Applied Optics Research at imec: introduction to the feature issue.

This feature issue provides an overview of the current applied optics research activities taking place at imec, Interuniversity Microelectronics Center, at its campuses in Leuven, Brussels and Ghent, Belgium. The issue contains articles covering wide range of topics on imaging systems, image processing, new materials, optical devices, sensors and detectors.

JPEG Pleno holography presents the numerical reconstruction software for holograms: an excursion in holographic views.

Digital reconstructions of numerical holograms enable data visualization and serve a multitude of purposes ranging from microscopy to holographic displays. Over the years, many pipelines have been developed for specific hologram types. Within the standardization effort of JPEG Pleno holography, an open-source MATLAB toolbox was developed that reflects the best current consensus. It can process Fresnel, angular spectrum, and Fourier-Fresnel holograms with one or more color channels; it also allows for diffraction-limited numerical reconstructions. The latter provides a way to reconstruct holograms at their intrinsic physical instead of an arbitrarily chosen numerical resolution. The Numerical Reconstruction Software for Holograms v10 supports all large public data sets featured by UBI, BCOM, ETRI, and ETRO, in their native and vertical off-axis binary forms. Through the release of this software, we hope to improve the reproducibility of research, thus enabling consistent comparison of data between research groups and the quality of specific numerical reconstructions.

Deep hologram converter from low-precision to middle-precision holograms.

We propose a deep hologram converter based on deep learning to convert low-precision holograms into middle-precision holograms. The low-precision holograms were calculated using a shorter bit width. It can increase the amount of data packing for single instruction/multiple data in the software approach and the number of calculation circuits in the hardware approach. One small and one large deep neural network (DNN) are investigated. The large DNN exhibited better image quality, whereas the smaller DNN exhibited a faster inference time. Although the study demonstrated the effectiveness of point-cloud hologram calculations, this scheme could be extended to various other hologram calculation algorithms.

Binary hologram compression using context based Bayesian tree models with adaptive spatial segmentation.

With holographic displays requiring giga- or terapixel resolutions, data compression is of utmost importance in making holography a viable technique in the near future. In addition, since the first-generation of holographic displays is expected to require binary holograms, associated compression algorithms are expected to be able to handle this binary format. In this work, the suitability of a context based Bayesian tree model is proposed as an extension to adaptive binary arithmetic coding to facilitate the efficient lossless compression of binary holograms. In addition, we propose a quadtree-based adaptive spatial segmentation strategy, as the scale dependent, quasi-stationary behavior of a hologram limits the applicability of the advocated modelling approach straightforwardly on the full hologram. On average, the proposed compression strategy produces files that are around 12% smaller than JBIG2, the reference binary image codec.

Pincushion point-spread function for computer-generated holography.

Point-spread functions (PSFs) are non-stationary signals whose spatial frequency increases with the radius. These signals are only meaningful over a small spatial region when being propagated over short distances and sampled with regular sampling pitch. Otherwise, aliasing at steep incidence angles leads to the computation of spurious frequencies. This is generally addressed by evaluating the PSF in a bounded disk-shaped region, which has the added benefit that it reduces the required number of coefficient updates. This significantly accelerates numerical diffraction calculations in, e.g., wavefront recording planes for high-resolution holograms. However, the use of a disk-shaped PSF is too conservative since it only utilizes about 78.5% of the total bandwidth of the hologram. We therefore derive a novel, to the best of our knowledge, optimally shaped PSF fully utilizing the bandwidth formed by two bounding hyperbola. A number of numerical experiments with the newly proposed pincushion PSF were performed, reporting over three-fold reductions of the signal error and significant improvements to the visual quality of computer-generated holograms at high viewing angles.

JPEG Pleno holography: scope and technology validation procedures.

JPEG Pleno is a standardization framework addressing the compression and signaling of plenoptic modalities. While the standardization of solutions to handle light field content is currently reaching its final stage, the Joint Photographic Experts Group (JPEG) committee is now preparing for the standardization of solutions targeting point cloud and holographic modalities. This paper addresses the challenges related to the standardization of compression technologies for holographic content and associated test methodologies.

Suitability analysis of holographic vs light field and 2D displays for subjective quality assessment of Fourier holograms.

Measuring the impact of compression on the reconstruction quality of holograms remains a challenge. A public subjectively-annotated holographic data set that allows for testing the performance of compression techniques and quality metrics is presented, in addition to a subjective visual quality assessment methodology. Moreover, the performance of the quality assessment procedures is compared for holographic, regular 2D and light field displays. For these experiments, a double-stimulus, multi-perspective, multi-depth testing methodology was designed and implemented. Analysis of the quality scores indicated that in the absence of a suitable holographic display and under the presented test conditions, non-holographic displays can be deployed to display numerically reconstructed holograms for visual quality assessment tasks.

