Jiangsheng Yu

Single-acquisition sequence for the measurement of oxygen partial pressure by hyperpolarized gas MRI

Magnetic resonance imaging (MRI) with hyperpolarized 3‐helium gas (HP 3He) offers the possibility of studying functional lung parameters such as the alveolar oxygen concentration and oxygen depletion rate. Until now, a double‐acquisition technique has been utilized to extract these parameters. A complicated single‐acquisition technique was previously developed to avoid the necessity of performing two identical breathing maneuvers. The results obtained with this technique were significantly less accurate than the results obtained with the double‐acquisition method. In this work, a novel, easily implemented single‐acquisition sequence is presented that provides results comparable to those obtained with the established double‐acquisition method. This method is demonstrated in a phantom and a pig model on a 1.5 T scanner using a 2D fast low‐angle shot (FLASH) gradient‐echo sequence. Numerical simulations of the time evolution of the oxygen concentration were performed. Simulation results are presented to support the experimental data. Various parameter regimes were experimentally and numerically investigated. Magn Reson Med 52:766–773, 2004.

Improved Technique for Measurement of Regional Fractional Ventilation by Hyperpolarized 3He MRI

Quantitative measurement of regional lung ventilation is of great significance in assessment of lung function in many obstructive and restrictive pulmonary diseases. A new technique for regional measurement of fractional ventilation using hyperpolarized 3He MRI is proposed, addressing the shortcomings of an earlier approach that limited its use to small animals. The new approach allows for the acquisition of similar quantitative maps over a shortened period and requires substantially less 3He gas. This technique is therefore a better platform for implementation in large species, including humans. The measurements using the two approaches were comparable to a great degree, as verified in a healthy rat lung, and are very reproducible. Preliminary validation is performed in a lung phantom system. Volume dependency of measurements was assessed both in vivo and in vitro. A scheme for selecting an optimum flip angle is proposed. In addition, a dead space modeling approach is proposed to yield more accurate measurements of regional fractional ventilation using either method. Finally, sensitivity of the new technique to model parameters, noise, and number of included images were assessed numerically. As a prelude to application in humans, the technique was implemented in a large animal study successfully. Magn Reson Med, 2010.

Comparison of lung T2* during free-breathing at 1.5 T and 3.0 T with ultrashort echo time imaging.

Assessment of lung effective transverse relaxation time (T2*) may play an important role in the detection of structural and functional changes caused by lung diseases such as emphysema and chronic bronchitis. While T2* measurements have been conducted in both animals and humans at 1.5 T, studies on human lung at 3.0 T have not yet been reported. In this work, ultrashort echo time imaging technique was applied for the measurement and comparison of T2* values in normal human lungs at 1.5 T and 3.0 T. A 2D ultrashort echo time pulse sequence was implemented and evaluated in phantom experiments, in which an eraser served as a homogeneous short T2* sample. For the in vivo study, five normal human subjects were imaged at both field strengths and the results compared. The average T2* values measured during free‐breathing were 2.11(±0.27) ms at 1.5 T and 0.74(±0.1) ms at 3.0 T, respectively, resulting in a 3.0 T/1.5 T ratio of 2.9. Furthermore, comparison of the relaxation values at end‐expiration and end‐inspiration, accomplished through self‐gating, showed that during normal breathing, differences in T2* between the two phases may be negligible. Magn Reson Med, 2011.

Hyperpolarized 13C MRI of the pulmonary vasculature and parenchyma

The study of lung perfusion in normal and diseased subjects is of great interest to physiologists and physicians. In this work we demonstrate the application of a liquid‐phase hyperpolarized (HP) carbon‐13 (13C) tracer to magnetic resonance imaging (MRI) of the pulmonary vasculature and pulmonary perfusion in a porcine model. Our results show that high spatial and temporal resolution images of pulmonary perfusion can be obtained with this contrast technique. Traditionally, pulmonary perfusion measurement techniques have been challenging because of insufficient signal for quantitative functional assessments. The use of polarized 13C in MRI overcomes this limitation and may lead to a viable clinical method for studying the pulmonary vasculature and perfusion. Magn Reson Med 57:459–463, 2007.

Quantitative assessment of emphysema using hyperpolarized 3He magnetic resonance imaging

In this experiment, Sprague–Dawley rats with elastase‐induced emphysema were imaged using hyperpolarized 3He MRI. Regional fractional ventilation r, the fraction of gas replaced with a single tidal breath, was calculated from a series of images in a wash‐in study of hyperpolarized gas. We compared the regional fractional ventilation in these emphysematous rats to the regional fractional ventilations we calculated from a previous baseline study in healthy Sprague–Dawley rats. We found that there were differences in the maps of fractional ventilation and its associated frequency distribution between the healthy and emphysematous rat lungs. Fractional ventilation tended to be much lower in emphysematous rats than in normal rats. With this information, we can use data on fractional ventilation to regionally distinguish between healthy and emphysematous portions of the lung. The successful implementation of such a technique on a rat model could lead to work toward the future implementation of this technique in human patients. Magn Reson Med 53:1341–1346, 2005.

