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A radial projection sliding-window sequence has been developed for imaging the rapid flow of 3 He gas in human lungs. The short echo time (TE) of the radial sequence lends itself to fast repetition times, and thus allows a rapid update in the image when it is reconstructed with a sliding window. Oversampling in the radial direction combined with angular undersampling can further reduce the time needed to acquire a complete image data set, without significantly compromising spatial resolution. Controlled flow phantom experiments using hyperpolarized 3 He gas exemplify the temporal resolution of the method. In vivo studies on three healthy volunteers, one patient with chronic obstructive pulmonary disease (COPD), and one patient with hemiparalysis of the right diaphragm demonstrate that it is possible to accurately resolve the passage of gas down the trachea and bronchi and into the peripheral lung. Hyperpolarized 3 He gas MRI has been shown to be effective in visualizing breath-hold images of ventilation in humans (1). With optical pumping techniques, polarization levels far in excess of those attainable at thermal equilibrium in a B 0 field of 1.5 T can be attained. Because this polarization is not constrained by processes of saturation recovery, imaging with very fast repetition times (TRs) at a high signal-to-noise ratio (SNR) is a realistic prospect in vivo. This has enabled the dynamic study of gas inhalation with repetitive-frame fast imaging techniques. The study of ventilation dynamics may provide insights into lung pathophysiology, including air-trapping in chronic obstructive pulmonary disease (COPD). Previous dynamic studies performed on human subjects have used low flip angle, short-TR spin warp gradient-echo sequences (2,3); gradient-echo EPI (4); and, most recently, interleaved spiral sequences (5). Single-shot EPI might appear to be the logical way to rapidly monitor the passage of inhaled gas, because a flip angle of 90°will convert all the polarization to transverse magnetization in one shot, and subsequent signal can therefore be equated to fresh influx of gas. However, diffusion attenuation constrains the spatial resolution attainable with EPI (4), and field inhomogeneity can severely distort the images in the coronal and sagittal planes. Salerno et al. (5) developed a 24-interleave spiral sequence for dynamic 3 He imaging that offers good spatial resolution, is robust to motion and susceptibility effects, and provides repeated sampling of central and outer kspace per RF excitation (view). This last feature means that fluoroscopic sliding-window reconstruction techniques (6) can be effectively applied, as dynamic contrast changes pertaining to gas flow dynamics are updated on each view. In a study of guinea pig lung ventilation, Viallon et al. (7) presented a radial projection cine sequence. This was used to sample k-space in a continuously revolving pattern. When combined with a sliding-window reconstruction this produced high-quality images with a fast pseudotemporal image refresh rate. The ...

A method for 3D volume-localized quantification of pO2 in the lungs is presented that uses repetitive frame 3D gradient-echo imaging of 3 He. The method was demonstrated by experiments on 3 He phantoms containing known concentrations of O 2 and in vivo on a group of three healthy human volunteers. The results were compared with those obtained by equivalent 2D thin-slice and 2D projection methodologies, and were found to be consistent with published results from the 2D projection methodologies (pO 2 ‫؍‬ 0.09 -0.18 bar). Studies performed on the same subject, on three separate occasions, demonstrated a repeatability of pO 2 measurement to within 14% using the 3D technique. Experimental differences between the 2D and 3D methods were substantiated with theoretical and numerical analyses of the signal decay, which took into account the effects of out-of-slice diffusion as a source of error in the thin-slice 2D experiments. It is shown that the 2D thin-slice technique systematically underestimates pO2 when there is significant gas diffusion (factor of 4 underestimate for D ‫؍‬ 0.9 cm MRI of hyperpolarized (HP)3 He gas can provide high spatial and temporal resolution images of gas ventilation in the lungs and airways, as well as useful functional information (1). Potentially one of the most useful functional parameters is an estimate of the partial pressure of oxygen (pO 2 ) in the lungs and the rate of oxygen extraction (2-6). Accurate spatially localized pO 2 measurement with 3 He MRI would be clinically useful as a means of assessing lung ventilation-perfusion (V/Q). When paramagnetic oxygen mixes with polarized 3 He in the lungs, a marked reduction in the 3 He T 1 arises through the electron-nuclear spin dipolar coupling. Saam et al. (7) quantified the dependence of the 3 He T 1 on oxygen concentration with phantom experiments. This O 2 dependence of the 3 He T 1 was used by Deninger et al. (2,3) for in vivo quantification of pO 2 in a series of breath-hold experiments on pigs and humans. These in vivo experiments used images acquired from whole-lung projections (very thick 2D slices). This was done to maximize the signal-to-noise ratio (SNR) and to circumvent diffusion of polarized gas out of the slice during the significant inter-image delay time (typically up to 7 s) as a possible source of change in the time course of the in vivo 3 He signal. Moller et al. (5) estimated the degree of partial volume mixing of polarization due to gas diffusion for a 2D thin-slice experiment in the guinea pig lung. Assuming a 24 s total acquisition time and an apparent diffusion coefficient of D ϭ 0.16 cm 2 s -1 , they predicted that only 5% of the in-slice magnetization would be exchanged between adjacent slices. In addition, the recently published results by Jalali et al. (6) from 2D pO 2 experiments in porcine lungs, with 2-cm-thick slices, are in fair agreement with the results from a 2D porcine study with 18-cm-thick slices (2). These results suggest that diffusion between slices may not be a considerable problem; however, a...