Basic principles of magnetic resonance imaging for beginner - Dr Sanjana Ravindra

BASIC PRINCIPLES OF MAGNETIC
RESONANCE IMAGING FOR
BEGINNER ORAL AND
MAXILLOFACIAL RADIOLOGISTS
Kagawa T, Yoshida S, Shiraishi T, Hashimoto M, Inadomi D, Sato M et al.
Oral Radiol 2017; 33(2): 92-100.
Journal Club No: 21
Dr Sanjana Ravindra
Oral Medicine & Radiology
Rajarajeswari Dental College
& Hospital, Bangalore
Dr Sanjana Ravindra

INTRODUCTION
Magnetic resonance imaging (MRI) began with the
discovery of nuclear magnetic resonance by Bloch
Magnetic resonance imaging (MRI) is a noninvasive tool to investigate the
internal anatomy and physiology of living subjects.
Dr Sanjana Ravindra

🏰
PHOTOGRAPHY RADIOGRAPHY MRI

“This review describes the basic principles
and diagnostic methods of MRI for
beginner oral and maxillofacial
radiologists

Soft-
tissue
diagnose
s
Benign and
malignant
Inflammation
Temporomandibul
ar joint disorders.
INDICATIONS
Dr Sanjana Ravindra

ADVANTAGES AND LIMITATIONS OF MRI
• Non-invasiveness and lack of
radiation exposure
• Ability to produce any given
tomographic image
• Ability to display blood vessels
without using a contrast agent.
• MRI scans provide higher tissue
resolution and a lower temporal
resolution than CT scans, which
also produce tomographic images
• Long scan time (approximately 30–
60 min)
• Inability to obtain a signal from
cortical bone and calcifications
• Inability to perform the test when
metal is present in the body
• Difficulty in scanning
claustrophobic patients.

The atom consists of two parts, i.e.,
 A central NUCLEUS
 Orbital ELECTRONS
BASIC PHYSICS
Dr Sanjana Ravindra

The NUCLEUS is made up
of PROTONS and NEUTRONS
PROTONS
have a positive charge.
NEUTRONS
have no electrical charge.
Inside the
atom
BASIC PHYSICS
U
Dr Sanjana Ravindra

Inside the
atom ELECTRONS
have a negative charge.
The number of electrons
in an atom usually
matches the number of
protons, making the
atom electrical neutral.
BASIC PHYSICS
Dr Sanjana Ravindra

The positive charge of
protons are analogous to
planets
The magnetic field or magnetization is
created with rotational motion of
positively charged protons
This magnetization can be represented
by a vector called magnetic vector.
When this proton is placed within a
magnetic field B o , they start rotation or
precessing around the axis (just like a
gyroscope) of magnetic field direction.
This interaction with the proton’s
magnetic vector and magnetic field B o
creates magnetic resonance.
BASIC PHYSICS
Dr Sanjana Ravindra

This electric current induces a magnetic field. Thus the proton has its own magnetic field
and behaves like a small bar magnet. It is because the body is made up of innumerable
protons and each proton in the body behaves like a bar magnet
A
Dr Sanjana Ravindra

When protons align, not only they rotate around themselves, but also their axis of rotation moves such that it forms a
CONE. This movement of axis of rotation of proton is called as PRECESSION.
Dr Sanjana Ravindra

PRECESSION FREQUENCY
Dr Sanjana Ravindra

PROTONS IN FREE
SPACE
PROTONS IN A MAGNETIC
FIELD
Dr Sanjana Ravindra

Precession frequency of the protons - not constant
Precession frequency α external magnetic field
Exact calculation of Precession frequency is done by means of the
LARMOR EQUATION
LARMOR EQUATION states that the precession frequency ( W =
gamma x Bo)
Bo = external magnetic field given in TESLA
Gamma = gyromagnetic ratio
The equation states that the Precession frequency becomes
higher as the strength of the External Magnetic field increases
0B 
Dr Sanjana Ravindra

