A critical study of ultrasound Doppler spectral analysis for detecting carotid disease

Novel flow quantification of the carotid bulb and the common carotid artery with vector flow ultrasound

Abnormal blood flow is usually assessed using spectral Doppler estimation of the peak systolic velocity. The technique, however, only estimates the axial velocity component, and therefore the complexity of blood flow remains hidden in conventional ultrasound examinations. With the vector ultrasound technique transverse oscillation the blood velocities of both the axial and the transverse directions are obtained and the complexity of blood flow can be visualized. The aim of the study was to determine the technical performance and interpretation of vector concentration as a tool for estimation of flow complexity. A secondary aim was to establish accuracy parameters to detect flow changes/patterns in the common carotid artery (CCA) and the carotid bulb (CB). The right carotid bifurcation including the CCA and CB of eight healthy volunteers were scanned in a longitudinal plane with vector flow ultrasound (US) using a commercial vector flow ultrasound scanner (ProFocus, BK Medical, Denmark) with a linear 5 MHz transducer transverse oscillation vector flow software. CCA and CB areas were marked in one cardiac cycle from each volunteer. The complex flow was assessed by medical expert evaluation and by vector concentration calculation. A vortex with complex flow was found in all carotid bulbs, whereas the CCA had mainly laminar flow. The medical experts evaluated the flow to be mainly laminar in the CCA (0.82 ± 0.14) and mainly complex (0.23 ± 0.22) in the CB. Likewise, the estimated vector concentrations in CCA (0.96 ± 0.16) indicated mainly laminar flow and in CB (0.83 ± 0.07) indicated mainly turbulence. Both methods were thus able to clearly distinguish the flow patterns of CCA and CB in systole. Vector concentration from angle-independent vector velocity estimates is a quantitative index, which is simple to calculate and can differentiate between laminar and complex flow.

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.

An in vitro system for Doppler ultrasound flow studies in the stenosed carotid artery bifurcation

To investigate the correlation between disease severity and Doppler spectral measurements in the carotid artery bifurcation, a unique in vitro system has been developed that mimics the human vasculature with respect to both anatomy and flow perfusion. Agar-based carotid phantoms are perfused with a blood-mimicking fluid using a computer-controlled pump and realistic pulsatile flow waveform. A three-axis translational stage allows the lumen to be interrogated with a 0.6-microL Doppler sample volume at the desired spatial intervals using a semiautomated acquisition system, to collect 10 cardiac cycles of gated quadrature data at each site. Off-line analysis, including a 1024-point FFT, produces a 4-D (i.e., time-varying 3-D) Doppler velocity data set with 1.3-cm/s velocity resolution and 12-ms temporal resolution. Using this system, in vitro flow in bifurcations with both normal and stenosed lumen geometry (from 30% to 80% stenosis by NASCET criteria) can be studied, along with the effect of factors, such as stenosis geometry (concentric vs. eccentric) and flow rate, on the observed Doppler ultrasound (US) spectra and haemodynamic patterns.

A transputer-based physiological signal processing system. Part 2--System testing and investigation of flow through models of very small arterial stenoses

This paper describes the performance testing of a novel transputer-based physiological digital signal processing (DSP) unit and its application in the interpretation of pulsed Doppler ultrasound signals, obtained from models of arterial stenoses. The first test used the DSP unit as a stand-alone spectrum analyser using (1) sinusoidal frequencies (50 Hz to 10 kHz) and (2) filtered white noise (centre frequency 3 kHz, bandwidth 2.5 kHz). For the second test, the DSP unit was attached to a 30-channel multi-gate Doppler ultrasound scanner (transmitting a 4.8 MHz pulse with a repetition frequency of 4.8 kHz) and a vessel tracking unit. The Doppler ultrasound signals obtained from steady flow (100-600 ml/min) in a rigid acrylic tube (internal diameter 6 mm) were then analyzed by the DSP unit and a commercially available system. Lastly, an in vitro investigation into the flow disturbances around very small stenoses (2-25% cross-sectional area reduction), using steady flow (100-600 ml/min), was undertaken. The results indicated that the system was capable of detecting stenoses as small as 5% cross-sectional area reduction.

