Whispering gallery modes (WGMs) have been exploited for a broad range of sensing applications. However, the vast majority of WGM sensors consist of passive resonators, requiring complex interrogation systems to be employed, ultimately limiting their practicality. Active resonators containing a gain medium, allowing remote excitation and collection of the WGM‐modulated fluorescence spectra, have emerged as an alternative to passive resonators. Although research is still in its infancy, recent progress has reduced the performance gap between the two paradigms, fueled by the potential for new applications that could not previously be realized. Here, recent developments in sensors based on active WGM microresonators are reviewed, beginning with a discussion of the theory of fluorescence‐based and lasing WGMs, followed by a discussion of the variety of gain media, resonator architectures, and emerging sensing applications. We conclude with a discussion of the prospects and future directions for improving active WGM sensors.

The platform presented here was devised to address the unmet need for real time label-free in vivo sensing by bringing together a refractive index transduction mechanism based on Whispering Gallery Modes (WGM) in dye doped microspheres and Microstructured Optical Fibers. In addition to providing remote excitation and collection of the WGM signal, the fiber provides significant practical advantages such as an easy manipulation of the microresonator and the use of this sensor in a dip sensing architecture, alleviating the need for a complex microfluidic interface. Here, we present the first demonstration of the use of this approach for biological sensing and evaluate its limitation in a sensing configuration deprived of liquid flow which is most likely to occur in an in vivo setting. We also demonstrate the ability of this sensing platform to be operated above its lasing threshold, enabling enhanced device performance.

This paper investigates the Q-factor limits imposed on the farfield detection of the whispering gallery modes of active microspherical resonators. It is shown that the Q-factor measured for a given active microsphere in the far-field using a microscope is significantly lower than that measured using evanescent field collection through a taper. The discrepancy is attributed to the inevitable small asphericity of microspheres that results in mode-splitting which becomes unresolvable in the far-field. Analytic expressions quantifying the Q-factor limits due to small levels of asphericity are subsequently derived.

Robotic C-arm Cone Beam Computed Tomography (CBCT) systems are playing an increasingly pivotal 6 role in interventional cardiac procedures and high precision radiotherapy treatments. One of the main 7 challenges in any form of cardiac imaging is mitigating the intrinsic motion of the heart, which causes 8 blurring and artefacts in the 3D reconstructed image. Most conventional 3D cardiac CBCT acquisition 9 techniques attempt to combat heart motion through retrospective gating techniques, whereby acquired 10 projections are sorted into the desired cardiac phase after the completion of the scan. However, this 11 results in streaking artefacts and unnecessary radiation exposure to the patient. Here, we present our 12 Adaptive CaRdiac cOne BEAm computed Tomography (ACROBEAT) acquisition protocol that uses 13 the patient's electrocardiogram (ECG) signal to adaptively regulate the gantry velocity and projection 14 time interval in real-time. It enables prospectively gated patient connected imaging in a single sweep of 15 the gantry. The XCAT digital software phantom was used to complete a simulation study to compare 16 ACROBEAT to a conventional multi-sweep retrospective ECG gated acquisition, under a variety of 17 different acquisition conditions. The effect of location and length of the acquisition window and total 18 number of projections acquired on image quality and total scan time were examined. Overall, 19 ACROBEAT enables up to a 5 times average improvement in the contrast-to-noise ratio, a 40% 20 reduction in edge response width and an 80% reduction in total projections acquired compared to 21 conventional multi-sweep retrospective ECG gated acquisition.

Cone-beam computed tomography (CBCT) imaging is becoming increasingly important for a wide range of applications such as image-guided surgery, image-guided radiation therapy as well as diagnostic imaging such as breast and orthopaedic imaging. The potential benefits of non-circular source-detector trajectories was recognized in early work to improve the completeness of CBCT sampling and extend the field of view (FOV). Another important feature of interventional imaging is that prior knowledge of patient anatomy such as a preoperative CBCT or prior CT is commonly available. This provides the opportunity to integrate such prior information into the image acquisition process by customized CBCT source-detector trajectories. Such customized trajectories can be designed in order to optimize task-specific imaging performance, providing intervention or patient-specific imaging settings. The recently developed robotic CBCT C-arms as well as novel multi-source CBCT imaging systems with additional degrees of freedom provide the possibility to largely expand the scanning geometries beyond the conventional circular source-detector trajectory. This recent development has inspired the research community to innovate enhanced image quality by modifying image geometry, as opposed to hardware or algorithms. The recently proposed techniques in this field facilitate image quality improvement, FOV extension, radiation dose reduction, metal artifact reduction as well as 3D imaging under kinematic constraints. Because of the great practical value and the increasing importance of CBCT imaging in image-guided therapy for clinical and preclinical applications as well as in industry, this paper focuses on the review and discussion of the available literature in the CBCT trajectory optimization field. To the best of our knowledge, this paper is the first study that provides an exhaustive literature review regarding customized CBCT algorithms and tries to update the community with the clarification of in-depth information on the current progress and future trends.