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).

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 (3He) gas. The FAVOR technique utilizes variable flip angles to remove the cumulative effect of RF pulses and T1 relaxation on the hyperpolarized gas signal and thereby eliminates the need for intervening air wash-out breaths and multiple cycles of 3He wash-in breaths before each image. The former allows an improvement in speed (by a factor of approximately 30) while the latter reduces the cost of each measurement (by a factor of approximately 5). The FAVOR and conventional ventilation methods were performed on six healthy male Brown Norway rats (190-270 g). Lobar measurements of ventilation, r, obtained with the FAVOR method were not significantly different from those obtained with the conventional method for the right middle and caudal and left lobes (P>0.05 by a Wilcoxon matched pairs test). A methacholine challenge test was also administered to an animal and reduction and recovery of r was detected by the FAVOR method. The reduced 3He consumption and the improvement in speed provided by FAVOR suggest that it may allow measurement of ventilation in human subjects not previously possible

Rapid 3-D mapping of hyperpolarized 3He spin-lattice relaxation times using variable flip angle gradient echo imaging with application to alveolar oxygen partial pressure measurement in rat lungs.

OBJECTIVE: The purpose of this work was to develop a rapid 3-D, variable flip angle (VFA) method for measurement of hyperpolarized (3)He T(1) which accounts for the effects of radiofrequency (RF) pulses without the need for additional flip angle information. MATERIALS AND METHODS: The 3-D, VFA method was validated in vitro over a range of oxygen partial pressures ranging from 0.04 to 0.52 atm. The approach was also tested in vivo in five healthy rats as a function of increasing number of wash-out breaths. The T(1) accuracy of the VFA method in the presence of flip angle mis-setting and RF field non-uniformity was compared with the CFA method using simulations and experiments. RESULTS: T(1) measurements were found to provide p(A)O(2) estimates, both in vitro and in vivo consistent with those predicted based on gas dilution and/or ventilation para- meters. For the RF pulse mis-setting (4%) and RF field non-uniformity (3%) used here, the VFA method provided a T(1) accuracy of better than 5% compared to 12% for the CFA method. CONCLUSION: With sufficient RF field homogeneity (3%) and proper calibration (4%), the VFA approach can provide rapid and reliable 3-D T(1) mapping of hyperpolarized (3)He without the need for additional flip angle information

Improvement in breast lesion characterization with dynamic contrast-enhanced MRI using pharmacokinetic modeling and bookend T-1 measurements

Dynamic contrast-enhanced breast MR imaging was performed on 14 patients (five cancerous lesions, nine benign) with slice-selective spoiled gradient-recalled echo (2D SPGR) imaging. Adiabatic saturation recovery T-1 measurements were performed before (T-1pre) and after (T-1post) 2D SPGR imaging. These two "book-end" T-1 measurements were used to calibrate the equations which were employed to convert the time course of the 2D SPGR signal strength to T-1-vs.-time, which in turn was used to compute the gadolinium concentration-vs.-time ([C](t)) in the lesion. The extraction-flow product (EF) was computed for each lesion by pharmacokinetic modeling of [C](t). For this study, EF provided a sensitivity and specificity for cancer of 100% and 78%, respectively. When only T-1pre was used to estimate [C](t) (which assumes a priori knowledge of the shape and amplitude of the slice profile), the sensitivity and specificity fell to 80% and 56%, respectively. This is presumably due to unexpected variations in the shape and/or amplitude of the slice profile, which could be caused by factors such as patient-to-patient variations in breast geometry or inconsistently set transmit gains. Therefore, both T-1pre and T-1post measurements are necessary for optimum sensitivity and specificity using pharmacokinetic analysis. (C) 2004 Wiley-Liss, Inc

Dissipation du quatrieme ordre sur les maillages non structures anisotropes pour des problemes de convection-diffusion

Theme 4 - Projet SinusAvailable at INIST (FR), Document Supply Service, under shelf-number : 14802 E, issue : a.1996 n.2953 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueSIGLEFRFranc

Comparison of hyperpolarized (3)He MRI rat lung volume measurement with micro-computed tomography.

In this study, the upper-limit volume (gas plus partial tissue volume) as well as absolute volume (gas only) of lungs measured with hyperpolarized (3)He-MR imaging is compared with that determined by micro-computed tomography (CT) under similar ventilation conditions in normal rats. Five Brown Norway rats (210-259 g) were ventilated with O(2), alternately with (3)He, using a computer-controlled ventilator, and 3D density-weighted images of the lungs were acquired during a breath hold after six wash-in breaths of (3)He. The rats were then transferred to a micro-CT scanner, and a similar experimental setup was used to obtain images of the lungs during a breath hold of air with an airway pressure equal to that of the MR imaging breath hold. The upper-limit and absolute volumes obtained from (3)He-MR and micro-CT methods were not significantly different (p > 0.05). The good agreement between the lung volumes measured with the two imaging methods suggests that (3)He-MR imaging can be used for quantitative analysis of lung volume changes in longitudinal studies without the exposure to the ionizing radiation which accompanies micro-CT imaging

Hyperpolarized noble gas magnetic resonance imaging of the animal lung: Approaches and applications

