the magnetic momentsAlignment of the magnetic moments

Random alignment in the absence of external magnetic field

Alignment External magnetic field

Parallel

Anti-parallel

The state of alignmentThe state of alignment

Quantum physics describes that the Quantum physics describes that the hydrogen nuclei only possesses two hydrogen nuclei only possesses two energy statesenergy states or populations – or populations – lowlow & & highhigh

Low energy nucleiLow energy nuclei align their magnetic align their magnetic moments moments parallelparallel to the external magnetic to the external magnetic fieldfield

High energyHigh energy nucleinuclei align their magnetic align their magnetic moments moments anti-parallelanti-parallel to the external to the external magnetic fieldmagnetic field

Energy levels & field strengthEnergy levels & field strength

Low energy population

high energy population

Energy difference depends on field strength

Energy levels & alignmentsEnergy levels & alignmentsThe energy level and the number of nuclei The energy level and the number of nuclei aligned in each direction is determined by aligned in each direction is determined by the strength of the external magnetic field the strength of the external magnetic field and the thermal energy level of the nucleiand the thermal energy level of the nucleiLow thermal energy nuclei do not have Low thermal energy nuclei do not have enough energy to oppose the field and enough energy to oppose the field and align parallelalign parallelHigh thermal energy nuclei have sufficient High thermal energy nuclei have sufficient energy to oppose and may align anti-energy to oppose and may align anti-parallelparallel

Alignment & field strengthAlignment & field strength

Thermal energy depends on the body Thermal energy depends on the body temperaturetemperatureThe main deciding factor to increase the The main deciding factor to increase the number of parallel alignments is the high number of parallel alignments is the high field strength of the external magnetic field field strength of the external magnetic field At thermal equilibrium the parallel At thermal equilibrium the parallel population is higher than the anti-parallel population is higher than the anti-parallel population population Therefore there is a Therefore there is a net magnetic momentnet magnetic moment parallel to the external magnetic fieldparallel to the external magnetic field

The net magnetization vectorThe net magnetization vector

B0

Net Magnetization Vector (NMV)

SummarySummary

The magnetic moment (of hydrogen in this The magnetic moment (of hydrogen in this case) is called the Net Magnetization case) is called the Net Magnetization Vector (NMV)Vector (NMV)

The static external magnetic field is called The static external magnetic field is called BB0 0

TheThe interaction of the NMV with Binteraction of the NMV with B0 0 is the is the

basis of MRIbasis of MRI

The unit of BThe unit of B00 is is TeslaTesla or or GaussGauss. .

1 Tesla (T) = 10000 Gauss (G)1 Tesla (T) = 10000 Gauss (G)

Summary continued…Summary continued…

When a patient is placed in the bore of the When a patient is placed in the bore of the magnet the hydrogen nuclei within the patient magnet the hydrogen nuclei within the patient align parallel and anti-parallel to Balign parallel and anti-parallel to B00..

A small A small excess of hydrogen nuclei line up excess of hydrogen nuclei line up parallel to Bparallel to B0 0 and constitute the NMV of the and constitute the NMV of the

patient.patient.

The energy difference between the two The energy difference between the two populations increases as Bpopulations increases as B00 increases. increases.

The magnitude of NMV is larger at high field The magnitude of NMV is larger at high field strengths(Bstrengths(B00 ) )

PrecessionPrecessionEach hydrogen nucleus Each hydrogen nucleus that makes up the NMV that makes up the NMV is spinning on its own is spinning on its own axisaxis

The influence of BThe influence of B00 produce an additional produce an additional spin or wobblespin or wobbleThis path is called the This path is called the precessional path and precessional path and the speed at which the the speed at which the NMV wobbles around NMV wobbles around BB00 is called the is called the precessional frequencyprecessional frequency

B0

Precession

Precessional path

Magnetic moment of the nucleus

Hydrogen nucleus

Precession continued….Precession continued….

