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Magnetic Resonance Imaging
Shahid Ali Murtza
1437-FET/BSEE/F10
Department of Electronic Engineering (DEE),
Faculty of Engineering and Technology (FET),
International Islamic University Islamabad (IIUI),
Islamabad, Pakistan.
(November 2013)
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Dedicated To My Dear ParentsAnd Teachers!
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ACKNOWLEGMENT
I am very grateful to Allah Almighty who bestowed me health and strength to accomplish this
research. I offer my deepest thankfulness to all those who helped me in this work. Special thanks
to Mr. Shafeeq Ahmed for helping me in making this document proofread.
Shahid Ali Murtza
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ABSTRACT
Magnetic resonance imaging (MRI) is an imaging technique used primarily in medical
settings to produce high quality images of the soft tissues of the human body. It is based on the
principles of nuclear magnetic resonance (NMR), a spectroscopic technique to obtain microscopic
chemical and physical information about molecules. It uses Radio Band for imaging. This
technique places a patient in a powerful magnet and passes radio waves through his or her body in
short pulses. Each pulse causes a responding pulse of radio waves to be emitted by thepatients
tissues. The location from which these signals originate and their strength are determined by a
computer, which produces a 2-D picture of a section of the patient. MRI can produce pictures in
any plane. The principles underlying nuclear magnetic resonance (NMR) are explained, and its
use in image formation is described in this document. MRI Test and merits and demerits of MRI
are also explained.
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TABLE OF CONTENTS
i. List of figures 5
ii. List of Tables 5
0. Summary .. 6
1. What is MRI? 7
2. Application of MRI 8
3. Principles 9
3.1 Magnetic Field Gradient....10
3.2 Frequency Encoding..11
3.3 Back Projection Imaging12
3.4 Slice Selection13
4. MRI Test 14
5. Merit and Demerits ...15
6. Final Word .16i. References.17
ii. List of Abbreviations .17
iii. Appendix ..18
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LIST OF FIGURES
Fig.1: MRI Scanner Cutaway [4]7
Fig.2:A cross-section of human skull with the region of interest marked in red [2].8
Fig.3: Aligning of Proton ....9
Fig.4:Precession and Energy Differential......9
Fig.5: Free Induction Decay and xy-plane11
Fig.6: MRI Scanner Gradients [4] ..13
Fig.7: Slices of Brain[4]
....14
Fig.8: Resistive Magnet. ...19
LIST OF TABLES
Table-1: Some Test References For MRI Test.15
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0. SUMMARY0.1 Magnetic Resonance Imaging:
Magnetic resonance imaging is widely used technique of diagnosis in medical imaging. The
human body contains hydrogen in the form of water and fats. Human have many diseases that can't
be diagnosed with ordinary methods. MRI is the useful technique that helps us in diagnosing many
diseases that are internally related to human structure of veins and bones.
0.2 Principles of MRI and NMR
MRI is same as nuclear magnetic resonance imaging. In industry NMR is used while in
medical the term MRI is used due the connotation of word "nuclear". The principles are explained
in the document are of NMR.
0.3 Applications:
MRI is widely used in industry and medical imaging. Diseases like cancer, brain tumors, bleeding
are using MRI for their early detection. On industrial levels it is also used with high magnetic
powers of order more than 10T.
0.4 MRI Test in Medical:
By using MRI test that is conducted by MRI Scanners, we can see the results of abnormalities and
moralities in tissues of a region of interest in human body. Blocking of blood clots or breakage of
bones are easily captured by this test.
0.5 MRI Merits and Demerits
MRI is safe to use, no radiation exposure, soft tissue structure is easily captured and blood
circulation in organs is found easily. On the other hand, MRI is expensive, a long time slow
process, prone to noises.
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1. WHAT IS MAGNETIC RESONANCE IMAGINGMagnetic resonance imaging (MRI) is a unique and powerful tool for medical diagnosis, in that it
is a noninvasive technique that allows visualization of soft tissues [1]. Magnetic resonance imaging
(MRI) uses a magnetic field and pulses of radio wave energy to capture images of organs and
structures inside the body. In many cases MRI gives different information about structures in the
body than can be seen with anX-ray,ultrasound, orcomputed tomography (CT) scan.
