Dynamic radial projection MRI of inhaled hyperpolarized ³He gas

Abstract

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 ...

Design and evaluation of a 32‐channel phased‐array coil for lung imaging with hyperpolarized 3‐helium

“…11). Indeed, 3 He imaging with high temporal resolution was reported 10 years ago (29)(30)(31)(32), but temporal resolution was obtained at the expense of spatial resolution. Our results demonstrate dynamic imaging with a resolution (2.3 Â 2.5mm 2 ) that was previously not possible in combination with high temporal resolution (eight images per second).…”

Functional lung imaging using hyperpolarized gas MRI

“…Fast MRI acquisitions for dynamic imaging typically employ non-Cartesian k-space trajectories including spiral and radial acquisition with SPGR sequences (27,78). These dynamic studies use a single, thicker slice from 2 to 3 cm to improve temporal resolution, and a lower flip angle of 2 to 7°to preserve signal over the duration of the scan.…”

A system for open‐access 3He human lung imaging at very low field

“…3 He imaging of human lungs is now performed commonly in clinical MRI scanners to explore a variety of lung disorders (5) using techniques such as spin-density imaging (6), flow mapping (7,8), and diffusion imaging (9,10). Additional notable achievements include monitoring of 3 He relaxation as an indicator of the local alveolar O 2 concentration (11), which has recently been directly linked to the physiologically relevant ventilation/perfusion ratio (V/Q) (12).…”