The cardiac Purkinje network plays a critical role in maintaining synchronized ventricular activation but remains difficult to image due to its fine and complex structure. Conventional MRI techniques lack sufficient contrast to distinguish the structural composition of Purkinje fibers (PFs). This study investigates the potential of inhomogeneous magnetization transfer (ihMT) as a novel contrast mechanism for visualizing and differentiating subregions of the Purkinje network. Five fixed ex vivo sheep hearts containing free-running PFs were scanned using a 9.4 T MRI system with a 2D ihMT RARE sequence. ihMTR maps were analyzed using manually defined regions of interest (ROIs) corresponding to free-running fibers, the Purkinje-myocardial junction (PMJ), and the surrounding myocardium. Histological analysis was performed on matched tissue sections to quantify collagen types I and III, adipocytes, Purkinje cells, and cardiomyocytes. Three ihMT protocols that produced high ihMTR values in free-running fibers (9.25-10.83%) and strong contrast relative to myocardium (2.00-2.17%) and the PMJ (2.99-3.40%) in 1 sample were selected and applied to all samples. Across all hearts, mean ihMTR values were consistently higher in free-running fibers compared to the PMJ (11.5 ± 1.5% vs 9.0 ± 2.9%). Histological analysis revealed significantly greater collagen content in free-running regions compared with the PMJ (72.4 ± 15.9% vs 31.1 ± 13.1%; p = 0.001), along with higher adipocyte content at the PMJ compared to free-running regions (12.3 ± 6.1% vs 3.8 ± 2.7%, not significant). Collagen type III was more prominent at the PMJ but remained a minor component overall. These findings demonstrate that ihMT imaging can distinguish PF subregions based on underlying microstructural differences, particularly collagen and adipocyte distribution. This study lays the groundwork for developing biophysical models to interpret ihMT signals in terms of tissue composition and microstructure, providing a foundation for future studies.

Purpose: The primary purpose of this study was to evaluate the accuracy of an MR-thermometry sequence for monitoring prostate temperature. The secondary purposes were to analyze clinical and technical factors that may affect accuracy and testing the method in a realistic setting, with MR-guided Laser ablation on an ex vivo muscle sample. Materials and methods: An ex vivo muscle sample was subjected to Laser ablation while using a two-dimensional multislice segmented echo planar imaging sequence for MR thermometry. The MR thermometry measurements were compared with invasive sensor temperature readings to assess accuracy. Subsequently, 56 men with a median age of 70 years (age range: 53-84 years) who underwent prostate MRI examinations at 1.5- (n = 27) or 3 T (n = 24) were prospectively included. For each patient, the proportion of 'noisy voxels' (i.e., those with a temporal standard deviation of temperature [SD(T)] > 2 degrees C) in the prostate was calculated. The impact of clinical and technical factors on the proportion of noisy voxels was also examined. Results: MR-thermometry showed excellent correlation with invasive sensors during MR-guided Laser ablation on the ex vivo muscle sample. The median proportion of noisy voxels per patient in the entire cohort was 1 % (Q1, 0.2; Q3, 4.9; range: 0-90.4). No significant differences in median proportion of noisy voxels were observed between examinations performed at 1.5 T and those at 3 T (P = 0.89 before and after adjustment). No clinical or technical factors significantly influenced the proportion of noisy voxels. Conclusion: Two-dimensional real time multislice MR-thermometry is feasible and accurate for monitoring prostate temperature in patients.

A gradient in the density of SR-Ca 2+ pumps appears from the center to the periphery of Purkinje cells (Pcells) after MI. We found that this post-MI rearrangement could result from the peripheral expression of SERCA2b pump, which is absent in healthy hearts. The additional expression of SERCA2b to the existing cardiac pump SERCA2a, and possibly more efficient Ca 2+ -transport properties of SERCA2b, are consistent with the proarrhythmic elevation of SR-Ca 2+ uptake previously proposed in Pcells after MI.

