Mirror Artifact Research Articles

Mirror artefact as a diagnostic tool in identifying occult liver lesions on ultrasound

Introduction: Mirror artefacts often can be seen in abdominal ultrasound. Their efficacy in detecting sonographically occult lesions has been overlooked. Case report: We present two cases of abdominal ultrasound examination, where the mirror image artefact was utilised in the diagnosis of sonographically occult lesions in segment VII of the liver. Discussion: The physical principle behind the mirror image artefacts in these cases has been explored and explained. Suggestion on utilising this artefact is recommended. Conclusion: These cases demonstrate the key role that mirror artefacts played in detecting occult liver lesions. Sonographers are advised to extend their survey of the right liver and pay close attention to displayed mirror images above the diaphragm especially from potential sonographically occult lesions in segment VII of the liver.

Research on electrical capacitance tomography (ECT) detection of cerebral hemorrhage based on symmetrical cancellation method

Currently, there is an urgent need for a fast and portable intracerebral hemorrhage (ICH) detection technology for pre-hospital emergency scenarios. Owing to the disproportionately elevated permittivity of blood compared to other brain tissues, Electrical Capacitance Tomography (ECT) offers a viable modality for mapping the spatial distribution of permittivity within the brain, thus facilitating the imaging-based identification of ICH. Currently, ECT is confined to time-differential imaging due to limited sensitivity, and this methodology requires non-hemorrhagic measurements for comparison, data that are frequently inaccessible in clinical contexts. To overcome this limitation, in accordance with the natural bilateral symmetry of the cerebral hemispheres, a symmetrical cancellation scheme is introduced. In this method, electrodes are uniformly arrayed around the cranial periphery and strategically positioned in a symmetrical manner relative to the sagittal suture. Subsequently, the measured capacitances for each electrode pair are subtracted from those of their symmetrical counterparts aligned with the sagittal suture. As a result, this process isolates the capacitance attributable solely to hemorrhagic events within a given hemisphere, permitting the absolute imaging of ICH. To assess the feasibility of this method, simulation and empirical imaging were conducted respectively on a numerical hemorrhage model and three physical models (a water-wrapped hemorrhage model, an isolated porcine fat-wrapped hemorrhage model, and an isolated porcine brain tissue-wrapped hemorrhage model). Traditional absolute imaging, time-differential imaging and symmetrical cancellation imaging were performed on all models. The results substantiate that the proposed imaging modality is capable of obtaining absolute imaging of ICH. But a mirrored artifact, symmetrical to the site of the actual hemorrhage image appeared in each of the imaging results. This mirror artifact was characterized by identical dimensions and an inverted pixel-value schema, an intrinsic consequence of the symmetrical cancellation imaging algorithm. The real image of hemorrhage can be ascertained through pre-judgment with the symptoms of the patient. Additionally, the quality of this imaging is seriously dependent on the precise alignment between the electrodes and the sagittal suture of the brain; even a minor deviation in symmetry could introduce excessive noises. Thus, the complicated operational procedures remain as challenges for practical application.

Calibration procedure for enhanced mirror artifact removal in full-range optical coherence tomography using passive quadrature demultiplexing.

SignificancePassive quadrature demultiplexing allows full-range optical coherence tomography (FR-OCT). However, imperfections in the wavelength- and frequency-response of the demodulation circuits can cause residual mirror artifacts, which hinder high-quality imaging on both sides of zero delay.AimWe aim at achieving high mirror artifact extinction by calibrated postprocessing of the FR-OCT signal.ApproachWe propose a mathematical framework for the origin of the residual mirror peaks as well as a protocol allowing the precise measurement and correction of the associated errors directly from mirror measurements.ResultsWe demonstrate high extinction of the mirror artifact over the entire imaging range, as well as an assessment of the method's robustness to time and experimental conditions. We also provide a detailed description of the practical implementation of the method to ensure optimal reproducibility.ConclusionThe proposed method is simple to implement and produces high mirror artifact extinction. This may encourage the adoption of FR-OCT in clinical and industrial systems or loosen the performance requirements on the optical demodulation circuit, as the imperfections can be handled in postprocessing.

Investigation Study of Ultrasound Practitioners' Awareness about Artefacts of Hepatobiliary Imaging in Almadinah Almunawwarah.

