Maxim Terekhov

Lung ventilation- and perfusion-weighted Fourier decomposition magnetic resonance imaging: In vivo validation with hyperpolarized 3He and dynamic contrast-enhanced MRI

The purpose of this work was to validate ventilation‐weighted (VW) and perfusion‐weighted (QW) Fourier decomposition (FD) magnetic resonance imaging (MRI) with hyperpolarized 3He MRI and dynamic contrast‐enhanced perfusion (DCE) MRI in a controlled animal experiment. Three healthy pigs were studied on 1.5‐T MR scanner. For FD MRI, the VW and QW images were obtained by postprocessing of time‐resolved lung image sets. DCE acquisitions were performed immediately after contrast agent injection. 3He MRI data were acquired following the administration of hyperpolarized helium and nitrogen mixture. After baseline MR scans, pulmonary embolism was artificially produced. FD MRI and DCE MRI perfusion measurements were repeated. Subsequently, atelectasis and air trapping were induced, which followed with FD MRI and 3He MRI ventilation measurements. Distributions of signal intensities in healthy and pathologic lung tissue were compared by statistical analysis. Images acquired using FD, 3He, and DCE MRI in all animals before the interventional procedure showed homogeneous ventilation and perfusion. Functional defects were detected by all MRI techniques at identical anatomical locations. Signal intensity in VW and QW images was significantly lower in pathological than in healthy lung parenchyma. The study has shown usefulness of FD MRI as an alternative, noninvasive, and easily implementable technique for the assessment of acute changes in lung function. Magn Reson Med, 2013.

Design and evaluation of a 32-channel phased-array coil for lung imaging with hyperpolarized 3-helium.

Imaging with hyperpolarized 3‐helium is becoming an increasingly important technique for MRI diagnostics of the lung but is hampered by long breath holds (>20 sec), which are not always applicable in patients with severe lung disease like chronic obstructive pulmonary disease (COPD) or α‐1‐anti‐trypsin deficiency. Additionally, oxygen‐induced depolarization decay during the long breath holds complicates interpretation of functional data such as apparent diffusion coefficients. To address these issues, we describe and validate a 1.5‐T, 32‐channel array coil for accelerated 3He lung imaging and demonstrate its ability to speed up imaging 3He. A signal‐to‐noise ratio increase of up to a factor of 17 was observed compared to a conventional double‐resonant birdcage for unaccelerated imaging, potentially allowing increased image resolution or decreased gas production requirements. Accelerated imaging of the whole lung with one‐dimensional and two‐dimensional acceleration factors of 4 and 4 × 2, respectively, was achieved while still retaining excellent image quality. Finally, the potential of highly parallel detection in lung imaging is demonstrated with high‐resolution morphologic and functional images. Magn Reson Med, 2010.

Heparin-polynitroxides: Synthesis and preliminary evaluation as cardiovascular EPR/MR imaging probes and extracellular space-targeted antioxidants

We report here the synthesis of heparin-polynitroxide derivatives (HPNs) in which nitroxide moieties are linked either to uronic acid or glycosamine residues of the heparin macromolecule. HPNs have low anticoagulant activity, possess superoxide scavenging properties, bind to the vascular endothelium/extra-cellular matrix and can be detected by EPR and MRI techniques. As the vascular wall-targeted redox-active paramagnetic compounds, HPNs may have both diagnostic (molecular MRI) and therapeutic (ecSOD mimics) applications.

Visualization of inert gas wash‐out during high‐frequency oscillatory ventilation using fluorine‐19 MRI

High‐frequency oscillatory ventilation is looked upon as a lung‐protective ventilation strategy. For a further clarification of the physical processes promoting gas transport, a visualization of gas flow and the distribution of ventilation are of considerable interest. Therefore, fluorine‐19 magnetic resonance imaging of the imaging gas octafluorocyclobutane (C4F8) during high‐frequency oscillatory ventilation was performed in five healthy pigs. For that, a mutually compatible ventilation‐imaging system was set up and transverse images were acquired every 5 sec using FLASH sequences on a 1.5 T scanner. Despite a drop in signal‐to‐noise ratio after the onset of high‐frequency oscillatory ventilation, for each pig, the four experiments could be analyzed. A mean wash‐out time (τ) at 5 Hz of 52.7 ± 18 sec and 125.9 ± 39 sec at 10 Hz, respectively, were found for regions of interest including the whole lung. This is in agreement with the clinical findings, in that wash‐out of respiratory gases is significantly prolonged for increased high‐frequency oscillatory ventilation frequencies. Our study could be a good starting‐point for a further optimization of high‐frequency oscillatory ventilation. Magn Reson Med, 2010.

Three-dimensional mapping of the B1 field using an optimized phase-based method: Application to hyperpolarized 3He in lungs

A novel method is presented for the three‐dimensional mapping of the B1‐field of a transmit radio‐frequency MR coil. The method is based on the acquisition of phase images, where the effective flip angle is encoded in the phase of the nonselective hard pulse excitation. The method involves the application of a rectangular composite pulse as excitation in a three‐dimensional gradient recall echo to produce measurable phase angle variation. However, such a pulse may significantly increase the radio‐frequency power deposition in excess of the standard acceptable SAR limits, imposing extremely long TRs (>100 msec), which would result in acquisition times significantly greater than a single breath‐hold. In this study, the phases of the radio‐frequency excitation are modified, resulting in a different pulse sequence scheme. It is shown that the new method increases sensitivity with respect to radio‐frequency inhomogeneities by up to 10 times, and reduces the total duration of the pulse so that three‐dimensional B1 mapping is possible with 3He in lungs within a single breath‐hold. Computer simulations demonstrate the increase in sensitivity. Phantom results with 1H MRI are used for validation. In vivo results are presented with hyperpolarized 3He in human lungs at 1.5T. Magn Reson Med, 2010.

