Mapping and correcting hyperpolarized magnetization decay with radial keyhole imaging

Evaluation of an integrated variable flip angle protocol to estimate coil B1 for hyperpolarized MRI

PURPOSE

The purpose of this work is to validate a simple and versatile integrated variable flip angle (VFA) method for mapping B1 in hyperpolarized MRI, which can be used to correct signal variations due to coil inhomogeneity.

THEORY AND METHODS

Simulations were run to assess performance of the VFA B1 mapping method compared to the currently used constant flip angle (CFA) approach. Simulation results were used to inform the design of VFA sequences, validated in four volunteers for hyperpolarized xenon-129 imaging of the lungs and another four volunteers for hyperpolarized carbon-13 imaging of the human brain. B1 maps obtained were used to correct transmit and receive inhomogeneity in the images.

RESULTS

Simulations showed improved performance of the VFA approach over the CFA approach with reduced sensitivity to T1. For xenon-129, the B1 maps accurately reflected the variation of signal depolarization, but in some cases could not be used to correct for coil receive inhomogeneity due to a lack of transmit-receive reciprocity resulting from suboptimal coil positioning. For carbon-13, the B1 maps showed good agreement with a separately acquired B1 map of a phantom and were effectively used to correct coil-induced signal inhomogeneity.

CONCLUSION

A simple, versatile, and effective VFA B1 mapping method was implemented and evaluated. Inclusion of the B1 mapping method in hyperpolarized imaging studies can enable more robust signal quantification.

Analytical corrections for B1-inhomogeneity and signal decay in multi-slice 2D spiral hyperpolarized 129Xe MRI using keyhole reconstruction

PURPOSE

Hyperpolarized xenon MRI suffers from heterogeneous coil bias and magnetization decay that obscure pulmonary abnormalities. Non-physiological signal variability can be mitigated by measuring and mapping the nominal flip angle, and by rescaling the images to correct for signal bias and decay. While flip angle maps can be calculated from sequentially acquired images, scan time and breath-hold duration are doubled. Here, we exploit the low-frequency oversampling of 2D-spiral and keyhole reconstruction to measure flip angle maps from a single acquisition.

METHODS

Flip angle maps were calculated from two images generated from a single dataset using keyhole reconstructions and a Bloch-equation-based model suitable for hyperpolarized substances. Artifacts resulting from acquisition and reconstruction schemes (e.g., keyhole reconstruction radius, slice-selection profile, spiral-ordering, and oversampling) were assessed using point-spread functions. Simulated flip angle maps generated using keyhole reconstruction were compared against the paired-image approach using RMS error (RMSE). Finally, feasibility was demonstrated for in vivo xenon ventilation imaging.

RESULTS

Simulations demonstrated accurate flip angle maps and B1-inhomogeneity correction can be generated with only 1.25-fold central-oversampling and keyhole reconstruction radius = 5% (RMSE = 0.460°). These settings also generated accurate flip angle maps in a healthy control (RSME = 0.337°) and a person with cystic fibrosis (RMSE = 0.404°) in as little as 3.3 s.

CONCLUSION

Regional lung ventilation images with reduced impact of B1-inhomogeneity can be acquired rapidly by combining 2D-spiral acquisition, Bloch-equation-based modeling, and keyhole reconstruction. This approach will be especially useful for breath-hold studies where short scan durations are necessary, such as dynamic imaging and applications in children or people with severely compromised respiratory function.

B1 and magnetization decay correction for hyperpolarized 129 Xe lung imaging using sequential 2D spiral acquisitions

PURPOSE

To mitigate signal variations caused by inhomogeneous RF and magnetization decay in hyperpolarized 129 Xe ventilation images using flip-angle maps generated from sequential 2D spiral ventilation images acquired in a breath-hold. Images and correction maps were compared with those obtained using conventional, 2D gradient-recalled echo.

THEORY AND METHODS

Analytical expressions to predict signal intensity and uncertainty in flip-angle measurements were derived from the Bloch equations and validated by simulations and phantom experiments. Imaging in 129 Xe phantoms and human subjects (1 healthy, 1 cystic fibrosis) was performed using 2D gradient-recalled echo and spiral. For both sequences, consecutive images were acquired with the same slice position during a breath-hold (Cartesian scan time = 15 s; spiral scan time = 5 s). The ratio of these images was used to calculate flip-angle maps and correct intensity inhomogeneities in ventilation images.

