Fundamentals of Magnetic Resonance

Imaging

-- Pulse Sequence & Safety

Outline

Image formation

Basic Pulse Sequence

Safety

Image Formation

Gradients

A gradient is simply a deliberate change in the magnetic field

Gradients are used in MRI to linearly modify the magnetic field from one point in space to another

Gradients are applied along an axis (i.e. Gx

along the x-axis, Gy

along the y-axis, Gz

along the z-axis)

What happens to the frequency of precession when we turn on a gradient?

Start, 16/2 8:18pm

Effect of Gradient on Rate of Precession Effect of a Gradient

Spatial Localization

Gradients, linear change in magnetic field, will provide additional information needed to localize signal

Makes imaging possible/practical

Couldnt spatially localize MRI signal instead moved subject to get each voxel

Nobel prize awarded for this idea!

Slice Selection

Use magnetic gradient to modify

frequency of the protons

precession

A slice will be selected with the

protons precess with the same

frequency as that of the RF

pulse.

Slice location vs slice thickness

The slice selection gradient is

always applied perpendicular to

the slice plane.

As we need to know where the signal come from

Just like a tuning fork!

Slice Orientation

Inplane Spatial Localization

Phase encodingFrequency encoding

Could be

the other

way round!

An example

Phase Encoding

Apply gradient in one direction

Protons decrease or increase their

rate of precession

Turn off the gradient

All of the protons will again precess

at the same rate

Difference is that they will have

different phase from each other

1. Initial state

2. Gradient turned on

3. Gradient turned off

Only the same thing will resonance

Before slice selection (only spin) At selected slice (precess with Larmor frequency)

phase encoding gradient is applied phase encoding gradient is turned off

A 2D Scenario Phase encoding in practice

F

r

e

q

u

e

n

c

y

e

n

c

o

d

i

n

g

Phase encoding

Phase

encoding

gradient of

signal 128

Phase

encoding

gradient of

signal 1

Phase

encoding

gradient of

signal 256

Frequency Encoding

Apply gradient in one direction to modify the rate at which the

protons spin based on location of the proton

Leave it on

Result:

Protons that experience a decrease in the net magnetic field

precess slower

Protons that experience an

increase in the net magnetic

field precess faster

= x G

Frequency offset from the center

x position relative to center

G frequency encoding gradientBefore frequency

encoding

Frequency encoding

turned on

Nobel Prize in Physiology or Medicine

in 2003 for discoveries concerning magnetic resonance imaging

Make MRI applicable to imaging human

Paul C. Lauterbur

USA

Sir Peter Mansfield

UK

Developed a mathematical

process to speed the image

reading, i.e., EPI

Introduced frequency and phase

encoding principle in the magnetic field

for spatial localization in MRI

Signal Detection Detecting Net Magnetization using Coil

C

Electromagnetic Induction!

K-Space

k-space is what is actually measured in MRI (i.e.,

the signal from M0 is transformed into x and y

values via k-space)

kPE

K-Space and MR Image

Echo

k-space lines

=

Inverse Fourier

transform

Mxy signal does not become 0 instantly, the spins merely diphase

Can rephase spins to form a symmetrical MR signal -> echo

MR data (k-space) from scanner is a line by line acquisition of echoes

Inverse Fourier Transform of the data gives required image

Rane SD. Texas A&M University, 2005.

K-Space and MR Image

original image full k-space data

high

signal

Imaging and K-Space

Center of k-space

Imaging and K-Space

Everything else

Imaging and K-Space

Full Frequency Half Phase

Nobel Prize in Chemistry in 1991

for the development of Fourier

Transform nuclear magnetic

resonance spectroscopy and

the development of multi-

dimensional NMR techniques

Richard Robert Ernst

Switzerland

Nobel Prize in Chemistry in 2002

For his development of nuclear magnetic

resonance spectroscopy for determining

the three-dimensional structure of

biological macromolecules in solution"

Honorary Professor, The Chinese

University of Hong Kong

Kurt Wthrich

Switzerland

Basic Pulse Sequences

Timing Diagram for an Imaging Sequence

as a function of time

a 90o slice selective pulse

a slice selection gradient pulse

a phase encoding gradient pulse

a frequency encoding gradient pulse

a signal

Repeat it!

TR: Repetition Time (the time between repetitions)

Phase encoding gradient changes: magnitude varies in equal steps between

the maximum amplitude of the gradient and the minimum value.

time

The simplest signal form generated in MRI

The magnetization component has a non-zero component in

the xy plane

The precessing magnetisation will induce a corresponding

oscillating voltage in a detection coil surrounding the sample

Free Induction Decay (FID)

Tissue 1 Tissue 2

MR signal intensity

very flexible and allow the user to acquire images in

which either T1 or T2 (dominantly) influences the signal

intensity displayed in the MR images.

