Alan C. F. Colchester

Development and preliminary evaluation of VISLAN, a surgical planning and guidance system using intra-operative video imaging

VISLAN is an integrated neurosurgical planning and guidance system. New segmentation and rendering techniques have been incorporated. A stereo video system is used intra-operatively and fulfils four roles. First, the video display is overlaid with graphical outlines showing the position of the planned craniotomy or the target (enhanced reality displays). Second, a skin surface patch is reconstructed from the stereo video images using patterned light (mean errors of surface point location are < 0.15 mm). Third, a freely mobile, hand-held localizer is tracked in real time (position errors are < 0.5 mm and with improved calibration < 0.2 mm), with its position superimposed on the pre-operative patient representation to assist surgical guidance. Fourth, markers fixed to the skull bone next to the cranial opening are used to detect intra-operative movement and to update registration. Initial results from phantom experiments show an overall system accuracy of better than 0.9 mm for intra-operative localization of features defined in pre-operative images. The prototype system has been tested during six neurosurgical operations with very good results.

Scale and segmentation of grey-level images using maximum gradient paths

We present a technique for the construction of multi-scale representations of grey-level images. Unlike conventional representations the scales are discrete as opposed to continuous and their level is solely determined by the data. The technique is based upon connecting singular points in the image with maximum gradient paths. We also describe two segmentation methods which use the maximum gradient paths generated during the construction of the multi-scale representation. In both segmentation techniques the paths are used to determine significant ridges and troughs. The first technique operates directly on the image, while the second technique uses the magnitude of the image derivative.

Superficial and deep structure in linear diffusion scale space: isophotes, critical points and separatrices

The concepts of structural stability and linear diffusion scale space are reviewed. The possible arrangements and interrelationships of isophotes, critical points and separatrices in single structurally stable images are detailed. The behaviour of these structures with changing resolution in a linear diffusion scale space is examined. This behaviour includes not only periods of smooth change, but also four catastrophic changes: shoe surface, balanced saddle, double-saddle isophote and heteroclinic separatrix.

Computer-aided interpretation of SPECT images of the brain using an MRI-derived 3D neuro-anatomical atlas

Nuclear medicine images have comparatively poor spatial resolution, making it difficult to relate the functional information which they contain to precise anatomical structures. A 3D neuro-anatomical atlas has been generated from the MRI data set of a normal, healthy volunteer to assist in the interpretation of nuclear medicine scans of the brain. Region growing and edge-detection techniques were used to semi-automatically segment the data set into the major tissue types within the brain. The atlas was then labelled interactively by marking points on each 2D slice. Anatomical structures useful in the interpretation of SPECT images were labelled. Additional, more detailed information corresponding to these structures is provided via an interactive index which allows access to images, diagrams and explanations. Registration of patient SPECT studies with the atlas is accomplished by using the position of the skull vertex and four external fiducial markers attached to the skin surface. The 3D coordinates determined from these points are used to calculate the transformation required to rotate, scale and translate the SPECT data, in 3D, to match the atlas. Corresponding 2D slices from the two 3D data sets are then displayed side-by-side on a computer screen. A cursor linking the two images allows the delineation of regions of interest (ROIs) in the SPECT scan based on anatomical structures identified from the atlas. Conversely regions of abnormal isotope distribution in the SPECT image can be localized by reference to corresponding structures in the atlas.

A new algorithm for deriving pulsatile blood flow waveforms tested using simulated dynamic angiographic data

SummaryIn vascular pathology the assessment of disease severity and monitoring of treatment requires quantitative and reproducible measurements of arterial blood flow. We have developed a new technique for processing sequences of dynamic digital X-ray angiographic images. We have tested it using computer simulated angiographic data which includes the effect of pulsatile blood flow and X-ray quantum noise. A parametric image was formed in which the image grey-level represents dye concentration as a function of time and distance along a vessel segment. Adjacent concentration — distance profiles in the parametric image were re-registered along the vessel axis until a match occurred. A match was defined as the point where the sum of squares of the differences in the two profiles was a minimum. The distance translated per frame interval is equal to the bolus velocity. We have tested several contrast medium injection methods including constant flow and a range of discrete pulses per second. The technique proved to be robust and independent of injection technique. Average blood flow was measured for simulated pulsatile waveforms with mean flows of up to 650 ml/min (peak velocities up to 186 cm/s) in a range of diameters from 2 mm to 6 mm. The standard deviation of the error in the mean flow estimates over the whole range of velocities and vessel sizes was ±1.4 cm/s.

