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RAPACCHI Stanislas

Postdoctoral Fellow
PhD in Physics
Key Words
- Magnetic Resonance Imaging (MRI) - Cardiovascular MRI
- Ultra-high field (7 Tesla) MRI of the in vivo human heart
- Non-invasive perfusion imaging of the myocardium by Arterial Spin Labeling (ASL)
- Sodium (23Na) cardiac MRI - Phosphorus (31P) cardiac MRI and MR-Spectroscopy (MRS)
- Accelerated MRI image reconstruction techniques: non-Cartesian acquisitions, parallel imaging and Compressed Sensing

Current Research Interest and projects

I am a MRI physicists involved in sequence programming (Siemens MR), images processing and reconstruction (Matlab, C/C++, Objective-C), acquisition strategy and experimental set-up (clinical & large animals) and statistical analysis.

My current project at Aix-Marseille University involves the development, validation and application of novel MRI diagnostic techniques for cardiac imaging with a strong focus on novel techniques at high (3T) and ultra-high (7T) magnetic field.

My work includes the implementation, optimization and validation of non-contrast perfusion MRI using Arterial Spin Labeling, Sodium MRI and Phosphorus MRI/MRS for advanced clinical diagnosis of the in vivo human heart.

Specific topics:
- MR sequence simulation and optimization. RF pulse design (Matlab/C)
- MR sequence programming (Siemens platform, C++)
- Evaluation in vivo and statistical analysis
- Implementation of advanced reconstruction techniques for sodium MRI (2D ultra-short TE MR sequence)



Journal Article

  • Mchinda, S, Varma, G, Prevost, VH, Le Troter, A, Rapacchi, S, Guye, M, Pelletier, J, Ranjeva, J-P, Alsop, DC, Duhamel, G & Girard, OM 2018, “Whole brain inhomogeneous magnetization transfer (ihMT) imaging: Sensitivity enhancement within a steady-state gradient echo sequence”, Magnetic Resonance in Medicine, vol. 79, no. 5, p. 2607-2619.
    Résumé : PURPOSE: To implement, characterize, and optimize an interleaved inhomogeneous magnetization transfer (ihMT) gradient echo sequence allowing for whole-brain imaging within a clinically compatible scan time. THEORY AND METHODS: A general framework for ihMT modelling was developed based on the Provotorov theory of radiofrequency saturation, which accounts for the dipolar order underpinning the ihMT effect. Experimental studies and numerical simulations were performed to characterize and optimize the ihMT-gradient echo dependency with sequence timings, saturation power, and offset frequency. The protocol was optimized in terms of maximum signal intensity and the reproducibility assessed for a nominal resolution of 1.5 mm isotropic. All experiments were performed on healthy volunteers at 1.5T. RESULTS: An important mechanism driving signal optimization and leading to strong ihMT signal enhancement that relies on the dynamics of radiofrequency energy deposition has been identified. By taking advantage of the delay allowed for readout between ihMT pulse bursts, it was possible to boost the ihMT signal by almost 2-fold compared to previous implementation. Reproducibility of the optimal protocol was very good, with an intra-individual error < 2%. CONCLUSION: The proposed sensitivity-boosted and time-efficient steady-state ihMT-gradient echo sequence, implemented and optimized at 1.5T, allowed robust high-resolution 3D ihMT imaging of the whole brain within a clinically compatible scan time. Magn Reson Med 79:2607-2619, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
    Mots-clés : crmbm, dipolar order, dual frequency RF saturation, ihMT, inhomogeneous magnetization transfer, magnetization transfer model, myelin, Provotorov theory of radiofrequency saturation, snc.

  • Zhou, Z, Han, F, Yu, S, Yu, D, Rapacchi, S, Song, HK, Wang, DJJ, Hu, P & Yan, L 2018, “Accelerated noncontrast-enhanced 4-dimensional intracranial MR angiography using golden-angle stack-of-stars trajectory and compressed sensing with magnitude subtraction”, Magnetic Resonance in Medicine, vol. 79, no. 2, p. 867-878.
    Résumé : PURPOSE: To evaluate the feasibility and performance of compressed sensing (CS) with magnitude subtraction regularization in accelerating non-contrast-enhanced dynamic intracranial MR angiography (NCE-dMRA). METHODS: A CS algorithm was introduced in NCE-dMRA by exploiting the sparsity of the magnitude difference of the control and label images. The NCE-dMRA data were acquired using golden-angle stack-of-stars trajectory on six healthy volunteers and one patient with arteriovenous fistula. Images were reconstructed using (i) the proposed magnitude-subtraction CS (MS-CS); (ii) complex-subtraction CS; (iii) independent CS; and (iv) view-sharing with k-space weighted image contrast (KWIC). The dMRA image quality was compared across the four reconstruction strategies. The proposed MS-CS method was further compared with KWIC for temporal fidelity of depicting dynamic flow. RESULTS: The proposed MS-CS method was able to reconstruct NCE-dMRA images with detailed vascular structures and clean background. It provided better subjective image quality than the other two CS strategies (P < 0.05). Compared with KWIC, MS-CS showed similar image quality, but reduced temporal blurring in delineating the fine distal arteries. CONCLUSIONS: The MS-CS method is a promising CS technique for accelerating NCE-dMRA acquisition without compromising image quality and temporal fidelity. Magn Reson Med 79:867-878, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
    Mots-clés : arterial spin labeling, compressed sensing, KWIC, magnitude subtraction, noncontrast MR angiography, view sharing.


Journal Article

  • Bydder, M, Rapacchi, S, Girard, O, Guye, M & Ranjeva, J-P 2017, “Trimmed autocalibrating k-space estimation based on structured matrix completion”, Magnetic Resonance Imaging, vol. 43, p. 88-94.
    Résumé : PURPOSE: Parallel imaging allows the reconstruction of undersampled data from multiple coils. This provides a means to reject and regenerate corrupt data (e.g. from motion artefact). The purpose of this work is to approach this problem using the SAKE parallel imaging method. THEORY AND METHODS: Parallel imaging methods typically require calibration by fully sampling the center of k-space. This is a challenge in the presence of corrupted data, since the calibration data may be corrupted which leads to an errors-in-variables problem that cannot be solved by least squares or even iteratively reweighted least squares. The SAKE method, based on matrix completion and structured low rank approximation, was modified to detect and trim these errors from the data. RESULTS: Simulated and actual corrupted datasets were reconstructed with SAKE, the proposed approach and a more standard reconstruction method (based on solving a linear equation) with a data rejection criterion. The proposed approach was found to reduce artefacts considerably in comparison to the other two methods. CONCLUSION: SAKE with data trimming improves on previous methods for reconstructing images from grossly corrupted data.
    Mots-clés : Artefacts, crmbm, IRLS, Parallel imaging, Robust, snc, Structured low rank approximation.