There are three conditions we need to maintain
to efficiently tilt the magnetization from Z -axis
to XY-plane. These are:
The frequency of B 1 rf-pulse should be the
same as the precession or resonance frequency
of w o .
B 1 rf-pulse should be applied long enough to
create the desired tilt (flip angle) of the
magnetization.
B 1 rf-pulse should be perpendicular to main
static field.
Dr Sanjana Ravindra

 Requirement is that they should have spin and should have odd number of
protons in the nucleus
 Hence theoretically 13C, 19F, 23Na, 31P can be used for MR imaging
 Hydrogen atom
o Only one proton
o H+ is equivalent to a proton
o Present in abundance in body water
o Best and most intense signal among all nuclei
Dr Sanjana Ravindra

PRINCIPLES OF MAGNETIC RESONANCE
 The quantity and behaviour of the protons in each tissue can be measured-
Resonance
 Resonance is a transfer of vibration energy from one system to another
 Every system has a frequency called resonance frequency
 Resonance frequency is a frequency at which energy transfer is most efficient
In MRI, the principle of resonance is used to transfer energy to the spinning hydrogen protons
Resonance frequency for the protons lies
within the radiofrequency band of EM
spectrumDr Sanjana Ravindra

 Patient is placed inside a large magnet
which induces a relatively strong external
magnetic field (usually 0.5 – 1.5 Tesla)
 Radiowaves are pulsed into the patient
by the body coil transmitter at 90 degree
to the magnetic field
S
Dr Sanjana Ravindra

 A radiofraquency pulse produced from a scanner
is directed into the patient, causing some hydrogen
nuclei to absorb energy (resonate).
 The RF pulse is turned off causing the release of
stored energy, detected as a signal by the receiver
coil. These signals are used to construct the MR
image.
Dr Sanjana Ravindra

COIL
A coil consists of one or more loops of conductive wire used to create uniform magnetic
field or to detect a changing magnetic field by voltage induced in the wire.
Gradient
coils
TYPES OF MRI COILS
Dr Sanjana Ravindra

GRADIENT COIL
The gradient coil produces large
static external magnetic field (0.02- 4
tesla).
Large static external magnetic fields
are classified into three types based
on magnitude of the magnetic field.
Gradient coils are three separate coils
one for each relevant field(X,Y,Z axis)
with its own power supply and under
independent computer control.
Used to code position information
into MRI signal and to permit the
imaging of thin anatomic slices
•Low field magnet system- < 0.2 T
•Mid field magnet system- 0.2- 1 T
•High field magnet system- >1T.
Dr Sanjana Ravindra

RADIOFREQUENCY COIL
Radiofrequency
coils are used for
transmitting and
receiving signals at
the resonance
frequency of the
protons within the
patient.
Can be
differentiated by
their functions
into:
• Transmit receive
coil
• Receive coil
• Transmit only
coil
• Multiply tuned
coil.
RF coil is selected on the
basis of the region of
interest.
i. A head coil-
examination of the
head, including the
oral and
maxillofacial
regions.
ii. Neck coil-
examination of the
neck.
iii. Surface coil-
examination of the
TMJ.
Dr Sanjana Ravindra

Head coil Neck coil
Surface coil
Head coil or bird cage coil
Dr Sanjana Ravindra

SHIM COIL
Provide auxiliary magnetic fields in order to compensate for in homogeneities in the main magnetic
field of the MRI machine.
• In addition, the space in which the patient reclines (the gantry) is a narrow tube, leading to a
significant feeling of restriction during scans.
• Open gantry style MRI machines using permanent magnets have been developed
• Their application is limited, because the magnetic field that can be obtained using
permanent magnets is only approximately 0.5 T.
GANTRY
Dr Sanjana Ravindra