RESULTS

Mean measured flip angle showed excellent agreement with the applied flip angle in simulations (R2  = 0.99) for both sequences. Mean measured flip angle agreed well with the globally applied flip angle (∼15% difference) in 129 Xe phantoms and in vivo imaging using both sequences. Corrected images displayed reduced coil-dependent signal nonuniformity relative to uncorrected images.

CONCLUSIONS

Flip-angle maps were obtained using sequentially acquired, 2D spiral, 129 Xe ventilation images. Signal intensity variations caused by RF-coil inhomogeneity can be corrected by acquiring sequential single-breath ventilation images in less than 5-s scan time. Thus, this method can be used to remove undesirable heterogeneity while preserving physiological effects on the signal distribution.

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.

Bias field correction in hyperpolarized 129 Xe ventilation MRI using templates derived by RF-depolarization mapping

PURPOSE

To correct for RF inhomogeneity for in vivo 129 Xe ventilation MRI using flip-angle mapping enabled by randomized 3D radial acquisitions. To extend this RF-depolarization mapping approach to create a flip-angle map template applicable to arbitrary acquisition strategies, and to compare these approaches to conventional bias field correction.

METHODS

RF-depolarization mapping was evaluated first in digital simulations and then in 51 subjects who had undergone radial 129 Xe ventilation MRI in the supine position at 3T (views = 3600; samples/view = 128; TR/TE = 4.5/0.45 ms; flip angle = 1.5; FOV = 40 cm). The images were corrected using newly developed RF-depolarization and templated-based methods and the resulting quantitative ventilation metrics (mean, coefficient of variation, and gradient) were compared to those resulting from N4ITK correction.

RESULTS

RF-depolarization and template-based mapping methods yielded a pattern of RF-inhomogeneity consistent with the expected variation based on coil architecture. The resulting corrected images were visually similar, but meaningfully distinct from those generated using standard N4ITK correction. The N4ITK algorithm eliminated the physiologically expected anterior-posterior gradient (-0.04 ± 1.56%/cm, P < 0.001). These 2 newly introduced methods of RF-depolarization and template correction retained the physiologically expected anterior-posterior ventilation gradient in healthy subjects (2.77 ± 2.09%/cm and 2.01 ± 2.73%/cm, respectively).

CONCLUSIONS

Randomized 3D 129 Xe MRI ventilation acquisitions can inherently be corrected for bias field, and this technique can be extended to create flip angle templates capable of correcting images from a given coil regardless of acquisition strategy. These methods may be more favorable than the de facto standard N4ITK because they can remove undesirable heterogeneity caused by RF effects while retaining results from known physiology.

Noninvasive diagnosis of pulmonary hypertension with hyperpolarised 129Xe magnetic resonance imaging and spectroscopy

Background

The diagnosis of pulmonary hypertension (PH) remains challenging. Pre- and post-capillary PH have different signatures on noninvasive ¹²⁹Xe gas-exchange magnetic resonance imaging (MRI) and dynamic MR spectroscopy (MRS). We tested the accuracy of ¹²⁹Xe MRI/MRS to diagnose PH status compared to right heart catheterisation (RHC).

Methods

¹²⁹Xe MRI/MRS from 93 subjects was used to develop a diagnostic algorithm, which was tested in 32 patients undergoing RHC on the same day (n=20) or within 5 months (42±40 days) (n=12). Three expert readers, blinded to RHC, used ¹²⁹Xe MRI/MRS to classify subjects as pre-capillary PH, post-capillary PH, no PH and no interstitial lung disease (ILD), or ILD.

Results

For pre-capillary PH, ¹²⁹Xe MRI/MRS diagnostic accuracy was 75% (95% CI 66–84) with a sensitivity of 67% (95% CI 54–79) and a specificity of 86% (95% CI 75–96); for post-capillary PH accuracy was 69% (95% CI 59–78) with sensitivity of 54% (95% CI 34–74) and specificity of 74% (95% CI 63–84). The model performed well in straightforward cases of pre-capillary PH but was less accurate in its diagnosis in the presence of mixed disease, particularly in the presence of ILD or combined post- and pre-capillary PH.