Spin Echo (SE)

Gf

G

Gs

TI

TE

TRhttp://www.youtube.com/watch?v=dFp2Z3wjrmo&list=PLCD41685D8499AAB1

http://www.youtube.com/watch?v=vMh11VtUA5o&list=PLCD41685D8499AAB1

For SE, relatively long imaging times

But not for GE, due to

flip angle is typically smaller than 90 (i.e., 20 ~ 60)

They have no spin-echo because there is no 180 pulse

Rephasing is done by means of gradient reversal only

Gradient Echo (GE)

http://www.youtube.com/watch?v=Rvpa1gqG06g&list=PLAE12114468910462

The fastest 2D imaging sequence currently available

Could be a SE or a GE sequence, multiple echoes are generated in one

excitation

Acquisition time TA for one image is 100ms and even lower

Acquisition time TA2D =Nph*TR/ETL (Nph is inplane phase encoding steps)

ETL is the Echo Train Length (i.e., the number of echoes per excitation)

Used in functional MRI, diffusion and perfusion imaging

Echo planar imaging (EPI)

Gf

G

Gs

Higher induced voltage can give a brighter signal

TE= only can give you one piece of data

TR =repetition time for if 3D image is needed

Major Pulse Sequence Parameters

Echo Time (TE) time after 90o RF pulse until readout. Determines

how much transverse relaxation will occur before reading one row

of the image.

Repetition Time (TR) Determines how much longitudinal

relaxation will occur before constructing the next row of the image.

In spin echo, TR is the time between two successive 90o RF

pulses

In gradient echo, TR is the time between centers of two small

angle pulse

Contrast in MRI: T1-Weighting

500ms 3000ms

Contrast in MRI: T2-Weighting

180ms30ms

50ms

50ms30ms 180 ms

Biological Effect of MRI and Safety

1. Static Magnetic Field Bo

2. Gradient Magnetic Fields

3. Radiofrequency Electromagnetic Fields

Safety Concerns Recall: Magnetic Properties of Material

iron, nickel, or cobalt

e.g., aneurysm clips, parts of pacemakers, shrapnel, etc.

large positive magnetic susceptibility

remain magnetized after an external magnetic field is removed

oxygen and ions of various metals like Fe, Mg, and Gd

positive magnetic susceptibility

Contrast agent

magnetic susceptibility between ferromagnetic and paramagnetic

e.g., iron containing contrast agents for bowel, liver, and lymph

node imaging.

Contrast agent

no intrinsic atomic magnetic moment

small negative magnetic susceptibility

e.g. water, copper, nitrogen, barium sulfate, and most tissues are

diamagnetic

The main magnetic field of a 1.5 T magnet is about 30,000

times the strength of the earth's magnetic field.

Ferromagnetic Objects Projectile Effect

Effect on implants

pacemakers is disturbed

intracranial aneurysm clips could be ferromagnetic and experience a

torque or twisting in a magnetic field

aneurysm clip might experience a fatal hemorrhage

heart valves (e.g., Star-Edwards) could be torqued in a magnetic field,

but not necessarily

Safety Concern 1 -- Static Magnetic FieldSafety

The whopping strength of the magnet makes safety

essential.

Things fly Even big things!

Safety Concern 1 -- Static Magnetic Field

SafetySafety Concern 1 -- Static Magnetic Field

Changing magnetic field induce electrical currents in conductors.

Effect:

For surgical metal implant, the potential exists for electrical currents being

induced in the metal with subsequent heating

Very rapidly changing magnetic fields as may be achieved with EPI

could cause nerve stimulation, which can affect motor nerves --

muscle contraction

Safety Concern 2 Varying Gradient Fields

Specific Absorption Rate (SAR)

defined as the power absorbed per mass of tissue

Unit: W/kg

the heating is normally insignificant

But EPI and MRS are possible of over heating tissue

Solution:

Shorter ETL

Longer TR

Less slices

Revise the RF sequence to reduce the energy

Safety Concern 3 -- Radio Frequency

Proportional to

the square of Bo

!

Burn from looped cables

Safety

Anyone going near the magnet subjects, staff and visitors

must be thoroughly screened:

Subjects must have no metal in their bodies:

pacemaker, aneurysm clips, metal implants (e.g.,

cochlear implants), interuterine devices (IUDs)

some dental work (fillings okay)

Subjects must remove metal from their bodies

jewellery, watch, piercings, coins, wallet, etc.

any metal that may distort the field (e.g., underwire

bra)

Subjects must be given ear plugs (acoustic noise can reach

120 dB)

Guidelines to Observe

Functional MRI not suggest

What the radiologist say

http://www.radiologyinfo.org/en/photocat/ga

llery3.cfm?image=mr-safety-

explained.jpg&pg=sfty_mr

What does radiologist say?Reference

The Essential Physics of Medical Imaging by Bushberg et al.

(Basics).

The Basics of MRI, Joseph P. Hornak, Ph.D. (principle)

Magnetic Resonance Imaging by Stark and Bradley, second

edition (Artifacts, Basics, Instrumentation, Pulse Sequences).

Safety Considerations in MR Imaging by Kanal et al. Radiology

176:593-606, 1990 (Safety).

Reference