The Basics of MRI

Chapter 1

INTRODUCTION



NMRI or MRI ?

Magnetic resonance imaging (MRI) is an imaging technique used primarily in medical settings to produce high quality images of the inside of the human body. MRI is based on the principles of nuclear magnetic resonance (NMR), a spectroscopic technique used by scientists to obtain microscopic chemical and physical information about molecules. The technique was called magnetic resonance imaging rather than nuclear magnetic resonance imaging (NMRI) because of the negative connotations associated with the word nuclear in the late 1970's. MRI started out as a tomographic imaging technique, that is it produced an image of the NMR signal in a thin slice through the human body. MRI has advanced beyond a tomographic imaging technique to a volume imaging technique. This package presents a comprehensive picture of the basic principles of MRI.

Before beginning a study of the science of MRI, it will be helpful to reflect on the brief history of MRI. Felix Bloch and Edward Purcell, both of whom were awarded the Nobel Prize in 1952, discovered the magnetic resonance phenomenon independently in 1946. In the period between 1950 and 1970, NMR was developed and used for chemical and physical molecular analysis.

In 1971 Raymond Damadian showed that the nuclear magnetic relaxation times of tissues and tumors differed, thus motivating scientists to consider magnetic resonance for the detection of disease. In 1973 the x-ray-based computerized tomography (CT) was introduced by Hounsfield. This date is important to the MRI timeline because it showed hospitals were willing to spend large amounts of money for medical imaging hardware. Magnetic resonance imaging was first demonstrated on small test tube samples that same year by Paul Lauterbur. He used a back projection technique similar to that used in CT. In 1975 Richard Ernst proposed magnetic resonance imaging using phase and frequency encoding, and the Fourier Transform. This technique is the basis of current MRI techniques. A few years later, in 1977, Raymond Damadian demonstrated MRI called field-focusing nuclear magnetic resonance. In this same year, Peter Mansfield developed the echo-planar imaging (EPI) technique. This technique will be developed in later years to produce images at video rates (30 ms / image).

Edelstein and coworkers demonstrated imaging of the body using Ernst's technique in 1980. A single image could be acquired in approximately five minutes by this technique. By 1986, the imaging time was reduced to about five seconds, without sacrificing too much image quality. The same year people were developing the NMR microscope, which allowed approximately 10 µm resolution on approximately one cm samples. In 1987 echo-planar imaging was used to perform real-time movie imaging of a single cardiac cycle. In this same year Charles Dumoulin was perfecting magnetic resonance angiography (MRA), which allowed imaging of flowing blood without the use of contrast agents.

In 1991, Richard Ernst was rewarded for his achievements in pulsed Fourier Transform NMR and MRI with the Nobel Prize in Chemistry. In 1992 functional MRI (fMRI) was developed. This technique allows the mapping of the function of the various regions of the human brain. Five years earlier many clinicians thought echo-planar imaging's primary applications was to be in real-time cardiac imaging. The development of fMRI opened up a new application for EPI in mapping the regions of the brain responsible for thought and motor control. In 1994, researchers at the State University of New York at Stony Brook and Princeton University demonstrated the imaging of hyperpolarized 129Xe gas for respiration studies.

In 2003, Paul C. Lauterbur of the University of Illinois and Sir Peter Mansfield of the University of Nottingham were awarded the Nobel Prize in Medicine for their discoveries concerning magnetic resonance imaging. MRI is clearly a young, but growing science.

Opportunities in MRI

In 2003, there were approximately 10,000 MRI units worldwide, and approximately 75 million MRI scans per year performed. As the field of MRI continues to grow, so do the opportunities in MRI.

There will always be a need for radiologists trained in MRI to read the magnetic resonance images. A radiologist is a medical doctor that has specialized in the field of radiology. The need is expected to grow so much that there will be and increased use of Radiology Practitioner Assistants and Radiology Physician Assistants.

An MRI technologist is an individual that operates the MRI scanner to obtain the images that a radiologist prescribes. Based on the number of current MRI systems, it is estimated that there will be a constant need for over 1000 MRI technologists per year. A good resource for MRI technologists is the Society for Magnetic Resonance Technologists (SMRT).

