Diffusion weighted hyperpolarized 129 Xe MRI of the lung with 2D and 3D (FLORET) spiral

PURPOSE

To enable efficient hyperpolarized ¹²⁹ Xe diffusion imaging using 2D and 3D (Fermat Looped, ORthogonally Encoded Trajectories, FLORET) spiral sequences and demonstrate that ¹²⁹ Xe ADCs obtained using these sequences are comparable to those obtained using a conventional, 2D gradient-recalled echo (GRE) sequence.

THEORY AND METHODS

Diffusion-weighted ¹²⁹ Xe MRI (b-values = 0, 7.5, 15 s/cm² ) was performed in four healthy volunteers and one subject with lymphangioleiomyomatosis using slice-selective 2D-GRE (scan time = 15 s), slice-selective 2D-Spiral (4 s), and 3D-FLORET (16 s) sequences. Experimental SNRs from b-value = 0 images ( SNR 0 EX $$ SNR{0}_{EX} $$ ) and mean ADC values were compared across sequences. In two healthy subjects, a second b = 0 image was acquired using the 2D-Spiral sequence to map flip angle and correct RF-induced, hyperpolarized signal decay at the voxel level, thus improving regional ADC estimates.

RESULTS

Diffusion-weighted images from spiral sequences displayed image quality comparable to 2D-GRE and produced sufficient SNR 0 EX $$ SNR{0}_{EX} $$ (16.8 ± 3.8 for 2D-GRE, 21.2 ± 3.5 for 2D-Spiral, 20.4 ± 3.5 for FLORET) to accurately calculate ADC. Whole-lung means and SDs of ADC obtained via spiral were not significantly different (P > 0.54) from those obtained via 2D-GRE. Finally, 2D-Spiral images were corrected for signal decay, which resulted in a whole-lung mean ADC decrease of ˜15%, relative to uncorrected images.

CONCLUSIONS

Relative to GRE, efficient spiral sequences allow ¹²⁹ Xe diffusion images to be acquired with isotropic lung coverage (3D), higher SNR $$ SNR $$ (2D and 3D), and three-fold faster (2D) within a single breath-hold. In turn, shortened breath-holds enable flip-angle mapping, and thus, allow RF-induced signal decay to be corrected, increasing ADC accuracy.

An approach to evaluation of the point-spread function for 23 Na magnetic resonance imaging

Despite the technical challenges that require lengthy acquisitions to overcome poor signal-to-noise ratio (SNR), sodium (²³ Na) magnetic resonance imaging (MRI) is an intriguing area of research due to its essential role in human metabolism. Low SNR images can impact the measurement of the point-spread function (PSF) by adding uncertainty into the resulting quantities. Here, we present methods to calculate the PSF by using the modulation transfer function (MTF), and a 3D-printed line-pair phantom in the context of ²³ Na MRI. A simulation study investigated the effect of noise on the resulting MTF curves, which were derived by direct modulation (DM) and a method utilizing Fourier harmonics (FHs). Experimental data utilized a line-pair phantom with nine spatial frequencies, filled with different concentrations (15, 30, and 60 mM) of sodium in 3% agar. MTF curves were calculated using both methods from data acquired from density-adapted 3D radial projections (DA-3DRP) and Fermat looped orthogonally encoded trajectories (FLORET). Simulations indicated that the DM method increased variability in the MTF curves at all tested noise levels over the FH method. For the experimental data, the FH method resulted in PSFs with a narrower full width half maximum with reduced variability, although the improvement in variability was not as pronounced as predicted by simulations. The DA-3DRP data indicated an improvement in the PSF over FLORET. It was concluded that a 3D-printed line-pair phantom represents a convenient method to measure the PSF experimentally. The MTFs from the noisy images in ²³ Na MRI have reduced variability from a FH method over DM.

Improved pulmonary 129 Xe ventilation imaging via 3D-spiral UTE MRI

PURPOSE

Hyperpolarized ¹²⁹ Xe MRI characterizes regional lung ventilation in a variety of disease populations, with high sensitivity to airway obstruction in early disease. However, ventilation images are usually limited to a single breath-hold and most-often acquired using gradient-recalled echo sequences with thick slices (~10-15 mm), which increases partial-volume effects, limits ability to observe small defects, and suffers from imperfect slice selection. We demonstrate higher-resolution ventilation images, in shorter breath-holds, using FLORET (Fermat Looped ORthogonally Encoded Trajectories), a center-out 3D-spiral UTE sequence.

METHODS

In vivo human adult (N = 4; 2 healthy, 2 with cystic fibrosis) ¹²⁹ Xe images were acquired using 2D gradient-recalled echo, 3D radial, and FLORET. Each sequence was acquired at its highest possible resolution within a 16-second breath-hold with a minimum voxel dimension of 3 mm. Images were compared using ¹²⁹ Xe ventilation defect percentage, SNR, similarity coefficients, and vasculature cross-sections.