  • Donadieu, M, Le Fur, Y, Maarouf, A, Gherib, S, Ridley, B, Pini, L, Rapacchi, S, Confort-Gouny, S, Guye, M, Schad, LR, Maudsley, AA, Pelletier, J, Audoin, B, Zaaraoui, W & Ranjeva, J-P 2017, “Metabolic counterparts of sodium accumulation in multiple sclerosis: A whole brain (23)Na-MRI and fast (1)H-MRSI study”, Multiple Sclerosis (Houndmills, Basingstoke, England), p. 1352458517736146.
    Résumé : BACKGROUND: Increase of brain total sodium concentrations (TSC) is present in multiple sclerosis (MS), but its pathological involvement has not been assessed yet. OBJECTIVE: To determine in vivo the metabolic counterpart of brain sodium accumulation. MATERIALS/METHODS: Whole brain (23)Na-MR imaging and 3D-(1)H-EPSI data were collected in 21 relapsing-remitting multiple sclerosis (RRMS) patients and 20 volunteers. Metabolites and sodium levels were extracted from several regions of grey matter (GM), normal-appearing white matter (NAWM) and white matter (WM) T2 lesions. Metabolic and ionic levels expressed as Z-scores have been averaged over the different compartments and used to explain sodium accumulations through stepwise regression models. RESULTS: MS patients showed significant (23)Na accumulations with lower choline and glutamate-glutamine (Glx) levels in GM; (23)Na accumulations with lower N-acetyl aspartate (NAA), Glx levels and higher Myo-Inositol (m-Ins) in NAWM; and higher (23)Na, m-Ins levels with lower NAA in WM T2 lesions. Regression models showed associations of TSC increase with reduced NAA in GM, NAWM and T2 lesions, as well as higher total-creatine, and smaller decrease of m-Ins in T2 lesions. GM Glx levels were associated with clinical scores. CONCLUSION: Increase of TSC in RRMS is mainly related to neuronal mitochondrial dysfunction while dysfunction of neuro-glial interactions within GM is linked to clinical scores.
    Mots-clés : 23Na-MRI, crmbm, demyelination, MRSI, Multiple sclerosis, neurodegeneration, snc, stepwise regression.

  • Han, F, Rapacchi, S & Hu, P 2017, “Prospective cardiac motion self-gating”, Quantitative Imaging in Medicine and Surgery, vol. 7, no. 2, p. 215-226.
    Résumé : BACKGROUND: To develop a prospective cardiac motion self-gating method that provides robust and accurate cardiac triggers in real time. METHODS: The proposed self-gating method consists of an "imaging mode" that acquires the k-space segments and a "self-gating mode" that captures the cardiac motion by repeatedly sampling the k-space centerline. A training based principal component analysis algorithm is utilized to process the self-gating data where the projection onto the first principal component was used as the self-gating signal. Retrospective studies using a sequence with self-gating mode only was performed on 8 healthy subjects to validate the accuracy and reliability of the self-gating triggers. Prospective studies using both ECG-gated and self-gated cardiac CINE sequences were conducted on 6 healthy subjects to compare the image quality. RESULTS: Using the ECG as the reference, the proposed method was able to detect self-gating triggers within ±10 ms accuracy on all 8 subjects in the retrospective study. The prospectively self-gated CINE sequence successfully detected 100% of the cardiac triggers and provided excellent CINE image quality without using ECG signals. CONCLUSIONS: The proposed cardiac self-gating method is a robust and accurate alternative to conventional ECG-based gating method for a number of cardiac MRI applications.
    Mots-clés : Cardiac MRI, motion correction using multiple coil array (MOCCA), principal component analysis (PCA), prospective gating, Self-gating.

  • Zhou, Z, Han, F, Rapacchi, S, Nguyen, K-L, Brunengraber, DZ, Kim, G-HJ, Finn, JP & Hu, P 2017, “Accelerated ferumoxytol-enhanced 4D multiphase, steady-state imaging with contrast enhancement (MUSIC) cardiovascular MRI: validation in pediatric congenital heart disease”, NMR in biomedicine, vol. 30, no. 1.
    Résumé : The purpose of this work was to validate a parallel imaging (PI) and compressed sensing (CS) combined reconstruction method for a recently proposed 4D non-breath-held, multiphase, steady-state imaging technique (MUSIC) cardiovascular MRI in a cohort of pediatric congenital heart disease patients. We implemented a graphics processing unit accelerated CS-PI combined reconstruction method and applied it in 13 pediatric patients who underwent cardiovascular MRI after ferumoxytol administration. Conventional breath-held contrast-enhanced magnetic resonance angiography (CE-MRA) was first performed during the first pass of ferumoxytol injection, followed by the original MUSIC and the proposed CS-PI MUSIC during the steady-state distribution phase of ferumoxytol. Qualities of acquired images were then evaluated using a four-point scale. Left ventricular volumes and ejection fractions calculated from the original MUSIC and the CS-PI MUSIC were also compared with conventional multi-slice 2D cardiac cine MRI. The proposed CS-PI MUSIC reduced the imaging time of the MUSIC acquisition to 4.6 ± 0.4 min from 8.9 ± 1.2 min. Computationally intensive image reconstruction was completed within 5 min without interruption of sequential clinical scans. The proposed method (mean 3.3-4.0) provided image quality comparable to that of the original MUSIC (3.2-4.0) (all P ≥ 0.42), and better than conventional breath-held first-pass CE-MRA (1.1-3.3) for 13 anatomical structures (all P ≤ 0.0014) with good inter-observer agreement (κ > 0.46). The calculated ventricular volumes and ejection fractions from both original MUSIC (r > 0.90) and CS-PI MUSIC (r > 0.85) correlated well with 2D cine imaging. In conclusion, PI and CS were successfully incorporated into the 4D MUSIC acquisition to further reduce scan time by approximately 50% while maintaining highly comparable image quality in a clinically practical reconstruction time.
    Mots-clés : cardiac cine, compressed sensing, Ferumoxytol, Magnetic Resonance Angiography, Parallel imaging, pediatric MRI.


Journal Article

  • Rashid, S, Rapacchi, S, Shivkumar, K, Plotnik, A, Finn, JP & Hu, P 2016, “Modified wideband three-dimensional late gadolinium enhancement MRI for patients with implantable cardiac devices”, Magnetic Resonance in Medicine, vol. 75, no. 2, p. 572-584.
    Résumé : PURPOSE: To study the effects of cardiac devices on three-dimensional (3D) late gadolinium enhancement (LGE) MRI and to develop a 3D LGE protocol for implantable cardioverter defibrillator (ICD) patients with reduced image artifacts. THEORY AND METHODS: The 3D LGE sequence was modified by implementing a wideband inversion pulse, which reduces hyperintensity artifacts, and by increasing bandwidth of the excitation pulse. The modified wideband 3D LGE sequence was tested in phantoms and evaluated in six volunteers and five patients with ICDs. RESULTS: Phantom and in vivo studies results demonstrated extended signal void and ripple artifacts in 3D LGE that were associated with ICDs. The reason for these artifacts was slab profile distortion and the subsequent aliasing in the slice-encoding direction. The modified wideband 3D LGE provided significantly reduced ripple artifacts than 3D LGE with wideband inversion only. Comparison of 3D and 2D LGE images demonstrated improved spatial resolution of the heart using 3D LGE. CONCLUSION: Increased bandwidth of the inversion and excitation pulses can significantly reduce image artifacts associated with ICDs. Our modified wideband 3D LGE protocol can be readily used for imaging patients with ICDs given appropriate safety guidelines are followed. Magn Reson Med 75:572-584, 2016. © 2015 Wiley Periodicals, Inc.
    Mots-clés : delayed enhancement, hyperintensity artifact, implantable cardiac devices, implantable cardioverter defibrillator, metal artifact reduction, Three dimensional late gadolinium enhancement, wideband inversion.

