Jean-Noël Hyacinthe

Inflow effect correction in fast gradient-echo perfusion imaging.

The purposes of this study were to assess the extent of the inflow effect on signal intensity (SI) for fast gradient‐recalled‐echo (GRE) sequences used to observe first‐pass perfusion, and to develop and validate a correction method for this effect. A phantom experiment with a flow apparatus was performed to determine SI as a function of Gd‐DTPA concentration for various velocities. Subsequently a flow‐sensitive calibration method was developed, and validated on bolus injections into an open‐circuit flow apparatus and in vivo. It is shown that calibration methods based on static phantoms are not appropriate for accurate signal‐to‐concentration conversion in images affected by high flow. The flow‐corrected calibration method presented here can be used to improve the accuracy and robustness of the arterial input function (AIF) determination for tissue perfusion quantification using MRI and contrast media. Magn Reson Med 50:885–891, 2003.

Hyperpolarization without persistent radicals for in vivo real-time metabolic imaging

Significance Hyperpolarization is a significant development in MRI because it allows for imaging different metabolites in real time in vivo. There are no fundamental obstacles to rapid translation of this technique. Yet, to date, it has been necessary to use persistent radicals that need to be filtered out before injection and require pharmacological tests, which slow down the overall protocol, leading to reduced sensitivity. The demonstration that it is possible to prepare purely endogenous MRI agents to probe metabolism in vivo without using any potentially toxic compounds is a substantial step forward toward clinical radiology free of side effects. Hyperpolarized substrates prepared via dissolution dynamic nuclear polarization have been proposed as magnetic resonance imaging (MRI) agents for cancer or cardiac failure diagnosis and therapy monitoring through the detection of metabolic impairments in vivo. The use of potentially toxic persistent radicals to hyperpolarize substrates was hitherto required. We demonstrate that by shining UV light for an hour on a frozen pure endogenous substance, namely the glucose metabolic product pyruvic acid, it is possible to generate a concentration of photo-induced radicals that is large enough to highly enhance the 13C polarization of the substance via dynamic nuclear polarization. These radicals recombine upon dissolution and a solution composed of purely endogenous products is obtained for performing in vivo metabolic hyperpolarized 13C MRI with high spatial resolution. Our method opens the way to safe and straightforward preclinical and clinical applications of hyperpolarized MRI because the filtering procedure mandatory for clinical applications and the associated pharmacological tests necessary to prevent contamination are eliminated, concurrently allowing a decrease in the delay between preparation and injection of the imaging agents for improved in vivo sensitivity.

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.

In vivo labelling of resting monocytes in the reticuloendothelial system with fluorescent iron oxide nanoparticles prior to injury reveals that they are mobilized to infarcted myocardium.

AIMS To evaluate the feasibility of loading resting monocytes/macrophages by intravenous (i.v.) injection of fluorescent iron oxide nanoparticles prior to injury and tracking of these cells in the very same animal to myocardial infarction (MI) by magnetic resonance imaging (MRI) and optical imaging. METHODS AND RESULTS Rats were injected with fluorescent iron oxide nanoparticles (10 mg/kg) (n = 15) prior to injury. After disappearance of the nanoparticles from the blood, MI was induced. Monocytes/macrophages were then tracked in the very same animal by MRI and optical imaging. Control groups were (i) non-injected animals (n = 15), (ii) injected animals associated with a sham operation (n = 8), and (iii) animals treated with an anti-inflammatory agent (n = 6). The presence of iron-loaded cells can be detected by MRI in vivo in the infarcted myocardium. Here, we showed that the detection of inflammatory cells in vivo correlated well with ex vivo imaging (MRI and reflectance fluorescence) and histology. We also showed that the method is robust enough to depict changes in the inflammatory response. CONCLUSION This study demonstrates that resting monocytes/macrophages can be loaded in vivo by a simple i.v. injection of fluorescent superparamagnetic iron oxide nanoparticles prior to injury and then tracked, in the same animal, in a model of ischaemia-reperfusion leading to myocardial infarct. Although previous studies of macrophages infiltration following MI have labelled the macrophages after injury, this study, for the first time, has pre-load the resting monocytes with fluorescent iron oxide nanoparticles.

Cine and tagged cardiovascular magnetic resonance imaging in normal rat at 1.5 T: a rest and stress study

BackgroundThe purpose of this study was to measure regional contractile function in the normal rat using cardiac cine and tagged cardiovascular magnetic resonance (CMR) during incremental low doses of dobutamine and at rest.MethodsFive rats were investigated for invasive left ventricle pressure measurements and five additional rats were imaged on a clinical 1.5 T MR system using a cine sequence (11–20 phases per cycle, 0.28/0.28/2 mm) and a C-SPAMM tag sequence (18–25 phases per cycle, 0.63/1.79/3 mm, tag spacing 1.25 mm). For each slice, wall thickening (WT) and circumferential strains (CS) were calculated at rest and at stress (2.5, 5 and 10 μg/min/kg of dobutamine).ResultsGood cine and tagged images were obtained in all the rats even at higher heart rate (300–440 bpm). Ejection fraction and left ventricular (LV) end-systolic volume showed significant changes after each dobutamine perfusion dose (p < 0.001). Tagged CMR had the capacity to resolve the CS transmural gradient and showed a significant increase of both WT and CS at stress compared to rest. Intra and interobserver study showed less variability for the tagged technique. In rats in which a LV catheter was placed, dobutamine produced a significant increase of heart rate, LV dP/dtmax and LV pressure significantly already at the lowest infusion dose.ConclusionRobust cardiac cine and tagging CMR measurements can be obtained in the rat under incremental dobutamine stress using a clinical 1.5 T MR scanner.

