Determination of regional V_A/Q by hyperpolarized ³He MRI

Abstract

Alveolar ventilation/perfusion ratio (V A /Q) is a key parameter in functional imaging of the lung. Herein, regional V A /Q was calculated from regional values of alveolar partial pressure of oxygen (P A O 2 ) measured by hyperpolarized 3 He gas MRI (HP 3 He MRI). Yorkshire pigs (n ‫؍‬ 7, mean weight ‫؍‬ 25 kg) were paralyzed and maintained under isoflurane anesthesia. Animals were placed into a birdcage coil, then transferred to the bore of a 1.5 T MRI unit. Prior to imaging, animals were manually ventilated with room air for 5 min, then a 3 He gas mixture was administered during breathhold and imaging performed. P A O 2 was measured based on the decay rate of 3 He signal. Subjects' blood gas concentrations were measured and these values and Imaging techniques for pulmonary disorders have traditionally focused on anatomic and pathologic descriptions of lung parenchyma, airways, and vasculature. Functional imaging of the lung is a relatively recent development in pulmonary imaging that seeks to image physiologic variables. Recent developments in imaging methods for regional measurements of function have generated considerable interest because of the potential for high-resolution, regional distributions of functional variables to enhance detection of lung pathologies. Additionally, regional measurements of function can provide fundamental insights into the pathophysiologic mechanisms of disease.Several methods have been reported recently for imaging of functional variables: local compliance (1-3), local ventilation (4 -6), local perfusion from arterial spin tagging techniques (7-9) and contrast washout kinetics (5,10,11), and regional determinations of the alveolar ventilation/perfusion ratio (V A /Q) by PET (12-16), CT (17), and MRI-based methods (8,18,19). Regional determinations of V A /Q are of particular interest because of the key role of V A /Q in gas exchange (20), and because V A /Q can be disturbed by pathology that affects either ventilation or perfusion. Local measurements and imaging of V A /Q therefore have the potential to serve as a sensitive marker for lung disease.MR-based functional imaging methods offer a unique combination of potentially high spatial resolution and high temporal resolution. Although proton MRI can be used for lung imaging (21), this approach is generally at a disadvantage in that contrast in signal between tissue and gas phases is low (22). Recently, several investigators have used hyperpolarized (HP) 3 He gas for high-resolution imaging of the airways (23,24), and most recently as a method to assess regional ventilation (4). One complicating factor that all HP 3 He methods share is the oxygen-dependent decay of signal due to spin transfer (25).The Mainz group used this oxygen-dependence of polarized spin decay to measure regional alveolar partial pressure of oxygen (P A O 2 ) in the lung, and suggested that these local measurements of P A O 2 and their correlation with decrease rate could reflect matching of ventilation and perfusion (26 -30). It has long been known that the...

Functional lung imaging using hyperpolarized gas MRI

“…Similar measures, combined with quantitative measures of airway lumen size and wall thickness, may also allow measurement of gas trapping and airway narrowing associated with other obstructive lung diseases such as asthma (24). Although HR CT can provide quantitative assessment and diagnosis of the anatomical features of lung diseases, HP MRI has the means to perform a range of functional measures including regional ventilation (25)(26)(27), and V A /Q mapping (28,29), while also providing images of regional microstructure-in some applications at length scales similar to or smaller than those depicted on CT (30,31)-and all of these measures are performed without ionizing radiation.…”

“…More recently, the paramagnetic effects of O 2 itself have been exploited to provide a quantitative estimate of the PO 2 (28,87,88) that can then be used to calculate V A /Q (29). In this method, the paramagnetic effect of alveolar oxygen causes loss of HP gas polarization over time that is proportional to the concentration of alveolar PO 2 (Eq.…”

“…[1]). The same slice is imaged at multiple phases and at two different delay times to separate RF saturation of signal from signal loss due to PO 2 concentration (29). This has typically required two matched breathholds, but recent work has indicated that the RF saturation can be estimated using a two-point method, allowing measurement within a single breathhold (88).…”

A system for open‐access 3He human lung imaging at very low field

“…3 He imaging of human lungs is now performed commonly in clinical MRI scanners to explore a variety of lung disorders (5) using techniques such as spin-density imaging (6), flow mapping (7,8), and diffusion imaging (9,10). Additional notable achievements include monitoring of 3 He relaxation as an indicator of the local alveolar O 2 concentration (11), which has recently been directly linked to the physiologically relevant ventilation/perfusion ratio (V/Q) (12). These advances are enabled by the high nuclear spin polarization obtainable from optical pumping methods, typically greater than 10% (1,2).…”

Measurement of alveolar oxygen partial pressure in the rat lung using Carr‐Purcell‐Meiboom‐Gill spin–spin relaxation times of hyperpolarized 3He and 129Xe at 74 mT

“…Specifically, HNG relaxation due to the paramagnetic influence of molecular oxygen can be used to estimate regional alveolar oxygen partial pressure (p A O 2 ) in the lung (11). Regional variations in p A O 2 may be expected in lung disease which impairs ventilation and/ or perfusion and can be used to compute ventilation/perfusion mismatch (12). Currently, HNG MR imaging is the only method of determining regional oxygen concentration in the lung in vivo making this area of research quite unique for the study of lung physiology and pathophysiology.…”