  • Shao, J, Rapacchi, S, Nguyen, K-L & Hu, P 2016, “Myocardial T1 mapping at 3.0 tesla using an inversion recovery spoiled gradient echo readout and bloch equation simulation with slice profile correction (BLESSPC) T1 estimation algorithm”, Journal of magnetic resonance imaging: JMRI, vol. 43, no. 2, p. 414-425.
    Résumé : BACKGROUND: To develop an accurate and precise myocardial T1 mapping technique using an inversion recovery spoiled gradient echo readout at 3.0 Tesla (T). THEORY AND METHODS: The modified Look-Locker inversion-recovery (MOLLI) sequence was modified to use fast low angle shot (FLASH) readout, incorporating a BLESSPC (Bloch Equation Simulation with Slice Profile Correction) T1 estimation algorithm, for accurate myocardial T1 mapping. The FLASH-MOLLI with BLESSPC fitting was compared with different approaches and sequences with regards to T1 estimation accuracy, precision and image artifact based on simulation, phantom studies, and in vivo studies of 10 healthy volunteers and three patients at 3.0 Tesla. RESULTS: The FLASH-MOLLI with BLESSPC fitting yields accurate T1 estimation (average error = -5.4 ± 15.1 ms, percentage error = -0.5% ± 1.2%) for T1 from 236-1852 ms and heart rate from 40-100 bpm in phantom studies. The FLASH-MOLLI sequence prevented off-resonance artifacts in all 10 healthy volunteers at 3.0T. In vivo, there was no significant difference between FLASH-MOLLI-derived myocardial T1 values and "ShMOLLI+IE" derived values (1458.9 ± 20.9 ms versus 1464.1 ± 6.8 ms, P = 0.50); However, the average precision by FLASH-MOLLI was significantly better than that generated by "ShMOLLI+IE" (1.84 ± 0.36% variance versus 3.57 ± 0.94%, P < 0.001). CONCLUSION: The FLASH-MOLLI with BLESSPC fitting yields accurate and precise T1 estimation, and eliminates banding artifacts associated with bSSFP at 3.0T. J. Magn. Reson. Imaging 2016;43:414-425.
    Mots-clés : high field T1 mapping, MOLLI, MR relaxometry, quantitative cardiac MRI.

  • Skelton, RJP, Khoja, S, Almeida, S, Rapacchi, S, Han, F, Engel, J, Zhao, P, Hu, P, Stanley, EG, Elefanty, AG, Kwon, M, Elliott, DA & Ardehali, R 2016, “Magnetic Resonance Imaging of Iron Oxide-Labeled Human Embryonic Stem Cell-Derived Cardiac Progenitors”, Stem Cells Translational Medicine, vol. 5, no. 1, p. 67-74.
    Résumé : Given the limited regenerative capacity of the heart, cellular therapy with stem cell-derived cardiac cells could be a potential treatment for patients with heart disease. However, reliable imaging techniques to longitudinally assess engraftment of the transplanted cells are scant. To address this issue, we used ferumoxytol as a labeling agent of human embryonic stem cell-derived cardiac progenitor cells (hESC-CPCs) to facilitate tracking by magnetic resonance imaging (MRI) in a large animal model. Differentiating hESCs were exposed to ferumoxytol at different time points and varying concentrations. We determined that treatment with ferumoxytol at 300 μg/ml on day 0 of cardiac differentiation offered adequate cell viability and signal intensity for MRI detection without compromising further differentiation into definitive cardiac lineages. Labeled hESC-CPCs were transplanted by open surgical methods into the left ventricular free wall of uninjured pig hearts and imaged both ex vivo and in vivo. Comprehensive T2*-weighted images were obtained immediately after transplantation and 40 days later before termination. The localization and dispersion of labeled cells could be effectively imaged and tracked at days 0 and 40 by MRI. Thus, under the described conditions, ferumoxytol can be used as a long-term, differentiation-neutral cell-labeling agent to track transplanted hESC-CPCs in vivo using MRI. SIGNIFICANCE: The development of a safe and reproducible in vivo imaging technique to track the fate of transplanted human embryonic stem cell-derived cardiac progenitor cells (hESC-CPCs) is a necessary step to clinical translation. An iron oxide nanoparticle (ferumoxytol)-based approach was used for cell labeling and subsequent in vivo magnetic resonance imaging monitoring of hESC-CPCs transplanted into uninjured pig hearts. The present results demonstrate the use of ferumoxytol labeling and imaging techniques in tracking the location and dispersion of cell grafts, highlighting its utility in future cardiac stem cell therapy trials.
    Mots-clés : Cardiac stem cell biology, Cardiovascular progenitors, Cell labeling, Cell Tracking, Embryonic Stem Cells, Ferric Compounds, Ferrosoferric Oxide, Heterografts, Humans, Magnetic Resonance Imaging, Myoblasts, Cardiac, Stem cell therapy, Stem Cell Transplantation.


Journal Article

  • Han, F, Rapacchi, S, Khan, S, Ayad, I, Salusky, I, Gabriel, S, Plotnik, A, Finn, JP & Hu, P 2015, “Four-dimensional, multiphase, steady-state imaging with contrast enhancement (MUSIC) in the heart: a feasibility study in children”, Magnetic Resonance in Medicine, vol. 74, no. 4, p. 1042-1049.
    Résumé : PURPOSE: To develop a technique for high resolution, four-dimensional (4D), multiphase, steady-state imaging with contrast enhancement (MUSIC) in children with complex congenital heart disease. METHODS: Eight pediatric patients underwent cardiovascular MRI with controlled mechanical ventilation after ferumoxytol administration. Breath-held contrast-enhanced MRA (CE-MRA) was performed during the first-pass and delayed phases of ferumoxytol, followed by a respiratory gated, 4D MUSIC acquisition during the steady state distribution phase of ferumoxytol. The subjective image quality and image sharpness were evaluated. Assessment of ventricular volumes based on 4D MUSIC was compared with those based on multislice 2D cardiac cine MRI. RESULTS: The 4D MUSIC technique provided cardiac-phase-resolved (65-95 ms temporal resolution) and higher spatial resolution (0.6-0.9 mm isotropic) images than previously achievable using first-pass CE-MRA or 2D cardiac cine. When compared with Ferumoxytol-based first-pass CE-MRA, the 4D MUSIC provided sharper images and better definition of the coronary arteries, aortic root, myocardium, and pulmonary trunk (P < 0.05 for all). The ventricular volume measurements were in good agreement between 4D MUSIC and 2D cine (concordance correlation coefficient >0.95). CONCLUSION: The 4D MUSIC technique may represent a new paradigm in MR evaluation of cardiovascular anatomy and function in children with complex congenital heart disease.
    Mots-clés : 4D steady state, congenital heart disease, Ferumoxytol, intravascular contrast agent, MR angiography, pediatric MRI.

