*4.4.4. Illumination of human thorax by multiple antenna groups*

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

to aim for one central point at a distance of 100 cm.

measurement set-up.

channels, **c)** the 24 "good channels".

By integration of a MIMO UWB device (MEODAT GmbH, Ilmenau, Germany) containing four modules, each with one *Tx* and two *Rx* channels [76], up to 32 channels became available. In a 1 *Tx* \* 8 *Rx* configuration a sampling frequency up to 530.4 Hz can be realized. Using sequentially activated transmitters the set-up can be extended to 32 channels (4 *Tx* \* 8 *Rx*) at a reduced sampling frequency of up to 132.6 Hz. For cardiac motion detection, the four *Tx* and eight *RX* antennas are placed over the chest as depicted in Fig. 34 and adjusted

**Figure 34.** a) Scheme of the UWB radar set-up with 8 *Rx* and 4 *Tx* antennas b) MR compatible

**Figure 35.** Cardiac signal and detected trigger events for **a)** two hand-picked best channels, **b)** all 32

The procedure to identify the most useful channels for triggering starts with a short preparatory measurement, where each channel is analyzed by the BSS to decompose the complex UWB signals [80], extracts the relevant cardiac component and calculates the trigger events as described in Section 4.4.2. The quality of each measurement channel is assessed by calculating the variation of the time interval between trigger events. For comparison, Fig. 35.a depicts the result of the BSS analysis by the best two channels, manually selected for the smallest variation between the trigger events. The cardiac signal based on these two hand-picked channels represents the best achievable result for a set-up like in the section before. By utilizing all 32 channels for the BSS, a smoother cardiac signal is detected, and the motion amplitude shows less variation over the time. However, the sharpness of the trailing slopes is also reduced. Due to this fact, the third trigger event escaped detection (s. Fig. 35.b). Some of the 32 channels contained much noise resulting in a Stand-alone UWB radar enables the detection of cardiac activities by different illumination conditions as shown in [68] for the radiographic standard position. The illumination of the heart from only one side at a time, like the frontal direction for motion detection as depicted in Fig. 33, was extended to the simultaneous illumination of two sides. No averaging was performed to enable the comparison of single heart beats [30]. This approach can open the field for new diagnostic applications by detecting differences and disturbances in comparative measurements of the left and right ventricle, thus recognizing potentially pathological patterns [69]. Two groups of four *Rx* and one *Tx* antennas were applied for this purpose. The first was placed in the left lateral and the second in the right anterior oblique position.

Each antenna group consisted of a single *Tx* antenna surrounded by four *Rx* antennas. All antennas were directed towards the estimated center position of the heart. The challenge was to measure the cardiac motion even from the lateral position, where the attenuation of the reflected signals from the heart is much higher due to the prolonged propagation path in tissue. The data analysis by BSS was applied for both antenna groups separately. For comparison, the data of only two or all four *Rx* channel per group were analyzed.

For lateral position, the UWB signal from the cardiac motion is considerably weaker and much more affected by noise. However, by increasing the number of *Rx*- channels, the signal quality improved substantially, effectively compensating the strong signal attenuation (s. Fig. 36.b). Only healthy volunteers were examined in this particular study but even among them characteristic peculiarities can be found. In both ventricles, the contraction velocity (trailing edge of the UWB motion curve) is higher than the velocity of ventricle dilatation. The duration of the dilation period, on the other hand, is longer for the right ventricle compared to its counterpart on the left. More characteristic features are expected to be visible in patients with cardiac diseases or malfunctions.
