Week 4 to 5

Objective
Basic Physics concepts will be
discussed
Define Precession
Larmor equation will be stated
Understand basics of nuclear
magnetism and resonance
Discuss difference between NMV and
field strength
Understand MR and Free Induction

• The earth spins on
its own axis
• The moon rotates
around the earth
• 70% of water is
housed in the
earth

Basically,
the earth
is a
gigantic
spinning
magnetic
bar!

B0 is the static external magnetic field
• Net magnetic moment is when
protons align with the external
magnetic field.
• NMV (net magnetization vector)
stands for net magnetic moment of
the patient.
• B0 is expressed in units called tesla
(T).
• NMV and field strength go hand and
hand. When field strength increases
NMV is larger, resulting in better
signal.
When NMV
and B0
interact
this is your
basis for
MRI

Magnetic Fields
• B0 the main magnetic field
• B1 – an RF field that excites the
spin

1. Quantum Theory
consider mechanical
properties of
individual nuclei.
1. Classical Theory
considers the net
magnetization
effects in whole
objects.

Known as the
building blocks of
the world!
The human body is filled
with atoms, which are
organized into
molecules or two or
more atoms arranged
together.
The body is abundantly
made up of hydrogen
molecules.
Atoms interaction with
magnetic fields are
important to MRI

Nucleus the positively charged central
core of an atom, consisting of protons
and neutrons and containing nearly all
its mass.
Electron are negatively charged
particles spinning around the nucleus
and are very light.
Neutron has no electric charge and are
large and heavy. Protons and
Neutron make an atom.
Proton has a positive charge and are
larger and weigh more.
Atomic Mass originates in the nucleus
and comes from particles called
nucleons.
Nucleons are subdivided into protons

Atomic Number
 Gives the atom its
chemical identity.
 The number of protons
in the nucleus.
Atomic Mass
 The number of protons
and neutrons in the
nucleus.
 This number is normally
balanced meaning this
number will be even.

Hydrogen atoms are
most abundant in the
human body.
The body is made up
of water. (H2O)
The basis of MRI
imaging is the
movement of
hydrogen protons in
the magnetic field.
RF coil stimulates the
protons
Then they relax in the
direction of the
magnetic field with the
energy.

Atoms have the same
number of protons,
but different numbers
of neutrons.
The way to alter a
balanced atom is to apply
external energy to knock
electrons from the atom.
The result is electrical
instability, which means a
deficiency in the amount of
electrons compared to
protons.

When two or more atoms are
arranged together

Electrons spin on
their own axis
Electrons orbit the
nucleus
Nucleus spins on its
own axis

Nucleus has an odd mass number where
the amount of neutrons is more or less
than your amount of protons. They spin
in opposite directions and the nucleus
has Angular Momentum (net spin):
how much ump and object has when
going in circles or the net spin of the
nucleus.
Also called MR Active Nuclei

 This is when the nuclei align their axis toward the magnetic field
because angular momentum is present.
 Protons are present = electrical charge
 The Law of Electromagnetic Induction states that if two of the three
individual forces (motion, magnetism, and charge) are present, then
the third automatically will occur. A magnetic field is created when
a charged particle moves.
 Magnetic Moment: tells the direction of the magnets north/south
axis and the amplitude of the magnetic field.
 Even though neutrons have no net charge, if the nucleus has an odd
mass number, then the nucleus is MRI active.

SPIN UP: main
magnetic field
are low energy
nuclei
SPIN DOWN: main
magnetic field is a
little higher energy
nuclei

What if
magnetism and
acquiring MRI
imaging was so
simple?

• A magnetic field is created when a
charged particle moves.
• Hydrogen nucleus contains a positive
proton that moves, which creates a
magnetic field around it making it act as
a magnet.
• The north & south axis have equal
strengths
• Each nucleus have their own magnetic
moment with vector properties:
- size or length
- direction of the magnetic moment
- an arrow

Classical Alignment
Hydrogen nuclei are
randomly oriented
when no magnetic field
is present.
When you introduce a
strong external magnetic
field the hydrogen nuclei
align with the magnetic
field.

Quantum
AlignmentHydrogen nuclei have two
energy states: low and high
Low: align parallel to the
eternal field (spin up)
High: align anti parallel (spin
down)
The strength of the external
magnetic field and thermal
energy level of the nuclei
determine the energy state.

• Thermal energy level of the nuclei
determines whether hydrogen nuclei
align parallel or anti parallel.
• Increase the field strength and fewer
nuclei can oppose the field.
• The thermal energy of a nucleus is
determined by the temperature of the
patient.(this is totally out of the control
of the technologist hands.)
• At this state the hydrogen nucleus
doesn’t change direction , but spins on
its axis.
• Always less high energy nuclei than low
energy.
• More spins are in line with the
magnetic field than opposed.
• During this time spin excess in the low
energy state forming net magnetization
represented by a vector (NMV).