  • Han, F, Zhou, Z, Rapacchi, S, Nguyen, K-L, Finn, JP & Hu, P 2015, “Segmented golden ratio radial reordering with variable temporal resolution for dynamic cardiac MRI”, Magnetic Resonance in Medicine.
    Résumé : PURPOSE: Golden ratio (GR) radial reordering allows for retrospective choice of temporal resolution by providing a near-uniform k-space sampling within any reconstruction window. However, when applying GR to electrocardiogram (ECG)-gated cardiac imaging, the k-space coverage may not be as uniform because a single reconstruction window is broken into several temporally isolated ones. The goal of this study was to investigate the image artifacts caused by applying GR to ECG-gated cardiac imaging and to propose a segmented GR method to address this issue. METHODS: Computer simulation and phantom experiments were used to evaluate the image artifacts resulting from three k-space sampling patterns (ie, uniform radial, conventional GR, and segmented GR). Two- and three-dimensional cardiac cine images were acquired in seven healthy subjects. Imaging artifacts due to k-space sampling nonuniformity were graded on a 5-point scale by an experienced cardiac imaging reader. RESULTS: Segmented GR provides more uniform k-space sampling that is independent of heart-rate variation than conventional GR. Cardiac cine images using segmented GR have significantly higher and more reliable image quality than conventional GR. CONCLUSION: Segmented GR successfully addresses the nonuniform sampling that occurs with combining conventional GR with ECG gating. This technique can potentially be applied to any ECG-gated cardiac imaging application to allow for retrospective selection of a reconstruction window. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.
    Mots-clés : Cardiac MRI, golden ratio, RF golden angle radial, segmented k-space.

  • Rapacchi, S, Natsuaki, Y, Plotnik, A, Gabriel, S, Laub, G, Finn, JP & Hu, P 2015, “Reducing view-sharing using compressed sensing in time-resolved contrast-enhanced magnetic resonance angiography”, Magnetic Resonance in Medicine, vol. 74, no. 2, p. 474-481.
    Résumé : PURPOSE: To study temporal and spatial blurring artifacts from k-space view-sharing in time-resolved MR angiography (MRA) and to propose a technique for reducing these artifacts. METHODS: We acquired k-space data sets using a three-dimensional time-resolved MRA view-sharing sequence and retrospectively reformatted them into two reconstruction frameworks: full view-sharing via time-resolved imaging with stochastic trajectories (TWIST) and minimal k-space view-sharing and compressed sensing (CS-TWIST). The two imaging series differed in temporal footprint but not in temporal frame rate. The artifacts from view-sharing were compared qualitatively and quantitatively in nine patients in addition to a phantom experiment. RESULTS: CS-TWIST was able to reduce the imaging temporal footprint by two- to three-fold compared with TWIST, and the overall subjective image quality of CS-TWIST was higher than that for TWIST (P < 0.05). View sharing caused a delay in the visualization of small blood vessels, and the mean transit time of the carotid artery calculated based on TWIST reconstruction was 0.6 s longer than that for CS-TWIST (P < 0.01). In thoracic MRA, the shorter temporal footprint decreased the sensitivity to physiological motion blurring, and vessel sharpness was improved by 8.8% ± 6.0% using CS-TWIST (P < 0.05). CONCLUSION: In time-resolved MRA, the longer temporal footprint due to view-sharing causes spatial and temporal artifacts. CS-TWIST is a promising method for reducing these artifacts. Magn Reson Med 74:474-481, 2015. © 2014 Wiley Periodicals, Inc.

  • Rapacchi, S, Smith, RX, Wang, Y, Yan, L, Sigalov, V, Krasileva, KE, Karpouzas, G, Plotnik, A, Sayre, J, Hernandez, E, Verma, A, Burkly, L, Wisniacki, N, Torrington, J, He, X, Hu, P, Chiao, P-C & Wang, DJJ 2015, “Towards the identification of multi-parametric quantitative MRI biomarkers in lupus nephritis”, Magnetic Resonance Imaging, vol. 33, no. 9, p. 1066-1074.
    Résumé : PURPOSE: To identify potential biomarkers of the renal impairment in lupus nephritis using a multi-parametric renal quantitative MRI (qMRI) protocol including diffusion weighted imaging (DWI), blood oxygen level dependent (BOLD), arterial spin labeling (ASL) and T1rho MRI between a cohort of healthy volunteers and lupus nephritis (LN) patients. MATERIALS AND METHODS: The renal qMRI protocol was performed twice with repositioning in between on 10 LN patients and 10 matched controls at 1.5 T. Navigator-gated and breath-hold acquisitions followed by non-rigid image registration were used to control respiratory motion. The repeatability of the 4 MRI modalities was evaluated with the intra-class correlation coefficient (ICC) and within-subject coefficient of variation (wsCV). Unpaired t-test and stepwise logistic regression were carried out to evaluate qMRI parameters between the LN and control groups. RESULTS: The reproducibility of the 4 qMRI modalities ranged from moderate to good (ICC=0.4-0.91, wsCV≤12%) with a few exceptions. T1rho MRI and ASL renal blood flow (RBF) demonstrated significant differences between the LN and control groups. Stepwise logistic regression yielded only one significant parameter (medullar T1rho) in differentiating LN from control groups with 95% accuracy. CONCLUSION: A reasonable degree of test-retest repeatability and accuracy of a multi-parametric renal qMRI protocol has been demonstrated in healthy volunteers and LN subjects. T1rho and ASL RBF are promising imaging biomarkers of LN.
    Mots-clés : Arterial spin labeling (ASL), Blood oxygen level dependent (BOLD), Lupus nephritis (LN), Multi-parametric quantitative MRI (qMRI), Renal function, T1rho MRI.

  • Wang, D, Shao, J, Rapacchi, S, Middione, MJ, Ennis, DB & Hu, P 2015, “Phase contrast MRI with flow compensation view sharing”, Magnetic Resonance in Medicine, vol. 73, no. 2, p. 505-513.
    Résumé : PURPOSE: To develop and evaluate a technique for accelerating phase contrast MRI (PC-MRI) acquisitions without significant compromise in flow quantification accuracy. METHODS: PC-MRI is commonly acquired using interleaved flow-compensated (FC) and flow-encoded (FE) echoes. We hypothesized that FC data, which represent background phase, do not change significantly over time. Therefore, we proposed to undersample the FC data and use an FC view sharing (FCVS) approach to synthesize a composite FC frame for each corresponding FE frame. FCVS was evaluated in a flow phantom and healthy volunteers and compared with a standard FC/FE PC-MRI. RESULTS: The FCVS sequence resulted in an error of 0.0% for forward flow and 2.0% for reverse flow volume when compared with FC/FE PC-MRI in a flow phantom. Measurements in the common carotid arteries showed that the FCVS method had -1.16 cm/s bias for maximum peak velocity and -0.019 mL bias in total flow, when compared with FC/FE with the same temporal resolution, but double the total acquisition time. These results represent ≤1.3% bias error in velocity and volumetric flow quantification. CONCLUSION: FCVS can accelerate PC-MRI acquisitions while maintaining flow and velocity measurement accuracy when there is limited temporal variation in the FC data.