MR pulsatility measurements in peripheral arteries: preliminary results

Phase contrast flow velocity measurements were performed in six healthy volunteers and 30 patients with arteriosclerotic disease. The iliac arteries were investigated in 8 cases and the femoral arteries in 28 cases. In the first 24 patients, 16 evenly distributed data sets were acquired during one cardiac cycle. In the last 12 patients, a trigger pulse followed by the acquisition of 30 evenly distributed data sets was applied every second heart beat. This procedure allowed data to be acquired over a full heart cycle without any acquisition gap. The measured flow velocities were displayed as function of time. Systolic acceleration, postsystolic deceleration and pulsatility of flow velocity were calculated and compared with stenosis grades determined from DSA angiograms. Flattening of the flow velocity patterns was found to correlate with the local severity of arteriosclerotic disease.

Phase-contrast pulsatility measurements: preliminary results in normal and arteriosclerotic iliofemoral arteries. Work in progress

Magnetic resonance (MR) flow measurements were obtained in six healthy volunteers and 30 patients with arteriosclerotic disease with a 1.5-T imager and a pulse sequence for flow quantification based on flow-induced phase shifts. The iliac arteries were investigated in eight and the femoral arteries in 28 subjects. A trigger pulse, followed by the acquisition of 30 evenly distributed data sets, was applied every second heartbeat, thus eliminating any acquisition gap in a full heart cycle. For quantitative analysis, flow velocity was plotted as a function of time. Systolic acceleration, postsystolic deceleration, and pulsatility of flow were calculated and compared with stenosis grades determined from recent intraarterial digital subtraction angiograms. The flattening of the temporal flow patterns correlated with local severity of vascular occlusive disease.

Signal processors in duplex sonography: in vitro comparison between analog and digital methods

Using a new flow-test phantom, which respects the acoustic properties of real blood as well as the proximal and distal impedances of body circulation, we assessed the performance of two duplex sonography signal processors on blood-flow measurements. With both the analog and the dynamic signal processor (Fast Fourier Transform), the correlation between duplex sonography and quantitative flow measurements was high (0.96-0.99) for different dynamic conditions (steady or pulsatile blood flow, varying heart rate, blood pressure, and hematocrit) and for different mechanical conditions (silicon tube or animal vessel). The real blood flow was overestimated by duplex sonography; the over-estimation was more pronounced with the analog processor (factor 1.87-4.20) than with the digital processor (factor 1.22-1.64, P < 0.05). Applied to the study of asymmetric stenoses, the digital processor was not superior to the analog processors described in the literature.

Between-method correlation in quantifying internal carotid stenosis

BACKGROUND AND PURPOSE

The degree of internal carotid stenosis has emerged as the most important predictor of ischemic stroke in extracranial carotid artery disease. The purpose of this study was to assess the validity of the noninvasive techniques for quantifying internal carotid stenosis with respect to the accepted standard of intra-arterial angiography.

METHODS

We measured the maximum percentage reduction in luminal diameter on the intra-arterial digital subtraction angiograms of 56 symptomatic patients with extracranial internal carotid stenosis (n = 77) or occlusion (n = 20). These data were compared with independent measurements based on continuous-wave Doppler ultrasonography, pulsed-wave Doppler spectrum analysis, color Doppler-assisted duplex imaging, and magnetic resonance angiography.

