Long-range diffusion of hyperpolarized 3 He in rats

Lung function measurements in preclinical research: What has been done and where is it headed?

Due to the close interaction of lung morphology and functions, repeatable measurements of pulmonary function during longitudinal studies on lung pathophysiology and treatment efficacy have been a great area of interest for lung researchers. Spirometry, as a simple and quick procedure that depends on the maximal inspiration of the patient, is the most common lung function test in clinics that measures lung volumes against time. Similarly, in the preclinical area, plethysmography techniques offer lung functional parameters related to lung volumes. In the past few decades, many innovative techniques have been introduced for in vivo lung function measurements, while each one of these techniques has their own advantages and disadvantages. Before each experiment, depending on the sensitivity of the required pulmonary functional parameters, it should be decided whether an invasive or non-invasive approach is desired. On one hand, invasive techniques offer sensitive and specific readouts related to lung mechanics in anesthetized and tracheotomized animals at endpoints. On the other hand, non-invasive techniques allow repeatable lung function measurements in conscious, free-breathing animals with readouts related to the lung volumes. The biggest disadvantage of these standard techniques for lung function measurements is considering the lung as a single unit and providing only global readouts. However, recent advances in lung imaging modalities such as x-ray computed tomography and magnetic resonance imaging opened new doors toward obtaining both anatomical and functional information from the same scan session, without the requirement for any extra pulmonary functional measurements, in more regional and non-invasive manners. Consequently, a new field of study called pulmonary functional imaging was born which focuses on introducing new techniques for regional quantification of lung function non-invasively using imaging-based techniques. This narrative review provides first an overview of both invasive and non-invasive conventional methods for lung function measurements, mostly focused on small animals for preclinical research, including discussions about their advantages and disadvantages. Then, we focus on those newly developed, non-invasive, imaging-based techniques that can provide either global or regional lung functional readouts at multiple time-points.

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.

3He diffusion MRI in human lungs

Hyperpolarized 3He gas allows the air spaces of the lungs to be imaged via MRI. Imaging of restricted diffusion is addressed here, which allows the microstructure of the lung to be characterized through the physical restrictions to gas diffusion presented by airway and alveolar walls in the lung. Measurements of the apparent diffusion coefficient (ADC) of 3He at time scales of milliseconds and seconds are compared; measurement of acinar airway sizes by determination of the microscopic anisotropy of diffusion is discussed. This is where Dr. JJH Ackerman's influence was greatest in aiding the formation of the Washington University 3He group, involving early a combination of physicists, radiologists, and surgeons, as the first applications of 3He ADC were to COPD and its destruction/modification of lung microstructure via emphysema. The sensitivity of the method to early COPD is demonstrated, as is its validation by direct comparison to histology. More recently the method has been used broadly in adult and pediatric obstructive lung diseases, from severe asthma to cystic fibrosis to bronchopulmonary dysplasia, a result of premature birth. These applications of the technique are discussed briefly.

Fluorine-19 MRI Contrast Agents for Cell Tracking and Lung Imaging

Fluorine-19 (¹⁹F)-based contrast agents for magnetic resonance imaging stand to revolutionize imaging-based research and clinical trials in several fields of medical intervention. First, their use in characterizing in vivo cell behavior may help bring cellular therapy closer to clinical acceptance. Second, their use in lung imaging provides novel noninvasive interrogation of the ventilated airspaces without the need for complicated, hard-to-distribute hardware. This article reviews the current state of ¹⁹F-based cell tracking and lung imaging using magnetic resonance imaging and describes the link between the methods across these fields and how they may mutually benefit from solutions to mutual problems encountered when imaging ¹⁹F-containing compounds, as well as hardware and software advancements.

Measurement of 129Xe gas apparent diffusion coefficient anisotropy in an elastase-instilled rat model of emphysema

Hyperpolarized noble gas ((3)He and (129)Xe) apparent diffusion coefficient (ADC) measurements have shown remarkable sensitivity to microstructural (i.e., alveolar) changes in the lung, particularly emphysema. The ADC of hyperpolarized noble gases depends strongly on the diffusion time (Δ), and (3)He ADC has been shown to be anisotropic for Δ ranging from a few milliseconds down to a few hundred microseconds. In this study, the anisotropic nature of (129)Xe diffusion and its dependence on Δ were investigated both numerically, in a budded cylinder model, and in vivo, in an elastase-instilled rat model of emphysema. Whole lung longitudinal ADC (D(L)) and transverse ADC (D(T)) were measured for Δ = 6, 50, and 100 ms at 73.5 mT, and correlated with measurements of the mean linear intercept (L(m)) obtained from lung histology. A significant increase (P = 0.0021) in D(T) was measured for Δ = 6 ms between the sham (0.0021 ± 0.0005 cm(2)/s) and elastase-instilled (0.005 ± 0.001 cm(2)/s) cohorts, and a strong correlation was measured between D(T) (Δ = 6 ms) and L(m), with a Pearson's correlation coefficient of 0.90. This study confirms that (129)Xe D(T) increases correlate with alveolar space enlargement due to elastase instillation in rats.

Regional determination of oxygen uptake in rodent lungs using hyperpolarized gas and an analytical treatment of intrapulmonary gas redistribution

A method is presented which allows for the accurate extraction of regional functional metrics in rodent lungs using hyperpolarized gas. The technique is based on the combination of measured T(1) decay, an independent measure of specific ventilation and mass balance considerations to extract the regional oxygen levels and uptake. In phantom and animal experiments, it is demonstrated that the redistribution of gas during the measurement is a significant confounding factor, and this effect is addressed analytically. The resulting parameterization of gas flow increases the accuracy of oxygen-sensitive MRI, and may also be used independently to assess air trapping and airway constriction. Limitations of the technique with respect to spatial resolution and robustness are also discussed.

Mapping of (3) He apparent diffusion coefficient anisotropy at sub-millisecond diffusion times in an elastase-instilled rat model of emphysema

Hyperpolarized (3) He gas can provide detailed anatomical maps of the macroscopic airways in the lungs (i.e., ventilation) as well as insight into the lung microstructure through the apparent diffusion coefficient. In particular, the apparent diffusion coefficient of (3) He in the lung exhibits anisotropic effects that depend on diffusion time (δ), and it has been shown to be extraordinarily sensitive to enlargement in terminal airways and alveoli associated with emphysema. In this study, the anisotropic nature of the (3) He apparent diffusion coefficient is studied in a rat model of emphysema, based on elastase instillation, specifically for δ values less than one millisecond. Longitudinal (D(L) ) and transverse (D(T) ) diffusion coefficients were mapped at δ = 360 μs and δ = 800 μs based on a cylinder model of lung structure and correlated with histological measurement of alveolar damage based on mean linear intercept (L(m) ). Whole-lung mean D(T) measured at δ = 360 μs in the elastase-instilled rat lungs (0.14 ± 0.09 cm(2) /s) demonstrated the most significant increase (p = 0.00195) compared to the sham-instilled cohort (0.06 ± 0.06 cm(2) /s) and had a strong linear correlation with L(m) (Pearson's correlation coefficient of 0.9). These results suggest that measurement of (3) He apparent diffusion coefficient anisotropy, specifically D(T) , can provide a sensitive indicator of emphysema, particularly at very short diffusion times (δ = 360 μs).