MRI Guide for Technologists: A Step by Step Approach

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.

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

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 -469^o F, -269^o C or 4^o 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 B_o, 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=YB_o. 

P=Y (Y=42.6Mhz constant for a 1.0 Tesla magnet)

B_o=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

FID-Free induction decay due to relaxation

T₂^* is shorter than T₂ time.