MRI Principles 01

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Medical Imaging Systems

Magnetic Resonance

Imaging

D. Asemani

April 2012

Part 01

General View


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2D. Asemani Magnetic Resonance Imaging

Diagnostic imaging modalities

2. ultrasound

1. standard plane-view x-rays

billions

no ionizing radiation

most frequently used

ionizing radiation

computed tomography (CT) scanning

display of pulse-echoes backscattered from tissues

second most frequently used


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3. Magnetic resonance imaging (MRI)

detailed anatomic information

without using ionizing radiation

sensing the spin of their atoms

most abstract and complicated

technically

precise anatomical capability

often used for presurgery planning

for cancer detection

images of brain activity in response to

various stimuli

successfully to medical imaging of the body because

of its high water content

60% of the body by weight

exceptional soft-tissue contrast

now supplanting many conventional invasive procedures

most important imaging sequences

Diagnostic imaging modalities


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A complex molecule is placed in a strong, highly uniform magnetic field. Electronicshielding produces microscopic field variations within the molecule so that geometrically

isolated nuclei rotate about distinct fields.

Each distinct magnetic environment produces:

a peak in the spectra of the received signal.

relative size of the spectral peaks : ratio of nuclei in each magnetic environment

NMR spectrum : extremely useful for elucidating molecular structure

NMR signal from a human is due predominantly to water protons

NMR signal is simply proportional to the volume of the water

key innovation for MRI:

impose spatial variations on the magnetic field to distinguish spins by their location.

Diagnostic imaging modalities3. Magnetic resonance imaging (MRI)

Origin: NMR - Nuclear Magnetic Resonance

NMR has been used for decades in chemistry

a magnetic field gradient each region of the volume to oscillate at a distinct frequency


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Principle:

protons of the nuclei of hydrogen atoms subjected to radio frequency

pulses in a strong magnetic field. The protons get thereby “excited” tohigher energy level. Protons also get “relaxed” to the lower energy level

on the switching off radio frequency pulses. The protons emit radio

frequency signals when they move from “excited” to “relaxed” state.

These radio signals can be detected by a receiver and a computer canfurther process the output into an image

In body tissues; protons of hydrogen are most abundant as

hydrogen atoms of water molecules (H of H2O).

MRI image shows difference in the water content anddistribution in various body tissues.


Each different type of tissues within the same region can be easily

distinguished


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A 1990 study : principal applications for MRI are examinations of the head (40%),

spine (33%), bone and joints (17%), and the body (10%).

The percentage of bone and joint studies was growing in 1990.

typical imaging studies range from 1 to 10 minutes

new fast imaging techniques acquire images in less than 50 ms.

MRI research :

fundamental tradeoffs between resolution, imaging time, and signal-to-noise ratio

(SNR).

Most commonly protons (1H) are imaged, although carbon (13 C), phosphorous (31P),

sodium ( 23Na),and fluorine (19F) are also of significant interest.

Typical field strengths for imaging range between 0.2 and 1.5 T,

although spectroscopic and functional imaging work is often performed with higher field

strengths.


both gradient and receiver coil hardware innovations


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Magnetic resonance imaging (MRI)

Introduction

hydrogen atoms in water (H2O) and fat make up approximately

60% of the body by weight

a proton in the nucleus of each hydrogen atom

nucleus spins a small magnetic field or moment is created

When hydrogen is placed in a large static magnetic field, the magnetic moment

of the atom spins around it like a tiny gyroscope at the Larmor frequency, which

is a unique property of the material.


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a radio frequency rotating field in a plane perpendicular to the static

field is needed

frequency of this field : identical to the Larmor frequency

once the atom is excited, the applied field is shut off and the

original magnetic moment decays to equilibrium and emits a signal

longitudinal magnetization constant, T1, :

more sensitive to the thermal properties of tissue

transversal magnetization relaxation constant, T2, :

affected by the local field inhomogeneities

T1 weighted images are used most often

Magnetic resonance imaging (MRI)Introduction


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MRI finds widespread application:

detection of disease and surgical planning

highly detailed representations of internal anatomy

(called parameterized images ) cosiderable skill is involved in adjusting theinstrument to obtain images that emphasize

different types of tissue contrast, thediscrimination among different organ types and

between healthy and pathological tissues.