High-resolution complementary spatial modulation of magnetization (CSPAMM) rat heart tagging on a 1.5 Tesla Clinical Magnetic Resonance System: a preliminary feasibility study.

The purpose of this study was to assess the feasibility of cardiac magnetic resonance (MR) tagging in rats on a standard clinical 1.5T MR system. Small animal models have been largely used as an experimental model in cardiovascular disease studies but mainly on high field systems (>4T) dedicated to research. Given the larger availability of routine clinical MR systems in centers with active cardiac research programs, it is of great interest to perform small animal imaging on whole-body MR systems of moderate field strength. The feasibility study was performed on 7 rats within 6 to 8 hours after myocardial infarction and 3 normal control rats. Myocardial strain was measured successfully in normal rats using the harmonic phase (ie, HARP) method, and a transmural gradient was demonstrated. In a rat model of acute occlusion/reperfusion, the myocardial circumferential strains were decreased, but the transmural strain gradient was preserved. This study demonstrated the feasibility of cardiac MR tagging in rats with a subendocardial resolution using a clinical 1.5T system.

Myocardial infarction quantification with Manganese‐Enhanced MRI (MEMRI) in mice using a 3T clinical scanner

Manganese (Mn2+) was recognized early as an efficient intracellular MR contrast agent to assess cardiomyocyte viability. It had previously been used for the assessment of myocardial infarction in various animal models from pig to mouse. However, whether Manganese‐Enhanced MRI (MEMRI) is also able to assess infarction in the acute phase of a coronary occlusion reperfusion model in mice has not yet been demonstrated. This model is of particular interest as it is closer to the situation encountered in the clinical setting. This study aimed to measure infarction volume taking TTC staining as a gold standard, as well as global and regional function before and after Mn2+ injection using a clinical 3T scanner. The first step of this study was to perform a dose‐response curve in order to optimize the injection protocol. Infarction volume measured with MEMRI was strongly correlated to TTC staining. Ejection fraction (EF) and percent wall thickening measurements allowed evaluation of global and regional function. While EF must be measured before Mn2+ injection to avoid bias introduced by the reduction of contrast in cine images, percent wall thickening can be measured either before or after Mn2+ injection and depicts accurately infarct related contraction deficit. This study is the first step for further longitudinal studies of cardiac disease in mice on a clinical 3T scanner, a widely available platform. Copyright

SNR enhancement of highly-accelerated real-time cardiac MRI acquisitions based on non-local means algorithm

Real-time cardiac MRI appears as a promising technique to evaluate the mechanical function of the heart. However, ultra-fast MRI acquisitions come with an important signal-to-noise ratio (SNR) penalty, which drastically reduces the image quality. Hence, a real-time denoising approach would be desirable for SNR amelioration. In the clinical context of cardiac dysfunction assessment, long acquisitions are required and for most patients the acquisition takes place with free breathing. Hence, it is necessary to compensate respiratory motion in real-time. In this article, a real-time and interactive method for sequential registration and denoising of real-time MR cardiac images is presented. The method has been experimented on 60 fast MRI acquisitions in five healthy volunteers and five patients. These experiments assessed the feasibility of the method in a real-time context.

Extrema temporal chaining: a new method for computing the 2d-displacement field of the heart from tagged MRI

This work takes is part of a medical research project which intends to induce and study cardiac hibernation in rats. The underlying goal is to understand the physiology of heart disease. We present here a novel method to compute the 2D-deformation field of the heart (rat or human) from tagged MRI. Previous work is not suitable for wide clinical use for different reasons, including important computing time and lack of robustness. We propose an original description of tags as local minima of 1D signals. This leads us to a new formulation of the tag tracking problem as an Extrema Temporal Chaining (ETC) and a 2D-rendering. 2D-displacements are then interpolated on a dense field. The developed method is fast and robust. Its performances are compared to those of HARP, a leading method in this field.

The role of imaging and molecular imaging in the early detection of metabolic and cardiovascular dysfunctions

Despite intense effort, obesity is still rising throughout the world. Links between obesity and cardiovascular diseases are now well established. Most of the cardiovascular changes related to obesity can be followed by magnetic resonance imaging (MRI) or by magnetic resonance spectroscopy (MRS). In particular, we will see in this review that MRI/MRS is extremely well suited to depict (1) changes in cardiac mass and function, (2) changes in stroke volume, (3) accumulation of fat inside the mediastinum or even inside the cardiomyocytes, (4) cell viability and (5) molecular changes during early cardiovascular diseases.