NUCLEAR BASIS OF MR IMAGE FORMATION
Magnetic field induced by spinning hydrogen nucleus.
Dynamic alteration of the hydrogen nuclei and production of magnetization
vector in a large external magnetic field .
Kinetic alteration of magnetization vector according to application of
radiofrequency pulse.
Relaxation phenomena of the magnetization vector after 90˚ RF pulse has been
switched offDr Sanjana Ravindra

MAGNETIC FIELD INDUCED BY SPINNING HYDROGEN NUCLEUS
Dynamic alteration of the hydrogen nuclei and production of magnetization vector in a large external magnetic
field .
Kinetic alteration of magnetization vector according to application of radiofrequency pulse.
Relaxation phenomena of the magnetization vector after 90˚ RF pulse has been switched off
Dr Sanjana Ravindra

Magnetic field induced by spinning hydrogen nucleus
 Spin is a fundamental property of nature like electrical charge or
mass and expressed in multiples of 1/2 and can be + or –
 Protons, electrons, and neutrons possess spin.
 Individual unpaired electrons, protons, and neutrons each possess
a spin of 1/2.
 As spin is associated with an electrical charge, a magnetic field is
generated in nuclei with impaired nucleons, causing these nuclei
to act as magnets with North and South poles (magnetic dipoles)Dr Sanjana Ravindra

• When an external magnetic field is applied, hydrogen nuclei
have two orientations in the field corresponding to two
different energy states:
 Spin up- in the direction of the magnetic field and are in lower energy state.
 Spin down- opposite to the direction of the field and are in higher energy
state.
Dr Sanjana Ravindra

DYNAMIC ALTERATION OF THE HYDROGEN NUCLEI AND PRODUCTION
OF MAGNETIZATION VECTOR IN A LARGE EXTERNAL MAGNETIC FIELD .
Dr Sanjana Ravindra

DYNAMIC ALTERATION OF THE HYDROGEN NUCLEI AND PRODUCTION OF
MAGNETIZATION VECTOR IN A LARGE EXTERNAL MAGNETIC FIELD
 The tilting or wobbling of spinning protons from a position
which was parallel with external magnet is called
precession.
 The rate or frequency of precession is called the Resonant
or Larmor frequency, which is proportional to the strength
of the applied magnetic field.
 The Larmor frequency of hydrogen is 42.58 MHZ in a
magnetic field of 1 Tesla.
 The magnetic field strengths used for MR imaging range
from 0.1 to 4.0T.Dr Sanjana Ravindra

Magnetization vector in the X, Y and Z planes
Dr Sanjana Ravindra

field .
KINETIC ALTERATION OF MAGNETIZATION VECTOR ACCORDING TO
APPLICATION OF RADIOFREQUENCY PULSE.
Dr Sanjana Ravindra

KINETIC ALTERATION OF MAGNETIZATION VECTOR ACCORDING TO APPLICATION OF
RADIOFREQUENCY PULSE
 When a radiofrequency pulse is applied, the hydrogen nuclei
precesssing in the direction of the external magnetic field(Z axis)
absorb the energy and begin to precess in the direction of the
applied radiofrequency field(X- axis).
 The phenomenon of energy transmission from RF pulse to the
hydrogen nuclei is termed as Resonance.
 The magnetization vector now precess in the new plane(XY axis) at
the larmor frequency. This process is termed flip of the
magnetization vector.
 The change in the angle is called as “flip angle”. If the flip angle is
90˚ or 180˚, the RF pulse applied to the body is called as 90˚ RF
pulse or 180˚ RF pulse
Kinetic alteration of the
magnetization vector in the
X, Y and Z planes following
application of a 90˚ RF
pulse
Dr Sanjana Ravindra

field .
RELAXATION PHENOMENA OF THE MAGNETIZATION VECTOR AFTER 90˚ RF
PULSE HAS BEEN SWITCHED OFFDr Sanjana Ravindra