examine most of the body, including :

brain, abdomen, heart, large vessels, breast, bones, as well as soft

tissue, joints, cartilage, muscle, and the head and neck

for both children and adults

for detecting cancer pathologies, tumors, and hemorrhaging



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early precedent to MRI : nuclear magnetic resonance (NMR),

determining composition of materials through unique frequency

shifts associated with different chemical compounds

, and soon, detailed spectral information from phosphorus, carbon, and hydrogen

nuclei were obtained. Specialized magnets were designed to accommodate

parts of the body for study

The nuclear magnetization is very weak;

the ratio of the induced magnetization to the applied fields is only 4×10 –9


biological NMR experiments

were underway


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Isidor Rabi

Nobel Prize for Physics in 1944

invention of the atomic and molecular beam magnetic resonancemethod of observing atomic spectra

Columbia University

magnetic resonance (NMR)

History

Felix Bloch,

Stanford University

first successful nuclear magnetic resonance

(NMR) experiment 1946 independently by two

scientists in the US:

Edward Purcell,

Harvard University

Nobel Prize for Physics in 1951

1946: atomic nuclei absorb and re-

emit radio frequency energy



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In 1973, Paul Lauterbur

NMR pioneer at the State University of New York

first NMR image

On July 3, 1977, nearly five hours after the start of the first MRI test, the

first human scan was made as the first MRI prototype

1973: Lauterbur suggests NMR could be used to

form images

Raymond Damadian,physician and experimenter working atBrooklyn's Downstate Medical Center

hydrogen signal in cancerous tissue is different from that of

healthy tissue because tumors contain more water

medical practitioner

first MR (Magnetic Resonance) Scanning Machine

1977: clinical MRI scanner patented

History Magnetic resonance imaging (MRI)


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fMRI

-1990: Ogawa observes BOLD effect with T2*blood vessels became more visible as blood oxygen

decreased

1991: Belliveau observes first functional images using a contrast agent

1992: Ogawa & Kwong publish first functional images using BOLD signal

-1977: Mansfield proposes echo-planar imaging (EPI) to acquire images faster

a mathematical model to analyze signals from within the human body in response to a

strong magnetic field, as well as a very fast imaging method

Lauterbur and Mansfield shared the 2003 Nobel prize for

medicine for their MRI discoveries

History Magnetic resonance imaging (MRI)


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Magnetism and electromagnetism

magnetic susceptibility of a substance : ability of external magnetic fields to

affect the nuclei of a particular atom, and is related to the electron configurations

of that atom

nucleus of an atom

paired electrons

unpaired electrons

external magnetic field

more protected

More affected

magnetic susceptibility

Paramagnetism

Diamagnetism

ferromagnetism



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Paramagnetism

unpaired electronssmall magnetic field about themselves known as the

magnetic moment

presence of an external magnetic field

align with the direction

of the field magnetic moments add

together positive way

local increase in the magnetic field

Example: oxygen

Magnetism and electromagnetism Magnetic resonance imaging (MRI)


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Diamagnetism show no net magnetic moment as the electron currentscaused by their motions add to zero

external magnetic field show a small magnetic moment that

opposes the applied field

slightly repelled by the magnetic field

negative magnetic susceptibilities

example:

water and inert gases



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Ferromagnetism

contact with a magnetic field strong attraction and alignment

•retains its magnetization

permanent magnetsExample: iron

Magnets :bipolar

two poles, north and south

magnetic lines of flux

Like poles repel and opposite poles attract



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: strength of the magnetic field

units

gauss (G)

kilogauss (kG)

tesla (T)

number of lines per unit area is called the magnetic flux density

B



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Electromagnetism

Magnetic fields are generated by moving charges (electrical current)

direction clockwise or counter-clockwise with respect to the direction of flow of the current

Ampere’s law or Fleming’s Right hand rule

magnitude and direction of the magnetic field due to a current

changing magnetic fields generate electric currents

induced electric current

a closed circuit

Faraday’s law of induction electromotive force (emf ) in the circuit



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laws of electromagnetic induction : induced emf

proportional to the rate of change of magnetic field and the area of the circuit

in a direction so that it opposes the change in magnetic field which causes it (Lenz’s law).

motion of electrically charged particles results in a magnetic

force orthogonal to the direction of motion

protons (nuclear constituent of atom) have a

property of angular momentum known as spin

Electromagnetism Magnetic resonance imaging (MRI)


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a stable atom:number of negatively charged electrons equals the number of positively charged protons

Atoms with a deficit or excess number of electrons are called ions.

