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Magnetic Resonance Imaging (MRI) Back to Course Index

                                     MAGNETIC RESONANCE IMAGING

 

       MRI1

 

 

INTRODUCTIONMRI2

 

Merriam Webster’s Medical desk Dictionary defines Magnetic Resonance Imagine (MRI) as: “A non-invasive diagnostic technique that produces computerized images of internal body tissue and is, based on nuclear magnetic resonance of atoms within the body, induced by the application of radio waves”.  The Wikipedia, the free encyclopedia, further expands the definition of MRI as:  Magnetic resonance Imaging, formerly referred to as magnetic resonance tomography (MRT) or Nuclear Magnetic resonance (NMR), is a method used to visualize the inside of living organisms as well as to detect the composition of geological structures.  Wikipedia goes on to say that it is primarily used to demonstrate pathological or other physiological alterations of living tissues and is a commonly used form of medical imaging.  MRI has also found many novel applications outside of the medical and biological fields such as rock permeability to hydrocarbons and certain non-destructive testing methods such as produce and timber quality characterization.”  A third (and possibly more direct definition related to the medical field) comes form the Phoenix5 website.  They define MRI as: “A procedure in which a magnet linked to a computer is used to create detailed pictures of areas (images) inside the body”.

 

 

MRI QUICK-LOOK

 

An overview of the MRI process is as follows:

 

o       MRI images are produced using magnetism, radio waves and a computer.   The computer generates visual images of the area of the body that was scanned and transfers the information to film.  A radiologist interprets the results and provides information to the attending physician.

o       MRI scanning is safe, non-invasive, painless, and does not involve X-ray radiation.

o       MRI exposes patients to a strong magnetic field. Consequently, it may be unsafe to scan patients with heart pacemakers, metal implants, or other metal objects in or on their body.  Also, it may be unsafe to scan any patient with any metal in their eyes.

o       MRI is performed in an enclosure; accordingly, anyone who is claustrophobic may have a severe psychological reaction to the process.  A sedative medication may be ordered by a physician to moderate this discomfort.

 

The MRI procedure exposes patients to an uncomfortable and noisy environment (patients lie within a closed chamber on a firm table for about ½ hour; scan times of up to 1.5 hours are not uncommon) and requires them to remain still during the scanning.  Typically, the area being scanned is stabilized (held in position) while undergoing the test.  Also, patients may be given a sedative medication to decrease their anxiety and help them relax them during the scan.  The patient can communicate via intercom with an MRI technologist throughout the test.

 

The patient is briefed in advance of what he or she will experience during the test.  Noise is often mentioned as an area of concern as it is often loud and random in nature.  The patent’s anxiety level can be significantly reduced if he or she is informed of this prior to the actual test.    

 

 

HISTORY

 

A brief history of the major events in the development of the MRI technology is shown below. It also identifies the timeframe in which the event occurred, as well as some of the major contributors to the technological breakthroughs.  This data gives one an appreciation of how rapidly the MRI technology and applications have grown in the past 50 years.  Also, the expansion and growth appear to be unlimited as the medical application as well as non-medical applications are continuing to expand on a regular basis:

 

1946                              Magnetic Resonance phenomenon was discovered by Felix Block

and Edward Purcell  (independently);

         

1950-70                  Nuclear Magnetic Resonance (NMR) was developed and used for chemical and physical molecular analysis.  Various contributors;

 

1971                              Nuclear magnetic relaxation times of tissues and tumors differed.  These lead scientists to consider magnetic resonance for the detection of disease.  Raymond Damadian was the primary contributor to the discovery of this phenomenon;

 

1973                Computerized technology (X-ray based) was introduced

                     by Hounsfield.  Also, magnetic resonance imaging on

                     test tube samples was demonstrated.  Paul Lauterbur was

                     the primary contributor;

 

1974                Magnetic resonance imaging using phase and

                     frequency coding and the Fourier Transform was

                     introduced by Richard Ernst.  The current MRI

                     technology is based on this concept. 

 

1977                              Raymond Damadian performed the first MRI image on the human body with assistance from Dr. Larry Minkoff and Dr. Michael Goldsmith.  The trio and others had labored several years to accomplish this milestone.  The image was unsophisticated when compared to today’s standards.   Also, the echo-planar imaging (EPI) was developed by Peter Mansfield;

 

1980                              Imaging of the body was demonstrated using phase and frequency coding and the Fourier Transform by Edelstein.  A single image could be processed in approximately five minutes.  The processing time was later reduced to a few seconds.  Also, the NMR microscope was developed.