RESULTS

Correlations with intra-arterial angiography were equally strong (r > .90) for magnetic resonance angiography, continuous-wave Doppler, and color duplex analysis. Positive and negative predictive values for (therapeutically relevant) 70% to 99% stenosis were higher for continuous-wave Doppler (.82, .97) and color duplex (.84, .98) than for magnetic resonance angiography (.79, .81). Also, accuracy in quantifying high-grade stenosis was better for both of these ultrasonographic techniques, mainly due to the frequent occurrence of a "flow gap" on the magnetic resonance angiograms. Continuous-wave Doppler and magnetic resonance angiography, but not color duplex, failed to detect slow residual arterial flow in one and two cases of symptomatic "pseudo-occlusion" of the internal carotid, respectively.

CONCLUSIONS

(1) Several noninvasive methods compare well with intra-arterial angiography in identifying and quantifying high-grade internal carotid stenosis; (2) the use of these noninvasive methods may suffice for treatment decisions; and (3) because residual between-method disagreement is partly explained by principles of physics, the validity of continuous-wave Doppler and color duplex in quantifying 60% to 99% stenosis is likely to be underestimated by correlation with intra-arterial angiography.

Experimental evaluation of intrinsic and nonstationary ultrasonic Doppler spectral broadening in steady and pulsatile flow loop models

Intrinsic and nonstationary Doppler spectral broadening, and the skewness of the spectral representation, were evaluated experimentally using porcine red cell suspensions as ultrasonic scatterers. Theoretically, the relative Doppler bandwidth, defined as the intrinsic bandwidth divided by the mean Doppler frequency shift, should be velocity independent. The relative Doppler bandwidth invariance theorem was experimentally verified with an in vitro steady laminar blood flow model. It is shown that the relative bandwidth is both independent of the flow velocity and blood hematocrit. Using a pulsatile laminar flow model, the authors demonstrated that the relative Doppler bandwidth invariance theorem did not hold during flow acceleration and deceleration. In addition, a positive skewness of the Doppler spectra was observed during acceleration while a negative skewness was measured during the deceleration of blood. The effect of the window duration used in the Fourier spectral computation, on nonstationary broadening, is characterized.

A flexible FFT algorithm for processing biomedical signals using a personal computer

The aim was to demonstrate the possibility of using personal computer PC-DOS (or generally MS-DOS) for real-time (or quasi real-time) biomedical signal processing by adding a simple A/D conversion card and the mathematical coprocessor XXX87. We have realized an assembly written fast Fourier transform (FFT) routine derived from a radix-4 algorithm, which is autogenerated, i.e. an algorithm modified by another algorithm running off-line according to the number of FFT points. The program is implemented as a subroutine to be called upon by high-level language in different procedures. This approach reduces the computational time, which is particularly useful when many Fourier transforms on different data arrays are required. Reported here are two different applications of the routine as applied to the spectral analysis of Doppler ultrasound velocimetry and surface electromyography.

Detection of moving flow separation in pulsatile flow and the degree of stenosis by power of Doppler shift signals

Power ratios were derived from the principle of ultrasonic Doppler velocimetry to determine a ratio of the volume of vortices to the total vessel volume under the ultrasonic beam. This ratio also equals the ratio of the cross-sectional area of vortices to the vessel lumen. In vitro pulsatile flow experiments were performed in a Reynolds number range of 1,230-4,320 with axisymmetric constrictions with area reductions of 55%, 65%, 70%, and 85% to model carotid stenosis. Flow separation downstream from the model stenoses was detected, and the power ratio fluctuated when vortices with the forward- and reverse-flow velocity components passed by the measurement position. The power ratio estimated the degree of stenosis within 10% of error. Ensemble average of the power ratio was computed to obtain the statistically averaged separated flow region. The moving flow reattachment point was revealed downstream from the 85% stenosis at a Reynolds number of 900.

