Sébastien Roujol

Accuracy, Precision, and Reproducibility of Four T1 Mapping Sequences: A Head-to-Head Comparison of MOLLI, ShMOLLI, SASHA, and SAPPHIRE

PURPOSE To compare accuracy, precision, and reproducibility of four commonly used myocardial T1 mapping sequences: modified Look-Locker inversion recovery (MOLLI), shortened MOLLI (ShMOLLI), saturation recovery single-shot acquisition (SASHA), and saturation pulse prepared heart rate independent inversion recovery (SAPPHIRE). MATERIALS AND METHODS This HIPAA-compliant study was approved by the institutional review board. All subjects provided written informed consent. Accuracy, precision, and reproducibility of the four T1 mapping sequences were first compared in phantom experiments. In vivo analysis was performed in seven healthy subjects (mean age ± standard deviation, 38 years ± 19; four men, three women) who were imaged twice on two separate days. In vivo reproducibility of native T1 mapping and extracellular volume (ECV) were measured. Differences between the sequences were assessed by using Kruskal-Wallis and Wilcoxon rank sum tests (phantom data) and mixed-effect models (in vivo data). RESULTS T1 mapping accuracy in phantoms was lower with ShMOLLI (62 msec) and MOLLI (44 msec) than with SASHA (13 msec; P < .05) and SAPPHIRE (12 msec; P < .05). MOLLI had similar precision to ShMOLLI (4.0 msec vs 5.6 msec; P = .07) but higher precision than SAPPHIRE (6.8 msec; P = .002) and SASHA (8.7 msec; P < .001). All sequences had similar reproducibility in phantoms (P = .1). The four sequences had similar in vivo reproducibility for native T1 mapping (∼25-50 msec; P > .05) and ECV quantification (∼0.01-0.02; P > .05). CONCLUSION SASHA and SAPPHIRE yield higher accuracy, lower precision, and similar reproducibility compared with MOLLI and ShMOLLI for T1 measurement. Different sequences yield different ECV values; however, all sequences have similar reproducibility for ECV quantification.

Real-time MR-thermometry and dosimetry for interventional guidance on abdominal organs

The use of proton resonance frequency shift–based magnetic resonance (MR) thermometry for interventional guidance on abdominal organs is hampered by the constant displacement of the target due to the respiratory cycle and the associated thermometry artifacts. Ideally, a suitable MR thermometry method should for this role achieve a subsecond temporal resolution while maintaining a precision comparable to those achieved on static organs while not introducing significant processing latencies. Here, a computationally effective processing pipeline for two‐dimensional image registration coupled with a multibaseline phase correction is proposed in conjunction with high‐frame‐rate MRI as a possible solution. The proposed MR thermometry method was evaluated for 5 min at a frame rate of 10 images/sec in the liver and the kidney of 11 healthy volunteers and achieved a precision of less than 2°C in 70% of the pixels while delivering temperature and thermal dose maps on the fly. The ability to perform MR thermometry and dosimetry in vivo during a real intervention was demonstrated on a porcine kidney during a high‐intensity focused ultrasound heating experiment. Magn Reson Med 63:1080–1087, 2010.

Real-time 3D target tracking in MRI guided focused ultrasound ablations in moving tissues

Magnetic resonance imaging‐guided high intensity focused ultrasound is a promising method for the noninvasive ablation of pathological tissue in abdominal organs such as liver and kidney. Due to the high perfusion rates of these organs, sustained sonications are required to achieve a sufficiently high temperature elevation to induce necrosis. However, the constant displacement of the target due to the respiratory cycle render continuous ablations challenging, since dynamic repositioning of the focal point is required. This study demonstrates subsecond 3D high intensity focused ultrasound‐beam steering under magnetic resonance‐guidance for the real‐time compensation of respiratory motion. The target is observed in 3D space by coupling rapid 2D magnetic resonance‐imaging with prospective slice tracking based on pencil‐beam navigator echoes. The magnetic resonance‐data is processed in real‐time by a computationally efficient reconstruction pipeline, which provides the position, the temperature and the thermal dose on‐the‐fly, and which feeds corrections into the high intensity focused ultrasound‐ablator. The effect of the residual update latency is reduced by using a 3D Kalman‐predictor for trajectory anticipation. The suggested method is characterized with phantom experiments and verified in vivo on porcine kidney. The results show that for update frequencies of more than 10 Hz and latencies of less then 114 msec, temperature elevations can be achieved, which are comparable to static experiments. Magn Reson Med, 2010.

Motion correction in MR thermometry of abdominal organs: a comparison of the referenceless vs. the multibaseline approach.

