Biomedical subjects

D W Holdsworth

Publications and source records attributed to D W Holdsworth.

At least 19 recordsLinked to original sources

PubMed1999

Anthropomorphic carotid bifurcation phantom for MRI applications.

Anthropomorphic carotid bifurcation flow phantoms that incorporate different stenotic geometries within the internal carotid artery have been developed. This technique produces high-fidelity, life-size vascular flow models that are compatible with magnetic resonance techniques. The models, in conjunction with a computer-controlled flow pump, address the need for a complex vascular geometry that can be used to verify magnetic resonance angiography (MRA) techniques that quantify stenosis severity and blood flow. Stenotic geometries, with up to 80% diameter reduction, have been fabricated in two different phantom materials. Plastic phantoms provide a durable, rigid geometry where the absolute dimensions of the model are well known. Agar gel phantoms provide tissue-like signal (T1, T2) up to the lumen boundary and are also compatible with ultrasound techniques. In this paper the technique to produce vascular flow phantoms is outlined and the compatibility of these phantoms with MRA techniques is demonstrated. J. Magn. Reson. Imaging 1999;10:533-544.

Carotid Arteries↗

PubMed1999

Application of dynamic computed tomography for measurements of local aortic elastic modulus.

A novel computed tomographic (CT) technique used for the instantaneous measurement of the dynamic elastic modulus of intact excised porcine aortic vessels subjected to physiological pressure waveforms is described. This system was comprised of a high resolution X-ray image intensifier based computed tomographic system with limiting spatial resolution of 3.2 mm-1 (for a 40 mm field of view) and a computer-controlled flow simulator. Utilising cardiac gating and computer control, a time-resolved sequence of 1 mm thick axial tomographic slices was obtained for porcine aortic specimens during one simulated cardiac cycle. With an image acquisition sampling interval of 16.5 ms, the time sequences of CT slices were able to quantify the expansion and contraction of the aortic wall during each phase of the cardiac cycle. Through superficial tagging of the adventitial surface of the specimens with wire markers, measurement of wall strain in specific circumferential sectors and subsequent calculations of localised dynamic elastic modulus were possible. The precision of circumferential measurements made from the CT images utilising a cluster-growing segmentation technique was approximately +/- 0.25 mm and allowed determination of the dynamic elastic modulus E(dyn) with a precision of +/- 8 kPa. Dynamic elastic modulus was resolved as a function of the harmonics of the physiological pressure waveform and as a function of the angular position around the vessel circumference. Application of this dynamic CT (DCT) technique to seven porcine thoracic aortic specimens produced a circumferential average (over all frequency components) E(dyn) of 373 +/- 29 kPa. This value was not statistically different (p < 0.05) from the values of 430 +/- 77 and 390 +/- 47 kPa obtained by uniaxial tensile testing and volumetric measurements respectively.

Animals↗

PubMed1999

An investigation of the flow dependence of temperature gradients near large vessels during steady state and transient tissue heating.

Temperature distributions measured during thermal therapy are a major prognostic factor of the efficacy and success of the procedure. Thermal models are used to predict the temperature elevation of tissues during heating. Theoretical work has shown that blood flow through large blood vessels plays an important role in determining temperature profiles of heated tissues. In this paper, an experimental investigation of the effects of large vessels on the temperature distribution of heated tissue is performed. The blood flow dependence of steady state and transient temperature profiles created by a cylindrical conductive heat source and an ultrasound transducer were examined using a fixed porcine kidney as a flow model. In the transient experiments, a 20 s pulse of hot water, 30 degrees C above ambient, heated the tissues. Temperatures were measured at selected locations in steps of 0.1 mm. It was observed that vessels could either heat or cool tissues depending on the orientation of the vascular geometry with respect to the heat source and that these effects are a function of flow rate through the vessels. Temperature gradients of 6 degrees C mm(-1) close to large vessels were routinely measured. Furthermore, it was observed that the temperature gradients caused by large vessels depended on whether the heating source was highly localized (i.e. a hot needle) or more distributed (i.e. external ultrasound). The gradients measured near large vessels during localized heating were between two and three times greater than the gradients measured during ultrasound heating at the same location, for comparable flows. Moreover, these gradients were more sensitive to flow variations for the localized needle heating. X-ray computed tomography data of the kidney vasculature were in good spatial agreement with the locations of all of the temperature variations measured. The three dimensional vessel path observed could account for the complex features of the temperature profiles. The flow dependences of the transient temperature profiles near large vessels during the pulsed experiments were consistent with the temperature distributions measured in the steady state experiments and provided unique insights into the process of convective heat transfer in tissues. Finally, it was shown that even for very short treatment times (3-20 s), large vessels had significant effects on the tissue temperature distributions.

