How Magnetic Resonance Imaging (MRI) Works

Magnetic Resonance Imaging (MRI)

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ABSTRACT

The technique of functional magnetic resonance imaging is rapidly moving from one of technical

interest to wide clinical application. MRI stands for Magnetic Resonance Imaging, a noninvasive diagnostic technique that uses harmless radio waves rather than x-rays to create images. It is particularly useful for imaging of soft tissues such as the brain, spinal cord, muscles and ligaments and detecting abnormal tissues.

An MRI scanner is a cylindrical machine, used to get images of the human body. An MRI machine consists of a round tunnel within where the patient lies on a narrow table. An image of this is seen to the right. Surrounding the tube is a large cylindrical magnet.

In an MRI scanner, the human body is subjected to a very strong magnetic field and it is made to absorb and then radiate energy at the Larmor precession frequency of the hydrogen nuclei present in water. The intensity with which these radiations are emitted by the body are recorded by the MRI scanner which displays an image of the portion of the body under test on its monitor. The portions containing larger amount of water look darker on the screen as compared to those having little water. If different colours are assigned to different intensities, then a colored picture can be obtained. Since water content in the bones and teeth etc. is very small, these parts of the body are nearly invisible and therefore it is very easy to see through the bones in the case of MRI scanners. For this purpose along with a few others, makes MRI scanners very useful for medical science.

Introduction

Magnetic resonance imaging (MRI), nuclear magnetic resonance imaging (NMRI), or magnetic resonance tomography (MRT) is a medical imaging technique used in radiology to visualize detailed internal structures. MRI makes use of the property of nuclear magnetic resonance (NMR) to image nuclei of atoms inside the body.

The property of Nuclear Magnetic Resonance (NMR) was first described by Purcell and Bloch  in 1946, work for which they received the Nobel prize in 1952. Since then NMR has become a

powerful tool in the analysis of chemical composition and structure. In 1973  Lauterbur and

Mansfield used the principles of NMR to describe a technique for determining physical structure.

Since then Magnetic Resonance Imaging (MRI) has been used in many biomedical, chemical and

engineering applications.

An MRI machine uses a powerful magnetic field to align the magnetization of some atoms in the body, and radio frequency fields to systematically alter the alignment of this magnetization. This causes the nuclei to produce a rotating magnetic field detectable by the scanner—and this information is recorded to construct an image of the scanned area of the body. Strong magnetic field gradients cause nuclei at different locations to rotate at different speeds. 3-D spatial information can be obtained by providing gradients in each direction.

During an MRI Scan, the patient is within a stable magnetic field which is 10,000 - 30,000 times stronger than the earth's magnetic field. Protons are tiny particles that are present in water molecules throughout the body. These are aligned by the incredibly strong magnetic field, noting that there are no water molecules in the human skeleton, only in bodily tissue. Radio waves are transmitted in pulses, and these protons produce echoes that are emitted out of the body. These echoes are received by the MRI scanner, and are then reconstructed into images of the body by a computer. These images are very precise and give a clear anatomical view of the body from any angle.

Background

Magnetic Resonance Imaging (MRI) uses the magnetic properties of hydrogen and its interaction with both a large external magnetic field and radio waves to produce highly detailed images of the human body.

Some Basic principles of magnetism-

The magnetic properties of the hydrogen nucleus, and its interaction with the externally applied  magnetic field . In its early days, MRI was known as NMR. This stands for Nuclear Magnetic Resonance. Although the name has changed (primarily due to the negative connotation of the word “nuclear”), the basic principles are the same. . We derive our images from the magnetic resonance properties of nuclear particles (specifically hydrogen).

In order to perform MRI, we first need a strong magnetic field. 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. The type of magnets used for MR imaging usually belongs to one of three types; permanent, resistive, and superconductive .A permanent magnet is sometimes referred to as a vertical field 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 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.

Some vertical field systems are based on resistive magnets. The main advantages of these types of magnets are:                                                                                                                                                    #No liquid cryogen,

# The ability to “turn off” the magnetic field,

# Relatively small fringe field. 

Superconducting magnets are the most common. They 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.  

Magnetic Properties of Matter

Magnetism is a fundamental property of matter. The three types of magnetic properties are: diamagnetic, paramagnetic, and ferromagnetic.

Outside a magnetic field, diamagnetic substances exhibit no magnetic properties. When placed in a magnetic field,diamagnetic substances will exhibit a negative interaction with the external magnetic field. In other words they are not attracted to, but rather slightly repelled by the magnetic field. These substances are said to have a negative  magneticsusceptibility

.

Paramagnetic

Substances also exhibit no magnetic properties outside a magnetic field. When placed in a magnetic field, however, these substances exhibit a slight positive interaction with the external magnetic field and are slightly attracted. The magnetic field is intensified within the sample causing an increase in the local magnetic field. These substances are said to have a positive magnetic  susceptibility

.

Ferromagnetic

Substances are quite different. When placed in a magnetic field they exhibit an extremely strong attraction to the magnetic field. The local magnetic field in the center of the substance is greatly increased. These substances (such as iron) retain magnetic properties when removed from the magnetic field. Objects made of ferromagnetic substances should not be brought into the scan room as they can become projectiles; being pulled at great speed toward the center of the MR imager. An object that has become permanently magnetized is referred to as a permanent magnet. A permanent magnet, such as a bar magnet, has two poles and is referred to as a dipole.

Atomic Structure

The nucleus of an atom consists of two particles; protons and neutrons. The protons have a positive charge and the neutrons have a neutral charge. The atomic number represents the number of protons in the nucleus. The atomic mass number is the total number of protons and neutrons. Orbiting the nucleus are the electrons, which carry a negative charge.

