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MRI MRI Physics Physics Dr Mohamed El Safwany, MD .

MRI Physics Dr Mohamed El Safwany, MD.. MRI Magnetic Resonance Imaging Magnetic Resonance Imaging

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MRIMRI

PhysicsPhysics

Dr Mohamed El Safwany, MD.

MRIMRI

MMagnetic agnetic RResonance esonance IImagingmaging

Intended learning outcomeIntended learning outcome

►The student should learn at the end of The student should learn at the end of this lecture MRI physics .this lecture MRI physics .

M is for MagneticM is for Magnetic

►Atomic structure:Atomic structure: ProtonProton NeutronNeutron ElectronElectron

M is for MagneticM is for Magnetic

►Motion within the atom:Motion within the atom:

SPINSPIN

M is for MagneticM is for Magnetic

►MR active nuclei:MR active nuclei: H1 H1 C13 C13 F19 F19 P31 P31 N15 N15 O17O17

M is for MagneticM is for Magnetic

►Why H1?Why H1? 1000 Billion Billion Atom in adult1000 Billion Billion Atom in adult Solitary Proton gives it a large magnetic Solitary Proton gives it a large magnetic

momentmoment

M is for MagneticM is for Magnetic

""NMVNMV""

Net Magnetization VectorNet Magnetization Vector

M is for MagneticM is for Magnetic

PrecessionPrecession

Larmor EquationLarmor Equation

Frequency Frequency αα Magnetic FieldMagnetic Field

ωω αα ββ

MM►Atomic structureAtomic structure►SPINSPIN►MR active nuclei MR active nuclei H1H1►What happen when placed in What happen when placed in

external magnetic field (external magnetic field (Spin UpSpin Up and and Spin Down Spin Down))

►NMVNMV►PrecessionPrecession►Larmor EquationLarmor Equation

R is for ResonanceR is for Resonance

Def:Def:

Energy transitionEnergy transition that occur that occur when object is subjected to when object is subjected to frequency frequency the samethe same as its as its ownown

R is for ResonanceR is for Resonance

Here;Here;

Radio FrequencyRadio Frequency " "RFRF""Same Frequency of Same Frequency of HH nuclei nuclei

At At 90 degree to B090 degree to B0

R is for ResonanceR is for Resonance

►RF Removed RF Removed Signal decreased Signal decreased Amplitude of MR Signal decreasedAmplitude of MR Signal decreased

I is for ImagingI is for Imaging

►Areas of Areas of High SignalHigh Signal►Areas of Areas of Low SignalLow Signal►Areas of Areas of Intermediate SignalIntermediate Signal

I is for ImagingI is for Imaging

NMVNMV can be can be separatedseparated in to in to

Individual Vectors of tissue present Individual Vectors of tissue present in the patientin the patient

►Such as Such as FatFat, , CSFCSF & & MuscleMuscle

I is for ImagingI is for Imaging

High High SignalSignal

Low Low SignalSignal

IntermediatIntermediatee

WhiteWhite BlackBlack GreyGrey

I is for ImagingI is for Imaging

Gradient MagnetsGradient Magnets►Used to Used to varyvary magnetic field in known magnetic field in known

mannermanner►Each point has slightly different rate of Each point has slightly different rate of

precession & Larmor Frequency.precession & Larmor Frequency.►Variety of signalVariety of signal released by Protons released by Protons

returning to z-plane can used to returning to z-plane can used to determine the composition of exact determine the composition of exact locationlocation of each point. of each point.

