MRI Physics RV

MRI PHYSICS-1

MRI PHYSICS-1Roshan Valentine

Overview of steps The patient is placed in a magnet,Radio wave is sent in,Radio wave is turned off,Patient emits a signal,Signal is received Image is reconstructed

But before that lets learn some BASIC PHYSICS

Human body is ~53% water, and water is ~11% hydrogen by mass but ~67% hydrogen by atomic percent. Thus, most of the mass of the human body is oxygen, but most of the atoms in the human body are hydrogen atoms.The average 70kg adult human body contains approximately 3 x 1027 atoms of which 67% are hydrogen atoms!!!!

But why we bothered about

ONLY HYDROGEN

Simplest element with atomic number of 1 and atomic weight of 1When in ionic state (H+), it is nothing but a proton.Proton is not only positively charged, but also has magnetic spin (wobble)!MRI utilizes this magnetic spin property of protons of hydrogen to elicit images!!

THUS

But why we cant act like magnets?The protons (i.e. Hydrogen ions) in body are spinning in a haphazard fashion, and cancel all the magnetism. That is our natural state!

Structure of an ATOMAtom consistes of a centre nucleus which Contains a proton and neutronAround which an electron rotates around it and its own axisThus its analogous to our solar system In the nucleus - besides other things - there are protons, little particles, that have a positive electrical chargeAny moving electrical charge is an electrical currentAny moving electric current induces a magnetic field

Thus, the proton has its own magnetic field and it can be seen as a little bar magnet

SPINProton in its natural state are arranged haphazardly with no net charge.But when exposed to an external magnetic field(B0) they arrange parallel or antiparallel to the external field.Naturally anti parallel needs more energy compared to parallel

In NMR it is the unpaired nuclear spins that produce a signal in a magnetic field

Insert walking slide14

PrecessionProtons as we all think doesnot just kay thereInstead they undergo PRECESSION

NOW WHAT IS PRECESSION

LARMOR EQUATION

Gyro for hydrogen is 42.5Mhz/T18

PHASErefers to the position of the magnetic moments on their circular precessional path Out of phase or incoherent means that the magnetic moments of hydrogen are at different places on the precessional path. In phase or coherent means that the magnetic moments of hydrogen are at the same place on the precessional path.

Measuring magnetization

TIME FOR SOME VECTORS

For easier understanding every proton can be considered as vectors.A vector can represent the direction of the force and the magnitude by its size.Here the force is referred to as the MAGNETIC FORCEMagnetization along an external magnetic field cannot be measured. For this a magnetization transverse to the external magnetic field is necessary.

Human body can be considered as a large magnet with its vector along the Longitudinal axis called LONGITUDINAL MAGNETIZATION.In a strong external magnetic field a newmagnetic vector is induced in the patient,who becomes a magnet himself. Thisnew magnetic vector is aligned with theexternal magnetic field.

TRANSVERSE MAGNETIZATIONLONGTIUDINAL

MAGNETIZATION

DUE TO PROTON ARRANGING ALONG THE DIRECTION

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Magnetization along an external magnetic field cannot be measured. For this a magnetization transverse to the external magnetic field is necessary.

RESONANCEPhenomenon that occurs when an object is exposed to a frequency close to its natural frequency of oscillation-LARMOR FREQUENCY

Lets take an example

ENERGYRF LARMOR FREQUENCY

But when RF =

For resonance of hydrogen to occur, an RF pulse of energy at exactly the Larmor frequency of hydrogen must be applied.Excitation application of RF pulse that causes resonance to occur.

Flip angleMagnitude of flip angle depends on the amplitude and duration of RF pulse.The plane at 90 to B0 is termed transverse plane.When resonance occurs, all the magnetic moments are in phase with each other.

MR SignalAs the NMV precesses at Larmor frequency in the transverse plane, a voltage is induced in the coil. This voltage constitutes the MR signal.

The magnitude of the signal depends on the amount of magnetisation present in the transverse plane.

Free Induction DecayRecovery gradual increase of magnetisation in longitudinal plane.Decay gradual decrease of magnetisation in transverse plane.

The magnitude of voltage induced in the receiver coil also decreases FID signal This is called free induction decay (FID): free because of the absence of the RF pulse; induction decay because of the decay of the induced signal in the receiver coil

CONTRAST MECHANISMHigh Intensity ( white)- High TmLow intensity ( dark) Low Tm

TR(Repitition time ) and TE(Echo time)

What determines the contrastThe inherent energy of the tissue How closely packed the molecules areHow well the molecular tumbling rate matches the Larmor frequency of hydrogen

T1 RecoverySPIN-LATTICE energy transferAs they loose energy , they regain their LmT1 time - dened as the time it takes for 63% of the Lm to recover.The TR determines how muchT1 recovery occurs in a particular tissue as it occurs during TR

T1 Recovery

T1 Recovery

Short T1 (Fat)Long T1 (Water)contrast

T2 DecaySPIN-SPIN Energy TransferDue to the intrinsic magnetic fields of the nuclei interacting with each other.Time it takes for 63% of the transverse magnetization to be lost due to dephasing.TE determines the T2 decay as dephasing occurs during thenDepends on how closely the molecular motion of the atomsmatches the Larmor frequency and the proximity of other spins.