Objectives:To investigate the knowledge and awareness of ultrasound practitioners’ concerning ultrasound artefacts in evaluating the hepatobiliary system.Methods:This electronic questionnaire-based comparative study involved the ultrasound practitioners’ who work in the radiology departments in Almadinah Almunawwarah governmental hospitals during the period from 1 November 2020 to 30 April 2021. Spearman’s rho correlation test was used to correlate between knowledge and job, academic qualification, and years of experience. A T-test and cross tabulation test were done to compare the knowledge about artefacts among radiologists and radiologic technologists.Results:This study involved 94 participants distributed as 22 (23.4%) radiologists and 72 (76.6%) radiologic technologists. The results shows that 85%, 71%, 73%, 69%, 54% and 53% of the participants assigned the acoustic shadowing, acoustic enhancement, ring down, side lobe, reverberation and mirror artefacts, as artefacts respectively. However, 68%, 53%, 19%, 19%, 18%, and 40% of the participants gave correct final diagnosis of acoustic shadowing, acoustic enhancement, ring down, side lobes, reverberation, and mirror artifacts, respectively. Spearman’s rho correlation test shows significant correlation between participants with more than three years experience and knowledge related mirror artefacts (r=0.328, p=0.001). It shows significant correlation between radiologists with knowledge related mirror artefacts (r=0.367, p<0.001). A significant correlation was found between highly qualified participants and knowledge related mirror artefacts (r=0.336, p=0.001) and side lobe artefacts (r=0.237, p=0.008).Conclusion:The questionnaire-based comparative study of knowledge about artefacts of hepatobiliary ultrasound imaging reveals a high level of Ultrasound practitioners’ knowledge in differentiating artefacts from pathology with a high level of knowledge in identifying hepatobiliary acoustic shadowing and acoustic enhancement artefacts. However, insufficient knowledge was noted in identifying mirror, side lobe, reverberation and ring down artefacts. A direct link was found between academic qualification, years of experience and practioners’ knowledge among.

Assessment of Scleral Contour Changes in High Myopia by Optical Coherence Tomography.

PurposeThis study aimed to establish and validate optical coherence tomography (OCT) based diagnostic criteria of high myopia.MethodsThis was a cross-sectional study including 100 eyes of high myopia with axial length larger than 26.5 mm and 100 control eyes, which were examined by spectral-domain OCT. Vertical and horizontal OCT of 9 mm scanning across fovea were analyzed. OCT characteristics including mirror artifact, scleral sink due to steep inclination of posterior sclera, scleral visibility, abnormality of outer retinal layer, foveoschisis, and dome-shaped macula were assessed in each group.ResultsThe mean axial length was 28.65 ± 2.07 mm (range, 26.51–34.59 mm) in high myopia group and 23.79 ± 0.99 mm (range, 21.26–25.94 mm) in control group. Among the OCT characteristics noted at high frequency in the eyes with high myopia, three criteria achieving high sensitivity and specificity were determined: scleral sink over 500 μm, scleral visibility over 100 μm, and dome-shaped macula. Under conditions of presence of any of three criteria in either horizontal or vertical OCT scanning, the diagnostic sensitivity, specificity, positive predictive value, and negative predictive value for high myopia were found to be 95.0%, 98.0%, 98.0%, and 95.1%, respectively. In a new set of OCT images from 50 eyes with high myopia and 50 eyes of nonhigh myopia cases, the OCT-based criteria also proved similar level of diagnostic validity.ConclusionsThe OCT-based criteria, which directly addressed posterior scleral contour changes, may lead to an intuitive and accurate diagnosis of high myopia. Also, the criteria may contribute to early detection and monitoring of eyes that cannot be defined as high myopia but can progress. OCT may be useful for monitoring high myopia patients as OCT can detect myopia-associated retinal pathologies as well as scleral contour changes.

Comparison between Widefield Optical Coherence Tomography Devices in Eyes with High Myopia.