Reliable temperature and thermal‐dose measurements using proton resonance frequency shift‐based magnetic resonance (MR) thermometry for MR‐guided ablation of abdominal organs require a robust correction of artefacts induced by the target displacement through an inhomogeneous and time‐variant magnetic field. Two correction approaches emerged recently as promising candidates to allow continuous real‐time MR‐thermometry under free‐breathing conditions: The multibaseline correction method, which relies on a pre‐recorded correction table allowing to correct for periodic phase changes, and the referenceless method, which depends on a background phase estimation in the target area based on the assumption of a smooth spatial variation of the phase across the organ. This study combines both methods with real‐time in‐plane motion correction to permit both temperature and thermal‐dose calculations on the fly. Subsequently, the practical aspects of both methods are compared in two application scenarios, a radio frequency‐ablation and a high‐intensity focused ultrasound ablation. A hybrid approach is presented that exploits the strong points of both methods, allowing accurate and precise proton resonance frequency‐thermometry measurements during periodical displacement, even in the presence of spontaneous motion and strong susceptibility variations in the target area. Magn Reson Med, 2010.

Improvement of MRI-Functional measurement with automatic movement correction in native and transplanted kidneys

To improve 2D software for motion correction of renal dynamic contrast‐enhanced magnetic resonance imaging (DCE‐MRI) and to evaluate its effect using the Patlak–Rutland model.

Adaptive registration of varying contrast-weighted images for improved tissue characterization (ARCTIC): Application to T1 mapping

To propose and evaluate a novel nonrigid image registration approach for improved myocardial T1 mapping.

Free-breathing multislice native myocardial T1 mapping using the slice-interleaved T1 (STONE) sequence.

To develop a novel pulse sequence for free‐breathing, multislice, native myocardial T1 mapping.

High-Resolution Mapping of Ventricular Scar Comparison Between Single and Multielectrode Catheters

Background—Mapping resolution is influenced by electrode size and interelectrode spacing. The aims of this study were to establish normal electrogram criteria for 1-mm multielectrode-mapping catheters (Pentaray) in the ventricle and to compare its mapping resolution within scar to standard 3.5-mm catheters (Smart-Touch Thermocool). Methods and Results—Three healthy swine and 11 swine with healed myocardial infarction underwent sequential mapping of the left ventricle with both catheters. Bipolar voltage amplitude in healthy tissue was similar between 3.5- and 1-mm multielectrode catheters with a 5th percentile of 1.61 and 1.48 mV, respectively. In swine with healed infarction, the total area of low bipolar voltage amplitude (defined as <1.5 mV) was 22.5% smaller using 1-mm multielectrode catheters (21.7 versus 28.0 cm2; P=0.003). This was more evident in the area of dense scar (bipolar amplitude <0.5 mV) with a 47% smaller very low–voltage area identified using 1-mm electrode catheters (7.1 versus 15.2 cm2; P=0.003). In this region, 1-mm multielectrode catheters recorded higher voltage amplitude (0.72±0.81 mV versus 0.30±0.12 mV; P<0.001). Importantly, 27% of these dense scar electrograms showed distinct triphasic electrograms when mapped using a 1-mm multielectrode catheter compared with fractionated multicomponent electrogram recorded with the 3.5-mm electrode catheter. In 8 mapped reentrant ventricular tachycardias, the circuits included regions of preserved myocardial tissue channels identified with 1-mm multielectrode catheters but not 3.5-mm electrode catheters. Pacing threshold within the area of low voltage was lower with 1-mm electrode catheters (0.9±1.3 mV versus 3.8±3.7 mV; P=0.001). Conclusions—Mapping with small closely spaced electrode catheters can improve mapping resolution within areas of low voltage.

Online real-time reconstruction of adaptive TSENSE with commodity CPU/GPU hardware

Adaptive temporal sensitivity encoding (TSENSE) has been suggested as a robust parallel imaging method suitable for MR guidance of interventional procedures. However, in practice, the reconstruction of adaptive TSENSE images obtained with large coil arrays leads to long reconstruction times and latencies and thus hampers its use for applications such as MR‐guided thermotherapy or cardiovascular catheterization. Here, we demonstrate a real‐time reconstruction pipeline for adaptive TSENSE with low image latencies and high frame rates on affordable commodity personal computer hardware. For typical image sizes used in interventional imaging (128 × 96, 16 channels, sensitivity encoding (SENSE) factor 2‐4), the pipeline is able to reconstruct adaptive TSENSE images with image latencies below 90 ms at frame rates of up to 40 images/s, rendering the MR performance in practice limited by the constraints of the MR acquisition. Its performance is demonstrated by the online reconstruction of in vivo MR images for rapid temperature mapping of the kidney and for cardiac catheterization. Magn Reson Med, 2009.

Improved quantitative myocardial T2 mapping: Impact of the fitting model

To develop an improved T2 prepared (T2prep) balanced steady‐state free‐precession (bSSFP) sequence and signal relaxation curve fitting method for myocardial T2 mapping.