Angiography↗

PubMed1999

Characterization of common carotid artery blood-flow waveforms in normal human subjects.

Knowledge of human blood-flow waveforms is required for in vitro investigations and numerical modelling. Parameters of interest include: velocity and flow waveform shapes, inter- and intra-subject variability and frequency content. We characterized the blood-velocity waveforms in the left and right common carotid arteries (CCAs) of 17 normal volunteers (24 to 34 years), analysing 3560 cardiac cycles in total. Instantaneous peak-velocity (Vpeak) measurements were obtained using pulsed-Doppler ultrasound with simultaneous collection of ECG data. An archetypal Vpeak waveform was created using velocity and timing parameters at waveform feature points. We report the following timing (post-R-wave) and peak-velocity parameters: cardiac interbeat interval (T(RR)) = 0.917 s (intra-subject standard deviation = +/- 0.045 s); cycle-averaged peak-velocity (V(CYC)) = 38.8 cm s(-1) (+/-1.5 cm s(-1)); maximum systolic Vpeak = 108.2 cm s(-1) (+/-3.8 cm s(-1)) at 0.152 s (+/-0.008 s); dicrotic notch Vpeak = 19.4 cm s(-1) (+/-2.9 cm s(-1)) at 0.398 s (+/-0.007 s). Frequency components below 12 Hz constituted 95% of the amplitude spectrum. Flow waveforms were computed from Vpeak by analytical solution of Womersley flow conditions (derived mean flow = 6.0 ml s(-1)). We propose that realistic, pseudo-random flow waveform sequences can be generated for experimental studies by varying, from cycle to cycle, only T(RR) and V(CYC) of a single archetypal waveform.

Adult↗

PubMed1999

Techniques to alleviate the effects of view aliasing artifacts in computed tomography.

Due to practical limitations in data acquisition, 3-D computed tomography systems must attempt to provide rapid reconstructions of acceptable quality from a limited number of views. The use of convolution backprojection (CBP) for image reconstruction from an inadequate number of projections, results in view aliasing artifacts. In this paper we investigate different post-processing methods of alleviating the effects of view aliasing artifacts. Two distinct methods and their variants are considered. The first, termed the intermediate view reprojection (IVR) method, involves estimating a set of intermediate views by reprojection, followed by a reconstruction using the augmented set of views. The second, termed the error-correction (EC) method, incorporates a correction on the initial reconstruction based on the projection-domain error. Suitable modifications and variants of the above methods are indicated. Of the methods discussed, the IVR method is simple, tends to reduce the effects of artifacts with less susceptibility to secondary effects, and is applicable to region-of-interest reconstructions.

Algorithms↗

PubMed1999

A three-dimensional cerebrovascular flow phantom.

We have constructed a life-sized fully three-dimensional (3D) rigid flow-through model of the cerebral vasculature. Average vessel diameters and lengths, taken from published values in the literature, were used to describe the geometry of our phantom; numerically controlled machining techniques were used to fabricate the model. Inflow to the phantom is provided through two internal carotid arteries and two vertebral arteries. Outflow is provided through the anterior cerebral arteries, the middle cerebral arteries, and the posterior cerebral arteries. The phantom includes the circle of Willis, and aneurysms of variable size may be attached at different locations. We have tested the model for geometric accuracy using high-resolution MR and CT imaging protocols, and have found that measured and prescribed diameters agree to within better than 4%. Flow dynamics, including waveform shape and flow division between branches, also mimic that seen in vivo, with flows within 16% (on average) of the prescribed values. We present 3D magnetic resonance angiography, digital subtraction angiography, and computed rotational angiography images of the phantom under conditions that mimic physiological situations.

Angiography, Digital Subtraction↗

PubMed1999

Quantitative angiographic blood-flow measurement using pulsed intra-arterial injection.