Background: To compare four different optical coherence tomography (OCT) devices for visualization of retinal and subretinal layers in highly myopic eyes. Methods: In this prospective, observational, cross-sectional study, consecutive patients with high myopia and control subjects were imaged by four OCT devices: Spectralis OCT2, PlexElite 2.0 100 kHz, PlexElite 2.0 200 kHz and the Canon Xephilio OCT-S1. The acquisition protocol for comparison consisted of single vertical and horizontal line scans centered on the fovea. Comparison between the devices in the extent of visible retina, presence of conjugate image or mirror artifacts, visibility of the sclerochoroidal interface and retrobulbar tissue. Results: 30 eyes with high myopia and 30 control subjects were analyzed. The visualized RPE length was significantly different between the OCT devices with Xephilio OCT-S1 imaging the largest extent (p < 0.0001). The proportion of eyes with conjugate image artifact was significantly higher with the Spectralis OCT (p < 0.0001), and lower with the PlexElite 200 kHz (p < 0.0001). No difference in visibility of the sclerochoroidal interface was noted among instruments. The retrobulbar tissue was visible in a higher proportion of eyes using swept-source PlexElite 100 kHz and 200 kHz (p < 0.007) compared to the other devices. Conclusions: In highly myopic eyes, the four OCT devices demonstrated significant differences in the extent of the retina imaged, in the prevalence of conjugate image artifact, and in the visualization of the retrobulbar tissue.

Effect of image artefacts on phase conjugation with spectral domain optical coherence tomography.

Recently the acquisition of the time-resolved reflection matrix was demonstrated based on spectral domain optical coherence tomography. In principle, the matrix describes the linear dependence of the OCT signal received from different depths on the field which is incident to the scattering sample. Knowledge of the matrix, hence, enables beam shaping to selectively enhance the received signal, for example to increase the penetration depth when imaging turbid media. We investigate the impact of image artefacts on the approach. Phase conjugation is shown to enhance the OCT signal, but not autocorrelation and mirror artefacts. Imaging applications are demonstrated indicating the potential for future in-vivo studies on biotissues.

Reverse time migration imaging of tunnels via the finite element method using an unstructured mesh

Wavefield extrapolation is critical in reverse time migration (RTM). The finite difference method is primarily used to achieve wavefield extrapolation in case of the RTM imaging of tunnels. However, complex tunnel models, including those for karsts and fault fracture zones, are constructed using regular grids with straight curves, which can cause numerical dispersion and reduce the imaging accuracy. In this study, wavefield extrapolation was conducted for tunnel RTM using the finite element method, wherein an unstructured mesh was considered to be the body-fitted partition in a complex model. Further, a Poynting vector calculation equation suitable for the unstructured mesh considered in the finite element method was established to suppress the interference owing to low-frequency noise. The tunnel space was considered during wavefield extrapolation to suppress the mirror artifacts based on the flexibility of mesh generation. Finally, the influence of the survey layouts (one and two sidewalls) on the tunnel imaging results was investigated. The RTM results obtained for a simple tunnel model with an inclined interface demonstrate that the method based on unstructured meshes can effectively suppress the low-frequency noise and mirror artifacts, obtaining clear imaging results. Furthermore, the two-sidewall tunnel survey layout can be used to accurately obtain the real position of the inclined interface ahead of the tunnel face. The complex tunnel numerical modeling and actual data migration results denote the effectiveness of the finite element method in which an unstructured mesh is used.

Artifacts in Macular Optical Coherence Tomography.

Purpose:To identify and explain different artifacts in macular optical coherence tomography (OCT).Methods:For this comprehensive review, a PubMed and Google Scholar (January 1995–October 2018) search was conducted by the researchers, using the keywords such as OCT, artifacts, artefact, and macula.Results:We reviewed the main OCT artifacts including software break-down or misidentification of retinal layers, incomplete segmentation error, complete segmentation failure, mirror artifact (inverted artifact), cut edge artifact, degraded image scan, out-of-register artifact, off-center artifact, motion artifact, foveal duplication, segmentation shift, blink artifact, static or fixed image artifact, linear artifact, and perfluorocarbon liquid-producing artifact.Conclusions:There are various artifacts in OCT image scans. The identification of these artifacts may help in accurate interpretations of OCT images in clinical settings that can affect the diagnosis and management of different retinal disorders.

Artificial intelligence deep learning algorithm for discriminating ungradable optical coherence tomography three-dimensional volumetric optic disc scans.

