Magalie Viallon

A combined 1H perfusion/3He ventilation NMR study in rat lungs

The assessment of both pulmonary perfusion and ventilation is of crucial importance for a proper diagnosis of some lung diseases such as pulmonary embolism. In this study, we demonstrate the feasibility of combined magnetic resonance imaging lung ventilation and perfusion performed serially in rat lungs. Lung ventilation function was assessed using hyperpolarized 3He, and lung perfusion proton imaging was demonstrated using contrast agent injection. Both imaging techniques have been implemented using projection‐reconstruction sequences with free induction decay signal acquisitions. The study focused on fast three‐dimensional (3D) data acquisition. The projection‐reconstruction sequences used in this study allowed 3D data set acquisition in several minutes without high‐performance gradients. 3D proton perfusion/helium ventilation imaging has been demonstrated on an experimental rat model of pulmonary embolism showing normal lung ventilation associated with lung perfusion defect. Assuming the possibility, still under investigation, of showing lung obstruction pathologies using 3He imaging, these combined perfusion/ventilation methods could play a significant clinical role in the future for diagnosis of several pulmonary diseases. Magn Reson Med 41:645–648, 1999.

Laser-polarized 3He as a probe for dynamic regional measurements of lung perfusion and ventilation using magnetic resonance imaging

Magnetic resonance imaging (MRI) using laser‐polarized noble gases, such as 129Xe and 3He, allows unparalleled noninvasive information on gas distribution in lung airways and distal spaces. In addition to pulmonary ventilation, lung perfusion assessment is crucial for proper diagnosis of pathological conditions, such as pulmonary embolism. Magnetic resonance perfusion imaging usually can be performed using techniques based on the detection of water protons in tissues. However, lung proton imaging is extremely difficult due to the low proton density and the magnetically inhomogeneous structure of the lung parenchyma. Here we show that laser‐polarized 3He can be used as a noninvasive probe to image, in a single MRI experiment, not only the ventilation but also the perfusion state of the lungs. Blood volume maps of the lungs were generated based on the 3He signal depletion during the first pass of a superparamagnetic contrast agent bolus. The combined and simultaneous lung ventilation and perfusion assessments are demonstrated in normal rat lungs and are applied to an experimental animal model of pulmonary embolism. Magn Reson Med 44:1–4, 2000.

Combined Use of Pulsed Arterial Spin-Labeling and Susceptibility-Weighted Imaging in Stroke at 3T

Background and Purpose: In acute stroke it is no longer sufficient to detect simply ischemia, but also to try to evaluate reperfusion/recanalization status and predict eventual hemorrhagic transformation. Arterial spin labeling (ASL) perfusion may have advantages over contrast-enhanced perfusion-weighted imaging (cePWI), and susceptibility weighted imaging (SWI) has an intrinsic sensitivity to paramagnetic effects in addition to its ability to detect small areas of bleeding and hemorrhage. We want to determine here if their combined use in acute stroke and stroke follow-up at 3T could bring new insight into the diagnosis and prognosis of stroke leading to eventual improved patient management. Methods: We prospectively examined 41 patients admitted for acute stroke (NIHSS >1). Early imaging was performed between 1 h and 2 weeks. The imaging protocol included ASL, cePWI, SWI, T2 and diffusion tensor imaging (DTI), in addition to standard stroke protocol. Results: We saw four kinds of imaging patterns based on ASL and SWI: patients with either hypoperfusion and hyperperfusion on ASL with or without changes on SWI. Hyperperfusion was observed on ASL in 12/41 cases, with hyperperfusion status that was not evident on conventional cePWI images. Signs of hemorrhage or blood-brain barrier breakdown were visible on SWI in 15/41 cases, not always resulting in poor outcome (2/15 were scored mRS = 0–6). Early SWI changes, together with hypoperfusion, were associated with the occurrence of hemorrhage. Hyperperfusion on ASL, even when associated with hemorrhage detected on SWI, resulted in good outcome. Hyperperfusion predicted a better outcome than hypoperfusion (p = 0.0148). Conclusions: ASL is able to detect acute-stage hyperperfusion corresponding to luxury perfusion previously reported by PET studies. The presence of hyperperfusion on ASL-type perfusion seems indicative of reperfusion/collateral flow that is protective of hemorrhagic transformation and a marker of favorable tissue outcome. The combination of hypoperfusion and changes on SWI seems on the other hand to predict hemorrhage and/or poor outcome.

