The nuclear spin polarization of the noble gas isotopes 3 He and 129 Xe can be increased using optical pumping methods by four to five orders of magnitude. This extraordinary gain in polarization translates directly into a gain in signal strength for MRI. The new technology of hyperpolarized (HP) gas MRI holds enormous potential for enhancing sensitivity and contrast in pulmonary imaging. This review outlines the physics underlying the optical pumping process, imaging strategies coping with the nonequilibrium polarization, and effects of the alveolar microstructure on relaxation and diffusion of the noble gases. It presents recent progress in HP gas MRI and applications ranging from MR microscopy of airspaces to imaging pulmonary function in patients and suggests potential directions for future developments. MRI has been extremely successful at diagnosing soft tissue disease since its discovery in 1972 (1). However, MRI is not as sensitive in comparison with other biomedical imaging techniques, such as CT, positron-emission tomography, or single-photon emission computed tomography. This is a consequence of a very small signal from a small population difference between nuclear energy states. For a spin-1/2 system, the "nuclear spin polarization", P N , is defined as:where N ϩ and N Ϫ denote populations with magnetic spin quantum numbers ϩ1/2 and Ϫ1/2, respectively. Typically, the thermal energy of the sample at temperature T exceeds the energy difference between the nuclear spin states in a magnetic field B 0 by several orders of magnitude ("hightemperature approximation") and the equilibrium polarization can be written as:where ␥ is the magnetogyric ratio, ប is Planck's constant divided by 2 , and k B is Boltzmann's constant. As an example, P N,0 Ϸ 5 ppm is predicted with Eq.[2] for protons ( 1 H) at body temperature (T ϭ 37°C) and B 0 ϭ 1.5T. In view of the inherent sensitivity problem, increasing the signalto-noise ratio (SNR) has been a field of continuous research since the discovery of NMR. Recently, the use of optically polarized noble gas isotopes 3 He and 129 Xe has attracted increasing interest for use in a variety of promising MR applications. These systems exhibit polarizations exceeding the thermal levels by several orders of magnitude. While the use of such "hyperpolarized" (HP) gases for MRI is a recent development, it is based on a solid foundation of work in atomic physics. The groundwork was laid by Kastler (2) more than 50 years ago by demonstrating transfer of angular momentum from circularly polarized light to the electron and nuclear spins of atoms, a process called "optical pumping" (OP). Since 1991, exploitation of OP as a means of enhancing signal initiated the development of a novel field in NMR (3,4). Research involving HP noble gases has been exceptionally fruitful in biomedical MRI as well as providing applications for investigation of materials (5-8).In the context of proton MRI, the lung is a particularly challenging area to study (9). Even at end expiration, the overall density is ...
Vascular malformations and tumors comprise a wide, heterogeneous spectrum of lesions that often represent a diagnostic and therapeutic challenge. Frequent use of an inaccurate nomenclature has led to considerable confusion. Since the treatment strategy depends on the type of vascular anomaly, correct diagnosis and classification are crucial. Magnetic resonance (MR) imaging is the most valuable modality for classification of vascular anomalies because it accurately demonstrates their extension and their anatomic relationship to adjacent structures. A comprehensive assessment of vascular anomalies requires functional analysis of the involved vessels. Dynamic time-resolved contrast material-enhanced MR angiography provides information about the hemodynamics of vascular anomalies and allows differentiation of high-flow and low-flow vascular malformations. Furthermore, MR imaging is useful in assessment of treatment success and establishment of a long-term management strategy. Radiologists should be familiar with the clinical and MR imaging features that aid in diagnosis of vascular anomalies and their proper classification. Furthermore, they should be familiar with MR imaging protocols optimized for evaluation of vascular anomalies and with their posttreatment appearances. Supplemental material available at http://radiographics.rsna.org/lookup/suppl/doi:10.1148/rg.315105213/-/DC1.
Sarcoidosis is a multisystem disease characterized by noncaseating granulomas in the affected organs, including skin, heart, nervous system, and joints. Diagnosis of sarcoidosis is generally based upon a compatible history, demonstration of granulomas in at least two different organs, negative staining and culture for acid fast bacilli, absence of occupational or domestic exposure to toxins, and lack of drug-induced disease. Involvement of the hollow organs is rare. Rather than being due to sarcoidosis, some reported mucosal lesions may simply have incidental granulomas. Extrinsic compression from lymphadenopathy can occur throughout the gastrointestinal tract. The stomach, particularly the antrum, is the most common extrahepatic organ to be involved, while the small bowel is the least common. Liver involvement frequently occurs and ranges from asymptomatic incidental granulomas to portal hypertension from granulomas in the portal triad, usually with relatively preserved liver function. CT scans show hepatosplenomegaly and adenopathy, followed in frequency by focal low-attenuation lesions of the liver and spleen. Ascites is usually a transudate from right heart failure (because of pulmonary hypertension) or portal hypertension (because of biliary cirrhosis). Rarely, an exudative ascites may occur from studding of the peritoneum with nodules. Pancreatic involvement presents as a mass, usually in the head or a diffusely firm, nodular organ. Corticosteroids should be instituted when organ function is threatened, usually lungs, eyes, and central nervous system. Their role in the treatment of hepatic sarcoidosis is unclear. The overall prognosis is good although most patients will have some permanent organ impairment. Cardiac and pulmonary diseases are the main causes of death.
With the use of polarization-transfer pulse sequences and hyperpolarized 129 Xe NMR, gas exchange in the lung can be measured quantitatively. However, harnessing the inherently high sensitivity of this technique as a tool for exploring lung function requires a fundamental understanding of the xenon gas-exchange and diffusion processes in the lung, and how these may differ between healthy and pathological conditions. Toward this goal, we employed NMR spectroscopy and imaging techniques in animal models to investigate the dependence of the relative xenon gas exchange rate on the inflation level of the lung and the tissue density. The spectroscopic results indicate that gas exchange occurs on a time scale of milliseconds, with an average effective diffusion constant of about 3.3 ؋ 10 ؊6 cm 2 /s in the lung parenchyma. Polarization-transfer imaging pulse sequences, which were optimized based on the spectroscopic results, detected regionally increased gas-exchange rates in the lung, indicative of increased tissue density secondary to gravitational compression. By exploiting the gas-exchange process in the lung to encode physiologic parameters, these methods may be extended to noninvasive regional assessments of lung-tissue density and the alveolar surface-to-volume ratio, and allow lung pathology to be detected at an earlier stage than is currently possible.
One of the major goals of hyperpolarized‐gas MRI has been to obtain 129Xe dissolved‐phase images in humans. So far, this goal has remained elusive, mainly due to the low concentration of xenon that dissolves in tissue. A method is proposed and demonstrated in dogs that allows information about the dissolved phase to be obtained by imaging the gas phase following the application of a series of RF pulses that selectively destroy the longitudinal magnetization of xenon dissolved in the lung parenchyma. During the delay time between consecutive RF pulses, the depolarized xenon rapidly exchanges with the gas phase, thus lowering the gas polarization. It is demonstrated that the resulting contrast in the 129Xe gas image provides information about the local tissue density. It is further argued that minor pulse‐sequence modifications may provide information about the alveolar surface area or lung perfusion. Magn Reson Med 44:349–357, 2000. © 2000 Wiley‐Liss, Inc.
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