Sathya Vijayakumar

Left atrial strain and strain rate in patients with paroxysmal and persistent atrial fibrillation: relationship to left atrial structural remodeling detected by delayed-enhancement MRI.

Background—Atrial fibrillation (AF) is a progressive condition that begins with hemodynamic and/or structural changes in the left atrium (LA) and evolves through paroxysmal and persistent stages. Because of limitations with current noninvasive imaging techniques, the relationship between LA structure and function is not well understood. Methods and Results—Sixty-five patients (age, 61.2±14.2 years; 67% men) with paroxysmal (44%) or persistent (56%) AF underwent 3D delayed-enhancement MRI. Segmentation of the LA wall was performed and degree of enhancement (fibrosis) was determined using a semiautomated quantification algorithm. Two-dimensional echocardiography and longitudinal LA strain and strain rate during ventricular systole with velocity vector imaging were obtained. Mean fibrosis was 17.8±14.5%. Log-transformed fibrosis values correlated inversely with LA midlateral strain (r=−0.5, P=0.003) and strain rate (r=−0.4, P<0.005). Patients with persistent AF as compared with paroxysmal AF had more fibrosis (22±17% versus 14±9%, P=0.04) and lower midseptal (27±14% versus 38±16%, P=0.01) and midlateral (35±16% versus 45±14% P=0.03) strains. Multivariable stepwise regression showed that midlateral strain (r=−0.5, P=0.006) and strain rate (r=−0.4, P=0.01) inversely predicted the extent of fibrosis independent of other echocardiographic parameters and the rhythm during imaging. Conclusions—LA wall fibrosis by delayed-enhancement MRI is inversely related to LA strain and strain rate, and these are related to the AF burden. Echocardiographic assessment of LA structural and functional remodeling is quick and feasible and may be helpful in predicting outcomes in AF.

k-t GRAPPA: A k-space Implementation for Dynamic MRI with High Reduction Factor

A novel technique called “k‐t GRAPPA” is introduced for the acceleration of dynamic magnetic resonance imaging. Dynamic magnetic resonance images have significant signal correlations in k‐space and time dimension. Hence, it is feasible to acquire only a reduced amount of data and recover the missing portion afterward. Generalized autocalibrating partially parallel acquisitions (GRAPPA), as an important parallel imaging technique, linearly interpolates the missing data in k‐space. In this work, it is shown that the idea of GRAPPA can also be applied in k‐t space to take advantage of the correlations and interpolate the missing data in k‐t space. For this method, no training data, filters, additional parameters, or sensitivity maps are necessary, and it is applicable for either single or multiple receiver coils. The signal correlation is locally derived from the acquired data. In this work, the k‐t GRAPPA technique is compared with our implementation of GRAPPA, TGRAPPA, and sliding window reconstructions, as described in Methods . The experimental results manifest that k‐t GRAPPA generates high spatial resolution reconstruction without significant loss of temporal resolution when the reduction factor is as high as 4. When the reduction factor becomes higher, there might be a noticeable loss of temporal resolution since k‐t GRAPPA uses temporal interpolation. Images reconstructed using k‐t GRAPPA have less residue/folding artifacts than those reconstructed by sliding window, much less noise than those reconstructed by GRAPPA, and wider temporal bandwidth than those reconstructed by GRAPPA with residual k‐space. k‐t GRAPPA is applicable to a wide range of dynamic imaging applications and is not limited to imaging parts with quasi‐periodic motion. Since only local information is used for reconstruction, k‐t GRAPPA is also preferred for applications requiring real time reconstruction, such as monitoring interventional MRI. Magn Reson Med, 2005.

Real-time magnetic resonance imaging–guided radiofrequency atrial ablation and visualization of lesion formation at 3 Tesla

