The limited applicability of Wiener filtering to ECG signals disturbed by the MHD effect

The diagnostic information of an electrocardiogram (ECG) is essential to ensure patient safety in interventional procedures or medical diagnostics, e.g. during cardiac stress testing. When performing these interventional or diagnostic procedures inside an magnetic resonance imaging (MRI) environment, the ECG is directly and indirectly distorted by the several magnetic fields. While it is still possible to detect the QRS complex which is necessary for gating cardiac image sequences, a diagnostic interpretation of the ECG itself is not possible. In an attempt to remove the distortion due to the magneto-hydrodynamic (MHD) effect, a model of the undistorted ECG was constructed outside the scanner using a Wiener filter. The filter was then applied in a forward sense to the ECG recorded inside an MR scanner. Clinical features, such as the ST level and QT interval were measured before and after filtering and compared to the nominal values taken prior to MRI scanning. Errors in ST level estimation were sometimes found to be greater than 0.1mV, indicating that diagnostic accuracy cannot always be maintained. However, the results for QT interval analysis are sometimes within acceptable tolerances (10 ms-30 ms). This work therefore provides a standard benchmark for other approaches to beat.