F.W. Hersman

Human pulmonary imaging and spectroscopy with hyperpolarized 129Xe at 0.2T.

RATIONALE AND OBJECTIVES Using a novel (129)Xe polarizer with high throughput (1-2 L/hour) and high polarization (approximately 55%), our objective was to demonstrate and characterize human pulmonary applications at 0.2T. Specifically, we investigated the ability of (129)Xe to measure the alveolar surface area per unit volume of gas, S(A)/V(gas). MATERIALS AND METHODS Variable spin echo time (TE) gradient and radiofrequency (RF) echoes were used to obtain estimates of the lungs contribution to both T(2)* and T(2). Standard multislice ventilation images were obtained and signal-to-noise ratio (SNR) determined. Whole-lung, time-dependent measurements of (129)Xe diffusion from gas to septal tissue were obtained with a chemical shift saturation recovery (CSSR) method. Four healthy subjects were studied, and the Butler et al CSSR formalism (J Phys Condensed Matter 2002; 14:L297-L304) was used to calculate S(A)/V(gas). A single-breath version of the xenon transfer contrast (SB-XTC) method was implemented and used to image (129)Xe diffusion between alveolar gas and septal tissue. A direct comparison of CSSR and SB-XTC was performed. RESULTS T(2)*=135+/-29 ms amd T(2)=326.2+/-9.5 ms. Maximum SNR=36 for ventilation images from inhalation of 1L 86% (129)Xe and voxel volume =0.225 mL. CSSR analysis showed S(A)/V(gas) decreased with increasing lung volume in a manner very similar to that observed from histology measurements; however, the absolute value of S(A)/V(gas) was approximately 40% smaller than histology values. SB-XTC images in different postures demonstrate gravitationally dependent values. Initial comparison of CSSR with XTC showed fairly good agreement with expected ratios. CONCLUSIONS Hyperpolarized (129)Xe human imaging and spectroscopy are very promising methods to provide functional information about the lung.

Hyperpolarized Xenon-129 gas-exchange imaging of lung microstructure: First case studies in subjects with obstructive lung disease

To develop and test a method to noninvasively assess the functional lung microstructure.

3He lung imaging in an open access, very-low-field human magnetic resonance imaging system

The human lung and its functions are extremely sensitive to gravity; however, the conventional high‐field magnets used for most laser‐polarized 3He MRI of the human lung restrict subjects to lying horizontally. Imaging of human lungs using inhaled laser‐polarized 3He gas is demonstrated in an open‐access very‐low‐magnetic‐field (<5 mT) MRI instrument. This prototype device employs a simple, low‐cost electromagnet, with an open geometry that allows variation of the orientation of the imaging subject in a two‐dimensional plane. As a demonstration, two‐dimensional lung images were acquired with 4‐mm in‐plane resolution from a subject in two orientations: lying supine and sitting in a vertical position with one arm raised. Experience with this prototype device will guide optimization of a second‐generation very‐low‐field imager to enable studies of human pulmonary physiology as a function of subject orientation. Magn Reson Med 53:745–749, 2005.

Power scaling of a wavelength-narrowed diode laser system for pumping alkali vapors

We report a method for locking the output wavelength and reducing the spectral linewidth of diode lasers by feeding back light to the emitters from a wavelength selective external optical cavity. Ten years ago our team developed a stepped-mirror that allowed a single external cavity to lock the wavelength of a stack of diode array bars by equalizing path lengths between each emitter and the grating. Here we report combining one such step-mirror external cavity with an array of power dividers, each sending a portion of this feedback power to a separate diode array bar stack.

Scalable pump source for diode pumped alkali laser

External cavity diode laser systems are well-suited for diode pumped alkali laser (DPAL) systems due to their high power efficiency and excellent wavelength control under changing thermal loads. By conditioning the characteristics of feedback power, external cavities can narrow the spectral bandwidth and limit transverse modes of diode laser bars. Existing configurations typically use low-efficiency diffraction gratings at the Littrow angle to send back to the diodes a small fraction of the power, while directing the majority of the power forward in the output beam. We previously reported that a stepped mirror allows a single external cavity to condition the output beams of a stack of diode array bars. In this report, we describe a new approach that could use a single external cavity to condition the output beams of several hundred diode array bars. A high efficiency grating is used to feedback essentially all the power in the external cavity, and power splitters then distribute the power to multiple diode array stacks. A 384 bar module capable of 20 kW power output into a modelimited slowly diverging beam with a spectral width below 0.050 nm has been designed and proposed for use in a DPAL. A 50 bar 3 kW prototype is currently being assembled.