A technique for quantitative blood-flow measurement using a novel pulsed injection of radiographic contrast agent is reported. A pressurized source of contrast agent is interrupted by a rotary valve at rates ranging from 1 to 30 Hz, producing well-defined boli at the end of a catheter. The position of these boli can be recorded by a digital radiographic system and analyzed by one of several previously reported techniques, to produce quantitative measurements of blood velocity and flow rate throughout the cardiac cycle. The contrast-agent flow wave form produced by the pulsed injector has been measured with an electromagnetic flow meter, for driving pressures ranging from 600 to 1500 kPa. Excellent modulation of the contrast agent is observed for injection frequencies up to 20 Hz, through catheters up to 100 cm in length. Preliminary in vitro angiographic flow measurements have been performed using an x-ray image intensifier, coupled to a linear photodiode array as the digital detector. Both constant flow and pulsatile human blood-flow wave forms were simulated within a 6.4-mm-diam straight tube and monitored with an electromagnetic flow meter. These experiments indicate that the pulsed injector can be used to provide estimates of arterial blood flow over the entire cardiac cycle (including reverse flow), to within about +/-11%, following injection of less than 10 ml of iodinated contrast agent.

Angiography, Digital Subtraction↗

PubMed1998

Elastic response of human iliac arteries in-vitro to balloon angioplasty using high-resolution CT.

Previous angioplasty studies have used angiography and intravascular ultrasound to obtain vascular dimensions. These imaging methods do not always provide reliable measurements due to limitations in image orientation and resolution. In this study, high-resolution (0.1 x 0.1 x 0.5 mm) transverse CT slices were obtained from human common-iliac arteries in vitro to study their elastic response pre- and post-angioplasty. Seven iliacs from five patients were imaged over the physiological pressure range both pre- and post-angioplasty. Contrast was obtained with humidified air surrounding the artery. Angioplasty was done with 10 or 12 mm diameter Medi-Tech balloon catheters with a balloon pressure of 300 kPa held for 30 s. Lumen circumference, c, measured from the images, was plotted against pressure, P, and curve fitting showed c = A(1 - e(-KP)) + B where A, K, and B are fitting parameters. Six lesions appeared soft and were compressed, while one was calcified and partially lifted off the wall. Normalized changes in parameters B and K were much higher post-angioplasty in the calcified lesion, and were over 3 standard deviations from the means of the normalized changes in the six compressed lesions. Balloon/stenosed lumen diameter ratios greater than 1.2 produced a lumen area increase of 38.6 +/- 4.1%(S.D.)(n = 3); ratios less than 1.2 produced an increase of 4.4 +/- 5.1%(S.D.)(n = 4). There was no correlation between area increase and balloon/normal lumen diameter ratio (the value used clinically). Arteries with lesions containing stiffer plaques that tear from the artery wall during angioplasty appear more distensible over the physiological pressure range post-angioplasty.

Aged↗

PubMed1998

The effect of storage time and repeated measurements on the elastic properties of isolated porcine aortas using high resolution x-ray CT.

A Laboratory CT scanner with a resolution of (0.1 mm)3 was used to determine if storage up to 7 days in saline at 4 degrees C and (or) repeated measurements would alter the compliance, C, and incremental elastic modulus, Einc, of isolated porcine aortas. All specimens were obtained fresh, made pressure-tight, and then mounted in the scanner, with humidified air used to produce adequate x-ray contrast. The specimens were imaged at pressures of 4, 8, 12, 16, 20, and 24 kPa, and vessel measurements were then obtained with a computerized technique and analyzed. Seven thoracic aortas were studied on days 0, 3, 5, and 7, with a significant change (p < 0.05) in compliance first occurring after three imaging studies (i.e., day 5). Compliance of the fresh thoracic aortas (mean +/- SD) was 0.90 +/- 0.28 mm/kPa at 14.4 kPa and 0.85 +/- 0.31 mm/kPa at 22.5 kPa. Six thoracic aortas were studied only on days 0 and 6 with no intermediate measurements. They showed no change in either compliance (0.88 +/- 0.07 mm/kPa at 14.4 kPa and 0.64 +/- 0.09 mm/kPa at 22.5 kPa) or Einc (0.46 +/- 0.05 MPa at 14.4 kPa and 0.88 +/- 0.15 MPa at 22.5 kPa) from day 0 to day 6. Thus, number of measurements rather than time appears to be the important factor. Six abdominal aortas were studied similarly but on days 0, 3, and 6. No significant change occurred in compliance (0.15 +/- 0.06 mm/kPa at 14.4 kPa and 0.032 +/- 0.026 mm/kPa at 22.5 kPa) but Einc showed a change, possibly due to their viscoelastic properties. We conclude that this nondestructive CT measurement method is suitable for repeated studies on porcine thoracic aortas, but not abdominal aortas, if the measurement involves two consecutive imaging sessions separated by no more than 6 days.