.Spectral-domain optical coherence tomography (SDOCT) is a noncontact and noninvasive imaging technology offering three-dimensional (3-D), objective, and quantitative assessment of optic nerve head (ONH) in human eyes in vivo. The image quality of SDOCT scans is crucial for an accurate and reliable interpretation of ONH structure and for further detection of diseases. Traditionally, signal strength (SS) is used as an index to include or exclude SDOCT scans for further analysis. However, it is insufficient to assess other image quality issues such as off-centration, out of registration, missing data, motion artifacts, mirror artifacts, or blurriness, which require specialized knowledge in SDOCT for such assessment. We proposed a deep learning system (DLS) as an automated tool for filtering out ungradable SDOCT volumes. In total, 5599 SDOCT ONH volumes were collected for training (80%) and primary validation (20%). Other 711 and 298 volumes from two independent datasets, respectively, were used for external validation. An SDOCT volume was labeled as ungradable when SS was or when any artifacts influenced the measurement circle or of the peripheral area. Artifacts included (1) off-centration, (2) out of registration, (3) missing signal, (4) motion artifacts, (5) mirror artifacts, and (6) blurriness. An SDOCT volume was labeled as gradable when SS was , and there was an absence of any artifacts or artifacts only influenced peripheral area but not the retinal nerve fiber layer calculation circle. We developed and validated a 3-D DLS based on squeeze-and-excitation ResNeXt blocks and experimented with different training strategies. The area under the receiver operating characteristic curve (AUC), sensitivity, specificity, and accuracy were calculated to evaluate the performance. Heatmaps were generated by gradient-weighted class activation map. Our findings show that the presented DLS achieved a good performance in both primary and external validations, which could potentially increase the efficiency and accuracy of SDOCT volumetric scans quality control by filtering out ungradable ones automatically.

Finite element reverse time migration imaging in tunnels using an unstructured mesh

Wave field extrapolation is critical in reverse time migration (RTM). At present, wavefield extrapolation in RTM imaging for tunnels is mostly carried out via the finite difference method. However, complex tunnel models, such as those for karst and fault fracture zones, are constructed using regular grids with straight curves which can easily cause numerical dispersion and reduce imaging accuracy. In this study, the wavefield extrapolation for tunnel RTM was conducted using the finite element method, where an unstructured mesh is employed as the body-fitted partition in a complex model. The Poynting vector calculation equation suitable for the unstructured mesh finite element method was established to suppress low-frequency noise interference. The tunnel space was considered during the wavefield extrapolation to suppress mirror artefacts by using the flexibility of mesh generation. Finally, the influence of the survey layouts (one-sidewall and two-sidewall) on the tunnel imaging results was explored. The RTM results for a simple tunnel model with an inclined interface show that the method based on unstructured meshes can effectively suppress low-frequency noise and mirror artefacts, thus obtaining clearer imaging results. Also, the two-sidewall tunnel survey layout more accurately obtains the real position of the inclined interface ahead of the tunnel face. Complex tunnel numerical modelling and actual data migration results further illustrate the effectiveness of the finite element unstructured mesh method.

Artefacts in musculoskeletal ultrasound

Artefacts in musculoskeletal (MSK) ultrasound (US) comprise artefacts that mimic an abnormality while scanning normal structures and artefacts that occur together with abnormal conditions at grey-scale and Doppler imaging. Some of these can be avoided by correct scanning technique while other artefacts are the result of inherent characteristics of different tissues which may help in making a correct diagnosis. The origin and examples of common grey-scale and power/colour Doppler artefacts in ultrasound are explained with particular reference to MSK ultrasound. The commonly encountered Doppler artefacts such as the flash/twinkle artefacts and pseudo flow artefacts and their importance for image interpretation are discussed. Anisotropy is the sonographic artefact that is direction dependent and can be mistaken for a partial tear while scanning a tendon or nerve. Aliasing and random noise are well known artefacts, which depend upon grey-scale and overall gain. Mirror image artefacts refer to any highly reflecting smooth surface which may act as an acoustic mirror. In rheumatology, the mirrors will nearly always be bone surfaces. Blooming artefact displays colour outside of a vessel and makes vessels appear larger. Artefacts can also be due to ultrasound beam characteristics (side-lobe artefact), multiple echoes (reverberation, comet-tail, and mirror artefacts), and attenuation errors and enhancement, among others. Not all artefacts are confusing or unwanted. Certain artefacts are beneficial in assisting with the diagnosis. A better understanding of proper imaging techniques will allow correction or minimization of many of these artefacts.