BACKGROUND Magnetic resonance imaging (MRI) allows visualization of location and extent of radiofrequency (RF) ablation lesion, myocardial scar formation, and real-time (RT) assessment of lesion formation. In this study, we report a novel 3-Tesla RT -RI based porcine RF ablation model and visualization of lesion formation in the atrium during RF energy delivery. OBJECTIVE The purpose of this study was to develop a 3-Tesla RT MRI-based catheter ablation and lesion visualization system. METHODS RF energy was delivered to six pigs under RT MRI guidance. A novel MRI-compatible mapping and ablation catheter was used. Under RT MRI, this catheter was safely guided and positioned within either the left or right atrium. Unipolar and bipolar electrograms were recorded. The catheter tip-tissue interface was visualized with a T1-weighted gradient echo sequence. RF energy was then delivered in a power-controlled fashion. Myocardial changes and lesion formation were visualized with a T2-weighted (T2W) half Fourier acquisition with single-shot turbo spin echo (HASTE) sequence during ablation. RESULTS RT visualization of lesion formation was achieved in 30% of the ablations performed. In the other cases, either the lesion was formed outside the imaged region (25%) or the lesion was not created (45%) presumably due to poor tissue-catheter tip contact. The presence of lesions was confirmed by late gadolinium enhancement MRI and macroscopic tissue examination. CONCLUSION MRI-compatible catheters can be navigated and RF energy safely delivered under 3-Tesla RT MRI guidance. Recording electrograms during RT imaging also is feasible. RT visualization of lesion as it forms during RF energy delivery is possible and was demonstrated using T2W HASTE imaging.

Magnetic Resonance Imaging‐Confirmed Ablative Debulking of the Left Atrial Posterior Wall and Septum for Treatment of Persistent Atrial Fibrillation: Rationale and Initial Experience

LA Debulking for Atrial Fibrillation. Introduction: Though pulmonary vein (PV) isolation has been widely adopted for treatment of atrial fibrillation (AF), recurrence rates remain unacceptably high with persistent and longstanding AF. As evidence emerges for non‐PV substrate changes in the pathogenesis of AF, more extensive ablation strategies need further study.

Temporal left atrial lesion formation after ablation of atrial fibrillation.

BACKGROUND Atrial fibrillation (AF) ablation uses radiofrequency (RF) energy to induce thermal damage to the left atrium (LA) in an attempt to isolate AF circuits. This injury can be seen using delayed enhancement magnetic resonance imaging (DE-MRI). OBJECTIVE The purpose of this study was to describe DE-MRI findings of the LA in the acute and chronic stages postablation. METHODS Twenty-five patients were scanned at two time points postablation. The first group (n = 10) underwent DE-MRI at 24 hours and at 3 months. The second group (n = 16) was scanned at 3 months and at 6 or 9 months. One patient had three scans (24 hours, 3 months, 9 months) and was included in both groups. The location and extent of enhancement were then analyzed between both groups. RESULTS The median change in LA wall injury between 24 hours and 3 months was -6.38% (range -11.7% to 12.58%). The median change in LA wall injury between 3 months and later follow-up was +2.0% (range -4.0% to 6.58%). There appears to be little relationship between the enhancement at 24 hours and 3 months (R(2) = 0.004). In contrast, a strong correlation is seen at 3 months and later follow-up (R(2) = 0.966). Qualitative comparison revealed a stronger qualitative relationship between MRI findings at 3 months and later follow-up than at 24 hours and 3 months. CONCLUSION RF-induced scar appears to have formed by 3 months postablation. At 24 hours postablation, DE-MRI enhancement appears consistent with a transient inflammatory response rather than stable LA scar formation.

Identification and Acute Targeting of Gaps in Atrial Ablation Lesion Sets Using a Real-Time Magnetic Resonance Imaging System

Background— Radiofrequency ablation is routinely used to treat cardiac arrhythmias, but gaps remain in ablation lesion sets because there is no direct visualization of ablation-related changes. In this study, we acutely identify and target gaps using a real-time magnetic resonance imaging (RT-MRI) system, leading to a complete and transmural ablation in the atrium. Methods and Results— A swine model was used for these studies (n=12). Ablation lesions with a gap were created in the atrium using fluoroscopy and an electroanatomic system in the first group (n=5). The animal was then moved to a 3-tesla MRI system where high-resolution late gadolinium enhancement MRI was used to identify the gap. Using an RT-MRI catheter navigation and visualization system, the gap area was ablated in the MR scanner. In a second group (n=7), ablation lesions with varying gaps in between were created under RT-MRI guidance, and gap lengths determined using late gadolinium enhancement MR images were correlated with gap length measured from gross pathology. Gaps up to 1.0 mm were identified using gross pathology, and gaps up to 1.4 mm were identified using late gadolinium enhancement MRI. Using an RT-MRI system with active catheter navigation gaps can be targeted acutely, leading to lesion sets with no gaps. The correlation coefficient (R2) between the gap length was identified using MRI, and the gross pathology was 0.95. Conclusions— RT-MRI system can be used to identify and acutely target gaps in atrial ablation lesion sets. Acute targeting of gaps in ablation lesion sets can potentially lead to significant improvement in clinical outcomes.

