Optical polarization of 3 He generates large nuclear spin magnetizations, allowing MR imaging of the gas spaces of human and animal lungs despite the low number density of spins in the gas. The atomic physics of optical pumping and spin exchange is explained; the hardware for polarizing, transporting, and imaging is detailed. Pulse sequences for optimum use of the nonrenewable magnetization require a different imaging strategy than traditional proton MRI. Examples are discussed for static lung images, diffusion images for characterization of the local alveolar structure, and high temporal resolution images.

Brain functional magnetic resonance imaging (FMRI) is possible because of local changes in blood flow and oxygenation levels. Understanding the role of each mechanism is important for interpreting FMRI results as well as for the design of the experiment itself. In this review, we address the role of flow and blood oxygen level dependence (BOLD) and how they can be used in conjunction with each other to enhance the BOLD effect. We also discuss the role of intra/extravascular signal changes in the presence of a vessel for the imaging situation and show how the ratio of these two contributions is likely to change as the blood volume fraction changes. Using the BOLD model itself, along with 3-D phase imaging, we show that blood oxygenation level in veins in vivo is Y =0.544*0.029 as measured in 14 vessels in five subjects. Finally, we comment on the use of phase images themselves as a means to discriminate tissues with different levels of blood such as gray matter, white matter, and cerebrospinal fluid. 0 1995 John Wiley & Sons, lnc.

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Rapid repetitive MRI of hyperpolarized (HP) gases using echoplanar imaging (EPI) has been theoretically investigated and experimentally demonstrated for 3 He in human lung. A quantitative treatment of signal attenuation and magnetization consumption for the unique circumstance of a rapidly diffusing nonrenewable magnetization source has been performed. Rapid (compared to the human respiratory cycle) and repetitive imaging of the lung gas space with EPI and a single delivered bolus of HP-3 He is feasible using low flip angles, provided the voxels are not too small. A coarse-grid (32 ؋ 64) EPI pulse sequence has been developed and implemented to image the lungs of healthy volunteers during rebreathing of a HP-3 He/N 2 gas mixture. A set of three 10-mm axial slices was imaged every 0.12 sec for the 36 sec duration of rebreathing, yielding a real-time visualization of ventilation. Despite some mild artifacts, the images are of good quality and show changes in gas density related to respiratory physiology. Magn Reson Med 42:507-514,

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