New York, Igaku-Shoin, 1981, pp 101-127. 7. Crooks LE: Overview of NMR imaging techniques, chap 3,. In Kaufman L, Crooks LE, Margulis AR (Eds): Nuclear ...
Medical Progress
Refer to: Davis PL, Crooks LE, Margulis AR, et al: Nuclear magnetic resonance imnaging: Current capabilities (Medical Progress). West J Med 1982 Oct; 137:290-293
Nuclear Magnetic Resonance Imaging: Current Capabilities PETER L. DAVIS, MD; LAWRENCE E. CROOKS, PhD; ALEXANDER R. MARGULIS, MD, and LEON KAUFMAN, PhD, San Francisco
Nuclear magnetic resonance imaging can produce tomographic images of the body without ionizing radiation. Images of the head, chest, abdomen, pelvis and extremities have been obtained and normal structures and pathology have been identified. Soft tissue contrast with this method is superior to that with x-ray computerized tomography and its spatial resolution is approaching that of x-ray computerized tomography. In addition, nuclear magnetic resonance imaging enables us to image along the sagittal and coronal planes directly, and it does not produce obscuring artifacts by bone.
NOT SINCE THE INTRODUCTION of x-rays into
medicine has there been so much excitement as with the realization that imaging by nuclear magnetic resonance is possible. A presentation on nuclear magnetic resonance (NMR) to a medical audience packs an auditorium. Every major x-ray company and other companies in the business of medical imaging have major investments in programs to develop nuclear magnetic resonance equipment. In addition, almost every university in this country has either a program or an agreement with some major company to test their equipnment and to do research in nuclear magnetic resonance medical imaging. Although equipment costs range between $800,000 and $1.5 million, multiple orders have already been issued for such purchase.
Physics The principle of nuclear magnetic resonance is based on the property of nuclei that have an odd number of protons or neutrons, or both, to behave From the Radiologic Imaging Laboratory, Department of Radiology, University of California, San Francisco. Reprint requests to: Peter L. Davis, MD, UCSF-RIL, 400 Grandview Drive, South San Francisco, CA 94080.
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as magnets.' 2 In the absence of a magnetic field, these nuclei will rotate with their axes pointing in random directions. When they encounter a strong magnetic field the nuclei will tend to align themselves with that field. Hydrogen is generally used for nuclear magnetic resonance imaging as it is the most sensitive to NMR, is ubiquitous and is abundant in the body. If weak radio waves of the proper frequency are applied to nuclei aligned in a strong magnetic field, they will flip and align themselves against the field. In a uniform magnetic field, as the nuclei flip back to their original alignment they will emit a radio signal that has the same frequency as the one whose energy was absorbed. The frequency of the radio wave that causes resonance is specific for each element and is proportional to the strength of the magnetic field. In a field of 3.5 kilogauss, hydrogen will resonate at 15 MHz. Because the frequency of the radio waves emitted is proportional to the magnetic field strength, if the field is made nonuniform in a known manner, then the frequency of the emitted radio waves will be inhomogeneous and the frequency information locates where the signal originated. For example, the frequency of
NUCLEAR MAGNETIC RESONANCE IMAGING ABBREVIATIONS USED IN TEXT CT= computerized tomography NMR = nuclear magnetic resonance
radio signals emitted by nuclei in a strong magnetic field will be higher than the frequency of radio signals emitted by nuclei in a weak magnetic field. By varying the magnetic field in all three dimensions it is possible to map the distribution of hydrogen through the substance being imaged. In addition, the strength of the signal is proportional to the amount of hydrogen present in that location. The characteristics of the NMR radio signal are not only a reflection of the amount of hydrogen present but also depend on Ti and T2, the socalled relaxation times. These depend on local physical and chemical factors such as molecular structure, temperature, viscosity and elemental composition, among others. These will affect the rate at which nuclei will align with the external magnetic field (Ti) and the rate at which nuclear energy emission decays (T2) as the nuclei return to their original alignment. Tl and T2 are constants for different tissues. Pure liquids align at a slower rate and emit for a longer time than liquids containing proteins or emulsified fat. Although Ti and T2 are constant for various tissues, their imaging differences can be enhanced. Varying the interval between successive excitations will selectively enhance tissues according to Ti. If the interval between successive excitations is short, tissues with a longer TI will emit a weaker signal than tissues with a shorter Ti. Varying the time interval between nuclear excitation and the recording of the signal will selectively enhance tissues according to T2. Those tissues that have long T2 times will provide larger signals than those tissues that have a short T2. In addition to the relaxation measurements, it is also possible to measure flow in blood vessels.3 Because it takes about 30 ms to acquire the signal from hydrogen nuclei in the volume being imaged, if the nuclei move through the volume quickly no signal will be recorded. If the velocity of the hydrogen nuclei is slower, the signal will be correspondingly stronger. Human NMR Imaging Human .NMR imaging equipment has been in operation for more than two years. A large variety
Figure 1.-Transverse NMR pulse echo image through the head of a patient with an ischemic infarct. The gray scale is such that fat appears most intense, other soft tissues intermediate, cerebrospinal fluid darker and air and bone the darkest. On the right are Ti and T2 time images. These are calculated images wherein the brighter the picture element, the longer the relaxation time. The infarct has an abnormally long T2 time.
