MRIMAGNETIC RESONANCE IMAGING

CONTENTS

History

MRI System Components

MRI Physics

MRI Signal Types & Coding

Fundamental Clinical Sequences

Tissue Contrast

Image Quality

Artefacts

MRI Safety

WHAT WE PRESENT ?

This presentation aims to explain

What is Magnetic Resonance Imaging?

What are the componenst of MRI machine

What is the method of imaging?

How Visual Is Obtained ?

Pyhsic of MRI

Sick fuck motherfucker

HISTORY OF MRI

What is Magnet?

The word magnetism is derived from Magnesia, known

present name is Manisa, found by a shepherd at around

1000 BC. He was walking on the mountains and suddenly

he was drawn to the earth by the tracks in his sandals.

Investigating the cause, he discovered lodestone

according to Pliny The Elder, he could magnetize metal

by rubbing lodestone on it.

HISTORY OF MRI

Hans Christian Ørsted made first experiment in 1820

Observed a direction of coppass changes when

located in a field of wire carrying current

Then it was discovered that surrounding quite long

wire around an empty conductor cylinder makes a

bigger magnetic field around itself.

MRI machines rely on this concept and the section

which patients are put in surrounded current-carrying

wire.

HISTORY OF MRI

2/3 of Human body is water

Organs and tissues have different contents and lots of

disease change them

Water is a molecule which includes hydrogen and

oxygen atoms. When magnetic field is applied protons

and neutrons spin around themselves randomly and

irregularly.

That explains why MRI is used that commonly

HISTORY OF MRI

In imaging, strong magnetic field is creatived and when

body is exposed to that force, hydrogen’s nucleus’ are

sorted either along or opposite of magnetic fields

direction. If nucleuses are excited by appropriate

frequency radio wave, their energy level change and

they pass over higher energy level. When radio waves

are cut, reverse occurs and atoms give their energy to

the system. This process is called Magnetic Resonance.

The energy which is transferred during this process spread

as alternative current and by using those signals, 3D

images are obtained having detailed chemical structure

of cells.

HISTORY OF MRI

WHO ?

Felix Bloch and Edward Purcell

Worked individually in 1946

There is a simple relation between resonance,

magnetic field strength and frequency of radio waves.

For every type of atom nucleuses, there is a constant

number which detecting the amplitude of radio

waves after application of magnetic field.

Rewarded with Nobel Physics Prize in 1952

HISTORY OF MRI

NMR (Nuclear Magnetic Resonance)

Isidor Rabi and his friends were first observers

Nuclear Magnetic Resonance expression was used in

his book by the name of “A New Method of Measuring

Nuclear Magnetic Moment”

Rewarded with Nobel Pyhsics Prize in 1944

HISTORY OF MRI

NMR

Between 1950 and 1970, NMR was being used for

moleculer analysis and in 1970, Raymond Damadian

showed tumoured tissues responds different signals then

healty tissues. After this, NMR began used in detecting

unhealtly tissues

HISTORY OF MRI

Gradient System

In 1973 Paul Lauterbur found Gradient System (Gx,

Gy, Gz) to pick up and excite a particular section of

human body in three dimension then boradcast a first

visual of 2 tube in Nature Magazine. He also called this

imaging by the name of Zeugmatography.

HISTORY OF MRI

Fourier in Radon out !!

In 1975 Richard Ernst, from Zurich University built the

base of Magnetic Resonance by offering the method

of frequency and phase coding. He thought of using

Fourier Transfor instead of Radon Transform and

applied succesfully

HISTORY OF MRI

EPI

In 1977, Damadian accoplished imaging of entire

human body.

Same year, Peter Mansfield developed Echo Planar

Imaging (EPI) provided imaging very fast.

HISTORY OF MRI

In 1980, Hawkens discovered the multiplanar property

of MR and detected first lesion.

Edelstein and his friends began obtaining visuals by

using Ernst Technique.

Initial MR works based on weight of protons and T1

technique began in clinically.

HISTORY OF MRI

Between 1982-1983, it was figured out that T2 based

Spin-Echo technique was better for imaging

pathologic.

Contrast agent got attraction and started being used

in 1980s and fr the first time, Schering Company

applied for patent for Gd-DTPA.

HISTORY OF MRI

In 1987, EPI technique was used th imaging of heart’s

phase in real-time.

Same year, Charles Dumoulin accomplished imaging

of veins without constast agents by developing

Magnetic Resonance Anjiyografi.

In 1993, functional Magnetic Resonance Imaging

(fMRI) was discovered. Then by that time, it was

possible to explore deep parts of human braing by

usin fMRI.

HISTORY OF MRI

In 1944, New York State and Princeton University

achieved imaging of lungs.

Finally FDA (Food and Drug Ac.) allowed marketing up

yo 4 Tesla devices in 1998

Today, increasing strength of magnetic field enables

to increase the quality of visual and also decreases to

time of observation.

COMPONENTS OF MRI

MAIN MAGNET

GRADIENT COIL FEATURES

RF SARMALLARI

SHIELDING

COMPUTERS

COMPONENTS OF MRI

MAIN MAGNET

According to constructure of magnet

Really strong from 0,02 Tesla to 3 Tesla

FDA allows 7 Tesla MRI for only clinical researches on human body

For animal used experiments, it is 11 Tesla

According to structure of magnet

Permanent (Fixed) Magnets : 0,7 T

Resistive Magnet : 0,2 T

Superconductor Magnet : 0,5 T - 7,9..T

MAIN MAGNET

According to streght

Low Field

Midfield 0,2 T – 1 T

High Field 1,5 T and over

MAIN MAGNET

Magnets are built with superconductor mostly

Most expensive part of MRI

3-4 km wire, liquid Helium 4K (-269 C) to provide the

resistance of wire reaches zero

GRADIENT COILS

In magnet,there is Gradient Features along Gx, Gy

and Gz and inner side RF Coils.

