Mri physics

MRI PHYSICS

PEHCHAN KAUN

NOBEL PRIZE 2003Paul lauterburPETERMANSFIELD

MRI: What is it?

Magnetic Resonance ImagingMagnetRadio Frequency = ResonanceImaging

• NMR measures the net magnetization of atomic nuclei in the presence of magnetic fields

• Magnetization can be manipulated by changing the magnetic field environment (static, gradient, and RF fields)

• Static magnetic fields don’t change (< 0.1 ppm / hr): The main field is static and (nearly) homogeneous• RF (radio frequency) fields are electromagnetic fields that

oscillate at radio frequencies (tens of millions of times per second)

• Gradient magnetic fields change gradually over space and can change quickly over time (thousands of times per second)

WE ARE MAGNETS!

Really?

• The adult 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 70 kg adult human body contains approximately 3 x 1027 atoms of which 67% are hydrogen atoms!!!!

Why the fuss about hydrogen?

• Nucleus needs to have 2 properties:– Spin– charge

• Nuclei are made of protons and neutrons– Both have spin ½– Protons have charge

• Pairs of spins tend to cancel, so only atoms with an odd number of protons or neutrons have spin – Good MR nuclei are 1H, 13C, 19F, 23Na, 31P

Y HYDROGEN?? Cont…

• Simplest element with atomic number of 1 and atomic weight of 1

• When 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!!

• We are magnets!

A Single Proton

++

+

There is electric charge on the surface of the proton, thus creating a small current loop and generating magnetic moment m.

The proton also has mass which generates anangular momentumJ when it is spinning.

Jm

Thus proton “magnet” differs from the magnetic bar in that italso possesses angular momentum caused by spinning.

SPIN!!

• Protons and neutron 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

But why we can’t 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!

• We need to discipline them first, how?

• When placed in a large magnetic field, hydrogen atoms have a strong tendency to align in the direction of the magnetic field

• Inside the bore of the scanner, the magnetic field runs down the center of the tube in which the patient is placed, so the hydrogen protons will line up in either the direction of the feet or the head.

• The majority will cancel each other, but the net number of protons is sufficient to produce an image.

5-spin down 7spin up

Precession

• The static field causes the spinning proton to ‘wobble’ in a regular manner called ‘PRECESSION’

LARMOR EQUATION

TIME TO WAKE UP…REGARDING MRI SAY TRUE/FALSE• The gyro magnetic ratio of a hydrogen atom is

42.57MHz/T• The strength of magnetic field is measured in gauss• The Larmor frequency is defined as SUM of the

magnetic field and gyromagnetic ratio.• An MR active nucleus spins at its own precessional

frequency.• ONLY hydrogen nuclei are said to be MR active

SACH KA SAMNA….

REGARDING MRI SAY TRUE/FALSE• The gyro magnetic ratio of a hydrogen atom is

42.57MHz/T T• The strength of magnetic field is measured in gauss

T• The Larmor frequency is defined as SUM of the

magnetic field and gyromagnetic ratio. F• An MR active nucleus spins at its own precessional

frequency. T• ONLY hydrogen nuclei are said to be MR active F

IN A NUT SHELL

Now, its time to listen to radio in RESONANCE.

►Pushing a swing in time with natural interval of the swing will make the swing higher and higher.

►Similarly, radio frequency pulses in resonance push the aligned protons (H+) to a higher energy level.

WATS RESONANCE????Def:

Energy transition that occur when object is subjected to frequency the same as its own.

Here;

Radio Frequency "RF"Same Frequency of H nucleiAt 90 degree to B0

2 things happen at Resonance:1- Energy AbsorptionIncrease number of High energy Spin Up

nuclei2- Phase CoherenceNMV precesses in transverse plane at Larmor

Frequency

• Energy Absorption:–The MRI machine applies

radio frequency (RF) pulse that is specific to hydrogen.

–The RF pulses are applied through a coil that is specific to the part of the body being scanned.

MR Signal

• NMV rotates around transverse plane. It passes across Receiver Coil inducing voltage

in it.

• RF Removed Signal decreased Amplitude of MR Signal decreased

• Free Induction Decay "FID":– Free (No RF Pulse)– ID (because of Decay of Induced signal in Receiver Coil)

SUMMARY TILL NOW…..ENJOYY…

• Measuring the MR Signal:– the moving proton vector

induces a signal in the RF antenna

– The signal is picked up by a coil and sent to the computer system.

the received signal is sinusoidal in nature

– The computer receives mathematical data, which is converted through the use of a Fourier transform into an image.

Measuring the MR Signalz

y x

RF signal from precessing protons

RF antenna

Now, we re-transmit the energy for image processing

►The emitted energy is too small (despite 2500 times the magnetic field with resonance RF pulse) to convert them into images.

►Hence, repeated “ON-OFF” of RF pulses are required.

►The emitted energy is stored (K-space), analysed and converted into images.

