4144
The Impact of Echo-time on Hyperpolarized Xenon-129 CSSR Measurements1Radiology, University of Pennsylvania, Philadelphia, PA, United States, 2Anesthesiology and Critical Care, University of Pennsylvania, Philadelphia, PA, United States
Synopsis
Chemical Shift Saturation Recovery (CSSR) MR spectroscopy and imaging are powerful techniques for deriving a number of important physiological pulmonary parameters. We investigated the impact of measurement echo time on the spectral composition of the acquisition and the subsequent numerical data analysis in rabbits. We found that lengthening the TE resulted in a vastly improved separation between the dissolved-phase resonances by suppressing short T2* signal components. As consequence of this phenomenon, the measured septal wall thickness changed with TE by 1.0 ± 0.13 μm/ms. Similarly, we hypothesize that CSSR-derived measures of pulmonary physiology might be field strength-dependent.
Purpose
The quantification of xenon-129 gas uptake by the lung parenchyma using Chemical Shift Saturation Recovery (CSSR) MR spectroscopy1-4 and imaging5-8 are powerful techniques for deriving a number of important physiological pulmonary parameters, such as septal wall thickness, surface-to-volume ratios, and blood flow rate. Nevertheless, depending on the specifics of the employed RF excitation pulses, scanner capabilities, and the spatial encoding schemes chosen, echo times for the various CSSR implementations in use vary. In this study, we investigated the impact of changes in echo time on CSSR spectroscopy measurements and the calculations of septal wall thickness derived from them.Methods
Imaging experiments were performed in 4 sedated New Zealand rabbits (3.5 – 4.5 kg). Animals were ventilated with room air until imaging began, at which point the gas mix was switched to 20% oxygen and 80% HXe for 3 breaths (6 ml/kg tidal volume) followed by a 10-s breath-hold. All studies were approved by the Institutional Animal Care and Use Committee.
Gaussian RF pulses (1.2-ms duration) were applied during the CSSR acquisition to saturate the dissolved-phase resonances around 200 ppm. Following a variable delay time ranging from 2.5 to 500 ms, a 0.9-ms Gaussian RF excitation pulse was used to generate a free induction decay. This sequence was repeated 40 times during a single breath hold. The signal was sampled for 30.72 ms with 1024 sampling points, apodized by a squared cosine function and zero-filled to 2048 points. The native echo time of the measurement was 0.55 ms. However, to simulate the effect of an increase in echo time, a corresponding number of sampling points were discarded prior to Fourier transformation and phasing of the spectra. The dissolved- and gas-phase resonances were integrated numerically. The dissolved-to-gas-phase ratio was subsequently fitted to an analytical gas-uptake model9. All MR studies were performed at 1.5T (Avanto; Siemens), using a custom xenon-129 transmit/receive birdcage coil (Stark Contrast, Erlangen, Germany). Enriched xenon gas (87% xenon-129) was polarized using a prototype commercial system (XeBox-E10, Xemed LLC, Durham, NH).
Results and Discussion
Figure 1 shows representative dissolved-phase spectra for delay times of 10 ms (Fig. 1A) and 500 ms (Fig. 1B) reconstructed with 8 different echo times. Typically, only a single dissolved-phase resonance can be resolved in xenon spectra of the rabbit lung, but with increasing echo time a second peak emerged approximately 8 ppm downfield from the more prominent tissue-plasma resonance at 197 ppm. This phenomenon is much more apparent at longer delay times, most likely because the dissolved-phase resonance is a composite peak consisting of signal contributions from various locations and with different T2*. At long delay times, the dissolved-phase signal has a larger component of xenon that has been carried downstream by the blood to magnetically more homogeneous regions. As the TE is increased, the short T2* components are selectively suppressed, revealing the long T2* signal components underneath.
As illustrated by the TE-dependence of the derived apparent septal wall thickness in Fig. 2, lengthening the echo time to suppress the short T2* signal components, which presumably originate closer to the magnetically highly inhomogeneous gas exchange sites, has a considerable impact. In our preliminary measurements, we found the septal wall thickness to increase with TE by 1.0 ± 0.13 μm/ms. Using this slope to extrapolate the measurement curves in Fig. 2, we were able to obtain the “true” septal wall thickness for a hypothetical zero-TE acquisition. A corollary of our findings would be that the measured septal wall thickness should also be dependent on the main field strength due to its effect on T2*. On the positive side, deliberately varying the echo time of spectroscopic measurements might help to reveal or more precisely quantify additional xenon resonances that are obscured by strong but rapidly decaying dissolved-phase signal sources in the lung.
Conclusion
We demonstrated the multi-exponential nature of xenon dissolved-phase T2* decay in the rabbit lung. Since the T2* of the individual contributing signal sources is likely related to their distance from the magnetically inhomogeneous gas exchange sites, the echo time of a CSSR measurement controls the composition of the evaluated signal and the physiological lung parameters extracted therefrom.Acknowledgements
Supported by NIH grants R01 EB015767, R01 HL129805, S10 OD018203 and R01 CA193050.References
[1] Ruppert K et al. NMR of hyperpolarized 129Xe in the canine chest: spectral dynamics during a breath-hold. NMR Biomed 2000;13:220-228. [2] Butler JP et al. Measuring surface-area-to-volume ratios in soft porous materials using laser-polarized xenon interphase exchange nuclear magnetic resonance. J Phys Condens Matter 2002;14:L297-L304. [3] Qing et al. Assessment of lung function in asthma and COPD using hyperpolarized 129Xe chemical shift saturation recovery spectroscopy and dissolved-phase MRI. NMR in Biomed 2014;27(12):1490-1501. [4] Zhong et al. Simultaneous assessment of both lung morphometry and gas exchange function within a single breath‐hold by hyperpolarized 129Xe MRI. NMR in Biomed 2017 (epub). [5] Doganay et al. Quantification of regional early stage gas exchange changes using hyperpolarized 129Xe MRI in a rat model of radiation-induced lung injury. Med Phys 2016;43(5):2410-2420. [6] Kern et al. Regional investigation of lung function and microstructure 129Xe chemical shift saturation recovery parameters by localized and dissolved‐phase imaging: A reproducibility study. MRM 2018 (epub). [7] Kern et al. Mapping of regional lung microstructural parameters using 129 Xe dissolved‐phase MRI in healthy volunteers hyperpolarized and patients with chronic obstructive pulmonary disease. MRM 2018 (epub). [8] Zanette et al. Physiological gas exchange mapping of hyperpolarized 129Xe using spiral-IDEAL and MOXE in a model of regional radiation-induced lung injury. Med Phys 2018;45(2):803-816. [9] Patz et al. Diffusion of hyperpolarized 129Xe in the lung: a simplified model of 129Xe septal uptake and experimental results. New J Physics 2011;13:015009.Figures
