MRI guided ablation within the atrium has recently been reported by other groups6,7
. In one of these studies, MRI angiography of the atrium was acquired, the atrium surface was then segmented, and real time catheter navigation was then carried out using this 3D reconstruction; however, no images were acquired during ablation6
; rather, immediately post-ablation lesion formation was confirmed by LGE imaging. In the other study7
, the catheters were navigated using RT-MRI sequences; however, there was no immediate tissue visualization during RF delivery and lesion formation, although there was T2w evaluation of the ablation site just before and after the ablation of the cavo-tricuspid isthmus. Additionally, these two previous studies were done in 1.5-Tesla MRI.
To our knowledge, this is the first study to demonstrate real-time visualization of atrial ablation lesion formation using a 3-Tesla MRI. We demonstrated feasibility to navigate in the right and left atrium under 3-Tesla RT-MRI guidance with good catheter visualization, providing adequate temporal and spatial resolution. We were able to create lesions using RF energy delivered to the atrial myocardium while imaging the atrial tissue (with T2w-HASTE) as the lesions formed.
Imaging of lesion formation during and after ablation presented its challenges due to cardiac and respiratory motion; this was overcome with the use of the respiratory-gated single-shot HASTE sequence. This allowed us to visualize the effect of RF energy within the atrium. Brightness on T2w images corresponds to edema13
, with progression to tissue destruction14
, and scar formation, as evidenced by LGE12
. All the lesions visualized using T2w-HASTE were validated with post-ablation LGE-MRI and ex-vivo examination of the heart.
We were able to demonstrate a correlation between ablation time and lesion size under power-controlled RF energy delivery. This is in agreement with previously published data4
. However, in this previous study, the time frame for imaging was much longer and the substrate was the ventricle, which is an easier target due to the thicker myocardial wall. Also, for a 20W power delivery, the size of enhancement observed by T2w-HASTE imaging at 15–20 seconds from the beginning of ablation had a good correlation with the dimensions (longest diameter and depth) of the lesion on macroscopic examination. Based on our limited data, T2w-HASTE imaging at later times from the start of ablation tends to overestimate lesion size by showing the surrounding edema.
Accurate lesion visualization in the atrium is still very challenging, and in our study it relied on accurate detection of catheter tip position using a high resolution T1w-FLASH sequence and subsequently imaging this spatial location using a T2w-HASTE sequence. In some of our experiments, changes in catheter position occurred during the time interval between acquisition of T1w-FLASH images and T2w-HASTE scan. Thus, real time visualization of the lesion formation was not always possible, likely because the T2w-HASTE slices did not cover the ablation location. However, even when slice alignment with catheter-tip was not feasible, after a 90 to 240 sec time lapse, it was possible to see lesion formation a few millimeters away from the assumed location.
Even when no specific arrhythmogenic substrate was targeted for ablation in our study, we believe that its importance lies in the possibility, for the first time, to visualize lesion formation as it happens in the atrium.
In summary, we developed a series of tools to navigate and guide an ablation catheter in a 3-Tesla RT-MRI scanner within the right and left atrium in a porcine model, record intra-cardiac electrograms while scanning, and deliver RF energy while simultaneously visualizing the atrial myocardium to assess for the presence of lesion formation. We were also able to correlate these very early visualized lesions on T2w images with LGE-MRI obtained from live and ex-vivo hearts, and anatomic tissue examination ( and 8). This new technology presents many challenges, and there are many shortcomings to its application, such as inability to monitor catheter tip temperature, high fidelity EGM and surface EKG recording during MRI scanning, and difficulty visualizing lesion formation in real time in many cases. Nonetheless, we believe that the work presented here is a significant advancement in the field of delivery and monitoring of RF ablation lesions, which could be potentially used as an endpoint in cardiac ablation procedures to improve outcomes.
The initial prototypes of MRI compatible catheters have not been mechanically optimized to allow the operator to steer them to all parts of the atrium. Furthermore, only two standard catheter curves were available; this aspect is under continued development.
EGM recording and surface EKG recording
During EGM recording noise was present despite filtering. This prevented us from obtaining high fidelity EGM recordings. Due to scanner interference and non-standard surface lead placement, the elements of the surface EKG are difficult to distinguish. The QRS is visualized; however, its deflections, the P wave and T wave are not clearly seen. We are working towards a dedicated EKG system that would provide high fidelity recordings.
To identify catheter-tip position and to confirm catheter tip-tissue contact, a T1w-FLASH sequence was used. On occasion, this proved to be challenging and a lesion could not be delivered and/or clearly visualized while delivering RF energy due to lack of contact between catheter tip and myocardium or misalignment between catheter tip position and T2w-HASTE slices. This could explain why only 11 out of 20 ablations resulted in lesion formation and six of these lesions were visualized in real-time.
Catheter temperature monitoring
The inability to monitor temperature real-time was a significant limitation. Nevertheless, we showed that despite the lack of temperature monitoring an appropriate myocardial lesion could be delivered and its progression monitored in real-time.