MRI

Magnetic Resonance Imaging

Ravi Ranjan

Introduction

• What is MRI?– Magnetic resonance imaging (MRI) is a spectroscopic

imaging technique used in medical settings to produce images of the inside of the human body.

– MRI is based on the principles of nuclear magnetic resonance (NMR), which is a spectroscopic technique used to obtain microscopic chemical and physical data about molecules

– In 1977 the first MRI exam was performed on a human being. It took 5 hours to produce one image.

• How Does it Work?

– The magnetic resonance imaging is accomplished through

the absorption and emission of energy of the radio

frequency (RF) range of the electromagnetic spectrum.

The Components

• A magnet which produces a very powerful uniform magnetic field.

• Gradient Magnets which are much lower in strength.

• Equipment to transmit radio frequency (RF).

• A very powerful computer system, which translates the signals transmitted by the coils.

The Magnet

• The most important component of the MRI scanner is the magnet:– The magnets currently used in scanners today are in

the .5-tesla to 2.0-tesla range (5,000 to 20,000-gauss). Higher values are used for research.

– Earth magnetic field: 0.5-gauss

The Magnet

• There are three types of magnets used in MRI systems: – Resistive magnets – Permanent magnets– Super conducting magnets (the most commonly used

type in MRI scanners).

• In addition to the main magnet, the MRI machine also contains three gradient magnets. These magnets have a much lower magnetic field and are used to create a variable field.

What kinds of nuclei can be used for NMR?

• Nucleus needs to have 2 properties:– Spin

– charge

• Nuclei are made of protons and neutrons– Both have spin ½

– Protons have charge

• Pairs of spins tend to cancel, so only atoms with an odd number of protons or neutrons have spin – Good MR nuclei are 1H, 13C, 19F, 23Na, 31P

The Technology

• How Does It All Work?• Spin:

– The atoms that compose the human body have a property known as spin (a fundamental property of all atoms in nature like mass or charge).

– Spin can be thought of as a small magnetic field and can be given a + or – sign and a mathematical value of multiples of ½.

– Components of an atom such as protons, electrons and

neutrons all have spin.

The Technology (cont.)

• Spin (cont.):– Protons and neutron spins

are known as nuclear spins.

– An unpaired component has a spin of ½ and two particles with opposite spins cancel one another.

– In NMR it is the unpaired nuclear spins that produce a signal in a magnetic field.


• Human body is mainly composed of fat and water, which makes the human body composed of about 63% hydrogen.

• Why Are Protons Important to MRI? – positively charged – spin about a central axis– a moving (spinning) charge creates a

magnetic field.– the straight arrow (vector) indicates the

direction of the magnetic field.


• When placed in a large magnetic field, hydrogen atoms have a strong tendency to align in the direction of the magnetic filed

• Inside the bore of the scanner, the magnetic field runs down the center of the tube in which the patient is placed, so the hydrogen protons will line up in either the direction of the feet or the head.

• The majority will cancel each other, but the net number of protons is sufficient to produce an image.


• Energy Absorption:– The MRI machine applies radio

frequency (RF) pulse that is specific to hydrogen.

– The RF pulses are applied through a coil that is specific to the part of the body being scanned.

The Technology (Cont.)

Resonance (cont.)The gradient magnets are rapidly turned on and off

which alters the main magnetic field.

– The pulse directed to a specific area of the body causes the protons to absorb energy and spin in different direction, which is known as resonance

Frequency (Hz) of energy absorption depends on strength of external

magnetic field.

The Technology (cont.)• Imaging:

– When the RF pulse is turned off the hydrogen protons slowly return to their natural alignment within the magnetic field and release their excess stored energy. This is known as relaxation.

• What happens to the released energy?– Released as heat

OR– Exchanged and absorbed by other protons

OR– Released as Radio Waves.

The Technology (cont.)• Measuring the MR Signal:

– the moving proton vector induces a signal in the RF antenna

– The signal is picked up by a coil and sent to the computer system.

the received signal is sinusoidal in nature

– The computer receives mathematical data, which is converted through the use of a Fourier transform into an image.

Measuring the MR Signalz

y x

RF signal from precessing protonsRF signal from precessing protons

RF antennaRF antenna

MR ImageMR Image

detaildetail

single voxelsingle voxel

fat and water protonsfat and water protons

net magnetizationnet magnetization

The Image

Hydrogen atoms are best for MRI

• Biological tissues are predominantly 12C, 16O, 1H, and 14N

• Hydrogen atom is the only major species that is MR sensitive

• Hydrogen is the most abundant atom in the body• The majority of hydrogen is in water (H2O)

• Essentially all MRI is hydrogen (proton) imaging

Nuclear Magnetic Resonance Visible Nuclei

A Single Proton

++++

++

There is electric charge There is electric charge on the surface of the proton, on the surface of the proton, thus creating a small current thus creating a small current loop and generating magnetic loop and generating magnetic moment moment ..

