MRI PHYSICS (PART I)

BY

DR. ARIFKHAN S

MRI – MAGNETIC RESONANCE IMAGING

• ATOM : Nucleus + electrons

• Nucleus : Neutrons+ PROTONS

• PROTONS spin and creates magnetic field.

• The protons - being little magnets - align themselves in the external magnetic field like

a compass needle in the magnetic field of the earth.

• May align parallel or anti-parallel

LARMOR EQUATION

is the precession frequency

(in Hz or MHz),

B0 is the strength of the

external magnetic field, which

is given in Tesla (T)

and

is the so-called gyromagnetic

ratio.

the value for protons is 42.5 MHz/T

MAIN MAGNET FIELD BO

• Purpose is to align H protons in H2O (little magnets)

[Little magnets lining up with external lines of force]

[Main magnet and some of its lines of force]

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 a magnetic bar in that italso possesses angular momentum caused by spinning.

VECTOR & COORDINATE SYSTEM

Protons in a Magnetic Field

Bo

Parallel(low energy)

Anti-Parallel(high energy)

Spinning protons in a magnetic field will assume two states.If the temperature is 0o K, all spins will occupy the lower energy state.

• Naturally the preferred state of alignment is the one that needs less energy

• So more protons are on the lower energy level, parallel to the external magnetic field

• Proton’s are not stationary. Precession movement is seen

precession frequency : precess per second

Depends upon the strength of the magnetic field

The stronger the magnetic field, the faster the precession rate

and the higher the precession frequency.

Magnetic field : B0

NMR: NUCLEAR MAGNETIC RESONANCE

• Resonance here refer to the change in energy states of the NUCLEI in response to A RF wave of specific Radio Frequency.

• Resonance can also occur in an external magnetic field.

• Criteria:

• Must have ODD number of protons or ODD number of neutrons.

• Examples:

• 1H, 13C, 19F, 23N, and 31P with gyromagnetic ratio of 42.58, 10.71,

• 40.08, 11.27 and 17.25 MHz/T

• The main Resonating nuclei in Human body is H.

Net Magnetization

BoM

T

BcM o

Energy Difference Between States

g/2 p = 42.57 MHz / Tesla for proton in hydrogen atom

Knowing the energy difference allows us to use electromagnetic waves with appropriate energy level to irradiate the spin system so that some spins at lower energy level can absorb right amount ofenergy to “flip” to higher energy level.

Spin System Before Irradiation

BoLower Energy

Higher Energy

Basic Quantum Mechanics Theory of MR

RESONANCE AND RF PULSING

2 things happen to Resonating protons when RF pulse is applied

1- Energy Absorption

Increase number of High energy Spin Up nuclei and these may align anti parallel

2- Phase Coherence

NMV precesses in transverse plane at Larmor Frequency

• High Energy state but Not in phase High energy state but in phase : COHERENCE

The Effect of Irradiation to the Spin System

Lower

Higher

Basic Quantum Mechanics Theory of MR

Spin System After Irradiation

Basic Quantum Mechanics Theory of MR

THIS ENERGY IS JUST HANDED OVER TO THEIR SURROUNDINGS, THE SO CALLEDLATTICE. AND THIS IS WHY THIS PROCESS IS NOT ONLY CALLEDLONGITUDINAL RELAXATION, BUTALSO SPIN-LATTICE-RELAXATION. THIS RELAXATION PRODUCES THE THERMAL SIGNAL WHICH IS ACQUIRED FOR IMAGING IN MRI

• T1 relaxation Curve : after switching off RF change in longitudinal MAGNETIZATION

• This time constant is the transversal relaxation time T2 , OR

• spin-spin-relaxation TIME

IMAGING TRICKS

• B0 MAGNETIC FIELD IS NOT UNIFORM. • INTERNAL MAGNETIC FIELD OF PROTONS: TISSUE DIFFERENCE• RF PULSING : DURATION AND INTERVAL

TISSUE FEATURES

• LIQUIDS HAVE LONG T1 AND LONG T2

When the lattice consists of pure liquid/water, it is difficult

for the protons to get rid of their energy, • FATS HAVE SHORT T1 AND T2

The carbon bonds at the ends of the fatty acids have frequencies near the Larmor frequency, thus resulting in effective energy transfer.