 

1987                              Echo-planar imaging was used to perform real-time movie imaging of a single cardiac cycle.  Also, magnetic resonance angiography (MRA) was developed.  This enabled imaging of flowing blood without the use of a contrast agent.

 

1993                              Functional MRI (fMRI) was developed.  This technology allows the mapping of the function of the various regions of the human brain;

 

200X           MRI is a young and growing technology.  Hospitals and

                     other medical institutions have demonstrated willingness

                     to invest large amounts of money on MRI equipment and

                     training.

 

The aforementioned list of events and contributors is not exhaustive.  Other professional’s could consider other events and people more significant than the ones mentioned here.  I will leave this as an exercise for the reader to expand this list to any level desired.  The intent of this list is to inform the reader of how quickly the imaging technology has grown and expanded in multiple dimensions.  Also, the future of this technology is bright and challenging.

 

 

MRI FUNCTION

 

MRI technology is complex and one needs a strong technical background to master the technical aspects of how it works; consequently, this CEU will provide general information related to:

 

o       Conceptual operation of the machine.

o       You will learn how machines actually generate images of a section of the body and/or a selected body part.

o       You will also learn what happens to a patient while undergoing a MRI.

o       Safety issues/concerns.

 

 

 

MRI MACHINEMRI3

 

Most MRI machines look like a large cube.  The overall dimensions are approximately six feet tall, five feet wide and eight feet long.  The machine is constructed with a horizontal tube extending through the magnet from front to back.  The patient slides into the tube via a special table.  MRI machines vary in size and shape with newer models designed for more openness around the sides.  New models of MRI machines are also shrinking due to technology improvements.  The MRI systems consists of the following major components:

 

o       Field magnets

o       Gradient magnets (located inside main magnet; they are turned “on” and “off” very rapidly in a specific manner and alter the main magnetic field on a very local level). A picture or “slice” of a particular area can be generated.

o       Radio Frequency (RF) pulses; specific to hydrogen; cause protons to spin or precess; applied via coils that generally conform to areas being scanned.

o       Skilled technologists

o       Computer

 

 

 

MAGNET TECHNOLOGY

 

One of the most important components of the MRI machine is the field magnet.  Thus, it is necessary to study some of the characteristics of magnets to understand how the MRI machine operates.  A magnet is an object that has a magnetic field.  The field magnets in MRI machines are generally electromagnet.  Electromagnets rely upon electric current flowing through a coil to generate a magnetic field-when the current increases so does the magnetic field.  Conversely, when the current stops the magnetic field ceases.  The following types of field magnets have been used in MRI machines:

 

Electromagnets:  An electromagnet (in its simplest form) is a wire that has been coiled into one or more loops.  When electric current flows through the coil, a magnetic field is generated around the coil.  The strength of the field is influenced by several factors: the number of loops determines the surface area of interaction, the amount of current determines the amount of activity, and the characteristics of the wire determine the impedance (resistance).  The more loops of wire and the greater the current, the stronger the magnetic field will be.  Specifically, the electromagnets for MRI machines are constructed with large coils of wire that are wrapped around a cylinder or bore.  An electric current passes through the windings and generate a magnetic field. If the current is turned off the magnetic field collapses. This magnet requires a large amount of electricity to operate because of the natural resistance in the wire. This type of magnet is limited to approximately 0.3-tesla level due to cost.

 

Permanent magnets: Permanent magnets do not rely upon outside influences to generate their field.  The “field” occurs naturally in some rocks, but can also be manufactured.   Most materials we encounter have no obvious magnetic properties – they are said to be non-magnetic.  In these materials, the magnetic fields of the individual atoms are randomly aligned and thus tend to cancel out; consequently, the material is magnetically neutral.  However, in a permanent magnetic the magnetic fields of the individual atoms are aligned in one preferred direction, giving rise to a net magnetic field.  In a permanent magnet the magnetic field is always present and always at full strength.  A major obstacle to the use of permanent magnets in MRI machines is their weight.  For example, they may weigh several tons for a 0.5-tesla magnetic field level; as a result, they are not practicable for the modern 2.0-tesla MRI machines.

