Reproducibility of 19 F-MR ventilation imaging in healthy volunteers.

PURPOSE: To assess the reproducibility of percentage ventilated lung volume (%VV) measurements in healthy volunteers acquired by fluorine (19 F)-MRI of inhaled perfluoropropane, implemented at two research sites. METHODS: In this prospective, ethically approved study, 40 healthy participants were recruited (May 2018-June 2019) to one of two research sites. Participants underwent a single MRI scan session on a 3T scanner, involving periodic inhalation of a 79% perfluoropropane/21% oxygen gas mixture. Each gas inhalation session lasted about 30 seconds, consisting of three deep breaths of gas followed by a breath-hold. Four 19 F-MR ventilation images were acquired per participant, each separated by approximately 6 minutes. The value of %VV was determined by registering separately acquired 1 H images to ventilation images before semi-automated image segmentation, performed independently by two observers. Reproducibility of %VV measurements was assessed by components of variance, intraclass correlation coefficients, coefficients of variation (CoV), and the Dice similarity coefficient. RESULTS: The MRI scans were well tolerated throughout, with no adverse events. There was a high degree of consistency in %VV measurements for each participant (CoVobserver1 = 0.43%; CoVobserver2 = 0.63%), with overall precision of %VV measurements determined to be within ± 1.7% (95% confidence interval). Interobserver agreement in %VV measurements revealed a high mean Dice similarity coefficient (SD) of 0.97 (0.02), with only minor discrepancies between observers. CONCLUSION: We demonstrate good reproducibility of %VV measurements in a group of healthy participants using 19 F-MRI of inhaled perfluoropropane. Our methods have been successfully implemented across two different study sites, supporting the feasibility of performing larger multicenter clinical studies.

MR properties of 19 F C3 F8 gas in the lungs of healthy volunteers: T 2 ∗ and apparent diffusion coefficient at 1.5T and T 2 ∗ at 3T.

PURPOSE: To measure the transverse relaxation time ( T 2 ∗ ) and apparent diffusion coefficient (ADC) of 19 F-C3 F8 gas in vivo in human lungs at 1.5T and 3T, and to determine the representative distribution of values of these parameters in a cohort of healthy volunteers. METHODS: Mapping of ADC at lung inflation levels of functional residual capacity (FRC) and total lung capacity (TLC) was performed with inhaled 19 F-C3 F8 (eight subjects) and 129 Xe (six subjects) at 1.5T. T 2 ∗ mapping with 19 F-C3 F8 was performed at 1.5T (at FRC and TLC) for 8 subjects and at 3T (at TLC for seven subjects). RESULTS: At both FRC and TLC, the 19 F-C3 F8 ADC was smaller than the free diffusion coefficient demonstrating airway microstructural diffusion restriction. From FRC to TLC, the mean ADC significantly increased from 1.56 mm2 /s to 1.83 mm2 /s (P = .0017) for 19 F-C3 F8, and from 2.49 mm2 /s to 3.38 mm2 /s (P = .0015) for 129 Xe. The posterior-to-anterior gradient in ADC for FRC versus TLC in the superior half of the lungs was measured as 0.0308 mm2 /s per cm versus 0.0168 mm2 /s per cm for 19 F-C3 F8 and 0.0871 mm2 /s per cm versus 0.0326 mm2 /s per cm for 129 Xe. A consistent distribution of 19 F-C3 F8 T 2 ∗ values was observed in the lungs, with low values observed near the diaphragm and large pulmonary vessels. The mean T 2 ∗ across volunteers was 4.48 ms at FRC and 5.33 ms at TLC for 1.5T, and 3.78 ms at TLC for 3T. CONCLUSION: In this feasibility study, values of physiologically relevant parameters of lung microstructure measurable by MRI ( T 2 ∗ , and ADC) were established for C3 F8 in vivo lung imaging in healthy volunteers.

An asymmetrical whole-body birdcage RF coil without RF shield for hyperpolarized 129 Xe lung MR imaging at 1.5 T.

