Richard N. Rankin

Three-dimensional colour Doppler imaging.

We have developed a system to acquire in vivo three-dimensional (3D) colour velocity images of peripheral vasculature. A clinical ultrasound system was modified by mounting the transducer on a motor-driven translation stage, allowing planar ultrasound images to be acquired along a 37 mm long stroke. A 3D velocity image is acquired by digitizing, in synchrony with the cardiac cycle, successive video images as the transducer is moved over the skin surface. 3D images require about 1 min to acquire and 10 min to reconstruct before being viewed interactively. Image acquisition at several points in the cardiac cycle permits a cine-type reconstructed image. Geometrical, temporal and velocity accuracy of the acquisition and reconstruction have been quantified and found not to degrade the image.

Percutaneous Embolotherapy of Lower Gastrointestinal Hemorrhage

PURPOSE To evaluate percutaneous embolotherapy in the treatment of lower gastrointestinal hemorrhage. MATERIALS AND METHODS Twenty-one patients who underwent attempted percutaneous embolization for acute lower gastrointestinal bleeding between 1982 and 1997 were retrospectively studied. Hemorrhagic sites included jejunum (n = 4), ileum (n = 4), cecum (n = 4), and the remaining colon (n = 9). RESULTS Embolization was not technically possible in four patients (19%). Hemostasis was achieved in 15 patients (71%) with prolonged hemostasis in 10 (48%). All embolizations distal to the cecum resulted in prolonged hemostasis. Three of four patients with jejunal bleeding had recurrent bleeding after apparent successful embolization. Only one of four cecal embolizations achieved prolonged cessation of bleeding. No ischemic complications were identified. CONCLUSION Based on these data, it would appear that the risk of bowel ischemia/infarction in the lower gastrointestinal tract may not be as high as has been suggested. Two regions (cecum and proximal jejunum) were associated with poor results, suggesting these areas may not be as responsive to embolotherapy as other sites in the lower gastrointestinal tract.

Flow Patterns at the Stenosed Carotid Bifurcation: Effect of Concentric versus Eccentric Stenosis

AbstractCarotid stenosis severity is a commonly used indicator for assessing risk of stroke. However, the majority of individuals with severe carotid artery disease never suffer a stroke, and strokes can occur even with only mild or moderate stenosis. This suggests local factors (other than stenosis severity) at or near the carotid artery bifurcation may be important in determining stroke risk. In this paper we investigate the effect of stenosis geometry on flow patterns in the stenosed carotid bifurcation, using concentrically and eccentrically stenosed anthropomorphic carotid bifurcation models having identical stenosis severity. Computational simulations and experimental flow visualizations both demonstrate marked differences in flow patterns of concentric and eccentric stenosis models for moderately and severely stenosed cases, respectively. In particular, we identify post-stenotic recirculation zone size and location, and spatial extent of elevated wall shear stress as key factors differing between the two geometries. As these are also key biophysical factors promoting thrombogenesis, we propose that the stenosed carotid bifurcation geometry—or the induced flow patterns themselves—may provide more specific indicators for those plaques that are vulnerable to enhanced thromboembolic potential, and hence, increased risk of ischemic stroke.

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.

Three-dimensional ultrasound imaging of the vasculature

With conventional ultrasonography, the diagnostician must view a series of two-dimensional images in order to form a mental impression of the three-dimensional anatomy, an efficient and time consuming practice prone to operator variability, which may cause variable or even incorrect diagnoses. Also, a conventional two-dimensional ultrasound image represents a thin slice of the patients anatomy at a single location and orientation, which is difficult to reproduce at a later time. These factors make conventional ultrasonography non-optimal for prospective or follow-up studies. Our efforts have focused on overcoming these deficiencies by developing three-dimensional ultrasound imaging techniques that are capable of acquiring B-mode, colour Doppler and power Doppler images of the vasculature, by using a conventional ultrasound system to acquire a series of two-dimensional images and then mathematically reconstructing them into a single three-dimensional image, which may then be viewed interactively on an inexpensive desktop computer. We report here on two approaches: (1) free-hand scanning, in which a magnetic positioning device is attached to the ultrasound transducer to record the position and orientation of each two-dimensional image needed for the three-dimensional image reconstruction; and (2) mechanical scanning, in which a motor-driven assembly is used to translate the transducer linearly across the neck, yielding a set of uniformly-spaced parallel two-dimensional images.

Flow Patterns in Carotid Bifurcation Models Using Pulsed Doppler Ultrasound: Effect of Concentric vs. Eccentric Stenosis on Turbulence and Recirculation

Hemodynamics play a significant role in stroke risk, where thrombus formation may be accelerated in regions of slow or recirculating flow, high shear and increased turbulence. An in vitro investigation was performed with pulsed Doppler ultrasound (DUS) using the complete spectral data to investigate the three-dimensional (3-D) distribution of advanced parameters that may have potential for making a more specific in vivo diagnosis of carotid disease and stroke risk. The effect of stenosis symmetry and the potential of DUS spectral parameters for visualizing regions of recirculation or turbulence were explored. DUS was used to map pulsatile flow in four model geometries representing two different plaque symmetries (eccentricity) and two stenosis severities (mild, severe). Qualitative comparisons were made with flow patterns visualized using digital particle imaging. Color-encoded maps of DUS spectral parameters (mean velocity, spectral-broadening index and turbulence intensity) clearly distinguished regions of slow or recirculating flow and disturbed or turbulent flow. Distinctly different flow patterns resulted from stenoses of equal severity but different eccentricity. Noticeable differences were seen in both the size and location of recirculation zones and in the paths of high-velocity jets. Highly elevated levels of turbulence intensity were seen distal to severe stenosis. Results demonstrated the importance of plaque shape, which is typically not considered in standard diagnosis, in addition to stenosis severity.