Journal Article

  • Gou, S, Wu, J, Liu, F, Lee, P, Rapacchi, S, Hu, P & Sheng, K 2014, “Feasibility of automated pancreas segmentation based on dynamic MRI”, The British Journal of Radiology, vol. 87, no. 1044, p. 20140248.
    Résumé : OBJECTIVE: MRI-guided radiotherapy is particularly attractive for abdominal targets with low CT contrast. To fully utilize this modality for pancreas tracking, automated segmentation tools are needed. A hybrid gradient, region growth and shape constraint (hGReS) method to segment two-dimensional (2D) upper abdominal dynamic MRI (dMRI) is developed for this purpose. METHODS: 2D coronal dynamic MR images of two healthy volunteers were acquired with a frame rate of 5 frames per second. The regions of interest (ROIs) included the liver, pancreas and stomach. The first frame was used as the source where the centres of the ROIs were manually annotated. These centre locations were propagated to the next dMRI frame. Four-neighborhood region transfer growth was performed from these initial seeds before refinement using shape constraints. RESULTS from hGReS and two other automated segmentation methods using integrated edge detection and region growth (IER) and level set, respectively, were compared with manual contours using Dice's index (DI). RESULTS: For the first patient, the hGReS resulted in the organ segmentation accuracy as a measure by the DI (0.77) for the pancreas, superior to the level set method (0.72) and IER (0.71). The hGReS was shown to be reproducible on the second subject, achieving a DI of 0.82, 0.92 and 0.93 for the pancreas, stomach and liver, respectively. Motion trajectories derived from the hGReS were highly correlated to respiratory motion. CONCLUSION: We have shown the feasibility of automated segmentation of the pancreas anatomy on dMRI. ADVANCES IN KNOWLEDGE: Using the hybrid method improves segmentation robustness of low-contrast images.

  • Rapacchi, S, Han, F, Natsuaki, Y, Kroeker, R, Plotnik, A, Lehrman, E, Sayre, J, Laub, G, Finn, JP & Hu, P 2014, “High spatial and temporal resolution dynamic contrast-enhanced magnetic resonance angiography using compressed sensing with magnitude image subtraction”, Magnetic Resonance in Medicine, vol. 71, no. 5, p. 1771-1783.
    Résumé : PURPOSE: We propose a compressed-sensing (CS) technique based on magnitude image subtraction for high spatial and temporal resolution dynamic contrast-enhanced MR angiography (CE-MRA). METHODS: Our technique integrates the magnitude difference image into the CS reconstruction to promote subtraction sparsity. Fully sampled Cartesian 3D CE-MRA datasets from 6 volunteers were retrospectively under-sampled and three reconstruction strategies were evaluated: k-space subtraction CS, independent CS, and magnitude subtraction CS. The techniques were compared in image quality (vessel delineation, image artifacts, and noise) and image reconstruction error. Our CS technique was further tested on seven volunteers using a prospectively under-sampled CE-MRA sequence. RESULTS: Compared with k-space subtraction and independent CS, our magnitude subtraction CS provides significantly better vessel delineation and less noise at 4× acceleration, and significantly less reconstruction error at 4× and 8× (P < 0.05 for all). On a 1-4 point image quality scale in vessel delineation, our technique scored 3.8 ± 0.4 at 4×, 2.8 ± 0.4 at 8×, and 2.3 ± 0.6 at 12× acceleration. Using our CS sequence at 12× acceleration, we were able to acquire dynamic CE-MRA with higher spatial and temporal resolution than current clinical TWIST protocol while maintaining comparable image quality (2.8 ± 0.5 vs. 3.0 ± 0.4, P = NS). CONCLUSION: Our technique is promising for dynamic CE-MRA.
    Mots-clés : Algorithms, Arteries, Blood Flow Velocity, Contrast Media, Data Compression, Gadolinium, Humans, Image Enhancement, Image Interpretation, Computer-Assisted, Imaging, Three-Dimensional, Magnetic Resonance Angiography, Organometallic Compounds, Reproducibility of Results, Sensitivity and Specificity, Subtraction Technique.

  • Rashid, S, Rapacchi, S, Vaseghi, M, Tung, R, Shivkumar, K, Finn, JP & Hu, P 2014, “Improved late gadolinium enhancement MR imaging for patients with implanted cardiac devices”, Radiology, vol. 270, no. 1, p. 269-274.
    Résumé : PURPOSE: To propose and test a modified wideband late gadolinium enhancement (LGE) magnetic resonance (MR) imaging technique to overcome hyperintensity image artifacts caused by implanted cardiac devices. MATERIALS AND METHODS: Written informed consent was obtained from all participants, and the HIPAA-compliant study protocol was approved by the institutional review board. Studies in phantoms and in a healthy volunteer were performed to test the hypothesis that the hyperintensity artifacts that are typically observed on LGE images in patients with implanted cardiac devices are caused by insufficient inversion of the affected myocardial signal. The conventional LGE MR imaging pulse sequence was modified by replacing the nonselective inversion pulse with a wideband inversion pulse. The modified LGE sequence, along with the conventional LGE sequence, was evaluated in 12 patients with implantable cardioverter defibrillators (ICDs) who were referred for cardiac MR imaging. RESULTS: The ICD causes 2-6 kHz in frequency shift at locations 5-10 cm away from the device. This off-resonance falls outside the typical spectral bandwidth of the nonselective inversion pulse used in conventional LGE, which results in the hyperintensity artifact. In 10 of the 12 patients, the conventional LGE technique produced severe, uninterpretable hyperintensity artifacts in the anterior and lateral portions of the left ventricular wall. These artifacts were eliminated with use of the wideband LGE sequence, thereby enabling confident evaluation of myocardial viability. CONCLUSION: The modified wideband LGE MR imaging technique eliminates the hyperintensity artifacts seen in patients with cardiac devices. The technique may enable LGE MR imaging in patients with cardiac devices, in whom LGE MR imaging otherwise could not be used for diagnosis.
    Mots-clés : Artifacts, Computer Simulation, Contrast Media, Defibrillators, Implantable, Female, Gadolinium DTPA, Humans, Magnetic Resonance Imaging, Male, Phantoms, Imaging.