 

Super conducting magnets:  This is a special design of the electromagnet in that the coil wires are continuously bathed in helium at approximately 452 degrees below zero.  This environment causes the wire resistance to decrease to near zero; consequently, the power requirements for the system are dramatically reduced that in turn significantly reduces the systems operating cost.  The patient is protected from the helium via thermal insulation.  As a matter of fact, the patient would be unaware of this feature unless he was briefed during orientation.

 

Magnets in the MRI machine are rated using a measure of flux density known as tesla. The Wikipedia defines tesla as: “tesla is the SI (International system of units) derived unit of magnetic flux density (or magnetic induction).   It is used to define the intensity (density) of a magnetic field.  The tesla, equal to one weber per square meter, was defined in 1960”.  As such, an MRI magnet is rated by the amount of magnetic flux or tesla it generates.  The magnets in modern MRI machines are in the 0.5 to 2.0 tesla range.  A magnetic field greater than 2 tesla has not been approved for use in medical imaging; however, more powerful magnets (over 50 tesla) are used in research.  For comparison purposes, the Earth’s magnetic field is approximately 0.5 gauss (1 tesla equals 10,000 gauss); These magnets are many times more powerful than the Earth’s magnetic field.

 

The amount of attraction a magnet has is directly related to the distance the object is from the magnet. Specifically, the force increases exponentially as the object transcends through the field and approaches the magnet.   Consider the consequences should a person carry a large metal object into the scan area.  At 15 to 20 feet, there would be a slight pull on the object, take a few steps closer to the magnet and the force increases significantly.  When the individual gets to within two or three feet of the magnet the force would be strong enough to remove the object from the person’s grasp and accelerate it toward the magnet.

 

As stated previously, magnets are attracted to, or repelled by, other materials.  A material that is strongly attracted to a magnet is said to have a high permeability.  Iron and steel are examples of materials with very high permeability, and they are strongly attracted to magnets.  Whereas, water has a very low permeability and is actually slightly repelled by magnetic fields.  Everything has a measurable permeability: people, gases and even the vacuum of outer space.  Magnets are constructed in different ways and utilize different technologies. 

 

A comparison of magnetic field strength helps us understand the relative strength of one magnet versus another and also the natural magnetic field strength of planet earth.  It is also helpful to relate the effects when one is in the scan area. To begin with, it can be a very dangerous place if proper precautions are not taken. For example, the law that states, “opposites attract” is applicable in this situation. Consequently, any metal object in the scan area can be drawn toward the magnet and become a dangerous projectile.  The projectiles may cause a threat to the patient, staff or could damage the MRI machine. The scan area must be protected from the following and other similar items:

 

o       Stethoscopes

o       Hemostats

o       Scissors

o       Pens

o       Paperclips

o       Keys

o       IV poles

o       Vacuum cleaners

o       Oxygen tanks

o       Patent stretchers

o       Heart monitors

o       Any other metal objects

 

This is not an exhaustive list but should be sufficient to alerts MRI technologist of the risks of inadvertently permitting metal objects in the scan area.  Also, this list does not address the risk of metal objects in or on a patient.  This list will be presented in a later section.

 

 

 

HOW THE MRI WORKS

 

The human body consists of billions of atoms, the fundamental building blocks for all matter.  An atom is the smallest possible particle of a chemical element that retains its chemical properties.  Whereas the word atom originally denoted a particle that cannot be cut into smaller particles, the atoms of modern parlance are composed of subatomic particles:

 

o       Electrons, which have a negative charge and are the least massive of the three;

o       Protons, which have a positive charge and are about 1836 times more massive than electrons; and

o       Neutrons, which have no charge and are slightly larger than protons.

 

Protons and neutrons make up a dense, massive atomic nucleus, and are collectively called nucleons.  The electrons orbit around the nucleus and form a large electron cloud surrounding the nucleus (Wikipedia).  The body consists of billions of atoms and the nucleus of each atom spins, or precesses on an axis.  There are many different types of atoms in the body but for MRI purposes we are only concerned with the hydrogen atom.  It consists of one electron, one proton and a nucleus and has a large magnetic moment.   The large magnetic moment enables the hydrogen atom to easily line up with the direction of the magnetic field.

 

When a patient is placed inside the bore of the MRI machine, the magnetic field runs through the center of the tube in which the patient is placed; therefore, if the patient is lying on their back, the hydrogen protons will line up in the direction of either the feet or the head.  Also, the vast majority of these protons will cancel each other out—that is for each one lined toward the feet, one toward the head will cancel it out.  Only a couple of protons out of every million are not cancelled out.  This doesn’t sound like much, but the sheer number of hydrogen atoms in the body gives us what we need to create wonderful images (howstuffworks.com).     