MRI : THE MACHINE

EQUIPMENT

Magnet

Gradient Coil

RF Coil

RF Coil

4T magnet

gradient coil(inside)

MAIN COMPONENTS OF A SCANNER• Static Magnetic Field Coils

• Gradient Magnetic Field Coils

• Magnetic shim coils

• Radiofrequency Coil

• Subsystem control computer

• Data transfer and storage computers

• Physiological monitoring, stimulus display, and behavioral recording hardware

MRI SCANNER COMPONENTS

STATIC MAGNETIC FIELD COILS

• Earths magnetic field is 0.3-0.7G (G: Gauss)

• 1 Tesla is 10000 G

• Magnets used for imaging mostly between .5 to 1.5 Tesla

• PERMANENT AND ELECTROMAGNET (SUPERCONDUCTING )

• The field should be homogenous. Inhomogeniety reduces the signal to noise ratio ; i.e affects the image quality.

PERMANENT MAGNETS & ELECTROMAGNETS • Permanent magnet : is always magnetic and does not use any energy for work.

but is thermally instable, field strength is limited, weight (a .3T magnet will be around 100 Tons!!!!)

• Electromagnet:

1. Resistive Magnet: An electrical current is passed through a loop of wire and generates a magnetic field

2. Superconducting magnets: widely used in MR machines. Here the current carrying conductor is kept at very low temperatures called SUPERCONDUCTING TEMPERATURE (4 Kelvin or -269 Celsius).

At this temperature the conducting material looses its resistance for electricity. And so produces constan magnetic field s.

Cryogens are Helium and Nitrogen.

• Advantages of superconducting magnets are high magnetic field strength and excellent magnetic

field homogeneity

VOLUME COILS

• These completely surround the part of the body that is to be imaged.

• it is the transmitter for all types of examinations. It also receives the signal when larger parts

of the body are imaged

• E.g The helmet-type head coil acts as receiver coil, the body coil transmitting the RF pulses while imaging the head

SHIM COILS

• Shimming: the process by which an in-homogenous magnetic filed is converted to a homogenous magnetic filed by electrical as well as mechanical adjustments.

• Shim Coils are such devices used for shimming of the magnetic field (i.e the STATIC MAGNETIC FIELDS)

IMAGING 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.

Gradient Function:• Slice selection• Frequency encoding• Phase encoding

GRADIENT FUNCTIONS

• Slice selection : We can selectively excite nuclei in one slice of tissue by incorporating a third magnetic field: the “gradient” magnetic field.

• The gradient magnetic field produces a linear change in the total magnetic field. • Here, “gradient” means “change in field strength as a function of

location in the MRI bore”. • Since the gradient field changes in strength as a function of position,

we use the term “gradient amplitude” to describe the field

PHASE AND FREQUENCY ENCODING Consider an MRI image composed of 9 voxels (3 x 3 matrix)

All voxels have the same precessional frequency and are all “in phase” after the slice select gradient and RF pulse

• When the Y “phase encode gradient” is on, spins on the top row have relatively higher precessional frequency and advanced phase. Spins on the bottom row have reduced precessional frequency and retarded phase

3. Turn off the Y “phase encode gradient”

4. All nuclei resume precessing at the same frequency

5. All nuclei retain their characteristic Y coordinate dependent phase angles

6. A “read out” gradient is applied along the X axis, creating a distribution of precessional frequencies along the X axis.

7. The signal in the RF coil is now sampled in the presence of the X gradient.

• While the frequency encoding gradient is on, each voxel contributes a unique combination of phase and frequency. The signal induced in the RF coil is measured while the frequency encoding gradient is on.

• 8. The cycle is repeated with a different setting of the Y phase encoding gradient. For a 256 x 256 matrix, at least 256 samples of the induced signal are measured in the presence of an X frequency encoding gradient. The cycle is repeated with 256 values of the Y phase encoding gradient.

• 9. After the samples for all rows are taken for every phase-encode cycle, 2D Fourier Transformation is then carried out along the phase-encoded columns and the frequency-encoded rows to produce intensity values for all voxels.

K- SPACE• The Fourier transformation acts on the observed “raw data” to form an image. A

conventional MRI image consists of a matrix of 256 rows and 256 columns of voxels (an “image matrix”).

• The “raw data” before the transformation ALSO consists of values in a 256 x 256 matrix

• The raw data matrix is also called K-SPACE.

• The 2DFT of k-space produces an image.

• Each value in the resulting image matrix corresponds to a grey scale intensity indicative of the MR characteristics of the nuclei in the voxels. Rows and columns in the image are said to be “frequency encoded” or “phase encoded”.