PURPOSE: This study describes the development and testing of an asymmetrical xenon-129 (129 Xe) birdcage radiofrequency (RF) coil for 129 Xe lung ventilation imaging at 1.5 Tesla, which allows proton (1 H) system body coil transmit-receive functionality. METHODS: The 129 Xe RF coil is a whole-body asymmetrical elliptical birdcage constructed without an outer RF shield to enable 1 H imaging. B1+ field homogeneity and flip angle mapping of the 129 Xe birdcage RF coil and 1 H system body RF coil with the 129 Xe RF coil in situ were evaluated in the MR scanner. The functionality of the 129 Xe birdcage RF coil was demonstrated through hyperpolarized 129 Xe lung ventilation imaging with the birdcage in both transceiver configuration and transmit-only configuration when combined with an 8-channel 129 Xe receive-only RF coil array. The functionality of 1 H system body coil with the 129 Xe RF coil in situ was demonstrated by acquiring coregistered 1 H lung anatomical MR images. RESULTS: The asymmetrical birdcage produced a homogeneous B1+ field (±10%) in agreement with electromagnetic simulations. Simulations indicated an optimal detuning configuration with 4 diodes. The obtained g-factor of 1.4 for acceleration factor of R = 2 indicates optimal array configuration. Coregistered 1 H anatomical images from the system body coil along with 129 Xe lung images demonstrated concurrent and compatible arrangement of the RF coils. CONCLUSION: A large asymmetrical birdcage for homogenous B1+ transmission with high sensitivity reception for 129 Xe lung MRI at 1.5 Tesla has been demonstrated. The unshielded asymmetrical birdcage design enables 1 H structural lung MR imaging in the same exam.

An 8-element Tx/Rx array utilizing MEMS detuning combined with 6 Rx loops for 19 F and 1 H lung imaging at 1.5T.

PURPOSE: To firstly improve the attainable image SNR of 19 F and 1 H C3 F8 lung imaging at 1.5 tesla using an 8-element transmit/receive (Tx/Rx) flexible vest array combined with a 6-element Rx-only array, and to secondly evaluate microelectromechanical systems for switching the array elements between the 2 resonant frequencies. METHODS: The Tx efficiency and homogeneity of the 8-element array were measured and simulated for 1 H imaging in a cylindrical phantom and then evaluated for in vivo 19 F/1 H imaging. The added improvement provided by the 6-element Rx-only array was quantified through simulation and measurement and compared to the ultimate SNR. It was verified through the measurement of isolation that microelectromechanical systems switches provided broadband isolation of Tx/Rx circuitry such that the 19 F tuned Tx/Rx array could be effectively used for both 19 F and 1 H nuclei. RESULTS: For 1 H imaging, the measured Tx efficiency/homogeneity (mean ± percent SD; 6.79µT/kW±26% ) was comparable to that simulated ( 7.57µT/kW±20% ). The 6 additional Rx-only loops increased the mean Rx sensitivity when compared to the 8-element array by a factor of 1.41× and 1.45× in simulation and measurement, respectively. In regions central to the thorax, the simulated SNR of the 14-element array achieves ≥70% of the ultimate SNR when including noise from the matching circuits and preamplifiers. A measured microelectromechanical systems switching speed of 12 µs and added minimum 22 dB of isolation between Tx and Rx were sufficient for Tx/Rx switching in this application. CONCLUSION: The described single-tuned array driven at 19 F and 1 H, utilizing microelectromechanical systems technology, provides excellent results for 19 F and 1 H dual-nuclear lung ventilation imaging.

Dissolved hyperpolarized xenon-129 MRI in human kidneys.

