New Imaging Modalities for Evaluating Pulmonary Disease

Abstract

Several new imaging modalities that will be used clinically or in the research laboratory to study pulmonary diseases are being developed. These new techniques include single photon emission computed tomography (SPECT), positron emission tomography (PET), and nuclear magnetic resonance (NMR) imaging. These noninvasive techniques offer much promise for the development of quantitative metabolic, chemical, and blood flow studies of the pulmonary system.The early principles of tomographic imaging were developed for utilization in determining isotope distributions, 1 but the appropriate equations for quantitative studies were developed by Cormack 2 and applied by Hounsfield 3 to x-ray computed tomography. After x-ray computed tomography was developed, the new areas of PET and SPECT became possible. Although positron tomography has been discussed for many years, the development of the positron emission tomograph by Phelps, Ter-Pogossian, and Hoffman 4 permitted the field of positron tomography to develop. Positron emission tomography has been slow to develop in multiple centers owing to the necessity of having a cyclotron in addition to the positron tomograph. Furthermore, the chemistry for the development of appropriate radiopharmaceuticals has been developed only in the last few years. Since the chemistry has been simplified and smaller cyclotrons are becoming available to produce these radioisotopes, the techniques of positron tomography will be available to more centers in the future. PET studies can be used to gain additional information on pulmonary embolism, lung density changes, tumor metabolism, and possibly drug and hormone receptors and metabolism.Single photon emission computed tomography has developed along two lines: the scannerbased systems 5 and the camera-based systems. 6 An advantage of SPECT is that it can perform tomographic imaging of the radioisotopes available in nuclear radiology facilities. Studies with SPECT can be used to gain additional information in patients with pulmonary embolism and potentially to evaluate pulmonary metabolism and to quantitate pulmonary perfusion.The techniques of nuclear magnetic resonance have been utilized for many years by chemists. Recently these techniques have been applied to imaging of the body. 7 Although the initial studies have utilized information concerning hydrogen atoms (protons), a potential exists for doing quantitative metabolic studies such as phosphorus spectroscopy. 8 Nuclear magnetic resonance studies offer an advantage over the other techniques in that no ionizing radiation is utilized. There are no known effects from the large magnetic fields or the radiofrequency signals the patients are being exposed to. NMR will be applied to the determination of lung water and the evaluation and characterization of lung and mediastinal tumors.A discussion of the technical aspect of the three modalities is included in this review. Only a limited number of studies evaluating the pulmonary system have been performed with these new technologies. Many of the stud...