It uses Radio Band for imaging. This technique places a patient in a powerful magnet and
passes radio waves through his or her body in short pulses. Each pulse causes a responding pulse
of radio waves to be emitted by the patients tissues. The location from which these signals
originate and their strength are determined by a computer, which produces a two-dimensional
picture of a section of the patient. MRI can produce pictures in any plane.
Fig.1: MRI Scanner Cutaway [4]
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Fig.2: A cross-section of human skull (A), with the Region of
Interest (ROI) marked in red (B) [2].
2.APPLICATION OF MRIIn medical imaging diagnosis is key point. While X-rays remain useful for looking at bones, MRI
scans are the diagnostic tool of choice for soft tissue organs, ligaments, the circulatory system
and the spinal column and cord. They help physicians identify multiple sclerosis, tumors,
tendonitis, strokes and many other conditions. Whats more, MRI technology is still in its infancy.
Manufacturers are constantly improving scanner designs, and scientists are discovering new
applications, from monitoring wine quality to detecting lies; one MRI study revealed that people
used twice as many regions of the brain to tell lies as they did to tell the truth[].
MRI also may show problems that cannot be seen with other imaging methods. There is
a growing interest in using MRI for diagnosis of many diseases, such as brain tumors, multiple
sclerosis, bleeding, injury, blood vessel diseases, or infection.
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3. PRINCIPLE OF MRIMRI is an imaging modality used to construct pictures of the NMR signal from the hydrogen atoms
in an object. In medical MRI, radiologists are most interested in looking at the NMR signal from
water and fat, the major hydrogen containing components of the human body.
To perform MRI, we first need a strong magnetic field of about 3 Tesla (see appendix). The
type of magnets used for MR imaging usually belongs to one of three types; permanent, resistive,
and superconductive (see appendix). When protons are placed in a constant magnetic field, they
precess at a frequency proportional to the strength of the magnetic field (at typical radio
frequencies).They also align somewhat to generate a bulk magnetization. (see Fig.3 and 4).
Fig.3: Aligning of Proton
Fig.4:Precession and Energy Differential.
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The principle behind all MRI is the resonance equation, which shows that the resonance frequency
0of a spin is proportional to the magnetic field, Bo, it is experiencing.
0 = 0
Where is the gyromagnetic ratio.
For example, assume that a human head contains only three small distinct regions where there is
hydrogen spin density. In reality the entire head would contain signal. When these regions of spin
are experiencing the same general magnetic field strength, there is only one peak in the NMR
spectrum.
3.1 Magnetic Field Gradient
If each of the regions of spin is to experience a unique magnetic field we will be able to image
their positions. A gradient in the magnetic field is our interest. A magnetic field gradient is a
variation in the magnetic field w.r.t. position. The most useful type of gradient in MRI is a 1-D
linear magnetic field gradient. A 1-D magnetic field gradient along the x axis in a magnetic field,
Bo, indicates that the magnetic field is increasing in the x direction. Here the norm of the vectors
show the magnitude of the magnetic field. The symbols for a magnetic field gradient in the x, y,
and z directions are Gx, Gy, and Gz.
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.
Fig.5: MRI Scanner Gradients [4]
3.2 Frequency Encoding
The point in the center of the magnet where (x, y, z) =[0, 0, 0] is called the isocenter of the magnet.
The magnetic field at the isocenter is Boand the resonant frequency is wo. If a 1-D magnetic field
gradient is applied to human hypothetical head with three spin containing regions, the three regions
experience different magnetic fields. The result is an NMR spectrum with more than one signal.
The amplitude of the signal is proportional to the number of spins in a plane perpendicular to the
gradient. This procedure is called frequency encoding and causes the resonance frequency to be
proportional to the position of the spin.
w= (Bo + x Gx) = wo+ x Gx
x = ( w - wo) / ( Gx)
This principle forms the basis behind all magnetic resonance imaging. To demonstrate how an
image might be generated from the NMR spectra, the Backprojection method of imaging is
studied.
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3.3 Back Projection Imaging
Backprojection imaging is one of the first forms of MRI to be demonstrated. Backprojection is an
extension of the frequency encoding procedure just described. In the technique, the object is first
placed in a magnetic field. A 1-D field gradient is applied at several angles, and the NMR
spectrum is recorded for each gradient. For example, say you wished to produce an YZ plane image
of an object. A magnetic field gradient in the +Y direction is applied to the object and an NMR
spectrum is recorded.
