Science at the Frontier (1992) / Chapter Skim
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6 Magnetic Resonance Imaging: New Breakthroughs in Medical Diagnosis
6 Magnetic Resonance Imaging: New Breakthroughs in Medical Diagnosis
Pages 119-148
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From page 119... ...
The Frontiers symposium session on magnetic resonance imaging (MRI) provided a vivid illustration of how these two sometimes distinct aspects of medicine often merge.
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From page 120... ...
Teaching on the faculty at the UCLA Medical School and also serving as director of MRI at Santa Barbara Cottage Hospital, Crues told about the increasing value of MRI in imaging the musculoskeletal system, hitherto thought to be the province of the x ray. If there was an exception to the generalization that clinical MRI involves its practitioners in a continual process of analyzing and refining the pictures made of individual patients, Robert Balaban, chief of the Laboratory of Cardiac Energetics at NIH, is it.
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From page 121... ...
PET scanning techniques also rely on interactions with particles, but the emanating source comes from within the body by way of radioactive molecules introduced there by nuclear physicians. These chemicals are designed insofar as is possible to go to the area under scrutiny, and once there to begin to decay and emit positrons.
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From page 122... ...
As more sophisticated methods of scanning, more discerning contrast agents, and altogether new techniques are refined, the potential clinical scope of MRI will expand. In the world of modern clinical medicine, however, practitioners face a number of practical considerations.
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From page 123... ...
These factors presume a fundamental knowledge of the principles of magnetic resonance, which he summarized for the scientists in his 1990 Frontiers keynote presentation, "Clinical Magnetic Resonance Imaging and Spectroscopy." THE PHYSICS AND COMPUTATION BEHIND MRI Like its predecessor the x ray, MRI grew out of studies in basic physics. Physics itself, however, was undergoing a dramatic growth into the new territory of quantum electrodynamics, leading to exploration of particle behaviors undreamed of even in Roentgen's time.
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From page 124... ...
The spin of all protons produces a characteristic magnetic moment, in this case one that is inherent to hydrogen. These spinning particles respond only to inputs adjusted according to the basic formula, called the Larmor equation: ~ = ~ Bo This equation defines the frequency of the RF pulse (co)
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From page 125... ...
. The MRI operator intentionally creates and designs this nonuniform field by superimposing over the main field additional magnetic fields produced by resistive electromagnets coaxial to the axis of the main field, continued Bradley, adding, "These additional electromagnets can provide linear spatial variation in the net magnetic field along the x-, y-, or z-axis, creating magnetic field gradients, gx, gy, and gz.'' From this spatial arrangement comes another dramatic improvement in diagnostic power over CT, because the CT image is limited to the plane of the gantry and produces clinical images in the axial and semicoronal planes only.
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From page 126... ...
In losing this energy, all of the protons that were energized by the RF pulse are involved in two basic activities, and single MR images result from a series of pulse sequences that are designed by the operator to focus on one or the other. T.' and T2 Relaxation Times as Data Bradley referred to the fact that each atomic species has its own characteristic magnetic moment.
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From page 127... ...
The energy of this pulse, as any beam of energy will, provides its own magnetic moment, which can be seen to arrive at the main field from a perpendicular direction. "When the system is exposed to the oscillating magnetic component of a radio wave at the Larmor frequency, the magnetization will begin to precess about the axis of that second, smaller magnetic field at the Larmor frequency," explained Bradley.
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From page 128... ...
Each of the spinning protons producing its own little magnetic moment is exposed to a slightly different microscopic magnetic environment, which is itself changing rapidly. As an analogy, one wave will ripple smoothly outward on the surface of a lake, but a series of stones thrown in create multiple waves that eventually cancel out.
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From page 129... ...
In particular, hydration-induced changes in the motion of water at or near the surface of macromolecules have a profound influence on relaxation times. Bradley explained how the practitioner develops a pulse sequence a series of RF pulses based on a feeling for how the chemical environment in a given area will influence the local magnetic effects on the target protons.
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From page 131... ...
The combination of appearances on To- and T2-weighted images allows us to determine that the central portion of the hematoma (which remains bright on both T.- and proton-density-weighted images) contains extracellular methemoglobin, while on the proton-density-weighted image there is a dark rim surrounding the hematoma due to hemosiderin macrophages.
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From page 132... ...
Such T2-weighted images appear brighter where T2 is longer, as opposed to the T~-weighted images, where shorter To times produce a higher signal intensity and therefore a brighter image. Looking More Closely at the Complexities of Relaxation Robert Balaban and his team at the NIH are doing research into the question of how water molecules actually interact on or near the surface of the macromolecules that constitute so much of human tissue.
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From page 133... ...