Measurement of gas transport kinetics in high‐frequency oscillatory ventilation (HFOV) of the lung using hyperpolarized 3He magnetic resonance imaging

To protect the patient with acute respiratory distress syndrome from ventilator associated lung injury (VALI) high‐frequency oscillatory ventilation (HFOV) is used. Clinical experience has proven that HFOV is an efficient therapy when conventional artificial ventilation is insufficient. However, the optimal settings of HFOV parameters, eg, tidal volumes, pressure amplitudes and frequency for maximal lung protection, and efficient gas exchange are not established unambiguously.

Reproducibility and comparison of oxygen-enhanced T1 quantification in COPD and asthma patients.

T1 maps have been shown to yield useful diagnostic information on lung function in patients with chronic obstructive pulmonary disease (COPD) and asthma, both for native T1 and ΔT1, the relative reduction while breathing pure oxygen. As parameter quantification is particularly interesting for longitudinal studies, the purpose of this work was both to examine the reproducibility of lung T1 mapping and to compare T1 found in COPD and asthma patients using IRSnapShotFLASH embedded in a full MRI protocol. 12 asthma and 12 COPD patients (site 1) and further 15 COPD patients (site 2) were examined on two consecutive days. In each patient, T1 maps were acquired in 8 single breath-hold slices, breathing first room air, then pure oxygen. Maps were partitioned into 12 regions each to calculate average values. In asthma patients, the average T1,RA = 1206ms (room air) was reduced to T1,O2 = 1141ms under oxygen conditions (ΔT1 = 5.3%, p < 5⋅10−4), while in COPD patients both native T1,RA = 1125ms was significantly shorter (p < 10−3) and the relative reduction to T1,O2 = 1081ms on average ΔT1 = 4.2%(p < 10−5). On the second day, with T1,RA = 1186ms in asthma and T1,RA = 1097ms in COPD, observed values were slightly shorter on average in all patient groups. ΔT1 reduction was the least repeatable parameter and varied from day to day by up to 23% in individual asthma and 30% in COPD patients. While for both patient groups T1 was below the values reported for healthy subjects, the T1 and ΔT1 found in asthmatics lies between that of the COPD group and reported values for healthy subjects, suggesting a higher blood volume fraction and better ventilation. However, it could be demonstrated that lung T1 quantification is subject to notable inter-examination variability, which here can be attributed both to remaining contrast agent from the previous day and the increased dependency of lung T1 on perfusion and thus current lung state.

Magnetic Resonance Imaging and Computational Fluid Dynamics of High Frequency Oscillatory Ventilation (HFOV)

In order to better understand the mechanisms of gas transport during High Frequency Oscillatory Ventilation (HFOV) Magnetic Resonance Imaging (MRI) with contrast gases and numerical flow simulations based on Computational Fluid Dynamics(CFD) methods are performed.

Flip-angle measurement by magnetization inversion: Calibration of magnetization nutation angle in hyperpolarized 3He magnetic resonance imaging lung experiments

The aim of this work was to establish a new, fast, and robust method of flip‐angle calibration for magnetic resonance imaging of hyperpolarized 3He. The method called flip‐angle measurement with magnetization inversion is based on acquiring images from periodically inverted longitudinal magnetization created using the spatial modulation of magnetization technique. By measuring the width of the area where the magnetization was inverted by the spatial modulation of magnetization preparation in phase images, the flip angle can be generated using a simple equation. To validate and establish the limits of the proposed method, flip‐angle measurement with magnetization inversion acquisitions were simulated and applied to proton and hyperpolarized 3He phantoms. Then, the calibration procedure was applied during hyperpolarized 3He magnetic resonance imaging in a healthy volunteer. The advantage of the flip‐angle measurement with magnetization inversion method compared with the conventional method based on the assessment of radiofrequency‐decay is that it is free of errors induced by relaxation due to oxygen, by imperfect excitation slice profile and by any diffusion of 3He into and out of the slice. Another advantage is that it does not require image processing with external software and therefore can be performed using the implemented tools on the magnetic resonance workstation. Magn Reson Med, 2011.

Realization of administration unit for 3He with gas recycling

Hyperpolarized (HP) noble gases (3He,129Xe) are used for MR-imaging of the lung. In the majority of case the HP gas is filled in Tedlarbags and directly inhaled by the patients. Starting from an earlier pilot device, an administration unit was built respectively to the Medical Devices Law to administer patients HP noble gas boli in defined quantities and at a predefined time during inspiration with high reproducibility and reliability without reducing MR-quality. The patients airflows are monitored and recorded. It is possible to use gas admixtures, measure the polarization on-line and collect the exhaled gas for later recycling. The first images with healthy volunteers were taken with this setup in a clinical study. Current results will be presented.