Two-harmonic complex spectral-domain optical coherence tomography using achromatic sinusoidal phase modulation

We resolve the complex conjugate ambiguity in spectral-domain optical coherence tomography (SD-OCT) by using achromatic two-harmonic method. Unlike previous researches, the optical phase of the fiber interferometer is modulated by an achromatic phase shifter based on an optical delay line. The achromatic phase modulation leads to a wavelength-independent scaling coefficient for the two harmonics. Dividing the mean absolute value of the first harmonic by that of the second harmonic in a B-scan interferogram directly gives the scaling coefficient. It greatly simplifies the determination of the magnitude ratio between the two harmonics without the need of third harmonic and cumbersome iterative calculations. The inverse fast Fourier transform of the complex-valued interferogram constructed with the scaling coefficient, first and second harmonics yields a full-range OCT image. Experimental results confirm the effectiveness of the proposed achromatic two-harmonic technique for suppressing the mirror artifacts in SD-OCT images.

The mirror artifact effect on OCTA reconstructions of patients with high myopia

Optical coherence tomography angiography (OCTA) is a useful technique for the noninvasive study of retinal vascularization; nevertheless artifacts may occur, especially in myopic patients, and this may lead to diagnostic issues. Four myopic patients (2 males, mean age 45 years, axial length 27–30 mm) were analyzed using swept source OCT Triton Topcon (DRI Triton; Topcon, Tokyo, Japan) and 3 × 3, 4.5 × 4.5, and 9 × 9 mm scans were obtained. Topcon Image6 software was used to segment retinal layers and to extract both OCTA reconstructions of superficial, deep and choriocapillary plexa, and en face images. OCT analysis clearly revealed retinochoroidal structures, including both inner and outer choroidal segments, the external limiting membrane, and the retinal pigment epithelium in all patients. OCTA analysis was able to reconstruct superficial, deep and choriocapillary plexa in all patients. On 9 × 9 OCTA and en face reconstructions of every patient, a circular, linear, hyporeflective area was found circumscribing the retinal vascular arcades, as a frame; no vessels were detected in this area. In accordance with previous studies, vessel rarefaction was reported. The dark circular area found in 9 × 9 scans had characteristic aspects, as the presence in both eyes, specularity, absence of vascularization, and lack of ophthalmoscopic evidence. In the B‑scan images, a mirror reflection of retinal profile, appearing upside down, was observed. This aspect can be due to the reflection of retinal reconstruction caused by the high curvature of the eye and to the staphyloma. Differential diagnosis might be choroidal rupture, even though the specularity and the presence of the dark area in all the layers clear the picture. This study confirmed that OCTA in high myopia can be affected by a number of pitfalls and this should be taken into account for the right interpretation of OCTA reconstructions.

A new 3D observation system designed for a seismic ahead prospecting method in tunneling

The tunnel seismic ahead prospecting method is usually used to estimate adverse geology ahead of a tunnel face. The observation system plays an important role in tunnel seismic ahead prospecting, which helps to obtain accurate imaging results. Optimizing the observation system is a key issue for seismic ahead prospecting. In this paper, observation layouts are evaluated and a new three-dimensional (3D) observation system is proposed by imaging results analysis. For linear and 3D layouts, imaging result analysis in this paper indicates that the mirror artifact caused by the linear layout (Which sets sources and geophones on a straight line) can be suppressed by the 3D layout, due to its energy concentration and convergence. Moreover, reflections using 3D layout identify real abnormal bodies better than the linear layout. Moreover, conventional observation layouts often use shot point on the tunnel face or front sidewall, which makes it difficult to filter interference waves from the back of the tunnel face. To solve this problem, typical numerical examples are conducted, and imaging results of observation layouts with shot point on front sidewall, tunnel face or back sidewall are studied. Results show shot point on the back sidewall helps to separate waves, suppress interference waves and extract effective waves (reflected P-waves by discontinuities) by apparent velocity filtering method, so shot point is designed on the back sidewall to get high-quality records. To balance record quality and application convenience, a new 3D observation system is proposed with 12 geophones and 10 shot points in a special arrangement. Compared with a linear layout, practical testing and application using the new 3D system indicate the main fractured zones are accurately identified and located, which proves the reliability and practicability of the new observation system.

