Effects of Lens Motion and Uneven Magnification on Image Spectra

Counter to intuition, the images of an extended galaxy lensed by a moving galaxy cluster should have slightly different spectra in any metric gravity theory. This is mainly for two reasons. One relies on the gravitational potential of a moving lens being time-dependent (the $\text{Moving}$ $\text{Cluster}$ $\text{Effect}$, $\text{MCE}$). The other is due to uneven magnification across the extended, rotating source (the $\text{Differential}$ $\text{Magnification}$ $\text{Effect}$, $\text{DME}$). The time delay between the images can also cause their redshifts to differ because of cosmological expansion. This Differential Expansion Effect is likely to be small. Using a simple model, we derive these effects from first principles. One application would be to the Bullet Cluster, whose large tangential velocity may be inconsistent with the $\Lambda CDM$ paradigm. This velocity can be estimated with complicated hydrodynamic models. Uncertainties with such models can be avoided using the MCE. We argue that the MCE should be observable with ALMA. However, such measurements can be corrupted by the DME if typical spiral galaxies are used as sources. Fortunately, we find that if detailed spectral line profiles were available, then the DME and MCE could be distinguished. It might also be feasible to calculate how much the DME should affect the mean redshift of each image. Resolved observations of the source would be required to do this accurately. The DME is of order the source angular size divided by the Einstein radius times the redshift variation across the source. Thus, it mostly affects nearly edge-on spiral galaxies in certain orientations. This suggests that observers should reduce the DME by careful choice of target, a possibility we discuss in some detail.Comment: 15 pages, 8 figures, 2 tables. This is the peer-reviewed version which has been accepted for publication in Monthly Notices of the Royal Astronomical Societ

Surgical Video Motion Magnification with Suppression of Instrument Artefacts

Video motion magnification could directly highlight subsurface blood vessels in endoscopic video in order to prevent inadvertent damage and bleeding. Applying motion filters to the full surgical image is however sensitive to residual motion from the surgical instruments and can impede practical application due to aberration motion artefacts. By storing the temporal filter response from local spatial frequency information for a single cardiovascular cycle prior to tool introduction to the scene, a filter can be used to determine if motion magnification should be active for a spatial region of the surgical image. In this paper, we propose a strategy to reduce aberration due to non-physiological motion for surgical video motion magnification. We present promising results on endoscopic transnasal transsphenoidal pituitary surgery with a quantitative comparison to recent methods using Structural Similarity (SSIM), as well as qualitative analysis by comparing spatio-temporal cross sections of the videos and individual frames.Comment: Early accept to the Internation Conference on Medical Imaging Computing and Computer Assisted Intervention (MICCAI) 2020 Presentation available here: https://www.youtube.com/watch?v=kKI_Ygny76Q Supplementary video available here: https://www.youtube.com/watch?v=8DUkcHI149