A second spectrum is recorded with the gradient now at a one degree angle to the +Y
axis. The process is repeated for the 360obetween 0oand 359o. Once this data has been recorded
the data can be backprojected through space in computer memory.
Once the background intensity is suppressed an image can be seen. The actual
Backprojection scheme is called the inverse Radon transform. In a conventional 90-FID imaging
sequence this procedure might be applied with the aid of the following pulse sequence. Varying
the angle of the gradient is accomplished by the application of linear combinations of two
gradients. Here the Y and X gradients are applied in the following proportions to achieve the
required frequency encoding gradient Gf.
Gy= GfSin
Gx= GfCos
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greater frequencies will be rotated by lesser angles.
The application of this 90opulse with a magnetic field gradient in the x direction will
rotate some of the spins in a plane perpendicular to the x axis by 90 o. The word some was used
because some of the frequencies have a B1less than that required for a 90o rotation. As a
consequence the selected spins do not actually constitute a slice.
A solution to the poor slice profile is to shape the 90opulse in the shape of a sinc pulse. The sinc
pulse has a square frequency distribution.
Fig.7: Slices of Brain
4.
MRI TEST:
An MRI Test is a test obtained using NMR for diagnosis of diseases like tumors, cancer etc.
Theradiologistmay discuss initial results of the MRI with patient right after the test. Complete
results are usually ready for doctor in one to two days. An MRI can sometimes find a problem in
a tissue or organ even when the size and shape of the tissue or organ looks normal.
Table-1:Some Test References For MRI Test.
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Magnetic resonance imaging (MRI)[3]
Normal: The organs, blood vessels, bones, and joints are normal in size, shape, appearance,
and location.
No abnormal growths, such as tumors, are present.
No bleeding, abnormal fluid, blockage in the flow of blood, or bulges in the bloodvessels (aneurysms) are present.
No signs of inflammation or infection are present.
Abnormal: An organ is too large, too small, damaged, or absent.
Abnormal growths (such as tumors) are present.
Abnormal fluid from a cause such as bleeding or an infection is present. Fluid isfound around the lungs or heart. Fluid is found around the liver, bowel, or other
organ in the abdomen.
A blood vessel is narrowed or blocked. An aneurysm is present.
Blockage in the gallbladder bile ducts or in the tubes (ureters) that lead out of the
kidneys is present.
Damage to joints, ligaments, or cartilage is seen. Bones are broken or showinfection or disease.
Problems of the nervous system are present, such as multiple sclerosis(MS), dementia, Alzheimer's disease, or herniated disc.
5. MERITS AND DEMERITS:
5.1 MERITS
The main advantages of magnetic resonance imaging (MRI) scans are:
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a. They do not involve exposure to radiation, so they can be safely used in people who may
be vulnerable to the effects of radiation, such aspregnantwomen and babies
b. They are particularly useful for showing soft tissue structures, such as ligaments and
cartilage, and organs such as the brain, heart and eyes.
c. They can provide information about how the blood moves through certain organs and blood
vessels, allowing problems with blood circulation, such as blockages, to be identified.
5.2 DEMERITS
The main disadvantages of MRI scans are:
a. MRI scanners are very expensive. A single scanner can cost over 1 million. This means
that the number of scanners a primary care trust (PCT) can afford to fund is limited.
Therefore, if your condition is non-urgent, you may have to wait several months to have
an MRI scan.
b. The combination of being put in an enclosed space and the loud noises that are made by
the magnets can make some people feel claustrophobic while they are having a MRI scan.
c. MRI scanners can be affected by movement, making them unsuitable for investigating
problems such as mouth tumors because coughing or swallowing can make the images that
are produced less clear.
6. FINAL WORD:MRI has changed lives of doctors and patients. MRI is a slow process as it has to do more than
any other imaging technique. Now a days, struggles are made to make this process faster and more
convenient for both doctors and patients. In this regard, Parallel MRI and Compressed Sensing of
MRI are hot topics. In these techniques the key objective is to minimize the time of MRI
acquisition and computational process. This definitely will give a sigh to all concerns.
