History of magnetic resonance imaging

History of Magnetic Resonance Imaging (MRI)

Getting an MRI scan may someday become as common as getting an X-ray. - Davis Meltzer, 1987

According to Gould (2004), on July 3, 1977, an event took place that would forever alter the landscape of modern medicine, although outside the scientific research community, this event hardly attracted any notice at all. The event in question was the first MRI exam ever performed on a human being. The procedure required almost five hours to produce one image, and the images were, by today's standards, very primitive (this first MRI machine now occupies a special niche in the Smithsonian); however, its successors number if the thousands today (Gould, 2004). The advent of the MRI clearly represented the beginnings of a new standard in noninvasive radio imaging that continues to be refined. This paper provides the background and history of magnetic resonance imaging, including its discovery and evolution, as well as newly identified applications for the technique. A summary of the research is provided in the conclusion.

Review and Discussion

Background and Overview. According to an early report by Howard Suchurek (1987), "Like the director of a chorus, an MRI scanner conduct the 'singing' of hydrogen atoms within the human body. The scanner surrounds the body with powerful electromagnets. Supercooled by liquid helium, they create a magnetic field as much as 60,000 times as strong as that of the earth" (16).

Magnetic resonance imaging (MRI) is an imaging technique that is used primarily in medical settings to produce high quality images of the inside of the human body (Gould 2004). 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 (Gould, 2004).

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 began as a tomographic imaging technique; in other words, it produced an image of the NMR signal in a thin slice through the human body. Since those early efforts, MRI has advanced beyond such tomographic imaging techniques to a more sophisticated volume imaging technique. This improved approach provides a comprehensive picture of the basic principles of MRI (Gould, 2004).

Today, magnetic resonance imaging is an imaging modality that is used primarily to develop pictures of the NMR signal from the hydrogen atoms contained within an object. In medical MRI applications, for example, radiologists are most interested in looking at the NMR signal from water and fat, the major hydrogen containing components of the human body (Gould, 2004).

Brief History of MRI. Any investigation of the science of MRI must review its history to understand how the technology evolved to its state today. According to Dr. Hornak (2002), Felix Bloch and Edward Purcell (both of whom were awarded the Nobel Prize in 1952), discovered the magnetic resonance phenomenon independently in 1946. During the period between 1950 and 1970, NMR was developed and used for chemical and physical molecular analysis. In 1971, Raymond Damadian demonstrated that the nuclear magnetic relaxation times of tissues and tumors differed, thus motivating scientists to consider magnetic resonance for the detection of disease (Gould, 2004).

In 1973 the x-ray-based computerized tomography (CT) was introduced by Hounsfield. This date is important to the MRI timeline because it demonstrated conclusively that hospitals were amenable to investing large amounts of money for medical imaging hardware. Magnetic resonance imaging was first demonstrated on small test tube samples in 1973 by Paul Lauterbur; this scientist employed a back projection technique similar to that used in CT. According to Gould, in 1975, Richard Ernst first proposed magnetic resonance imaging using phase and frequency encoding, as well as the Fourier Transform.

This technique became the foundation for current MRI techniques. In 1977, Raymond Damadian first demonstrated an MRI field-focusing nuclear magnetic resonance; also that year, Peter Mansfield developed the echo-planar imaging (EPI) technique. This technique was expected to be refined in the future to produce images at video rates (30 ms / image) (Gould, 2004).

In 1980, Edelstein and his coworkers demonstrated imaging of the body using Ernst's technique; using this approach, a single image could be acquired in approximately five minutes. The imaging time was further reduced to about five seconds by 1986, an advance that was made without sacrificing too much image quality. In 1980, other researchers were also developing the NMR microscope; this device allowed approximately 10 mm resolution on approximately one cm samples. Gould reports that in 1987, echo-planar imaging was first used to perform real-time movie imaging of a single cardiac cycle; also in 1987, Charles Dumoulin was refining magnetic resonance angiography (MRA), a process that allowed imaging of flowing blood without the use of contrast agents.

In 1991, Richard Ernst was recognized for his achievements in pulsed Fourier Transform NMR and MRI as a recipient of the Nobel Prize in Chemistry; in 1992, functional MRI (fMRI), a technique that provided a means of mapping of the function of the various regions of the human brain, was first introduced (Gould, 2004).

In 1997, the innovation of fMRI formed the foundation for a new application for EPI in mapping the regions of the brain that were believed responsible for thought and motor control. 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 1994 (Gould, 2004). Finally, 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. Clearly, MRI is recent but importantly significant science. "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" (Gould 2004:5).

MRI Imaging. Magnetic resonance started out as a tomographic imaging modality for producing NMR images of a slice though the human body. Each slice had a thickness. Magnetic resonance imaging is based on the absorption and emission of energy in the radio frequency range of the electromagnetic spectrum (Hornak, 2002). Each year seems to bring a new application of MRI or a new pulse sequence which opens up new imaging opportunities with MRI. For those who have never seen an MRI machine, the basic design used in most is a giant cube, which in a typical system might be 7 feet tall by 7 feet wide by 10 feet long (2 m by 3 m) (although new models are rapidly shrinking). There is a horizontal tube running through the magnet from front to back. This tube is known as the bore of the magnet. The patient, lying on his or her back, slides into the bore on a special table. Whether or not the patient goes in head first or feet first, as well as how far in the magnet they will go, is determined by the type of exam to be performed. MRI scanners vary in size and shape, and newer models have some degree of openness around the sides, but the basic design is the same. Once the body part to be scanned is in the exact center or isocenter of the magnetic field, the scan can begin.

Together with radio wave pulses of energy, the MRI scanner can pick out a very small point inside the patient's body and ask it, essentially, "What type of tissue are you?" The point might be a cube that is half a millimeter on each side. The MRI system goes through the patient's body point by point, building up a 2-D or 3-D map of tissue types. It then integrates all of this information together to create 2-D images or 3-D models.

MRI provides an unparalleled view inside the human body. The level of detail we can see is extraordinary compared with any other imaging modality. MRI is the method of choice for the diagnosis of many types of injuries and conditions because of the incredible ability to tailor the exam to the particular medical question being asked. By changing exam parameters, the MRI system can cause tissues in the body to take on different appearances. This is very helpful to the radiologist (who reads the MRI) in determining if something seen is normal or not. We know that when we do "A," normal tissue will look like "B" -- if it doesn't, there might be an abnormality. MRI systems can also image flowing blood in virtually any part of the body. This allows us to perform studies that show the arterial system in the body, but not the tissue around it. In many cases, the MRI system can do this without a contrast injection, which is required in vascular radiology.

Magnetic Intensity. In order to understand how MRI works, an examination of the "magnetic" component will be useful. The biggest and most important component in an MRI system is the magnet. 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: