Presentation Report @Istanbul Aydin University

1. I
[ISTANBUL AYDIN UNIVERSITY]
[ENGINEERING FACULTY]
[ELECTRICAL&ELECTRONICS ENGINEERING]
[MICROWAVE THEORY]
[MRI TECHNOLOGY]
[SAEID KARAMZADEH]
[YAVUZ SELIMAK]
[STUD. NO: B1205.020050]
[OGUZ GENCER]
[STUD. NO: B1205.020050]

2. I
ABSTRACT
This article is a report on Magnetic Resonance Imaging Systems. MRI Systems are being
used for a long time in our life. There are also some other devices for medical diagnosis.
However MRI has more advantages than others. On the other hand, MRI system has many
components which have important role in this system. These systems include some sub-systems
and one of them is the human body as you guess. There are some detailed information about
MRI physics in this article to discussing deeply. Understanding the physics of MRI is the easiest
way to understand rest of this article. Simply, MRI works with RF unlike the Computer
Tomography. It sends some RF pulse to tissue of human body and generate some pictures from
the reflected signal from the tissue. There are some different type of signals and their coding
styles. Some of them were used in the earlier stages. Some others are still used. Clinical
sequences are also known by the MRI Technicians and Engineers. So this is another important
subject that we have to consider while working with MRI. On the other hand, the image
processing is a highly important subject in MRI. There are some structures and features that
directly or indirectly effect the quality of the image. Occassionally, some errors are occured on
the image that we call artefacts. Device’s errors or moving object in the MR coil can cause these
artefacts. There are also some other reasons that you can find in this article. The another
important subject is the safety. If do not complied with the rules or precautions, it would be so
dangerous while working with the MRI. To sum up, now it is more clear to see the main idea
behind the MRI system and the others.
Key Words
Magnetic Resonance Imaging System, MRI, physics, image, sequence

3. I
CONTENTS
ABSTRACT ………………………………………………………………………………………………………………………………………….. I
CONTENTS ………………………………………………………………………………………………………………………………………….. II
1. SECTION : HISTORY OF MRI ………………………………………………………………………………………………………… 1
2. SECTION : MRI SYSTEM COMPONENTS ……………………………………………………………………………………… 4
2.1. MAIN MAGNET ………………………………………………………………………………………………………………… 4
2.2. GRADIENT COIL FEATURES ……………………………………………………………………………………………….… 6
2.3. SHIMMING …………………………………………………………………………………………………………………….…… 6
2.4. RF COILS ………………………………………………………………………………………………………………………… 7
2.5. SHIELDING ………………………………………………………………………………………………………………………… 8
2.6. COMPUTERS ……………………………………………………………………………………………………………………… 8
3. SECTION : MRI SPHYSICS ………………………………………………………………………………………………………….… 9
3.1. Magnetic Elements ……………………………………………………………………………………………………………… 9
3.2. Precession …………………………………………………………………………………………………………………………… 9
3.3. Resonance ………………………………………………………………………………………………………………………… 10
3.4. Relaxation ………………………………………………………………………………………………………………………… 13
3.5. Gradient ……………………………………………………………………………………………………………………….… 13
3.6. K-space …………………………………………………………………………………………………………………………..…… 15
4. MRI SIGNAL TYPES&CODING TECHNIQUES …………………………………………………………………………………… 16
4.1. MR SIGNAL TYPES ………………………………………………………………………………………………………….……… 16
4.1.1.FID (Free Induction Decay) …………………………………………………………………………………….…… 16
4.1.2.Spin Echo …………………………………………………………………………………………………………………. 17
4.1.3.Stimulated Echo ……………………………………………………………………………………………………….… 17
4.2. SIGNAL CODING ……………………………………………………………………………………………………………….… 18
4.3. GRADIENT COIL FEATURES ……………………………………………………………………………………………….… 18
5. FUNDAMENTAL CLINICAL SEQUENCES ………………………………………………………………………………………… 19
5.1. Sequence Parameters ………………………………………………………………………………………………………….. 19
5.2. Multislice Working …………………………………………………………………………………………………………..…… 19
5.3. Sequences …………………………………………………………………………………………………………………….…..… 20
5.3.1.Spin Echo …………………………………………………………………………………………………………….……..… 20
5.3.2.Multi Echo ……………………………………………………………………………………………………………..….…. 20
5.3.3.Gradient Echo ……………………………………………………………………………………………………..……..... 21
5.3.4.Inversion Recovery ……………………………………………………………………………………………………..… 21