• This happens since the majority of
magnetic moments align parallel or in
a low energy state to the magnetic
field, or B0.
• Hydrogens net magnetic moment
allows us to produce a magnetic
vector or electric field for use in
clinical MRI.
• NMV is the magnetic field formed and
is aligned parallel to the external
magnetic field or B0.
• The higher the strength of the
magnetic field the better signal in

• When an external magnetic
field is introduced to a
hydrogen nucleus that is
spinning on its axis, it
begins precession or
wobbling like a spinning
top.
 The path they travel is known as
the precessional path.
 The speed at which a proton
moves around B0 or wobble is
the precessional frequency and
is measured in Megahertz (MHz).

http://youtu.be/p3WnFYBnghU

This is used to calculate
the precessional
frequency.
• The gyromagnetic ratio helps
determine the relationship of
angular momentum and
magnetic moment.
• The gyromagnetic ratio is always
constant and for hydrogen
is 42.57 MHz/T

Field
Strength
Precessional
Frequency
Calculation
1.5T 63.86MHz 42.57MHz X 1.5T
1.0T 42.57MHz 42.57MHz X 1.0T
0.5T 21.28MHz 42.57MHz X 0.5T

The nucleus of an
object experiences
an oscillating
disturbance from an
object that has a
frequency similar to
its own frequency= it
gains energy
The ability of two
objects to exchange
energy.
 No matter the field
strength, to produce
clinical MRI images the
energy at precessional
frequency of hydrogen
must match the
radiofrequency (RF) of
the electromagnetic
spectrum.
 For hydrogen to
experience
resonance it is
dependent upon
the atoms
properties to
absorb energy at
an RF pulse of
energy at the exact
Larmor Equation.

 RF radiofrequency
is a band on the
electromagnetic
spectrum
 RF pulse is a
magnetic field and is
the direction of
oscillation at the
Larmor frequency.
 Excitation is when an
RF pulse is applied
and resonance
occurs.
 Low energy nuclei
then are able to join
the high energy
nuclei because of
resonance.
 With an increase of
field strength more
energy is required.
 Thermal energy of
B0

Two results
occur:1. NMV moves out of
alignment away from Bo
2. The magnetic moments
of hydrogen nuclei move
into phase with each
other.

http://youtu.be/jUKdVBpCLHM

First result of resonance is it moves
NMV out of alignment . Instead of
being aligned with the B0, it’s now
at an angle with B0. This angle is
known as flip angle.
Typical flip angel is 90° and this
provides enough energy for the
longitudinal NMV to transfer to the
transverse NMV.
B0 is now known as the longitudinal
plane (Mz) = recovery occurs here
90° to the B0 plane is known as the
transverse plane (Mxy)= decay
occurs here
B0
Longitudinal
plane
Transverse
plane
Z
X

Two directions that tissue is magnetized are:
1. Longitudinal is magnetized in the direction of the B0 or main magnetic
field
2. Transverse is when the direction of the tissue magnetized at a 90° angle
compared to the main magnetic field or B0.

 Second result of resonance is
when a hydrogen nuclei
becomes magnetized they move
into phase or the position of the
precessional path according to
that magnetic moment with one
another around the B0.
Two States of Phase
1. In Phase (Coherent):these spin
at the same place on the path
around the B0.
-Resonance is always in phase
2. Out of Phase (Incoherent):
these don’t spin at the same
place on the precessional
path.

What happens with a hydrogen atom in MRI?
http://youtu.be/IGk3NAziVWs

The voltage induced in
the receiver coil.
This occurs when in
phase or coherent
magnetization cuts
across the coil.
The transverse
magnetization causes a
current of electrical
voltage in the coil, hence
Say
what?

Faraday’s Law:
If a receiver coil or conductive loop is placed near a moving magnetic
field a voltage will be induced in the receiver coil
http://youtu.be/vwIdZjjd8fo

http://youtu.be/SwH2OEB0DKU

RF Pulse applied Relaxation occurring
The hydrogen nuclei
losses the energy given it
by the RF pulse because
the RF pulse is turned
off. What do you call
As transverse magnetization decreases so does
the voltage in the receiver coil = reduced signal
is FID

How: Reduced
transverse
magnetization and
reduction of voltage
produced in the
receiver coil.
The loss of signal
due to relaxation.

http://youtu.be/MPXbDDRumwM

Objective
Define relaxation
T1 Recovery
T2 Decay
T1 relaxation time
T2 relaxation time
Spin lattice relaxation
 Spin spin relaxation
Vectors
 PULSE TIMING
PARAMETERS
TR
TE

Several things occur when relaxation takes
place because the RF pulse is turned off.
NMV realigns with the B0
Along with it goes the high energy nuclei now becoming low
energy
The magnetic moment or amplitude of the magnetic field is now
in the spin up direction.
Hydrogens magnetic moments become dephased because they
can no longer stay coherent or are able to unite.

1.Recovery /T1
The amount of
magnetization in the
longitudinal plane gradually
increases.
2. Decay/T2
Happens at the same time,
but independently, the
amount of magnetization in
the transverse plane
gradually decreases.

 Termed spin lattice
relaxation. When the nuclei give up their
energy to the surrounding
environment or lattice.
 Nuclei then recover their
magnetization in the longitudinal
plane as they release energy to the
surrounding lattice.
T1 Relaxation time
 The rate of recovery that’s
constant
 Defined as time taken for 63%
of the longitudinal
magnetization to recover.

Termed spin-spin relaxation
 When the magnetic fields of
neighboring nuclei interact with
each other.
 Results in loss or decay of
coherent transverse
magnetization in the transverse
plane only.
 T2 Relaxation Time
 Time taken for 63% of the
transverse magnetization to
decay or be lost and 37%
remains in the transverse
magnetization.

