Biomedical subjects
D J Hawkes
Publications and source records attributed to D J Hawkes.
The measurement of blood flow waveforms from X-ray angiography. Part 1: Principles of the method and preliminary validation.
The principles and implementation of a method for measurement of blood flow waveforms from X-ray angiography are described. Contrast medium mass values are obtained at multitudinous positions along individual vessels and from numerous images in a time sequence. These values are represented as a matrix of grey levels in a parametric image. This image is normalized to represent contrast medium concentration, and the movement over time of isoconcentration portions of the contrast bolus is recovered to determine blood flow. Preliminary validation has been undertaken using parametric images generated in two ways: synthesis from a computer model of vascular pulsatile flow and analysis of cine-angiograms of physical models (plastic and perspex tubes) carrying known pulsatile flows. Two distinct methods for interrogation of parametric images by digital image processing were employed; both provided accurate flow measurements.
Registration and display of the combined bone scan and radiograph in the diagnosis and management of wrist injuries.
A system has been developed for the registration and combined display of the X-ray image and isotope bone scan. This system has been evaluated by prospectively studying 23 patients who were referred for suspected injury of the wrist. The registration system has an inherent precision for registration of about 1 mm. When patient positioning errors are included, this increases to about 4 mm. Two observers evaluated the sets of images prior to registration and after registration and combined display. They judged that in 16 out of 17 cases judged abnormal (observer 1) and 12 out of 18 cases judged abnormal (observer 2), the registration technique improved localization of a lesion. One case was rejected as registration was not possible due to incorrect positioning of the hand.
Validation of a quantitative radiographic technique to estimate pulsatile blood flow waveforms using digital subtraction angiographic data.
We have validated a new radiographic technique for determining pulsatile volume flow in arteries following an intraarterial injection of contrast material. Instantaneous blood velocities were estimated by generating a parametric image from dynamic angiographic images in which the image grey level represents contrast material concentration as a function of time and distance along a vessel segment. Adjacent concentration--distance profiles in the parametric image were shifted with respect to distance 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 gives the instantaneous contrast material bolus velocity. We have validated the technique using an experimental phantom of blood circulation, consisting of a pump, flexible plastic tubing, the tubular probe of an electromagnetic flowmeter (EMF) and a solenoid, to simulate a pulsatile flow waveform, which includes reverse flow. Small boluses of contrast material can be injected at various positions in the circuit. Measurements of pulsatile velocity flow were taken at 40 ms intervals, using a tube of 6.6 mm internal diameter and an imaged tube length of 200 mm. The shape of the flow velocity waveform was faithfully reproduced but there was an overestimation of peak velocity of 40% at low velocities (peak velocity of 540 mm s-1), reducing to 19% at peak velocities of 964 mm s-1 with an underestimation of 16% at the peak velocities of 1899 mm s-1. The validation was repeated for distances ranging from 130 to 230 mm between injection and measurement sites and for imaged tube lengths varying from 200 to 20 mm.(ABSTRACT TRUNCATED AT 250 WORDS)
Analysis of texture in macroradiographs of osteoarthritic knees using the fractal signature.
Texture of regions of macroradiographs (x5) of six normal and five osteoarthritic knee joints, taken on a high resolution microfocal x-ray unit, are examined using mathematical morphology. Radiographs of bones are two-dimensional projections of attenuation coefficient through the three-dimensional (3D) joint structure. Visible texture represents the summation of the attenuation from numerous thin plates of bone. Where there is no organization in the trabeculae, resultant radiographs approximate a fractal surface. Varying structuring element size in mathematical morphology allows estimation of fractal dimension over a range of resolution. Variation of fractal dimension with resolution, the fractal signature, indicates how images deviate from fractal surfaces. By correct choice of structuring element, a texture analysis method using the fractal signature has been developed, tolerant to changes in image acquisition and digitization. Texture in regions of radiographs of normal tibia approximates a fractal surface with dimension 2.8 as does vertical structure in arthritic patients. In osteoarthritic knee joints, horizontal tibial trabeculae thicken. Horizontal structure in the tibia on radiographs of arthritic patients deviates from the fractal model. This is indicated by peaks in the fractal signature whose height and position match a visual assessment of the degree of arthritic change.
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 it. 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.
Registration of MR and CT images for skull base surgery using point-like anatomical features.