4. II
5.3.5.FLAIR ( Fluid Attenuated Inversion Recovery) ………………………………………………….…………… 21
5.3.6.STIR ( Short Inversion Recovery) ………………………………………………………………………………….. 21
5.3.7.Turbo Spin Echo – Fast Spin Echo ……………………………………………………………………………….… 21
6. TISSUE CONTRAST ………………………………………………………………………………………………………………..…… 23
6.1. T1 Weight Contrast …………………………………………………………………………………………………………..… 23
6.2. T2 Weight Contrast ……………………………………………………………………………………………………..……… 23
6.3. Proton Density Contrast ……………………………………………………………………………………………...……… 23
6.4. Gradient Contrast …………………………………………………………………………………………………….……….… 23
7. IMAGE QUALITY …………………………………………………………………………………………………………….…………… 25
7.1. Sequence Prameters ………………………………………………………………………………………………..…….…… 26
7.1.1.Voxel Volume ………………………………………………………………………………………...…….…..………… 26
7.1.2.Number of Signal Average (NSA) …………………………………………………………….………..….……… 26
7.2. Receiver Bandwidth ……………………………………………………………………………………….……………….…… 26
8. ARTEFACTS …………………………………………………………………………………………………………………..………..…… 28
9. MRI SAFETY ……………………………………………………………………………………………………………….……….….…… 31
10. CONCLUSION ……………………………………………………………………………………………………………..…………….… 32
11. CITATIONS ………………………………………………………………………………………………………………………….………. 33

5. 1
1. SECTION : HISTORY OF MRI
This presentation aims to explain Magnetic Resonance Imaging (MRI ) technology which
is based upon magnetism and electricity and it is divided into 3 parts which will be covered by
one of us.
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.
The first experiment to create an artificial magnet was made by Hans Christian Ørsted in
1820. He initially discovered that a compass’ direction changes when it is 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.
It is actually not possible to specify the very beginning point in the history of MRI but
Jean-Baptiste-Joseph Fourier must be mentioned who found Fourier Transform. Without this
equation, it would not have been possible to obtain MR visuals.
A human body contains 2/3 rate water. After discovering that, it can be comprehended
why MRI is been usable in medicine that much. The reason why MRI is helpful is that organs and
tissues have different contents and lots of disease change them, even %1 of change is enough
us to detect pathological changes by MRI. Water is a molecule which includes hydrogen and
oxygen atoms. Protons and neutrons spin around themselves randomly and irregularly.
Hydrogen’s nucleus resembles magnet’s end. 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.

6. 2
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. 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. This phenomenon was demonstrated by Felix
Bloch and Edward Purcell for protons. These to Scientifics ,working separately in 1946, defined
some characteristics of atom nucleuses which can be referred to physicochemical and they were
awarded with Nobel Prize in 1952. However, nuclear magnetic resonance first observed by
Isidor Rabi and his friends. Rabi was the fist scientist used “Nuclear Magnetic Resonance(NMR)”
expression in his book by the name of “A New Method of Measuring Nuclear Magnetic
Moment” and awarded with Nobel Physics Prize in 1944. Let us sort them in terminological
order;
 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 .
 ın 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.
 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.
 In 1977, Damadian accoplished imaging of entire human body.
 Same year, Peter Mansfield developed Echo Planar Imaging (EPI) provided
imaging very fast.
 In 1980, Hawkens discovered the multiplamar property of MR and detected fist
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.

7. 3
 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.ß
 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.
 In 1993, functional Magnetic Resonance Imaging (fMRI) was discovered. Then by
that time, it was possible to explore deep parts of human braing by using fMRI.
 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 and for this reason, high strength
MRI machines draw attention.
The advantage of MRI over CT (Computed Tomography) is not using radiation and not causing
cumulative troubles at the end. The other one is that MRI can demonstrate soft tissues better.
MRI was developped by most Nobel-winning chemists, physicists, engineers and doctors for 50
years of long period and today, every year over 2000 MRI are produced, over 60 million analysis
are carried out all around the world. In Turkey, first MRI was established at 9 September
University Medical Faculty in 1989 than following years, the numbers of machines increased
rapidly. [1]