We have learned the time it takes for
magnetization to be regained and
lost
The two extremes of contrast are Fat
& Water molecules in MRI
This will be discussed in a later week
in detail as to how the relaxation and
decay affect tissue characteristics.

 Describes how fast something moves and the
direction it moves in
 The NMV or net magnetization is expressed with
this

Pulse sequences are a necessary part of
MRI:
• Dephasing because the magnetic field has
inhomogeneities
• Regenerate signal
• They determine if an image is T1, T2, PD
weighted.
• So we are pulsing the RF and gradients at
specific times and a specific order to
achieve T1, T2 and PD contrast MR
images.
Definition: a series of RF pulses, gradient applications and intervening time
periods.
**Basically, this is how you acquire contrast to an image in MRI**
You basically are wanting to achieve a specific contrast quickly while not
having artifact and losing signal to noise.

With a pulse sequence you want
to acquire one of these contrast
weighted images.
PD

 TR or repetition
time is the time
between each
excitation pulse.
 Controls the amount
of time longitudinal
relaxation can occur
b/t RF pulses. (T1
Recovery)
 T1 & Proton Density
Weighting is
controlled here
Short TR Long
TR

TE or Echo
time is the time
when the RF pulse is
applied to the peak
of the signal as it is
applied to the coil.
determines how
much transverse
magnetization
occurs.
T2 weighting is
Long
TE
Short TE

Objective
Understand what imaging
parameters means
Understand how the technologis
will use them for imaging
Explain the various options a
technologist has in imaging

Image contrast is vitally important in MRI see
visualize abnormalities, anatomy and pathology.
Intrinsic: can’t be
changed and are inherent
in body tissues.
 T1 recovery time
 T2 decay time
 Proton density
 Flow
 (ADC) apparent
diffusion coefficient
Extrinsic: can be
changed
 TR
 TE
 Flip angle
 TI
 Turbo
factor/echo train
length
 B value
Two
categories of
factors :

• Contrast is mainly given to images by means of T1 recovery and T2
decay, but not to forget proton density.
• Proton density is pretty much the amount of protons in the area of
tissue being imaged. If there are lots of protons you have lots of
signal.
• T1 and T2 relaxations have 3 factors they depend on for better
signal:
 Inherent energy of tissues: low energy tissue easily absorb energy.
 How close the molecules are to one another: closer together work more efficient because they are
close to neighboring hydrogen for interactions.
 How well does the Larmor frequency of hydrogen match the molecular tumbling rate: if the rates
match there is efficient energy exchange.
• Fat & water are the best examples of opposite contrast in MRIFAT WATER
Absorbs energy quickly Inefficient at receiving energy
T1 is short T1 is long
Nuclei get rid of their energy by giving it to other
fat tissues close by, then returning to B0.
Nuclei take longer to get rid of energy to water
tissue close by.

Relaxation In
Different
Tissues
1.T1 Relaxation
2.T2 Relaxation

Fat and water
are two extremes of contrast for MRI
Fat recovers faster than water on the longitudinal axis.
Whereas, along the transverse axis its loss is quicker than
water.
T1 recovery in fat: T1 relaxation time is short because fat is able to
relax and get back to longitudinal magnetization fast.
T1 recovery in water: T1 relaxation time is long because water is a
log time to relax and get back to longitudinal magnetization.
T2 decay in fat: very efficient process since the molecules are so close
to one another making the spins dephase fast and transverse magnetization is
fast.
T2 decay in water: not as efficient simply because the molecules are
not close together and they dephase slow causing loss of transverse
magnetization is gradual.

B0
TIME
0 2
0
8
0
1004
0
6
0
LONGITUDINAL
DIRECTION
TRANSVERSE
DIRECTION
Short T1
time

B0
TIME
0 2
0
8
0
1004
0
6
0
LONGITUDINAL
DIRECTION
TRANSVERSE
DIRECTION
Long T1 time

B0
TIME
0 2
0
8
0
1004
0
6
0
LONGITUDINAL
DIRECTION
TRANSVERSE
DIRECTION

Imaging Parameters
A set of measurable factors that determine
the visual representation of something that is
scanned.
T1 Contrast
T2 Contrast
Proton Density Contrast

• TR controls T1
weighting
• Short TE and TR
values to
suppress T2
contrast
• Fat is bright

• TE controls T2
weighting
• Long TE and TR
values allowing fat
and water both to
decay
• Fat is dark
• Water bright

• Measures the amount
of hydrogen in a
specific tissue.
• The higher the
amount of protons in
a tissue the brighter
the image.
• Need to remove T1
and T2 contrast.

T2*
Decay
• Gradient echo sequences
• do not use the 180 degree
refocusing pulse and therefore
will only measure free
induction decay.
• Makes use of the susceptibility
of imperfections in the
magnetic field inhomogeneity
to generate a useful image.
• Once the signal or excitation
is taken away FID occurs.