RELAXATION PHENOMENA OF THE MAGNETIZATION
VECTOR AFTER 90˚ RF PULSE HAS BEEN SWITCHED OFF
Magnetizator vector moves back towards the
direction of the external magnetic field (Z axis).
Magnitude of the magnetization vector along XY
plane decreases and that of Z axis increases.
This phenomenon of return of nuclei to their
original spin state is called relaxation and the
energy loss is detected as a signal, which is called
free induction decay (FID).
Includes two independent processes:
• Spin lattice relaxation
• Spin- spin relaxation
Dr Sanjana Ravindra

• Spin- lattice relaxation- number of excess hydrogen nuclei with
a higher energy state return to the original state by releasing
their energy to the surrounding lattice.
• The time constant for this exponential process is termed as
“T1” or spin- lattice relaxation time.
• T1 is the time taken for 63% of the nuclei to return to the lower
energy state following termination of the 90˚ RF pulse.
• Mz (t)= MO (1-e-t/T1).
Dr Sanjana Ravindra

• Factors that influences T1 value of a tissue are:
• Particular chemical substance and its physical state.
• Field strength.
• Temperature.
• Liquid surrounding the protons.
• Mobility of the protons.
• Fat has short T1 (200-300 msec), realigns quickly after a RF pulse
and appears bright.
Dr Sanjana Ravindra

 Spin- spin relaxation time- the state of perfectly uniform or in
phase hydrogen nuclei changes to random phase as before the
application of the 90˚ RF pulse.
 The time constant for this exponential decay s termed as “T2” or
spin- spin relaxation time.
 My(t)= My e-t/T2 .
 Spin-spin relaxation depends on
• Large homogeneous external magnetic field.
• Very small magnetic fields induced around spinning hydrogen nuclei.
• Inhomogenities within the large external magnetic field.
 The real time from in phase to random phase is also a constant
value referred to as “ T2* ”.
Dr Sanjana Ravindra

Decay phenomenon
dependent on spin- spin
relaxation time, T2.
Decay phenomenon
dependent on spin- spin
relaxation time, T2*
Dr Sanjana Ravindra

BASIC MRI IMAGES
• T1- and T2-weighted images
The duration of time required to return to the vector quantity in
the longitudinal direction is known as the T1 value
Time required to attenuate to the vector quantity in the transverse
direction is the T2 value
Dr Sanjana Ravindra

• Graphs of vector quantity changes
in the longitudinal directions over
time are called the T1 curve
• T1-weighted images (T1WI)
represent tissues with a higher
signal of the shorter T1 value
(short longitudinal relaxation time
and rapid signal recovery).
• Graphs of vector quantity changes
in the transverse directions over
time are called T2 curve
• T2-weighted images (T2WI)
represent tissues with a higher
signal of the longer T2 value (long
transverse relaxation time and
slow signal attenuation).
T1 weighed images T2 weighed images
Dr Sanjana Ravindra

T1 WEIGHTED IMAGES (FAT IMAGES)
T1 weighted image is produced by a short repetition time between RF
pulses and a short signal recovery time .
A tissue with short T1 produces all intense MR signal and is displayed
as bright white in a T1 weighted image.
A tissue with long T1 produces a low intensity signal and appears
dark in MR image. Eg- CSF.
T1 gives good image contrast and are helpful for depicting small
anatomical regions like TMJ.
Dr Sanjana Ravindra

T2 WEIGHTED IMAGES(WATER IMAGES)
So called as water has longest T2 relaxation time and appears
bright in the image.
Images are obtained by using a long TR(2000 ms) and a
longer TE greater than 60 msec
Tissues with long T2 (CSF) appear bright and tissues with
short T2 appear dark
T2 weighted images frequently used for identifying
inflammatory and pathological changes in the tissue
Dr Sanjana Ravindra