OBJECTIVE:This study proposes analytical and hybrid models for fast and accurate temperature field reconstruction in microwave ablation (MWA), laser interstitial thermal therapy (LITT), and radiofrequency ablation (RFA), aiming at future real-time clinical use. MATERIALS AND METHODS:The proposed approach combines spatial variable transformation and the Laplace transform for time-dependent terms, with finite difference techniques. A 1 mm isotropic grid represents the voxel network. To ensure accurate temperature representation, voxel-averaged temperatures are computed by integrating the solution of the bioheat equation, under spherical symmetry, over voxel bounds. To approximate the elongated ablation zone, the central circumference of the spherical model is repeated and incorporated into a hemisphere-based geometry. Simulated temperature fields are aligned with experimental MRI data using Advanced Normalization Tools (ANTs). All experiments were conducted ex vivo: MWA in bovine liver, and LITT and RFA in agar phantoms. Regions of interest (ROIs) include voxels with significant thermal variation. Heat source parameters are estimated by minimizing the quadratic difference between simulated and MRI-derived temperatures via a spatiotemporal objective function. RESULTS:Across all modalities, over 83-98% of voxels presented RMSE ≤ 1°C, with few exceeding 10°C. LITT showed the best overall agreement. Total simulation and alignment per repetitions required under 0.3 s, significantly below MRI repetition time, enabling potential intraoperative use. CONCLUSION:Although approximate and not yet ready for in vivo clinical application, the proposed models offer fast, voxel-level temperature reconstructions. Their computational efficiency supports further development toward real-time monitoring and procedural adjustment during thermal ablation.

PURPOSE:Real-time monitoring of microwave liver ablation (MWA) using MRI thermometry can be hindered by boiling-induced susceptibility artifacts. These artifacts cause large temperature measurement errors that prevent accurate lesion size prediction. This study proposes a correction methodology based on removing the contribution of this susceptibility artifact using subvoxel sources of susceptibility. METHODS:In vivo microwave ablations (N = 23) were performed on seven pig livers and the temperature was monitored using the proton resonance frequency shift (PRFS) method. The boiling-induced artifacts were observed in 11 ablation cases. To validate the proposed methodology, the thermal dose was computed from the corrected temperature maps, and the resulting lesion estimates were compared in three dimensions with ground-truth lesion segmentations derived from post-ablation T1w images. A numerical simulation was also conducted to emphasize the need for a finer spatial discretization. RESULTS:After correction, a better agreement was noticed between thermal dose prediction and lesion size. Over the 11 cases observed with boiling-induced susceptibility artifacts, the median volumetric Dice, Total Overlap, and False Negative rates changed by 4.8%, 6.2%, and -11.4%. CONCLUSION:Comparison with T1w imaging showed improvements in prediction of lesion volume. The proposed methodology takes into account partial volume effects and the contribution of adjacent slices. It was able to simulate a diverse range of deformed dipole-like artifacts observed in experimental data.

The knowledge of the cardiac microstructure and the 3D myofiber architecture grow years after years with the multiplication and the upgrade of imaging technologies. However, the course of events of pathophysiological processes like cardiac remodeling, and the link with clinical phenotypes are not yet clearly understood. Some concerns have been raised regarding the interpretation of the late gadolinium enhancement (LGE) at the right ventricle attachment or insertion point (RVIP) however 3D microstructure organization of the RVIP has not been extensively described in the literature.