Fat is
bright
Water is
dark
Water is
bright
Fat is dark
Differences in signal
intensity between
tissues relative to the
number of mobile
hydrogen protons per
unit volume

T1
Weighted
Proton
Density
T2*
Weighted
T2Weighted
TR Short
TE Short
T2 decay
Inhomogeneitie
s
TE
Long
TR Long
Always present;
suppress T1 & T2
contrast
Long TR & Short
TE

 TR or
repetition
time is the time
between each
excitation pulse.
 Controls the amount
of time longitudinal
relaxation can occur
b/t RF pulses. (T1
Recovery)
 T1 & Proton Density
Weighting is
Short TR Long
TR

TIME: when you increase the TR it will increase the
scan time. This in turn allows you to add more slices.
If you decrease the TR your time decreases along with
the reduction in slices.
SNR: when you increase the TR your SNR
increases because the tissues are given a longer time
for T1 recovery. Decreasing the TR tissues now
increase how much they magnetize and the SNR goes
down.

 TE or Echo time is
the time when the RF
pulse is applied to the
peak of the signal hits
the coil.
 determines how much
transverse magnetization
occurs.
 T2 weighting is controlled
here
Long
TE
Short TE

TIME: doesn’t control how long the scan will last
SNR: when you increase the TE your SNR decreases
because you are increasing the amount of time
transverse magnetization decay can occur.
An artifact you will encounter as you increase TE will be
susceptibility. Because you are increasing protons to
be in a dephased state.

Introduction to
Pulse Sequences
Inhomogeneities cause loss of signal on our
image. Below are two ways to regenerate signal
for a quality image.
Spin echo
Gradient Echo

• The most common pulse sequences
used starts with a 90 degree RF pulse.
• Uses a 180° pulse to regenerate signal
as tissues are relaxing and desired
tissue contrast needs to be attained.
• The time between the 90° pulses is TR
• TAU: the time after the 180° RF pulse=
time to dephase when the 90° RF
pulse is withdrawn.
• Can use one or two echo’s to acquire
T1, T2 or PD weighted images
• Take the longest scanning time
because only one line of k-space is
filled.
• Sharper imaging capabilities than Fast
Spin Echo.

Protons spinning on their
axis along the B0
90º pulse directs it along
the transverse plane (x,y)
180° pulse is applied to
rephase the signal.
Signal decays because of
inhomogeneities in the
magnetic field. T2* dephasing
happens.
The slow and fast spins
catch up.
Transverse magnetization
in phase occurs and the
coil has signal or spin
echo.

• The most common pulse
sequences used.
• Uses multiple 180° refocusing
pulse per TR.
• Can use two or more echo’s to
acquire T1, T2 or PD weighted
images.
• (FSE)Fast Spin Echo times have
shorter imaging times than SE.
• High resolutions as one line of
k-space is filled per echo.
• Sometimes referred to as Turbo
Spin Echo (TSE)

Long TR = 2000ms or longer
Short TR = 300 – 700ms
Long TE = 60ms or longer
Short TE = 10 – 25ms
Values of TR and TE

• Use gradients fields to achieve
transverse magnetization instead of
180°.
• Makes use of flip angles less than
90° to achieve transverse
magnetization.
• Commonly used to achieve T1, PD &
T2*
• T2 relaxation is referred to often as
T2*
• Signal is sampled at TE
• Used to view hemorrhages

• Gradient coils are
attached to the
gradient power
amplifier that have the
task of taking the
available power and
generating gradients in
the magnetic field.
• Notice the thick bands
of conductive material
that covers it inside the
magnet. Can be used
to rephrase and

Dephase
Because the
precessional
frequency have
changed since the
gradient is no longer
applied.
Rephase
To get the spins
back in sink a
gradient, or
rewinder is

Scanner
Table
Magnetic Field Stable as
precessional frequencies
remain at centre frequency
Magnetic Field
Increases as
precessional
frequencies increase =
Strong
Magnetic Field
Decreases as
precessional
frequencies decrease
= Weak
Entranceto
Scanner
ISOCENTER

Advantages Disadvantage
s
• TR can be
shortened
resulting in
shorter scan
times
• Flip angles that
are less or more
than 90° can be
• Not able to
correct magnetic
field
inhomogeneities
• Contain magnetic
susceptibility
artifact

T1 Weighting T2* Weighting PD Weighting
Flip angle is large
with a short TR so
full recovery of fat
and water won’t
occur.
To lessen T2* the
TE is short not
allowing fat and
water to decay.
To increase T2* decay
your TE is long
allowing fat and water
time to decay.
To lessen T1 recovery
your flip angle is small
and a long TR so
recovery can occur.
Flip angle is small
with a long TR so
full recovery of fat
and water will be
minimal.
To lessen T2* the
TE is short not
allowing fat and
water to decay.

Values in Gradient Echo Imaging
Long TR = 100ms or greater
Short TR = less than 50ms
Short TE = 1 – 5ms
Long TE = 15 – 25ms
Low flip angles = 5 – 20 degrees
Large flip angles = 70 degrees or greater

Objective
• Understand encoding and gradients
• Know the x, y, z coordinate system
• Know the difference between the
1) slice select gradient
2) physical gradient
3) phase encoding gradient
4) frequency or readout gradient
• Be able to describe k-space (raw data)
• Able to know what Fast Fourier transform (FTT) does
to the image

• After resonance happens the result is signal
in the patient.
• Locating the signal is necessary now.
• Encoding is defined as locating the signal
once it is selected along both axes of the
image.
• There are 3 dimensions to locate signal.