Simultaneous measurement of pulmonary partial pressure of oxygen and apparent diffusion coefficient by hyperpolarized 3He MRI

Hyperpolarized 3He (HP 3He) MRI shows promise to assess structural and functional pulmonary parameters in a sensitive, regional, and noninvasive way. Structural HP 3He MRI has applied the apparent diffusion coefficient (ADC) for the detection of disease‐induced lung microstructure changes at the alveolar level, and HP 3He pulmonary partial pressure of oxygen (pO2) imaging measures the oxygen transfer efficiency between the lung and blood stream. Although both parameters are affected in chronic obstructive pulmonary disease (COPD), a quantitative assessment of the regional correlation of the two parameters has not been reported in the literature. In this work, a single acquisition technique for the simultaneous measurement of ADC and pO2 is presented. This technique is based on the multiple regression method, in which a general linear estimator is used to retrieve the values of ADC and pO2 from a series of measurements. The measurement uncertainties are also analytically derived and used to find an optimal measurement scheme. The technique was first tested on a phantom model, and then on an in vivo normal pig experiment. A case study was performed on a COPD patient, which showed that in a region of interest ADC was 29% higher while oxygen depletion rate was 61% lower than the corresponding global average values. Magn Reson Med, 2009.

Measurement of Pulmonary Partial Pressure of Oxygen and Oxygen Depletion Rate with Hyperpolarized Helium-3 MRI: A Preliminary Reproducibility Study on Pig Model

RATIONAL AND OBJECTIVES Pulmonary partial pressure of oxygen (pO(2)) and oxygen depletion rate (R) are two important parameters of lung function. The dependence of hyperpolarized (3)He (HP (3)He) T(1) on local oxygen concentration provides the basis for high-resolution mapping of the regional distributions of pO(2) and R in the lung. Although the oxygen-sensitive HP (3)He magnetic resonance imaging technique has been applied in human subjects and several animal species, reproducibility studies are rarely reported in the literature. This work presents a preliminary reproducibility study on a pig model. In this study, important scan parameters, such as measurement timing and flip angle, are optimized to minimize the noise-induced measurement uncertainty. MATERIALS AND METHODS In the in vivo study, five normal pigs and one diseased pig with simulated pulmonary emboli were scanned with a small flip angle gradient echo sequence. The pulmonary oxygen measurement was repeated two to four times in each pig. In each measurement, a series of six images were acquired with optimal timing and flip angle. The parametric maps were generated using a bin-based data processing procedure that applied the multiple regression fitting method to extract the pO(2) and R. Variations of global mean, percentiles, and regions of interest were calculated from the maps to analyze reproducibility. RESULTS The global statistical analyses show that average variation of global mean is 10.7% for pO(2) and 23.8% for R, and that the average variation of percentiles (10th, 25th, 50th, 75th, and 90th) and interquartile range is 14.8% for pO(2) and 30.4% for R. The region-of-interest analysis on the manually selected regions shows that the average variation of mean is 12.6% for pO(2) and 21.9% for R. CONCLUSION In this work, a preliminary study on the reproducibility of measuring pO(2) and R with HP (3)He magnetic resonance imaging on a pig model is presented.

Optimization of scan parameters in pulmonary partial pressure oxygen measurement by hyperpolarized 3He MRI

The dependence of hyperpolarized (HP) 3He T1 on local oxygen concentration provides the basis for measuring the partial pressure of oxygen (pO2) and oxygen depletion rate (R) in the lungs. Precise measurements of this type are difficult because the oxygen effect manifests itself through a decay of signal, leading to noisy images at the end of the series. The depolarization caused by RF excitation pulses further complicates the problem. It is therefore important to optimize scan parameters, such as measurement timing and flip angle, to obtain accurate and reproducible measurements. This work presents a new single‐acquisition technique in conjunction with the multiple regression fitting method for data evaluation. Analytical expressions for the measurement uncertainties are derived. A total of four types of single‐acquisition timing schemes are investigated; simulation shows a large uncertainty variation between these schemes (pO2: 7.5–30.2%; R: 47.4–173.7%). A basic procedure for optimizing scan parameters is then described. A phantom experiment was conducted to verify the simulation results. Repeated in vivo measurements with the optimal scheme in a rabbit experiment showed that average variation of global mean is 6.2% for pO2 and 12.0% for R, and that the average variation of percentiles (10th, 25th, 50th, 75th, and 90th) is 8.7% for pO2 and 19.0% for R. Magn Reson Med, 2007.

Automatic Coil Selection for Streak Artifact Reduction in Radial MRI

In radial MR imaging, streaking artifacts contaminating the entire field of view can arise from regions at the outer edges of the prescribed field of view. This can occur even when the Nyquist criterion is satisfied within the desired field of view. These artifacts become exacerbated when parts of the object lie in the superior/inferior regions of the scanner where the gradient strengths become weakened. When multiple coil arrays are used for signal reception, coils at the outer edges can be disabled before data acquisition to reduce the artifact levels. However, as the weakened gradient strengths near the edges often distort the object, causing the signal to become highly concentrated into a small region, the streaks are often not completely removed. Data from certain coils can also be excluded during reconstruction by visually inspecting the individual coil images, but this is impractical for routine use. In this work, a postprocessing method is proposed to automatically identify those coils whose images contain high levels of streaking for subsequent exclusion during reconstruction. The proposed method was demonstrated in vivo dynamic contrast enhanced MRI datasets acquired using a three‐dimensional hybrid radial sequence. The results demonstrate that the proposed strategy substantially improves the image quality and show excellent agreement with images reconstructed with manually determined coil selection. Magn Reson Med, 2012.

DWDM performance of a packaged reconfigurable optical add-drop multiplexer subsystem supporting modular systems growth

We describe dense wavelength-division-multiplexing (DWDM) operation of a reconfigurable optical add-drop multiplexer subsystem module employing silica waveguide technology that is suitable for use with both fixed-wavelength and wavelength-selectable add-drop transceivers. We illustrate its use in providing modular growth in high channel-count DWDM systems.