  • Sarma, M, Hu, P, Rapacchi, S, Ennis, D, Thomas, A, Lee, P, Kupelian, P & Sheng, K 2014, “Accelerating dynamic magnetic resonance imaging (MRI) for lung tumor tracking based on low-rank decomposition in the spatial-temporal domain: a feasibility study based on simulation and preliminary prospective undersampled MRI”, International Journal of Radiation Oncology, Biology, Physics, vol. 88, no. 3, p. 723-731.
    Résumé : PURPOSE: To evaluate a low-rank decomposition method to reconstruct down-sampled k-space data for the purpose of tumor tracking. METHODS AND MATERIALS: Seven retrospective lung cancer patients were included in the simulation study. The fully-sampled k-space data were first generated from existing 2-dimensional dynamic MR images and then down-sampled by 5 × -20 × before reconstruction using a Cartesian undersampling mask. Two methods, a low-rank decomposition method using combined dynamic MR images (k-t SLR based on sparsity and low-rank penalties) and a total variation (TV) method using individual dynamic MR frames, were used to reconstruct images. The tumor trajectories were derived on the basis of autosegmentation of the resultant images. To further test its feasibility, k-t SLR was used to reconstruct prospective data of a healthy subject. An undersampled balanced steady-state free precession sequence with the same undersampling mask was used to acquire the imaging data. RESULTS: In the simulation study, higher imaging fidelity and low noise levels were achieved with the k-t SLR compared with TV. At 10 × undersampling, the k-t SLR method resulted in an average normalized mean square error <0.05, as opposed to 0.23 by using the TV reconstruction on individual frames. Less than 6% showed tracking errors >1 mm with 10 × down-sampling using k-t SLR, as opposed to 17% using TV. In the prospective study, k-t SLR substantially reduced reconstruction artifacts and retained anatomic details. CONCLUSIONS: Magnetic resonance reconstruction using k-t SLR on highly undersampled dynamic MR imaging data results in high image quality useful for tumor tracking. The k-t SLR was superior to TV by better exploiting the intrinsic anatomic coherence of the same patient. The feasibility of k-t SLR was demonstrated by prospective imaging acquisition and reconstruction.
    Mots-clés : Feasibility Studies, Humans, Image Enhancement, Image Processing, Computer-Assisted, Lung Neoplasms, Magnetic Resonance Imaging, Movement, Respiration, Retrospective Studies.

  • Song, HK, Yan, L, Smith, RX, Xue, Y, Rapacchi, S, Srinivasan, S, Ennis, DB, Hu, P, Pouratian, N & Wang, DJJ 2014, “Noncontrast enhanced four-dimensional dynamic MRA with golden angle radial acquisition and K-space weighted image contrast (KWIC) reconstruction”, Magnetic Resonance in Medicine, vol. 72, no. 6, p. 1541-1551.
    Résumé : PURPOSE: To explore the feasibility of 2D and 3D golden-angle radial acquisition strategies in conjunction with k-space weighted image contrast (KWIC) temporal filtering to achieve noncontrast enhanced dynamic MRA (dMRA) with high spatial resolution, low streaking artifacts and high temporal fidelity. METHODS: Simulations and in vivo examinations in eight normal volunteers and an arteriovenous malformation patient were carried out. Both 2D and 3D golden angle radial sequences, preceded by spin tagging, were used for dMRA of the brain. The radial dMRA data were temporally filtered using the KWIC strategy and compared with matched standard Cartesian techniques. RESULTS: The 2D and 3D dynamic MRA image series acquired with the proposed radial techniques demonstrated excellent image quality without discernible temporal blurring compared with standard Cartesian based approaches. The image quality of radial dMRA was equivalent to or higher than that of Cartesian dMRA by visual inspection. A reduction factor of up to 10 and 3 in scan time was achieved for 2D and 3D radial dMRA compared with the Cartesian-based counterparts. CONCLUSION: The proposed 2D and 3D radial dMRA techniques demonstrated image quality comparable or even superior to those obtained with standard Cartesian methods, but within a fraction of the scan time.


Journal Article

  • Khan, SN, Rapacchi, S, Levi, DS & Finn, JP 2013, “Pediatric cardiovascular interventional devices: effect on CMR images at 1.5 and 3 Tesla”, Journal of Cardiovascular Magnetic Resonance, vol. 15, p. 54.
    Résumé : BACKGROUND: To predict the type and extent of CMR artifacts caused by commonly used pediatric trans-catheter devices at 1.5 T and 3 T as an aid to clinical planning and patient screening. METHODS: Eleven commonly used interventional, catheter-based devices including stents, septal occluders, vascular plugs and embolization coils made from either stainless steel or nitinol were evaluated ex-vivo at both 1.5T and 3T. Pulse sequences and protocols commonly used for cardiovascular magnetic resonance (CMR) were evaluated, including 3D high-resolution MR angiography (MRA), time-resolved MRA, 2D balanced-SSFP cine and 2D phase-contrast gradient echo imaging (GRE). We defined the signal void amplification factor (F) as the ratio of signal void dimension to true device dimension. F1 and F2 were measured in the long axis and short axes respectively of the device. We defined F3 as the maximum extent of the off-resonance dark band artifact on SSFP measured in the B0direction. The effects of field strength, sequence type, orientation, flip angle and phase encode direction were tested. Clinical CMR images in 3 patients with various indwelling devices were reviewed for correlation with the in-vitro findings. RESULTS: F1 and F2 were higher (p<0.05) at 3T than at 1.5T for all sequences except 3D-MRA. Stainless steel devices produced greater off-resonance artifact on SSFP compared to nitinol devices (p<0.05). Artifacts were most severe with the stainless steel Flipper detachable embolization coil (Cook Medical, Bloomington, IN), with F1 and F2 10 times greater than with stainless steel stents. The orientation of stents changed the size of off-resonance artifacts by up to two fold. Sequence type did influence the size of signal void or off-resonance artifact (p<0.05). Varying the flip angle and phase encode direction did not affect image artifact. CONCLUSION: Stainless steel embolization coils render large zones of anatomy uninterpretable, consistent with predictions based on ex-vivo testing. Most other commonly used devices produce only mild artifact ex-vivo and are compatible with diagnostic quality in-vivo studies. Knowledge of ex-vivo device behavior can help predict the technical success or failure of CMR scans and may preempt the performance of costly, futile studies.
    Mots-clés : Alloys, Analysis of Variance, Artifacts, Contrast Media, Embolic Protection Devices, Embolization, Therapeutic, Magnetic Resonance Imaging, Cine, Organometallic Compounds, Platinum, Polyesters, Polytetrafluoroethylene, Prostheses and Implants, Septal Occluder Device, Stainless Steel, Stents.


Journal Article

  • Lombaert, H, Peyrat, J-M, Croisille, P, Rapacchi, S, Fanton, L, Cheriet, F, Clarysse, P, Magnin, I, Delingette, H & Ayache, N 2012, “Human atlas of the cardiac fiber architecture: study on a healthy population”, IEEE transactions on medical imaging, vol. 31, no. 7, p. 1436-1447.
    Résumé : Cardiac fibers, as well as their local arrangement in laminar sheets, have a complex spatial variation of their orientation that has an important role in mechanical and electrical cardiac functions. In this paper, a statistical atlas of this cardiac fiber architecture is built for the first time using human datasets. This atlas provides an average description of the human cardiac fiber architecture along with its variability within the population. In this study, the population is composed of ten healthy human hearts whose cardiac fiber architecture is imaged ex vivo with DT-MRI acquisitions. The atlas construction is based on a computational framework that minimizes user interactions and combines most recent advances in image analysis: graph cuts for segmentation, symmetric log-domain diffeomorphic demons for registration, and log-Euclidean metric for diffusion tensor processing and statistical analysis. Results show that the helix angle of the average fiber orientation is highly correlated to the transmural depth and ranges from -41° on the epicardium to +66° on the endocardium. Moreover, we find that the fiber orientation dispersion across the population (±13°) is lower than for the laminar sheets (±31°) . This study, based on human hearts, extends previous studies on other mammals with concurring conclusions and provides a description of the cardiac fiber architecture more specific to human and better suited for clinical applications. Indeed, this statistical atlas can help to improve the computational models used for radio-frequency ablation, cardiac resynchronization therapy, surgical ventricular restoration, or diagnosis and followups of heart diseases due to fiber architecture anomalies.
    Mots-clés : Adolescent, Adult, Aged, Anatomy, Artistic, Atlases as Topic, Computer Graphics, Databases, Factual, Diffusion Magnetic Resonance Imaging, Female, Heart, Humans, Image Processing, Computer-Assisted, Male, Middle Aged.

