basic principles of magnetic resonance imaging for beginner - dr sanjana ravindra

BASIC PRINCIPLES OF MAGNETIC

RESONANCE IMAGING FOR

BEGINNER ORAL AND

MAXILLOFACIAL RADIOLOGISTSKagawa 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 RavindraOral 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

diagnoses

Benign and malignant

Inflammation

Temporomandibular 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

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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

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 off

Dr Sanjana Ravindra


MAGNETIC FIELD INDUCED BY SPINNING HYDROGEN NUCLEUS




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




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

Dr Sanjana Ravindra





RELAXATION PHENOMENA OF THE MAGNETIZATION VECTOR AFTER 90˚ RF PULSE HAS BEEN SWITCHED OFF

Dr 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

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 burnsDr 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

Dr Sanjana Ravindra

M

Dr Sanjana Ravindra

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

ARTICLEDr Sanjana Ravindra

REFERENCES

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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

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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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