Motion within the atom

Negatively charged electrons spin on their own axisNegatively charged electrons orbit the nucleus

nucleus spins on its own axis

Each type of motion produces a magnetic field

In MR we are concerned with the motion of the nucleus

Atomic structureMagnetic resonance imaging (MRI)


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MR active nuclei

Protons and neutrons spin about their own axes within the nucleus

direction of spin is random

even mass numberspins cancel each other out so the nucleus

has no net spin.

odd mass number spins do not cancel each other out and the

nucleus spins

a moving unbalanced charge induces amagnetic field around itself



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Nuclei with an odd number of protons : MR active tiny bar magnets

all have odd mass numbers

Hydrogen 1, Carbon 13, Fluorine 19, Phosphorous 31, Nitrogen 15, Oxygen 17, Sodium 23

hydrogen nucleus is the MR active nucleus used in MRI

a single proton (atomic number 1).

• it is abundant in the human body (e.g. in fat and water);

• its solitary proton gives it a large magnetic moment.

MR active nuclei



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Alignment and precession

Alignment

In a normal environment the magnetic

moments of MR active nuclei point in a

random direction

no overall magnetic effect

placed in an external magnetic field

magnetic moments line up with

the magnetic field flux linesalignment

using two theories: the classical theory and the quantum theory


In “field free” space

randomly oriented


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classical theory

uses the direction of the magnetic moments to illustrate alignment

Parallel alignment alignment of magnetic moments in the samedirection as the main field

Anti-parallel alignment alignment of magnetic moments in the oppositedirection to the main field

At room temperature there are always more nuclei with their magnetic moments aligned

parallel to the main field than aligned antiparallel.

net magnetization vector or NMV

balance between the parallel andantiparallel magnetic moments

Magnetic resonance imaging (MRI)Alignment

Alignment


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quantum theory

uses the energy level of the nuclei to illustrate alignment

certain factors that determine whether the magnetic moment of a nucleus

aligns in the parallel direction or the antiparallel direction

magnitude or strength of the external magnetic field,

termed B0 in tesla (T);

energy level of the nucleus

magnetic moments of hydrogen nuclei align in the presence of an external magnetic

field in the following two energy states or populations:

Spin up Spin down

nuclei have low energy

do not have enough energy to oppose

the main field

parallel

nuclei have high energy and have enough

energy to oppose the main field

antiparallel



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Inside magnetic field

oriented with or against B0M = net magnetization

M

Applied Magnetic

Field (B0)

In “field free” space

randomly oriented

there is a small difference (10:1 million) in the number of protons in the low and

high energy states – with more in the low state leading to a net magnetization

(M)

quantum theory



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magnetic moments of the nuclei actually align at an angle to B0 due to the force of repulsion

between B0 and the magnetic moments

Hydrogen can only have two energy states

– high or low

magnetic moments of hydrogen can only align in the parallel or

antiparallel directions

magnetic moments of hydrogen cannot orientate themselves in any

other direction

temperature of the sample being imaged is an important factor that

determines whether a nucleus is in the high or low energy population

In clinical imaging we discount thermal effects

quantum theory



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At any one moment in time there are a greater proportion of nuclei with

their magnetic moments aligned with the field than against it

excess aligned with B0 produces a net magnetic effect called the NMVwhich aligns with the main magnetic field

As the magnitude of the external magnetic field increases, more of the magnetic moments of

the nuclei line up in the parallel direction because the amount of energy they must possess to

oppose the field and line up antiparallel to the stronger magnetic field is increased

NMV gets larger

static nuclear moment is far too weak to be measured when it is aligned

with the strong static magnetic field

Physicists in the 1940s developed resonance techniques that permit thisweak moment to be measured

key idea : to measure the moment while it oscillates in a plane

perpendicular to the static field



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First one must tip the moment away from the static field.