Two new specialist positions have recently evolved in MRI: the post processing technologist and the health safety specialist. The MRI post processing technologist applies various post processing algorithms to magnetic resonance images to either extract more information from or enable better visualization of information in magnetic resonance images. An MRI health safety specialist assists hospitals and clinics in setting up and maintaining a safe MRI system.

Because of the complexity of the MRI system, there will always be a need for MRI service technicians. Service technicians are hired by both the MRI manufacturers and some larger sites to keep the MRI system operating properly. MRI service technicians usually have a BS or associates degree in electrical technology and a good knowledge of MRI.

In any field, there will be a need for scientists trained in the basic sciences of chemistry, biology, and physics to perform basic research and push back the frontiers of the science. Some specific needs for these scientists include contrast agent and molecular imaging development, and advanced imaging pulse sequence design. These individuals typically have an advanced degree in their respective field and have had significant training in MRI. A good resource for scientists is the International Society for Magnetic Resonance in Medicine (ISMRM).

Biomedical engineers and material scientists are needed for MRI subsystem development. One of these subsystems where continued demand is seen is imaging coil development. An emerging area requiring many skilled individuals is the development of MRI compatible devices. These devices include pacemakers, defibrillators, surgical clips and pins, and catheters. Many of these devices will require discoveries at the molecular level, such as biocompatible antireflective coatings for pacemaker wires and strong non metallic synthetic joints and pins.

Imaging scientists are needed for algorithm development for post processing of magnetic resonance images, and intelligent code for identifying and diagnosing pathology. Computer scientists are still needed to design user friendly efficient graphical user interfaces (GUI) for newly developed software.

Lastly, there is a need for architects to design safe and efficient MRI centers and clinics. The Basics of MRI is a good starting place for all the above individuals interested in starting their training in pursuit of a career in MRI or a related field.

If you are interested in one of these professions, familiarize yourself with the profession and MRI. Explore various options for obtaining the education needed for the profession.

Currently, there are approximately six major clinical MRI original equipment manufacturers (OEMs). In addition to these clinical OEMs, there are two major experimental MRI OEMs. Other MRI related subsystem manufacturers include RF coil, contrast agents, compatible devices, RF amps, and magnets. The following tables contain the names of some of the major manufacturers of these devices. Click on the name for an external link to the company. Because they are links external to The Basics of MRI, they are subject to change.

Clinical MRI OEMs
Esaote
Fonar
General Electric Healthcare
Hitachi Medical Systems
Millennium Technology
Odin Medical technologies
Philips Healthcare
Siemens Healthcare
Toshiba Medical Systems

Experimental High Field MRI OEMs
Agilent Technologies
Bruker Biospin MRI

Manufacturers of MRI Contrast Agents
Bracco Group
Bayer HealthCare Pharmaceuticals
GE Imaging Agents
Lantheus Medical Imaging
Sigma-Aldrich

MRI Compatible Device Manufacturers
Biophan Technologies
InVivo
Magmedix
Medrad
MR Resources
Schiller
SA Instruments

Magnet System Manufacturers
Bruker Biospin MRI
Magnex Scientific
Resonance Research / Stern Magnetics

RF Coil Manufacturers
Advanced Imaging Research
Doty Scientific
GE Healthcare
IGC Medical Advances
InVivo
Lammers Medical Technology
Machnet B.V.
MR Instruments
Nova Medical
RAPID Biomedical
Scanmed
XL Resonance

Gradient Supplies
Copley Controls
Tesla Engineering

RF Amps
Communication Power Corporation
Advanced Energy
Herley Medical Products

Tomographic Imaging

This computer based teaching package will provide you with an understanding of the principles of MRI form both the microscopic, macroscopic, and imaging system perspective. Let's begin with a pictorial introduction to some basic MRI. Magnetic resonance started out as a tomographic imaging modality for producing NMR images of a slice through the human body. Each slice had a thickness (Thk). This form of imaging is in some respects equivalent to cutting off the anatomy above the slice and below the slice. The slice is said to be composed of several volume elements or voxels. The volume of a voxel is approximately 2 mm3. The magnetic resonance image is composed of several picture elements called pixels. The intensity of a pixel is proportional to the NMR signal intensity of the contents of the corresponding volume element or voxel of the object being imaged.