 

 

 

RF TECHNOLOGY

 

Radio Frequencies (RF) pulses are applied to hydrogen atoms in the area to be examined.  The RF pulses cause the protons in that area to absorb energy sufficient to make them spin or precess in a different direction (particular frequency and direction).  As one might guess, this is the “resonance” part of MRI.  The specific frequency of resonance is called the Larmour frequency and is different depending on what particular tissue is to be imaged and the strength of the magnetic field.  MRI machines generally come with different coils designed for different parts of the body: knees, head, neck, wrists and so on.  These coils generally fit the contour of the body part being scanned.

 

The final element of the total MRI magnetic package is the gradient magnets.  They are positioned inside the main magnet and when they are turned “on” and “off” rapidly in a specific manner, they alter the main magnetic field on a very local level.  This enables a precise area to be chosen for examination.  This is typically referred to as a slice.  Current MRI technology allows us to slice any part of the body in any direction, giving it a huge advantage over any other imaging modality.  This also means that the machine does not have to be moved to get an image from a different direction- it can manipulate everything using the gradient magnets.

 

The final part of the process relates to what happens when the RF pulses are turned off.  The protons slowly return to their natural alignment within the magnetic field and release the excess stored energy (the energy they absorbed when they started to spin).  During the “return to normal” time, the hydrogen protons give off a signal that the coil picks up and sends to the computer system.  This data is converted (through the Fourier transform) into a picture that can be put on film and analyzed by the Radiologist.  Again, as one might guess, this is the “imaging” part of MRI.   

 

 

 

CONTRAST

 

MRI contrast agent is designed to alter the local magnetic field in the area being examined.  Normal and abnormal tissue responds differently to this slight alternation, providing differing signals that enable the radiologist to visualize many different types of tissue abnormalities better than could be seen without the contrast.

 

 

 

MRI APPLICATIONS

 

As noted previously, the MRI uses radio frequency waves and a strong magnetic field to provide clear and detailed images of internal organs and tissues.  It is a valuable and reliable modality for diagnosing a broad range of medical conditions in all parts of the body.  It has been stated that the only way to see inside your body better is to operate; consequently it is ideal for:

 

o       Diagnosing/evaluations of tumors, injuries (trauma), and infections in the brain; diagnosing/evaluating stroke damage; evaluation of chronic disorders of the Central Nervous System; 

o       Visualizing shoulder, wrist, knee and ankle injuries (including ligament damage);

o       Diagnosing tendonitis and related illnesses;

o       Evaluating masses of “soft” tissue; evaluating bone tumors, cysts and bulging or herniated discs in the spine;

o       Diagnosing Multiple Sclerosis (MS)

o       Diagnosis/evaluation of blood disorders;

o       Hundreds of other applications that are discovered on a daily basis.

 

 Let’s look a little closer at how MRI’s are actually performed and some of the unique features associated with the procedure.  We will use a head MRI as our example but we must recognize that all MRI procedures are similar.  

 

 

HEAD MRI   

 

MRI is the most reliable, non-invasive diagnostic tool for the head.  As noted previously, it is used to examine brain tumors, brain trauma, assessment of stroke damage; certain chronic disorders of the Central Nervous System and others illnesses.  It is also important for documenting and assessing brain abnormalities in patients with dementia, as well as for diagnosis of diseases related to eyes and the inner ear.

 

Patients undergo a pre-test interview (for this procedure and all other MRI procedures) where they are asked to identify any metal objects in or on their body.  For example, the patient will be asked if they have a pacemaker, implanted defibrillator, artificial heart valve, implanted port, infusion catheter, prosthetic hip, intrauterine device (IUD) or any other metal plate, pins, screws or surgical staples.  If there is any question regarding metal fragments in the body the patient may be asked to undergo an X-ray to determine their risk prior to the MRI.

 

An MRI can easily damage an eye if there are any metal objects in the eye; thus, the technologist will ask the patient if he or she has any metal objects in his or her eyes.  If so, the situation should be evaluated prior to undergoing an MRI.

