Download Seminar Report

ABSTRACT
Magnetic Resonance Imaging is a medical imaging technique used in radiology to
visualize detailed internal structures. It uses the properties of Nuclear Magnetic Resonance.
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 human body. MRI
provides good contrast between the soft tissues of the body. MRI images can also be
developed in 3D.
In MRI scanning generally we use three types of magnets i.e., permanent, resistive
and superconductive magnets, which are based on the magnetic properties of the
materials.MRI can be used in the applications such as radiation therapy simulation in multi
nuclear imaging, susceptibility weighted imaging, current density imaging and so on.
1
MAGNETIC RESONANCE IMAGING
1. 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.
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.
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. MRI provides good contrast between soft tissues of the
body, which makes it especially useful in imaging the brain, muscles, the heart and cancers
compared with other medical imaging techniques such as Computed Tomography (CT) or Xrays. MRI does not use ionizing radiation as in CT scans.
2
2. HISTORY:
In the 1950s, Herman Carr reported on the creation of a one-dimensional MR
image. Paul Lauterbur expanded on Carr's technique and developed a way to generate the
first MRI images, in 2D and 3D, using gradients. In 1973, Lauterbur published the first
nuclear magnetic resonance image and the first cross-sectional image of a living mouse was
published in January 1974. Nuclear magnetic resonance imaging is a relatively new
technology first developed at the University of Nottingham, England. Peter Mansfield, a
physicist and professor at the university, then developed a mathematical technique that would
allow scans to take seconds rather than hours and produce clearer images than Lauterbur had.
In a 1971 paper in the journal science, Dr. Raymond Damadian, an ArmenianAmerican physician, scientist and professor at the Downstate Medical Center State University
of New York (SUNY), reported that tumors and normal tissues can be distinguished in vivo
by nuclear magnetic resonance (NMR). While researching the analytical properties of
magnetic resonance, Damadian created the world's first magnetic resonance imaging machine
in 1972. He filed the first patent for an MRI machine, U.S. patent #3,789,832 on March 17,
1972, which was later issued to him on February 5, 1974. As the National Science
Foundation notes, "The patent included the idea of using NMR to 'scan' the human body to
locate cancerous tissue.” Damadian along with Larry Minkoff and Michael Goldsmith
subsequently went on to perform the first MRI body scan of a human being on July 3, 1977.
These studies performed on humans were published in 1977.
3
Fig 1. An MRI image
In recording the history of MRI, Mattson and Simon (1996) credit Damadian with
describing the concept of whole-body NMR scanning, as well as discovering the NMR tissue
relaxation differences that made this feasible.
3. TYPES OF MAGNETS USED:
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 20,000 times stronger than that of the
earth.
There are three types of magnets that can be used for MRI are as given below:
1. Permanent magnet,
2. Resistive magnet,
3. Superconductive magnet.
Permanent Magnet: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. They are permanent magnets.
4
Advantages:
1. Relatively low cost
2. No electricity or cryogenic liquids are needed to maintain the magnetic field
3. Nearly nonexistent fringe field
Resistive Magnet: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.
Advantages:
1. No liquid cryogen,
2. The ability to turn off the magnetic field,
3. Relatively small fringe field.
Superconductivity Magnet: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.
Advantages:
1. Their ability to attain field strengths of up to 3 Tesla for clinical imagers, and up to 10
Tesla or more.
2. No need of external power source.
4. MAGNETIC PROPERTIES OF MATERIALS:
Magnetism is a fundamental property of matter. There are three types’ magnetic
properties.
1. Diamagnetic properties
2. Paramagnetic properties
3. Ferromagnetic properties
5
Fig 2. Magnetic Properties
Diamagnetic Properties:
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 magnetic
susceptibility.
Paramagnetic Properties:
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.
6
Ferromagnetic Properties:
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.
5. WORKING OF MRI:
The body is largely composed of water molecules. Each water molecule has two
hydrogen nuclei or protons. When a person is inside the powerful magnetic field of the
scanner, the magnetic moments of some of these molecules become aligned with the direction
of the field. A radio frequency transmitter is briefly turned on, producing a further
varying electromagnetic field. The photons of this field have just the right energy, known as
the resonance frequency, to be absorbed and flip the spin of the aligned protons in the body.
The frequency at which the protons resonate depends on the strength of the applied magnetic
field. After the field is turned off, those protons which absorbed energy revert to the original
lower-energy spin-down state. A hydrogen dipole has two spins, 1 high spin and 1 low. In
low spin both dipole and field are in parallel direction and in high spin case it is antiparallel.