Is the ability to predict the
magnetic field strength and
the precessional frequencies
experienced by nuclei that
are along the linear axis of
the gradient after the
passage of current is applied
altering the magnitude of the
Bο.
Positive
2.005T
Negative
1.995 T2.0 T
Linear

• Can be physical
gradient
references
• Can be logical
gradient
references

Referencing the function of
the gradient
• Slice Selection Gradient is
“Z”
• Phase Encoding Gradient
is “Y”
• Frequency Gradient is “X”

Directly related to the
body
• Referencing its
direction or
orientation
• When the
technologist chooses
to use the z gradient
for slice selection, the

• The slope is the amplitude of
the magnetic field strength
along the axis.
• Showing how much change
occurs on this axis.
FACT:
Steep slopes alter the magnetic
field strength more than shallow
slopes = same for precessional
frequency
Steep Gradient
Shallow Gradient
15000
G
15000 G
15300 G14700 G
14900 G 15005 G
STRONG
STRONG
WEAK
WEAK
65.13MHz62.57MHz
63.87MHz63.42MHz

The magnetic gradient is applied
during the RF excitation pulse to
locate a slice position in the
desired scan plane and the RF
pulse excites this slice only.
• (Z) Axial
• (Y) Coronal A-P
• (X) Sagittal R-L
• Two or Three gradient fields are
applied when an oblique slice is
desired.

After selecting a slice you will need to
tell the machine if it is thin or thick.
The slope of the slice gradient makes
that difference. How?
Thick
Slices
Thin
Slices
• Steep gradient: results in a big difference between the
precessional frequency at two points of the gradient
Fig. 1.1
• Shallow Gradient: results in less of a difference in the
precessional frequency at two points of the gradient
Fig. 1.0
• Bandwidth is known as the matching frequency range
between the 2 points.
Shallow
gradient
Fig.1.0
Fig.1.1
Steep
gradient

• This gradient is used to send the
frequency signal along the long axis of
the body.
• The frequency gradient is switched on
once it receives the signal or echo
sampling time. Also, called readout
gradient.
• Frequency FOV decides the size of
anatomy that is covered along the
frequency encoding axis, determined
by the steepness of the slope.
• Fourier Transform will be used to
separate the many different
frequencies once the MR signal is
received.

• Turned on after the excitation pulse.
• Applied after the slice selection yet before
the frequency gradient changing the speed
& phase of nuclei.
• Steepness or shallowness of the slope
determines the degree of difference in phase
shift.
• Allows for the encoding or location of the
spatial signal along a second dimension by
different spin phases. (Basically, it has to
find the signal on the short axis of the
image.)
• The phase shift size (large or shallow) tells
the machine the difference in tissue that are

Y
Z
X
Z
Y
Y
X X
FREQUENCY
PHASE
FREQUENCY
PHASE
FREQUENCY
FREQUENCY
PHASE
PHASE
PHASE =short axis FREQUENCY= long
axis

The amount of data points you can acquire
during the acquisition window and frequency
matric that can be used.
Sampling
Time/
Acquisition
Window : Length
of the readout
gradient
Sampling Rate/
Sampling
Frequency: the
amount of data
points taken during a
given time.
Data Points:
stored samples

• Two directions exist here: phase
and frequency and signal, which
now must be made into an
image.
• An area in the array processor
where raw data about spatial
frequencies of the MR image are
stored.
• Despite k-space and the MR
image looking nothing alike, the
same info is there.
• Data points is determined by the
number of phase and frequencies
were selected.
• Fourier Transform is used to
convert them and takes place in

K-Space: is where
collected data is stored in
the array processor as
data points, which are
stored in k-space.
• Rectangular
• Has two axes (phase &
frequency)
• Data point contain info
for the whole slice.
• Measurements unit is
called radians per cm. FREQUENCY AXIS
PHASEAXIS

K-Space:
• Phase gradient is responsible for
deciding which drawer or line of K
space is filled during TR.
• TR is the deciding factor in the
amount of slices allowed.
• Positive polarity gradient: uses
lines that are along the top portion
of k space.
• Negative polarity gradient: uses
lines that are along the bottom
portion of k space.
• Outer lines = steep gradients
• Central lines = shallow gradients
• Filled from top to bottom or vice
versa (there are a variety of other
ways to fill k space)
Positive
Negative
Central
Outer
Outer
HIGH
SIGNAL OR
CONTRAST
IMAGE
DETAIL

Positive
lines
Negative
lines
Central
Outer
Outer
Data Points

• What sequence you choose
determines how k-space is filled
• For example Cartesian would
generally be used for gradient and
spin echo pulses.
• Linear (most common): horizontal
filling starting at the bottom and
going to the top.
• Centric: middle line first is
acquired working it’s way to the
top and bottom.

• Used for dynamic MR imaging
• The same image is collecting into k-space many times as
contrast goes in and out . The purpose is to see how tissue
respond to contrast over a time period.
• The center of k-space will contain the important information.
• Best when you want to acquire fast high resolution imaging

• Begins in the middle and fills out, as low spatial
frequencies are in the middle.
• Best when wanting acquire MRA imaging because
contrast media has high signal intensity data
1
2
4
5
3

• This is the most common way of
filling
• Filling begins at the top and continues
1
2
4
5
3

An example
of signal
echoes
being
sampled,
then stored
in k-space.

