Regional anisotropy of airspace orientation in the lung as assessed with hyperpolarized helium-3 diffusion MRI.

Purpose: To evaluate regional anisotropy of lung-airspace orientation by assessing the dependence of helium-3 ((3) He) apparent diffusion coefficient (ADC) values on the direction of diffusion sensitization at two field strengths.

Materials And Methods: Hyperpolarized (3) He diffusion-weighted magnetic resonance imaging (MRI) of the lung was performed at 0.43T and 1.

Rapid acquisition of helium-3 and proton three-dimensional image sets of the human lung in a single breath-hold using compressed sensing.

Purpose: To develop and validate a method for acquiring helium-3 ((3) He) and proton ((1) H) three-dimensional (3D) image sets of the human lung with isotropic spatial resolution within a 10-s breath-hold by using compressed sensing (CS) acceleration, and to assess the fidelity of undersampled images compared with fully sampled images.

Methods: The undersampling scheme for CS acceleration was optimized and tested using (3) He ventilation data. Rapid 3D acquisition of both (3) He and (1) H data during one breath-hold was then implemented, based on a balanced steady-state free-precession pulse sequence, by random undersampling of k-space with reconstruction by means of minimizing the L1 norm and total variance.

Characterization and detection of physiologic lung changes before and after placement of bronchial valves using hyperpolarized helium-3 MR imaging: preliminary study.

Rationale And Objectives: The aim of this study was to determine the efficacy of hyperpolarized (3)He (HHe) ventilation and apparent diffusion coefficient (ADC) HHe magnetic resonance imaging (MRI) in detecting changes in lung function and microstructure in emphysematous lung after bronchial valve (BV) placement.

Materials And Methods: One patient diagnosed with emphysema had nine BVs placed in upper lobe bronchi. Imaging was performed before and 6 months after BV placement.

A short-breath-hold technique for lung pO2 mapping with 3He 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, pO2 values are obtained by measuring the oxygen-induced T1 relaxation of inhaled hyperpolarized 3He. Unlike other methods, only two 3He images are acquired: one with reverse-centric and the other with centric phase-encoding order.

Assessment of in vitro vs. in vivo lung structure using hyperpolarized helium-3 diffusion magnetic resonance imaging.

The purpose of this study was to assess the properties of a model system for hyperpolarized He-3 (HHe) diffusion MR imaging created from the lungs of New Zealand white rabbits by drying the lungs while inflated at constant pressure. The dried lungs were prepared by sacrificing the animal, harvesting the lungs en bloc and dehydrating the lungs for several days using dry compressed air. In four rabbits, the apparent diffusion coefficient (ADC) of HHe gas was measured in vivo and, within 1 week, in vitro in the dried lungs.

Helium-3 diffusion MR imaging of the human lung over multiple time scales.

Rationale And Objectives: Diffusion magnetic resonance imaging (MRI) with hyperpolarized (3)He gas is a powerful technique for probing the characteristics of the lung microstructure. A key parameter for this technique is the diffusion time, which is the period during which the atoms are allowed to diffuse within the lung for measurement of the signal attenuation. The relationship between diffusion time and the length scales that can be explored is discussed, and representative, preliminary results are presented from ongoing studies of the human lung for diffusion times ranging from milliseconds to several seconds.

Extending the range of diffusion times for regional measurement of the 3He ADC in human lungs.

A stimulated-echo-based technique was developed to measure the regional apparent diffusion coefficient (ADC) of hyperpolarized 3He during a single breathhold for diffusion times of 25 ms or greater. Compared to previous methods, a substantially shorter minimum diffusion time was achieved by decoupling diffusion sensitization from image acquisition. A hyperpolarized-gas phantom was used to validate the method, which was then tested in four healthy subjects in whom regional ADC maps were acquired with diffusion times of 50, 200, and 1500 ms and a tag wavelength of 5 or 10 mm.

Hyperpolarized 3He MR imaging of the lung: effect of subject immobilization on the occurrence of ventilation defects.

Rationale And Objectives: To investigate immobilization-induced ventilation defects when performing hyperpolarized (3)He (H(3)He) magnetic resonance imaging (MRI) of the lung.

Methods And Materials: Twelve healthy subjects underwent MRI of the lungs after inhalation of H(3)He gas at three time points: 1) immediately after having been positioned supine on the MRI scanner table, 2) at 45 minutes while remaining supine, 3) and immediately thereafter after having turned prone. All image sets were reviewed in random order by three independent, blinded readers who recorded number, location, and size of H(3)He ventilation defects.

MR grid-tagging using hyperpolarized helium-3 for regional quantitative assessment of pulmonary biomechanics and ventilation.

