Transcript net magnetization

King Saud University
College of Dentistry
Magnetic Resonance
Imaging - MRI
Asma’a Al-Ekrish
BDS
Demonstrator ( OMF Radiology )
Sources
Source of illustrations:
Understanding MRIAn interactive guide to MRI principles
and applications
Philips Medical Systems
Best, The Netherlands
Sources
Some MRI images reproduced from:
Som, P.M. and H.D. Curtin (2003). Head and neck
imaging. St. Louis, Mosby.
White, S. C. and M. J. Pharoah (2004). Oral
radiology.Principles and interpretation. St. Louis,
Mosby.
Introduction
RF
pulse
Lecture Contents
 Magnetic Resonance
 Image Production
 Diagnostic Applications
Magnetic Resonance
Magnetic Nuclei
Electric charge + spinning
Tiny magnetic field
Magnetic Nuclei
magnetic
moment
Strength and direction of magnetic field
represented by a vector:
MAGNETIC MOMENT
Magnetic Nuclei
 Uneven number of protons which causes them to
have a net “spin”
 Spinning nuclei are MR active
Magnetic Nuclei
1H
 Abundant in the human body and have a
large magnetic moment
Magnetic Nuclei
Moments are constantly changing their
alignments making this a dynamic system
Precession
Precession
Magnetic nucleus placed in a scanner’s magnetic
field rotates in a cone around the main field’s
direction
Precession
 Larmor Frequency: speed or frequency of precession
 Proportional to the strength of the magnetic field
Precession
LARMOR EQUATION
Larmor
gyromagnetic x strength of
=
magnetic field
frequency
ratio
(MHz)
(MHz/T)
(T)
Every element has a specific gyromagnetic ratio
Magnetic Resonance
 Magnetic resonance induced by exposing nuclei to a second
magnetic field B1 and a radiofrequency pulse
 Resonance only occurs if the radiofrequency applied matches the
Larmor frequency of the nuclei
Magnetic Resonance
 MR signal is emitted
 After removal of the RF signal, nuclei gradually return
to their position relative to main magnetic field and the
MR signal “decays”
Magnetic Resonance
 Body coil
( fixed inside magnet )
VOLUME COILS
TMJ COILS
 Head coil
Net Magnetization
 Individual magnetic moments cannot be measured
 Signal produced in MRI is produced by the sum of all the magnetic
moments: the NET MAGNETIZATION (represented by the magnetic
vector)
Net Magnetization
 NET MAGNETIZATION points in the same direction as the main
magnetic field
Net Magnetization
LONGITUDINAL MAGNETIZATION
 Net magnetization is
situated on the z-axis of a
3-dimensional coordinate
system  the “static
frame”
 The z-axis indicates the
direction of the scanner’s
main field
Net Magnetization
LONGITUDINAL MAGNETIZATION
 Net magnetization
precesses at the larmour
frequency
 At equilibrium, precession
is not detectable and MR
signal cannot be
measured
Net Magnetization
TRANSVERSE MAGNETIZATION
 To detect precession, the
magnetization vector must
be tipped into the
horizontal plane
 Basis of MR signals
Net Magnetization
TRANSVERSE MAGNETIZATION
Larger net magnetization
Larger transverse
magnetization
Stronger MR signal
The MR Signal
 Nuclei at equilibrium with main magnetic field  only
longitudinal magnetization which cannot be measured
 RF pulse applied  transverse magnetization
 Net magnetization spirals down towards the horizontal
plane
The MR Signal
 Strength and
duration of the RF
pulse determine the
degree of tilt or flip
angle (a)
 Pulses are named
after the flip angle
they induce
The MR Signal
Receiver coil
As net magnetization precesses in the transverse
plane, an oscillating electrical signal is
generated which is detected by a receiver coil
The MR Signal
s
Current in receiver coil
t
This current is the MR signal measured in MRI
The MR Signal
s
Free Induction Decay
t
The MR signal immediately after an RF signal is
The Free Induction Decay
The MR Signal
s
Free Induction Decay
Decay
t
= amplitude
Properties:
 The signal strength or amplitude of the signal is the
largest value in one oscillation
 Dictated by size of transverse magnetization
vector
The MR Signal
s
Free Induction Decay
Decay
t
= amplitude
Properties:
 Measured in mVs
 The amplitude gradually decays as net
magnetization gradually returns to equilibrium
The MR Signal
s
Free Induction Decay
t
Frequency = cycles per second
(MHz)
The MR Signal
Free Induction Decay
 The signal’s phase is the position in the
signal’s cycle of oscillation
 Measured in degrees
RF Pulse
Flip Angles
90o pulse
180o pulse
Tips net magnetization to
the horizontal plane
Tips net magnetization to
the negative z-axis
RF Pulse
Repetition Time ( TR )
Time interval between individual RF pulses
RF Pulse
Echo Time
Time interval between
the RF pulse and
detection of the
image
Image Production
Image Production
 Tissues and lesions are differentiated
from eachother when they have different
signal intensities  tissue contrast

Strong signal  white areas

Weak signal  dark areas

Intermediate signal  gray
areas
Image Production
 Signal intensity depends on:
1.
Proton Density (PD) Contrast: Number of
hydrogen nuclei in tissue

H  high signal intensity

 H  low signal intensity

Determines size of the equilibrium
magnetization of a tissue
Image Production
 Signal intensity depends on:
2.
Differences in T1 and T2 relaxation rates
between tissues