PURPOSE: To assess the feasibility of using dissolved hyperpolarized xenon-129 (129 Xe) MRI to study renal physiology in humans at 3 T. METHODS: Using a flexible transceiver RF coil, dynamic and spatially resolved 129 Xe spectroscopy was performed in the abdomen after inhalation of hyperpolarized 129 Xe gas with 3 healthy male volunteers. A transmit-only receive-only RF coil array was purpose-built to focus RF excitation and enhance sensitivity for dynamic imaging of 129 Xe uptake in the kidneys using spoiled gradient echo and balanced steady-state sequences. RESULTS: Using spatially resolved spectroscopy, different magnitudes of signal from 129 Xe dissolved in red blood cells and tissue/plasma could be identified in the kidneys and the aorta. The spectra from both kidneys showed peaks with similar amplitudes and chemical shift values. Imaging with the purpose-built coil array was shown to provide more than a 3-fold higher SNR in the kidneys when compared with surrounding tissues, while further physiological information from the dissolved 129 Xe in the lungs and in transit to the kidneys was provided with the transceiver coil. The signal of dissolved hyperpolarized 129 Xe could be imaged with both tested sequences for about 40 seconds after inhalation. CONCLUSION: The uptake of 129 Xe dissolved in the human kidneys was measured with spectroscopic and imaging experiments, demonstrating the potential of hyperpolarized 129 Xe MR as a novel, noninvasive technique to image human kidney tissue perfusion.

Optimization of steady-state free precession MRI for lung ventilation imaging with 19 F C3 F8 at 1.5T and 3T.

PURPOSE: To optimize 19 F imaging pulse sequences for perfluoropropane (C3 F8 ) gas human lung ventilation MRI considering intrinsic in vivo relaxation parameters at both 1.5T and 3T. METHODS: Optimization of the imaging parameters for both 3D spoiled gradient (SPGR) and steady-state free precession (SSFP) 19 F imaging sequences with inhaled 79% C3 F8% and 21% oxygen was performed. Phantom measurements were used to validate simulations of SNR. In vivo parameter mapping and sequence optimization and comparison was performed by imaging the lungs of a healthy adult volunteer. T1 and T2* mapping was performed in vivo to optimize sequence parameters for in vivo lung MRI. The performance of SSFP and SPGR was then evaluated in vivo at 1.5T and 3T. RESULTS: The in vivo T2* of C3 F8 was shown to be dependent upon lung inflation level (2.04 ms ± 36% for residual volume and 3.14 ms ± 28% for total lung capacity measured at 3T), with lower T2* observed near the susceptibility interfaces of the diaphragm and around pulmonary blood vessels. Simulation and phantom measurements indicate that a factor of ~2-3 higher SNR can be achieved with SSFP when compared with optimized SPGR. In vivo lung imaging showed a 1.7 factor of improvement in SNR achieved at 1.5T, while the theoretical improvement at 3T was not attained due to experimental SAR constraints, shorter in vivo T1 , and B0 inhomogeneity. CONCLUSION: SSFP imaging provides increased SNR in lung ventilation imaging of C3 F8 demonstrated at 1.5T with optimized SSFP similar to the SNR that can be obtained at 3T with optimized SPGR.

Optimized and accelerated 19 F-MRI of inhaled perfluoropropane to assess regional pulmonary ventilation.

PURPOSE: To accelerate 19 F-MR imaging of inhaled perfluoropropane using compressed sensing methods, and to optimize critical scan acquisition parameters for assessment of lung ventilation properties. METHODS: Simulations were performed to determine optimal acquisition parameters for maximal perfluoropropane signal-to-noise ratio (SNR) in human lungs for a spoiled gradient echo sequence. Optimized parameters were subsequently employed for 19 F-MRI of inhaled perfluoropropane in a cohort of 11 healthy participants using a 3.0 T scanner. The impact of 1.8×, 2.4×, and 3.0× undersampling ratios on 19 F-MRI acquisitions was evaluated, using both retrospective and prospective compressed sensing methods. RESULTS: 3D spoiled gradient echo 19 F-MR ventilation images were acquired at 1-cm isotropic resolution within a single breath hold. Mean SNR was 11.7 ± 4.1 for scans acquired within a single breath hold (duration = 18 s). Acquisition of 19 F-MRI scans at shorter scan durations (4.5 s) was also demonstrated as feasible. Application of both retrospective (n = 8) and prospective (n = 3) compressed sensing methods demonstrated that 1.8× acceleration had negligible impact on qualitative image appearance, with no statistically significant change in measured lung ventilated volume. Acceleration factors of 2.4× and 3.0× resulted in increasing differences between fully sampled and undersampled datasets. CONCLUSION: This study demonstrates methods for determining optimal acquisition parameters for 19 F-MRI of inhaled perfluoropropane and shows significant reduction in scan acquisition times (and thus participant breath hold duration) by use of compressed sensing.