Reference-Free PRFS MR-Thermometry Using Near-Harmonic 2-D Reconstruction of the Background Phase

Proton resonance frequency shift (PRFS) MR thermometry (MRT) is the generally preferred method for monitoring thermal ablation, typically implemented with gradient-echo (GRE) sequences. Standard PRFS MRT is based on the subtraction of a temporal reference phase map and is, therefore, intrinsically sensitive to tissue motion (including deformation) and to external perturbation of the magnetic field. Reference-free (or reference-less) PRFS MRT has been previously described by Rieke and was based on a 2-D polynomial fit performed on phase data from outside the heated region, to estimate the background phase inside the region of interest. While their approach was undeniably a fundamental progress in terms of robustness against tissue motion and magnetic perturbations, the underlying mathematical formalism requires a thick unheated border and may be subject to numerical instabilities with high order polynomials. A novel method of reference-free PRFS MRT is described here, using a physically consistent formalism, which exploits mathematical properties of the magnetic field in a homogeneous or near-homogeneous medium.

Ultrasonography-based 2D motion-compensated HIFU sonication integrated with reference-free MR temperature monitoring: a feasibility study ex vivo

Magnetic resonance imaging (MRI) and ultrasonography have been used simultaneously in this ex vivo study for the image-guidance of high intensity focused ultrasound (HIFU) treatment in moving tissue. A ventilator-driven balloon produced periodic and non-rigid (i.e. breathing-like) motion patterns in phantoms. MR-compatible ultrasound (US) imaging enabled near real-time 2D motion tracking based on optical flow detection, while near-harmonic reference-free proton resonance frequency shift (PRFS) MR thermometry (MRT) was used to monitor the thermal buildup on line. Reference-free MRT was applied to gradient-echo echo-planar imaging phase maps acquired at the frame rate of 250 to 300 ms/slice with voxel size 1.25×1.25×5 mm(3). The MR-US simultaneous imaging was completely free of mutual interferences while minor RF interferences from the HIFU device were detected in the far field of the US images. The effective duty-cycle of the HIFU sonication was close to 100 % and no off-interval was required to temporally decouple it from the ultrasonography. The motion compensation of the HIFU sonication was achieved with an 8 Hz frame rate and sub-millimeter spatial accuracy, both for single-focus mode and for an iterated multi-foci line scan. Near harmonic reference-less PRFS MRT delivered motion-robust thermal maps perpendicular or parallel to the HIFU beam (0.7 °C precision, 0.5 °C absolute accuracy). Out-of-plane motion compensation was not addressed in this study.

State-of-the-art MRI techniques in neuroradiology: principles, pitfalls, and clinical applications

This article reviews the most relevant state-of-the-art magnetic resonance (MR) techniques, which are clinically available to investigate brain diseases. MR acquisition techniques addressed include notably diffusion imaging (diffusion-weighted imaging (DWI), diffusion tensor imaging (DTI), and diffusion kurtosis imaging (DKI)) as well as perfusion imaging (dynamic susceptibility contrast (DSC), arterial spin labeling (ASL), and dynamic contrast enhanced (DCE)). The underlying models used to process these images are described, as well as the theoretic underpinnings of quantitative diffusion and perfusion MR imaging-based methods. The technical requirements and how they may help to understand, classify, or follow-up neurological pathologies are briefly summarized. Techniques, principles, advantages but also intrinsic limitations, typical artifacts, and alternative solutions developed to overcome them are discussed. In this article, we also review routinely available three-dimensional (3D) techniques in neuro MRI, including state-of-the-art and emerging angiography sequences, and briefly introduce more recently proposed 3D quantitative neuro-anatomy sequences, and new technology, such as multi-slice and multi-transmit imaging.

Whole-body Mri for Metastases Screening: A Preliminary Study Using 3d Vibe Sequences With Automatic Subtraction Between Noncontrast and Contrast Enhanced Images

Objectives:To evaluate 3D Volumetric Interpolated Breath-hold Examination (VIBE) whole-body MRI (WB-MRI) acquisition for the metastases staging. Methods:Thirty-two consecutive patients with solid tumor were examined from head to feet before and after contrast injection. An automatic subtraction occurred between the 2 series of images. WB-MRI was compared with conventional staging techniques (CT, scintigraphy, brain MRI, and whole-body PET in 4 cases). Results:WB-MRI and the reference techniques depicted metastases in 25 patients. WB-MRI depicted more bone lesions in the spine, pelvis, skull, femur, and tibia, whereas scintigraphy detected more rib lesions. WB-MRI depicted 27 cerebral metastases, whereas brain MRI depicted 40 cerebral metastases. WB-MRI depicted a total of 8 hepatic metastases, 8 adrenal lesions, and conventional staging 7 hepatic metastases and 10 adrenal lesions. WB-MRI examination depicted lung metastases in 10 patients, and CT examination in 13 patients. Conclusion:The results of this study indicate that WB-MRI is a feasible and promising technique for tumor staging.