Animals↗

PubMed1997

X-ray imaging technique for in vitro tissue composition measurements using saline/iodine displacement: technique optimization.

An in vitro radiographic technique which uses saline/iodine displacement has been developed to study the thickness of bone-equivalent and soft-tissue-equivalent materials within atherosclerotic plaques in arterial specimens which have been cut open longitudinally and laid flat. Results concerning the optimization of the imaging parameters are presented and discussed. The technique consists of imaging arterial specimens under two different conditions: (1) when it is immersed in an isotonic saline solution, to estimate the calcium content, and (2) when it is immersed in a concentrated iodine solution, to estimate the total thickness of the specimen. Calibration step wedges made out of bone-mimicking and soft-tissue-mimicking materials are imaged simultaneously to generate calibration curves which are used to convert the radiographs into bone-equivalent and soft-tissue-equivalent thickness images. The optimal spectral parameters were determined to be 45 and 100 kVp for the saline and the iodine images, respectively, with a significant amount of added filtration for both images. Inherent systematic inaccuracies due to (1) the nonidealities due to linear attenuation coefficient mismatch between tissue and calibration materials and (2) beam hardening due to heel effect are determined theoretically, and can be used to correct a set of bone-equivalent and the soft-tissue-equivalent images to within +/- 6 microns with an ideal, noise-free imaging system.

Angiography↗

PubMed1997

A real vessel phantom for imaging experimentation.

Vascular phantoms are used to evaluate imaging techniques such as ultrasound (US), CT, and angiography. They are expected to mimic the vasculature, surrounding tissue, and blood, and therefore must meet specific requirements on the mimicking materials, with respect to x-ray attenuation and acoustic properties (velocity, attenuation). In the past, researchers have used a variety of vessel models, including walled (typically latex tube) and wall-less phantoms (obtained by moulding a lumen in a block of agar). These models lacked the exact geometry of human vessels as well as pathologic features such as plaques and calcifications. To overcome these disadvantages, this paper describes a real vessel phantom for US and x-ray studies. The phantom consists of an agar-filled acrylic box containing a formaldehyde fixed section of a real human vessel (obtained at autopsy) cannulated onto two acrylic tubes. This phantom was evaluated by comparing the images obtained with x-ray angiography, CT, and 3-D B-mode US. The images show good overall correlation based on the location of the geometrical features within the phantom, such as lumen, plaques, and calcifications. Discrepancies, artifacts, and difficulties were minor, and are discussed. The use of a real vessel, with its natural geometry and pathology, makes this phantom attractive for evaluation of imaging techniques including projection radiography, CT and US, and for extending its use to MR and US based flow studies.

Angiography↗

PubMed1997

Three-dimensional computed tomographic reconstruction using a C-arm mounted XRII: correction of image intensifier distortion.

X-ray image intensifiers (XRIIs) have many applications in diagnostic imaging including acquisition of near-real-time projection images of the intracranial and coronary vasculature. Recently, there has been some interest in using this projection data to generate three-dimensional (3-D) computed tomographic (CT) reconstructions. The XRII and x-ray tube are rotated around the object, acquiring sufficient data for the simultaneous reconstruction of many transverse slices. Three-dimensional reconstructions are compromised, however, if the projection data is geometrically distorted in any way. Previous studies have shown the distortion in XRIIs to be substantial and to be highly angular dependent. In this paper, we present a global correction technique which provides a table of correction coefficients for an image acquired at any arbitrary angle about the patient. The coefficients are generated using a linear least-squares fit between the detected and known locations of a grid of small steel beads which is attached to the XRII (27 cm nominal diameter). We have performed corrections on 100 images obtained during rotation of the gantry through 200 degrees and find that a fifth-order polynomial provides optimum image distortion reduction (mean residual distortion of 0.07 pixels), however, fourth-order polynomials provide sufficient distortion reduction for our application (mean residual displacement of 0.1 pixels). Using sixth-order polynomials does not provide a statistically significant reduction in image distortion. The spatial distribution of residual distortion did not demonstrate any particular pattern over the face of the XRII. Image angle and coefficient angle must be known to within +/- 2 degrees in order to keep the mean residual distortion be approximately 0.5 pixels.