Stronger source of contrast
3.
Flow
4.
Susceptibility
5.
Diffusion
6.
Perfusion
Relaxation
T1
relaxation
T2
relaxation
 Is the gradual return of net magnetization to the
longitudinal axis after excitation with an RF pulse
 Two independent components: T1 and T2
relaxation rates
Relaxation
 Different tissues have different T1 and T2
relaxation rates  their signal intensities
appear different
 Most MRI acquisition techniques are
influenced by T1 or T2 contrast
T2 Relaxation
T2- “Spin-spin” Relaxation
 Decay of transverse
magnetization after
RF pulse
 Fast
T2 Relaxation
 Occurs as magnetic moments interact with eachother
 They have different precession frequencies so they de-
phase  decay of transverse magnetization
T2 Relaxation
 T2 relaxation time: the time needed for a 63%
reduction of transverse magnetization
T2 Relaxation
 Water and abnormal
tissues: long T2
relaxation time 
bright
 Normal tissues:
Normal
Abnormal
63%
reduction
intermediate T2
relaxation time 
gray
T2 Relaxation
T2 weighted images are favored when
searching for pathological
conditions
T2* Relaxation
 Dephasing caused by spin-spin relaxation accelerated
by de-phasing caused by imperfections in the main
magnetic field
 Cumulative effect leads to “effective T2 ” or “
relaxation
T2* ”
T1 Relaxation
T1- “Spin lattice” Relaxation
 Recovery of longitudinal
magnetization
 Occurs due to interaction of
hydrogen nuclei with their
surroundings
 Slow
 Affected by flip angle (a) of
RF pulse

Small (a)  faster net
magnetization returns to
z-axis
T1 Relaxation
T1- “Spin lattice” Relaxation
 Time required for
recovery of 63% of
longitudinal
magnetization
T1 Relaxation
T1- “Spin lattice” Relaxation
 Fat: short T1
relaxation time 
high signal intensity
 Good image contrast
63%
recovery
High anatomic
detail
T1 Relaxation
T1 weighted images are useful for
demonstration of anatomy,
especially of small regions where
high spatial resolution is needed
(eg TMJ)
Pulse Sequences
Pulse sequences are carefully coordinated and
timed sequence of RF pulses, gradient
applications, and intervening time periods
which generate a particular type of image
contrast
Pulse Sequences
2 main categories:
1. Spin Echo sequences
2. Gradient Echo
sequences
Pulse Sequences
Spin Echo


Contrast can be
adjusted by variations
in:

Repetition time ( TR )

Echo time ( TE )
Transverse magnetization characterized by T2 because
extra relaxation due to field inhomogeneities is
counteracted
Pulse Sequences
Gradient Echo

Contrast may be
adjusted by
variations in:


Relaxation time
(TR )

Echo time ( TE )

Flip angle ( a )
Decay of transverse magnetization
characterized by T2* ( T1 and PD also ??)
Pulse Sequences
Image Production
T1
weighted
T2*
T2
PD
weighted
weighted
weighted
 Obtained by manipulating the parameters of a
sequence of RF pulses
Gradient Coils

3 sets of gradient coils
set at right angles to
eachother

Each coil produces a
magnetic gradient in a
particular direction ( xy-z axes)

When all three used together, a gradient can be
produced in any direction and images may be acquired
from any plane.

Provide 15 mT/meter
Slice Selection

Strength of secondary
magnetic field varies
linearly along the length
of the field to produce a
magnetic gradient

Therefore, Larmour frequency of nuclei varies along
length also

Select slice to be imaged by applying an RF pulse
whose frequency matches the Larmour frequency of
the nuclei in that area
Slice Selection

Orientation of the slice is perpendicular to the field
gradient
Slice Thickness
May be selected by
manipulating:
1.
Steepness of field
gradient
2.
Bandwidth of RF
pulse
Spatial Encoding
 Signals from individual voxels must be distinguished from
eachother
 Achieved by 2 gradient fields at right angles:

First, the Phase gradient

Second, the Frequency gradient
Spatial Encoding

Each voxel: unique phase and frequency
Multi-slice and Volumetric Imaging
Multi-slice
scanning
Volume ( 3D)
scanning
Multi-slice and Volumetric Imaging
Multi-slice
Volumetric
Diagnostic Applications
Diagnostic Applications
Axial
Acquisition of images
in any plane
Coronal
Sagittal
Diagnostic Applications
Obtaining corrected views of TMJ
Oblique sagittal
Oblique coronal
Diagnostic Applications
Evaluation of
articular disc
position and
morphology
Detection of
joint effusion
Diagnostic Applications
High Resolution MRI
Evaluation of articular disc
integrity
Diagnostic Applications
CT
CT
MRI
MRI
More accurate evaluation of internal
structure and extent of soft tissue lesions
Diagnostic Applications
Separation of
pathological and normal
soft tissues
Evaluation of effect of
lesions on adjacent soft
tissues
Diagnostic Applications
MR sialography
Diagnostic Applications
Functional Imaging Techniques
Utilize ultra-fast imaging sequences in
order to assess function and physiology
( eg of TMJ when opening and closing)
Diagnostic Applications
ADVANTAGES OF MRI
 Superior anatomic and pathological
details in soft tissues
 No ionizing radiation
 Non-invasive
 Imaging possible in several planes
without moving the patient
 Fewer artifacts
Diagnostic Applications
DISADVANTAGES OF MRI
 High cost
 Special site planning and shielding
 Patient claustrophobia
 Inferior images of bone
 Long scanning times
Diagnostic Applications
Contraindications to MR Imaging
 Cerebral aneurysm clips
 Cardiac pacemakers
 Ferromagnetic implants
 Metallic prosthetic heart valves
 Claustrophobic or uncooperative patients
 First trimester of pregnancy ( ?? )
Conclusion
 Image characteristics in MRI are
dependent on several factors
 These factors may be manipulated to
achieve the required quality and contrast
according to the specific diagnostic need
 Advances in MRI technology are allowing
the use of this modality an increasingly
versatile ways
Questions?
Thank
You