Two populations;Two populations;

High energy nuclei – High energy nuclei – spin downspin down

Low energy nuclei – Low energy nuclei – spin upspin up

Their magnetic Their magnetic moments precess on moments precess on a circular path around a circular path around BB00 as shown as shown

Spin up nuclei

Spin down nuclei

B0

Precession

The Larmor equationThe Larmor equation

The value of the precessional frequency is The value of the precessional frequency is governed by the Larmor equation i.egoverned by the Larmor equation i.e

The precessional frequency (The precessional frequency (ωω00) = Magnetic ) = Magnetic field strength(Bfield strength(B00) x Gyro-magnetic ratio() x Gyro-magnetic ratio(γγ))

ω ω 0 0 = B= B0 0 x x γγGyro-magnetic ratio is a constant for a Gyro-magnetic ratio is a constant for a specific MR active nucleus and is specific MR active nucleus and is expressed as the precssional frequency at expressed as the precssional frequency at 1.0 tesla. The unit is MHz / T1.0 tesla. The unit is MHz / T

Precessional frequencies of Precessional frequencies of HydrogenHydrogen

γγ BB00 ωω

1.5 T1.5 T 63.86 MHz63.86 MHz

42.57 Mhz/T42.57 Mhz/T 1.0 T1.0 T 42.57 MHz42.57 MHz

0.5 T0.5 T 21.28 MHz21.28 MHz

ResonanceResonance

Resonance is a phenomenon that occurs Resonance is a phenomenon that occurs when an object is exposed to an oscillating when an object is exposed to an oscillating perturbation that has a frequency close to perturbation that has a frequency close to its own natural frequency of oscillation. its own natural frequency of oscillation.

At resonance the object can absorb At resonance the object can absorb energy from the external sourceenergy from the external source

ThereforeTherefore Exchange of energy Exchange of energy between between two systems at a two systems at a specific specific frequencyfrequency is is called called resonanceresonance..

Nuclear ResonanceNuclear Resonance

When a nucleus is exposed to an external When a nucleus is exposed to an external perturbation that has an oscillation similar perturbation that has an oscillation similar to its own natural frequency, the nucleus to its own natural frequency, the nucleus gains energy from the external force. gains energy from the external force.

The nucleus gains energy and resonates if The nucleus gains energy and resonates if the energy is delivered at exactly its the energy is delivered at exactly its precessional frequency.precessional frequency.

RF signal & Nuclear magnetic ResonanceRF signal & Nuclear magnetic Resonance

Energy at the precessional frequency of hydrogen Energy at the precessional frequency of hydrogen at all field strengths in clinical MRI corresponds to at all field strengths in clinical MRI corresponds to the radio frequency (RF) band of the the radio frequency (RF) band of the electromagnetic spectrumelectromagnetic spectrum

For For resonance ofresonance of hydrogenhydrogen to occur, an to occur, an RFRF pulsepulse of energy at exactly the of energy at exactly the Larmor frequencyLarmor frequency of the of the hydrogenhydrogen NMV must be applied NMV must be applied

Other MR active nuclei that have aligned with BOther MR active nuclei that have aligned with B00

do not resonate because their precessional do not resonate because their precessional frequencies are different to that of hydrogenfrequencies are different to that of hydrogen

Excitation & RF frequencyExcitation & RF frequency

The application of an RF pulse that causes The application of an RF pulse that causes resonance to occur is termed resonance to occur is termed excitationexcitation..

The absorption of energy causes an The absorption of energy causes an increaseincrease in in the number of the number of spin downspin down hydrogen hydrogen nucleinuclei populations as some of the spin up nuclei gain populations as some of the spin up nuclei gain energy via resonance and become high energy energy via resonance and become high energy nuclei (next slide)nuclei (next slide)

The energy difference corresponds to the The energy difference corresponds to the energy required to produce resonance via energy required to produce resonance via excitationexcitation

Energy transfer during excitationEnergy transfer during excitation

Low energy population

High energy population

Some nuclei gain energy to join the high energy population

The results of resonanceThe results of resonance

The first result is the NMV moves out of The first result is the NMV moves out of alignment away from Balignment away from B00