Object-based digital hologram segmentation and motion compensation.

Digital video holography faces two main problems: 1) computer-generation of holograms is computationally very costly, even more when dynamic content is considered; 2) the transmission of many high-resolution holograms requires large bandwidths. Motion compensation algorithms leverage temporal redundancies and can be used to address both issues by predicting future frames from preceding ones. Unfortunately, existing holographic motion compensation methods can only model uniform motions of entire 3D scenes. We address this limitation by proposing both a segmentation scheme for multi-object holograms based on Gabor masks and derive a Gabor mask-based multi-object motion compensation (GMMC) method for the compensation of independently moving objects within a single hologram. The utilized Gabor masks are defined in 4D space-frequency domain (also known as time-frequency domain or optical phase-space). GMMC can segment holograms containing an arbitrary number of mutually occluding objects by means of a coarse triangulation of the scene as side information. We demonstrate high segmentation quality (down to ≤ 0.01% normalized mean-squared error) with Gabor masks for scenes with spatial occlusions. The support of holographic motion compensation for arbitrary multi-object scenes can enable faster generation or improved video compression rates for dynamic digital holography.

Exact global motion compensation for holographic video compression.

Holographic video requires impractical bitrates for storage and transmission without data compression. We introduce an end-to-end compression pipeline for compressing holographic sequences with known ground truth motion. The compression strategy employs a motion compensation algorithm based on the rotational transformation of an angular spectrum. Residuals arising from the compensation step are represented using short-time Fourier transforms and quantized with uniform mid-rise quantizers whose bit depth is determined by a Lagrangian rate-distortion optimization criterion where the distortion metric is the mean squared error. Experiments use computer-generated holographic videos, and we report Bjøntegaard delta peak signal-to-noise ratio gains of around 20 dB when compared to traditional image/video codecs.

Efficient holographic video generation based on rotational transformation of wavefields.

High-quality holographic video generation requires both high computational resources and time; therefore, algorithms that can generate holographic videos efficiently will be soon in demand for dynamic holographic 3D displays. In this work, we present two algorithms that use a motion compensation scheme - based on the rotational transformation of wavefields [Appl. Opt.47, D110 (2008)], to generate efficiently holographic frames. The effects of motions to the wavefield are analyzed, while video generation examples are provided to support our theory. We report that with the proposed motion compensation scheme combined with the Phong multiple wavefront recording plane CGH method, a 10 times acceleration was achieved with regard to the ray tracing method, with a trade-off of energy loss of 10%. This speedup can be further improved by parallelization of the computations.

APRIL and BAFF: novel biomarkers for central nervous system lymphoma.

BACKGROUND: Early diagnosis of CNS lymphoma (CNSL) is essential for successful therapy of this rapidly progressing brain tumor. However, in patients presenting with focal brain lesions, fast and reliable diagnosis of PCNSL remains a challenge. A proliferation-inducing ligand (APRIL) and B cell activating factor (BAFF) are important factors in the pathophysiology, diagnosis, and prognosis of systemic B cell malignancies. However, their utility as biomarkers for the diagnosis of CNSL and their effects on CNSL cells remain unclear. METHODS: In this prospective study, we analyzed the levels of APRIL and BAFF in the cerebrospinal fluid (CSF) of 116 patients with suspected focal brain lesions, including 53 CNSL patients. Additionally, we serially measured their levels during chemotherapy and relapse. Furthermore, we analyzed the effect of APRIL and BAFF on two B cell lymphoma cell lines using proliferation, viability, and chemotaxis assays. RESULTS: CSF levels of APRIL and BAFF reliably differentiated CNSL from other focal brain lesions (including primary and metastatic brain tumors, autoimmune-inflammatory lesions, and neuroinfectious lesions) with a specificity of 93.7% (APRIL, BAFF) and a sensitivity of 62.3% (APRIL) and 47.1% (BAFF). Serial CSF analysis of CNSL patients during chemotherapy and relapse demonstrates a close correlation of APRIL CSF levels and the course of this disease. In vitro, APRIL and BAFF showed anti-apoptotic effects during MTX treatment and mediated chemotaxis of malignant B cells. CONCLUSION: This study extends the spectrum of valuable diagnostic biomarkers in CNSL. In patients with focal brain lesions, measurement of APRIL in CSF could help accelerating the diagnosis of CNSL. Moreover, our results highlight an important role of APRIL and BAFF in the pathophysiology of CNSL.

Wave atoms for digital hologram compression.