RESULTS

The FLORET sequence obtained relative normalized SNR, 40% greater than 2D gradient-recalled echo (P = .012) and 26% greater than 3D radial (P = .067). Moreover, the FLORET images were acquired with 3-fold-higher nominal resolution in a 15% shorter breath-hold. Finally, vasculature was less prominent in FLORET, likely due to diminished susceptibility-induced dephasing at shorter TEs afforded by UTE sequences.

CONCLUSION

The FLORET sequence yields higher SNR for a given resolution with a shorter breath-hold than traditional ventilation imaging techniques. This sequence more accurately measures ventilation abnormalities and enables reduced scan times in patients with poor compliance and severe lung disease.

Implementation of the FLORET UTE sequence for lung imaging

PURPOSE

Magnetic resonance imaging of lungs is inherently challenging, but it has become more common with the use of UTE sequences and their relative insensitivity to motion. Spiral UTE sequences have been touted recently as having greater k-space sampling efficiencies than radial UTE, but few are designed for the shorter T2 * of the lung. In this study, FLORET (Fermat looped, orthogonally encoded trajectories), a recently developed spiral 3D-UTE sequence designed for the short T2 * species, was implemented in human lungs for the first time and the images were compared with traditional radial UTE images.

METHODS

The FLORET sequence was implemented with parameters optimized for lung imaging on healthy and diseased (cystic fibrosis) subjects. On healthy subjects, radial UTE images (3D-radial and 2D-radial with phase encoding) were acquired for comparison to FLORET. Various metrics including SNR, vasculature contrast, diaphragm sharpness, and parenchymal density ratios were acquired and compared among the separate UTE sequences.

RESULTS

The FLORET sequence performed similarly to traditional radial UTE methods with a much shorter total scan time for fully sampled images (FLORET: 1 minute 55 seconds, 3D-radial: 3 minutes 25 seconds, 2D-radial with phase encoding: 7 minutes 22 seconds). Additionally, the FLORET image obtained on the cystic fibrosis subject resulted in the observation of cystic fibrosis lung pathology similar or superior to that of the other UTE-MRI techniques.

CONCLUSION

The FLORET sequence allows for faster acquisition of high diagnostic-quality lung images and its short T2 * components without sacrificing SNR, image quality, or tissue/disease quantification.

Three-dimensional ultrashort echo-time imaging using a FLORET trajectory

PURPOSE

Three-dimensional ultrashort echo-time (UTE) imaging commonly makes use of an isotropic 3D radial projection acquisition. The FLORET sequence is proposed and evaluated as a more efficient alternative.

METHODS

The properties of the FLORET trajectory are contrasted with those of a 3D radial projection trajectory. The theoretical advantages of FLORET, including greater sampling and SNR efficiency, are evaluated based upon experimental data. The effect of T2* decay on FLORET is analyzed in comparison to the 3D radial, Cones, and Density Adapted Radial trajectories. FLORET UTE image quality is compared with 3D radial UTE image quality.

RESULTS

FLORET is shown to have several advantages over 3D radial acquisitions with respect to image quality, scan time, signal-to-noise, and off-resonance blurring for UTE data. The signal and resolution losses from T2* decay for a FLORET acquisition are shown to be comparable to those of Density Adapted Radial and Density Compensated Cones trajectories.

CONCLUSION

The FLORET sequence is recommended as an alternative to 3D radial projection sequences for musculoskeletal UTE imaging as well as other UTE applications that accommodate modest to long per shot sampling times. FLORET is not recommended for imaging extremely short T₂ species such as dentin. Magn Reson Med 78:1038-1049, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

A flexible nested sodium and proton coil array with wideband matching for knee cartilage MRI at 3T

PURPOSE

We describe a 2 × 6 channel sodium/proton array for knee MRI at 3T. Multielement coil arrays are desirable because of well-known signal-to-noise ratio advantages over volume and single-element coils. However, low tissue-coil coupling that is characteristic of coils operating at low frequency can make the potential gains from a phased array difficult to realize.

METHODS

The issue of low tissue-coil coupling in the developed six-channel sodium receive array was addressed by implementing 1) a mechanically flexible former to minimize the coil-to-tissue distance and reduce the overall diameter of the array and 2) a wideband matching scheme that counteracts preamplifier noise degradation caused by coil coupling and a high-quality factor. The sodium array was complemented with a nested proton array to enable standard MRI.

RESULTS

The wideband matching scheme and tight-fitting mechanical design contributed to >30% central signal-to-noise ratio gain on the sodium module over a mononuclear sodium birdcage coil, and the performance of the proton module was sufficient for clinical imaging.

CONCLUSION

We expect the strategies presented in this study to be generally relevant in high-density receive arrays, particularly in x-nuclei or small animal applications. Magn Reson Med 76:1325-1334, 2016. © 2015 Wiley Periodicals, Inc.