525 research outputs found
Beyond Low Rank + Sparse: Multi-scale Low Rank Matrix Decomposition
We present a natural generalization of the recent low rank + sparse matrix
decomposition and consider the decomposition of matrices into components of
multiple scales. Such decomposition is well motivated in practice as data
matrices often exhibit local correlations in multiple scales. Concretely, we
propose a multi-scale low rank modeling that represents a data matrix as a sum
of block-wise low rank matrices with increasing scales of block sizes. We then
consider the inverse problem of decomposing the data matrix into its
multi-scale low rank components and approach the problem via a convex
formulation. Theoretically, we show that under various incoherence conditions,
the convex program recovers the multi-scale low rank components \revised{either
exactly or approximately}. Practically, we provide guidance on selecting the
regularization parameters and incorporate cycle spinning to reduce blocking
artifacts. Experimentally, we show that the multi-scale low rank decomposition
provides a more intuitive decomposition than conventional low rank methods and
demonstrate its effectiveness in four applications, including illumination
normalization for face images, motion separation for surveillance videos,
multi-scale modeling of the dynamic contrast enhanced magnetic resonance
imaging and collaborative filtering exploiting age information
Parallel Magnetic Resonance Imaging as Approximation in a Reproducing Kernel Hilbert Space
In Magnetic Resonance Imaging (MRI) data samples are collected in the spatial
frequency domain (k-space), typically by time-consuming line-by-line scanning
on a Cartesian grid. Scans can be accelerated by simultaneous acquisition of
data using multiple receivers (parallel imaging), and by using more efficient
non-Cartesian sampling schemes. As shown here, reconstruction from samples at
arbitrary locations can be understood as approximation of vector-valued
functions from the acquired samples and formulated using a Reproducing Kernel
Hilbert Space (RKHS) with a matrix-valued kernel defined by the spatial
sensitivities of the receive coils. This establishes a formal connection
between approximation theory and parallel imaging. Theoretical tools from
approximation theory can then be used to understand reconstruction in k-space
and to extend the analysis of the effects of samples selection beyond the
traditional g-factor noise analysis to both noise amplification and
approximation errors. This is demonstrated with numerical examples.Comment: 28 pages, 7 figure
Deficiency and abelianized deficiency of some virtually free groups
Let be the HNN extension of where the stable letter
conjugates the first factor to the second. We explore small presentations of
the groups . We show that for certain choices of
(m,n), for example (2,3), the group has a relation gap unless it
admits a presentation with at most 3 defining relations, and we establish
restrictions on the possible form of such a presentation. We then associate to
each (m,n) a 3-complex with 16 cells. This 3-complex is a counterexample to the
D(2) conjecture if has a relation gap.Comment: 7 pages; no figures. Minor changes; now to appear in Math. Proc.
Camb. Phil. So
Beat Pilot Tone: Versatile, Contact-Free Motion Sensing in MRI with Radio Frequency Intermodulation
Motion in Magnetic Resonance Imaging (MRI) scans results in image corruption
and remains a barrier to clinical imaging. Motion correction algorithms require
accurate sensing, but existing sensors are limited in sensitivity, comfort, or
general usability. We propose Beat Pilot Tone (BPT), a radio frequency (RF)
motion sensing system that is sensitive, comfortable, versatile, and scalable.
BPT operates by a novel mechanism: two or more transmitted RF tones form
standing wave patterns that are modulated by motion and sensed by the same
receiver coil arrays used for MR imaging. By serendipity, the tones are mixed
through nonlinear intermodulation in the receiver chain and digitized
simultaneously with the MRI data. We demonstrate BPT's mechanism in simulations
and experiments. Furthermore, we show in healthy volunteers that BPT can sense
head, bulk, respiratory, and cardiac motion, including small vibrations such as
displacement ballistocardiograms. BPT can distinguish between different motion
types, achieve greater sensitivity than other methods, and operate as a
multiple-input multiple-output (MIMO) system. Thus, BPT can enable
motion-robust MRI scans at high spatiotemporal resolution in many applications
Printed Receive Coils with High Acoustic Transparency for Magnetic Resonance Guided Focused Ultrasound.
In magnetic resonance guided focused ultrasound (MRgFUS) therapy sound waves are focused through the body to selectively ablate difficult to access lesions and tissues. A magnetic resonance imaging (MRI) scanner non-invasively tracks the temperature increase throughout the tissue to guide the therapy. In clinical MRI, tightly fitted hardware comprised of multichannel coil arrays are required to capture high quality images at high spatiotemporal resolution. Ablating tissue requires a clear path for acoustic energy to travel but current array materials scatter and attenuate acoustic energy. As a result coil arrays are placed outside of the transducer, clear of the beam path, compromising imaging speed, resolution, and temperature accuracy of the scan. Here we show that when coil arrays are fabricated by additive manufacturing (i.e., printing), they exhibit acoustic transparency as high as 89.5%. This allows the coils to be placed in the beam path increasing the image signal to noise ratio (SNR) five-fold in phantoms and volunteers. We also characterize printed coil materials properties over time when submerged in the water required for acoustic coupling. These arrays offer high SNR and acceleration capabilities, which can address current challenges in treating head and abdominal tumors allowing MRgFUS to give patients better outcomes
- …