Conclusion

This study demonstrates the potential to develop ¹²⁹Xe MRI/MRS into a modality with good accuracy in detecting pre- and post-capillary PH. Furthermore, the combination of ¹²⁹Xe dynamic MRS and gas-exchange MRI uniquely provide concurrent, noninvasive assessment of both haemodynamics and gas-exchange impairment that may aid in the detection of PH.

Diagnostic models using ¹²⁹Xe MRI/MRS metrics can noninvasively detect pre-capillary PH, post-capillary PH and ILD. The combination of ¹²⁹Xe MRI/MRS provides a comprehensive assessment of haemodynamics and gas-exchange impairment in individual patients.https://bit.ly/3tDJw5P

Protocols for multi-site trials using hyperpolarized 129 Xe MRI for imaging of ventilation, alveolar-airspace size, and gas exchange: A position paper from the 129 Xe MRI clinical trials consortium

Hyperpolarized (HP) ¹²⁹ Xe MRI uniquely images pulmonary ventilation, gas exchange, and terminal airway morphology rapidly and safely, providing novel information not possible using conventional imaging modalities or pulmonary function tests. As such, there is mounting interest in expanding the use of biomarkers derived from HP ¹²⁹ Xe MRI as outcome measures in multi-site clinical trials across a range of pulmonary disorders. Until recently, HP ¹²⁹ Xe MRI techniques have been developed largely independently at a limited number of academic centers, without harmonizing acquisition strategies. To promote uniformity and adoption of HP ¹²⁹ Xe MRI more widely in translational research, multi-site trials, and ultimately clinical practice, this position paper from the ¹²⁹ Xe MRI Clinical Trials Consortium (https://cpir.cchmc.org/XeMRICTC) recommends standard protocols to harmonize methods for image acquisition in HP ¹²⁹ Xe MRI. Recommendations are described for the most common HP gas MRI techniques-calibration, ventilation, alveolar-airspace size, and gas exchange-across MRI scanner manufacturers most used for this application. Moreover, recommendations are described for ¹²⁹ Xe dose volumes and breath-hold standardization to further foster consistency of imaging studies. The intention is that sites with HP ¹²⁹ Xe MRI capabilities can readily implement these methods to obtain consistent high-quality images that provide regional insight into lung structure and function. While this document represents consensus at a snapshot in time, a roadmap for technical developments is provided that will further increase image quality and efficiency. These standardized dosing and imaging protocols will facilitate the wider adoption of HP ¹²⁹ Xe MRI for multi-site pulmonary research.

Improved preclinical hyperpolarized 129 Xe ventilation imaging with constant flip angle 3D radial golden means acquisition and keyhole reconstruction

Hyperpolarized (HP) 129 Xe MRI is increasingly used to noninvasively probe regional lung structure and function in the preclinical setting. As in human imaging, the primary barrier to quantitative imaging with HP gases is nonequilibrium magnetization, which is depleted by T1 relaxation and radio frequency excitation. Preclinical HP gas imaging commonly involves mechanically ventilating small animals and encoding k-space over tens or hundreds of breaths, with small subsets of k-space data collected within each breath. Breath-to-breath magnetization renewal enables the use of large flip angles, but the resulting magnetization decay generates large view-to-view differences in within-breath signal intensity, leading to artifacts and degraded image quality. This deleterious signal decay has motivated the use of variable flip angle (VFA) sampling schemes, in which the flip angle is progressively increased to maintain constant view-to-view signal intensity. However, VFA imaging complicates data acquisition and provides only a global correction that fails to compensate for regional differences in signal dynamics. When constant flip angle (CFA) imaging is used alongside 3D radial golden means acquisition, the center of k-space is sampled with every excitation, thereby encoding signal dynamics alongside imaging data. Here, keyhole reconstruction is used to generate multiple images to capture in-breath HP 129 Xe signal dynamics in mice and thus provide flip angle maps to quantitatively correct images without extra data collection. These CFA images display SNR that is not significantly different from VFA images, and further, high frequency k-space scaling can be used to mitigate decay-induced image artifacts. Results are supported by point spread function calculations and simulations of radial imaging with preclinical signal dynamics. Together, these results show that CFA 3D radial golden means ventilation imaging provides comparable image quality with VFA in small animals and allows for keyhole reconstruction, which can be used to generate flip angle maps and correct images for signal depletion.