  • Viallon, M, Mewton, N, Thuny, F, Guehring, J, O'Donnell, T, Stemmer, A, Bi, X, Rapacchi, S, Zuehlsdorff, S, Revel, D & Croisille, P 2012, “T2-weighted cardiac MR assessment of the myocardial area-at-risk and salvage area in acute reperfused myocardial infarction: comparison of state-of-the-art dark blood and bright blood T2-weighted sequences”, Journal of magnetic resonance imaging, vol. 35, no. 2, p. 328-339.
    Résumé : PURPOSE: To compare different state-of-the-art T2-weighted (T2w) imaging sequences combined with late gadolinium enhancement (LGE) for myocardial salvage area (MSA) assessment by cardiac magnetic resonance (CMR). T2w imaging has been used to assess the myocardial area at risk (AAR) in acute myocardial infarction (AMI) patients, but its clinical application is challenging due to technical and physical limitations. MATERIALS AND METHODS: Thirty patients with reperfused AMI underwent complete CMR imaging 2-5 days after hospital admission. Myocardial AAR and MSA were quantified on four different T2w sequences: (a) free-breathing T2-prepared single-shot balanced steady-state free precession (T2p_ssbSSFP); (b) breathhold T2-weighted acquisition for cardiac unified T2 edema (ACUTE); (c) breathhold T2w dark-blood inversion recovery turbo-spin echo (IR-TSE) (short-term inversion recovery: STIR); and (d) free-breathing high-resolution T2 dark-blood navigated BLADE. The diagnostic performance of each technique was also assessed. RESULTS: Quantitative analysis showed significant differences in myocardial AAR extent as quantified by the four T2w sequences (P < 0.05). There were also significant differences in sensitivity, specificity and overall diagnostic performance. CONCLUSION: Detection and quantification of AAR, and thus of MSA, by T2wCMR in reperfused AMI patients varied significantly between different T2w sequences in the same clinical setting.
    Mots-clés : Analysis of Variance, Chi-Square Distribution, Contrast Media, Coronary Angiography, Female, Humans, Image Interpretation, Computer-Assisted, Magnetic Resonance Imaging, Male, Meglumine, Middle Aged, Myocardial infarction, Myocardial Reperfusion, Organometallic Compounds, Reproducibility of Results, ROC Curve, Vectorcardiography.


Journal Article

  • Mewton, N, Rapacchi, S, Augeul, L, Ferrera, R, Loufouat, J, Boussel, L, Micolich, A, Rioufol, G, Revel, D, Ovize, M & Croisille, P 2011, “Determination of the myocardial area at risk with pre- versus post-reperfusion imaging techniques in the pig model”, Basic Research in Cardiology, vol. 106, no. 6, p. 1247-1257.
    Résumé : The purpose of this study was to compare the accuracy of post-reperfusion cardiac magnetic resonance (CMR) and pre-reperfusion multidetector computed tomography (MDCT) imaging to measure the size of the area at risk (AAR), using pathology as a reference technique in a porcine acute myocardial infarction model. Fifteen pigs underwent balloon-induced coronary artery occlusion for 40 min followed by reperfusion. The AAR was assessed with arterial enhanced MDCT performed during occlusion, while two different T2 weighted (T2W) CMR imaging sequences and the contrast-enhanced (ce-) CMR endocardial surface length (ESL) were performed after 90 min of reperfusion. Animals were euthanized and the AAR was assessed by pathology. Additional measurements of the myocardial water content in the AAR, remote and the AAR border zones were performed. AAR by pathology best correlated with measurements made by MDCT (R(2) = 0.88; p < 0.001) with little bias on Bland-Altman plots (bias 2.5%, SD 6.1% LV area). AAR measurements obtained by T2W STIR, T2W ACUTE sequences or the ESL on ce-CMR showed a fair correlation with pathology (R(2) = 0.72, R(2) = 0.65 and R(2) = 0.69, respectively; all p ≤ 0.001), but significantly overestimated the size of the AAR with important bias (17.4 ± 10.8% LV area; 11.7 ± 11.0% LV area; 13.0 ± 10.3% LV area, respectively). The myocardial water content in the AAR border zones was significantly higher than the remote (82.8 vs. 78.8%; p < 0.001). Our data suggest that post-reperfusion imaging methods overestimated the AAR likely due to the presence of edema outside of the boundaries of the AAR. Pre-reperfusion arterial enhanced MDCT showed the greatest accuracy for the assessment of the AAR.
    Mots-clés : Animals, Cardiac Imaging Techniques, Disease Models, Animal, Magnetic Resonance Imaging, Multidetector Computed Tomography, Myocardial infarction, Myocardium, Swine.

  • Pai, VM, Rapacchi, S, Kellman, P, Croisille, P & Wen, H 2011, “PCATMIP: enhancing signal intensity in diffusion-weighted magnetic resonance imaging”, Magnetic Resonance in Medicine, vol. 65, no. 6, p. 1611-1619.
    Résumé : Diffusion-weighted MRI studies generally lose signal intensity to physiological motion, which can adversely affect quantification/diagnosis. Averaging over multiple repetitions, often used to improve image quality, does not eliminate the signal loss. In this article, PCATMIP, a combined principal component analysis and temporal maximum intensity projection approach, is developed to address this problem. Data are first acquired for a fixed number of repetitions. Assuming that physiological fluctuations of image intensities locally are likely temporally correlated unlike random noise, a local moving boxcar in the spatial domain is used to reconstruct low-noise images by considering the most relevant principal components in the temporal domain. Subsequently, a temporal maximum intensity projection yields a high signal-intensity image. Numerical and experimental studies were performed for validation and to determine optimal parameters for increasing signal intensity and minimizing noise. Subsequently, a combined principal component analysis and temporal maximum intensity projection approach was used to analyze diffusion-weighted porcine liver MRI scans. In these scans, the variability of apparent diffusion coefficient values among repeated measurements was reduced by 59% relative to averaging, and there was an increase in the signal intensity with higher intensity differences observed at higher b-values. In summary, a combined principal component analysis and temporal maximum intensity projection approach is a postprocessing approach that corrects for bulk motion-induced signal loss and improves apparent diffusion coefficient measurement reproducibility.
    Mots-clés : Animals, Computer Simulation, Diffusion Magnetic Resonance Imaging, Image Enhancement, Image Processing, Computer-Assisted, Liver, Motion, Phantoms, Imaging, Principal Component Analysis, Swine.