BACKGROUND:Quantitative real-time MRI-based temperature mapping techniques are hampered by abdominal motion. Intrascan motion can be reduced by rapid acquisition sequences such as 2D echo planar imaging (EPI), and inter-scan organ displacement can be compensated by image processing such as optical flow (OF) algorithms. However, motion field estimation can be seriously affected by local variation of signal intensity on magnitude images inherent to tissue heating, potentially leading to erroneous temperature estimates. PURPOSE:This study aims to characterize, in the context of clinical MRI-guided microwave ablation (MWA), a novel deformable image registration (DIR) algorithm that enhances the generation of thermal maps aligned to a reference position, a critical step for calculating cumulative thermal dose and, consequently, for the real-time evaluation of interventional procedure progress. METHODS:A retrospective image analysis was performed on 11 patients that underwent MWA of a liver tumor (primary or metastasis). Ablation duration was set to 9 ± 2 min with a 14-gauge large antenna. A stack of 13-20 contiguous slices was acquired dynamically (350 repetitions) at 1.5T using a single-shot EPI sequence. Evaluation was first performed on motion-free datasets (5 gated acquisitions using a cushion positioned in the patient abdomen) then with ones with motion (8 fixed-frequency acquisitions at 0.5 Hz). Temperature, thermal dose and lesion size were computed using three workflows: (i) standard phase subtraction (gold standard), (ii) conventional OF motion compensation, (iii) PCA-based OF motion compensation. The impact of flow field, temperature and lesion volume estimation were compared using averaged endpoint error (AEE), NRMSE and bland Altman plot, respectively. RESULTS:Intensity signal decreases (close to 50%) were observed in the vicinity of the probe during MW energy delivery. Both motion correction algorithms reduce the NRMSE of magnitude images throughout the acquisition (p < 0.005) and achieve similar results between them. Gated acquisition results. Conventional OF produced erroneous vector fields compared to the PCA-based OF, leading to higher maximal EE (3 mm vs. 1 mm) and temperature errors up to 15°C-20°C. PCA-based OF algorithm significantly reduces the NRMSE of temperature (p < 0.005). The conventional OF method underestimated the final size of lesions with a bias of 0.93 cm3 while the PCA-based OF reported a bias of 0.5 cm3. Fixed frequency acquisition results. The temperature estimation without motion correction led to strong fluctuations or loss of temperature measurement while the proposed PCA-based OF recovered both a stable and precise measurement with null bias. CONCLUSION:The deformable image registration algorithm is less sensitive to local variations of the signal. Volumetric temperature imaging without gating (20 slices/2 s) could be performed with the same accuracy, and offer trade-offs in acquisition time or volume coverage. Such a strategy is expected to increase procedure safety by monitoring large volumes more rapidly for MR-guided thermotherapy on mobile organs.

Background Clinical Laser-Induced Thermotherapy (LITT) currently lacks precise control of tissue temperature increase during the procedure. This study presents a new method to automatically regulate the maximum temperature increase in vivo at different positions by adjusting LITT power delivered by multiple laser probes using real-time volumetric MR-thermometry. Methods The regulation algorithm was evaluated in vivo on a pig leg muscle. Temperature regulation was performed in volumes surrounding each laser probe tip. The power delivered to each laser probe was automatically adjusted every second using a feedback control algorithm by processing on-the-fly MR-thermometry images (10 slices/second) on a 1.5 T clinical scanner (1.56 mm × 1.56 mm x 3 mm resolution), using the proton-resonance frequency (PRF) shift technique. Several experimental conditions were tested with predefined temperature-time profiles corresponding to conditions of thermal ablation (+30 °C above body temperature) or moderate hyperthermia (+10 and + 15 °C). Control images were acquired after injection of Gadolinium at the end of experiment and were compared with the thermal dose images calculated from the thermometry images. Results The mean difference and root mean squared error between target temperatures and measured ones remained below 0.5 °C and 2 °C respectively, for 5 min duration. Lesion sizes observed on thermal dose and on images acquired after gadolinium injection were in good agreement. Conclusion Automatic regulation of in vivo temperature increase during LITT procedures with multiple laser emitters control is feasible. The method provides an adaptative solution to improve the safety and efficacity of such clinical procedures.

