Yinan Xu

A multislice single breath-hold scheme for imaging alveolar oxygen tension in humans

Reliable, noninvasive, and high‐resolution imaging of alveolar partial pressure of oxygen (pAO2) is a potentially valuable tool in the early diagnosis of pulmonary diseases. Several techniques have been proposed for regional measurement of pAO2 based on the increased depolarization rate of hyperpolarized 3He. In this study, we explore one such technique by applying a multislice pAO2‐imaging scheme that uses interleaved‐slice ordering to utilize interslice time‐delays more efficiently. This approach addresses the low spatial resolution and long breath‐hold requirements of earlier techniques, allowing pAO2 measurements to be made over the entire human lung in 10–15 s with a typical resolution of 8.3 × 8.3 × 15.6 mm3. PO2 measurements in a glass syringe phantom were in agreement with independent gas analysis within 4.7 ± 4.1% (R = 0.9993). The technique is demonstrated in four human subjects (healthy nonsmoker, healthy former smoker, healthy smoker, and patient with COPD), each imaged six times on 3 different days during a 2‐week span. Two independent measurements were performed in each session, consisting of 12 coronal slices. The overall pAO2 mean across all subjects was 95.9 ± 12.2 Torr and correlated well with end‐tidal O2 (R = 0.805, P < 0.0001). The alveolar O2 uptake rate was consistent with the expected range of 1–2 Torr/s. Repeatable visual features were observed in pAO2 maps over different days, as were characteristic differences among the subjects and gravity‐dependent effects. Magn Reson Med, 2012.

Accelerated fractional ventilation imaging with hyperpolarized Gas MRI.

To investigate the utility of accelerated imaging to enhance multibreath fractional ventilation (r) measurement accuracy using hyperpolarized gas MRI. Undersampling shortens the breath‐hold time, thereby reducing the O2‐induced signal decay and allows subjects to maintain a more physiologically relevant breathing pattern. Additionally, it may improve r estimation accuracy by reducing radiofrequency destruction of hyperpolarized gas.

Multislice fractional ventilation imaging in large animals with hyperpolarized gas MRI.

The noninvasive assessment of regional lung ventilation is of critical importance in the quantification of the severity of disease and evaluation of response to therapy in many pulmonary diseases. This work presents, for the first time, the implementation of a hyperpolarized (HP) gas MRI technique to measure whole‐lung regional fractional ventilation (r) in Yorkshire pigs (n = 5) through the use of a gas mixing and delivery device in the supine position. The proposed technique utilizes a series of back‐to‐back HP gas breaths with images acquired during short end‐inspiratory breath‐holds. In order to decouple the radiofrequency pulse decay effect from the ventilatory signal build‐up in the airways, the regional distribution of the flip angle (α) was estimated in the imaged slices by acquiring a series of back‐to‐back images with no interscan time delay during a breath‐hold at the tail end of the ventilation sequence. Analysis was performed to assess the sensitivity of the multislice ventilation model to noise, oxygen and the number of flip angle images. The optimal α value was determined on the basis of the minimization of the error in r estimation: αopt = 5–6º for the set of acquisition parameters in pigs. The mean r values for the group of pigs were 0.27 ± 0.09, 0.35 ± 0.06 and 0.40 ± 0.04 for the ventral, middle and dorsal slices, respectively (excluding conductive airways r > 0.9). A positive gravitational (ventral–dorsal) ventilation gradient effect was present in all animals. The trachea and major conductive airways showed a uniform near‐unity r value, with progressively smaller values corresponding to smaller diameter airways, and ultimately leading to lung parenchyma. The results demonstrate the feasibility of the measurement of the fractional ventilation in large species, and provide a platform to address the technical challenges associated with long breathing time scales through the optimization of acquisition parameters in species with a pulmonary physiology very similar to that of humans. Copyright

Regional Alveolar Partial Pressure of Oxygen Measurement with Parallel Accelerated Hyperpolarized Gas MRI

RATIONALE AND OBJECTIVES Alveolar oxygen tension (Pao2) is sensitive to the interplay between local ventilation, perfusion, and alveolar-capillary membrane permeability, and thus reflects physiologic heterogeneity of healthy and diseased lung function. Several hyperpolarized helium ((3)He) magnetic resonance imaging (MRI)-based Pao2 mapping techniques have been reported, and considerable effort has gone toward reducing Pao2 measurement error. We present a new Pao2 imaging scheme, using parallel accelerated MRI, which significantly reduces measurement error. MATERIALS AND METHODS The proposed Pao2 mapping scheme was computer-simulated and was tested on both phantoms and five human subjects. Where possible, correspondence between actual local oxygen concentration and derived values was assessed for both bias (deviation from the true mean) and imaging artifact (deviation from the true spatial distribution). RESULTS Phantom experiments demonstrated a significantly reduced coefficient of variation using the accelerated scheme. Simulation results support this observation and predict that correspondence between the true spatial distribution and the derived map is always superior using the accelerated scheme, although the improvement becomes less significant as the signal-to-noise ratio increases. Paired measurements in the human subjects, comparing accelerated and fully sampled schemes, show a reduced Pao2 distribution width for 41 of 46 slices. CONCLUSION In contrast to proton MRI, acceleration of hyperpolarized imaging has no signal-to-noise penalty; its use in Pao2 measurement is therefore always beneficial. Comparison of multiple schemes shows that the benefit arises from a longer time-base during which oxygen-induced depolarization modifies the signal strength. Demonstration of the accelerated technique in human studies shows the feasibility of the method and suggests that measurement error is reduced here as well, particularly at low signal-to-noise levels.