A short-breath-hold technique for lung pO₂mapping with³He MRI

A pulse-sequence strategy was developed for generating regional maps of alveolar oxygen partial pressure (pO2) in a single 6-sec breath hold, for use in human subjects with impaired lung function. Like previously described methods, pO2values are obtained by measuring the oxygen-induced T1relaxation of inhaled hyperpolarized3He. Unlike other methods, only two3He images are acquired: one with reverse-centric and the other with centric phase-encoding order. This phase-encoding arrangement minimizes the effects of regional flip-angle variations, so that an accurate map of instantaneous pO2can be calculated from two images acquired a few seconds apart. By combining this phase-encoding strategy with variable flip angles, the vast majority of the hyperpolarized magnetization goes directly into the T1measurement, minimizing noise in the resulting pO2map. The short-breathhold pulse sequence was tested in phantoms containing known O2concentrations. The mean difference between measured and prepared pO2values was 1 mm Hg. The method was also tested in four healthy volunteers and three lung-transplant patients. Maps of healthy subjects were largely uniform, whereas focal regions of abnormal pO2were observed in diseased subjects. Mean pO2values varied with inhaled O2concentration. Mean pO2was consistent with normal steady-state values in subjects who inhaled3He diluted only with room air.