SIGNAL INTENSITIES OF DIFFERENT TISSUES ON T1- AND T2-
WEIGHTED IMAGES
Dr Sanjana Ravindra

CAUTIONARY NOTES BEFORE MRI IMAGING
• MRI devices constantly utilize a powerful magnetic field, bringing magnetic
materials into the examination room is prohibited
• Medical equipment such as stretchers, wheelchairs, scissors, and gas cylinders -
same room as the MRI device - special-purpose nonmagnetic materials.
• MRI examinations - contraindicated - cardiac pacemakers, implantable
cardioverter defibrillators, and artery clips.
• Patients with tattoos or those wearing colored contact lenses, mascara, or eye
shadow, because all of these materials include minute iron particles that cause
image artifacts - become heated due to the magnetic field, potentially resulting
in patient burns
Dr Sanjana Ravindra

CONCLUSION
Dr Sanjana Ravindra

MRI ARTIFACTS CAUSED BY METAL
Because MRI examinations utilize a
magnetic field, artifacts can occur due
to the presence of magnetic metals in
the imaging area.
Even if the magnetic metals themselves
do not exhibit magnetism, they become
magnets in the magnetic field. As a
result, they form their own magnetic
fields that cause the local magnetic
field to become non-uniform.
Therefore, while artifacts only appear
in the direction of slices in CT scans,
they appear as three-dimensional
missing signals in MRI examinations
Dr Sanjana Ravindra

CONTRAST-ENHANCED MRI
 A gadolinium preparation is used as the contrast agent in MRI;
typically, 0.2 ml/kg is administered intravenously.
 Gadolinium has a high T1-shortening effect and is, therefore, used
as a contrast agent to increase the diagnostic ability
 The gadolinium contrast agent has an adverse effect rate of
approximately 1–2% and is thus considered safer than iodine
contrast agents.
 Capturing sequential images at fixed intervals while injecting the
contrast agent and then graphing the contrast effect along the time
axis produces a time–signal intensity curve (TIC).
 This curve is useful for identifying features such as malignant
neoplasms based on the graph patternDr Sanjana Ravindra

Magnetic resonance imaging
of a ranula
The lesion in the left submandibular region is
depicted as having a low signal in T1-weighted
images and high signal in T2-weighted images.
Therefore, the contents can be defined as water
Magnetic resonance imaging
of a lipoma.
The lesion in the right cheek is depicted as a
high signal in both T1-weighted and T2-
weighted images; therefore, the contents can be
diagnosed as fatty tissue
Dr Sanjana Ravindra

MRI of inflammation in the right cheek
In the T2-weighted image, fatty tissue and waterbased tissue are both depicted as
having high signals. Therefore, the range of inflammation is difficult to
determine. Because short-TI inversion recovery cancels the signal from fatty
tissue, the range of inflammation is easy to ascertain
Dr Sanjana Ravindra

DENTAL MRI
Conventional MRI techniques in dentistry have been restricted to
imaging pulp, attached periodontal membrane, and other
surrounding soft tissues or have required indirect imaging of
enamel and dentin through contrast produced by MRI-visible
medium
Images of the mineralized components of dental tissues have
been obtained from extracted teeth by using solid-state MRI
techniques, such as single-point imaging and stray-field imaging.
Imaging times in the 5- to 6-hour range, such methods are
unsuitable for in vivo applications
Main weakness of MRI methods in comparison with traditional
dental imaging is the high cost, differential and limited
accessibility of MRI equipment
Position of intraoral RF
coil for in vivo dental
imaging experiments on
the top of selected slice
Dr Sanjana Ravindra

maxillary left first
premolar with a
complete lingual cusp
fracture
Fracture (yellow
arrows), red
arrow delineates
what is most
likely air
entrapped in the
pulp canal
Conventional
radiography
Dr Sanjana Ravindra

INTRA-OPERATIVE MRI MACHNIE
Intraoperative magnetic resonance imaging
(iMRI) refers to an operating room configuration
that enables surgeons to image the patient via
an MRI scanner while the patient is undergoing
surgery, particularly brain surgery.
IMRI reduces the risk of damaging critical parts
of the brain and helps confirm that the surgery
was successful or if additional resection is
needed before the patient’s head is closed and
the surgery completed.
Higher field strengths, currently available in 1.5
and 3T options, provide better spatial and
contrast resolution enabling surgeons to more
accurately evaluate the findings on an image
Dr Sanjana Ravindra

