mri assignment trw

8/13/2019 MRI Assignment TRW

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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).

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