Johannes Krug

ECG-based gating in ultra high field cardiovascular magnetic resonance using an independent component analysis approach

BackgroundIn Cardiovascular Magnetic Resonance (CMR), the synchronization of image acquisition with heart motion is performed in clinical practice by processing the electrocardiogram (ECG). The ECG-based synchronization is well established for MR scanners with magnetic fields up to 3 T. However, this technique is prone to errors in ultra high field environments, e.g. in 7 T MR scanners as used in research applications. The high magnetic fields cause severe magnetohydrodynamic (MHD) effects which disturb the ECG signal. Image synchronization is thus less reliable and yields artefacts in CMR images.MethodsA strategy based on Independent Component Analysis (ICA) was pursued in this work to enhance the ECG contribution and attenuate the MHD effect. ICA was applied to 12-lead ECG signals recorded inside a 7 T MR scanner. An automatic source identification procedure was proposed to identify an independent component (IC) dominated by the ECG signal. The identified IC was then used for detecting the R-peaks. The presented ICA-based method was compared to other R-peak detection methods using 1) the raw ECG signal, 2) the raw vectorcardiogram (VCG), 3) the state-of-the-art gating technique based on the VCG, 4) an updated version of the VCG-based approach and 5) the ICA of the VCG.ResultsECG signals from eight volunteers were recorded inside the MR scanner. Recordings with an overall length of 87 min accounting for 5457 QRS complexes were available for the analysis. The records were divided into a training and a test dataset. In terms of R-peak detection within the test dataset, the proposed ICA-based algorithm achieved a detection performance with an average sensitivity (Se) of 99.2%, a positive predictive value (+P) of 99.1%, with an average trigger delay and jitter of 5.8 ms and 5.0 ms, respectively. Long term stability of the demixing matrix was shown based on two measurements of the same subject, each being separated by one year, whereas an averaged detection performance of Se = 99.4% and +P = 99.7% was achieved.Compared to the state-of-the-art VCG-based gating technique at 7 T, the proposed method increased the sensitivity and positive predictive value within the test dataset by 27.1% and 42.7%, respectively.ConclusionsThe presented ICA-based method allows the estimation and identification of an IC dominated by the ECG signal. R-peak detection based on this IC outperforms the state-of-the-art VCG-based technique in a 7 T MR scanner environment.

Improved ECG based gating in ultra high field cardiac MRI using an independent component analysis approach

Background Cardiac gating in ultra high field (UHF) MRI is a challenging task due to the magnetohydrodynamic (MHD) effect [1]. The MHD effect is particularly severe at such field strengths and severely distorts the electrocardiogram (ECG). State-of-the-art ECG based gating methods which use the vectorcardiogram (VCG) [2] are thus prone to errors [1]. This work presents an approach which separates the ECG and the MHD signal using Independent Component Analysis (ICA).

Limitations of VCG based gating methods in ultra high field cardiac MRI

Background The electrocardiogram (ECG) is important for gating purposes in cardiac magnetic resonance imaging (CMR). However, the magnetohydrodynamic (MHD) effect, which is caused by the flow of blood in the static magnetic field, makes it difficult to record clean ECG signals for gating. The vectorcardiogram (VCG), which can be derived from the ECG signal, is commonly used for gating purposes and has been well established for magnetic fields strength of up to 3T. However, for higher field strengths this method is prone to errors [1]. This work tries to explain the reasons for the VCG based methods not being suitable for cardiac gating in ultra high field MRI. Methods ECGs were recorded using a standard 12-lead Holter ECG (Getemed, Germany) inside a 7T MR scanner (Siemens Magnetom), a 3T MR scanner (Philips Achieva) and outside the MR scanner as references. Measurements were made on five healthy volunteers while MR imaging was switched off. The VCGs were derived from the 12-lead ECG using the inverse Dower matrix [2]. The VCG method [3] was implemented and applied to the acquired data sets. This method measures the distance in the VCG space between a static reference vector defined by the R-wave (R) peak of an ECG signal recorded outside the MR scanner (at 0T) and the VCG vector recorded inside the MR scanner. Those points where a minimal distance and angle between both vectors is reached are classified as R peaks.

MR-compatible RF ablation system for online treatment monitoring using MR thermometry

RF ablation (RFA) is used for thermal ablation of tumors in which the RF electrode is placed in the tissue under image-guidance. Because of the good tumor visibility and the lack of ionizing radiation, MR-guided RFA is the method of choice. Additionally, with the help of MR thermometry the RF ablation can be monitored during the intervention. Unfortunately, the imaging of an MR scanner is highly sensitive to interferences caused by external electrical signals. In this paper the high-power RF ablation signal of a commercially available medical therapy device is made MR-compatible. A design of a low-pass filter with high-power compatibility is presented. The filter performance is demonstrated by means of simulations and measurements.

Interventional MRI: Minimal-invasive Surgery under MR guidance

Magnetic resonance imaging (MRI) is a well established method in medical diagnostics. By providing real-time fluoroscopic images of arbitrarily cross sections and high soft tissue contrast resolution, MRI in principle is the modality of choice for image guided interventions. Features like MR thermometry or phase contrast MRI may be used for outcome control of the surgery. This paper gives an overview on the advantages and drawbacks of utilizing MRI for image guided interventions and about the most promising medical applications. Different MR scanner types as well as issues related to the MRI compatibility and visualization of surgical tools are presented. Several safety aspects related to the scanners static and radio frequency fields are discussed.

Simulation and experimental validation of resonant electric markers used for medical device tracking in magnetic resonance imaging

Magnetic resonance imaging (MRI), which was traditionally used for patient diagnosis, has gained in importance in minimally invasive interventions in the recent past. Hence, there is an increasing demand for medical devices compatible with the MR environment. One of the challenges is to visualize the medical devices, e.g. catheters, within the MR image. Several methods exist to cope with this task.

RF-ablation pattern shaping employing switching channels of dual bipolar needle electrodes: ex vivo results

PurposeRadiofrequency (RF) ablation with mono- or bipolar electrodes is a common procedure for hepatocellular carcinoma (HCC) with a low rate of recurrence for small size tumors. For larger lesions and/or non-round/ellipsoid shapes RF ablation has some limitations and generally does not achieve comparable success rates to microwave ablation or high-intensity focused ultrasound therapies.Materials and methodsTo shape RF ablations for matching a tumor size and geometry, we have developed an electronic channel switch box for two bipolar needles that generates multiple selectable ablation patterns. The setup can be used with commercially available mono- or bipolar RF generators. The switch box provides ten selectable ablation procedures to generate different ablation patterns without a relocation of a needle. Five patterns were exemplary generated in ex vivo tissue of porcine liver and chicken breast and visually characterized.ResultsDifferent ablation patterns, e.g., in a L- or U-shape, were achieved. In chicken breast a maximum ablation with a diameter of

Correction to: RF-ablation pattern shaping employing switching channels of dual bipolar needle electrodes: ex vivo results

4.3\, \hbox {cm}