MAGNETICRESONANCEIMAGINING

The machine

• The strength of the magnet varies from 0.5 – 4 tesla

• Average cost between 1-2.5 million

Timeline of MR Imaging

1920 1930 1940 1950 1960 1970 1980 1990 2000

1924 - Pauli suggests that nuclear particles

may have angular momentum (spin).

1937 – Rabi measures magnetic moment of

nucleus. Coins “magnetic resonance”.

1946 – Purcell shows that matter absorbs energy at a resonant

frequency.

1946 – Bloch demonstrates that nuclear precession can be

measured in detector coils.

1972 – Damadian patents idea for large

NMR scanner to detect malignant

tissue.

1959 – Singer measures blood flow

using NMR (in mice).

1973 – Lauterbur publishes method for

generating images using NMR gradients.

1973 – Mansfield independently

publishes gradient approach to MR.

1975 – Ernst develops 2D-Fourier transform for MR.

NMR renamed MRI

MRI scanners become clinically

prevalent.

1990 – Ogawa and colleagues create functional images using endogenous, blood-oxygenation

contrast.

1985 – Insurance reimbursements for MRI exams begin.

MRI is a very close relative of NMR, which allows clinicians to obtain chemical and physical information about certain molecules. In the 1970’s the name was changed from NMRI to MRI due to the negative connotations associated with the word “nuclear”. Many patients thought that the exam would expose them to radiation.

In 1952 Felix Bloch and Edward Purcell were awarded the Nobel Prize when they discovered the concepts surrounding NMR/MRI.

During the time between 1950-1970, the idea was used for chemical and physical analysis of molecules.

• In 1971, Raymond Damadian discovered that NMR could be used in the detection of diseases.

• In 1974, Damadian received a patent for the design of his MRI machine.

• In 1977, Damadian did his first scan on a human, his assistant, Larry Minkoff. He couldn’t go in himself due to his enormous size.

Damadian’s first prototype was called “Indomitable”, due to criticism and the seven years that it took to complete.

In 1978, Damadian established a new corporation called FONAR, which introduced the first commercial MRI scanner in 1980.

MRI machines look like a large block with a tube running through the middle of the machine, called the bore of the magnet.

The bore is where the patient is located for the duration of the scan.

The MRI machine picks points in the patients body, decideswhat type of tissue the pointsdefine, then compiles the points into 2 dimensional and 3 dimensional images.

Once the 3 dimensional image is created, the MRImachine creates a model of the tissue. This allows the clinician to diagnose without the use of invasive surgery.

The largest and most important components of the MRI machine are the magnets.The magnet strength is measured in units of Tesla or Gauss (1 Tesla = 10,000 Gauss).Today’s MRI machines have magnets with strengths from 5000 to 20,000 Gauss.To give perspective on the strength of these magnets, the earth’s magnetic field is about .5 Gauss, making the MRI machine 10,000 to 30,000 times stronger.

Background Information

• Our bodies are made up of roughly 63% water

• MRI machines use hydrogen atoms• The hydrogen atoms act like little

magnets, which have a north and south pole

• The atoms inside our body are aligned in all different directions

• An MRI consists of:– a primary magnet: creates the magnetic field

by coiling electrical wire and running a current through the wire

– gradient magnets: allow for the magnetic field to be altered precisely and allow image slices of the body to be created.

– a coil: emits the radiofrequency pulse allowing for the alignment of the protons.

• The signals are then ran through a computer and go through a Fourier equation to produce an image.

• Tissues can be distinguished from each other based on their densities.

A schematic representation of the major systems on a magnetic

resonance imager

Different types of MRI• Interventional MRI

– Used to guide in some noninvasive procedures

• Real Time MRI– Continuous filming/ monitoring of objects in real

time• Functional MRI

– Measures signal changes in the brain due to changing neural activity

MRI and Radio Frequencies• The RF coil produces a radio frequency

simultaneously to the magnetic field• This radio frequency vibrates at the perfect

frequency (resonance frequency) which helps align the atoms in the same direction

• the radio frequency coil sent out a signal that resonates with the protons. The radio waves are then shut off. The protons continue to vibrate sending signals back to the radio frequency coils that receive these signals.

