A. C. Tan

University College London, Londinium, England, United Kingdom

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Publications (19)4.99 Total impact

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    ABSTRACT: This paper investigates the potential use of a prototype micro-optic twin Liquid Crystal Display (LCD) monitor for stereomicroscopy in microsurgery. The new device displays stereoscopic images via a pair of colour Charge-Coupled Device (CCD) video cameras attached on to a surgical stereo microscope. The paper illustrates the 3D microscope-display system set-up and calibration for stereo viewing. A series of experimental techniques was devised to measure the user-response of the new display system in depth perception of a solid object against the traditional eyepieces of a stereo microscope. As a control, free viewing with the un-aided eyes was also measured. The positional data were collected using a passive mechanical arm. The results showed good correspondence between all three visualisation methods. Error analysis of our numerical findings suggests that the depth accuracy of the new device is well within the precision limits of hand movement for surgical operations. Our study also proves that there are small discrepancies within the sample population of operators using the system. A study based on the psychological and psychophysical factors influencing the system is planned.
    09/2006: pages 998-1009;
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    ABSTRACT: A low cost, high precision tracking device, developed for the Institute of Laryngology & Otology, University College London for use in augmented reality assisted skull base surgery, is presented. The device achieves sub-millimetric resolution that enables its use with microscope magnified three dimensional imaging. The algorithms used for achieving high resolution and the hardware structure are discussed by the authors. Special emphasis is placed on real time digital signal processing using a DSP processor and the hardware designed to achieve this.
    04/2006: pages 511-517;
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    ABSTRACT: We present experimental data on the use of autostereoscopic displays as complementary visualization aids to the surgical stereo microscope for augmented reality surgery. Five experts in the use of the microscope, and five non-experts, performed a depth experiment to assess stereo cues as provided by two autostereoscopic displays (DTI 2015XLS Virtual Window and SHARP micro-optic twin), the surgical microscope and the "naked" eye.
    Mixed and Augmented Reality, 2003. Proceedings. The Second IEEE and ACM International Symposium on; 11/2003
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    ABSTRACT: In the context of our research on Augmented Reality based surgical navigation for stereoscopic microscope based ENT surgery, we aim to use autostereoscopic displays as a complementary visual aid. We performed an experiment to evaluate the depth perception capabilities from four stereoscopic visual aids: the surgical microsope, the SHARP twin LCD autostereoscopic display, the DTI Virtual Window 2015XLS flat panel autostereoscopic display and the naked eye. Five expert and five non-expert subjects performed an unbiased depth test to assess the autostereoscopic displays using the naked eye and microscope as the gold standard. The SHARP display was considered to allow sufficient lateral and longitudinal freedom whilst providing accurate stereo vision. The DTI display, though much lighter and easy to manipulate than the SHARP, did show promising results, eventhough not all subjects were at ease with the overall display quality and corresponding stereoscopic quality.
    Medical Image Understanding and Analysis (MIUA), Sheffield; 07/2003
  • Rudy Jacques Lapeer, An Chek Tan
    International Symposium on Computer Aided Surgery around the Head (CAS-H), Interlaken, Switzerland; 02/2003
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    ABSTRACT: Active contours have been used extensively for medical image registration as they display good smoothing properties which allow to account for the discrete nature of the image and noise from the acquisition process. However, contours are generally difficult to initialise around the region of interest (ROI). The marker based watershed segmentation can segment unique boundaries from an image or stack of images, however it has no smoothing/generalisation properties. Combining the two approaches results in a segmentation method which both solves the contour initialisation and generalisation problem. In this paper we briefly explain the marker based watershed and active contour segmentation and the algorithm to convert watershed contours into active contours. We then illustrate the approach on the segmentation of abdominal organs from MR images and bony structures from CT images.
    Medical Image Understanding and Analysis (MIUA), Portsmouth; 07/2002
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    ABSTRACT: The three-dimensional segmentation of regions of interest in medical images, be it a 2D slice by slice based approach or directly across the 3D dataset, has numerous applications for the medical professional. These applications may involve something as simple as visualisation up to more critical tasks such as volume estimation, tissue quantification and classification, the detection of abnormalities and more. In this paper we describe a method which aims to combine two of the more popular segmentation techniques: the watershed segmentation and the active contour segmentation. Watershed segmentation provides unique boundaries for a particular image or series of images but does not easily allow for the discrete nature of the image and the image noise. Active contours or snakes do possess this generalisation or smoothing property but are difficult to initialise and usually require to be close to the boundary of interest to converge. We present a hybrid approach by segmenting a region of interest (ROI) using a 3D marker-based watershed algorithm. The resulting ROI’s boundaries are then converted into a contour, using a contour following algorithm which is explained during the course of the paper. Once the contours are determined, different parameter settings of internal/external forces allow the expert user to adapt the initial segmentation. The approach thus yields a fast initial segmentation from the watershed algorithm and allows fine-tuning using active contours. Results of the technique are illustrated on 3D colon, kidney and liver segmentations from MRI datasets.