Absolute velocity measurement using three-beam spectral-domain Doppler optical coherence tomography

We report the development of a three-beam spectral-domain Doppler optical coherence tomography setup that allows single interferometer-based measurement of absolute flow velocity. The setup makes use of galvo-based phase shifting to remove complex conjugate mirror artifact and a beam displacer in the sample arm to avoid cross talk image. The results show that the developed approach allows efficient utilization of the imaging range of the spectral-domain optical coherence tomography setup for three-beam-based velocity measurement.

TCT-657 Calibration of Intravascular Optical Coherence Tomography As Presented In Peer Reviewed Publications

Intravascular optical coherence tomography (OCT) is a new imaging modality in interventional cardiology providing highresolution intracoronary images. The modality is already established as an efficient and precise research tool and is increasingly used for guiding coronary intervention. The quantitative capabilities of marketed OCT systems have been validated and measurements are found accurate and precise in bench testing [1]. When using OCT it is important to carefully calibrate the system for accuratemeasurements [2,3]. Present standard in marketed systems is automated calibration. Still the calibration can be incorrect due to failed automated calibration, wrong manual adjustment, or changes to the length of the optic fiber due to altered bending of the imaging wire. Correct calibration and the identification of incorrect calibration can be challenging due to mirror artifacts, blood in the imaging wire and proximity to the vessel wall. Looking for guidance on calibration of intravascular OCT in published peer reviewed papers led to the identification of some inconsistencies similar to our own experience. Still we do not have a database of wrong calibrations owing to immediate corrections. Therefore we aimed at identifying patterns of failed calibration as presented in images in peerreviewed papers. Using the PubMed internet database, we built a search containing following MESH-terms: “Tomography, Optical Coherence” AND (“Percutaneous Coronary Intervention”OR “Coronary Occlusion”OR “Coronary Stenosis” OR “Coronary Restenosis” OR “Coronary Vessels” OR “Coronary Disease”OR “Coronary Circulation”OR “Coronary Thrombosis” OR “Coronary Artery Disease” OR “Myocardial Reperfusion”). With this search more than 300 articles published since 1 January 2011 were identified. Of these, 222 were included in the analysis giving a total of 1164 OCT images with the information needed for evaluating the calibration. Articles which were excluded from the analysis are due to lack of images (N = 57), lack of fiduciaries (N = 38) or no full-text subscription by the local university library (N = 27). Calibration errors were divided into 8 groups, ranging from slightly incorrect through errors with potentially clinical relevant impact on quantitative measurements. Errors in classes that may likely affect stent sizing if applied in clinical practice were termed serious calibration errors (Classes 3, 4 and 6). Frames which we were unable to classify were not counted and if a frame appearedmore than once or in more articles (e.g. editorials), it was only counted once. Our findings (shown in Table 1) were 1) 640 images produced by time domain OCT systems. Of these 15% were in-assessable. Worse than slightly incorrect calibration, was found in 35% of assessable cases. Serious calibration incorrectness was seen in 16% of images. 2) A total of 524 images had been obtained bymarketed frequency domain systems (482 by C7 system and 42 images by Illumien—both St. Jude Medical, USA). In images acquired by the C7 system serious calibration incorrectness was detected in 16% of cases. In images acquired by the Illumien system (42) only a single acquisition with 3 images was severely incorrect calibrated due to inverted calibration. Inverted calibration may cause the depicted catheter to resemble a correct calibrated OCT catheter. This is an occasional finding termed “inverted calibration” (Class 4 error). A related finding due to inverted calibration is the distortion of the image and appearance of vessel wall or wires inside the apparent imaging wire (Fig. 1 error Class 4). We propose the term “inverted calibration artifacts” for such findings. The “slightly incorrect”-group was expanded to contain also the Class 1 categorized images, as the error in these groups only result in minimal error of the measurements. For the C7 system 43% of images in articles presenting quantitative measurements showed any degree of incorrect calibration. For the M2 system, this was 40%. We are aware, that images shown in articles are chosen for illustrative purpose. This is a process different from the systematic core lab evaluation, andwe assume that all results derivedweremade from correctly calibrated images. Still, systematic errors inmany or all images in a paper might induce a slight uncertainty about derived quantitative results. Incorrect calibration in images presented for tissue characterization or similar could be seen as unimportant. Still, when presented in high impact journals, in expert reviews or consensus reports some users might potentially adopt this as the gold standard of calibration. In conclusion, a high incidence of incorrect calibrated images was found in systematic review of peer reviewed publications. Improved calibration facilities by newer generation OCT systems seems to reduce errors though not alleviating the risk associated with not identifying inverted calibration.