Humans↗

PubMed1997

X-ray imaging technique for in vitro tissue composition measurements using saline/iodine displacement: experimental verification.

A novel in vitro radiographic technique using saline/iodine displacement, which can be used to study the bone-equivalent and soft-tissue-equivalent thicknesses within vessel walls, was applied to imaging of arterial specimens. Results concerning the accuracy and precision of the bone-equivalent and soft-tissue-equivalent thickness measurements obtained with this technique are reported and discussed. Planar radiographs of a phantom were obtained under two different conditions: (1) when it is immersed in an isotonic saline solution using a 45-kVp spectrum with no added filtration, and (2) when it is immersed in a concentrated iodine solution using a 100-kVp spectrum with 12.5-mm aluminum-added filtration. Calibration step wedges made out of bone-mimicking and soft-tissue-mimicking materials are imaged simultaneously to generate calibration curves that are used to convert the radiographs into bone-equivalent and total-thickness images. A soft-tissue-thickness image is obtained from the subtraction of the bone-equivalent image from the total-thickness image. Thickness measurements obtained from these images yielded average accuracies of +/- 110 microns for both the bone-equivalent and the soft-tissue-equivalent images. The precision (one standard deviation) of the thickness measurements was +/- 60 and +/- 90 microns for the bone-equivalent and the soft-tissue-equivalent images, respectively. In conclusion, since calcified plaque can become as thick as 3-4 mm, the saline/iodine displacement technique has the potential to be a very useful technique for ex vivo studies of the progression of atherosclerosis because of its high accuracy and precision.

Angiography↗

PubMed1997

Use of a C-arm system to generate true three-dimensional computed rotational angiograms: preliminary in vitro and in vivo results.

PURPOSE: To evaluate the potential use of a C-arm mounted X-ray image intensifier (XRII) system to generate three-dimensional computed rotational angiograms during interventional neuroradiologic procedures. METHODS: A clinical angiographic system was modified to allow collection of sufficient views during selective intraarterial contrast injections for CT reconstruction of a 15 x 15 x 15-cm3 volume. Image intensifier distortion and C-arm instabilities were corrected by using image-based techniques. The impact of the pulsatile nature of the vessels during image data acquisition and of the presence of bone on the 3-D reconstructions was investigated by generating 3-D reconstructions of an anesthetized 20-kg pig and of a human skull phantom. RESULTS: A sequence of images sufficient for 3-D reconstruction was acquired in less than 5 seconds. Image intensifier distortion and C-arm instabilities were corrected to subpixel accuracy (0.035 mm and 0.07 mm, respectively). Both the intracranial vessels of the pig and the small, high-contrast structures in the skull were reconstructed with negligible artifacts. CONCLUSIONS: Using a C-arm mounted XRII system, computed rotational angiography can provide true 3-D images of diagnostic quality.

Animals↗

PubMed1996

Geometric characterization of stenosed human carotid arteries.

RATIONALE AND OBJECTIVES: The geometry of stenosed carotid bifurcations was analyzed to determine average representations for several stenosis grades. METHODS: Film angiograms of 62 patients with internal carotid artery stenoses were digitized. Residual lumen boundaries were manually outlined. The outlines were processed with a computer to extract geometric measurements. The measurements were grouped according to stenosis grade and used to create average representations. RESULTS: Accuracy and precision of the outlining technique were +/- 0.020 common carotid diameters (CCD) and +/- 0.025 CCD, respectively. Maximum narrowing of the internal carotid artery occurred at 0.3 CCD +/- 1.5 (mean +/- standard deviation) distal to the flow divider. The region of significant narrowing extended axially 1.2 CCD +/- 1.0. Poststenotic dilatations were observed, with enlargement of 1.3 +/- 0.7 times the normal diameter of the distal internal carotid artery. A tendency toward smaller bifurcation angles with increasing stenosis severity was observed. CONCLUSION: Three-dimensional geometric models could be created for carotid bifurcations that were disease free (normal) and of arbitrary stenosis grade.