We have developed a registration technique for combining magnetic resonance imaging (MRI) and computed tomography (CT) images of the skull base for use in surgical planning. The technique is based on user identification of point-like landmarks visible in both modalities. The combination of images involves a small amount of expert interaction, is relatively quick and preliminary evaluation indicates that it is accurate to within 1.5 mm. Registered or fused images can be viewed either on an image processing workstation, or fused images can be printed onto conventional film for convenience in clinical use. We present one patient in order to demonstrate the technique's indications and advantages.
Matching of geometric models in the analysis of lumen shape in digital X-ray angiography.
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Early visual representation using regions defined by maximum gradient profiles between singular points.
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A new algorithm for deriving pulsatile blood flow waveforms tested using stimulated dynamic angiographic data.
In 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.
Bone mineralisation in preterm infants measured by dual energy radiographic densitometry.
Dual energy radiographic densitometry was used to follow postnatal changes in the bone mineral content of the radii of 15 infants of less than 30 weeks' gestation. The system permitted bone mineral content to be measured with minimal disturbance to the infants in their incubators. Mean bone mineral content at birth was 2.4 mg/mm shaft length, decreasing to 1.9 mg/mm at 6 weeks of age, before starting to rise. Mineralisation was poor compared with that of a fetus at an equivalent postconceptual age. Mean intakes of calcium and phosphate were considerably less than the intrauterine accumulation of these minerals and it is postulated that this was the main cause of the poor mineralisation. Radiographic densitometry is both accurate and precise and has advantages over photon absorptiometry in that it can be used to measure bone mineral in infants who are not only preterm, but also ill enough to require intensive care.
Tissue analysis by dual-energy computed tomography.
A straightforward method of tissue analysis from dual-energy computed tomography (CT) is presented which does not rely on previous inaccurate or incorrect formulations of the X-ray attenuation coefficient. The attenuation coefficients of tissue and bone were represented by a mixture of two reference materials. For convenience, water and calcium chloride were chosen. After careful calibration of the CT scanner, a dual-energy CT scan yields the water and calcium chloride in units of kg/kg multiplied by the specific gravity of the tissue. For an error of +/- 2 HU on dual-energy scans at 140 kVp and 87 kVp on an EMI CT5005 general-purpose scanner, the error on the calcium chloride coefficients is +/- 0.004 kg/kg. Fat concentrations greater than 25% by weight may be detected when the coefficients are averaged over at least 200 pixels. Bone mineral content could be measured with a precision of 0.01 (in units of kg/kg multiplied by specific gravity) for averaging regions of 24 pixels. Iodine concentrations in tissue can be deduced if water and iodine are chosen as the reference materials. Clinical examples are presented to illustrate the technique for scans of both the head and the abdomen.
A simple method for correcting left ventricular equilibrium radionuclide angiography for the effects of arrhythmias.
A simple method of correcting equilibrium radionuclide angiographic data for the effects of arrhythmias is proposed. This involves no further acquisition time or inconvenience to the patient. This correction is necessary in patients with marked arrhythmias as the left ventricular time-activity curve (LVTAC) becomes distorted with greater sampling time in the first few frames of the LVTAC than in later frames. This will show on the LVTAC as greater counts in the first few frames and will lead to an overestimation of left ventricular ejection fraction (LVEF) and wall motion. The method proposed normalises each frame to make the contribution from a non-cardiac region constant over the cardiac cycle. Results of the correction are presented and discussed.
A preliminary investigation of nuclear resonant scattering as a new technique for the in vivo measurement of hepatic copper.
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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 robust and accurate method for calculating the fractal signature of texture in macroradiographs of osteoarthritic knees.
In osteoarthritis of the knee, horizontal trabeculae in the tibia thicken, leading to the appearance of horizontal striations on radiographs. Any attempt to understand and monitor the disease process must examine these changes. We have previously described a method of analysing the texture in these images that uses the mathematical morphology operations of dilation and erosion to calculate how image fractal dimension varies with resolution. This variation with resolution is the fractal signature. Calculation of fractal dimension should be independent of linear transformations in image values, and it is shown that careful choice of morphological structuring element is required for this. In this paper we describe the dependence of our method of calculating the fractal signature on image acquisition and digitization parameters. Errors due to variations in pixel size, X-ray exposure, and patient repositioning are small compared to the changes seen in the signature due to osteoarthritic change. Using pairs of orthogonal structuring elements, one tuned to calculate the signature due to horizontal image structure and the other for vertical image structure, texture directionality is examined. The difference between the two resulting signatures in individual knees is shown to be a measure of the change in texture seen in osteoarthritis.