MRI Guide for Technologists
A Step by Step Approach
by
Book Cover & Preview Text
BASIC PHYSICS
The physics of Magnetic Resonance Imaging is very
complex; in fact, Physicists specializing in this area of MRI may spend their
entire career on this subject. The American Association of Physicists in
Medicine (AAPM) has special certification for Physicists specializing in MRI.
It would therefore be quite presumptuous of me to
expect to teach the physics of MRI in one chapter. This chapter is geared to
explaining the basic physics of MRI so that you, the technologist, can apply
this knowledge when performing an MRI examination. knowledge of basic MRI
physics may enable you to recognize routine problems and perhaps troubleshoot
them without incurring downtime and delays while waiting for service.
DEFINITIONS
Magnet
Definition-An
object that is surrounded by a magnetic field and that has the property, either
natural or induced, of attracting iron or steel.
In MRI, such materials must always be stable,
homogeneous and strongly magnetic. There are certain conditions needed to
maintain the magnetic environment.
Cryogens are used to maintain stability by keeping
the magnetic windings cool. Both Helium and Nitrogen have been used however
most modern magnets only use Helium. As Helium is a cryogenic material, its
temperature is -469o F, -269o C or 4o
K.
Shimming coils are used to maintain homogeneity.
There are two types of shimming coils.
Active, which is on
constantly and is electrical.
Passive which are sheets of cores stacked together
Flux density is large enough to maintain field
strength up to 2 Tesla
Resonance
Definition-
The increase in amplitude of oscillation of an electric or mechanical system
exposed to a periodic force whose frequency is equal or very close to the
natural undamped (Not tending toward a state of rest) frequency of the system.
In MRI the effect is that a nuclei acted upon by an
external force will spin at a frequency equal to its own.
Imaging
Definition-The
production of images of good diagnostic quality for purposes of interpretation
There are three major requirements for MRI imaging
Protons of the human body-Used because they are
abundant in the body and possess a magnetic moment of their own.
Radio frequency (RF)-Termed B1 With the
application of Radio frequency, protons are stimulated and move from a low to
high energy. They now possess a magnitude and direction. RF is electro magnetic radiation, the same
used for AM radios and television news stations.
Strong magnetic field-This main magnet is termed Bo,
which is horizontal in design in most modern magnets. In permanent magnets the
magnetic field is vertical.
In order to obtain a signal, the RF must be applied
perpendicular to the main magnet, B0. The speed at which the protons
resonate must match the magnetic field strength and the Gyromagnetic ratio. The
Gyromagnetic ratio is the ratio of the magnetic moment to the mechanical angular
momentum of a system).
The Larmor equation is now applied P=YBo.
P=Y (Y=42.6Mhz constant for a 1.0 Tesla magnet)
Bo=Field strength (e.g. 1.0 T)
If a magnet is 1.5T then it is 63.9 MHz.
FREE INDUCTION DECAY
The very first signal produced is called Free
Induction Decay (FID) and is unavoidable. When performing an MRI examination a
set of pulse sequences is selected. A pulse sequence is a set of instructions
telling the computer how the images should appear.
T2*
T2*- A form of Free Induction Decay which
occurs as result of the following:
Dephasing caused by magnetic field in homogeneities
Formats
Book Details
About the Book
The book includes chapters on MRI Physics, Patient
preparation, four glossaries and head to foot instructions on how to perform an
MRI scan.
The handbook is geared to the practicing MRI
technologist and student MRI technologists.
The handbook was written as training tool for the
student MRI technologist and as a reference handbook for the practicing MRI
Technologist. The book is not a textbook, but rather a daily reference tool to
supplement a bona-fide course of study along with an appropriate amount of
clinical training. It is expected that practicing MRI technologists can use
this handbook well after a training program is completed.
The approach is quite practical in that an
individual with appropriate clinical experience can perform scans of any
anatomy. It is comprehensive in that it takes into account virtually every MRI
examination performed.
The handbook depends on illustrations to convey the
subject matter. The images used are actual images from MRI examinations which
demonstrate anatomy and illustrate the desired outcome of an MRI examination.
Color illustrations are provided for diagrams.
The main feature of the handbook is in its approach
to the material. The handbook begins with preliminary sections. Sections on
scanning using a step-by-step “Cook Book” approach, from the tools to use, the
landmarks to identify and the protocols to be used follow, and are the crux of
the handbook. The Illustrations bring it all together so that the reader can
identify the expected end result.
About the Author
Born March 25, 1949 in a small village called
“Tamarind Town” in Guyana, South America, he immigrated to the United States in
1970 and began working in the retail auto parts industry.
In 1987, he began his education in Radiologic
Technology at Hostos Community College. The main motivator to enter this profession was his childhood
recollection of the only x-ray technologist in the Berbice area of South
America being picked up from a Cricket game and transported by helicopter to a
hospital to take x-rays on a patient waiting to undergo surgery.
After graduation from the Radiologic Technology
program he began working at Bronx Lebanon Hospital and was quickly promoted to
supervisor. He had a keen interest in
MRI and read everything that he could lay his hands on while initially
volunteering in MRI facilities to gain as much clinical knowledge as possible.
Registered in Diagnostic and CT, became registered
in MRI in 1995 and ultimately received a Masters Degree in 1996 in Health Care
Science.
He was involved in the start up of the Jacobi
Medical Center, Bronx New York MRI Division, to include equipment selection,
policy implementation and working with regulatory agencies. Mootoo continues to be involved in MRI both
clinically and academically as instructor and clinical site director for
several colleges.
He is the founder and sole instructor of “MRI
Seminar Services Inc” an ASRT approved preparation course whose goal is to
prepare students for the MRI Registry examination