Three-dimensional color Doppler imaging of the carotid artery

Stroke is the third leading cause of death in the United States. It is caused by ischemic injury to the brain, usually resulting from emboli from atherosclerotic plaques. The carotid bifurcation in humans is prone to atherosclerotic disease and is a site where emboli may originate. Currently, carotid stenoses are evaluated by non-invasive duplex Doppler ultrasound, with preoperative verification by intra-arterial angiography. We have developed a system that uses a color Doppler ultrasound imaging system to acquire in-vivo 3-D color Doppler images of the human carotid artery, with the aim of increasing the diagnostic accuracy of ultrasound and decreasing the use of angiography for verification. A clinical TL Ultramark 9 color Doppler ultrasound system was modified by mounting the hand-held ultrasound scan head on a motor-driven translation stage. The stage allows planar ultrasound images to be acquired over 45 mm along the neck between the clavicle and the mandible. A 3- D image is acquired by digitizing, in synchrony with the cardiac cycle, successive color ultrasound video images as the scan head is stepped along the neck. A complete volume set of 64 frames, comprising some 15 megabytes of data, requires approximately 2 minutes to acquire. The volume image is reformatted and displayed on a Sun 4/360 workstation equipped with a TAAC-1 graphics accelerator. The 3-D image may be manipulated in real time to yield the best view of blood flow in the bifurcation.

Clinical Doppler ultrasound for the assessment of plaque ulceration in the stenosed carotid bifurcation by detection of distal turbulence intensity: A matched model study

The assessment of flow disturbances due to carotid plaque ulceration may provide added diagnostic information to Doppler ultrasound (DUS) of the carotid stenosis, and indicate whether the associated hemodynamics are a potential thromboembolic source. We evaluated the effect of ulceration in a moderately stenosed carotid bifurcation on distal turbulence intensity (TI) measured using clinical DUS in matched anthropomorphic models. Several physiologically relevant ulcer geometries (hemispherical, mushroom-shaped, and ellipsoidal pointing distally and proximally) and sizes (2-mm, 3-mm and 4-mm diameter hemispheres) were investigated. An offline analysis was performed to determine several velocity-based parameters from ensemble-averaged spectral data, including TI. Significant elevations in TI were observed in the post-stenotic flow field of the stenosed carotid bifurcation by the inclusion of ulceration (P < 0.001) in a region two common carotid artery diameters distal to the site of ulceration during the systolic peak and the diastolic phase of the cardiac cycle. Both the size and shape of ulceration had a significant effect on TI in the distal region (P < 0.001). Due to the use of a clinical system, this method provides the means to evaluate for plaque ulcerations in patients with carotid atherosclerosis using DUS.

Rapid volume flow rate estimation using transverse colour Doppler imaging

A system is described in which the volume flow rate of blood in a vessel is determined using transverse colour Doppler ultrasound imaging. The system measures rapidly the two-dimensional velocity profile of blood flowing through a vessel. By integration of the measured velocity profiles the volume flow rate of blood in the vessel is obtained. The Doppler angle is obtained from the included angle between two imaging planes, and their respective average measured flows. This technique yields instantaneous and average flow rate in real time, and permits long flow recordings to be made and stored digitally. The error is less than 5% over a 8:1 flow rate range.

Prospective comparison of magnetic resonance angiography with selective renal angiography for living kidney donor assessment.

OBJECTIVES For years, the reference standard in the evaluation of living donor vascular anatomy has been selective renal angiography (SRA). Because of the potential morbidity associated with SRA, we prospectively evaluated magnetic resonance angiography (MRA) in the assessment of renal donors. METHODS All patients had SRA and 53 renal units were prospectively evaluated by MRA. We used SRA supplemented by findings at donor nephrectomy (DN) as our standard. We defined a positive test as the detection of any abnormality in the number of renal arteries. RESULTS Selective renal angiography yielded a sensitivity of 86%, specificity of 95%, positive predictive value (PPV) of 75%, and negative predictive value (NPV) of 97% compared with findings at DN. MRA had a sensitivity of 64%, 88% specificity, 58% PPV, and 90% NPV. MRA correctly identified only 7 of 11 renal units with accessory arteries. MRA also incorrectly identified 5 accessory arteries not present on SRA or DN. Two patients diagnosed with fibromuscular dysplasia by SRA were missed using MRA. CONCLUSIONS We have shown that MRA is not capable of replacing SRA as the reference standard in renal donor imaging.