This document provides an overview of the basic principles of magnetic resonance imaging (MRI) for beginner oral and maxillofacial radiologists. It discusses MRI physics including proton spin and precession in magnetic fields. It describes how MRI uses radiofrequency pulses and magnetic field gradients to encode spatial information and generate tomographic images of tissues based on proton relaxation properties following excitation. The document also outlines the basic components of an MRI system including the main magnet, gradient coils, and radiofrequency coils. It provides a brief overview of MRI sequences and contrast.

1. 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.
Presented by –Dr.Zareesh Akhtar
3rd MDS OMR

2. INTRODUCTION
• Magnetic resonance imaging (MRI) is a noninvasive
tool to investigate living subjects' internal anatomy
and physiology.

3. • Coinciding with the discovery of X-rays by Roentgen
in 1895, field of electromagnetic spectrum was also
opened to scientists who can use it for medical
imaging.
• In case of magnetic resonance imaging (MRI), radiant
energy is in the form of radio-frequency (RF) wave
rather than X-ray.

4. photography Radiography MRI
DIFFRACTION
REFLECTION

5. Benign and
malignant
Temporomandibular
joint disorders.
Soft tissue
diagnoses
Inflammation

6. 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.

7. BASIC PHYSICS
• The atom consists of two parts, i.e.,
• A central NUCLEUS
• Orbital ELECTRONS

8. BASIC PHYSICS-Inside the atom
PROTONS have a positive charge.
NEUTRONS have no electrical charge.
ELECTRONS have a negative charge.
The NUCLEUS is made up of PROTONS
and NEUTRONS
The number of electrons in an
atom usually matches the
number of protons, making
the atom electrically neutral.

9. • 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

10. • The magnetic field or magnetization is created
with the rotational motion of positively charged
protons
• This magnetization is represented by a vector
called a 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 the
magnetic field direction.
• This interaction with the proton’s magnetic
vector and magnetic field creates magnetic
resonance.

11. • When protons align, not only they rotate around
themselves, but also their axis of rotation such that
it forms a CONE.
• This movement of the axis of rotation of the
proton is called PRECESSION.

12. PRECESSION FREQUENCY
The frequency at which the proton spin is called the
precession frequency.

13. PRECESSION
• PROTONS IN FREE
SPACE
• PROTONS IN A
MAGNETIC FIELD

14. • Precession frequency of the protons - not constant
• Exact calculation of Precession frequency is done by
means of the LARMOR EQUATION-Υ
• 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

15. • There are three conditions we need to maintain to
efficiently tilt the magnetization from Z –the 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.
• rf-pulse should be perpendicular to the main static
field.

16. • Requirement is that they should have spin and
should have an odd number of protons in the
nucleus
• Hydrogen atom
-Only one proton
-H+ is equivalent to a proton
-Present in abundance in body water
-Best and most intense signal among all nuclei

17. 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

18. • 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 spectrum

19. PRINCIPLES OF MAGNETIC RESONANCE
• 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

20. • A radiofrequency 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.

21. MAGNETIC RESONANCE IMAGING SYSTEM
INSTRUMENTATION
1.5 T whole body MR system (VISART®, Toshiba, Japan) showing the bore an
patient bed (A) and operator’s control station (B).

22. 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.
• Types of coil
- Gradient coil
- Radiofrequency coil
- Shim coil

23. 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.
•Low field magnet system- < 0.2 T
•Mid field magnet system- 0.2- 1 T
•High field magnet system- >1T.

24. • 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

25. 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

26. • 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.

28. • Provide auxiliary magnetic fields in order to
compensate for in homogeneities in the main
magnetic field of the MRI machine.
SHIM COIL
GANTRY
• The space in which the patient reclines (the gantry)
is a narrow tube, leading to a significant feeling of
restriction during scans.

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

30. 1.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 +ve or -ve
• 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)

31. • 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.

32. 2.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.

33. • 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.

34. Magnetization vector in the X, Y and
Z planes

35. 3.Kinetic alteration of magnetization vector according
to the application of radiofrequency pulse
• When a radiofrequency pulse is applied, the
hydrogen nuclei precessing 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).
Kinetic alteration of the
magnetization vector in
the X, Y and Z planes
following application of a
90˚ RF pulse

36. • 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 the 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

38. 4.Relaxation phenomena of the magnetization vector after
90˚ rf pulse has been switched off
• Magnetic 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

39. • 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
•

40. • 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.

41. • 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.

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

43. 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

45. • Fat has a short T1 and T2 time.
• Water has a long T1 and T2 time.
• T1 weighted images are characterized by bright fat and dark
water.
• T2 weighted images are characterized by bright water and
intermediate fat.

46. T1 weighed images T2 weighed images
• 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).

47. 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 signals
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 the MR image. Eg- CSF.
• T1 gives good image contrast and is helpful for
depicting small anatomical regions like TMJ.

48. T2 WEIGHTED IMAGES(WATER IMAGES)
• So-called water has the 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

49. SIGNAL INTENSITIES OF DIFFERENT TISSUES
ON T1- AND T2- WEIGHTED IMAGES
Ongole ,Clinical Manual for Oral Medicine and Radiology

51. 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.

52. • 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

54. 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.
• Because MRI examinations utilize a magnetic field,
artifacts can occur due to the presence of magnetic
metals in the imaging area.
• Therefore, while artifacts only appear in the direction
of slices in CT scans, they appear as three-dimensional
missing signals in MRI examinations

65. 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.

66. • 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 pattern

67. 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

68. 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.

69. 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

70. Uses of MRI in Dentistry
1. Use of MRI for head and neck lesions: T1 weighted
images are useful for defining the anatomy of the lesion
T2 weighted images are useful for assessing invasion of
the lesion into surrounding structures. Extension of the
lesion into muscles, brain or blood vessels can be
precisely studied using contrast enhanced MRI. Coronal
scan helps in the assessment of lesions involving the base
of the skull and the perineural extensions of tumors.
2. Congenital disorders: T1 weighted sequences with
coronal and axial images demonstrate abnormalities such
as cleft lip and palate.

71. 3. Infections: AIDS- Generalized cervical
lymphadenopathy with cystic lesions in the parotid
can be demonstrated in an MRI
4. Sinusitis: MRI of the sinus is indicated only when
sinusitis is complicated by a serious that condition like
a tumor, venous sinus thrombosis or an intracranial
extension of the infection

72. 5. Benign Tumors: Hemangiomas, Lymphangiomas,
Neurofibromas and Schwannomas can be studied.
6. Malignant tumors: MRI can be used for the diagnosis,
staging and for the monitoring of malignant tumors
affecting the head and neck region.
7. TMJ: It is the best imaging technique to study the TMJ
(articular disc perforations and disc displacements can be
evaluated on a MRI.)

73. 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.

74. • 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

75. INTRAOPERATIVE MRI MACHINE IN
INDIA

76. Summary
Pre -scan excitation Relaxation
T1relaxation
• Spin lattice
RELAXATION
• Occurs along z-axis
Ti relaxation time
Time required for 63%
of the longitudinal
magnetization to be
regained –slow process
T2 relaxation
• Spin –spin relaxation
• Occurs along the X-Y
axis
T2 relaxation time
Time required for 63 %
of transverse
magnetization to be lost
–a fast process

77. Summary
Pre -scan excitation Relaxation Acquisition
Computer
and
display

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