A new technique is demonstrated in six healthy human subjects that combines grid-tagging and hyperpolarized helium-3 MRI to assess regional lung biomechanical function and quantitative ventilation. 2D grid-tagging, achieved by applying sinc-modulated RF-pulse trains along the frequency- and phase-encoding directions, was followed by a multislice fast low-angle shot (FLASH)-based acquisition at inspiration and expiration. The displacement vectors, first and second principal strains, and quantitative ventilation were computed, and mean values were calculated for the upper, middle, and lower lung regions.

XTC MRI: sensitivity improvement through parameter optimization.

Xenon polarization Transfer Contrast (XTC) MRI pulse sequences permit the gas exchange of hyperpolarized xenon-129 in the lung to be measured quantitatively. However, the pulse sequence parameter values employed in previously published work were determined empirically without considering the now-known gas exchange rates and the underlying lung physiology. By using a theoretical model for the consumption of magnetization during data acquisition, the noise intensity in the computed gas-phase depolarization maps was minimized as a function of the gas-phase depolarization rate.

Direct measurement of lung motion using hyperpolarized helium-3 MR tagging.

Purpose: To measure lung motion between end-inhalation and end-exhalation using a hyperpolarized helium-3 (HP (3)He) magnetic resonance (MR) tagging technique.

Methods And Materials: Three healthy volunteers underwent MR tagging studies after inhalation of 1 L HP (3)He gas diluted with nitrogen. Multiple-slice two-dimensional and volumetric three-dimensional MR tagged images of the lungs were obtained at end-inhalation and end-exhalation, and displacement vector maps were computed.

The variability of regional airflow obstruction within the lungs of patients with asthma: assessment with hyperpolarized helium-3 magnetic resonance imaging.

Background: It is unknown whether focal changes of airflow obstruction within the lungs of patients with asthma vary or are fixed in location with time or repeated bronchoconstriction. With hyperpolarized helium-3 magnetic resonance (H(3)HeMR) imaging, the airspaces are depicted and focal areas of airflow obstruction are shown as "ventilation defects."

Objective: To investigate the regional changes of airflow obstruction with time and repeated bronchoconstriction.

Evaluation of the reproducibility of lung motion probability distribution function (PDF) using dynamic MRI.

Treatment planning based on probability distribution function (PDF) of patient geometries has been shown a potential off-line strategy to incorporate organ motion, but the application of such approach highly depends upon the reproducibility of the PDF. In this paper, we investigated the dependences of the PDF reproducibility on the imaging acquisition parameters, specifically the scan time and the frame rate. Three healthy subjects underwent a continuous 5 min magnetic resonance (MR) scan in the sagittal plane with a frame rate of approximately 10 f s-1, and the experiments were repeated with an interval of 2 to 3 weeks.

Evaluation of emphysema severity and progression in a rabbit model: comparison of hyperpolarized 3He and 129Xe diffusion MRI with lung morphometry.

The apparent diffusion coefficients (ADCs) of hyperpolarized (3)He and (129)Xe gases were measured in the lungs of rabbits with elastase-induced emphysema and correlated against the mean chord length from lung histology. In vivo measurements were performed at baseline and 2, 4, 6, and 8 wk after instillation of elastase (mild and moderate emphysema groups) or saline (control group). ADCs were determined from acquisitions that used two b values.

Assessment of lung development using hyperpolarized helium-3 diffusion MR imaging.

Purpose: To determine whether hyperpolarized helium-3 (HHe) diffusion MR can detect the expected enlargement of alveoli that occurs with lung growth during childhood.

Materials And Methods: A total of 29 normal subjects aged four to 30 years underwent HHe diffusion MR imaging with the b-value pair 0, 1.6 second/cm(2).

A computer simulated phantom study of tomotherapy dose optimization based on probability density functions (PDF) and potential errors caused by low reproducibility of PDF.

Lung tumor motion trajectories measured by four-dimensional CT or dynamic MRI can be converted to a probability density function (PDF), which describes the probability of the tumor at a certain position, for PDF based treatment planning. Using this method in simulated sequential tomotherapy, we study the dose reduction of normal tissues and more important, the effect of PDF reproducibility on the accuracy of dosimetry. For these purposes, realistic PDFs were obtained from two dynamic MRI scans of a healthy volunteer within a 2 week interval.

Hyperpolarized HHe 3 MRI of the lung in cystic fibrosis: assessment at baseline and after bronchodilator and airway clearance treatment.

Rationale And Objectives: The purpose of this study is to determine hyperpolarized helium 3 (HHe) magnetic resonance (MR) findings of the lung in patients with cystic fibrosis (CF) compared with healthy subjects and determine whether HHe MR can detect changes after bronchodilator therapy or mechanical airway mucus clearance treatment.