All of these particles are in motion. Both the neutrons and protons spin about their axis. The electrons, in addition to orbiting the nucleus, also spin about their axis. The spinning of the nuclear particles produces angular momentum. If an atom has an even number of both protons and neutrons, then the angular momentum is zero. If an atom has an uneven number of neutrons or protons, then the atom has a certain angular momentum. The angular momentum is expressed as a vector quantity having both magnitude and direction.

In addition to spin angular momentum, certain nuclei exhibit magnetic properties. Because a proton has mass, a positive charge, and spins, it produces a small magnetic field much like a bar magnet. This small magnetic field of the proton is referred to as the magnetic moment. The magnetic moment is also a vector quantity having both magnitude and direction and is oriented in the same direction as the angular momentum. The ratio between the angular momentum and the magnetic moment gives us a constant known as the gyromagnetic ratio, which is specific to each magnetically active nuclei. There are several nuclei, which are magnetically active.Hydrogen has a significant magnetic moment and is nearly 100% abundant in the human body. For these reasons, we use only the hydrogen proton in routine clinical imaging, and that is where we will focus our attention from here on.The nucleus of the hydrogen atom contains a single proton. Because ofthis, as previously mentioned, it possesses a significant magnetic moment. The proton will behave as a tiny bar magnet.

Creating an MR Signal

A radio wave is actually an oscillating electromagnetic field. The RF field is also referred to as the B1 field. It is oriented perpendicular to the main magnetic field. If we apply a pulse of RF energy into the tissue at the Larmor frequency, we first find the individual spins begin to precess in phase, as will the net magnetization vector. As the RF pulse continues, some of the spins in the lower energy state absorb energy from the RF field and make a transition into the higher energy state. This has the effect of “tipping” the net magnetization toward the transverse plane.

For the purpose of this explanation, we will assume sufficient energy isapplied to produce a 90-degree flip of the net magnetization. In such an example, it is said that a 90-degree  flip  angle, or a 90-degree  pulse has been applied

Application

MRIs are a relatively new technology to hit the medical world, and have completely revolutionized medical imaging and the diagnosing process as we know it. In-vivo images can be taken of the human body, meaning that internal images can be seen without making any incisions. Completely non-intrusive procedures are used, which makes MRI's very effective, but somewhat expensive, for doctors to use.

MRIs are administered to patients suffering from the following:

#inflammation or infection in an organ

#degenerative diseases

#strokes

#musculoskeletal disorders

#tumors

#other irregularities that exist in tissue or organs in their body

High-resolution images of organs or any area of the body can be made without the need for using x-rays because MRIs use radio frequency (RF) light. Since they use RF light, MRIs do not present any known health risks to the patients; however anyone with metal implants could not receive a MRI. If a person's nervous system needed to be studied, an MRI image would be the best imaging method to use, especially if the brain or spinal cord needed to be investigated.

Functional MRI's are done to determine which parts of the brain have control over which uses of the human body. These MRIs are critical in determining motor imagery, speech portions of the brain, and diagnosing which parts of the brain may be affected by a tumor. Some operations are deferred because a portion of the brain that is vital may be removed, and this is only determined via functional MRIs.

Benefits of MRI

The MRI scan is a painless and safe scan that produces clearer images of the body and its tissues, at any angle. This is particularly useful in detecting soft tissue tumours throughout the body. An MRI is nearly twice as sensitive as X-ray mammography in detecting breast cancer in women that have a high genetic risk of the disease. It uses no radiation for scanning and therefore eliminates the health risk of x-rays that do use radiation.

Risks of MRI

While an MRI scan is a relatively safe procedure as there is no damaging radiation involved, there are still several risks. If the patient is pregnant, or suspects they may be, a doctor should be informed because the effects on an unborn baby are poorly understood. There is also the risk of patients being injured if they forget to remove pieces of metal from their body or their clothing. There have been cases where patients have been injured due to metal left behind by the previous patient. If sedation is required due to claustrophobia, then there are associated risks of over-medication. If a contrast dye is used, which helps to show up some parts of the body more clearly, there is a small risk of allergic reaction.

Limitations of MRI

An MRI is a very expensive and time consuming investigation compared to other methods such as x-ray and CT scan. Some parts of the body, like bone, are better examined using simpler techniques such as an X-Ray. An MRI may not always be able to tell the difference between some disease processes. It is also not a very good investigation for emergencies or accidents because of the long time it takes and the fact that all equipment has to be removed from the room while the machine is running.

Specialized Types of MRI

Magnetic Resonance Angiography (MRA): This type of MRI is designed to show the blood vessels. It is most often used to examine the arteries and veins of the head, neck, brain and heart. Usually, a contrast dye is injected, which cannot leave the blood vessels meaning that they show up much brighter than the areas around them. An example of this technique is seen to the right. Functional MRI: This type of MRI is done on the brain, and not only shows the structure of the brain, but also how much activity is taking place in each part. This has been used to find out what parts of the brain are most active during certain situations or tasks. Cardiac MRI: This can be used for several different conditions and is dealt with separately.

Conclusion

It is possible that an MRI may show that everything is completely normal; however, there are several things that could be seen on an MRI and this will vary depending on where in the body the scan is being done. An MRI is very good at showing up problems with soft tissues such as muscles and ligaments and is the most sensitive investigation for spinal and joint problems. For this reason, MRI is often used in sports-related injuries, allowing the doctor to see even very small tears or areas of swelling and inflammation. Almost all other organs can be examined in some detail using an MRI, showing problems in structure and function. MRI is most often used to show problems with the soft tissues of the body which can be genetic or caused by some disease process like a tumor. This includes areas like the brain, and an MRI can give a particularly clear image of the brain's structure. This is particularly useful for conditions such as a brain tumor where an MRI is often used to find out exactly where in the brain it is, allowing much more effective surgery.