Term Paper: History of Magnetic Resonance Imaging

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[. . .] The magnet in an MRI system is rated using a unit of measure known as a tesla. Another unit of measure commonly used with magnets is the gauss (1 tesla = 10,000 gauss). The magnets in use today in MRI are in the 0.5-tesla to 2.0-tesla range, or 5,000 to 20,000 gauss. Magnetic fields greater than 2 tesla have not been approved for use in medical imaging, though much more powerful magnets -- up to 60 tesla -- are used in research. Compared with the Earth's 0.5-gauss magnetic field, you can see how incredibly powerful these magnets are.

These types of results help to provide an intellectual understanding of the magnetic strength; however, Gould points out that everyday examples are also useful to understand the fundamentals involved in MRI. According to Gould, the MRI clinical site is potentially a very dangerous place if strict precautions are not observed since metal objects can become dangerous projectiles if they are taken into the scan room. For instance, even otherwise-harmless objects such as paperclips, pens, keys, scissors, hemostats, stethoscopes and any other small objects can be pulled out of pockets and off the body without warning, at which point they fly toward the opening of the magnet (where the patient is placed) at very high speeds, thereby posing a potential threat to everyone in the treatment area; likewise, credit cards, bank cards and anything else with magnetic encoding can be erased by most MRI systems (Gould, 2004).

The Magnets. According to Gould (2004), there are three basic types of magnets used in MRI systems today:

Resistive magnets consist of many windings or coils of wire wrapped around a cylinder or bore through which an electric current is passed. This causes a magnetic field to be generated. If the electricity is turned off, the magnetic field dies out. These magnets are lower in cost to construct than a superconducting magnet (see below), but require huge amounts of electricity (up to 50 kilowatts) to operate because of the natural resistance in the wire; however, operating this type of magnet above about the 0.3-tesla level would be prohibitively expensive.

A permanent magnet is as it name implies, permanent. Gould notes that a permanent magnet's field is constant in terms of activity and strength; therefore, it costs nothing to maintain the field; the major constraint is that these magnets are extremely heavy. Gould reports that most weigh several tons at the 0.4-tesla level, but a stronger field would require a magnet so heavy it would be difficult to even build. While permanent magnets are getting smaller, they are still limited to low field strengths.

Superconducting magnets are still by far the most commonly used in MRI applications today (Gould, 2004). Superconducting magnets are similar to a resistive magnet in that they have coils or windings of wire through which a current of electricity is passed create the magnetic field; the important distinction is that the wire is continually bathed in liquid helium at 452.4 degrees below zero (Gould, 2004).

MRI vs. Other Imaging Techniques. A number of imaging techniques exist today, but determining which is most appropriate in terms of providing the best approach for the patient can be challenging. According to Dr. Kurt Albertine, the positron emission tomography (PET) imaging approach is most appropriate for assessing such things as muscle damage after a heart attack and the effects of chemotherapy drugs on body tissue (2001, pp. 568-9). Based on their research, Ioannidis and Lau's conclusions in this regard were:

FDG-PET has very good to excellent sensitivity but decreased ability to differentiate benign lesions from low-grade malignant tumors; thus, although FDG-PET has very good to excellent performance in differentiating high- or intermediate-grade tumors from low-grade tumors, it is not a good diagnostic test for differentiating low-grade tumors from benign lesions.

There are no good quality data on the comparative diagnostic performance of FDG-PET against CT or MRI for diagnosis of primary soft tissue lesions, since comparative evidence has been obtained with tests used in series. However, there is limited evidence suggesting approximately equivalent diagnostic performance of FDG-PET and MRI for diagnosing local recurrence and even more limited evidence suggesting approximately equivalent performance of FDG-PET and CT scan for diagnosing distant metastatic disease.

Evidence on the impact of FDG-PET on clinical outcomes and on the management of patients is extremely limited, and there are no controlled studies to answer these important questions.

There is very limited data on the usefulness of FDG-PET in assessing the response to therapy; the evidence that does exist suggests that FDG-PET can be used to follow therapeutic responses, but it is difficult to separate complete from partial response, and there is insufficient evidence to compare the performance of FDG-PET against CT or MRI in this regard (Ioannidis & Lau, 2002, p. 6).

Other researchers, however, point to the superior efficacy of PET imaging over MRI techniques for assessing neural activity (Ioannidis & Lau, 2002).

Conclusion.

The future of MRI seems limited only by resources and human imagination. Certainly, the technology remains relatively new, since it has been in widespread use for less than 20 years (compared with over 100 years for X-rays). Very small scanners that can be used for imaging specific body parts are being developed; for example, a scanner that clinicians (or the patient) can simply place on the arm, knee or foot in are currently in use in some areas. The ability to visualize the arterial and venous system continues to be refined. Neurophysiological methods certainly will further scientists' understanding of the workings of the human central nervous system (Maruish & Moses, 1997). Functional brain mapping which involves scanning a person's brain while he or she is performing a certain physical task such as squeezing a ball, or looking at a particular type of picture, is also helping researchers better understand how the brain works (Gould, 2004). Today, research is being conducted in some institutions to image the ventilation dynamics of the lungs through the use of hyperpolarized helium-3 gas. The development of new and improved ways to image strokes in their earliest stages also continues to be pursued.

References

Albertine, K. (2001). Anatomica. Willoughby, NSW, Australia: Global Book Publishing.

Gould, T.A. (2004). How MRI Works. (2004). How Stuff Works. Available: http://www.howstuffworks.com/mri.htm/printable.

Hornak, J.P. (2002). The Basics of MRI. Available: http://www.cis.rit.edu/htbooks/mri/inside.htm.

Ioannidis,… [END OF PREVIEW]

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