3D volume‐localized pO₂ measurement in the human lung with ³He MRI

Abstract

A method for 3D volume-localized quantification of pO2 in the lungs is presented that uses repetitive frame 3D gradient-echo imaging of 3 He. The method was demonstrated by experiments on 3 He phantoms containing known concentrations of O 2 and in vivo on a group of three healthy human volunteers. The results were compared with those obtained by equivalent 2D thin-slice and 2D projection methodologies, and were found to be consistent with published results from the 2D projection methodologies (pO 2 ‫؍‬ 0.09 -0.18 bar). Studies performed on the same subject, on three separate occasions, demonstrated a repeatability of pO 2 measurement to within 14% using the 3D technique. Experimental differences between the 2D and 3D methods were substantiated with theoretical and numerical analyses of the signal decay, which took into account the effects of out-of-slice diffusion as a source of error in the thin-slice 2D experiments. It is shown that the 2D thin-slice technique systematically underestimates pO2 when there is significant gas diffusion (factor of 4 underestimate for D ‫؍‬ 0.9 cm MRI of hyperpolarized (HP)3 He gas can provide high spatial and temporal resolution images of gas ventilation in the lungs and airways, as well as useful functional information (1). Potentially one of the most useful functional parameters is an estimate of the partial pressure of oxygen (pO 2 ) in the lungs and the rate of oxygen extraction (2-6). Accurate spatially localized pO 2 measurement with 3 He MRI would be clinically useful as a means of assessing lung ventilation-perfusion (V/Q). When paramagnetic oxygen mixes with polarized 3 He in the lungs, a marked reduction in the 3 He T 1 arises through the electron-nuclear spin dipolar coupling. Saam et al. (7) quantified the dependence of the 3 He T 1 on oxygen concentration with phantom experiments. This O 2 dependence of the 3 He T 1 was used by Deninger et al. (2,3) for in vivo quantification of pO 2 in a series of breath-hold experiments on pigs and humans. These in vivo experiments used images acquired from whole-lung projections (very thick 2D slices). This was done to maximize the signal-to-noise ratio (SNR) and to circumvent diffusion of polarized gas out of the slice during the significant inter-image delay time (typically up to 7 s) as a possible source of change in the time course of the in vivo 3 He signal. Moller et al. (5) estimated the degree of partial volume mixing of polarization due to gas diffusion for a 2D thin-slice experiment in the guinea pig lung. Assuming a 24 s total acquisition time and an apparent diffusion coefficient of D ϭ 0.16 cm 2 s -1 , they predicted that only 5% of the in-slice magnetization would be exchanged between adjacent slices. In addition, the recently published results by Jalali et al. (6) from 2D pO 2 experiments in porcine lungs, with 2-cm-thick slices, are in fair agreement with the results from a 2D porcine study with 18-cm-thick slices (2). These results suggest that diffusion between slices may not be a considerable problem; however, a...

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

“…In the COPD patient, negative pO 2 values were returned from several regions of the lung (d). The pO 2 maps from all HVs were relatively homogeneous and predominantly had values similar to those published previously for HVs (1,4). A small region with negative pO 2 was seen near the diaphragm in HV3 due to a slight downward movement of the left lung base during breath-hold.…”

“…The time dependence of pO 2 during a breath-hold was assumed to be linear and given by pO 2 (t) ¼ p 0 À Rt, where p 0 is the initial partial pressure and R is the rate of oxygen extraction by perfusion (1,4). To observe the temporal signal behavior of the imaging data, ROIs were prescribed.…”

Compressed sensing in hyperpolarized3He Lung MRI

“…As the quality of reconstruction may depend upon the SNR of the original undersampled data, the effect of SNR on CS reconstruction was studied by introducing a Gaussian white noise to the highest SNR image (initial SNR $19) and changing the variance to obtain a set of images of SNR (17,14,10,9,7,6, and 5). The SNR was computed by taking the mean signal from a homogeneous region of interest (ROI) in the lung field and background noise regions.…”

“…1b. This should be the method of choice when the acquisition of a 3D lung volume is desired, and the method promises to be useful for future 3D experiments using interleaved acquisitions for ADC computation or short acquisition time for 3D oxygen partial pressure mapping with 3 He (17).…”

Functional Imaging: CT and MRI

“…(204)(205)(206); It is now feasible to obtain 3D maps of oxygen uptake ratios (or ventilationperfusion maps) using a single breath-hold technique. (207) The method is still under development, but may be able to assist, among many other options, the assessment of pulmonary thromboembolic disease (in particular chronic disease patients in the pre-operative assessment of thromboendarterectomy), the planning of surgical or endobronchial interventions and the pre-operative assessment of patients with borderline respectability of lung cancer. Several groups have demonstrated the feasibility of combining perfusion and ventilation studies in animal model settings .…”