They release the difference in energy as a photon, and the released photons are detected by
the scanner as an electromagnetic signal, similar to radio waves.
7
Fig 3. Structure of MRI
As a result of conservation of energy, the resonant frequency also dictates the
frequency of the released photons. The photons released when the field is removed have
energy — and therefore a frequency — which depends on the energy absorbed while the field
was active. It is this relationship between field-strength and frequency that allows the use of
nuclear magnetic resonance for imaging. An image can be constructed because the protons in
different tissues return to their equilibrium state at different rates, which is a difference that
can be detected.
The 3D position from which photons were released is learned by applying additional
fields during the scan. This is done by passing electric currents through speciallywound solenoids, known as gradient coils. These fields make the magnetic field strength vary
depending on the position within the patient, which in turn makes the frequency of released
photons dependent on their original position in a predictable manner, and the original
locations can be mathematically recovered from the resulting signal by the use of
inverse Fourier transform.
8
Contrast agents may be injected intravenously to enhance the appearance of blood
vessels, tumors or inflammation. Contrast agents may also be directly injected into a joint in
the case of arthrograms, MRI images of joints. Unlike CT, MRI uses no ionizing
radiation and is generally a very safe procedure. Nonetheless the strong magnetic fields and
radio pulses can affect metal implants, including cochlear implants and cardiac pacemakers.
In the case of cochlear implants, the US FDA has approved some implants for MRI
compatibility. In the case of cardiac pacemakers, the results can sometimes be lethal, so
patients with such implants are generally not eligible for MRI.
Since the gradient coils are within the bore of the scanner, there are large forces
between them and the main field coils, producing most of the noise that is heard during
operation. Without efforts to damp this noise, it can approach 130 decibels (dB) with strong
fields.
MRI is used to image every part of the body, and is particularly useful for tissues with
many hydrogen nuclei and little density contrast, such as the brain, muscle, connective
tissue and most tumors.
6. 3D OBJECT RECONSTRUCTION:
The Principle:Because contemporary MRI scanners offer isotropic, or near isotropic, resolution,
display of images does not need to be restricted to the conventional axial images. Instead, it is
possible for a software program to build a volume by 'stacking' the individual slices one on
top of the other. The program may then display the volume in an alternative manner.
3D Rendering Techniques:The 3D rendering techniques used in MRI are given below:
1. Surface rendering
2. Volume rendering
9
Surface Rendering:
A threshold value of grey scale density is chosen by the operator (e.g. a level that
corresponds to fat). A threshold level is set, using edge detection image processing
algorithms. From this, 3-dimensional model can be constructed and displayed on screen.
Multiple models can be constructed from various thresholds, allowing different colors to
represent each anatomical component such as bone, muscle, and cartilage. However, the
interior structure of each element is not visible in this mode of operation.
Volume Rendering:
Surface rendering is limited in that it only displays surfaces that meet a threshold
density, and only displays the surface closest to the imaginary viewer. In volume rendering,
transparency and colors are used to allow a better representation of the volume to be shown in
a single image - e.g. the bones of the pelvis could be displayed as semi-transparent, so that
even at an oblique angle, one part of the image does not conceal another.
Image Segmentation:Where different structures have similar threshold density, it can become impossible to
separate them simply by adjusting volume rendering parameters. The solution is
called segmentation, a manual or automatic procedure that can remove the unwanted
structures from the image.
10
7. APPLICATIONS:
The applications of the MRI are given below:
Basic MRI scans
1.

T1-weighted MRI

T2-weighted MRI

T*2-weighted MRI

Spin density weighted MRI
Specialized MRI scans
2.

Diffusion MRI

Magnetization Transfer MRI

Fluid attenuated inversion recovery (FLAIR)

Magnetic resonance angiography

Functional MRI

Real-time MRI
3.
Interventional MRI
4.
Radiation therapy simulation

5.
Current density imaging

Magnetic resonance guided focused ultrasound

Multinuclear imaging

Susceptibility weighted imaging (SWI)
Portable instruments
11
CONCLUSION
Magnetic Resonance Imaging instead of using ionizing radiation uses the property of
nuclear magnetic resonance of a strong magnetic field. As a result even the soft tissues of the
body can be imaged in a detail, which makes it especially useful in imaging the brain,
muscles, the heart and cancers compared with other medical imaging techniques such as
Computed Tomography (CT) or X-rays.
As the cryogenic liquids are not used, allergies caused by them are reduced and due to
the open structure the anxiety of the patients can be reduced to some extent. As it is also
implemented in portable size it is easy to image the body without any clumsiness.
12