History and Science of Magnetic Resonance Imaging (MRI)

Introduction

"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 attracted hardly 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 results were, by today's standards, very primitive. (This first MRI machine now occupies a special niche in the Smithsonian.) Its successors, however, number in 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.

Background and Overview of MRI

According to an early report by Howard Sochurek (1987), "Like the director of a chorus, an MRI scanner conducts 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" (p. 16).

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 (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 1970s. MRI began as a tomographic imaging technique — that is, it produced an image of the NMR signal in a thin slice through the human body. Since those early efforts, MRI has advanced beyond tomographic imaging to a more sophisticated volume imaging technique. This improved approach provides a comprehensive picture of the underlying principles of MRI (Gould, 2004).

Brief History of MRI

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

Any investigation of the science of MRI must review its history to understand how the technology evolved to its current state. According to Dr. Hornak (2002), Felix Bloch and Edward Purcell — both of whom were awarded the Nobel Prize in 1952 — independently discovered the magnetic resonance phenomenon 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, thereby motivating scientists to consider magnetic resonance as a tool for the detection of disease (Gould, 2004).

In 1973, the X-ray-based computerized tomography (CT) scan was introduced by Hounsfield. This date is important to the MRI timeline because it demonstrated conclusively that hospitals were willing to invest large sums of money in medical imaging hardware. That same year, magnetic resonance imaging was first demonstrated on small test tube samples by Paul Lauterbur, who employed a back-projection technique similar to that used in CT. According to Gould (2004), 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 methods. 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 per image) (Gould, 2004).

In 1980, Edelstein and his colleagues demonstrated whole-body imaging using Ernst's technique; with this approach, a single image could be acquired in approximately five minutes. Imaging time was further reduced to about five seconds by 1986, an advance achieved without sacrificing significant image quality. Also in 1980, researchers were developing the NMR microscope, a device that allowed approximately 10 mm resolution on approximately 1 cm samples. Gould (2004) 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 with the Nobel Prize in Chemistry. In 1992, functional MRI (fMRI) — a technique that enabled mapping of the functional regions of the human brain — was first introduced (Gould, 2004).

How MRI Imaging Works

By 1997, fMRI formed the foundation for a new application of EPI in mapping the brain regions 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 Physiology or Medicine for their discoveries concerning magnetic resonance imaging. As Gould (2004, p. 5) notes, "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."

Magnetic resonance imaging began as a tomographic modality for producing NMR images of a slice through the human body, with each slice having a defined thickness. MRI 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 that opens up new imaging opportunities.

For those who have never seen an MRI machine, the basic design used in most systems is a large cube — in a typical system, approximately 7 feet tall by 7 feet wide by 10 feet long (roughly 2 m × 3 m), although newer models are rapidly shrinking. A horizontal tube runs through the magnet from front to back; this tube is known as the bore of the magnet. The patient lies on his or her back and slides into the bore on a special table. Whether the patient enters head-first or feet-first, and how far into the magnet they travel, depends on the type of exam being performed. MRI scanners vary in size and shape, and newer models offer some degree of openness around the sides, but the basic design remains the same. Once the body part to be scanned is positioned at the exact center — or isocenter — of the magnetic field, the scan can begin.

Together with radio wave pulses of energy, the MRI scanner can isolate a very small point inside the patient's body and determine, essentially, what type of tissue it is. That point might be a cube just half a millimeter on each side. The MRI system moves through the patient's body point by point, building a 2-D or 3-D map of tissue types, and then integrates all of this information to create 2-D images or 3-D models.

MRI provides an unparalleled view inside the human body. The level of detail it reveals is extraordinary compared with any other imaging modality. MRI is the method of choice for diagnosing many types of injuries and conditions because of its remarkable ability to be tailored to the specific medical question being asked. By changing exam parameters, the MRI system can cause tissues in the body to take on different appearances — a feature that helps the radiologist determine whether a finding is normal or abnormal. MRI systems can also image flowing blood in virtually any part of the body, allowing studies that visualize the arterial system without capturing the surrounding tissue. In many cases, this can be accomplished without a contrast injection, which is typically required in vascular radiology.

Conclusion

The future of MRI seems limited only by resources and human imagination. The technology remains relatively new, having been in widespread use for less than 20 years at the time of this writing — compared with over 100 years for X-rays. Very small scanners designed to image specific body parts are being developed; for example, scanners that clinicians or patients can place directly on the arm, knee, or foot are already in use in some settings. The ability to visualize the arterial and venous systems continues to be refined. Neurophysiological methods will certainly further scientists' understanding of the human central nervous system (Maruish & Moses, 1997).