In vivo methods and applications of xenon-129 magnetic resonance

Hyperpolarised gas lung MRI using xenon-129 can provide detailed 3D images of the ventilated lung airspaces, and can be applied to quantify lung microstructure and detailed aspects of lung function such as gas exchange. It is sensitive to functional and structural changes in early lung disease and can be used in longitudinal studies of disease progression and therapy response. The ability of 129Xe to dissolve into the blood stream and its chemical shift sensitivity to its local environment allow monitoring of gas exchange in the lungs, perfusion of the brain and kidneys, and blood oxygenation. This article reviews the methods and applications of in vivo129Xe MR in humans, with a focus on the physics of polarisation by optical pumping, radiofrequency coil and pulse sequence design, and the in vivo applications of 129Xe MRI and MRS to examine lung ventilation, microstructure and gas exchange, blood oxygenation, and perfusion of the brain and kidneys.

Generalized Linear Binning to Compare Hyperpolarized 129Xe Ventilation Maps Derived from 3D Radial Gas Exchange Versus Dedicated Multislice Gradient Echo MRI

RATIONALE

Hyperpolarized ¹²⁹Xe ventilation MRI is typically acquired using multislice fast gradient recalled echo (GRE), but interleaved 3D radial ¹²⁹Xe gas transfer MRI now provides dissolved-phase and ventilation images from a single breath. To investigate whether these ventilation images provide equivalent quantitative metrics, we introduce generalized linear binning analysis.

METHODS

This study included 36 patients who had undergone both multislice GRE ventilation and 3D radial gas exchange imaging. Images were then quantified by linear binning to classify voxels into one of four clusters: ventilation defect percentage (VDP), Low-, Medium- or High-ventilation percentage (LVP, MVP, HVP). For 3D radial images, linear binning thresholds were generalized using a Box-Cox rescaled reference histogram. We compared the cluster populations from the two ventilation acquisitions both numerically and spatially.

RESULTS

Interacquisition Bland-Altman limits of agreement for the clusters between 3D radial vs GRE were (-7% to 5%) for VDP, (-10% to 14%) for LVP, and (-8% to 8%) for HVP. While binning maps were qualitatively similar between acquisitions, their spatial overlap was modest for VDP (Dice = 0.5 ± 0.2), and relatively poor for LVP (0.3 ± 0.1) and HVP (0.2 ± 0.1).

CONCLUSION

Both acquisitions yield reasonably concordant VDP and qualitatively similar maps. However, poor regional agreement (Dice) suggests that the two acquisitions cannot yet be used interchangeably. However, further improvements in 3D radial resolution and reconciliation of bias field correction may well obviate the need for a dedicated ventilation scan in many cases.

Preclinical hyperpolarized 129 Xe MRI: ventilation and T2 * mapping in mouse lungs at 7 T using multi-echo flyback UTE

Fast apparent transverse relaxation (short T2 *) is a common obstacle when attempting to perform quantitative 1 H MRI of the lungs. While T2 * times are longer for pulmonary hyperpolarized (HP) gas functional imaging (in particular for gaseous 129 Xe), T2 * can still lead to quantitative inaccuracies for sequences requiring longer echo times (such as diffusion weighted images) or longer readout duration (such as spiral sequences). This is especially true in preclinical studies, where high magnetic fields lead to shorter relaxation times than are typically seen in human studies. However, the T2 * of HP 129 Xe in the most common animal model of human disease (mice) has not been reported. Herein, we present a multi-echo radial flyback imaging sequence and use it to measure HP 129 Xe T2 * at 7 T under a variety of respiratory conditions. This sequence mitigates the impact of T1 relaxation outside the animal by using multiple gradient-refocused echoes to acquire images at a number of effective echo times for each RF excitation. After validating the sequence using a phantom containing water doped with superparamagnetic iron oxide nanoparticles, we measured the 129 Xe T2 * in vivo for 10 healthy C57Bl/6 J mice and found T2 * ~ 5 ms in the lung airspaces. Interestingly, T2 * was relatively constant over all experimental conditions, and varied significantly with sex, but not age, mass, or the O2 content of the inhaled gas mixture. These results are discussed in the context of T2 * relaxation within porous media.

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.