of normal and abnormal images have been published. When looking at these images, we quickly become aware that the gray scales are not consistent among the images produced by different investigators or even within the same group of investigators.4-6 This results from the fact that there are several NMR imaging techniques and each technique combines the hydrogen density, Tl and T2 components in different proportions and relations.5'7 In addition, depending on which technique is used, imaging time can vary from 1 to 30 minutes. Further experience will be needed to determine which techniques are more useful under what conditions. Extensive imaging of the head has been carried out.4'6'8-'0 NMR images of the brain show spatial resolution approaching that of computerized tomography (CT) and much more soft tissue contrast than with CT. Gray and white matter can be easily separated. The extent of this improved contrast has already been shown in a survey of ten patients with multiple sclerosis.' On the average, the use of NMR has provided detection of almost seven times as many lesions as the use of CT has. NMR has also proved useful for imaging the posterior fossa.4"2 CT images are limited here by artifacts of the skull. These artifacts do not occur with NMR as very little signal is obtained from the skull. In fact, tumors, hematomas and vascular abnormalities have also been imaged in the head (Figures 1, 2). NMR imaging of the chest has shown several THE WESTERN JOURNAL OF MEDICINE
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types of lesions.6 Masses in the lung can be identified. The extent of tumors infiltrating the mediastinum can be determined because on NMR images a tumor can be easily separated from the great vessels inasmuch as flowing blood usually provides a weaker NMR signal than solid tissues (Figure 3). Esophageal tumors have also been
identified.'3 Cardiac imaging has been surprising. One would suppose that the heart would move too fast to be imaged, but we have been able to obtain satisfactory images of the heart.5"4 In addition, if part of the myocardium moves slower than its surrounds due to ischemia or infarction, it will produce a stronger signal and a brighter image. Using gating techniques NMR images of the heart have been obtained throughout the cardiac cycle.
Figure 2.-Transverse NMR pulse echo image through the neck of a patient with squamous cell carcinoma of the right side of the neck. The tumor can be seen just adjacent to the right common carotid artery and internal jugular vein, which appear as dark lumens.
Figure 3.-Transverse NMR pulse echo image tnrougn the chest of a patient with a tumor just left of the mediastinum. The lumens of the great vessels can be seen.
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Abdominal NMR imaging continues to improve at a rapid rate.4-6 5"6 Spatial resolutions of 2 by 2 mm are currently reported (Figure 4), and even better resolution is expected. Again the tissue contrast is superior to that with the use of CT. Normal anatomic structures are readily recognized. Several groups have imaged the liver and other abdominal organs and have published examples of hepatomas, hepatic metastasis, hepatic infarct, steatosis, cirrhosis, Wilson's disease and dilated bile ducts.617 For some diseases NMR provided a better image, for some CT was better and for some both were equally useful. Other diseases imaged include pancreatitis, pancreatic carcinomas and adrenal tumors. NMR images of the kidneys visualize the cortex and medulla.4'5 In a patient with chronic glomerulonephritis, the cortex was noted to be virtually absent in the shrunken kidneys.5 Bone is not well seen by NMR because it is relatively hydrogen poor. The marrow cavities usually contain fat, however, which images quite brightly. So whereas bone cannot be seen, it is usually well defined by the surrounding soft tissues and bone marrow. In fact, in patients with giant cell tumors, the NMR images have shown expansion of the marrow cavity, cortical thinning and replacement of the marrow by tumor.'8 Prospects In addition to images, NMR provides other information such as Ti and T2 relaxation times. Several groups have shown in vivo in humans and in animals that many pathologic diseases such as
Figure 4.-Transverse NMR pulse echo image through the lower abdomen. The right kidney, muscle planes, aorta and inferior vena cava can be seen, as can the marrow within the vertebral body. Peristalsis blurs the small bowel but does not create artifacts.
