MRI magnetic resonance imaging1

What is MRI?primarily in medical settings to produce high quality images of the inside of the human body magnetic resonance NMR organs and soft tissues tissues... Orient all the nucl

Magnetic Resonance Imaging (MRI)

Naomi Kim

Outline of Presentation

• Background & Brief Description

• Sample MRI Images

• Understanding Technology (Procession,

Larmor Frequency, Radio Frequency,

Gradient Magnets)

• Advantages & Disadvantages

• Distortions

• Figures of Merit

• Current Research & Future Works

What is MRI?

primarily in medical settings

to produce high quality

images of the inside of the human body

magnetic resonance (NMR)

(organs and soft tissues )

tissues

How Does MRI look like?

• a giant cube

• horizontal tube

running through the

magnet from front to

back “bore of the

magnet.”

• The patient, lying on

his or her back,

slides into the bore

on a special table

MR Image

Sagittal section through a normal human knee

Sagittal section through a

normal human face

MR Image

Sagittal section through

a normal human

What are its components?

MRI system consists of:

• Radio Frequency (RF) Transmitter

• Main Magnet 0.5 to 2.0-tesla or 5,000 to 20,000 gauss

(Resistive, Permanent, or Superconducting)

• RF coils

• 3 Gradient Magnets 18 to 27-millitesla or 180 to 270

gauss

• RF amplifier

• Fourier Transformer

• Computer

How Does it Work? – Brief Outline

1 Orient all the nuclear spins in a patient’s

body in parallel with a strong magnetic field (Main Magnet)

2 Locate the point to be examined (Gradient

Magnets) and flip the spins of the hydrogen atom in the point in the other direction with a strong pulse of radio frequency of exactly the right frequency

3 Collect the electromagnetic signal when the

spins relax to their original state and

transform the signal to produce image.

Spin (Precession)

• Hydrogen Atom –

Magnetic Dipole Moment

(MDM)

• High water content of

non-bony tissues

• A symmetric body with

spin angular momentum

and some torque that is

perpendicular to the

angular momentum

precesses

• All of the hydrogen

protons will align with

the magnetic field in

one direction or the

other

• Vast majority of these

protons will cancel each

other out

• The excess nuclei in the

lower energy state give

a net MDM component

along the field

Larmor (Resonance)

Frequency

• Frequency of procession

• Depends on Magnetic field and Gyromagnetic ratio –

ratio of the MDM to the nuclear spin angular momentum

 L = H/2 where L = Larmor frequency,  =

gyromagnetic ratio, and H = applied magnetic field

• Unique value for each type of nucleus - each type of

nucleus will precess at a unique frequency in a given

magnetic field -> we can distinguish between nuclear types!

• exactly equal to the frequency of radiation absorbed in a

transition from one spin state to another

Radio Frequency Magnetic Field H 1

• We want to displace M (tiny magnet: Hydrogen

atom) from its direction along H and watch M as

it tries to go back to its alignment along H

• Apply a second magnetic field H1 to displace M

• Little dipole magnets realigning themselves

and beginning to precess about the net

magnetic field (vector sum of H1 and H)

• H1 turned on and off quickly (90º pulse) -> get

a small displacement of M

• M precessing about H and gradually realigning

itself along H

Free Induction Decay (FID) & Fourier Transformation

• FID: signal produced by the free

return of M to H direction

• MR signal (the FID) -> amplitude

vs time

• FT of the FID -> signal strength vs

frequency

BUT How do we select a “slice”?

• Selection: apply gradient magnets so that protons in one

slice precess at a unique Larmor frequency, different

from all other protons in the imaging field

• Apply a range of frequencies to the RF coil to observe

slightly different Larmor frequencies in selected slice

NOTE:

A proton precessing at a certain Larmor frequency will

respond to an RF pulse only if the RF pulse frequency is

exactly the same as the Larmor frequency

Local environmental magnetic fields cause the dipoles to

precess at slightly different frequencies (L = H/2)

Basis of NMR - identical nuclei in slightly different magnetic

fields having different Larmor frequency

potentially harmful ionizing radiation, as

do standard x-ray and CT scans

gas, or body waste, which can hinder

other imaging techniques

deliver high quality pictures of the brain's delicate soft tissue structures

• Tremendous amount of noise during a scan

• MRI scans require patients to hold very still

for extended periods of time MRI exams can range in length from 20 minutes to 90 minutes

or more

• Orthopedic hardware (screws, plates, artificial

joints) in the area of a scan can cause severe

artifacts

• High cost

perpendicular to the slice direction

Current Research & Future

Works

• Still in its infancy - in widespread use for less than

20 years (compared with over 100 years for X-rays)

• Very small scanners for imaging specific body

parts are being developed

• Functional brain mapping

through the use of hyperpolarized helium-3 gas

strokes in their earliest stages is ongoing

blood flow

Figures of Merit

• Producing and holding H constant – most

difficult job

• H-field value should be as large as possible

because the signal-to-noise ratio (S:N) of the output information depends on H

• Repetitive pulsing improves the S:N ratio

• Detection Limit? – How large a tumor has to be

to be detected by MRI?

• A point (horizontal points build up slice) is a

each side

America: Academic Press, 2000

Lea & Febiger, 1990.

America: Prentice-Hall, Inc., 2000.

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• <http://www.cis.rit.edu/htbooks/mri/inside.htm >