Contrast MechanismsContrast Mechanisms

High High SignalSignal

Low Low SignalSignal

IntermediatIntermediatee

WhiteWhite BlackBlack GreyGrey

Contrast Mechanisms Contrast Mechanisms ((Relaxation Relaxation Process)Process)

Relaxation ProcessRelaxation Processafter removal of RF pulseafter removal of RF pulse

Signal induced in Receiver Coil Signal induced in Receiver Coil decreasedecrease

Contrast Mechanisms Contrast Mechanisms ((DefinitionsDefinitions))

Repetition Time "TR"Repetition Time "TR"TimeTime from from application of one RF pulseapplication of one RF pulse

To the To the application of the nextapplication of the next

(it affects the length of relaxation period(it affects the length of relaxation period

after application of one RF excitation after application of one RF excitation pulsepulse

to the beginning of the next)to the beginning of the next)

Contrast Mechanisms Contrast Mechanisms ((DefinitionsDefinitions))

Echo Time "TE"Echo Time "TE"TimeTime between between RF excitation pulseRF excitation pulse and andcollection of signalcollection of signal

(it affects the length of relaxation period(it affects the length of relaxation periodafter removal of RF excitation pulseafter removal of RF excitation pulseand the peak of signal received in and the peak of signal received in

receiver coil)receiver coil)

Contrast Mechanisms Contrast Mechanisms ((DefinitionsDefinitions))

Flip AngleFlip AngleAngleAngle throw which the throw which the NMV movedNMV moved as as

result of a result of a RF excitation pulseRF excitation pulse

Contrast Mechanisms Contrast Mechanisms ((ParametersParameters))

Image contrast controlled by:Image contrast controlled by:

1- 1- Extrinsic Contrast parametersExtrinsic Contrast parameters::TRTR, , TETE & & Flip AngleFlip Angle

2- 2- Intrinsic Contrast parametersIntrinsic Contrast parameters::T1 RecoveryT1 Recovery, , T2 DecayT2 Decay, , Proton DensityProton Density, ,

FlowFlow & & Apparent Diffusion CoefficientApparent Diffusion Coefficient

Contrast Mechanisms Contrast Mechanisms ((T1 T1 Recovery)Recovery)

Short TR Short TR T1 contrast T1 contrast

((T1 WeightedT1 Weighted))

►TR 300-600 msTR 300-600 ms►TE 10-30 msTE 10-30 ms

Contrast Mechanisms Contrast Mechanisms ((T2 T2 Decay)Decay)

FatFat much much better at energy exchangebetter at energy exchange than than WaterWater

Because this T2 depend on:Because this T2 depend on:

1-How closely molecular motion of atoms matches 1-How closely molecular motion of atoms matches Larmor FrequencyLarmor Frequency

2-Proximity of other spins2-Proximity of other spins

So;So;

Fat's T2 time is very short compared to Fat's T2 time is very short compared to waterwater

FAT

WATER

Contrast Mechanisms Contrast Mechanisms ((T2 T2 Decay)Decay)

Long TE Long TE T2 contrastT2 contrast

(T2 Weighted)(T2 Weighted)

►TR 2000 msTR 2000 ms►TE 70 msTE 70 ms

Contrast Mechanisms Contrast Mechanisms ((Proton Proton Density)Density)

►Proton DensityProton Density

Long TR Long TR Proton densityProton density

►TR 2000 msTR 2000 ms►TE 10-30 msTE 10-30 ms

Contrast Mechanisms Contrast Mechanisms ((Contrast Contrast Media)Media)

►Contrast MediaContrast Media►as as GadoliniumGadolinium► local magnetic field fluctuation occur local magnetic field fluctuation occur

near Larmor frequencynear Larmor frequency►T1 Relaxation times of nearby protons T1 Relaxation times of nearby protons

are reducedare reduced►So they So they appear brighter in T1 weighted appear brighter in T1 weighted

ImageImage

Text BookText Book

►David Sutton’s RadiologyDavid Sutton’s Radiology►Clark’s Radiographic positioning and Clark’s Radiographic positioning and

techniquestechniques

AssignmentAssignment

►Two students will be selected for Two students will be selected for assignment.assignment.

QuestionQuestion

►Define echo time TE in MRI?Define echo time TE in MRI?

Thank YouThank You

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