Saturday, August 22, 2009

What Is An MRI (Magnetic Resonance Imaging)?

After the development of computed tomography (aka the CT scan), the next big jump in radiology was the development of the imaging modality known as magnetic resonance imaging, or MRI.

The History of Magnetic Resonance Imaging (MRI) Development

The basis of MRI lies in understanding how to apply the chemical principle of nuclear magnetic resonance to living tissue to produce an image (discussed more fully below). While there are many people involved in the research that brought MRI to fruition, the Nobel Prize Committee recognized two individuals in particular: Paul C. Lauterbur and Sir Peter Mansfield. In Lauterbur's own words, here is how he came upon the idea of using NMR for biomedical imaging:
After I returned to Stony Brook, by a long, leisurely automobile drive from California with my family, and settled in again to my department (where I found the same arguments continuing that had been going on when I left) another unexpected event occurred. It had its beginning several years earlier, when a field service engineer for Varian, the leading NMR company, saw an opportunity and asked for my opinion on his idea of starting his own company to make or distribute specialized NMR equipment and supplies. His business plan seemed reasonable, and I encouraged him to go ahead. For a time the company thrived, and I was a member of the Board of Directors.

In May of 1971, however, some other members of the board compared notes with the company's banker and found that the company had engaged in some very dubious business practices and was, in fact, bankrupt. At a hastily-called Board meeting, appropriate actions were weighed, and the banker, there as a guest, threatened to close the company that day unless someone he trusted could be persuaded to take over as President, Chairman of the Board, and Chief Executive Officer. I was the only academic on the Board, the semester had just ended, and the others believed that I was free for the summer, so that I was asked to take the job. I agreed, flew to the company headquarters in New Kensington, PA, near Pittsburgh, at the beginning of each week and back to Stony Brook and my family and students for the weekend.

The developments at the company could supply the plot for a novel, but the incident that is important for my purpose here is that a post-doc arrived with tumor-bearing rats to check the proton NMR relaxation times of their tumors and normal tissues and organs. I was there to observe the experiments, and noted that large and consistent differences were observed for specimens from all parts of the sacrificed animals and that the experiments seemed well-done. Some individuals were speculating that similar measurements might supplement or replace the observations of cell structure in tissues by pathologists, but the invasive nature of the animal procedure was distasteful to me, the data too complex, and the sources of differences too obscure, to be relied upon for medical decisions. As I pondered the problem that evening, I realized that there might be a way to locate the precise origins of NMR signals in complex objects, and hence to form an image of their distributions in two or even three dimensions. That story, and its consequences, is told more fully elsewhere.
As for the first MRI image itself, here is Mansfield's account:
I was still very much concerned with imaging speed and also the question of sliced definition. After a lot of thought and discussion with Peter Grannell we came up with a method of slice selection which looked as though it might work reasonably well. Alan Garroway also came up with a different method of slice selection using a string of short pulses to define the slice and between us we thought that the sensible approach would be to combine our efforts and publish a short note on the general technique of slice selection. This was sent to the Journal of Physics and was published in the form of a letter. The question of imaging times was still concerning me and during the course of 1974 I spent a great deal of time turning over my thoughts on how this may be achieved. One way forward was what I called line scan imaging. In this method a line of magnetization in a specimen was selectively excited and read out. This process was repeated until the object had been scanned. The technique was much faster than the sensitive point scan method of Hinshaw and also turned out to be faster than the projection reconstruction method of Paul Lauterbur, but I was still not satisfied. Nevertheless, line scanning was used to produce a number of images and in particular it was used to scan the finger of one of my early research students, Dr Andrew Maudsley. The scan times for these finger images were 15-23 minutes. These were the first images of a live human subject and were presented at a special meeting of the Medical Research Council which was convened in 1976 to review the work of the several imaging groups that had sprung up both at Nottingham and also in Aberdeen. All groups were vying for MRC support and this meeting was called specially to review the topic and to decide how best to support the work. The images demonstrating live human anatomy were annotated by Professor Rex Coupland, then head of the Department of Human Morphology. They produced a startling response at this meeting and convinced the MRC that our work should be supported. We produced a grant application requesting a substantial sum of money to produce a whole body MRI machine.
Since the 1970s, the rapid development of technology in broad terms has fostered the development of MRI as a practical, and in some cases now, essential tool for physicians, changing the standard of care forever.

The Principles Behind Magnetic Resonance Imaging (MRI)

A detailed explanation of the physics and chemistry that go into using magnets to produce images is beyond the scope of this blog. However, Wikipedia has a nice section that explains MRI in plain and easy-to-follow terms:
The body is largely composed of water molecules which each contain two hydrogen nuclei or protons. When a person goes inside the powerful magnetic field of the scanner, the magnetic moments of these protons align with the direction of the field.
A radio frequency electromagnetic field is then briefly turned on, causing the protons to alter their alignment relative to the field. When this field is turned off the protons return to the original magnetization alignment. These alignment changes create a signal which can be detected by the scanner. The frequency of the emitted signal depends on the strength of the magnetic field. The position of protons in the body can be determined by applying additional magnetic fields during the scan which allows an image of the body to be built up. These are created by turning gradients coils on and off which creates the knocking sounds heard during an MR scan.
Diseased tissue, such as tumors, can be detected because the protons in different tissues return to their equilibrium state at different rates. By changing the parameters on the scanner this effect is used to create contrast between different types of body tissue.

Contrast agents may be injected intravenously to enhance the appearance of blood vessels, tumors or inflammation. Contrast agents may also be directly injected into a joint in the case of arthrograms, MR images of joints. Unlike CT, MRI uses no ionizing radiation and is generally a very safe procedure. Patients with some metal implants, cochlear implants, and cardiac pacemakers are prevented from having an MRI scan due to effects of the strong magnetic field and powerful radio frequency pulses.

The Future of Magnetic Resonance Imaging (MRI)

MRI has found many applications in the medical field. Functional MRIs are used to assess how differential delivery of glucose reflects neurological function in conscious subjects. There are many potential applications of functional MRIs, ranging from studies in psychology and economics to more medically-related applications in psychiatry. In oncology, MRIs are being paired with PET scans to further improve the sensitivity of tumor detection. If the last few decades are any indication, the next ten years will likely see an explosion of MRI installations and application development, to the degree that one day an MRI might be as routine as a plain x-ray.

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