Intraoperative MRI machine
Dr Sanjana Ravindra

INTRAOPERATIVE MRI MACHINE IN INDIA.
Dhirubhai Ambani Hospital and Medical Research Institute
Dr Sanjana Ravindra

😉
REASON FOR
CHOOSING THIS
ARTICLE
Dr Sanjana Ravindra

REFERENCES
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3. Lund G, Wirtschafter JD, Nelson JD, Williams PA. Tattooing of eyelids: magnetic resonance imaging artifacts.
Ophthalmic Surg. 1986;17:550–3.
4. Wagle WA, Smith M. Tattoo-induced skin burn during MR imaging. AJR. 2000;174:1795.
5. Ross JR, Matava MJ. Tattoo-induced skin “burn” during magnetic resonance imaging in a professional football player: a
case report. Sports. Health (London). 2011;3:431–4.
6. Tokue H, Taketomi-Takahashi A, Tokue A, Tsushima Y. Incidental discovery of circle contact lens by MRI: you can’t
scan my poker face, circle contact lens as a potential MRI hazard. BMC Med Imaging. 2013;13:11.
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8. Hou H, Xu Z, Xu D, Zhang H, Liu J, Zhang W. CT and MRI findings of primitive neuroectodermal tumor in the
maxillofacial region. Oral Radiol. 2016;32:14–21.
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and MRI findings in 13 cases. Oral Radiol. 2015;31:81–9.
10. Thomsen HS, Marckmann P. Extracellular Gd-CA: differences in prevalences of NSF. Eur J Radiol. 2008;66:180–3
11. Katti G, Ara SA, Ayesha S, Magnetic Resonance Imaging (MRI)- Review, Int J Dent Clin. 2011:3(1):65-70.Dr Sanjana Ravindra

REFERENCES
13. Westbrook C. MRI at a Glance. USA: Blackwell Science Ltd. 2002.
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15. Schild HH. MRI Made Easy. Germany: Nationales Druckhaus Berlin. 1990.
16. Reimer P, Parizel PM, Stichnoth FA. Clinical MR Imaging- A Practical Approach. 2nd ed. New York: Springer Berlin Heidelber. 2006.
17. Moeller TB, Reif E. MRI Parameters and Positioning. New York: Thieme Stuttgart. 2003.
18. Jezzard P, Matthews PM, Smith SM. Functional MRI: An Introduction to Methods. New York: Oxford University Press. 2001.
19. Liney G. MRI from A to Z A Definitive Guide for Medical Professionals. UK: Cambridge University Press. 2005.
20. Moeller TB, Reif E. Pocket Atlas of Sectional Anatomy Computed Tomography and Magnetic Resonance Imaging. Vol I Head and
Neck. New York: Georg Thieme Verlag. 2007.
21. Whaites E. Essentials of Dental Radiography and Radiology. 4th ed. Spain: Churchill Livingstone Elsevier. 2007. p. 239-241.
22. Frederiksen NL. Specialized Radiographic Techniques. In: White SC, Pharoah MJ. Oral Radiology Principles and Interpretation. 5th
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23. Magnetic Resonance Imaging. In: Karjodkar FR ed. Textbook of Dental and Maxillofacial radiology. 2nd ed. New Delhi: Jaypee
Brothers Medical Publishers (P) Ltd. 2009. p. 286-323.
24. Nuclear Magnetic Resonance. Curry TS, Dowdey JE, Murry RC eds. Christensen’s Physics of Diagnostic Radiology. 4th ed. USA: Lea
& Febiger. 1990.
25. Magnetic resonance imaging. Farr’s Physics for Medical Imaging. p. 169-195.
Dr Sanjana Ravindra

THANK
YOU!
👍
Dr Sanjana Ravindra

Basic principles of magnetic resonance imaging for beginner - Dr Sanjana Ravindra

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