Assessment of a single‐acquisition imaging sequence for oxygen‐sensitive ³He‐MRI

Abstract

MRI of the lungs using hyperpolarized helium-3 ( 3 He) allows the determination of intrapulmonary oxygen partial pressures (p O2 ). The need to separate competing processes of signal loss has hitherto required two different imaging series during two different breathing maneuvers. In this work, a new imaging strategy to measure p O2 by a single series of consecutive scans is presented. Within the last 5 years, helium-3 magnetic resonance imaging ( 3 He-MRI) of the lungs has been employed for both morphological and functional imaging. The first encompasses studies of overt or subclinical lung disease, e.g., in chronic obstructive pulmonary disease (COPD) and emphysema patients (1,2), asthma patients (3), and smokers (4). The latter includes the use of fast imaging sequences (5-7), diffusion studies (8 -10), and the determination of intrapulmonary oxygen partial pressures (11-13). All of these methods provide exciting new approaches to lung function analysis. Complementary to the diagnostic information obtained from morphological imaging, rapid lung imaging allows time-resolved ventilation studies. Diffusion-weighted imaging and oxygen-sensitive MRI yield physiological information that was previously unavailable or was obtainable only by invasive means. (Comprehensive reviews are given in Refs. 14 and 15.)Oxygen-sensitive 3 He-MRI makes use of the oxygen-induced nuclear relaxation of 3 He (16) to compute the intrapulmonary oxygen partial pressure p O2 during short breath-holds. However, true relaxation due to molecular oxygen must be distinguished from RF-induced signal loss, i.e., the two depolarization effects must be decoupled. As neither is known a priori, this requires two different imaging sequences: with different RF excitation amplitudes and, hence, flip angles (11), or with different interimage time intervals (12). A subtraction of the logarithmic intensities of both image series serves to mathematically eliminate one unknown and allows precise determination of the other. To date, the two image series have been acquired during two separate breathing maneuvers.The analyses described in Refs. 11 and 12 rely on identical physiological conditions during paired image acquisition. It has to be assumed that the p O2 at the start of the imaging sequence and its temporal development are equal in both breath-holds. The imaged subject thus has to perform two identical breathing maneuvers. However, preliminary studies in healthy human volunteers (13) and patients (17) have shown that this condition is not always met.A second disadvantage of the double-acquisition approach lies in its 3 He consumption. In previous experiments (12,13), a 3 He amount of ϳ2 ϫ 200 cm 3 was required for each examination. In view of the limited resources of 3 He, a more economical measurement technique is desirable.In this work, we present a new imaging algorithm which permits the determination of intrapulmonary p O2 using a single-breath, single-bolus imaging sequence. Its strategy is to employ a simultaneous change in both excitation ampli...

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

“…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.…”

“…Nevertheless, inter-pixel diffusion is still a source of error in 3D pO 2 mapping if the pixel size Dz, is such that Dz % z rms ¼ ffiffiffiffiffiffiffiffiffiffiffi ffi 2DDt p , where D is the characteristic diffusion coefficient on the time and microstructural length scale of the experiment. For free 3 He diffusion in air D ¼ 0.86 cm 2 /s (7), whereas smaller apparent diffusion coefficients (ADCs) are characteristic of longer diffusion times in healthy acinar tissue when measured with longitudinal diffusion weighted magnetization pulse sequences (8)(9)(10)(11). Similarly, any other change in signal decay, which is not related to oxygen concentration or RF depolarization, such as a delayed in-flow of gas into the region of interest (ROI), could cause errors in the pO 2 values calculated with the technique.…”

“…Similarly, any other change in signal decay, which is not related to oxygen concentration or RF depolarization, such as a delayed in-flow of gas into the region of interest (ROI), could cause errors in the pO 2 values calculated with the technique. 3 He pO 2 mapping has been used to image patients with bronchiolitis obliterans (12) and lung transplant (12,13), and several studies have performed pO 2 mapping in patients with chronic obstructive pulmonary disease (COPD) (13)(14)(15)(16)(17). COPD is characterized by abnormalities of air-flow and elevation in regional gas ADC related to emphysema in the alveolar spaces (18).…”

“…The aim of this work was to investigate the effects of delayed-ventilation and diffusion in 3 He pO 2 mapping in patients with moderate to severe COPD. It was hypothesized that gas movement and delayed ventilation during static breath-hold in regions of emphysematous and partially obstructed lung would cause errors in the pO 2 values returned by the technique.…”

“…It was hypothesized that gas movement and delayed ventilation during static breath-hold in regions of emphysematous and partially obstructed lung would cause errors in the pO 2 values returned by the technique. 3 He 3D pO 2 mapping was performed in patients with moderate to severe COPD as defined by the Global initiative for chronic Obstructive Lung Disease guidelines. Paradoxical findings in the measured regional pO 2 values were observed confirming the technique is prone to error in diseased lungs where gas ventilation is delayed or inter-voxel gas diffusion is spatially unconstrained by emphysematous disruption to the lung microstructure.…”