Enable us to adjust the strenght of magnetic field.

SI Unit is Tesla

Rise Time is time arrival that spirals can reach top level

Main noisy comes from Gradiant Features

Motorbycles 90 Db, drill 100 Db, MRI 110 Db

RF COILS

According to function

Sends RF Pulse and detects return signals

(Transmitter-Receiver)

Only sends (Transmitter)

Only Receives (Receiver)

RF COILS

According to anatomical region

Head

Body

Breast

RF COILS

According to mechanical structures

Linear Coils

Volume Coils

According to working principle

Linear Coil

Quadrature Coil

RF COILS

RF COILS

SHIELDING

It is a shield to prevent stronge RF pulses to damage

other devices

Also prevents other devices’ waves to distract RF Pulse

Can be Passive and Active

Passive is built Faraday Case

Active is built by producing reverse magnetic fields but

expensive and can damage elevators or cars

Computers

Enables the system work properly

Adjusts RF pulses

Provides power amplifiers and Gradiant Coils

work effective and sensitive

Physics of MRI

Magnetic Field

Paramagnetic

Diamagnetic

Supermagnetic

Ferromagnetic

Precession

Resonance

Larmor Freq. (γ) = 42.58 Mhz/T (H+)

𝜔0 = γ 𝐵0𝜔0 = Angular Velocity

𝐵0 = External Magnetic Field

𝑀0 = Net Magnetic Field

Relaxation

𝑀𝑧 = 𝑀0 1 − 𝑒−𝑡/𝑇1 (Longitudinal)

𝑀𝑥𝑦 = 𝑀0 𝑒−𝑡/𝑇2 (Transvers)

Gradient

Basicly, to create a gradient is actually creating some

increment or decrement on any constant value. By

calculating, the value of this alteration at any point

divided by maximum alteration, gives us the gradient

measurement.

In MRI systems, gradients is used to change the

external magnetic field’s value at any point as

required.

K-Space

3 things that effects creating the image in MRI

K-Space is a 2D place where we put signals coming from coils

K Normal Decreased Normal

Res Normal Decreased Normal

FOV Normal Normal Decreased

Time

ResolutionSignal/Noise

Ratio

MR Signals Types

FID (Free Induction Decay)

Spin-Echo

Stimulated Echo

Signal Coding

Slice Selection Gradient (Z axis)

Phase Encoding Gradient (Y axis)

Frequency Encoding Gradient (X axis)

Gradient Coil Features

Max. Strength(T/m) , Rise Time(s) , Slew Rate(T/m*s)

Slice thickness

K-Space: Center = Low Freq. contrast density

Outer = HF Resolution

Filling K-Space

Fundamental Clinical

Sequences

Sequence parameters (TR, TE)

Multislice working

Sequences

Spin-Echo

Multiecho

Gradient Echo

Inversion Recovery

FLAIR(Fluid Attenuated Inversion Recovery – BOS) 1900-2500ms

STIR(Short TI Inverison Recovery – Fat Tissue)

Turbo spin-echo & Fast spin-echo

Tissue Contrast

T1 Contrast

T2 Contrast

Proton Density Contrast

Gradient Echo

Image Quality

SNR (Signal Noise Ratio)

Sequence Parameters

NSA(Number Of Signal Average) or NEX (Number of Excitations)

Matrix (Acquisition)

FOV (Field of View)

Slice Thickness

Slice Gap

Choosing Phase Coding Direction

Receiver Bandwidth

MRI ARTEFACTS

MR hardware and room shielding

Herringbone artifact

Moire fringes

Zebra stripes

Central point artifact

RF overflow artifacts

Inhomogeneity artifacts

Zipper artifact

MRI ARTIFACTS

MR software

Slice-overlap artifact aka cross-talk artifact

Cross excitation

Patient and physiologic motion

Phase-encoded motion artifact

Entry slice phenomenon

MRI ARTIFACTS

Tissue heterogeneity and foreign bodies

Black-Border Artifact

Magic Angle Effect

Susceptibility artifact / magnetic Susceptibility

artifact

Chemical shift artifact

MRI ARTIFACTS

Fourier transform & Nyquist sampling

theorem

Gibbs artifact / truncation artifact

Zero-fill artifact

Aliasing / wrap around artefact

Gibbs Artifact / Truncation

Artifact

Black-Border Artifact

MRI SAFETY

Pregnants and are not allowed to get in to MRI room

All kind of metal items are prohibited like paper-clip,

hairgrip, neckless etc., those can reach a speed of 60

km/h under impact of 1,5 T magnetic field

Patients must fill in a form including safety instructions

Cardiac pacemaker, biomedical implants and

devices, aneurysm clips, Stents, infusion pumps etc.

Patients working on metal jobs who may have swarfs

in espicelly their eyes or other possible parts of body

Metalic implants either damage patients and make

artefact on image

Created by

Oğuz GENÇER B1205.020099

Yavuz Selim AK B1205.020050

Thanks for all

audiences