  • Rapacchi, S, Wen, H, Viallon, M, Grenier, D, Kellman, P, Croisille, P & Pai, VM 2011, “Low b-value diffusion-weighted cardiac magnetic resonance imaging: initial results in humans using an optimal time-window imaging approach”, Investigative Radiology, vol. 46, no. 12, p. 751-758.
    Résumé : OBJECTIVES: Diffusion-weighted imaging (DWI) using low b-values permits imaging of intravoxel incoherent motion in tissues. However, low b-value DWI of the human heart has been considered too challenging because of additional signal loss due to physiological motion, which reduces both signal intensity and the signal-to-noise ratio (SNR). We address these signal loss concerns by analyzing cardiac motion during a heartbeat to determine the time-window during which cardiac bulk motion is minimal. Using this information to optimize the acquisition of DWI data and combining it with a dedicated image processing approach has enabled us to develop a novel low b-value diffusion-weighted cardiac magnetic resonance imaging approach, which significantly reduces intravoxel incoherent motion measurement bias introduced by motion. MATERIALS AND METHODS: Simulations from displacement encoded motion data sets permitted the delineation of an optimal time-window with minimal cardiac motion. A number of single-shot repetitions of low b-value DWI cardiac magnetic resonance imaging data were acquired during this time-window under free-breathing conditions with bulk physiological motion corrected for by using nonrigid registration. Principal component analysis (PCA) was performed on the registered images to improve the SNR, and temporal maximum intensity projection (TMIP) was applied to recover signal intensity from time-fluctuant motion-induced signal loss. This PCATMIP method was validated with experimental data, and its benefits were evaluated in volunteers before being applied to patients. RESULTS: Optimal time-window cardiac DWI in combination with PCATMIP postprocessing yielded significant benefits for signal recovery, contrast-to-noise ratio, and SNR in the presence of bulk motion for both numerical simulations and human volunteer studies. Analysis of mean apparent diffusion coefficient (ADC) maps showed homogeneous values among volunteers and good reproducibility between free-breathing and breath-hold acquisitions. The PCATMIP DWI approach also indicated its potential utility by detecting ADC variations in acute myocardial infarction patients. CONCLUSIONS: Studying cardiac motion may provide an appropriate strategy for minimizing the impact of bulk motion on cardiac DWI. Applying PCATMIP image processing improves low b-value DWI and enables reliable analysis of ADC in the myocardium. The use of a limited number of repetitions in a free-breathing mode also enables easier application in clinical conditions.
    Mots-clés : Adult, Coronary Artery Disease, Diffusion Magnetic Resonance Imaging, Female, Heart, Humans, Image Processing, Computer-Assisted, Male, Middle Aged, Motion, Myocardial infarction, Reproducibility of Results, Respiration, Signal-To-Noise Ratio.

  • Yang, F, Zhu, YM, Rapacchi, S, Luo, JH, Robini, M & Croisille, P 2011, “Interpolation of vector fields from human cardiac DT-MRI”, Physics in Medicine and Biology, vol. 56, no. 5, p. 1415-1430.
    Résumé : There has recently been increased interest in developing tensor data processing methods for the new medical imaging modality referred to as diffusion tensor magnetic resonance imaging (DT-MRI). This paper proposes a method for interpolating the primary vector fields from human cardiac DT-MRI, with the particularity of achieving interpolation and denoising simultaneously. The method consists of localizing the noise-corrupted vectors using the local statistical properties of vector fields, removing the noise-corrupted vectors and reconstructing them by using the thin plate spline (TPS) model, and finally applying global TPS interpolation to increase the resolution in the spatial domain. Experiments on 17 human hearts show that the proposed method allows us to obtain higher resolution while reducing noise, preserving details and improving direction coherence (DC) of vector fields as well as fiber tracking. Moreover, the proposed method perfectly reconstructs azimuth and elevation angle maps.
    Mots-clés : diffusion, Heart, Humans, Image Processing, Computer-Assisted, Magnetic Resonance Imaging.


Journal Article

  • Wen, H, Bennett, EE, Hegedus, MM & Rapacchi, S 2009, “Fourier X-ray scattering radiography yields bone structural information”, Radiology, vol. 251, no. 3, p. 910-918.
    Résumé : PURPOSE: To characterize certain aspects of the microscopic structures of cortical and trabecular bone by using Fourier x-ray scattering imaging. MATERIALS AND METHODS: Protocols approved by the National Institutes of Health Animal Care and Use Committee were used to examine ex vivo the hind limb of a rat and the toe of a pig. The Fourier x-ray scattering imaging technique involves the use of a grid mask to modulate the cone beam and Fourier spectral filters to isolate the harmonic images. The technique yields attenuation, scattering, and phase-contrast (PC) images from a single exposure. In the rat tibia cortical bone, the scattering signals from two orthogonal grid orientations were compared by using Wilcoxon signed rank tests. In the pig toe, the heterogeneity of scattering and PC signals was compared between trabecular and compact bone regions of uniform attenuation by using F tests. RESULTS: In cortical bone, the scattering signal was significantly higher (P < 10(-15)) when the grid was parallel to the periosteal surface. Trabecular bone, as compared with cortical bone, appeared highly heterogeneous on the scattering (P < 10(-34)) and PC (P < 10(-27)) images. CONCLUSION: The ordered alignment of the mineralized collagen fibrils in compact bone was reflected in the anisotropic scattering signal in this bone. In trabecular bone, the porosity of the mineralized matrix accounted for the granular pattern seen on the scattering and PC images.
    Mots-clés : Animals, Bone and Bones, Bone Density, Fourier Analysis, Rats, Scattering, Radiation, Statistics, Nonparametric, Swine, X-Rays.


Journal Article

  • Frindel, C, Robini, M, Rapacchi, S, Stephant, E, Zhu, Y-M & Croisille, P 2007, “Towards in vivo diffusion tensor MRI on human heart using edge-preserving regularization”, Conference proceedings: .. Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Annual Conference, vol. 2007, p. 6008-6011.
    Résumé : We investigate the noise sensitivity in various Diffusion Tensor MRI acquisition protocols in sixteen human ex vivo hearts. In particular, we compare the accuracy of protocols with various numbers of excitations and diffusion sensitizing directions for estimating the principal diffusion directions in the myocardium. It is observed that noise sensitivity decreases as the number of excitations and the number of sensitizing directions increase (and hence as the acquisition time increases). To reduce the effects of noise and to improve the results obtained with a smaller number of excitations and/or a smaller number of sensitizing directions, we introduce a 3-D edge-preserving regularization method operating on diffusion weighted images. It allows to maintain the quality of the principal diffusion direction field while minimizing the acquisition time, which is a necessary step for in vivo diffusion tensor MR imaging of the human heart.
    Mots-clés : Algorithms, Cardiomyopathy, Dilated, Diffusion Magnetic Resonance Imaging, Humans, Image Enhancement, Image Interpretation, Computer-Assisted, Imaging, Three-Dimensional, Myocardium, Reproducibility of Results, Sensitivity and Specificity.
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