MAGNETIC RESONANCE IMAGING

Magnet Gradient Coil RF Coil

RF Coil

4T magnet

gradient coil(inside)

B0

Magnetic Resonance Imaging

• Magnetic nuclei are abundant in the human body (H,C,Na,P,K) and spin randomly

• Since most of the body is H2O, the Hydrogen nucleus is especially prevalent

• Patient is placed in a static magnetic field

• Magnetized protons (spinning H nuclei) in the patient align in this field like compass needles

• Radio frequency (RF) pulses then bombard the magnitized nuclei causing them to flip around

•The nuclei absorb the RF energy and enter an excited state

• When the magnet is turned off, excited nuclei return to normal state & give off RF energy

•The energy given off reflect the number of protons in a “slice” of tissue

• Different tissues absorb & give off different amounts of RF energy (different resonances)

• The RF energy given off is picked up by the receiver coil & transformed into images

• MRI offers the greatest “contrast” in tissue imaging technology (knee, ankle diagnosis)

• cost: about $1450 - $2000

• time: 30 minutes - 2 hours, depending on the type of study being done

Open MRIClosed (traditional) MRI scanner

MRI machines have come a long way since Indomitable. Previously, it took up to five hoursto get an image, whereas today, it takes minutes.In 1992, functional magnetic resonance imaging(fMRI) was discovered, which allowed clinicians tosee various regions of the brain, their functions, and their specific locations.

Slide shows which area of the brain is responsible for touch.

X- Rays• X-rays are generated from the interaction of accelerated e-’s & a target metal

(tungsten)

• Patient is placed between X-ray tube and silver halide film

• X-rays passed through the body are absorbed in direct proportion to tissue density

• X-rays penetrating the body strike the silver halide film and turn it dark

•The more x-rays that penetrate, the darker the area inscribed on the film

• Bones & metal absorb or reflect X-rays r inscribed film is “lighter” or “more white”

• Soft tissues allow more X-rays to penetrate r inscribed film is “darker”

• Visualizing tissues of similar density can be enhanced using “contrast agents”

•Contrast agents: dense fluids containing elements of high atomic number (barium, iodine)

•Contrast agents absorbs more photons than the surrounding tissue r cavity appears lighter

•These contrast agents can be injected, swallowed, or given by enema

electron beam generator

tungsten target metal

resultant X-ray beam

silver halide film

Magnetic Resonance TechniquesNuclear Spin Phenomenon:• NMR (Nuclear Magnetic Resonance)• MRI (Magnetic Resonance Imaging) • EPI (Echo-Planar Imaging)• fMRI (Functional MRI)• MRS (Magnetic Resonance Spectroscopy)• MRSI (MR Spectroscopic Imaging)

Electron Spin Phenomenon (not covered in this course):• ESR (Electron Spin Resonance)

or EPR (Electron Paramagnetic Resonance)• ELDOR (Electron-electron double resonance)• ENDOR (Electron-nuclear double resonance)

There are three types of magnets:

1.Resistive Magnets

2.Permanent Magnets

3.Superconducting Magnets

The resistive magnet has many coils of wire that wrap around the bore, through which electrical currents are passed, creating a magnetic field. This particular magnet requires a large amount of electricity to run, but are quite cheap to produce.

The permanent magnet is one that delivers a magnetic field, which is always on at full strength and therefore, does not require electricity. The cost to run the machine is low due to the constant magnetic force. However, the major drawback of these magnets is the weight in relation to the magnetic field they produce.

The superconducting magnets are very similar to the design of the resistive magnets, in that they too have coils through which electricity is passed creating a magnetic field. However, the major difference between the resistive magnet and the superconducting magnet is the fact that the coils are constantly bathed in liquid helium at -452.4ºC. This cold temperature causes the resistance of the wire to be near zero, therefore reducing the electrical requirement of the system. All of these factors allow for the machine to remain a manageable size, have the ability to create high quality images, and still operate at a reasonable cost.