High-pass GRAPPA: an image support reduction technique for improved partially parallel imaging.

Partially parallel imaging (PPI) is a widely used technique in clinical applications. A limitation of this technique is the strong noise and artifact in the reconstructed images when high reduction factors are used. This work aims to increase the clinical applicability of PPI by improving its performance at high reduction factors. A new concept, image support reduction, is introduced. A systematic filter‐design approach for image support reduction is proposed. This approach shows more advantages when used with an important existing PPI technique, GRAPPA. An improved GRAPPA method, high‐pass GRAPPA (hp‐GRAPPA), was developed based on this approach. The new technique does not involve changing the original GRAPPA kernel and performs reconstruction in almost the same amount of time. Experimentally, it is demonstrated that the reconstructed images using hp‐GRAPPA have much lower noise/artifact level than those reconstructed using GRAPPA. Magn Reson Med, 2008.

Self-calibration method for radial GRAPPA/k-t GRAPPA

Generalized autocalibrating partially parallel acquisitions (GRAPPA), an important parallel imaging technique, can be easily applied to radial k‐space data by segmenting the k‐space. The previously reported radial GRAPPA method requires extra calibration data to determine the relative shift operators. In this work it is shown that pseudo‐full k‐space data can be generated from the partially acquired radial data by filtering in image space followed by inverse gridding. The relative shift operators can then be approximated from the pseudo‐full k‐space data. The self‐calibration method using pseudo‐full k‐space data can be applied in both k and k‐t space. This technique avoids the prescans and hence improves the applicability of radial GRAPPA to image static tissue, and makes k‐t GRAPPA applicable to radial trajectory. Experiments show that radial GRAPPA calibrated with pseudo‐full calibration data generates results similar to radial GRAPPA calibrated with the true full k‐space data for that image. If motion occurs during acquisition, self‐calibrated radial GRAPPA protects structural information better than externally calibrated GRAPPA. However, radial GRAPPA calibrated with pseudo‐full calibration data suffers from residual streaking artifacts when the reduction factor is high. Radial k‐t GRAPPA calibrated with pseudo‐full calibration data generates reduced errors compared to the sliding‐window method and temporal GRAPPA (TGRAPPA). Magn Reson Med 57:1075–1085, 2007.

An 11-Channel Radio Frequency Phased Array Coil for Magnetic Resonance Guided High Intensity Focused Ultrasound of the Breast

In this study, a radio frequency phased array coil was built to image the breast in conjunction with a magnetic resonance guided high‐intensity focused ultrasound (MRgHIFU) device designed specifically to treat the breast in a treatment cylinder with reduced water volume. The MRgHIFU breast coil was comprised of a 10‐channel phased array coil placed around an MRgHIFU treatment cylinder where nearest‐neighbor decoupling was achieved with capacitive decoupling in a shared leg. In addition a single loop coil was placed at the chest wall making a total of 11 channels. The radio frequency coil array design presented in this work was chosen based on ease of implementation, increased visualization into the treatment cylinder, image reconstruction speed, temporal resolution, and resulting signal‐to‐noise ratio profiles. This work presents a dedicated 11‐channel coil for imaging of the breast tissue in the MRgHIFU setup without obstruction of the ultrasound beam and, specifically, compares its performance in signal‐to‐noise, overall imaging time, and temperature measurement accuracy to that of the standard single chest‐loop coil typically used in breast MRgHIFU. Magn Reson Med, 2013.

Improved partial k-space reconstruction technique for dynamic myocardial perfusion MRI

The advent of partially parallel k-space data acquisition and reconstruction techniques has resulted in a drastic reduction in acquisition time. In this work, we have shown that the use of a new hybrid technique for reconstruction of partial k-space data can reduce acquisition time without loss of spatiotemporal resolution. The technique is termed hybrid k-t GRAPPA since it is a combination of k-t GRAPPA and an image-k-space keyhole technique. The hybrid method with an acceleration factor 4 generated better results than standard GRAPPA with acceleration factor 2 in preliminary results