The angle to which the NMV moves out of The angle to which the NMV moves out of alignment is called the alignment is called the flip angleflip angle The magnitude of the flip angle depends The magnitude of the flip angle depends upon the amplitude and duration of RF upon the amplitude and duration of RF pulsepulseUsually Usually the flip angle is 90the flip angle is 9000 (see next (see next slide). The transverse NMV rotates at the slide). The transverse NMV rotates at the Larmor frequencyLarmor frequency

The flip angle & Transverse planeThe flip angle & Transverse plane

BB0 0 is now termed the is now termed the longitudinal planelongitudinal plane

The plane at 90The plane at 9000 to B to B00 is termed the is termed the

transverse planetransverse planeLongitudinal plane

Longitudinal plane

Transverse plane

Transverse plane

Flip angle

NMV

NMV

Flip angle 900

B0

In phase / out of phaseIn phase / out of phase

The second result of resonance is that the The second result of resonance is that the magnetic moments within the transverse NMV magnetic moments within the transverse NMV move into phase with each othermove into phase with each other

Phase is the position of each magnetic moment Phase is the position of each magnetic moment on the precessional path around Bon the precessional path around B00

Magnetic momentsMagnetic moments that are that are in phasein phase are in the are in the same placesame place on the precessional path around B on the precessional path around B00

at any given timeat any given time

MM MM that are that are out of phaseout of phase are are notnot in the in the same same placeplace on the precessional path on the precessional path

Phase of magnetic moments Phase of magnetic moments around the precessional patharound the precessional path

Out of phase

In phase

SummarySummary

For resonance of hydrogen to occur, RF at For resonance of hydrogen to occur, RF at exactly the Larmor frequency of hydrogen exactly the Larmor frequency of hydrogen must be appliedmust be applied

The result of resonance is an NMV in the The result of resonance is an NMV in the transverse plane that is in phasetransverse plane that is in phase

This NMV precesses in the transverse This NMV precesses in the transverse plane at the Larmor frequencyplane at the Larmor frequency

The MR signalThe MR signal

Formation of MR signal after Formation of MR signal after removal of RF pulseremoval of RF pulse

The MR signalThe MR signal

As a result of resonance the NMV is As a result of resonance the NMV is precessing in phase in the transverse precessing in phase in the transverse plane.plane.

According to Faraday’s laws of induction, According to Faraday’s laws of induction,

When a receiver coil (a conductive loop) is When a receiver coil (a conductive loop) is placed in the area of moving magnetic placed in the area of moving magnetic field a voltage is induced in it. field a voltage is induced in it.

This Signal is produced when coherent (in This Signal is produced when coherent (in phase) magnetization cut across the coil.phase) magnetization cut across the coil.

MR signal continued….MR signal continued….

Therefore the moving NMV produces Therefore the moving NMV produces magnetic field fluctuations inside the coil magnetic field fluctuations inside the coil

As the NMV precesses at the Larmor As the NMV precesses at the Larmor frequency in the transverse plane a frequency in the transverse plane a voltage is induced in the coil.voltage is induced in the coil.

This voltage constitutes the MR signalThis voltage constitutes the MR signal

The The frequencyfrequency of the MR signal is the of the MR signal is the same as the same as the Larmor frequencyLarmor frequency

TheThe magnitudemagnitude of the MR signal depends of the MR signal depends on the on the amount of magnetization presentamount of magnetization present in in the transverse planethe transverse plane

Generation of the MR signal in the Generation of the MR signal in the receiver coilreceiver coil

B0

NMV

Receiver coil

Pre

cess

ion o

f N

MV

Relaxation & The free Relaxation & The free induction decay signalinduction decay signal

Switching off RF pulseSwitching off RF pulseRelaxationRelaxationRecovery & decayRecovery & decayFIDFID

RelaxationRelaxation

When the RF pulse is turned off the NMV When the RF pulse is turned off the NMV is again influenced by Bis again influenced by B0 0 , and, it tries to , and, it tries to

realign with it. realign with it.