The superconducting magnet is the most commonly used in machines today, giving

the highest quality images of all three magnet types.

There is another type of magnet that is found in all MRI machines, called gradient magnets

These magnets are responsible for altering the magnetic field in the area to be scanned and can magnetically “slice” the tissue to be examined from every angle.

MRI’s of the heart can be done to look at many different areas including: vessels, chambers, and valves.

The MRI can detect problems associated with different heart diseases including plaque build up and other blockages in blood vessels due to coronary artery disease or heart attacks.

MRI’s of the brain can evaluate how the brain is working, whether normal or abnormal.

Brain MRI’s can show damage resulting from different problems such as: damage due to stroke, abnormalities associated with dementia and/or Alzheimer’s, seizures, and tumors.fMRI are done prior to brain surgery, to give a map of the brain, and help plan the procedure.

MRI’s can be done on the knee to evaluate damage to the meniscus, ligaments, and tendons.

Tears in the ligaments are given a grade 1-3 depending on their severity: 1-fluid around the ligament2-fluid around the ligament with partial disruption of the ligament fibers3-complete disruption of the ligament fibers

Often prior to a MRI scan, a patient would need to have a contrast dye, either injected or taken orally, usually gadolinium as seen here.

The Procedure…

Once the contrast dye has been injected, the patient enters the bore of the MRI machine on their back lying on a special table.

The patient will enter the machine head first or feet first, depending on the area to be scanned.

Once the target is centered, the scan can begin.

•The scan can last anywhere from 20-30 minutes.

•The patient has a coil that is placed in the target area, to be scanned.

•A radio frequency is passed through the coils that excites the hydrogen protons in the target area.

•The gradient magnets are then activated in the main magnet and alter the magnetic field in the area that is being scanned.

The patient must hold completely still in order to get a high quality image.

(This is hard for patients with claustrophobia, and often times a

sedative will be given, if appropriate.)The radio frequency is then turned-off and the hydrogen protons slowly begin

to return to their natural state.

The magnetic field runs down the center of the patient, causing the slowing hydrogen protons to line-up.

The protons either align themselves pointed towards the head or the feet of the patient, and most cancel each other out.

The protons that are not cancelled create a signal and are the ones responsible for the image.

The contrast dye is what makes the target area stand out and show any irregularities that are present.

The dye blocks the X-Ray photons from reaching the film, showing different densities in the tissue.

The tissue is classified as normal or abnormal based on its response to the magnetic field.

The tissues with the help of the magnetic field send a signal to the computer.

The different signals are sent and modified into images that the clinician can evaluate, and label as normal or abnormal.

If the tissue is considered abnormal, the clinician can often detect the abnormality, and monitor progress and treatment of the abnormality.

Uses for an MRI

Used to image a large variety of tissues and substances.

• Brain imaging: to define anatomy, identify bleeding, swelling, tumors, or the presence of a stroke

• To locate glands, organs, joint structures, muscles and bone

Some diseases manifest themselves in having an increase in water content

The MRI can detect inflammation (tumors) in many tissues

Helpful in diagnosing problems with eyes, ears, heart, circulatory system, lungs, pelvis, spinal cord, etc.

MRI Results

• Creates a 2D and sometimes 3D image that comes from the information of the radio waves of the protons

• Example heart scan

• https://www.youtube.com/watch?v=G4dFVeP9Vdo

MRI treatment is a wonderful option for most patients, but there are some people who are

not candidates.

Those include:1) Patients with pacemakers cannot have the

scan done as the magnet from the MRI

interferes with the signal sent from the pacemaker, and deactivates it.2) Patients who are too tall, or too obese3) Patients who have orthopedic hardware

can get distortion in the image, and the scan

quality is not as high.

THE FUTURE OF MRI:

•The possibility of having very small machines that scan specific parts of the body.• The continuing improvements on seeing the venous and arterial systems.• Brain mapping while the patient does specific tasks, allowing clinician’s to see what part of the brain is responsible for that task/activity.• Improvements on the ability to do MRI’s of the lungs.• ETC.

THANK YOU

Garima Kotnala

Mtech (NST)-1st Sem Enroll.

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