Materials And Methods: Thirty-one subjects, 16 healthy volunteers and 15 patients with CF, underwent HHe lung ventilation MR imaging and spirometry at baseline. Eight patients with CF then were treated with nebulized albuterol, after which a follow-up HHe MR scan was obtained.

Pulmonary embolism: comprehensive evaluation with MR ventilation and perfusion scanning with hyperpolarized helium-3, arterial spin tagging, and contrast-enhanced MRA.

Purpose: Development of a comprehensive magnetic resonance (MR) examination consisting of MR angiography (MRA) and MR ventilation and perfusion (MR V/Q) scan for the detection of pulmonary emboli (PE) and assessment of the technique in a rabbit model.

Materials And Methods: Reversible PE was induced by inflating a non-detachable silicon balloon in the left pulmonary artery of five New Zealand White rabbits. MR V/Q scans were obtained prior to, during, and after balloon deflation.

Hyperpolarized 3He lung imaging at 0.5 and 1.5 Tesla: a study of susceptibility-induced effects.

Hyperpolarized (3)He MRI of the human lung was performed at 0.54 and 1.5 T using identical software and hardware (except for RF coils) at both field strengths.

Hyperpolarized 3He lung ventilation imaging with B1-inhomogeneity correction in a single breath-hold scan.

The signal-to-noise ratio (SNR) of hyperpolarized noble gas MR images is sensitive to the flip angle used. Variations in flip angle due to B1-inhomogeneity of the RF coil cause intensity variation artifacts in lung ventilation images which may mask or mimic disease. We show these artifacts can be minimized by using the optimal flip angle and corrected if the local flip angle is known.

Exploring lung function with hyperpolarized (129)Xe nuclear magnetic resonance.

With the use of polarization-transfer pulse sequences and hyperpolarized (129)Xe NMR, gas exchange in the lung can be measured quantitatively. However, harnessing the inherently high sensitivity of this technique as a tool for exploring lung function requires a fundamental understanding of the xenon gas-exchange and diffusion processes in the lung, and how these may differ between healthy and pathological conditions. Toward this goal, we employed NMR spectroscopy and imaging techniques in animal models to investigate the dependence of the relative xenon gas exchange rate on the inflation level of the lung and the tissue density.

Imaging the lungs in asthmatic patients by using hyperpolarized helium-3 magnetic resonance: assessment of response to methacholine and exercise challenge.

Background: Imaging of gas distribution in the lungs of patients with asthma has been restricted because of the lack of a suitable gaseous contrast agent. Hyperpolarized helium-3 (HHe3) provides a new technique for magnetic resonance imaging of lung diseases.

Objective: We sought to investigate the use of HHe3 gas to image the lungs of patients with moderate or severe asthma and to assess changes in gas distribution after methacholine and exercise challenge.

Rapid hyperpolarized 3He diffusion MRI of healthy and emphysematous human lungs using an optimized interleaved-spiral pulse sequence.

Purpose: To develop and validate an interleaved-spiral diffusion pulse sequence capable of hyperpolarized (3)He MR imaging of the whole lung in less than 10 seconds.

Materials And Methods: Hyperpolarized (3)He diffusion measurements were performed in seven healthy volunteers and five patients with emphysema using an interleaved-spiral pulse sequence that provided 11 contiguous 15-mm thick coronal ADC maps, with an in-plane resolution of 3.9 mm, covering the whole lung in 5.

Merging parametric active contours within homogeneous image regions for MRI-based lung segmentation.

Inhaled hyperpolarized helium-3 (3He) gas is a new magnetic resonance (MR) contrast agent that is being used to study lung functionality. To evaluate the total lung ventilation from the hyperpolarized 3He MR images, it is necessary to segment the lung cavities. This is difficult to accomplish using only the hyperpolarized 3He MR images, so traditional proton (1H) MR images are frequently obtained concurrent with the hyperpolarized 3He MR examination.

Emphysema: hyperpolarized helium 3 diffusion MR imaging of the lungs compared with spirometric indexes--initial experience.

Purpose: To quantitatively evaluate hyperpolarized helium 3 ((3)He) diffusion magnetic resonance (MR) images of the lung in patients with emphysema and to determine whether apparent diffusion coefficients (ADCs) measured with MR imaging correlate with spirometric indexes.

Materials And Methods: Hyperpolarized (3)He diffusion MR imaging was performed in 16 healthy volunteers and 11 patients. Coronal diffusion-sensitized MR images were obtained during suspended respiration after inhalation of laser-polarized (3)He gas, and images of the ADC were calculated.