• Imaging:– When the RF pulse is turned off the

hydrogen protons slowly return to their natural alignment within the magnetic field and release their excess stored energy. This is known as relaxation.

• What happens to the released energy?– Released as heat

OR– Exchanged and absorbed by other protons

OR– Released as Radio Waves.

MR ImageMR Image

detaildetail

single voxelsingle voxel

fat and water protonsfat and water protons

net magnetizationnet magnetization

The Image

Relaxation Process

2-Nuclei loose Precessional coherence or dephaseand NMV decay in the transverse planethis process called "T2 Decay"

1-NMV recovers and realign to B0this process called "T1 Recovery"

T1 time & T2 Decay are an intrinsic contrast parameter that are inherent to tissue being imaged.

T1WI

Definitions

Echo Time "TE"Time between RF excitation pulse andcollection of signal

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

coil)

• Repetition Time "TR"Time from application of one RF pulseTo the application of the next

(it affects the length of relaxation periodafter application of one RF excitation pulseto the beginning of the next).

Flip AngleAngle throw which the NMV moved as result of a RF excitation pulse

Parameters

Image contrast controlled by:

1- Extrinsic Contrast parameters:TR, TE & Flip Angle

2- Intrinsic Contrast parameters:T1 Recovery, T2 Decay, Proton Density, Flow

& Apparent Diffusion Coefficient

T1 Recovery

T1 RecoveryCaused by exchange of energy fromnuclei to their surrounding environment or lattice"Spin Lattice Energy Transfer"and realign in B0this occur in exponential processat different rates in different tissue

NB: Molecules are constantly in motion; Rotational and Transitional

T1 Recovery

T1 in Water T1 in Fatinefficient at receiving energyT1 is longeri.e. nuclei take a lot longer to dispose energy to surrounding water tissueWATER

absorb energy quicklyT1 is very shorti.e. nuclei dispose their energy to surrounding fat tissue and return to B0 in very short time FAT

T1 Recovery

Short TR T1 contrast

(T1 Weighted)

• TR 300-600 ms• TE 10-30 ms

T2 Decay

Fat much better at energy exchange than WaterBecause T2 depends on:

1-How closely molecular motion of atoms matches Larmor Frequency

2-Proximity of other spinsSo;

Fat's T2 time is very short compared to water

FAT WATER

T2 Decay

Long TE T2 contrast

(T2 Weighted)

• TR 2000 ms• TE 70 ms

T2* DECAY

• T2* relaxation – Disturbances in magnetic field (magnetic susceptibility) increase the rate of spin coherence T2 relaxation

• The total NMR signal is a combination of the total number of nuclei (proton density), reduced by the T1, T2, and T2* relaxation components

Proton Density

• Proton Density

Long TR Proton density

• TR 2000 ms• TE 10-30 ms

Gradient Magnets• Used to vary magnetic field in known

manner• Each point has slightly different rate of

precession & Larmor Frequency.• Variety of signal released by Protons

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

• function of "Gradient Magnets” – Slice selection– Frequency encoding– Phase encoding

BRAIN STIMULATOR

REGARDING MRI……• T1 Recovery is due to interactions of

intrinsic magnetic fields of adjacent nuclei.• T2 Decay is due to dephasing caused by

inhomogenities of external magnetic field.• Fat has short T1 Recovery time.• Water has a short T1 Recovery time.• On T1 fat appears as high signal.

ANSWERS

• T1 Recovery is due to interactions of intrinsic magnetic fields of adjacent nuclei F

• T2*Decay is due to dephasing caused by inhomogenities of external magnetic fieldT

• Fat has short T1 Recovery time T• Water has a short T1 Recovery time F• On T1 fat appears as high signal T

IMAGE QUALITY

• Factors that determine image quality Signal: noise ratio , Contrast , Spatial Resolution, Scan time.

SIGNAL : NOISE RATIO

• Noise is a random variation in the MR signal, occurring at all frequencies and all the time, due to -1. PATIENT(white noise)

2.ENVIRONMENT. 3. SCANNER.• All noise reduces and obscures contrast

bbetween tissues and is worst in areas of low PD and low signal.

IMPROVE SNR…

• INCREASING THE SIGNAL BY

• INCREASING THE VOXEL SIZE

• DECREASING TE AND INCREASING TR/FLIP ANGLE.

• SE SEQUENCES > GRE• HIGH TESLA MACHINE

• REDUCING NOISE BY• INCREASING THE

NUMBNER OF EXCITATIONS

• REDUCING BANDWIDTH

• REDUCING CROSS TALK

The great headache—ARTIFACTS!!• Image Artifact is something observed in a scientific

investigation that is not naturally present but occurs as a result of the investigative procedure.

• A structure not normally present, but visible as a result of malfunction in the hardware or software of the device, or a consequence of environmental influences as heat or humidity or can be caused by the human body itself.

WHICH ARTIFACT????

Motion artifacts( ghost)

SPECIAL THANKS TO…….