**4.7. UWB triggered cardiac MRI**

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 approach with another established triggering technique for cardiac MRI.

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

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

shows that both methods give virtually undistinguishable results, thus establishing the feasibility of CMR imaging utilizing non-contact UWB radar for triggering. In contrast to established techniques like ECG or PO, however, contact-less UWB-sensing provides cardiac and respiratory information simultaneously and, thus, a sequence-independent external navigator signal.

ultraMEDIS – Ultra-Wideband Sensing in Medicine 301

*Non-contact breast imaging:* The most significant reason for non-contact breast measurements is the size of the antennas compared with the breast size. Thereby, it is impossible to mount a sufficient number of antennas on the breast surface in order to achieve an adequate image quality. The displacement of the antennas from the breast increases the area where additional antennas can be localized. Besides that, it allows mechanical scanning where the antennas can be rotated around the breast in order to create a synthetic aperture. On the other hand, this non-contact strategy is accompanied by a lot of other problems and

Breast Breast

Tumor Tumor

Mold

**Figure 43.** Schematization of non-contact breast imaging using a liquid contact medium (left side) and

Depending on the dielectric contrast between the medium surrounding the antennas and the breast tissue, only a fraction of the radiated signal energy will penetrate the breast. The major part will be reflected at the breast surface. It provides clutter which has to be eliminated since it perturbs the signals of interest. In order to reduce the reflection coefficient, several approaches use a liquid coupling medium in which the breast has to be immersed and in which the antennas can surround the breast. The same energy reduction effect appears for reflected components from inside of the breast passing the dielectric boundary in the opposite direction. Furthermore, in the opposite direction (from dielectric dense medium into a less dense medium) waves can only leave the breast below the angle of total reflection which

The individual breast shape plays an important role in connection with these effects as well as for image processing. In section 5.3, we describe a method for breast and whole body

*Contact-mode breast imaging:* Contact-based breast imaging avoids the disadvantages described above. The antennas are localized directly at the breast surface. Understandably, they have to be small enough in order to arrange a sufficient number of antennas around the breast. The corresponding number of signal channels will be obtained by electronic scanning, that means sequential feeding of all transmitter antennas with simultaneous signal acquisition of all receiving antennas. This strategy involves the problem of individual breast shapes and sizes which influences the contact pressure of the breast skin onto the antenna

implies an additional reduction of the detectable signal energy outside the breast.

surface reconstruction based on the reflected UWB signals.

aperture and, thus, the signal quality [86].

contact-based breast imaging (right side) in the prone examination position

Liquid tank Antennas Antennas

challenges.

**Figure 42. a)** Cardiac UWB signal with selected trigger events; **b)** Top: Reconstructed images utilizing PO trigger, Bottom: Image reconstruction by UWB trigger events applied.