Pulsed and color Doppler analysis of normal carotid bifurcation flow dynamics using an in-vitro model

An anatomically accurate model of the human carotid bifurcation was studied by using color Doppler flow mapping and pulsed Doppler signal analysis. In concordance with dye injection studies, a zone of flow separation was clearly demonstrated at the origin of the internal carotid artery opposite the flow divider. This zone of flow reversal was detected when the ratio of external to common carotid flow rates was greater than 0.27 and became progressively larger as the ratio increased, finally evolving into a zone of heterogenous flow at a ratio greater than 0.5. The authors conclude that the zone of flow reversal seen in the human carotid artery is a geometric property of the bifurcation and that its size depends on the relative flow between both branches. Doppler color imaging can be used to noninvasively map out its extent.

Measurement of arterial blood flow by Doppler ultrasound

A review is given of quantitative techniques and clinical applications of arterial Doppler ultrasound. The currently available Doppler equipment of stand-alone continuous wave and pulsed wave units, duplex systems and colour flow systems is briefly described. Doppler ultrasound can be divided into procedures concerned with waveform analysis, volume flow measurement and more recently colour flow imaging. Arterial Doppler waveform analysis is considered for a number of areas including carotid, lower limb, renal and renal transplant, obstetrics, adult cerebral, neonatal cerebral, and tumour studies. Using a duplex scanner volume flow in arteries can be measured from estimates of vessel cross sectional area, mean Doppler frequency and beam-vessel angle. The errors associated with each of these measurements is discussed, and reports of experimentally determined in vivo accuracy of volume flow measurements made using this technique are considered. Other volume flow measurement techniques including the promising attenuation compensation method are also explored.

Real-time system for robust spectral parameter estimation in Doppler signal analysis

In assessing the level of stenosis in extracranial Doppler analysis, spectral analysis has until now been used qualitatively, for the most part. Owing to the many variables affecting the measurements (mainly noise level and instrument setting made subjectively by the operator), the reliability of the inferences on the degree of stenosis is not clearly definable. Under such conditions the need arises for algorithms and systems that can estimate spectral parameters with a higher degree of accuracy, to verify whether reliable inferences can indeed by made or if this technique is only a qualitative one. In the paper a real-time spectral analysis system is described. The system relies on a new spectral estimation algorithm which gives estimates with good robustness with respect to noise. Moreover, a clear measurement procedure which eliminates the many subjective factors affecting the estimates has also been proposed and used. The system has been evaluated with simulated signals and in clinical trials and has shown better performance than the commonly used commercial analysers.

Determination of volume of vortices in poststenotic pulsatile flow by ultrasonic Doppler power ratio and spectrum analysis

Based on the principle of ultrasonic Doppler flowmetry, a power ratio was derived from independent forward and reverse flow Doppler shift signals to measure a ratio of the volume of vortices to the total vessel volume in poststenotic separated flow. The ratio was also proportional to the ratio of the cross-sectional areas of vortices to the vessel lumen. In vitro pulsatile flow experiments were performed to test the methodology and to study flow separation and vortex shedding downstream from model stenoses. The averaged flow cross-sectional area ratio linearly correlated (r = 0.91) with the actual area reduction of the stenosis.

Range-gated pulsed Doppler power frequency spectrum analysis for the diagnosis of carotid arterial occlusive disease

Carotid arterial disease was investigated with a Duplex Scanner using Power Frequency Spectrum Analysis. Sixty-one carotid systems were evaluated noninvasively and angiographically, while 20 controls were examined and assumed to be normal. Peak frequency and 50% frequency bandwidth, a quantitative index of spectral broadening, were correlated with the percentage of stenosis. Peak frequency predicted the presence or absence of hemodynamically significant stenoses (greater than or equal to 50% diameter reduction) with 90.8% accuracy, while 50% frequency bandwidth correctly identified similar lesions with 93.2% accuracy (p = NS). Also, with the latter results, carotid systems were grouped into less than 25%, 25% to 49%, and greater than or equal to 50% stenosis categories with an 86.4% accuracy. Similar statistical evaluation was attempted for peak frequency results. It was not possible to separate hemodynamically insignificant lesions (less than 50% diameter reduction) into distinct groups because of the overlap of results among those arteries with less than 50% stenosis. Finally, all eight occluded internal carotid arteries were identified with combined Doppler/imaging analysis. However, with imaging alone, only 5 of 8 (63%) occluded arteries were correctly identified.