NUCLEAR MAGNETIC RESONANCE IMAGING
tumors, abscesses and hematomas have Ti relaxation times different from normal tissues.6"17'1920 In rats it has been shown that the T2 times, along with the Ti times, improve tissue differentiation.' 920 Further investigations in humans will be required to determine whether the relaxation times can be used for human tissue characterization. Because NMR can image and differentiate fat from other types of soft tissues and flowing blood, we expect that atherosclerotic plaques and other abnormalities of blood vessels can be imaged and studied. Such plaques have already been imaged in excised vessels.3 Although NMR innately provides good soft tissue contrast, several groups are already developing contrast agents, and two are already in use. Mineral oil has been used to increase the NMR signal from the bowel and oxygen has been used to change the signal from the heart.5'2' Because it is difficult to increase the hydrogen content of most tissues, most contrast agents will be paramagnetic compounds that will instead affect the Ti and T2 times of the hydrogen already present. It is possible that in the future other NMRsensitive nuclei such as phosphorus will be imaged in vivo.22 These other nuclei are less abundant, however, and less sensitive than hydrogen and therefore will not be capable of the resolution of contrast attainable with hydrogen. REFERENCES 1. Bradley WG, Tosteson H: Basic principles of NMR, chap 2, In Kaufman L, Crooks LE, Margulis AR (Eds): Nuclear Magnetic Resonance Imaging in Medicine. New York, Igaku-Shoin, 1981, pp 11-29 2. Gore JC, Emery EW, Orr JS, et al: Medical nuclear magnetic resonance imaging-I. Physical principles. Invest Radiol 1981 Jul; 16:269-274 3. Kaufman L, Crooks LE, Sheldon PE, et al: Evaluation of NMR imaging for detection and quantification of obstructions in vessels. Invest Radiol, in press
4. Crooks L, Arakawa M, Hoenninger J, et al: NMR whole body imager operating at 3.5 KGauss. Radiology 1982 Apr; 143: 169-174 5. Young IR, Bailes DR, Burl M, et al: Initial clinical evaluation of a whole body nuclear magnetic resonance (NMR) tomograph. J Comput Assist Tomogr 1982 Feb; 6:1-18 6. Hutchison JMS, Smith FW: Human NMR imaging, chap 6, In Kaufman L, Crooks LE, Margulis AR (Eds): Nuclear Magnetic Resonance Imaging in Medicine. New York, Igaku-Shoin, 1981, pp 101-127 7. Crooks LE: Overview of NMR imaging techniques, chap 3, In Kaufman L, Crooks LE, Margulis AR (Eds): Nuclear Magnetic Resonance Imaging in Medicine. New York, Igaku-Shoin, 1981, pp 30-52 8. Doyle FH, Gore JC, Pennock JM, et al: Imaging of the brain by nuclear magnetic resonance. Lancet 1981 Jul 11; 2:53-57 9. Holland GN, Hawkes RC, Moore WS: Nuclear magnetic resonance (NMR) tomography of the brain: Coronal and sagittal sections. J Comput Assist Tomogr 1980 Aug; 4:429-433 10. Hawkes RC, Holland GN, Moore WS, et al: Nuclear magnetic resonance (NMR) tomography of the brain: A preliminary clinic assessment with demonstration of pathology. J Comput Assist Tomogr 1980 Oct; 4:577-586 11. Young IR, Hall AS, Pallis CA, et al: Nuclear magnetic resonance imaging of the brain in multiple sclerosis. Lancet 1981 Nov 14; 2:1063-1066 12. Young IR, Burl M, Clarke GJ, et al: Magnetic resonance properties of hydrogen: Imaging the posterior fossa. AJR 1981 Nov; 137:895-901 13. Smith FW, Hutchison JMS, Mallard JR, et al: Oesophageal carcinoma demonstrated by whole-body nuclear magnetic resonance imaging. Br Med J [Clin Res] 1981 Feb 14; 282(6263): 510-512 14. Hawkes RC, Holland GN, Moore WS, et al: Nuclear magnetic resonance (NMR) tomography of the normal heart. J Comput Assist Tomogr 1981 Oct; 5:605-612 15. Hawkes RC, Holland GN, Moore WS, et al: Nuclear magnetic resonance (NMR) of the normal abdomen. J Comput Assist Tomogr 1981 Oct; 5:613-618 16. Edelstein WA, Hutchison JM, Smith FW, et al: Human whole-body NMR tomographic imaging: Normal sections. Br J Radiol 1981 Feb; 54(638):149-151 17. Doyle FH, Pennock JM, Banks LM, et al: Nuclear magnetic resonance imaging of the liver: Initial experience. AJR 1982 Feb; 138:193-200 18. Brady TJ, Gebhardt MC, Pykett IL, et al: Nuclear magnetic resonance imaging of upper extremities: Evaluation of normal appearance and appearance of giant cell tumors (Abstract). Presented at the Radiological Society of North America, 67th scientific assembly and annual meeting, Chicago, November 15-20, 1981 19. Herfkens RJ, Davis PL, Crooks LE, et al: NMR imaging of the abnormal live rat and correlation with tissue characteristics. Radiology 1981 Oct; 141:211-218 20. Davis PL, Sheldon P, Kaufman L, et al: Nuclear magnetic resonance imaging of mammary adenocarcinomas in the rat. Cancer, 1983, in press 21. Newhouse JH, Pykett IL, Brady TJ, et al: NMR scanning of the abdomen: Preliminary results in small animals, In Witcofski RL, Karstaedt N, Partain CT (Eds): NMR Imaging. Winston-Salem, NC, Bowman Gray School of Medicine of Wake Forest University, 1982, pp 121-124 22. Kramer DM: Imaging elements other than hydrogen, chap 9, In Kaufman L, Crooks LE, Margulis AR (Eds): Nuclear Magnetic Resonance Imaging in Medicine. New York, IgakuShoin, 1981, pp 184-203
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