8. 4
2. SECTION : MRI SYSTEM COMPONENTS
2.1.MAINMAGNET
The biggest and most important component of an MRI system is the magnet. There is a
horizontal tube -- the same one the patient enters -- running through the magnet from front to
back. This tube is known as the bore. But this isn't just any magnet -- we're dealing with an
incredibly strong system here, one capable of producing a large, stable magnetic field.
The strength of a magnet in an MRI system is rated using a unit of measure known as
a tesla. Another unit of measure commonly used with magnets is the gauss (1 tesla = 10,000
gauss). The magnets in use today in MRI systems create a magnetic field of 0.5-tesla to 2.0-tesla,
or 5,000 to 20,000 gauss. When you realize that the Earth's magnetic field measures 0.5 gauss,
you can see how powerful these magnets are.
Most MRI systems use a superconducting magnet, which consists of many coils or
windings of wire through which a current of electricity is passed, creating a magnetic field of up
to 2.0 tesla. Maintaining such a large magnetic field requires a good deal of energy, which is
accomplished by superconductivity, or reducing the resistance in the wires to almost zero. To do
this, the wires are continually bathed in liquid helium at 452.4 degrees below zero Fahrenheit
(269.1 below zero degrees Celsius). This cold is insulated by a vacuum. While superconductive
magnets are expensive, the strong magnetic field allows for the highest-quality imaging, and
superconductivity keeps the system economical to operate. [1]

9. 5
Magnets classified in two main group :
1. According to Structure
a. Permanent (Fixed) Magnets : 0,7 T
b. Resistive Magnet : 0,2 T
c. Superconductive Magnet : 0,5T – 7,9 T
2. According to Strength of Magnetic Field
a. Low Field
b. Midfield : 0,2T – 1 T
c. High Field : 1,5 T – Higher

10. 6
2.2.GRADIENTCOIL FEATURES
In magnet, there are coils called Gx, Gy, Gz and RF Coils at inner side. These magnets are
much smaller that the primary magnet (about 1/1000 as strong), but they allow the
magnetic field to be altered very precisely.
Rise time is the required interval that coils reach up to maximum strength measured in Tesla
and Slew Rate gives maximum strength and rise time together ( T / m / sn ).
During rapid sequences, gradients need to be changed and magnetic field strength needs to
be reached to top rapidly and main noisy occurs at this process causing by interaction of
external magnetic field and gradient magnetic field. To give an example, a bike makes 90 Db,
a drill makes 100 Db and MRI machine makes 110 Db while 140 Db considered traumatic
level. [1]
2.3.SHIMMING
It is correction to provide homogenize the magnetic field in magnet. Two ways to
construct :
 Passive Shimming; it is made to put metal plates at the inside and outside of
magnet.
 Active Shimming; it fulfilled by additional coils in homogenite hellium section in
magnetic field. [1]

11. 7
2.4. RF COILS
RF coils can be divided into three general categories;
 According to Function
o Transmit and receive Coils
o Receive Only Coils
o Transmit Only Coils
 According to Body Section RF Used
o Body, head and breast surface coils.
 According to mechanical structure
o Linear Coil
o Volume Coil
 According to Operate Principles
o Linear Coil
o Quadrature Coil : A coil that produces an RF field with circular polarization. The
RF power received from the RF power amplifier comes in two signals (quadrature detection),
which have a phasedifference of 90°. The RF transmit coil converts the power into a circularly
polarized RF magnetic field. Quadrature coils can be used as both, transmit and/or receive coil.
When used as a transmitter coil a factor of two power reduction over a linearcoil results; as
a receiver an increase in SNR of up to a factor of √2, can be achieved. [1]

12. 8
2.5.SHIELDING
Shielding prevents either RF pulses to damage other devices and surrounding waves
interfere and distorte the image so that makes shielding extremely important for MRI machines
.
Shielding can be constructed in two ways;
 Passive ; Faraday Cage
 Active ; this method is made by producting reverse magnetic field at out of
magnet however it is expensive and can damage elevators and cars surrounding.
[1]
2.6.COMPUTER
Computers are used to adjust the strength of magnetic field and sequences timing, to
provide operating efficienty and sensivity of RF Coils, amplificators and gradients. [1]

