Investigation of the Pancharatnam-Berry phase element for the generation of the top-hat beam

Within optics, the Pancharatnam-Berry phase enables the design and creation of various flat special optical elements such as top-hat converters. We present a study on engineering efficient vectorial top-hat converters inscribed in glass by high-power femtosecond laser pulses. We phase-encode a top-hat converter and demonstrate how its quality is influenced by various parameters. We investigate theoretically the generation of the top-hat beam under imperfect conditions such as the mismatch of the incident beam width or the misalignment of the center of the converter. Experimental verification of the concept is also presented.

Segmented K-Space Blipped-Controlled Aliasing in Parallel Imaging (Skipped-CAIPI) for High Spatiotemporal Resolution Echo Planar Imaging

PurposeA segmented k-space blipped-CAIPI (skipped-CAIPI) sampling strategy for echo planar imaging (EPI) is proposed, which allows for a flexible choice of EPI factor and phase encode bandwidth independent of the controlled aliasing (CAIPIRINHA) pattern.Theory and MethodsWith previously proposed approaches, exactly two EPI trajectories were possible given a specific CAIPIRINHA pattern: either with slice gradient blips (blipped-CAIPI), or following a shot-selective approach (higher resolution). Recently, interleaved multi-shot segmentation along shot-selective CAIPI trajectories has been applied for high-resolution anatomical imaging. For more flexibility and a broader range of applications, we propose segmentation along any blipped-CAIPI trajectory. Thus, all EPI factors and phase encode bandwidths available with traditional segmented EPI can be combined with controlled aliasing.ResultsTemporal signal-to-noise ratios of moderate-to-high-resolution time series acquisitions at varying undersampling factors demonstrate beneficial sampling alternatives to blipped-CAIPI or shot-selective CAIPI. Rapid high-resolution scans furthermore demonstrate SNR-efficient and motion-robust structural imaging with almost arbitrary EPI factor and minimal noise penalty.ConclusionsSkipped-CAIPI sampling increases protocol flexibility for high spatiotemporal resolution EPI. In terms of signal-to-noise ratio and efficiency, high-resolution functional or structural scans benefit vastly from a free choice of the CAIPIRINHA pattern. Even at moderate resolutions, the independence of sampling pattern, echo time and image matrix size is valuable for optimized functional protocol design. Although demonstrated with 3D-EPI, skipped-CAIPI is also applicable with simultaneous multislice EPI.

Distinctions among real and apparent respiratory motions in human fMRI data

Head motion estimates in functional magnetic resonance imaging (fMRI) scans appear qualitatively different with sub-second image sampling rates compared to the multi-second sampling rates common in the past. Whereas formerly the head appeared still for much of a scan with brief excursions from baseline, the head now appears to be in constant motion, and motion estimates often seem to divulge little information about what is happening in a scan. This constant motion has been attributed to respiratory oscillations that do not alias at faster sampling rates, and investigators are divided on the extent to which such motion is “real” motion or only “apparent” pseudomotion. Some investigators have abandoned the use of motion estimates entirely due to these considerations. Here we investigate the properties of motion in several fMRI datasets sampled at rates between 720-1160 ms, and describe 5 distinct kinds of respiratory motion: 1) constant real respiratory motion in the form of head nodding most evident in vertical position and pitch, which can be very large; 2) constant pseudomotion at the same respiratory rate as real motion, occurring only in the phase encode direction; 3) punctate real motions occurring at times of very deep breaths; 4) a low-frequency pseudomotion in only the phase encode direction following very deep breaths; 5) slow modulation of vertical and anterior-posterior head position by the respiratory envelope. We reformulate motion estimates in light of these considerations and obtain good concordance between motion estimates, physiologic records, image quality measures, and events evident in the fMRI signals.Highlights- Examines several fast-TR datasets with sampling rates of 720-1160 ms- Identifies 7 kinds of motion in fMRI scans, 5 of them related to respiration- Identifies 2 forms of pseudomotion occurring only in phase encode direction- Pseudomotion is a function of soft tissue mass, not lung volume- Reformulates motion estimates to draw out particular kinds of motion