Complex conjugate artifact-free adaptive optics optical coherence tomography of in vivo human optic nerve head

We acquired in vivo images of the human optic nerve head (ONH) using an adaptive optics-optical coherence tomography (AO-OCT) system. In order to improve imaging of the lamina cribrosa in the ONH with high lateral resolution and sensitivity, we implemented a complex conjugate artifact-free Fourier domain OCT (Fd-OCT) acquisition scheme with a reference arm-based phase shifting method. This allowed positioning of the lamina cribrosa structures near the zero path length difference where AO-OCT imaging achieves highest sensitivity. Implementation of our complex conjugate artifact removal (CCR) method required constant phase shifts between consecutive axial scans (A-scans), generated by continuous beam path-length changes from offsetting the pivot point of the scanning mirror placed in the reference arm. Fourier transform along the transverse axis and a filtering algorithm allowed reconstruction of CCR AO-OCT images. The suppression ratio of the mirror artifact was approximately 22 dB (at 18,000 A-scans per second acquisition speed) with a paperboard test target and an optimum phase-shift value. Finally, we reconstructed the three-dimensional structure of human ONH with enhanced depth range and sensitivity using CCR AO-OCT.

An advanced algorithm for dispersion encoded full range frequency domain optical coherence tomography

Dispersion encoded full range (DEFR) optical coherence tomography (OCT) has become highly attractive as it is a simple way to increase the measurement range of OCT systems. Full range OCT is especially favorable as it does not only increase the measurement range but also shifts the highest sensitivity into the center of the measurement range. While the early versions of DEFR were highly computational expensive, new versions reduce the number of necessary Fourier transforms. Recently it has been shown that a GPU based algorithm can perform DEFR with more than 20,000 A-lines per second. We present a new version of the DEFR algorithm that requires only one Fourier transform per A-scan and uses convolution in z-space instead of multiplication in k-space, therefore reducing the computational effort considerably. While dispersion encoding has so far only been used to suppress mirror artifacts, we show that, with dispersion encoding and only one more Fourier transform, autocorrelation terms can be removed likewise. Since very high values of dispersion reduce the effective measurement range in dispersion encoded OCT, we present an estimate for a sufficient amount of dispersion for a successful image recovery, which is depending on the thickness of the scattering layers. Furthermore, we demonstrate the usability of ZnSe as a new dispersive material with a very high dispersion and describe a simple method to extract the dispersive phase from the measurement of a single reflex of a glass surface. Using a standard consumer PC, an artifact-free recovery of 1000 - 2000 A-scans per second with 2048 depth values including autocorrelation removal was achieved. The dynamic range (sensitivity) is not reduced and the suppression ratio of mirror artifacts and autocorrelation signals is more than 50dB using ZnSe.

Spectral-Domain-optische-Kohärenztomographie in der Behandlung der myopen choroidalen Neovaskularisationsmembran

Spectral domain optical coherence tomography (SD-OCT) investigations provide additional information about the morphological characteristics of myopic choroidal neovascularization (mCNV). Reproducible measurements of intraretinal and subretinal fluid are of growing importance for an evaluation of progression. The non-invasive technique reduces the need for frequent fluorescence angiography after individual assessment. Appropriate correction of the reference arm is mandatory. Automatic adjustment of transversal measured values due to alterations in the paraxial field (depending on the axial length) has been implemented in a new device. Mirror artefacts and excess curvature can be avoided by reducing the length of the OCT cross-section (15°). New possibilities to record the choriocapillaris and choroid have expanded the knowledge of potential pathomechanisms and risk factors.