When perpendicular to the static field, the moment feels a torque

proportional to the strength of the static magnetic field

The torque always points perpendicular to the magnetization and causes the

spins to oscillate or precess in a plane perpendicular to the static field.



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gyromagnetic ratio : precessional frequency of a specific

nucleus at 1 T and therefore has units of MHz/T

precessional frequency is proportional to the strength of the external

magnetic field

precessional frequencies of hydrogen (gyromagnetic

ratio 42.57 MHz/T) commonly found in clinical MRI are:

At equilibrium the magnetic moments of the nuclei are out of phase with each other. Phase

refers to the position of the magnetic moments on their circular precessional path

PrecessionMagnetic resonance imaging (MRI)


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Out of phase or incoherent : magnetic moments

of hydrogen are at different places on the

precessional path

In phase or coherent : magnetic moments of hydrogen are

at the same place on the precessional path

What is actually aligned with the B0 is the axis around which the proton precesses

the decay of precession (i.e., it is the rate of precession out of alignment with B0together with the proton density of the tissue concerned that is crucial in MRI)

nuclei not aligned but still precessing

in the same direction.all nuclei aligned and precessing

in the same direction.

Precession Magnetic resonance imaging (MRI)


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Resonance and signal generation

Resonance an energy transition that occurs when an object is subjectedto a frequency the same as its own

In MR, resonance is induced by applying a radiofrequency (RF) pulse

at the same frequency as the precessing hydrogen nuclei; at 90° to B0

hydrogen nuclei to resonate (receive energy from the RF pulseMR active nuclei do not resonate because their gyromagnetic ratios are

different from that of hydrogen

Larmor equation: their precessional frequency is different and therefore

they only resonate if RF at their specific precessional frequency is applied



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Two things happen at resonance:

energy absorption and phase coherence.

Energy absorption

The hydrogen nuclei absorb energy from the RF pulse (excitation pulse)

If just the right amount of energy is applied the number of nuclei in the spin up

position equals the number in the spin down position. As a result the NMV (which

represents the balance between spin up and spin down nuclei) lies in the transverse

plane as the net magnetization lies between the two energy states

As the NMV has been moved through 90°

from B0, it has a flip or tip angle of 90

Resonance and signal generationMagnetic resonance imaging (MRI)


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Phase coherence

The magnetic moments of the nuclei move into phase with each other

As the magnetic moments are in phase both in the spin up and spin down positions and

the spin up nuclei are in phase with the spin down nuclei, the net effect is one of

precession so the NMV precesses in the transverse plane at the Larmor frequency

hydrogen nuclei do not move

nuclei are not flipped onto their sides in the transverse plane and neither

are their magnetic moment

Only the magnetic moments of the nuclei move, either aligning with or

against B0. This is because hydrogen can have only two energy states,

high or low. It is the NMV that lies in the transverse plane not the

magnetic moments, nor the nuclei themselves



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RF Excitation

• protons can flip between low and high energy states (i.e., flip

between being aligned with or against B0)

• to do so the energy transfer must be of a precise amount

and must be facilitated by another force (e.g., other protonsor molecules)

• in MRI, RF (radio frequency) pulses are used to excite the

RF field – the Swing analogy – tipping the net magnetization

out of alignment with B0


M ti i i (MRI)


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It can be shown :

a rotating RF field introduces a fictitious field in the Z direction of strength ω/γ

By tuning the frequency of the RF field to ω0 , we effectively delete the B0 field.

RF slowly nutates the magnetization away from the z axis. The Larmor relation still

holds in this “rotating frame,” so the frequency of the nutation is γB1,where B 1 is the

amplitude of the RF field

Since the coils receive x and y (transverse) components of induction, thesignal is maximized by tipping the spins completely into the transverse plane

This is accomplished by a π/2 RF pulse, which requires γB1τ=π/2, where τ

is the duration of the RF pulse.