To do that it must lose the energy given to To do that it must lose the energy given to it by the RF pulse.it by the RF pulse.

The process by which the NMV loses The process by which the NMV loses energy is called energy is called relaxationrelaxation

As relaxation occurs the NMV returns to As relaxation occurs the NMV returns to align with Balign with B0 0

When this happens,When this happens,

The amount of magnetization in the The amount of magnetization in the longitudinal planelongitudinal plane gradually increases – gradually increases – this is called this is called recoveryrecovery

The amount of magnetization in the The amount of magnetization in the transverse planetransverse plane gradually decreases – gradually decreases – this is called this is called decaydecay

Recovery & DecayRecovery & Decay

The free induction decay signalThe free induction decay signal

As the magnitude of transverse As the magnitude of transverse magnetization decreases so does the magnetization decreases so does the voltage induced in the receiver coil. voltage induced in the receiver coil.

The induction of this reduced signal is The induction of this reduced signal is called the called the free induction decayfree induction decay (FID)(FID) signal signal

Result of relaxationResult of relaxationDuring relaxation During relaxation

The NMV gives up absorbed energy and The NMV gives up absorbed energy and returns to Breturns to B00

The magnetic moments of the NMV lose The magnetic moments of the NMV lose the transverse magnetization due to the transverse magnetization due to dephasing dephasing

In phase Dephasing

Out of phase

Looking down on to transverse plane

T1 Recovery & T2 Decay T1 Recovery & T2 Decay Relaxation results in Relaxation results in

recovery ofrecovery of magnetization in the longitudinal planemagnetization in the longitudinal plane

and and

decay of magnetization in the transverse planedecay of magnetization in the transverse plane..

The The recovery of longitudinal magnetization recovery of longitudinal magnetization is is caused by a process calledcaused by a process called T1 recovery T1 recovery

The The decay of transverse magnetizationdecay of transverse magnetization is caused is caused by a process called by a process called T2 decayT2 decay

T1 RecoveryT1 Recovery

T1 recovery is caused by the nuclei giving T1 recovery is caused by the nuclei giving up their energy to the surrounding up their energy to the surrounding environment or lattice and it is often environment or lattice and it is often termed termed spin - lattice relaxationspin - lattice relaxation

The rate of recovery is an exponential The rate of recovery is an exponential process with a process with a recovery time constantrecovery time constant called called T1T1

Recovery time constant -T1Recovery time constant -T1

T1 is the time T1 is the time it takes 63% of it takes 63% of the longitudinal the longitudinal magnetization magnetization to recover in to recover in the tissuethe tissue

Time

100%

63%

Sig

nal

inte

nsi

ty

T1

T2 decayT2 decay

This is caused by nuclei exchanging This is caused by nuclei exchanging energy with neighbouring nuclei. energy with neighbouring nuclei. The energy exchange is caused by the The energy exchange is caused by the magnetic fields of each nucleus magnetic fields of each nucleus interacting with its neighbour. interacting with its neighbour. It is often termed It is often termed spin-spin relaxationspin-spin relaxation results in a decay or loss of transverse results in a decay or loss of transverse magnetizationmagnetizationThe rate of decay is also an exponential The rate of decay is also an exponential process so that the process so that the T2T2 relaxation time is relaxation time is its its time constant of decaytime constant of decay

Time constant of decay – T2Time constant of decay – T2

T2 is the time T2 is the time it takes 63% of it takes 63% of the transverse the transverse magnetization magnetization to be lostto be lost

100%

37%

T2 Time

Sig

nal

inte

nsi

ty

Dephasing of the FIDDephasing of the FID

A signal or voltage is only induced in the A signal or voltage is only induced in the receiver coil if there is magnetization in the receiver coil if there is magnetization in the transverse plane that is in phasetransverse plane that is in phase

Dephasing (T2)

Signal (FID)

Pulse timing parametersPulse timing parameters

The magnitude and timing of the The magnitude and timing of the RF pulses form the basis of MRI RF pulses form the basis of MRI and are discussed in Next lessonand are discussed in Next lesson