*4.5.2. Comparison of cardiac UWB Signal and one dimensional MRI*

296 Ultra-Wideband Radio Technologies for Communications, Localization and Sensor Applications

**4.5. Simultaneous cardiac UWB/ECG, UWB/MRI measurements** 

UWB and ECG were simultaneously acquired. The radar system was equivalent to Section 4.4.2 with one *Tx* and two *Rx* channels. The ECG was recorded with two channels (left arm and left leg against right arm) at a sampling frequency of 8 kHz. For the UWB signals sampled at 44.2 Hz, the same data analysis (see Section 4.4.2) was applied to extract the cardiac signal and determine the trigger events. The usual R-peak detection was applied to trigger on the ECG signal. Cardiac UWB and ECG signals were both re-sampled at 1 kHz

The point of maximum mechanical contraction of the heart in the cardiac UWB signals (s. Fig. 37.a) is delayed to the ECG R-peak, indicating the point of the myocardium's peak electrical activity. Therefore, we have to be aware of the difference between detecting cardiac mechanics by UWB radar and the heart's electrical activity by ECG. For the goal of MRI gating, however, the important thing is the existence of a fixed temporal relationship between ECG and UWB signals with as little jitter as possible. For the time lag between ECG and UWB trigger events, we obtained a standard deviation of less than 20 ms which is already smaller than the UWB sampling time of 22.6 ms. This result proves the consistency

**Figure 36. a)** Measurement set-up with two antenna groups for separate monitoring of the heart's left

**Figure 37. a)** ECG signal with R-peak trigger events and UWB signal with trigger events located at the

maximum of mechanical contraction; **b)** Measurement with an extra-systole.

and right ventricle; **b)** Cardiac signal for left and right ventricle applying four *Rx* channels.

*4.5.1. UWB radar and high resolution ECG* 

to retain more detailed information of the ECG.

and robustness of our procedure.

For better understanding the relationship between actual cardiac mechanics and UWB motion-detection signals, a fast MR-sequence was developed with the aim to monitor myocardial landmarks inside the human body in real time. We implemented a very fast 1D gradient echo sequence for low RF power deposition in tissue and high scan repetition frequency on our MR scanner [77]. One dimensional MR profiles and motion sensitive UWB data were acquired simultaneously allowing the comparison of both techniques and hence a verification of the UWB radar navigator. MR compatible UWB antennas [32] mounted above the chest were directed towards the heart (s. Fig. 38). A flexible RF coil with large openings was used to detect the MRI signal. The UWB data were sampled at 132.6 Hz. Using one *Tx* and five *Rx* UWB antennas 500 virtual channels could be constructed from the IRFs.

**Figure 38. a)** Scheme of the antenna configuration; **b)** Set-up of simultaneous UWB and MRI measurement.

In the MRI sequence, the one-dimensional 'pencil-like' imaging region is selected by the intersecting volume of two perpendicular slices (s. Fig. 39.a). Both slices are excited in short succession resulting in a saturation effect in the region of the intersection. When the experiment is repeated with a different delay time between both excitation pulses, the two images differ only in the strength of this saturation effect, and subtraction yields the desired 1D image. Placed through the heart in antero-posterior direction, this 'pencil' was scanned at a repetition frequency of 25.4 Hz. The motion components in both data sets, the 500 virtual UWB channels and the MR pencil, were once again separated by applying BSS decomposition.

298 Ultra-Wideband Radio Technologies for Communications, Localization and Sensor Applications

ultraMEDIS – Ultra-Wideband Sensing in Medicine 299

depicted, and especially in Fig. 41.b it becomes obvious that these are different processes. However, the UWB detected abdominal respiration correlates well with heart motion due to respiration. The correlation factor in measurement a) is 0.932 and 0.81 in measurement b).

**Figure 40. a)** Extended antenna configuration with second group over the abdominal region; **b)**

**Figure 41.** Breast and abdominal respiration by UWB radar and mechanical heart shift in head foot direction monitored by MR pencil **a)** with changed breast and abdominal respiration **b)** with fading

CMR and UWB signals were acquired simultaneously and synchronously to enable UWB triggering [81]. The UWB antennas were mounted in the same frontal position related to the subject as in Section 4.4.1. Simultaneous pulse oximetry (PO) was applied to compare our

After acquiring a series of CMR images using a clinical sequence with conventional PO gating, we retrospectively reconstructed the *k*-space data a second time but now using trigger points derived from the simultaneously acquired UWB radar signals [81]. Figure 42.b

approach with another established triggering technique for cardiac MRI.

Placement of the two slices for the 'pencil-like' MRI (Head Foot).

breast but changed abdominal component.

**4.7. UWB triggered cardiac MRI** 

**Figure 39. a)** Selection of the 'pencil' by two crossing slices in antero-posterior direction through the heart; **b)** Detected cardiac motion component by UWB radar and MR "pencil".

The trigger events (squares in Fig. 39.b) in the UWB cardiac motion data - representing the point of maximum contraction of the myocardium - were determined by applying the algorithm proposed by us. This did not work with the cardiac components of the MR signal due to the pronounced double peaks in this data set. Comparing the cardiac components simultaneously gained by UWB and MR data, we observe perfectly matching slopes of both signals. However, in contrast to UWB radar the MR signal is affected by the blood velocity in the heart producing the double peaks. Keeping this in mind, we can conclude that both modalities render the same motion. Thus, we can assume the cardiac motion detection by UWB radar to be verified.