• The raw data is sent to the
array processor and a
mathematical calculation
changes the MR signal from
time to frequency domain.
• Necessary because gradients
can only spatially locate
signal by frequency not time.
• Raw data is in k space and
needs to be turned into an
image. Hence, an image is
produced.

• Once it is filled with data from
signal and resolution an image is
displayed.
• Image quality is determined on
how k-space is filled.
• Central portion: high signal
amplitude and low resolution;
mainly for contrast of image
• Outer portion: low signal
amplitude and high resolution;
mainly for spatial resolution
• Looks like a
grid
• Each data
point in k-
space maps…
To every
point on the
MR image.

Scan Time, TR and Filling K Space
Repetition time (TR) slices are selected here as well as
phase and frequency encoded, but one slice per k space at a
time. Controls how many slices you can have per TR. A larger
TR will allow more slices and in turn increase scan time.
Phase matrix controls the number of lines of k space that need
to be filled to finish a scan. 512 phase matrix selected you will
need 512 lines filled to complete the scan.
Number of excitations (NEX) how many times each line of k
space get data filling it. No need to change the TR here, but
increasing the NEX does increase the scan time and higher
signal to noise (better image quality).

Partial Echo
Imaging
• Used when you only want to use part of the echo or signal that
is read and to save scanning time. Why and how can you do
this?
• In k space the right mirrors the left. So take the info from the
right side and use it on the left side instead of imaging the left
side.
RIGHT LEFT

Partial Fractional
Averaging or
Half Fourier
• Used when you only want to use part of the echo or signal
that is read to save scanning time. Why and how can you
do this?
• In k space the negative and positive portions mirrors of
each other. So there is enough info that can be used to
complete the scan.
• Down side since signal loss does occur, but scan time is
00000000000000000
0
00000000000000000
0
00000000000000000
0
00000000000000000
0
00000000000000000
0

Types of Acquisition
• Sequential: all the date of slice 1 is acquired and displayed
upon acquisition like CT imaging. Then it does the same
for slice 2, 3 on so on.
• 2D: most common; fills one line of k space at slice 1, then
slice 2 and so on until the line is filled. Then it moves on
to the next line the same way.
• 3D: the complete volume of tissue is imaged. Not useful if
patient moves because there is no slice gap and scanning
takes much longer.

OBJECTIVE:
Radiofrequency & Gradient
Systems
1. Learn how a coil is made up
2. Discuss transmit and receive coils & bandwidth
3. Understand the workings of the operators console and
technologist work station
Electromagnetism
1. Understand the differences between diamagnetic,
paramagnetic, superparamagnetic, ferromagnetic
2. Define Faraday’s Law
3. Learn what magnets are used in MRI
4. Discuss MRI field strength

1. Magnet
2. RF source
3. Magnetic field gradient system
4. Computer system
5. Image processor

• Susceptibility measures
how much a substance
becomes magnetized
when it experiences a
magnetic field. So
basically it’s ability to
become magnetized.
• Polarization occurs when
matter and the magnetic
field meet, it either
opposes or increases the
external field.
Diamagnetic
Paramagnetic
Ferromagnetic
Super Paramagnetic

• Are repelled
against the
magnetic field
• Have a negative
or low magnetic
susceptibly
• Examples would
be glass, wood,
plastic and
gold.

• Have low and positive
magnetic susceptibility
• Lose their magnetic
susceptibility when the
magnetic field is lost
• Example is gadolinium
contrast, aluminum,
platinum

• Has positive
magnetic
susceptibility
properties
• Remain
magnetized after
a magnetic field is
removed
• Example is iron,
nickel, cobalt

• Have positive susceptibility
properties
• Do not retain magnetic memory
• Stronger than paramagnetic
substances yet weaker than
ferromagnetic substances
• Contrast agents that reduce the
tissues ability to absorb T2
signal.
• Previously used to enhance liver
tumor and reticuloendothelial
system.

• Known as a dipole magnet
because it has a north and
a south pole
• Field strength is measured
in Guass (G) or Tesla (T)
• Guass is used to measure
low fields and the fringe
field that extends beyond
the main magnet bore.
• Tesla is to measure higher
field strengths.

Permanent Magnet Better known as “OPEN MRI UNITS”
 The magnetic field is confined to the magnetic plates as they are above and below the patient.
 Constructed of block of slab of natural ferrous material.
 0.06 T to 0.35T strengths
 Sensitive to the room temperature and affect scanning abilities of unit.
 They can be closer to waiting areas as well as there are fewer safety concerns required when it comes to a
fringe field.
 No power supply needed.
Resistive
 The right hand thumb rule applies here.
 The main magnetic field can be horizontal or vertical.
 Field strengths up to 0.3T.
 Need constant current to maintain its static field.
 Can be turn off when not in use allowing for low capital cost yet high operational cost due to the power
needed to operate it.
Superconductive
 To turn these magnets off the current has to be slowly directed away from the coils, which takes several
minutes. Only in an emergency you will quench it to turn it off.
 To acquire a static field a direct current is applied to the coils and these coils are kept cool and so the don’t
experience resistance or heat by housing them in cryogen.
 0.5T-3T for clinical use and 4T in research

N
S
Bo
Permanent Magnet Better
known as “OPEN MRI UNITS”
 The magnetic field is confined to the
magnetic plates as they are above
and below the patient.
 Constructed of block of slab of
natural ferrous material.
 0.06 T to 0.35T strengths
 Sensitive to the room temperature
and affect scanning abilities of unit.
 They can be closer to waiting areas
as well as there are fewer safety
concerns required when it comes to
a fringe field.