Compression of macroscopic digital holograms is a major research problem, which if unresolved will continue to limit the possible applications of holography in multimedia contexts. The quest of searching for the most suitable representation for compression is still an open problem. In this work, we study sparsification by the wave atom transform, introduced in 2006 by Demanet et al., and experiment on four large-scale representative diffuse macroscopic holograms while testing compressibility in the object plane, Fourier plane, and defocused plane representations, respectively. We demonstrate that it is a suitable nonadaptive, sparsifying transform for Fourier or defocused content, and by integration into the wave atom coding (WAC) method, we sketch a full-fledged codec for the compression of macroscopic holograms. WAC is compared to two variants of JPEG 2000, with equal complexity of coding tools, and the more recent High Efficiency Video Coding (H.265/HEVC). For Fourier and defocused holograms, WAC outperforms the JPEG 2000 variants by 0.9-7.9 dB Bjøntegaard-Delta peak signal to noise ratio, especially in the former case, while it is as good as or better than even H.265/HEVC for very deep computer-generated holograms, thus improving on existing approaches.

Randomized phase III study of whole-brain radiotherapy for primary CNS lymphoma.

OBJECTIVE: This is the final report of a phase III randomized study to evaluate whole-brain radiotherapy (WBRT) in primary therapy of primary CNS lymphoma (PCNSL) after a median follow-up of 81.2 months. METHODS: Patients with newly diagnosed PCNSL were randomized to high-dose methotrexate (HDMTX)-based chemotherapy alone or followed by WBRT. We hypothesized that the omission of WBRT would not compromise overall survival (OS; primary endpoint), using a noninferiority design with a margin of 0.9. RESULTS: In the per-protocol population (n = 320), WBRT nonsignificantly prolonged progression-free survival (PFS) (median 18.2 vs 11.9 months, hazard ratio [HR] 0.83 [95% confidence interval (CI) 0.65-1.06], p = 0.14) and significantly PFS from last HDMTX (25.5 vs 12.0 months, HR 0.65 [95% CI 0.5-0.83], p = 0.001), but without OS prolongation (35.6 vs 37.1 months, HR 1.03 [95% CI 0.79-1.35], p = 0.82). In the intent-to-treat population (n = 410), there was a prolongation by WBRT of both PFS (15.4 vs 9.9 months, HR 0.79 [95% CI 0.64-0.98], p = 0.034) and PFS from last HDMTX (19.4 vs 11.9 months, HR 0.72 [95% CI 0.58-0.89], p = 0.003), but not of OS (32.4 vs 36.1 months, HR 0.98 [95% CI 0.79-1.26], p = 0.98). CONCLUSION: Although the statistical proof of noninferiority regarding OS was not given, our results suggest no worsening of OS without WBRT in primary therapy of PCNSL. CLASSIFICATION OF EVIDENCE: This study provides Class II evidence that in PCNSL HDMTX-based chemotherapy followed by WBRT does not significantly increase survival compared to chemotherapy alone. The study lacked the precision to exclude an important survival benefit or harm from WBRT.

Prognostic impact of intraocular involvement in primary CNS lymphoma: experience from the G-PCNSL-SG1 trial.

The impact of intraocular involvement (IOL) in primary CNS lymphoma (PCNSL) has not been sufficiently evaluated. Here, we present the analysis of IOL in the only completed randomized phase III trial in PCNSL. The G-PCNSL-SG1 study evaluated the role of whole-brain radiotherapy in primary therapy of PCNSL. Data of the 526 eligible study patients were checked, and clinical characteristics, therapy, and outcome of patients with IOL diagnosed at study inclusion were analyzed. Ophthalmologic examination at study inclusion was performed in 297 patients (56.5 %) of whom IOL was diagnosed in 19 (6.4 %). Clinical characteristics did not significantly differ between patients with IOL (IOL+) and those without (IOL-). The median progression-free survival (PFS) in the IOL+ group was 3.5 months (95 % CI 0.0-7.07) as compared to 8.3 months (95 % CI 4.78-11.78) in the IOL- group (P = 0.004), the median overall survival (OS) was 13.2 months (95 % CI 0.86-25.62) and 20.5 months (95 % CI 15.56-25.5), respectively (P = 0.155). In multivariate analysis, a significantly inferior PFS and OS for IOL+ patients were found. IOL at diagnosis of PCNSL was an independent negative prognostic indicator for PFS and OS in this analysis.

Optical and magneto-optical properties of metal phthalocyanine and metal porphyrin thin films.

The optical constants together with the magneto-optical Voigt constants of several phthalocyanine (Pc) and methoxy functionalized tetraphenylporphyrin (TMPP) thin films prepared on silicon substrates are presented. The materials investigated are MePc with Me = Fe, Co, Ni, Cu, Zn and MeTMPP with Me = Cu, Ni. We also compared our results to the metal-free H2Pc, H2TPP and H2TMPP. The experimental results will be supported by electronic structure calculations based on density functional theory (DFT) and interpreted using the perimeter model initially proposed by Platt. The model allows for qualitative understanding of the forbidden character of transitions in planar, aromatic molecules, and is able to qualify differences between Pc and TMPP type materials.