Another useful RF pulse rotates spins by π radians. This can be

used to invert spins. It also can be used to refocus transverse

spins that have dephased due to B0 field inhomogeneity.

spin echo

widely used in imaging


M ti i i (MRI)


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MR signal

A receiver coil is situated in the transverse plane

As the NMV rotates around the transverse plane as a result of resonance,it passes across the receiver coil inducing a voltage in it. This voltage is the

MR signal.

After a short period of time the

RF pulse is removed

The signal induced in the receiver coil begins to decrease

amplitude of the voltage induced in the receiver coil

therefore decreases. This is called free induction decay

(FID):

„free‟ : absence of the RF pulse

“induction decay‟ : the decay of the induced signal in the receiver coil


MR i l M ti i i (MRI)


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signals produced during relaxation (move from higher energy to lower

energy) is dependent on:

density of hydrogen the velocity of flowing fluid through the tissue

the rate at which the excited nucleus are relaxed

relaxation parameters are marked T1 and T2

T1 and T2 : depend on the physical properties of the tissues when exposed to a series of pulses at predetermined time intervals

Different tissues have different T 1 and T 2 properties based on the response of

their hydrogen nuclei to radio frequency pulses in the strong magnetic field

These differential properties are made use of by setting equipment parameters (TR

and TE ) in order to generate images either based on T1 or T2 properties of the tissues.

MR signal Magnetic resonance imaging (MRI)

MR i l M ti i i (MRI)


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D. Asemani Magnetic Resonance Imaging

images of the tissues are known as either T1 or T2 weighted.

TR : the time to repeat RF pulses while

TE : time to receive echo i.e., time interval between application of pulse and listeningof the signal.

signal intensity pertains to the brightness of signal generated by specific tissue:

The tissues that are bright (white) : hyperintense

darker signal tissues : hypointense.

The tissues which are in between bright and dark : isointense.


MR i l Magnetic resonance imaging (MRI)


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fat : bright on T 1 weighted images and less bright on T 2

weighted images

water : dark on T 1 weighted images and bright on T 2 weighted

images

gas : dark on T 2 weighted images


C t t h i Magnetic resonance imaging (MRI)


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Contrast mechanisms

An image has contrast if there are areas of high signal (white on the

image), as well as areas of low signal (dark on the image).

intermediate signal (shades of grey in-between white and black).

NMV can be separated into the individual vectors of the tissues present

in the patient such as fat, cerebro-spinal fluid (CSF) and muscle

A tissue has a high signal (white) if it has a large transverse

component of magnetization

A tissue gives a low signal (black), if it has a small transverse

component of magnetization

A tissue gives an intermediate signal (grey), if it has a

medium transverse component of magnetization


Contrast mechanisms Magnetic resonance imaging (MRI)


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Image contrast is controlled by extrinsic contrast parameters (those that are

controlled by the system operator). These include :

• Repetition time (TR).

time from the application of one RF pulse

to the application of the nextmilliseconds (ms).

affects the length of a relaxation period

after the application of one RF excitationpulse to the beginning of the next

Echo time (TE). This is the time between an RF excitation

pulse and the collection of the signal

affects the length of the relaxation period after the

removal of an RF excitation pulse and the peak of

the signal received in the receiver coilmilliseconds (ms).




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• Flip angle. This is the angle through which the NMV is moved as a result of aRF excitation pulse.

• Turbo-factor or echo train length (ETL/TF)

• Time from inversion (TI)

• ‘b’ value

intrinsic contrast mechanisms :

do not come under the operators control

• T1 recovery

• T2 decay

• proton density• flow

• apparent diffusion coefficient (ADC).


extrinsic contrast parameters :


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48

MRI At A Glance, Catherine Westbrook, 2002 By Blackwell Science Ltd,

Introduction To Biomedical Engineering, John D. Enderle, 2005, Elsevier

Fundamentals of Biomedical Engineering, G.S. Sahney, New Age

International P.

The Biomedical Engineering HandBook, Second Edition. Ed. , Joseph D.

Bronzino, Boca Raton: CRC Press LLC, 2000

Selected References:

Documents

MRI Principles 01