Magnetic resonance imaging is based on the absorption and emission of energy in the radio frequency range of the electromagnetic spectrum. It is clear from the attenuation spectrum of the human body why x-rays are used, but why did it take so long to develop imaging with radio waves, especially with health concerns associated with ionizing radiation such as x-rays? Many scientists were taught that you can not image objects smaller than the wavelength of the energy being used to image. MRI gets around this limitation by producing images based on spatial variations in the phase and frequency of the radio frequency energy being absorbed and emitted by the imaged object.

Microscopic Property Responsible for MRI

The human body is primarily fat and water. Fat and water have many hydrogen atoms which make the human body approximately 63% hydrogen atoms. Hydrogen nuclei have an NMR signal. For these reasons magnetic resonance imaging primarily images the NMR signal from the hydrogen nuclei. Each voxel of an image of the human body contains one or more tissues. For example here is a voxel with one tissue inside. Zooming in on the voxel reveals cells. Within each cell there are water molecules. Here are some of the water molecules. Each water molecule has one oxygen and two hydrogen atoms. If we zoom into one of the hydrogens past the electron cloud we see a nucleus comprised of a single proton. The proton possesses a property called spin which:

  1. can be thought of as a small magnetic field, and
  2. will cause the nucleus to produce an NMR signal.

Not all nuclei possess the property called spin. A list of these nuclei will be presented in Chapter 3 on spin physics.

Units Review

Before you can begin learning about MRI, you must be versed in the language of MRI. MRI scientists and clinicians use a set of units when describing temperature, energy, frequency, etc. Please review these units before advancing to subsequent chapters in this text.

Units of time are seconds (s).

Angles are reported in degrees (o) and in radians (rad). There are 2π radians in 360o.

The absolute temperature scale in Kelvin (K) is used in MRI. The Kelvin temperature scale is equal to the Celsius scale reading plus 273.15. 0 K is characterized by the absence of molecular motion. There are no degrees in the Kelvin temperature unit.

Magnetic field strength (B) is measured in Tesla (T). The earth's magnetic field in Rochester, New York is approximately 5x10-5 T.

The unit of energy (E) is the Joule (J). In MRI one often depicts the relative energy of a particle using an energy level diagram.

The frequency of electromagnetic radiation may be reported in cycles per second or radians per second. Frequency in cycles per second (Hz) have units of inverse seconds (s-1) and are given the symbols ν or f. Frequencies represented in radians per second (rad/s) are given the symbol ω. Radians tend to be used more to describe periodic circular motions. The conversion between Hz and rad/s is easy to remember. There are 2π radians in a circle or cycle, therefore

2π rad/s = 1 Hz = 1 s-1.

Power is the energy consumed per time and has units of Watts (W).

Finally, it is common in science to use prefixes before units to indicate a power of ten. For example, 0.005 seconds can be written as 5x10-3 s or as 5 ms. The m implies 10-3. The animation window contains a table of prefixes for powers of ten.

In the next chapter you will be introduced to the mathematical beckground necessary to begin your study of NMR.


Problems

  1. What is the abbreviation for slice thickness in MRI? 
  2. What is the smallest information element of a picture? 
  3. The tomographic image plane contains many small volume elements called? 
  4. The two transmission windows for electromagnetic energy through the human body are in which regions of the spectrum? 
  5. Most MRI looks at NMR signals from what nuclei?  
  6. Who first demonstrated MRI using the backprojection technique?  
  7. Who first demonstrated MRI using the Fourier technique?  
  8. Who showed that the nuclear relaxation times of healthy tissues differed for tumors, and later produced a whole body MRI?  
  9. What two types of molecules are responsible for the signal in MRI?  
  10. What is meant by a tomographic image?  

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