. Thus any change in the water proton signal would be due to magnetization transfer between the macromolecule and water protons.
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From page 134... ...
MAGNETIC RESONANCE IMAGING IN MEDICINE Soft Tissues Even Bone and Moving JointsAre Prime MRI Targets John Crues specializes in diagnosing musculoskeletal problems. When MRI began to reach clinical trials in the early 1980s, he reported, not many of his colleagues in orthopedics were optimistic that it would be of much use to their specialty because of an apparent major hurdle.
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From page 135... ...
Crues.) Biomechanical joints such as the human knee and shoulder represent systems with a very precise design, produced by evolution for purposes that people living in modern times now routinely exceed.
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From page 136... ...
This has virtually eliminated diagnostic surgery in the musculoskeletal system," Crues continued, and now allows surgeons to devise a more definitive and sophisticated approach to specific pathologies. A compact and accessible area like the human knee allows physicians to use a variation on the large, body-size RF coils, which often produces very sharp pictures.
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From page 137... ...
"A major breakthrough in understanding this disease stemmed from the surgical findings of Charles Neer, who postulated that 95 percent of rotator cuff tears were due to chronic trauma," continued Crues, caused by a congenitally abnormally shaped acromion, the bone at the top of the shoulder. Normally this bone surface is flat, but "some people are born with a congenital abnormal shape," said Crues, a kind of bony hook that often abrades and produces chronic trauma on the rotator cuff muscle laid over it.
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From page 138... ...
Even more recently, PET studies (such as those that send a solution of glucose molecules labeled with fluorine-18 atoms to the brain) have provided real-time pictures of local activity as various regions of the brain metabolize other molecules for the energy to transmit their electrical signals.
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From page 139... ...
Deoxybemoglobin can be clearly distinguished from me/hemoglobin, said Bradley, because its associated "water molecules are unable to approach the heme iron within 3 angstroms for a dipole-dipole interaction," the mechanism whereby To is shortened. Paramagnetic substances act as contrast agents to enhance MR images.
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From page 140... ...
Before MRI practitioners could produce images of flow, techniques had to be developed to overcome a number of obstacles. Bradley pointed out that several "independent factors can result in decreased signal intensity of flowing blood: high velocity, turbulence, and oddecho dephasing." Sometimes known as flow void, this decrease in signal can per se be revelatory, indicating under certain conditions the presence of aneurysms and arteriovenous malformations.
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From page 141... ...
Turbulence, a form of chaotic and hence unpredictable motion, may also develop when blood flows rapidly through expanding vessels and changes in direction. Just as some situations produce a tendency to decrease the MRI signal, "three independent factors also result in increased signal intensity," wrote Bradley: "flow-related enhancement, even-echo rephasing, and diastolic pseudogating" (Stark and Bradley, 1988, p.
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From page 142... ...
Each of the diagnostic imaging modalities nuclear medicine, ultrasound, x-ray-based techniqueshas its own family of dedicated contrast media." The contrast agents used in MRI in certain situations can improve accuracy, reduce the number of possible diagnostic procedures, or shorten the course (and therefore the cost) of an MRI sequence.
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From page 143... ...
Gadolinium, which has a magnetic effect orders of magnitude larger than the effect of hydrogen nuclei on one another, provides a "built-in amplification sten" said Stark, and can significantly shorten 1~ and T2 relaxation times, To more than T2. Since "tissue To relaxation is inherently slow compared with T2 relaxation, the predominant effect of paramagnetic contrast agents is on To," explained Stark.
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From page 144... ...
Therefore, even a single displaced electron will alter a target atom sufficiently to produce a slightly different resonant frequency, referred to as the chemical shift. Such a change in the atomic configuration will also alter the local magnetic neighborhood, so to speak, though the change in magnetism is very small compared to the power of the gradient field used in standard MRI.
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From page 145... ...
Bydder's latest research interest involves imaging the movement in viva of anisotropically weighted water, what he described as "diffusion-weighted imaging," a very novel approach that Bydder nonetheless relates to the field: "Ideas in magnetic resonance have come from a lot of different sources, and this is another one that has actually been borrowed from spectroscopy." Bydder credited Michael Mosley from the University of California, San Francisco, with the important work necessary to turn the idea into a clinical application. The motion of the water molecules due to diffusion causes signal loss that allows MRI to differentiate particular tissues.
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From page 146... ...
· Fast scanning techniques. Because the utility of MRI in part depends on its ability to pay its way, faster scan times could lead to more images at a lower cost.
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From page 147... ...
A host of oral and intravenous contrast agents are being developed and tested. · Clinical efficacy.
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From page 148... ...
1987. Magnetic Resonance Imaging of the Knee.
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