Preliminary Work on the Interpretation of SPECT Images with the Aid of Registered MR Images and an MR Derived 3D Neuro-anatomical Atlas

This paper describes two methods to aid interpretation and quantification of SPECT or PET images. In the first method 3D SPECT or PET data sets are aligned and scaled to a 3D MRI data set of the same patient using 4 skin markers visible on each modality. Three display schemes have been implemented for viewing the aligned slices. Examples of these displays are provided. The second method uses a labelled 3D MRI reference data set from a volunteer to identify major anatomical structures. The MR reference data set is aligned with the isotope image using the same 4 markers plus a marker on the vertex of the skull. The reference data set is segmented approximately into the major tissue types - cerebrospinal fluid (CSF) and grey and white matter. Major structures are identified via labels in the 3D data set. A linked cursor aids delineation of anatomical regions on the isotope image using the outline of structures on the reference data set as a template. Directions for future research in the generation of complete digital anatomical atlases, which include inter-individual variations, are outlined.

Registration of 3D Surface Data for Intra-Operative Guidance and Visualization in Frameless Stereotactic Neurosurgery

We describe a technique for registering 3-D multimodal image data, acquired preoperatively, with intraoperative surface data derived from stereo video during neurosurgery. Ultimately, our aim is to provide a system that supplants traditional frame-based stereotactic techniques while achieving comparable accuracy. For registration we employ chamfer-matching in conjunction with a cost function that is robust to ‘outliers’. To balance robustness and computation speed, we employ a quasi-stochastic search of parameter space that includes pursuing multiple start points. This paper describes the registration problem as it pertains to our application. We discuss our approach to optimization and carry out a computational evaluation of the technique under various conditions.

Measurement of Liver Blood Flow: A Review

The study of hepatic haemodynamics is of importance in understanding both hepatic physiology and disease processes as well as assessing the effects of portosystemic shunting and liver transplantation. The liver has the most complicated circulation of any organ and many physiological and pathological processes can affect it1,2. This review surveys the methods available for assessing liver blood flow, examines the different parameters being measured and outlines problems of applicability and interpretation for each technique. The classification of these techniques is to some extent arbitrary and several so called “different” methods may share certain common principles. The methods reviewed have been classified into two groups (Table 1): those primarily reflecting flow through discrete vessels or to the whole organ and those used to assess local microcirculatory blood flow. All techniques have their advantages and disadvantages and in some situations a combination may provide the most information. In addition, because of the many factors affecting liver blood flow and sinusoidal perfusion, readings in a single subject may vary depending on positioning, recent food intake, anxiety, anaesthesia and drug therapy. This must be borne in mind if different studies are to be meaningfully compared.

VALIDATION OF VOLUME BLOOD FLOW MEASUREMENTS USING THREE-DIMENSIONAL DISTANCE-CONCENTRATION FUNCTIONS DERIVED FROM DIGITAL X-RAY ANGIOGRAMS

RATIONALE AND OBJECTIVES.The authors present phantom validation of a method for computing pulsatile flow waveforms in arterial vessels from high–frame-rate biplane x-ray angiograms. METHODS.The three-dimensional course of a blood vessel is constructed from biplane digital x-ray angiograms. A parametric image of contrast mass versus time and true three-dimensional path length is generated. Adjacent contrast mass-distance profiles are matched to compute instantaneous velocity, which is multiplied by cross-sectional area to yield volume flow. An electromagnetic flowmeter was used to validate flow estimates in a phantom consisting of 150-mm tubes 3, 4, and 6 mm in diameter, orientated 15°, 30°, and 35° to the imaging plane, with flow rates and waveforms expected in vivo. RESULTS.Mean and peak flows were accurate to within 9% and 10%, respectively, for velocities of less than 1 meter/second at a frame rate of 25 frames per second. CONCLUSIONS.A practical method for computing highly pulsatile flow waveforms in vivo in tortuous vessels is presented.

Craniotomy simulation and guidance using a stereo video based tracking system (VISLAN)

Image guided neurosurgery is becoming more widely used as conventional stereotactic techniques are replaced by frameless systems which allow flexible but accurate positioning of surgical approach routes with reduced invasiveness. We have developed a surgical planning and guidance system (VISLAN) which uses stereo video to construct an Intra-Operative Patient Representation (IOPR) and to track a hand-help locator in real time. The IOPR is automatically registered with the Pre-Operative Patient Representation (POPR), which is constructed from MR and other modalities and also includes the surgical plan. Images combining POPR and IOPR objects guide the surgeon on his pre-planned path towards the target.