Successful long-term control of lymphomatous meningitis with intraventricular rituximab.

Most primary central nervous system lymphomas (PCNSL) are highly malignant B-cell non-Hodgkin lymphomas. First-line therapy consists of high-dose methotrexate-based chemotherapy with or without whole brain radiotherapy. However, no standard of care is defined for refractory or recurrent PCNSL. We report a patient with lymphomatous meningitis, which was refractory to standard treatment but was successfully controlled for more than 6 years with intraventricular rituximab. Apart from two infections of the Ommaya reservoir, no severe side effects of intraventricular long-term treatment could be detected. This report indicates that intraventricular administration of rituximab might be beneficial in select patients with refractory or recurrent lymphomatous meningitis.

Expression of B-cell activating factor, a proliferating inducing ligand and its receptors in primary central nervous system lymphoma.

B-cell activating factor belonging to the tumor necrosis factor family (BAFF) and a proliferating inducing ligand (APRIL) might play an important role in the pathogenesis of systemic B-cell malignancies. However, the BAFF/APRIL system has not been systematically evaluated in primary central nervous system lymphoma (PCNSL) to date. We assessed the expression of BAFF, APRIL and its receptors BAFF-R (BAFF receptor), BCMA (B-cell maturation antigen) and TACI (transmembrane activator and calcium modulator cyclophilin ligand interactor) in five PCNSL specimens by immunohistochemical staining. We found extensive expression of BAFF and weak to moderate expression of APRIL, BAFF-R, BCMA, and TACI in all specimens. CD20 positive cells showed expression of both ligands and receptors at the same time. Our results indicate that autocrine stimulation of the BAFF/APRIL system might be involved in the pathogenesis of PCNSL.

Early relapses in patients with primary CNS lymphoma treated with methotrexate-based chemotherapy without consolidating whole brain irradiation.

Methotrexate (MTX)-based chemotherapy is used as upfront treatment for most patients with primary CNS lymphoma. Whether consolidating whole brain irradiation (WBI) should be recommended for patients who achieve complete remission (CR) is still a matter of debate. Patients who are predicted to experience an early relapse (ER, ≤12 months from diagnosis) might especially benefit from consolidating treatment. We therefore evaluated the incidence and prognostic impact of ER in patients with CR following chemotherapy without WBI. We identified 40 patients between 2000 and 2010 who had achieved CR following MTX-based chemotherapy. Of 36 evaluable patients 11 (31 %) experienced an ER. These patients had significantly impaired overall survival (46.0 vs. 79.0 months, p = 0.001). Normal cerebrospinal fluid cell count (11.0 vs. 76.0 months, p = 0.001) and frequent reduction of MTX dose due to impaired creatinine clearance (10.0 vs. 48.0 months, p = 0.005) had a significantly negative impact on relapse-free survival. Patients with CR following MTX-based induction chemotherapy represent a heterogeneous population. In these patients, ER is an independent risk factor for impaired overall survival. Therefore, these patients might especially benefit from consolidating treatment. These results have to be validated by prospective trials.

Rituximab significantly improves complete response rate in patients with primary CNS lymphoma.

Rituximab improves outcome for patients with systemic diffuse large B-cell lymphoma (DLBCL) and has therefore become a standard component of the treatment of this disease. However, it is unclear whether rituximab is efficacious in patients with primary CNS lymphoma (PCNSL), a rare DLBCL variant, also. The purpose of this study was to evaluate the effect of rituximab on the complete response (CR) rate after chemotherapy with methotrexate (MTX) and ifosfamide (IFO) of patients with PCNSL. This is a retrospective, observational, single-center trial analyzing 36 consecutive patients with newly diagnosed, CD-20-positive PCNSL who were treated primarily with chemotherapy between March 2007 and December 2010. We compared 19 patients who were treated with MTX and IFO with 17 patients who were treated with the same regimen plus rituximab. The addition of rituximab to MTX and IFO was correlated with a significant increase in the CR rate (100.0 vs. 68.4 %, p = 0.02). Furthermore, six-month progression-free survival was significantly higher for the rituximab group (94.1 vs. 63.2 %, p = 0.04). Toxicity did not differ significantly between the groups. Our results indicate that rituximab might be efficacious in the treatment of PCNSL and support addition of this drug to current treatment protocols until data from randomized controlled trials becomes available. Immunochemotherapy with MTX, IFO, and rituximab seems to have excellent activity as induction chemotherapy and should be further tested in prospective trials.