In short, if a current is passed through a straight
wire, then a magnetic field is created.
B
T
V

Faraday’s Law:
If a receiver coil or conductive loop is placed near a moving magnetic
field a voltage will be induced in the receiver coil

• Slinky configuration
• Wire is aligned side by side
to allow for magnetic
imaging to occur.
• Problems like resistance
(Ohm’s Law) or heat can
occur along this wire, hence
constant current must be
flowing or cryogen must be
used to cool the wire.
V=IR

Resistive
 The right hand thumb rule applies
here.
 The main magnetic field can be
horizontal or vertical.
 Field strengths up to 0.3T.
 Need constant current to maintain
its static field.
 Can be turn off when not in use
allowing for low capital cost yet
high operational cost due to the
power needed to operate it.

Superconductive
 To turn these magnets off the
current has to be slowly directed
away from the coils, which takes
several minutes. Only in an
emergency you will quench it to turn
it off.
 To acquire a static field a direct
current is applied to the coils and
these coils are kept cool and so they
don’t experience resistance or heat
by housing them in cryogen.
 0.5T-3T for clinical use and 4T in

• Dedicated
Extremity
MRI/Niche high
field 1.0 T
• Spacious
• High SNR
• The magnets are
above and below
the patient =
vertical magnetic
field.

• Cryogens are super cooled liquid gases and without this
very important gas the magnetic field is lost.
• At room temperature liquid helium boils away very fast
so the MRI machine has to be able to prevent it from
leaking into the air.
• The MRI machine contains the helium by having a
cryostat that has a helium condenser that recycles any
boil off.
• They include helium and sometimes nitrogen.
• The concern is helium will over take oxygen in the room.
• During a quench the boil off happens and technologist

The rapid boiling off of
the cryogens that are
responsible for keeping
the magnet cool and as a
superconducting magnet.

Permanent Resistive Superconductive
Are not able to be turned on
and off
Field strength is normally no
more than 0.2-0.3 Tesla
Low power need
Heavy Quick shut down Ability to have a high Tesla strength of 3
Tesla routinely
SNR low & longer scan times High power need = high
operating cost
High SNR & Short scan times
Normally have an open design Can not be turned off quickly
Low cost to operate as they
require no cryogen
Main magnetic field runs horizontal or from
head to feet with the patient.
No power supply needed
Limit magnetic field with a Tesla
strength of about 0.35

High Field 1.0T & greater
Mid Field 0.5T-0.9T
Low Field 0.5T and less

• These are the stray magnetic field outside the bore of the
magnet. This is a concern because stronger magnets can pose
a danger to patients outside the scanner room.
• Magnetic Shielding is necessary to protect patient with devices
such as projectile objects or pacemakers who may be sitting in
the waiting room.
1. Passive: Line the MR room walls with steel. Less expensive
2. Active: Uses additional solenoid magnets outside the cryogen
bath that restricts the magnetic field, especially used in a mobile
MR unit. Otherwise, the unit must be placed in a large building
and fringe fields are placed around the unit.

• Prevents RF noise
from entering scanner
room and causing the
image to be distorted.
• Copper is best for this
type of shielding.
• Used to block the
magnetic field of the
magnet from
interfering with
pacemakers and other
equipment outside of
the scan room.

• Needed because it is not possible to
build a MR unit that is totally
homogeneous. So this is used to
correct inhomogeneities in the magnet.
• A metal piece (passive shimming) or loop
of wire that has current and an additional
solenoid magnet (active shimming) in it is
placed around the magnet bore.
• The actual method is called shimming or
balancing or leveling.
• Negative= is it requires a separate power
supply and if this fails image quality will
be affected.
Passive Shielding
• Done when the magnet is
installed
Active Shielding
• Done for each patient or
sequence to make it more
homogeneous for all patients
regardless of anatomic
makeup.

Windows: (image A & B)
consisting of a stainless steel
tube frame, stainless steel
screen frame, two layers of
brass screen, and two layers of
1/4" tempered glass. The two
layers of R.F. screen are offset
in order to prevent the Moiré
effect and also blackened to
achieve maximum visibility.
Doors: Be careful not to hit the
RF seals (image C & D), which
could rip them off and degrade
the RF shielding protection for
the room.
(A)
(B)
(C
)
(D)

Inside the bore of the
magnet is a
transmit/receive coil
Bird Cage Coil is
Transmit/Receive

• Defined: a slope that is measuring change
in the physical quantity over a period of
time.
• Uses 3 individual electromagnets.
• Used for spatial encoding, to rephrase
spins & produce echoes.
• Power supplied by one or more amplifiers.
Factors that alter the strength
of the electromagnet:
• Current passing through the wire
• The number of windings in the coil
• Diameter of the wire used in the
windings
• Distance or spacing between the

• Slew Rate: the strength of the gradient over distance or how quick
the gradient coils turn on and off from its max gradient amp.
Measured in T/m/sec
• Amplitude: how severe the slope is. Measured as mT/m
(milliTesla per meter).
• Rise Time: the time it takes to a gradient to switch on, achieve the
required gradient slope and switch off again. Or it’s speed.
Measured in microseconds or μs.
• Duty Cycle or %: time during which the gradient system can be
run at maximum power or is allowed to work.