Hyperpolarized noble gas (HNG) magnetic resonance (MR) imaging is a very promising noninvasive tool for the investigation of animal models of lung disease, particularly to follow longitudinal changes in lung function and anatomy without the accumulated radiation dose associated with x rays. The two most common noble gases for this purpose are 3He (helium 3) and 129Xe (xenon 129), the latter providing a cost-effective approach for clinical applications. Hyperpolarization is typically achieved using spin-exchange optical pumping techniques resulting in ∼10 000 -fold improvement in available magnetization compared to conventional Boltzmann polarizations. This substantial increase in polarization allows high spatial resolution (<1 mm) single-slice images of the lung to be obtained with excellent temporal resolution (<1 s). Complete three-dimensional images of the lungs with 1 mm slice thickness can be obtained within reasonable breath-hold intervals (<20 s). This article provides an overview of the current methods used in HNG MR imaging with an emphasis on ventilation studies in animals. Special MR hardware and software considerations are described in order to use the strong but nonrecoverable magnetization as efficiently as possible and avoid depolarization primarily by molecular oxygen. Several applications of HNG MR imaging are presented, including measurement of gross lung anatomy (e.g., airway diameters), microscopic anatomy (e.g., apparent diffusion coefficient), and a variety of functional parameters including dynamic ventilation, alveolar oxygen partial pressure, and xenon diffusing capacity. © 2009 American Institute of Physics

Comparison of hyperpolarized (3)He MRI with Xe-enhanced computed tomography imaging for ventilation mapping of rat lung.

Lung ventilation was mapped in five healthy Brown Norway rats (210-377 g) using both hyperpolarized (3)He MRI and Xe-enhanced computed tomography (Xe-CT) under similar ventilator conditions. Whole-lung measurements of ventilation r obtained with (3)He MRI were not significantly different from those obtained from Xe-CT (p = 0.1875 by Wilcoxon matched pairs test). The ventilation parameter r is defined as the fraction of refreshed gas per unit volume per breath. Regional ventilation was also measured in four regions of the lung using both methods. A two-tailed paired t-test was performed for each region, yielding p > 0.05 for all but the upper portion of the right lung. The distribution of regional ventilation was evaluated by calculating ventilation gradients in the superior/inferior (S/I) direction. The average S/I gradient obtained using the (3)He MRI method was found to be 0.17 ± 0.04 cm(-1) , whereas the average S/I gradient obtained using the Xe-CT method was found to be 0.016 ± 0.005 cm(-1) . In general, S/I ventilation gradients obtained from both methods were significantly different from each other (p = 0.0019 by two-tailed paired t-test). These regional differences in ventilation measurements may be caused by the manner in which the gas contrast agents distribute physiologically and/or by the imaging modality

A comparison of T-2(*)-weighted magnitude and phase imaging for measuring the arterial input function in the rat aorta following intravenous injection of gadolinium contrast agent

The arterial input function (AIF) is important for quantitative MR imaging perfusion experiments employing Gd contrast agents. This study compared the accuracy of T-2*-weighted magnitude and phase imaging for noninvasive measurement of the AIF in the rat aorta. Twenty-eight in vivo experiments were performed involving simultaneous arterial blood sampling and MR imaging following Gd injection. In vitro experiments were also performed to confirm the in vivo results. At 1.89 T and TE = 3 ms, the relationship between changes in 1/T-2* in blood (estimated from MR signal magnitude) and Gd concentration ([Gd]) was measured to be similar to 19 s(-1) mM(-1), while that between phase and [Gd] was similar to 0.19 rad mM(-1). Both of these values are consistent with previously published results. The in vivo phase data had approximately half as much scatter with respect to [Gd] than the in vivo magnitude data (r(2) = .34 vs. r(2)= .17, respectively). This is likely due to the fact that the estimated change in 1/T2* is more sensitive than the phase to a variety of factors such as partial volume effects and T, weighting. Therefore, this study indicates that phase imaging may be a preferred method for measuring the AIF in the rat aorta compared to T2*-weighted magnitude imaging. (c) 2005 Elsevier Inc. All rights reserved

Measurement of alveolar oxygen partial pressure in the rat lung using Carr-Purcell-Meiboom-Gill spin-spin relaxation times of hyperpolarized 3He and 129Xe at 74 mT.

Regional measurement of alveolar oxygen partial pressure can be obtained from the relaxation rates of hyperpolarized noble gases, (3) He and (129) Xe, in the lungs. Recently, it has been demonstrated that measurements of alveolar oxygen partial pressure can be obtained using the spin-spin relaxation rate (R(2) ) of (3) He at low magnetic field strengths (<0.1 T) in vivo. R(2) measurements can be achieved efficiently using the Carr-Purcell-Meiboom-Gill pulse sequence. In this work, alveolar oxygen partial pressure measurements based on Carr-Purcell-Meiboom-Gill R(2) values of hyperpolarized (3) He and (129) Xe in vitro and in vivo in the rat lung at low magnetic field strength (74 mT) are presented. In vitro spin-spin relaxivity constants for (3) He and (129) Xe were determined to be (5.2 ± 0.6) × 10(-6) Pa(-1) sec(-1) and (7.3 ± 0.4) × 10(-6) Pa(-1) s(-1) compared with spin-lattice relaxivity constants of (4.0 ± 0.4) × 10(-6) Pa(-1) s(-1) and (4.3 ± 1.3) × 10(-6) Pa(-1) s(-1), respectively. In vivo experimental measurements of alveolar oxygen partial pressure using (3) He in whole rat lung show good agreement (r(2) = 0.973) with predictions based on lung volumes and ventilation parameters. For (129) Xe, multicomponent relaxation was observed with one component exhibiting an increase in R(2) with decreasing alveolar oxygen partial pressure