13. 9
3. SECTION : MRI PHYSICS
3.1.Magnetic Elements
As it is known, there are magnetic elements in the universe. Their type are vary with their
behavior inside of a magnetic field. When we put a magnetic element into a magnetic field; if it
increase the strength of the magnetic field, it is called as paramagnetic. If decrease the strength,
it is called as diamagnetic. If the element increase the magnetic field strength too much, it is
called as supermagnetic. On the other hand, after we take the element out from the magnetic
field, if it still has a magnetic property, it is called as ferromagnetic element. [1]
3.2.Precession
Precession is a change in the orientation of the rotational axis of a rotating body. In an
appropriate reference frame it can be defined as a change in the first Euler angle, whereas the
third Euler angle parameterizes the rotation itself. In other words, the axis of rotation of a
precessing body rotates itself around another axis. A motion in which the second Euler angle
changes is called nutation. In physics, there are two types of precession: torque-free and
torque-induced. [2]
3.3.Resonance
In physics, resonance is the tendency of a system to oscillate with
greater amplitude at some frequencies than at others. Frequencies at
which the response amplitude is a relative maximum are known as the
system's resonant frequencies, or resonance frequencies. At these
frequencies, even small periodic driving forces can produce large
amplitude oscillations, because the system stores vibrational energy. [3]
I
mage 1 [12]

14. 10
Resonance, could be used to transmit the energy into a system by using equal frequencies. This
frequency is called as Larmor Frequency (γ). The hydrogen atom has a huge importance in our
life. It is another moot point but the hydrogen is a big part of a water molecule and also water
molecule constitutes approximately %70 of human body. So in the MRI Systems work based on
Larmor Frequency of the hydrogen atom which is 42.58 MHz in a 1 Tesla Magnetic Field.
Image 2[3]

15. 11
The RF signal is applied to the target area on tissue in Larmor frequency. In that case the angle
of precession is changed until become 900. So the effect of magnetic field strength on target
tissue will be changed for a while. This period is called generally as Resonance Period.
ωo = γ Bo
ωo = Angular Velocity
Bo = External Magnetic Field
Mo = Net Magnetic Field
Image 3 [1]
3.4.Relaxation
After that transmission, the RF signal is stopped and the angle of precession will take the
%63 of the first value a short time later. This period is called as Relaxation Period. There are
2 type of relaxation which are T1 (Longitudinal) and T2 (Transvers). [1] They are calculated
by that formulas;
Mz = Mo (1 − e−t/T1
) (Longitudinal) [1]
Mxy = Mo e−t/T2
(Transvers) [1]
Image 4 [1]

16. 12
3.5.Gradient
Simply, 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. [1]
3.6.K-space
In MRI physics, k-space is the 2D or 3D Fourier transform of the MR image measured. Its
complex values are sampled during an MR measurement, in a premeditated scheme
controlled by a pulse sequence, i.e. an accurately timed sequence of radiofrequency and
gradient pulses. In practice, k-space often refers to the temporary image space, usually a
matrix, in which data from digitized MR signals are stored during data acquisition. When k-
space is full (at the end of the scan) the data are mathematically processed to produce a
final image. Thus k-space holds raw data before reconstruction. [4]
There are 3 main factor that directly effect while creating the image of MR. The resolution
effect is seen on Image 5.
Time
Resolution
Signal/Noise
Ratio

17. 13
At (a) K is Normal, Resolution is Normal and FOV is Normal.
At (b) K was decreased, Resolution was decreased and FOV is Normal.
At (c) K is Normal, Resolution is Normal, FOV was decreased. [1]
Image 5 [1]
4. MRI SIGNAL TYPES&CODING TECHNIQUES
4.1.MR SIGNAL TYPES
4.1.1. FID (FreeInduction Decay)
The constituted signal when the hydrogen atoms are stimulated is called as FID. It is
usually used in 90o. [10]
Image 6 [1]

18. 14
Spin Echo
This signal is a combination of 2 signal which are 90o FID signal and 180o signals. In magnetic
resonance, a spin echo is the refocusing of spin magnetization by a pulse of resonant
electromagnetic radiation. Modern nuclear magnetic resonance and magnetic resonance
imaging make use of this effect. [11]
Image 7 [1]
4.1.2. Stimulated Echo
The stimulated echo signal is a combination of 3 signals which are usually 90o [1]

19. 15
4.1.3. SIGNAL CODING
During the RF signal transmission, it would not be known where the reflected signal
comes from. So the signal coding is an important process to determine signal source in
3D space.
As it is known 3D coordinates has x, y and z axes. There are 3 gradient to determine
exact source. [1]
 Slice Selection Gradient (z axis)
 Phase Encoding Gradient (y axis)
 Frequency Encoding Gradient (x axis)
Image 8 [1]
4.2.GRADIENTCOIL FEATURES
There are 3 features of gradient coil which has an important role in imaging process; [1]
 Maximum gradient strength:
The maximum gradient difference which be able to do in the main magnetic
field.
 Rise Time:
The time to reach the
maximum gradient strength.
More less is better.
 Slew Rate:
Acceleration, which is equal to
Max Strength/Rise Time
Image 9 [1]

