³He pO₂ mapping is limited by delayed‐ventilation and diffusion in chronic obstructive pulmonary disease

Abstract

Purpose: Lung pO 2 mapping with 3 He MRI assumes that the sources of signal decay with time during a breath-hold are radiofrequency depolarization and oxygen-dependent T 1 relaxation, but the method is sensitive to other sources of spatiotemporal signal change such as diffusion. The purpose of this work was to assess the use of 3 He pO 2 mapping in patients with chronic obstructive pulmonary disease. Methods: Ten patients with moderate to severe chronic obstructive pulmonary disease were scanned with a 3D single breath-hold pO 2 mapping sequence. Results: Images showed signal increasing over time in some lung regions due to delayed ventilation during breath-hold. Regions of physically unrealistic negative pO 2 values were seen in all patients, and regional mean pO 2 values of À0.3 bar were measured in the two patients most affected by delayed ventilation (where mean time to signal onset was 3-4 s). Conclusions: Movement of gas within the lungs during breath-hold causes regional changes in signal over time that are not related to oxygen concentration, leading to erroneous pO 2 measurements using the linear oxygen-dependent signal decay model. These spatio-temporal sources of signal change cannot be reliably separated at present, making pO 2 mapping using this methodology unreliable in chronic obstructive pulmonary disease patients with significant bullous emphysema or delayed ventilation. Magn Reson Med 71:1172-1178, 2014. V C 2013 Wiley Periodicals, Inc.Key words: hyperpolarized 3 He; partial pressure of oxygen; chronic obstructive pulmonary disease; delayed-ventilation; diffusionThe partial pressure of oxygen (pO 2 ) in the lung airspaces depends on ventilation and perfusion matching and is a regional indicator of gas exchange. Hyperpolarized 3 He MRI pO 2 mapping has been shown to provide quantitative, regional measures of pO 2 in healthy volunteers (HVs) (1-4), which are physiologically plausible. The method assumes that all signal depletion during a breath-hold is due to radiofrequency (RF) depolarization and oxygen-dependent T 1 effects (5) and these effects can be separated to calculate the regional pO 2 using typically a linear (1), or exponential (6), oxygen-dependent signal decay model.The method is, however, also sensitive to other sources of temporal signal change during the breath-hold acquisition time. pO 2 mapping methods by definition require time resolved images to be separated by a delay time Dt, which is of the same order of magnitude as the 3 He T 1 in air % 20 s (1,5). Hence, inter-frame delays of 2-5 s are typically used to measure the pO 2 with 3 He. On this time scale, inter-slice diffusion of gas with different polarization has been shown to be a source of significant error in slice selective 2D pO 2 acquisitions leading to an underestimation of pO 2 values (4). This can be mitigated by using a 3D sequence (4) where the whole lung experiences the same RF history. Nevertheless, inter-pixel diffusion is still a source of error in 3D pO 2 mapping if the pixel size Dz, is such that, where D i...

A hybrid multibreath wash-in wash-out lung function quantification scheme in human subjects using hyperpolarized3He MRI for simultaneous assessment of specific ventilation, alveolar oxygen tension, oxygen uptake, and air trapping

“…Third, the signal uniformity achieved after the ventilation series minimizes the chance that gas flow or diffusion between regions with differing HP gas concentration may mimic the effect of abnormal oxygen concentration, thereby causing error in the P A O 2 measurement. This bias has been discussed previously and is most noticeable in diseased regions. In the most extreme case, HP gas arrives at a voxel after the first set of images due to slow filling or an extremely long ventilation time constant.…”

Magnetic resonance imaging with hyperpolarized agents: methods and applications

“…Each measurement is performed within a single breath-hold [79, 141], and modified centric and reverse-centric view orders mitigate effects due to regionally variable flip angle (B 1 -field variation) [79]. Gas flow effects within the lungs during a breath-hold have recently been characterized using 3 He, underscoring that this method of pO 2 measurement reflects both regional pO 2 and local patterns of ventilation and flow [143]. Regional pO 2 measures can potentially be used to calculate the ratio of ventilation-to-perfusion [142], and pO 2 measurements have recently been used as a marker of disease severity in a study of ventilation-to-perfusion ratio heterogeneity in smokers [144, 145].…”

Sparse Reconstruction Techniques in Magnetic Resonance Imaging

“…Acceleration enables accurate calculation of ventilation, with initial application in imaging ventilation changes in asthma 165 . The same group has previously explored hyperpolarized 3 He for chronic obstructive pulmonary disease (COPD) 166 and application to this pathology is also likely feasible. Alternatively, an ultrashort echo time (UTE) approach has been used with compressed sensing for imaging COPD patients, where UTE MR signal intensity correlates to CT density and can be related to pulmonary function metrics such as FEV1/FRC 167 .…”