Background The minimum admissible detuning efficiency (DE) of a receive coil is an essential parameter for coil designers. A receive coil with inefficient detuning leads to inhomogeneous B 1 during excitation. Previously proposed criteria for quantifying the DE rely on indirect measurements and are difficult to implement. Purpose To present an alternative method to quantify the DE of receive‐only surface coils. Study Type Theoretical study supported by simulations and phantom experiments. Phantoms Uniform spherical (100 mm diameter) and cylindrical (66 mm diameter) phantoms. Field Strength/Sequence Dual repetition time B 1 mapping sequence at 1.5T, and Bloch‐Siegert shift B 1 mapping sequence at 3.0T. Assessment One non‐planar (80 × 43 mm 2 ) and two planar (40 and 57 mm diameter) surface coils were built. Theoretical analysis was performed to determine the minimum DE required to avoid B 1 distortions. Experimental B 1 maps were acquired for the non‐planar and planar surface coils at both 1.5T and 3.0T and visually compared with simulated B 1 maps to assess the validity of the theoretical analysis. Statistical Tests None. Results Based on the theoretical analysis, the proposed minimum admissible DE, defined as DE thr = 20 Log ( Q ) + 13 dB, depended only on the quality factor ( Q ) of the coil and was independent of coil area and field strength. Simulations and phantom experiments showed that when the DE was higher than this minimum threshold level, the B 1 field generated by the transmission coil was not modified by the receive coil. Data Conclusion The proposed criterion for assessing the DE is simple to measure, and does not depend on the area of the coil or on the magnetic field strength, up to 3T. Experimental and simulated B 1 maps confirmed that detuning efficiencies above the theoretically derived minimal admissible DE resulted in a non‐distorted B 1 field. Evidence Level 2 Technical Efficacy Stage 1

Motivation: The architecture of the cardiac conduction system is implicated in cardiac arrhythmias such as ventricular fibrillation and characterizing this structure will help better understand and diagnose these arrhythmias. Goal(s): Optimizing the inhomogeneous magnetization transfer technique for 3D imaging of conducting fibers. Approach: A range of parameters for ihMTRARE sequence were explored for imaging samples from sheep’s left ventricles containing free-running Purkinje fibers and myocardium. Results: Across samples, ihMTR signal was found consistently higher in fibers compared to the myocardium for the chosen saturation parameters. This finding holds promise for future 3D imaging of the cardiac conduction system. Impact: The results of this study hold promise for future 3D imaging of larger samples of the left ventricle containing free-running and intramural Purkinje fibers using the optimized sequence which would allow the segmentation and characterization of these fibers.

Background Studies of Purkinje cells (Pcells) from canine hearts have suggested an increase of Ca 2+ -release by the sarcoplasmic reticulum (SR) but also reported a potential augmentation of SR-Ca 2+ -uptake after MI. Abnormal increase of SR-Ca 2+ -uptake in heart cells is novel and contrasts with the reduction of this function in cells of failing heart. Our study examined the origin of this increased SR-Ca 2+ -uptake by considering a change in SR-Ca 2+ pump (SERCA2) expression in Purkinje fibers (PFs) post MI. Methods Pcells were isolated from canine hearts 48Hrs post MI. Intracellular Ca 2+ -activity was captured by confocal microscopy. Purkinje-typical Ca 2+ events were analyzed to probe the regional Ca 2+ -dynamics within Pcells. A Purkinje-specific numerical model assisted in the interpretation of Ca 2+ -anomalies detected in Pcells Ca 2+ -transients. SR-Ca 2+ -uptake system was studied by immunofluorescence in Pcells from canine, ovine and human hearts post MI. SERCA protein and gene expressions in PFs and myocardium were measured by Western Blots and RT-qPCR in a classical porcine model of MI. Results 48Hrs after MI, Pcells showed 60% increase in spark-rate and 37% acceleration of Ca 2+ wave decay. In the model of normal wave, 35% increase of Ca 2+ -uptake rate reproduced the actual post-MI wave alterations. In apparent contrast with increased Ca 2+ -uptake rate, SERCA2 protein expression was reduced in canine, sheep, and human Pcells after MI. In pig MI model, the protein level of cardiac-specific SERCA2-splicing variant SERCA2a was reduced by 52% in the whole infarcted ventricle whereas the “non-cardiac” SERCA2b level was increased by 120%. In the infarcted regions, PFs showed 30% downregulation of SERCA2a gene expression and 630% upregulation of SERCA2b. Conclusion Our results confirm that elevated spontaneous Ca 2+ -activity in post-MI PFs is due to increased SR-Ca 2+ -uptake within Pcells. Data suggest that a replacement of “cardiac” SERCA2a by the “non-cardiac” SERCA2b sub-isoform in cardiac cells in response to ischemia is implicated in this alteration.