20. 16
5. SECTION : FUNDAMENTAL CLINICAL SEQUENCES
5.1.Sequence Parameters
TR: For filling the K-space, it must be repeated in every row. The time between two
repetition is called as TR time.
TE: The time between 90o RF pulse and consist of Spin-Echo signal. It changes between 5-
250 miliseconds.
FA (Flip Angle): It changes proportionally with the power of RF Pulse which is used to consist
net magnetization in x-y plane. [1]
5.2.Multislice Working
While waiting for a TR time, a new TR time starts. This kind of processes are called as
multislice working.
Image 10 [1]

21. 17
5.3.Sequences
5.3.1. Spin Echo
The basic sequence to consist spin echo signal. [1]
Image 11 [1]
5.3.2. Multi Echo
To consist more than one spin echo signal [1]
Image 12 [1]
5.3.3. Gradient Echo
For re-phasing the hydrogen atoms, by using the gradient instead of 180o RF pulse. [1]
Image 13 [1]

22. 18
5.3.4. Inversion Recovery
The main difference from spin echo sequence is the 180o RF pulse applied at first. [1]
Image 14 [1]
5.3.5. FLAIR( Fluid Attenuated Inversion Recovery)
The pulse sequence is an inversion recovery technique that nulls fluids. For example, it
can be used in brain imaging to suppress cerebrospinal fluid (CSF) effects on the image, so as
to bring out the periventricular hyperintense lesions, such as multiple sclerosis (MS) plaques.
[5]
5.3.6. STIR( ShortInversion Recovery)
In the standard STIR sequence, the spin echo sequence is completed by a previous 180°
inversion pulse. Fat has a short T1. Hence by choosing a short TI of 140 milliseconds, the
fat signal can be suppressed . The combination of short TI inversion-recovery and fast spin
echo sequences reduces acquisition time to acceptable limits for clinical practice. [6]

23. 19
5.3.7. Turbo Spin Echo – Fast Spin Echo
Similar in concept to EPI, is Turbo Spin Echo (TSE) in which multiple 180° RF pulses
are used to continually refocus the decaying Mxy magnetization. In this way, multiple
MRI signals may be recorded from each excitation pulse. (TSE is also known as FSE and
RARE.)[7]
6. TISSUE CONTRAST
6.1.T1 Weight Contrast
A “T1-weighted” spin-echo sequence is designed to produce contrast mainly based on
the T1 characteristics of tissues by de-emphasizing T2 contributions. This can be
accomplished by using relatively short repetition times TR (~500ms) to maximize the
difference in longitudinal relaxation during the return to equilibrium, and a short echo time
TE (<15ms) to minimize T2 dependency during signal acquisition. [8]
6.2.T2 Weight Contrast
A “T2-weighted” spin-echo sequence is designed to produce contrast mainly based on
the T2 characteristics of tissues by de-emphasizing T1 contributions. This can be
accomplished by using relatively long echo time TE (~100 ms) and long repetition times TR
(~3000 ms). Compared with the T1-weighted image, we observe a image contrast inversion.
As TE is increased, more T2 contrast is achieved, at the cost of a reduced transverse
magnetization signal. [8]
6.3.Proton Density Contrast
A spin-density weighted image relies mainly on differences in the number of
magnetizable protons per volume. This can be accomplished by minimizing the impact of T1
and T2 differences with long TR and short TE in the spin-echo pulse sequence, respectively.
[8]

