The present work addresses critical issues in constructing high suppression, narrowband spectral filters in rare-earth-ion-doped crystals, mainly targeting the application of ultrasound optical tomography but is also applicable for areas such as quantum memories, self-filtering of laser frequencies, and other applications relying on high absorption in rare-earth-ion-doped crystals. The polarization of light transmitted through a highly absorbing crystal is experimentally analyzed. Additionally, an existing wave propagation method is used to simulate beam propagation through a spectral hole to study the high étendue requirements of ultrasound optical tomography.