Aged↗

PubMed1994

Turbine flow sensor for volume-flow rate verification in MR.

A turbine flow sensor for MR flow experiments has been evaluated using reference volume-flow rate measurements obtained using an electromagnetic (EM) flow meter measurements and simultaneous phase contrast (PC) MR acquisitions. After calibration, the device was found to have accuracy (compared with the EM flow meter), linearity, and precision of better than +/- 1%, +/- 3.5%, 3.5%, respectively, in constant flow mode (0 to 30 ml s-1). The frequency response of the flow sensor was flat (within +/- 10%) up to 13.9 Hz. Volume-flow rate measurements on constant and simulated physiologic flow waveforms were in close agreement with both the electromagnetic (EM) flow meter and the gated MR PC estimates.

Calibration↗

PubMed1994

Dual-energy x-ray imaging technique for in vitro tissue composition measurement.

A dual-energy in vitro radiographic technique has been developed to study the thickness of tissue and bone within atherosclerotic plaques. Results concerning the accuracy and precision of the thickness measurements using this technique are presented and discussed. Planar radiographs of phantoms were obtained with a low-energy spectrum (45 kVp, no added filtration) and a high-energy spectrum (100 kVp, 2.88-mm copper-added filtration), and then decomposed into bone-equivalent and Lucite basis-material images. Thickness measurements from these images yielded average accuracies of +/- 750 microns for the Lucite images, and +/- 25 microns for the bone-equivalent images. The imprecision (one standard deviation) of the thickness measurements was +/- 192 and +/- 47 microns for the Lucite and the bone-equivalent images, respectively (for thin sections). Although the accuracy and precision of Lucite thickness measurements were not as good as those obtained with other techniques, such as the iodine displacement technique, the accuracy and precision of the bone thickness measurements are shown to be much better. The high accuracy and precision of the bone measurement makes dual energy a very appealing technique for analyzing the physical properties of calcified atherosclerotic plaques in excised arterial specimens.

Absorptiometry, Photon↗

PubMed1994

Further comments on the measurement of carotid stenosis from angiograms. North American Symptomatic Carotid Endarterectomy Trial (NASCET) Group.

BACKGROUND AND PURPOSE: Three different methods for estimating the percentage of reduction in the diameter of the internal carotid artery (ie, stenosis) have been proposed in the literature. Further comparisons of the methods were carried out with the intent of recommending a current standard for determining the percentage of stenosis from angiograms. METHODS: Angiograms from 112 patients were obtained. For each angiogram, stenosis was estimated in the manner of the European Carotid Surgery Trial (ECST method), the North American Symptomatic Carotid Endarterectomy Trial (NASCET method), and by a method using the common carotid artery lumen diameter (CC method). RESULTS: Although there is much discrepancy among the estimates of stenosis arising from the three different methods for any particular patient, it is possible to predict (on average) the percentage of stenosis from one method to another. The relationship between the NASCET and CC methods is linear, with a mean ratio of distal internal carotid artery to common carotid diameter of 0.62 (SD of 0.11). The variability in the diameter of the common carotid artery lumen stabilizes only beyond 2.5 common carotid diameter units (approximately 20 to 30 mm by conventional angiography) proximal to the bifurcation. Unexpectedly, the relationships between both the ECST and NASCET methods and ECST and CC methods were parabolic (P < .001). The reasons underlying these departures from linearity are uncertain. CONCLUSIONS: The comparability of our results with those reported in the literature regarding the CC and NASCET methods provides further evidence of the reproducibility of methods measuring anatomic features that can be visualized on an angiogram. Disease of the internal carotid artery is one of the important causes of ischemic symptoms. Measuring the narrowest portion of the internal artery relative to the normal portion of the same artery, well beyond the bulb, is a logical method. Moreover, benefits of carotid endarterectomy for patients with 70% to 99% stenosis as determined by the NASCET method have been well established in a clinical trial. Converting from the NASCET method to the CC method, given that the CC method is neither superior nor easier to calculate, is not recommended.

Angiography↗