Rapid and efficient mapping of regional ventilation in the rat lung using hyperpolarized ³He with Flip angle variation for offset of RF and relaxation (FAVOR)

Abstract

A novel imaging method is presented, Flip Angle Variation for Offset of RF and Relaxation (FAVOR), for rapid and efficient measurement of rat lung ventilation using hyperpolarized helium-3 ( 3 He) gas. The FAVOR technique utilizes variable flip angles to remove the cumulative effect of RF pulses and T 1 relaxation on the hyperpolarized gas signal and thereby eliminates the need for intervening air wash-out breaths and multiple cycles of 3 He wash-in breaths before each image. The former allows an improvement in speed (by a factor of Ϸ30) while the latter reduces the cost of each measurement (by a factor of Ϸ5). MR imaging with hyperpolarized 3 He gas has been proposed for measurement of regional ventilation in the rodent lung (1) and has been recently validated with xenon-enhanced CT imaging (2). It is anticipated that 3 He MR will provide a favorable approach for measurement of ventilation in animal cohorts to track lung disease over time without the complications associated with accumulated x-ray dose (3). This method measures the dynamic change in lung 3 He signal as a function of breath number and extracts the relative refreshment of gas in a given lung voxel per breath. However, the conventional 3 He ventilation measurement requires knowledge of the longitudinal relaxation time, T 1 , of the 3 He gas in the ventilator system and in the lung, the latter requiring knowledge of the alveolar oxygen partial pressure (p A O 2 ).Furthermore, without accurate knowledge of the RF pulse history the method requires multiple 3 He breathing cycles (i.e., 3 He wash-in breaths) with air wash-out breaths between cycles in order to completely clear the lung of 3 He gas, which is time-consuming (i.e., several minutes), an inefficient use of hyperpolarized 3 He gas (i.e., costly), and can lead to imprecision due to variations in tidal volume from the ventilator.Perhaps most important, the conventional technique involving multiple 3 He and air breathing cycles requires several minutes (Ϸ10 min) for a single ventilation map, which is likely too slow to capture rapid changes in ventilation associated with short-duration bronchoconstriction such as a methacholine (MCh) challenge (Ͻ1 min), similar to asthma (4,5). Previous work has been limited to measurement of pre-and postsensitization effects or postlong-term challenge in part due to the time required to measure ventilation using the conventional method. The ability to detect changes in ventilation over time scales of less than 1 min may provide improved sensitivity to shortterm challenge and insight into disease which more closely resembles asthma (6 -9). As well, rapid measurement of ventilation may prove critical for evaluation of fast-acting drug therapies for asthma (10,11).We propose a novel approach, Flip Angle Variation for Offset of RF and Relaxation (FAVOR), for obtaining regional ventilation in a single set of breathing cycles (i.e., only one set of 3 He wash-in breaths and no air wash-out breaths). This approach utilizes variable flip angle (VFA) RF pulses that comp...

Comparison of hyperpolarized3He MRI rat lung volume measurement with micro-computed tomography

“…The RF pulse flip angle was varied throughout the k-space acquisition as described by Zhao et al to compensate for gas depolarization due to RF pulse history (32,33) and TR, TE and b were small compared to T 1, T 2 Ã and ADC respectively (as described below) such that the resulting MR image signal strength (Eq. [1]) was weighted only by the density of 3 He gas, thus permitting measurement of absolute lung volume for comparison with micro-CT.…”

Litz wire radiofrequency receive coils for hyperpolarized noble gas MR imaging of rodent lungs at 73.5 mT

“…129 Xe and 3 He lung imaging was performed in five Sprague Dawley rats (500 6 25 mg, Charles River Laboratories, Saint-Constant, Canada) ventilated with a custom-built ventilation system using a University-approved animal care protocol described previously (29,30). Imaging was performed during a 7 s breath-hold interval following eight wash-out breaths with either hyperpolarized 129 Xe or 3 He gas.…”

A novel intubation technique for minimally invasive longitudinal studies of rat lungs using hyperpolarized 3He magnetic resonance imaging

“…The ET tube was then attached directly to a small animal custom ventilator (GEHC, Malmo, Sweden) compatible with HNG MRI. 15 Insertion of the Foley catheter required no surgery. The Foley cuff was inflated using the assembly shown in Figure 1.…”

“…The SNR measured is comparable with other 2D images acquired using the standard method after a single breath of 3 He with an SNR of 185. 15 …”