• Each gradient pulse is balanced by the use of an
equal but opposite gradient pulse.
• Referred to as bipolar
• A gradient waveform, which will act on any
stationary spin on resonance between two
consecutive RF pulses and return it to the
same phase it had before the gradients were
applied.

• Converts the analog signal to a series of digital values by
measurement at a set of particular times; making use of
the analog to digital converter.
• When the rate of sampling is less than twice the
highest frequency in the signal = aliasing.
• The duration of sampling determines how small a difference of
frequencies can be separated.
• Ramp sampling occurs by reducing the time of the sequence by
performing this function while the frequency gradient changes
collecting data points when the rise time is about complete.

Radiofrequency System
Radiofrequency Coil
Static Magnetic Field

Transmit and
Receive Coils:
Receive Only
Coils:
These operate
as the receiver
and transmitter
or receive only
of
radiofrequency
signal in MR.

3 Functions:
• Transmit Receive: sends waves into
the person, and then receives the waves
back which determine the image.
 The main bore acts as a transmit, but is
able to do both by transmitting RF then
changing to receive MR signal.
• Receive Only: only accept MR signal
and are put right on the surface of the
body part you want to image.
Known as surface coils. Some are also
placed in the body such as endorectal
coils for prostate imaging.

Volume Coil:
• Transmit Receive: sends waves into
the person, and then receives the waves
back which determine the image.
 The main bore acts as a transmit, but is
able to do both by transmitting RF then
changing to receive MR signal.
• Receive Only: only accept MR signal
and are put right on the surface of the
body part you want to image.
Known as surface coils. Some are also
placed in the body such as endorectal
coils for prostate imaging.

• Phased Array is an
antenna theory of antennas
coupled together to enhance
transmission of MRI signal.
Works off the idea that each
individual coil receives the
signal from it’s FOV. Then
each coils signal is processed
and forms one large FOV.
Benefits: noise is limited
because of the small FOV
used. All data can be
captured in one sequence.

Transmit
Bandwidth
:
Receive
Bandwidth
The range of frequencies
transmitted in an RF pulse.
Responsible for slice location
and thickness in a pulse
sequence.
The range of frequencies
accepted by the receiver to
sample the MR signal. (read-
gradient)
# of frequencies + the time to
obtain the samples

SNR, SCAN
TIME, ARTIFACT,
CONTRAST
TRANSMIT
BANDWIDTH
RECEIVE
BANDWIDTH

Gradient coils are
attached to the
gradient power
amplifier that have
the task of taking
the available power
and generating
gradients in the
magnetic field.

Notice the
thick bands
of conductive
material that
covers it
inside the
magnet.

X axis runs horizontal
axis of the magnet or left
to right.
Y axis runs vertical to
the magnet or anterior
posterior.
Z axis runs the long
axis of the magnet or
head to foot.
 Hence, the reason for the noise in
MRI
 Known as physical gradients

Magnet
A. Receiver Amplifier
B. RF Coils
C. Gradient Coils
Equipment
A. RF & Gradient Power Amplifier
B. Analog & Digital Convertor
C. Pulse Sequence Controller
Console
A. Array processor
B. Host Computer
C. Storage Device

• Host Computers job is to take the information given by the technologist
and send it to the pulse sequence controller.
• Now these digital commands need to be turned into analog
commands.
• The digital commands are sent to the RF and gradient power amplifiers
as amplifiers increase the signal strength.
• Between the magnet where the patient is and the RF and gradient coils
will transmit RF pulses and receiving echoes. The result, spatially
encoded echoes from the patient.
• Now they have to be amplified by the receiver amplifier.
• These echoes need to be digitized so the computer can process them
for the technologist. Once this is done they are stored as raw data in
the host computer.
• The array processor turns it into images for you the technologist.

Receiver
Amplifier
Increase the
strength of the MR
echo's before being
digitized.
Array
Processor
Reconstructs
images by using
Fourier transforms.
(ADC) Analog to
Digital
Converter
Responsible for
conversion of the
analog echo’s to
digital information
stored as raw data.

This is where the technologist is
able to choose items such as:
• Scan setup
• Scanning
• Post processing
• Reformatting images
• View images
Operator
Interface

• Filming the images are
highly unusual now.
CD storage is normally
used for permanent
copy of images.
• PACS is used to store
studies and are able
to be manipulated
and to view for future

Protocol (a set
of pulse
sequences and
parameters) is
what
technologist
choose when
they need to
perform specific
MRI exams.

Protocol (a set
of pulse
sequences and
parameters) is
what
technologist
choose when
they need to
perform
specific MRI
exams.

Operators
Console
which is your
connection to
the host
computer.
Here the
technologist
can view, send
to PACS, film,
save and post
process.

Houses the
1) gradient and
radiofrequency
cabinet
2) universal power
supply
3) water pump/chiller
4) helium pump.
5) Desktop computer
system are more
common today and
are located in the
control room

Pulse Profile
Best used when doing
quality control of the RF
signal of a coil
The coil is tuned by the
manufacture for the specific
machine and can not be
used on other machines.
Tuning is highly important for
the technologist to achieve
the best quality image known
as (SNR) signal to noise
ratio.

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