RT 200 Finals Topic 4

by **Joshua** Mon Oct 12, 2009 5:28 pm

Topic 4: Magnetic Resonance Imaging (MRI)

Magnetic Resonance Imaging (MRI)

History:
1940’s - Felix Bloch and Edward Purcell investigate how nuclei (nuclei of atoms and nuclei of cells) of materials behave in magnetic field. They discovered that these nuclei will absorb energy from radio waves at certain frequency. Bloch and Purcell receives Nobel prize 1952.

1970’s – Raymond Damandian published the first cross-sectional image of a living mouse in

1971-73. The first NMR image was published by Paul Lauterbur in 1973.
The first studies performed on humans were published in 1977.

The 2003 Nobel Prize in Physiology or Medicine was were awarded to Paul Lauterbur and Sir Peter Mansfield for the mathematical formalism and developing
techniques for fast imaging.

1980’s – The year Magnetic Resonance Imaging, an imaging modality based on nuclear magnetic resonance (NMR) spectroscopy.

MRI:
Magnetic Resonance Imaging (MRI), or nuclear magnetic resonance imaging (NMRI),is primarily a medical imaging technique most commonly used in radiology to visualize the internal structure and function (physiologic) of the
body.

The machine provides cross-sectional, or three dimensional, images without using x-rays or radioactive material. It produces images by the use of a strong magnetic field and radio waves.

It was employed in chemistry and physics to obtain information about complex molecules and molecular motion.

Molecule – structure form of atoms of various elements.

The composition of human body is the ultimate basis in atoms (nuclei and protons).

The Person who study cell:

1665 - Robert Hooke – Study cell and name as the basic biologic building
block.
1673 - Anton Van Leeuwenhoek – Living cell based on microscopic observation.
1938 - Schneider & Schwann – study on plants and animals cells.
1953 - Watson & Crick – Living cell “DNA” deoxyribonucleic s acid

Molecular Composition of the Human Body:

5 principal types

Water – 80% (hydrogen atoms)
Protein – 15%
Lipids (fats) – 2%
Carbohydrates (sugar) – 1%
Nucleic Acid – 1%
Other – 1%

Four (4) of these molecules (protein, lipids, carbohydrates, nucleic acid) are macromolecules (Very large molecules consist of thousand of atoms).
Scanner Construction and Its Operation:

Schematic of construction of a cylindrical superconducting MR scanner:

The 4 major components of an MRI scanner are (the whole system is
controlled by one or more computers):

The Main Magnet
The Shim Coils
The Gradient System
The RF System

The Magnet

The largest and most expensive component of the scanner
The strength of the magnet is measured in tesla (T).
It polarizes(setting direction) the sample

The Shim Coils

The principal requirement for magnet imaging.
Main function: To restore or correct field homogeneity (nuclei placements
- same kind are form in the same kind as others).
Sub-function: Localize the MR signal

The Gradient coils (system)

Sub-function: It makes nuclei line-up
Main functions:
- Electric coils to obtain spatially encode the positions of protons or the information.
- Localize the MR signal

The Radio Frequency (RF) transmission system (probe

It consists of an RF synthesizer, power amplifier and transmitting coil.
It excites sample (transmits pulse) and detect the signal (makes detectable by scanner).

MRI
Fundamental Concepts:

RF system is turned on makes the protons alter their alignment to the
field.
RF system is turned off return to their alignment but these alignment
changes the position of protons which create signal.
The signal depends on the strength of magnetic field.
The position of protons can be determined by applying additional magnetic fields created by turning gradient coils on and off (knocking sound during scan).
These signal which detect by the scanner.

Why MRI?
Best-low contrast resolution
No ionizing radiation
Direct Multiplanar Imaging
No bone or air artifacts – It produce image by bone in clear, unobstructed detail. (Brain, Spinal Cord – nerve damage)
Totally non-invasive

Between MRI and CT Scan:

MRI

It uses non-ionizing radio frequency (RF) signals to acquire its images.
It provides greater contrast between the different soft tissues (brain, musculoskeletal, cardiovascular and oncological [cancer]) of the body.
Contrast agents for MRI are those which have paramagnetic properties.

CT

It uses X-rays, a type of ionizing radiation, to acquire its images composed of elements of a higher atomic number).
CT may be enhanced by use of contrast agents containing elements of a higher atomic number

Both CT and MRI scanners can generate multiple two-dimensional cross-sections (slices) of tissue and three-dimensional reconstructions.

Safety

Contraindication:

Implants and foreign bodies

Pacemakers
Patient with Prosthesis plates

Specialized MRI Scans

Magnetic resonance angiography - is used to generate pictures of the arteries in order to evaluate them (stenosis [abnormal narrowing] or aneurysms [vessel wall dilatations, at risk of rupture]).

Interventional MRI - used to guide minimally-invasive procedures.

Radiation therapy simulation – is used to locate tumors within the body in preparation for radiation therapy treatments.