Comparison of MEMS switches and PIN diodes for switched dual tuned RF coils.

PURPOSE: To evaluate the performance of micro-electromechanical systems (MEMS) switches against PIN diodes for switching a dual-tuned RF coil between 19 F and 1 H resonant frequencies for multi-nuclear lung imaging. METHODS: A four-element fixed-phase and amplitude transmit-receive RF coil was constructed to provide homogeneous excitation across the lungs, and to serve as a test system for various switching methods. The MR imaging and RF performance of the coil when switched between the 19 F and 1 H frequencies using MEMS switches, PIN diodes and hardwired configurations were compared. RESULTS: The performance of the coil with MEMS or PIN diode switching was comparable in terms of RF measurements, transmit efficiency and image SNR on both 19 F and 1 H nuclei. When the coil was not switched to the resonance frequency of the respective nucleus being imaged, reductions in the transmit efficiency were observed of 32% at the 19 F frequency and 12% at the 1 H frequency. The coil provides transmit field homogeneity of ±12.9% at the 1 H frequency and ±14.4% at the 19 F frequency in phantoms representing the thorax with the air space of the lungs filled with perfluoropropane gas. CONCLUSION: MEMS and PIN diodes were found to provide comparable performance in on-state configuration, while MEMS were more robust in off-state high-powered operation (>1 kW), providing higher isolation and requiring a lower DC switching voltage than is needed for reverse biasing of PIN diodes. In addition, clear benefits of switching between the 19 F and 1 H resonances were demonstrated, despite the proximity of their Larmor frequencies.

Experimental verification of SNR and parallel imaging improvements using composite arrays.

Composite MRI arrays consist of triplets where two orthogonal upright loops are placed over the same imaging area as a standard surface coil. The optimal height of the upright coils is approximately half the width for the 7 cm coils used in this work. Resistive and magnetic coupling is shown to be negligible within each coil triplet. Experimental evaluation of imaging performance was carried out on a Philips 3 T Achieva scanner using an eight-coil composite array consisting of three surface coils and five upright loops, as well as an array of eight surface coils for comparison. The composite array offers lower overall coupling than the traditional array. The sensitivities of upright coils are complementary to those of the surface coils and therefore provide SNR gains in regions where surface coil sensitivity is low, and additional spatial information for improved parallel imaging performance. Near the surface of the phantom the eight-channel surface coil array provides higher overall SNR than the composite array, but this advantage disappears beyond a depth of approximately one coil diameter, where it is typically more challenging to improve SNR. Furthermore, parallel imaging performance is better with the composite array compared with the surface coil array, especially at high accelerations and in locations deep in the phantom. Composite arrays offer an attractive means of improving imaging performance and channel density without reducing the size, and therefore the loading regime, of surface coil elements. Additional advantages of composite arrays include minimal SNR loss using root-sum-of-squares combination compared with optimal, and the ability to switch from high to low channel density by merely selecting only the surface elements, unlike surface coil arrays, which require additional hardware.

The noise factor of receiver coil matching networks in MRI.

In typical MRI applications the dominant noise sources in the received signal are the sample, the coil loop and the preamplifier. We hypothesize that in some cases (e.g. for very small receiver coils) the matching network noise has to be considered explicitly. Considering the difficulties of direct experimental determinations of the noise factor of matching networks with sufficient accuracy, it is helpful to estimate the noise factor by calculation. A useful formula of the coil matching network is obtained by separating commonly used coil matching network into different stages and calculating their noise factor analytically by a combination of the noise from these stages. A useful formula of the coil matching network is obtained. ADS simulations are performed to verify the theoretical predictions. Thereafter carefully-designed proof-of-concept phantom experiments are carried out to qualitatively confirm the predicted SNR behavior. The matching network noise behavior is further theoretically investigated for a variety of scenarios. It is found that in practice the coil matching network noise can be improved by adjusting the coil open port resonant frequency.