FT

FT-1

k-space

kx

ky

Acquired Data

Image space

x

y

Final Image

MRI task is to acquire k-space image then transform to a spatial-domain image. kx is sampled (read out) in real time to give N samples. ky is adjusted before each readout.

MR image is the magnitude of the Fourier transform of the k-space image

• The top row of k-space would be measured in the presence of a strong positive phase encode gradient.

• A middle row of k-space would be measured with the phase encode gradient turned off.

• The bottom row of k-space would conventionally be measured in the presence of a strong negative phase encode gradient.

• While the frequency encoding gradient is on, the voltage in the RF coil is measured at least 256 times. The 256 values measured during the first RF pulse are assigned to the first row of the 256 x 256 “raw data” matrix. The 256 values measured for each subsequent RF pulse are assigned to the corresponding row of the matrix.

RF BANDWIDTH• There are actually two RF bandwidths are associated with MRI:

Transmit and Receive

• RF Transmit bandwidth ~ +1 kHz affects

slice thickness

• RF Receive bandwidth ~ +16 kHz is

sometimes adjusted to optimize signal-to-noise in images

SURFACE COILS

• Surface coils are placed directly on the area of interest, and have different shapes corresponding

to the part to be examined. • They are receiver coils only, most of the received signal coming

from tissues near by; deeper structures cannot be examined with these coils

BASIC MRI TECHNIQUE

• 1. PLACE A PATIENT IN A UNIFORM MAGNETIC FIELD

• 2. DISPLACE THE EQUILLIBRIUM MAGNETIZATION VECTOR WITH RF – PULSE.

• 3. OBSERVE THE SIGNAL AS THE MAGNETIZATION VECTOR RETURNS TO EQULLIBRIUM

MR SignalWhen RF pulse is given

NMV rotates around transverse plane It passes across Receiver Coil

Inducing voltage in it thus producing a signal

• 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)

IMAGING

NMV can be separated in toIndividual Vectors of tissue present in the patient

• Such as Fat, CSF & Muscle

IMAGING

Intermediate

Low Signal

High Signal

Grey Black White

Small transverse

component of magnetization

Large transverse

component of magnetization

CONTRAST MECHANISMS

Intermediate

Low Signal

High Signal

Grey Black White

Small transverse

component of magnetization

Large transverse

component of magnetization

CONTRAST MECHANISMS (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

CONTRAST MECHANISMS (RELAXATION PROCESS)

after removal of RF pulse

Signal induced in Receiver Coil decreased

T1-relaxation time

NMV recovers and realign to B0 this process called "T1 Recovery"

T1 time is

an intrinsic contrast parameter that inherent to tissue being imaged

T2- relaxation time

2-Nuclei loss Precessional coherence or diphase and NMV decay in

the transverse plane this process called "T2 Decay“

T2 Decay is

an intrinsic contrast parameter and is inherent to the tissue being imaged

CONTRAST MECHANISMS (T1 RECOVERY)

Short TR DIFFERNCE IN SIGNAL INTENSITY T1 contrast

(T1 Weighted)

• TR 300-600 ms

CONTRAST MECHANISMS (DEFINITIONS)

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)SHORT TR : <500ms LONG TR : > 1500ms

CONTRAST MECHANISMS (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)

•TR determines T1 contrast

•TE determines T2 contrast.

CONTRAST MECHANISMS (DEFINITIONS)

Flip Angle

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

CONTRAST MECHANISMS (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

CONTRAST MECHANISMS (T1 RECOVERY)

T1 in Water T1 in Fatinefficient at receiving energyT1 is longeri.e. nuclei take allot longer to dispose energy to surrounding water tissue

WATER

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

FAT

PROPERTIES OF BODY TISSUES

Tissue T1 (ms) T2 (ms)

Grey Matter (GM) 950 100

White Matter (WM) 600 80

Muscle 900 50

Cerebrospinal Fluid (CSF) 4500 2200

Fat 250 60

Blood 1200 100-200

T1 values for B0 ~ 1Tesla.T2 ~ 1/10th T1 for soft tissues

CONTRAST MECHANISMS (T2 DECAY)

T2 DecayCaused by exchange of energy from one nucleus to another"Spin-spin Energy Transfer“

as result of intrinsic magnetic fields of nuclei interlacing with each other

this energy exchange loss of coherence or dephasingand as result NMV decay in transverse planeT2 is exponential process occur at different rates in different tissues

THANK YOU