Neuro-imaging of cerebral ischemic stroke

Major progress has recently been made in the neuro-imaging of stroke as a result of improvements in imaging hardware and software. Imaging may be based on either magnetic resonance imaging (MRI) or computed tomography (CT) techniques. Imaging should provide information on the entire vascular cervical and intracranial network, from the aortic arch to the circle of Willis. Equally, it should also give information on the viability of brain tissue and brain hemodynamics. CT has the advantage in the detection of acute hemorrhage whereas MRI offers more accurate pathophysiological information in the follow-up of patients.

Hybrid Ultrasound/Magnetic Resonance Simultaneous Acquisition and Image Fusion for Motion Monitoring in the Upper Abdomen

ObjectivesThe combination of ultrasound (US) and magnetic resonance imaging (MRI) may provide a complementary description of the investigated anatomy, together with improved guidance and assessment of image-guided therapies. The aim of the present study was to integrate a clinical setup for simultaneous US and magnetic resonance (MR) acquisition to obtain synchronized monitoring of liver motion. The feasibility of this hybrid imaging and the precision of image fusion were evaluated. Materials and MethodsUltrasound imaging was achieved using a clinical US scanner modified to be MR compatible, whereas MRI was achieved on 1.5- and 3-T clinical scanners. Multimodal registration was performed between a high-resolution T1 3-dimensional (3D) gradient echo (volume interpolated gradient echo) during breath-hold and a simultaneously acquired 2D US image, or equivalent, retrospective registration of US imaging probe in the coordinate frame of MRI. A preliminary phantom study was followed by 4 healthy volunteer acquisitions, performing simultaneous 4D MRI and 2D US harmonic imaging (Fo = 2.2 MHz) under free breathing. ResultsNo characterized radiofrequency mutual interferences were detected under the tested conditions with commonly used MR sequences in clinical routine, during simultaneous US/MRI acquisition. Accurate spatial matching between the 2D US and the corresponding MRI plane was obtained during breath-hold. In situ fused images were delivered. Our 4D MRI sequence permitted the dynamic reconstruction of the intra-abdominal motion and the calculation of high temporal resolution motion field vectors. ConclusionsThis study demonstrates that, truly, simultaneous US/MR dynamic acquisition in the abdomen is achievable using clinical instruments. A potential application is the US/MR hybrid guidance of high-intensity focused US therapy in the liver.

ARFI-prepared MRgHIFU in liver: simultaneous mapping of ARFI-displacement and temperature elevation, using a fast GRE-EPI sequence

MR acoustic radiation force imaging (ARFI) is an elegant adjunct to MR‐guided high intensity focused ultrasound for treatment planning and optimization, permitting in situ assessment of the focusing and targeting quality. The thermal effect of high intensity focused ultrasound pulses associated with ARFI measurements is recommended to be monitored on line, in particular when the beam crosses highly absorbent structures or interfaces (e.g., bones or air‐filled cavities). A dedicated MR sequence is proposed here, derived from a segmented gradient echo‐echo planar imaging kernel by adding a bipolar motion encoding gradient with interleaved alternating polarities. Temporal resolution was reduced to 2.1 s, with in‐plane spatial resolution of 1 mm. MR‐ARFI measurements were executed during controlled animal breathing, with trans‐costal successively steered foci, to investigate the spatial modulation of the focus intensity and the targeting offset. ARFI‐induced tissue displacement measurements enabled the accurate localization, in vivo, of the high intensity focused ultrasound focal point in sheep liver, with simultaneous monitoring of the temperature elevation. ARFI‐based precalibration of the focal point position was immediately followed by trans‐costal MR‐guided high intensity focused ultrasound ablation, monitored with a conventional proton resonance frequency shift MR thermometry sequence. The latter MR thermometry sequence had spatial resolution and geometrical distortion identical with the ARFI maps, hence no coregistration was required. Magn Reson Med, 2012.