 

The technologist may also ask if the patent is pregnant (MRI is generally not recommended when pregnant, although no specific risk has been identified).  The patient will also be asked about drug allergies and claustrophobia (abnormal dread of being in closed or narrow spaces).  If a patient is claustrophobic a physician may order a mild sedative.

 

 

How the procedure is performed: 

 

The patient is placed on a sliding table (typically on his or her back) and a coil (generates RF pulses) is positioned on the head.  The patient is then placed in the MRI machine.  An intercom is available if the patient or technologists need to communicate.  The exam will generally take about ½ hour; however, the procedure may take longer depending upon the unique characteristics required.  The patient will be asked to remain still through the imaging process (some movement is allowed between sequences).  Some patients will require an injection of a contrast agent to enhance the visibility of certain tissue or blood vessels.  The injection is made via an intravenous line in the patient’s arm or hand vein.  The injection of the contrast agent is made after the initial set of images so the radiologist has a “with and without contrast” under a similar set of conditions.

 

The MRI process is not painful, however some discomfort may be experienced due to claustrophobia, remaining still and lying on the hard table.  The other item often mentioned as troublesome is the noise and vibration that is common during the MRI procedure.  Some patients are given earplugs to soften the noise.

 

A Radiologist will analyze the MRI images and report his findings to the primary care physician.  The results are generally available within days of the MRI.

 

Benefits

         

o       Images are non-invasive, more detailed and clearer than for any other modality.

o        No exposure to radiation.

o       Milder contrast agent (less likely to produce allergic reaction than conventional agents used for x-ray and CT scanning.

o       Enables detection of abnormalities that might be missed by other imaging methods.

 

 

Risks/Limitations

 

o       Other modalities are better under specific conditions (conventional X-ray provides better clarity for bone; CT is better with patients with severe bleeding due to trauma to the head or other similar head injuries; others who are unable to undergo the MRI procedure).

o       MRI is more expensive than other imaging modalities.

o       Generally not recommended during pregnancy.

 

 

 

RISKS OF MRI SCAN

 

As noted previously, an MRI scan is painless and there are no known “side affects or after affects” (either psychologically or physiology) due to the procedure.  The major benefits include precise accuracy in detecting structural abnormalities of the body, reliability of diagnosis and prognoses, and enhanced treatment modalities.  A major risk is undergoing a scan with unidentified metal in or on the body.  For example, metallic clips, surgical clips or other metal can seriously injury the patient or distort the image.  Most MRI facilities will not scan patients with a pacemaker, metal implants or metal chips or clips in or around the eyeball.  The risk that the metal will be moved by the magnetic field of the MRI is too great.  The “no scan” label also fits for patient’s artificial heart valves, metallic ear implants, bullet fragments and chemotherapy or insulin pumps.  In most cases surgical staples, plates, pins and screws pose no risk during MRI if they have been in place for more than four to six weeks. Tattoos and permanent eyeliner may also create a problem.

 

The superconducting magnets are always on so everyone entering the scan room should be aware or made aware of the dangers of the scan and equipment.

 

 

PRE-MRI QUESTIONNAIRE

 

A Patient Relations Representative will contact the patient and conduct a safety screening prior to the exam.  The patient is encouraged to communicate freely and openly and ask questions if not absolutely clear on all issues.  Typical question asked are:

 

o       Cardiac pacemaker or a metallic artificial heart valve

o       Aneurysm clips

o       Inner ear implant

o       Intrauterine device (IUD)

o       Metal implant, pin or any metallic implant or any foreign metal in the eye

o       Surgical staples, shrapnel or bullet wounds

o       Permanent eyeliner or body piercing

o       Pregnancies

o       Neurostimulator TENS Unit

o       Insulin pump or infusion pump

o       Intracranial or intraorbital implants

o       Joint replacement

o       Fractured bone treated with metal rods, plates, pins, screws or nails

o       Harrington rod

o       Prosthetic, penal implant

o       Dental implants with magnetic posts

o       Medicated patch

o       Pessary (female patients for lumbar spine, pelvic or hip scans)

 

 

 

 

SUMMARY

 

The MRI has proven very valuable for the diagnosis of a broad range of pathologic conditions in all parts of the body The technique provides an unparalleled view inside the human body. The level of detail we can see is extraordinary compared with any other medical device. An MRI is the method of choice for the diagnosis of many types of injuries and conditions because of the incredible ability to modify the exam to the particular medical question being asked. This ever improving technique has catapulted us forward in the category of science technology.