24. 20
6.4.Gradient Echo
GRE is based on a field gradient to induce the formation of an echo, instead of using a
180 deg RF pulse and is used e.g. for fast imaging procedures. The gradient changes the local
magnetic field slightly resulting in a dephasing of the transverse magnetization rapidly.
Reversing the field gradient at time TE/2 the spins will rephase and produce a gradient echo
at time TE. GRE is not a true spin-echo technique but purposeful de- and re-phasing of the
FID In contrast to conventional spin-echo, GRE emphasizes field inhomogeneities and tissue
susceptibilities. The decay of the transverse magnetization is fast and a strong function of
T2* [8]
A major variable determining tissue contrast is the flip angle :
 A small flip angle in combination with short TR results in enhanced transverse
magnetization (spin density weighting)
 With short TR, a equilibrium between longitudinal and transversal magnetization
can be achieved
7. IMAGEQUALITY
The quality of an MR image depends on several factors:
 Spatial resolution and image contrast
 (SNR) Signal to noise ratio (and contrast to noise ratio)
 Artifacts
An MR exploration is a compromise between scan time and image quality. An MR
exploration protocol and its sequence parameters will have to be optimized in function of
the organs and pathology. Spatial resolution corresponds to the size of the smallest
detectable detail. The smaller the voxels are, the higher the potential spatial resolution will
be. Voxel volume is determined by the matrix size (256 x 256 or 512 x 512 etc..), the field of
view (10 cm, 20 cm, etc....), and slice thickness.
Image contrast varies with the type of pulse sequence and its parameters. Tissue-contrast is
also modified by pre-saturation pulses or contrast agents. Image contrast and signal

25. 21
weighting have to be adapted to the imaging objectives: anatomy, edema, tissue-
characterization (fat, hemorrhage, water), vascularization... Noise is like interferences which
present as a irregular granular pattern. This random variation in signal intensity degrades
image information. The main source of noise in the image is the patient's body (RF emission
due to thermal motion). The whole measurement chain of the MR scanner (coils,
electronics...) also contributes to the noise. This noise corrupts the signal coming from the
transverse magnetization variations of the intentionally excited spins (on the selected slice
plane). The signal to noise ratio (SNR) is equal to the ratio of the average signal intensity
over the standard deviation of the noise. [9]
The signal to noise ratio depends both on some factors that are beyond the operator's
control (the MR scanner specifications and pulse sequence design) and on factors that the
user can change:
 Fixed factors : static field intensity, pulse sequence design, tissue characteristics
 Factors under the operator's control
o RF coil to be used
o Sequence parameters : voxel size (limiting spatial resolution), number of
averagings, receiver bandwidth
7.1.Sequence Prameters
7.1.1. Voxel Volume
The signal comes from the excited protons on the selected slice plane. The number of
spins in parallel state in excess is proportional to the static magnetic field intensity. The
larger the field intensity is, the higher the excess number of spins in parallel state (available
to make the MR signal) will be. Thus, the signal intensity varies almost linearly with the main
field intensity. Assuming a uniform proton density, the number of excited spins is
proportional to the voxel size and so is the signal intensity. The signal goes up linearly with
the voxel size. [9]
7.1.2. Number of Signal Average (NSA) or Number of Excitations (NEX)

26. 22
When the number of excitations (or Signal Average) for the same slice increases:
 The signal is identical for each measure
 The noise is random and is not the same for each measure
Therefore, the signal sum goes up linearly with the number of excitations but the noise
only goes up with the square root of the number of excitations. In other words, the
average signal remains constant, but the average noise goes down with the square root
of the number of excitations. The signal to noise ratio goes up with the square root of
the number of excitations. [9]
7.2.Receiver Bandwidth
Given a voxel size and static field strength, the number of excited spins is defined and so
is the amount of MR signal. The readout of the MR signal can take more or less time,
depending on the receiver bandwidth. The relation between the receiver bandwidth
and the strength of the readout gradient is such that:
 A broad bandwidth corresponds to a fast sampling of the MR signal and a high-
intensity readout gradient
 A narrow bandwidth corresponds to a slow sampling of the MR signal and a low-
intensity readout gradient
Background noise has a constant intensity at all frequencies (white noise). Therefore,
the larger the receiver bandwidth is, the more noise is recorded (and the higher is the
readout gradient intensity and the faster the MR signal is sampled). [9]
8. ARTEFACTS
Many different artefacts can occur during magnetic resonance imaging , some affecting
the diagnostic quality, while others may be confused with pathology. An artefact is a
feature appearing in an image that is not present in the original object. Artefacts can be
classified as patientrelated, signal processing-dependent and hardware (machine)-related.
Only some of them will be covered in this section.