The proton also has The proton also has mass which generates mass which generates ananangular momentumangular momentumJJ when it is spinning. when it is spinning.

JJ

Thus proton “magnet” differs from the magnetic bar in that itThus proton “magnet” differs from the magnetic bar in that italso possesses angular momentum caused by spinning.also possesses angular momentum caused by spinning.

How do protons interact with a magnetic field?

• Moving (spinning) charged particle generates its own little magnetic field– Such particles will tend to line up with external

magnetic field lines (think of iron filings around a magnet)

• Spinning particles with mass have angular momentum– Angular momentum resists attempts to change

the spin orientation (think of a gyroscope)

MRI

X-Ray, CT

Electromagnetic Radiation Energy

MRI uses a combination of Magnetic and Electromagnetic Fields

• NMR measures the net magnetization of atomic nuclei in the presence of magnetic fields

• Magnetization can be manipulated by changing the magnetic field environment (static, gradient, and RF fields)

• Static magnetic fields don’t change (< 0.1 ppm / hr): The main field is static and (nearly) homogeneous• RF (radio frequency) fields are electromagnetic fields that

oscillate at radio frequencies (tens of millions of times per second)

• Gradient magnetic fields change gradually over space and can change quickly over time (thousands of times per second)

Radio Frequency Fields

• RF electromagnetic fields are used to manipulate the magnetization of specific types of atoms

• This is because some atomic nuclei are sensitive to magnetic fields and their magnetic properties are tuned to particular RF frequencies

• Externally applied RF waves can be transmitted into a subject to perturb those nuclei

• Perturbed nuclei will generate RF signals at the same frequency – these can be detected coming out of the subject

Recording the MR signal

• Need a receive coil tuned to the same RF frequency as the exciter coil.

• Measure “free induction decay” of net magnetization

• Signal oscillates at resonance frequency as net magnetization vector precesses in space

• Signal amplitude decays as net magnetization gradually realigns with the magnetic field

• Signal also decays as precessing spins lose coherence, thus reducing net magnetization

NMR signal decays in time

• T1 relaxation – Flipped nuclei realign with the magnetic field

• T2 relaxation – Flipped nuclei start off all spinning together, but quickly become incoherent (out of phase)

• T2* relaxation – Disturbances in magnetic field (magnetic susceptibility) increase the rate of spin coherence T2 relaxation

• The total NMR signal is a combination of the total number of nuclei (proton density), reduced by the T1, T2, and T2* relaxation components

Different tissues have different relaxation times. These relaxation time differences can be used to generate image contrast.

• T1 - Gray/White matter• T2 - Tissue/CSF• T2* - Susceptibility (functional MRI)

MR ImageMR Image

detaildetail

single voxelsingle voxel

fat and water protonsfat and water protons

net magnetizationnet magnetization

The Image

• Why MRI ?

– Utilizes non ionizing radiation. (unlike x-rays).

– Ability to image in any plane. (unlike CT scans).

– Very low incidents of side effects.– Ability to diagnose, visualize, and

evaluate various illnesses.

The only better way to see the insides of your body is to cut you open!

Application:Head. MRI can look at the brain for tumors, bleeding in the brain, nerve injury, and other problems, such as damage caused by a stroke. MRI can also find problems of the eyes and optic nerves, and the ears and auditory nerves.

Chest. MRI of the chest can look at the heart, the valves, and coronary blood vessels. It can show if the heart or lungs are damaged. MRI of the chest may also be used to look for breast or lung cancer.

Spine. MRI can check the discs and nerves of the spine for conditions such as spinal stenosis, disc bulges, and spinal tumors.

find problems such as tumors, bleeding, injury, blood vessel diseases, or infection.

Application:

•Blood vessels. MRI to look at blood vessels and the flow of blood through them is called magnetic resonance angiography (MRA). It can find problems of the arteries and veins, such as an aneurysm, a blocked blood vessel, or the torn lining of a blood vessel (dissection). •Abdomen and pelvis. find problems in the organs and structures in the belly, such as the liver, gallbladder, pancreas, kidneys & bladder. Also finding tumors, bleeding, infection, and blockage. In women, it can look at the uterus and ovaries. In men, it looks at the prostate.

•Bones and joints. Check for problems of the bones and joints, such asarthritis, problems with the temporomandibular joint, bone marrow problems, bone tumors, cartilage problems, torn ligaments or tendons, or infection. MRI may also be used to tell if a bone is broken when X-ray results are not clear.

THANKS….

MRI

Technology

Transcript of MRI