We present here a method to automatically regulate heat deposition during Laser Interstitial Thermal Therapy to precisely control temperature evolution during the procedure. The method relies on real-time rapid volumetric thermometry using the Proton Resonance Frequency Shift technique and a regulation algorithm that adjusts every second the emitted power by the laser to force temperature to follow a predefined temperature-time profile.

Receive-only coils must be decoupled from the transmit coil during excitation. For conventional coils, decoupling is achieved using a resonant trap, which is switched during B1 transmit. However, in situations with very limited space like intravascular coils, this method remains problematic. We aimed to address efficient remote decoupling of a receive-only coil. We implemented an alternative approach by adding a “negative resistance” to the trap. This negative resistance is tailored in such a way that cancels out the positive resistance of the blocking trap, hence augmenting the quality factor of the trap and thus the decoupling efficiency.

While the microstructure of the left ventricle (LV) has been largely described, only a few studies investigated the right ventricular insertion point (RVIP). It was accepted that the aggregate cardiomyocytes organization was much more complex due to the intersection of the ventricular cavities but a precise structural characterization in the human heart was lacking even if clinical phenotypes related to right ventricular wall stress or arrhythmia were observed in this region. MRI-derived anatomical imaging (150 µm3) and diffusion tensor imaging (600 µm3) were performed in large mammalian whole hearts (human: N = 5, sheep: N = 5). Fractional anisotropy, aggregate cardiomyocytes orientations and tractography were compared within both species. Aggregate cardiomyocytes orientation on one ex-vivo sheep whole heart was then computed using structure tensor imaging (STI) from 21 µm isotropic acquisition acquired with micro computed tomography (MicroCT) imaging. Macroscopic and histological examination were performed. Lastly, experimental cardiomyocytes orientation distribution was then compared to the usual rule-based model using electrophysiological (EP) modeling. Electrical activity was modeled with the monodomain formulation. The RVIP at the level of the inferior ventricular septum presented a unique arrangement of aggregate cardiomyocytes. An abrupt, mid-myocardial change in cardiomyocytes orientation was observed, delimiting a triangle-shaped region, present in both sheep and human hearts. FA’s histogram distribution (mean ± std: 0.29 ± 0.06) of the identified region as well as the main dimension (22.2 mm ± 5.6 mm) was found homogeneous across samples and species. Averaged volume is 0.34 cm3 ± 0.15 cm3. Both local activation time (LAT) and morphology of pseudo-ECGs were strongly impacted with delayed LAT and change in peak-to-peak amplitude in the simulated wedge model. The study was the first to describe the 3D cardiomyocytes architecture of the basal inferoseptal left ventricle region in human hearts and identify the presence of a well-organized aggregate cardiomyocytes arrangement and cardiac structural discontinuities. The results might offer a better appreciation of clinical phenotypes like RVIP-late gadolinium enhancement or uncommon idiopathic ventricular arrhythmias (VA) originating from this region.

A novel thermometry acquisition and a fast deep learning based image reconstruction were combined for cardiac interventional thermometry at high spatial (0.86×0.86mm 2 ) and temporal (0.97s) resolutions, robust to motion and susceptibility artefact and independent of external ECG-gating. The method was tested in phantom and in-vivo in a sheep. The proposed deep learning method outperformed the state-of-the-art algorithm in terms of SNR and paves the way for clinical studies.

A key component of the cardiac conduction system is the atrioventricular (AV) node where the His bundle (HB) divides into the right (RBB) and left (LBB) bundles branches. The RBB is connected to the right ventricle (RV) via the moderator band (MB).