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8.1.MR Hardwareand Room Shielding
8.1.1. Herringbone Artifact
8.1.2. Moire Fringes
8.1.3. Zebra Stripes
8.1.4. Central Point Artifact
8.1.5. RF Overflow Artifact
8.1.6. Inhomogeneity Artifact
8.1.7. Zipper Artifact
8.2.MR Software
8.2.1. Slice-overlap Artifact
8.2.2. Cross Excitation
8.3.Patient and Physiologic Motion
8.3.1. Phase-encoded Motion Artifact
8.3.2. Entry Slice Phenomenon
8.4.TissueHeterogenetiy Foreign Bodies
8.4.1. Black Boundary Artifact
8.4.2. Magic Angle Effect
8.4.3. Susceptibility Artifact/Magnetic Susceptibility Artifact
8.4.4. Chemical Shift Artifact
8.5.Fourier Transform
8.5.1. Gibbs Artifact/Truncation Artifact
8.5.2. Zero-fill Artifact
8.5.3. Aliasing /Wrap Around Artifact
8.5.1 Gibbs Artifact
This is caused by the under-sampling of high spatial frequencies at sharp
boundaries in the image. Lack of appropriate high-frequency components leads to an
oscillation at a sharp transition known as a ringing artefact. The artefact occurs near the

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sharp boundaries, where high contrast transitions in the object occur. It appears as
multiple, regularly spaced parallel bands of alternating bright and dark signal that slowly
fade with distance (Fig. ). Ringing artefacts are more prominent in smaller digital matrix
sizes. Methods employed to correct Gibbs artefact include filtering the Kspace data prior
to Fourier transform, increasing the matrix size for a given field of view, the Gegenbauer
reconstruction and Bayesian approach.
Image 15[13]

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8.4.1 Black Boundary Effect
Black boundary artifact or india ink artifact is an artificially created black line located at fat-
water interfaces such as those between muscle and fat. This results in a sharp delineation of the
muscle-fat boundary that is sometimes visually appealing but not an anatomical structure.
Case 1 is a coronal image through the upper body with an echo time of 7 ms. A black line is seen
surrounding the muscles of the shoulder girdle as well as around the liver.
This artifact occurs in gradient echo sequences as a result of selecting an echo time (TE) in
which the fat and water spins (located in the same pixel at an interface) are out of phase,
cancelling each other. Using a SE sequence instead of GE will also eliminate the artifact. []
Image [14]

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9. MRI SAFETY
It is very important to provide a safe atmosphere şn daily applications for radiologists,
technicians, nurses and physicists. Everyday, using of medical implants and additional
equipmants and therefore patients need to be checked carefully and closerly. Most of the
injuries occur by lack of scanning methods and misinforming.
If we estipulate all cautions
 Patients must be informed carefully for every effect of MRI and magnetic force
and undressed up dailiy clothes
 Patients who has cardiac pacemaker, biomedical implants and devices, aneurysm
clips, stents, infusion pumps etc. not allowed, metal materials either damage
patients and cause artifacts on image.
 Patients working on metal jobs who may have swarfs in espicelly their eyes or
other possible parts of body.
 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
Pregnants and are not allowed to get in to MRI room even though any of harm proved on
fetus.
10.CONCLUSION
In this report, we tried to explain what is the effect of magnetic force on water molecules
and it’s results, how a visual can obtained, which scientists contributed, the revolution of the
system, physic of MRI, components of MRI machine, distortion factors on imaging process and
safety instructions which need to be followed during scan by authorized personnel. Also we
tried to give some examples about artefacts and slightly touched to SAR effect of MRI on
patients.

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11.CITATIONS
[1] – Türk Manyetik Rezonans Derneği - MR Fizik Kursu El Kitabı
[2] - http://en.wikipedia.org/wiki/Precession
[3] - http://en.wikipedia.org/wiki/Resonance
[4] - http://en.wikipedia.org/wiki/K-space_(MRI)
[5] - http://en.wikipedia.org/wiki/Fluid_attenuated_inversion_recovery
[6] - http://www.imaios.com/en/e-Courses/e-MRI/MRI-Sequences/inversion-recovery-stir-
flair
[7] - http://www.revisemri.com/questions/pulse_sequences/tse
[8] - http://inst.nuc.berkeley.edu/NE107/Lectures/MRI_III_Contrast_NE107_Fall10.pdf
[9] - http://www.imaios.com/en/e-Courses/e-MRI/Image-quality-and-artifacts
[10] - http://en.wikipedia.org/wiki/Free_induction_decay
[11] - http://en.wikipedia.org/wiki/Spin_echo
[12] - http://crab0.astr.nthu.edu.tw/~hchang/ga1/ch02-02.htm
[13] - http://radiopaedia.org/
[14] - http://radiopaedia.org/