Ronald Mooiweer1, Rainer Schneider2, Radhouene Neji1,3, Rahul K Mukherjee1, Steven Williams1, Li Huang1, Valéry Ozenne4, Pierre Bour4, Jason Stroup5, Tom Lloyd5, Pierre Jaïs4, Bruno Quesson4, Mark O'Neill1, Tobias Schaeffter1,6, Reza Razavi1, and Sébastien Roujol1
1Biomedical Engineering, King's College London, London, United Kingdom, 2Siemens Healthcare GmbH, Erlangen, Germany, 3MR Research Collaborations, Siemens Healthcare Limited, Frimley, United Kingdom, 4IHU-Liryc, Pessac, France, 5Imricor Medical Systems, Burnsville, MN, United States, 6Physikalisch-Technische Bundesanstalt, Berlin, Germany
Synopsis
MR thermometry can offer real-time temperature
information during RF ablation in the heart. As ECG-triggering can be
unreliable in these situations, cardiac triggering based on the position of the
ablation catheter could provide an alternative. Active tracking was used to
continuously measure the position of microcoils inside the catheter. Cardiac
triggers were determined after respiratory motion filtering. Temperature
stability over time was below 2.5 ˚C.
Introduction
MR thermometry can offer real-time feedback
during MRI-guided catheter ablation to treat cardiac arrhythmias [1-5].
ECG-triggering can be negatively affected by RF ablation equipment and rapid
gradient switching associated with the thermometry’s EPI-based acquisition [4].
Cardiac triggering based on continuous position determination of active
tracking microcoils in the catheter (AT-triggering) has been suggested and
demonstrated in phantom in the absence of respiratory motion [6]. This method is
now expanded to filter out respiratory motion and was tested in-vivo in an animal model.Methods
Trigger
determination
Active tracking (AT) modules (determining 3D
position in 25 ms) were repeated until variations in their signal triggered the
execution of single-shot MR-thermometry sequences. Trigger determination was
implemented in the reconstruction software on the scanner. At the start of the
sequence, at least 11 seconds of continuous AT signal were recorded to capture the
AT signal over several breathing cycles. The following steps were then applied to
all incoming AT measurements:
- For
each new AT measurement at time t, a
high-pass filter was applied to the last 10 seconds of AT signal to generate a
respiratory filtered AT signal. The filtered signal at time t (i.e one time point) was stored as the entry of the AT triggering
signal at time t (Figure 1).
-
A
trigger was detected when a peak was detected in the last 3 entries in the AT
triggering signal and when the peak was within the highest 50 percentile, to
avoid local maxima.
In-vivo
Evaluation
Thermometry with AT-based cardiac triggering (prototype) was performed within the left ventricle of a sedated and mechanically
ventilated sheep (5 seconds breathing cycle). The stability of thermometry with
AT-triggering was evaluated at two positions without RF energy deposition (100
dynamics), as well as during ablation (40W for 50 seconds, 150 dynamics). The experiments
were conducted at 1.5 T (MAGNETOM Aera, Siemens Healthcare, Erlangen Germany)
using an MR-compatible ablation catheter with active tracking microcoils in
the tip (Vision-MR Ablation Catheter, Imricor Medical Systems, USA)). Proton
resonance frequency shift based thermometry using EPI readout was performed as
in [5], acquiring 4 slices per heartbeat. MR-thermometry reconstruction
consisted of non-rigid motion correction, multi-baseline correction for
respiratory-induced phase variation, as well as temporal filtering of
temperature maps, as previously described in [7].
Results
The time interval between the AT-based cardiac
triggers is fairly stable and consistent with the heart rate (Figure 2), but
outliers (>1200ms) indicate that some heart beats were missed. These mis-triggers
were attributed to AT measurements in which there was insufficient SNR to determine
the AT position correctly, causing occasional outlier AT signal positions.
The stability of thermometry was calculated in
6 slices in which the myocardium was visible (Figure 3). Median values were
below 1.5˚C and outliers within 2.5˚C.
A localised
elevation in temperature and associated lethal thermal dose could be observed
during an ablation experiment (see Figure 4). Discussion
AT-based cardiac triggered MR-thermometry was
successfully demonstrated during stability and ablation experiments. No
ECG-triggering was used in these experiments and it may be insightful to
compare ECG-triggering to AT-triggering in the future. AT could also be used for
prospective respiratory motion correction (i.e. slice tracking), instead of
cardiac triggering only, which will be the focus of future work.Conclusion
Active
tracking was successfully used to perform cardiac triggering during
MR-thermometry in an animal model.Acknowledgements
This work
was supported by the EPSRC grant (EP/R010935/1) and the Health Innovation
Challenge Fund (grant number HICF-R10-698), a parallel funding partnership
between the Department of Health, and the Wellcome Trust. This work was also
supported by the Wellcome EPSRC Centre for Medical Engineering at Kings College
London (WT 203148/Z/16/Z) and by the National Institute for Health Research
(NIHR) Biomedical Research Centre based at Guy’s and St Thomas’ NHS Foundation
Trust and King’s College London. The views expressed are those of the authors
and not necessarily those of the NHS, the NIHR or the Department of Health.References
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