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i. REFERENCES1. Lerski R., Straughan K., Shad L., et al. MR image texture analysis - an approach
to tissue characterization. MRI. 1993; 11:8738872. Texture analysis methodologies for magnetic resonance imaging: Dialogues Clin
Neurosci. 2004 June; 6(2): 243250.3. Table: Magnetic Resonance Imaging (MRI), by Healthwise Staff, (May 16, 2011)4. MRI: A Guided Tour By Kristen Coyne(Magnet Labs, UoF)5. Basic Principles of MRI by Wm. Faulkner, B.S.,R.T.(R)(MR)(CT)
ii. LIST OF ABBREVIATIONS1-D : One Dimensional2-D : Two Dimensional
CT : Computed Tomography
MR : Magnetic ResonanceMRI : Magnetic Resonance Imaging
NMR : Nuclear Magnet Resonance
ROI : Region of Interest
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iii. APPENDIX1-D vs. 2-D:A 1-D magnetic field gradient is a variation with respect to one direction, while
a 2-D gradient is a variation with respect to two.
Computer Tomography: Tomography literally means "slice" or "cross-sectional"imaging. Computed means related to computer calculations. An imaging technique that uses X-
rays to produce 2-,3-D images ..
NMR-nuclear magnetic resonance:In research and industry, MRI is known as
NMR nuclear magnetic resonance. It's more or less the same process, but the medical
establishment prefers the term MRI because some patients are scared off by the word nuclear.
Radio Waves (RF):Radio waves in the relevant frequency range are often simply called
RF (radio frequency).The employed frequencies overlap with the ones used in radio
communication. MR must therefore be carried out in an RF shielded room (also known as an RF
cabin or a Faraday cage). The shielding can be tackled a number of ways. In animal experimental
scanners, the plugs in the ends of the scanner act as boundaries forthe shielded volume.
Tesla:The field strength of the magnets used for MR is measured in units of Tesla. One (1)
Tesla is equal to 10,000 Gauss. The magnetic field of the earth is approximately 0.5 Gauss. Given
that relationship, a 1.0 T magnet has a magnetic field approximately 20,000 times stronger than
that of the earth.
Permanent Magnet:A permanent magnet is sometimes referred to as a vertical field
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magnet. These magnets are constructed of two magnets (one at each pole).The patient lies on a
scanning table between these two plates.
Advantages of these systems are:
1) Relatively low cost,
2) No electricity or cryogenic liquids are needed to maintain the magnetic field,
3) Their more open design may help alleviate some patient anxiety,
4) Nearly non-existant fringe field.
It should be noted that not all vertical field magnets are permanent magnets.
Resistive Magnets:Resistive magnets are constructed from a coil of wire. The more turns
to the coil, and the more current in the coil, the higher the magnetic field. These types of magnets
are most often designed to produce a horizontal field due to their solenoid design
.
Fig.8 Resistive Magnet
Some vertical field systems are based on resistive magnets. The main advantages of these types of
magnets are:
1) No liquid cryogen,
2) The ability to "turn off" the magnetic field,
3) Relatively small fringe field
Superconducting Magnets: Superconducting magnets are the most common. They
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are made from coils of wire (as are resistive magnets) and thus produce a horizontal field. They
use liquid helium to keep the magnet wire at 4 degrees Kelvin where there is no resistance. The
current flows through the wire without having to be connected to an external power source. The
main advantage of superconducting magnets is their ability to attain field strengths of up to 3 Tesla
for clinical imagers, and up to 10 Tesla or more for small bore spectroscopy magnets.
Larmor Equation: 0 = 0
The Larmor Equation tells us that the precessional frequency (0) is equal to the strength of the
external static magnetic field (0) multiplied by the gyromagnetic ratio (). Increasing B0 will
increase the precessional frequency and conversely, decreasing B0 will decrease the precessional
frequency. This is analogous to a spinning top. It will precess due to the force of gravity. If the
gravity were to be decreased (as it is on the moon), then the top would precess slower.
Energy States:Placing many protons in a magnetic field, we can find that some align anti-
parallel and a slight majority aligns parallel. Protons aligned in the parallel orientation are said to
be in a low energy state. Protons in the anti-parallel orientation are said to be in a high-energy state
(see Fig.4).