T2 DecayFat has better energy exchange compared to water HENCE T2 is SHORT FOR FAT.

T2 Decay

contrast

T1 Weighting T2 Weighting

3006001030

msmsmsms>2000>70

T1T2

Proton Density(PD)WeightingContrast is predominantly due to differences in the proton density of the tissuesLow PD darkHigh PD brightCortical bone and air are always darkPDW Decreasing T1 and T2 effects

T1 Weighting T2 Weighting

3006001030

msmsmsms>2000>70

Pulse Sequence MechanismsMagnetic eld inhomogeneities cause the NMV to dephase before intrinsic magnetic elds of the nuclei can produceThe main purposes of pulse sequences are: to rephase spins and remove inhomogeneity effects and therefore produce a signal or echo ; to enable manipulation of the TE and TR to produce different types of contrast.

Pulse Sequence MechanismsTo measure relaxation times and produce an image with good contrast we need to regenerate the signal .Hence pulse sequences

TypesSpin Echo Pulse SequenceGradient Echo Pulse Sequence

Spin Echo Pulse Sequence

A basic rephasing sequence.

The time taken to rephase after the application of the 180 RF pulse equals the time to dephase when the 90 RF pulse was withdrawn.This time is called the TAU time.

Spin EchoSpin echo pulse sequences produce either T1, T2 or proton density weighting TR controls the T1 weighting Short TR maximizes T1 weighting Long TR maximizes proton density weighting TE controls the T2 weighting Short TE minimizes T2 weighting Long TE maximizes T2 weighting

Spin Echo (SE) Using Single Echo

The timing parameters used are selected to produce a T1 weighted image.

Spin Echo With Two Echoes

Produce both a PD and a T2WI PD

T2WI

FAST SPIN ECHO (TURBO SPIN ECHO)Faster version of conventional spin echo. More than one phase encoding is performed per TR, reducing the scan time.FSE employs a train of 180 rephasing pulses, each one producing a spin echo. This train of spin echoes is called an echo train. The number of 180 RF pulses and resultant echoes is called the echo train length (ETL) or turbo factor.

FAST SPIN ECHO (TURBO SPIN ECHO)The higher the turbo factor the shorter the scan time

In T2 weighted scans, water and fat are hyperintense (bright). This is because the succession of 180 RF pulses reduces the spinspin interactions in fat thereby increasing its T2 decay time.

Spatial encoding in conventional spin echo.

The echo train.

Inversion Recovery Sequence

Inversion Recovery SequenceInversion recovery (IR) was developed in the early days of MRI to provide good T1 contrast on low field systems.But became obsolete due to high scanning timeNow back when combined with fast spin sequences

Fast Inversion Recovery Sequence180 inverting pulse is followed after the TI time by the 90 excitation pulse and the train of 180 RF pulses to fill out multiple lines of K space as in fast spin echo.instead of being used to produce T1 weighted images, fast inversion recovery is usually used to suppress signal from certain ti ssues in conjuncti on with T2 weighti ng so that water and pathology return a high signal

Fast Inversion Recovery Sequence2 TypesSTIR( short tau inversion recovery)FLAIR ( fluid attenuated inversion recovery)

STIRthe time it takes fat to recover from full inversion to the transverse plane so that there is no longitudinal magnetizati on corresponding to fat. This is called the null pointUsesSTIR is an extremely important sequence in musculoskeletal imaging because normal bone, whichcontains fatty marrow, is suppressed and lesions within bone such as bone bruising and tumors are seen more clearly (Figures 5.18 and 5.19 ). It is also a very useful sequence for suppressing fat in general MR imaging ( see Chapter 6).

FLAIRFLAIR is used to suppress the high CSF signal in T2 weighted imagesTI corresponding to the time of recovery of CSF from 180 to the transverse plane nulls the signal from CSFTo see periventricular and cord lesions more clearly

STIR FLAIR

Gradient Echo Pulse SequenceGradient is used to reduce magnetic homogeneityPrinciple based on Larmor equation

Gradient Echo Pulse SequenceAfter the RF pulse is withdrawn, FID signal is immediately produced due to magnetic field inhomogeneities and T2* dephasing occurs.The gradient rephases the magnetic moments so that a signal can be received by the coil, which contains T1 and T2 information.This signal is called a gradient echo

How gradients dephase

Something like the rabbit and turtle race74

How gradients rephase

Post dephasing aftr RF is switched off75

AdvantagesRephase faster.Low flip angles (