    Medical Image Computing and Computer-Assisted Intervention - MICCAI 2002, 5th International Conference, Tokyo, Japan, September 25-28, 2002, Proceedings, Part I; 01/2002
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    ABSTRACT: The CAESAR (Computer Assisted ENT Surgery using Augmented Reality) project aims to augment the visual information, as perceived by the surgeon during surgical intervention, with anatomical information, reconstructed from medical images of the patient. This allows the surgeon to see structures in the augmented operating scene which were previously not visible thus decreasing the risk of complications, reducing the length of the intervention and allow interventions which were previously not possible. In this paper we first give an overview of earlier and related work and then present a framework for the augmentation of images of a surgical scene as obtained from a binocular stereo operating microscope. The framework is hierarchical and it allows us to adapt and improve individual steps, i.e. calibration, registration and tracking, in the process without affecting other parts of the system. A laboratory experiment is described and preliminary results on augmentation accuracy (overlay error) are reported.
    International Conference on Interactive and Collaborative Computing (ICC), Wollongong, Australia; 12/2000
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    ABSTRACT: The ‘Computer Assisted ENT Surgery using Augmented Reality’ (CAESAR) project aims to improve ENT surgical procedures through augmentation of the real operative scene during surgery: a virtual scene, which shows structures that are normally hidden to the eye of the surgeon, is superimposed onto the real scene. The main distinction of this project as opposed to previous work in the field is to create a hierarchical and stepwise implemented system which allows operations such as calibration, tracking and registration to be assessed on an individual basis. This allows us to compare different alternatives for each operation and eventually apply the best solution without interfering with the performance of other parts of the system. In this paper, we present a framework for the alignment of the objects/subject in the real and virtual operating environment before the onset of surgery, and test its performance on a phantom skull. The operations involved are thus based on a static system and include calibration of the stereo microscope and registration of the virtual patient (as reconstructed from CT data) with the real patient. The final alignment of all objects in the real and virtual operating scene is assessed by cumulating maximum errors of each individual step.
    Medical Image Computing and Computer-Assisted Intervention - MICCAI 2000, Third International Conference, Pittsburgh, Pennsylvania, USA, October 11-14, 2000, Proceedings; 01/2000
  • Medical Image Computing and Computer-Assisted Intervention - MICCAI'99, Second International Conference, Cambridge, UK, September 19-22, 1999, Proceedings; 01/1999
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    ABSTRACT: The virtual medical laboratory (VML) was conceived to provide an Internet-accessible resource, offering access for clinicians and scientists to an invaluable data archive at the institute of Laryngology and Otology, London. The Institute is home to the largest collection of temporal bone, laryngeal, skull and sinus sections in Europe. The skull and sinus collections include an extensive section consisting of animal material. These were contributions from zoos around the world. Over the last 50 years, samples have been carefully sectioned and stained by specialized technicians to produce histology slices of most regions of the head and neck. The aim of the project is to create a virtual medical laboratory, which will provide access to archived histological material as well as computerized tomography and magnetic resonance data. Central to this aim is the reconstruction of the internal anatomy of the temporal bone from two-dimensional histology slices, to create three-dimensional views that can be used for anatomical simulation and surgical training in otolaryngology. State-of-the-art three-dimensional reconstruction and rendering technology allows us to develop such a model. Computer-generated simulation could be made available to all hospitals in which otolaryngology is practised, via digital communication networks. We aim to develop core technology in our own specialty that is applicable to other fields of higher education, which have not been exposed to such modern teaching modalities. This has the potential to become an invaluable teaching resource for anatomists, surgeons and other scientists.
    Journal of Telemedicine and Telecare 02/1997; 3 Suppl 1:79-81. · 1.47 Impact Factor
  • CVRMed-MRCAS'97, First Joint Conference Computer Vision, Virtual Reality and Robotics in Medicine and Medial Robotics and Computer-Assisted Surgery, Grenoble, France, March 19-22, 1997, Proceedings; 01/1997
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    ABSTRACT: Knowledge of the segmental anatomy and intersegmental biliary connections is an essential prerequisite to the effective management of patients with complex biliary strictures. Three dimensional (3D) imaging has the ability to demonstrate complex anatomical relationships that are difficult to appreciate on simple non-invasive two dimensional (2D) imaging. Our aim was to develop a technique for accurate, non-invasive 3D computed tomography (CT) cholangiography. Contiguous 4 mm CT sections were obtained through the liver during a dynamic bolus of 200 ml IV contrast. 3D surface reconstructions were then performed, the biliary system was isolated from surrounding hepatic parenchyma using segmentation and contrast threshold algorithms. 14 patients (six females, eight males, median age 68 years (range 48-82)) were studied. 13/14 had malignant biliary obstruction and one had obstruction secondary to a pancreatic pseudocyst. Obstruction was at the liver hilum in eight, the common bile duct in five and the common hepatic duct in one. Four patients had biliary endoprostheses but were symptomatic from inadequate drainage. There was good demonstration of the biliary anatomy, obstructed segments and intersegmental biliary connections in 13/14; irregular biliary dilatation secondary to primary sclerosing cholangitis rendered interpretation difficult in one. 3D cholangiography provided a useful adjunct to other imaging techniques. In particular, in patients with complex hilar strictures it aided implementation of appropriate interventional drainage procedures.
    British Journal of Radiology 06/1994; 67(797):445-8. · 1.22 Impact Factor
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    ABSTRACT: A system is described which produces displays of anatomical surfaces from sets of X-ray computed tomography (CT), magnetic resonance imaging (MRI) and ultrasonographic scans (US). The displays are created to show the three-dimensional character of the internal and external anatomy. The images may be manipulated on the screen to simulate dissection of the three-dimensional object they represent. Thus it is possible to increase the diagnostic value of the original data and to plan surgery by simulation. To allow long-term studies on the human face for surgical planning and assessment a non-hazardous optical scanning system has been devised. The display system also produces data to drive a numerically controlled milling machine for the production of models, prostheses and implants.
    Journal of Audiovisual Media in Medicine 02/1993; 16(1):4-10.
  • Clinical Radiology - CLIN RADIOL. 01/1992; 45(1):52-52.
  • A. C. Tan, R. Richards
    Proceedings of the world transputer user group (WOTUG) conference on Transputing '91; 05/1991
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    ABSTRACT: Facial reconstruction has until now been carried out by the sculpting technique. This method involves building a face with clay or other suitable material on to a skull or its cast, taking into account appropriate facial thickness measurements together with information provided by anthropologists such as approximate age, sex, race and other individual idiosyncrasies. A method for facial reconstruction is presented using 3-D computer graphics and is compared with the manual technique. The computer method involves initially digitising a skull using a laser scanner and video camera interfaced to a computer. A face, from a data bank which has previously digitised facial surfaces, is then placed over the skull in the form of a mask and the skin thickness is altered to conform with the underlying skull. The advantage of the computer method is its speed and flexibility. We have shown that the computer method for reconstructing a face is feasible and furthermore has the advantage over the manual technique of speed and flexibility. Nevertheless, the technique is far from perfect. Further facial thickness data needs collecting and the method requires evaluation using both known control skulls and later unknown remains.
    Forensic Science International 08/1989; 42(1-2):69-84. · 2.31 Impact Factor
  • A C Tan, R Richards
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    ABSTRACT: Three-dimensional (3D) medical graphics is becoming popular in clinical use on tomographic scanners. Research work in 3D reconstructive display of computerized tomography (CT) and magnetic resonance imaging (MRI) scans on conventional computers has produced many so-called pseudo-3D images. The quality of these images depends on the rendering algorithm, the coarseness of the digitized object, the number of grey levels and the image screen resolution. CT and MRI data are fundamentally voxel based and they produce images that are coarse because of the resolution of the data acquisition system. 3D images produced by the Z-buffer depth shading technique suffer loss of detail when complex objects with fine textural detail need to be displayed. Attempts have been made to improve the display of voxel objects, and existing techniques have shown the improvement possible using these post-processing algorithms. The improved rendering technique works on the Z-buffer image to generate a shaded image using a single light source in any direction. The effectiveness of the technique in generating a shaded image has been shown to be a useful means of presenting 3D information for clinical use.
    Medical informatics = Médecine et informatique 01/1989; 14(2):149-56.
  • Proceedings of the 8th technical meeting of the OUG, Sheffield; 01/1988