Continuous-wave versus range-gated pulsed Doppler power frequency spectrum analysis in the detection of carotid arterial occlusive disease

Two types of ultrasonic Doppler velocity metering devices currently used in the detection of extracranial carotid artery disease, the continuous-wave (CW) and the range-gated pulsed (RP) Doppler systems, were compared in the present study. Power frequency spectrum analysis (PFSA) was performed on 130 carotid arterial bifurcations with a CW Doppler and 81 carotid arteries with an RP Doppler system. All results were compared with angiographic findings. The frequency bandwidth at 50% peak power (f50%), a quantitative index for defining spectral broadening, detected stenoses equal to or greater than 50% diameter reduction with 93% sensitivity, 92% specificity, and 92% accuracy with the CW system. With the RP Doppler, the same degree of stenosis was identified with 94% sensitivity, 93% specificity, and 93% accuracy. Compared with angiographic classification into 0-24%, 25-49%, and 50-99% diameter reduction categories, CW Doppler PFSA and an 85% overall accuracy, and the RP Doppler overall accuracy was 86%. CW Doppler also correctly identified 15 of 16 internal carotid artery (ICA) occlusions; 8 of 8 ICA occlusions were correctly identified with the RP Doppler. Thus, both techniques detected carotid artery disease with comparable results. For research and ease of operation, an RP Doppler system with a variable sampling volume appears to be most desirable. However, a standard CW system is superior if utility and cost-effectiveness are of prime importance.

Reproducibility of carotid artery Doppler frequency measurements

Doppler sonography of 29 extracranial carotid arteries was performed twice within three days or less. Angiography revealed stenoses with from 10 to 95% diameter reduction in 25 internal carotid arteries whereas four vessels were found to be normal. The systolic peak frequency of the internal carotid artery read from the Doppler spectrum (n = 29) could be reproduced very well. This was shown by a linear regression nearest the line of identical values with a coefficient of correlation r = 0.97 (p less than 0.001). Other values derived from the spectral analysis of the Doppler shift signal were not so well reproduced including the peak frequency ratio (systolic peak frequency of the internal carotid artery/systolic peak of the common carotid artery) (n = 22; r = 0.81; p less than 0.001). The mean frequencies read from the zero-crossing detector recordings (n = 20) could not be reproduced as demonstrated by a linear regression far away from the line of identical measurements with a coefficient of correlation r = 0.43 (p less than 0.05).

Influence of contralateral obstructions on Doppler-frequency spectral analysis of ipsilateral stenoses of the carotid arteries

Contralateral obstructions have been suggested to be responsible for inaccuracy in Doppler sonographic diagnosis and estimation of severity of internal carotid artery stenoses. Therefore correlations of the systolic peak frequency of the internal carotid artery (Doppler-Frequency-Spectrum-Analysis) and severity of stenoses found by angiography have been compared in subgroups with (n = 36) and without (n = 48) additional contralateral obstructions. The linear regressions in both subgroups were found to be very similar (Y = 0.0098X - 12.4 and Y = 0.0099X - 14.0), the coefficients of correlation identical (r = 0.83). In 26 patients, the systolic peak frequency of the internal carotid artery did not demonstrate a significant change due to operation of the contralateral vascular lesion; thus indicating that contralateral obstructions do not influence CW-Doppler sonographic findings of an ipsilateral vessel wall lesion concerning diagnostic accuracy and non-invasive estimation of the severity of stenoses.

Non-invasive detection of carotid bifurcation disease by continuous-wave Doppler with spectral analysis

The diagnostic accuracy of 5 MHz continuous-wave (C-W) Doppler with spectral analysis for detecting carotid bifurcation disease was evaluated. In a first phase of the study, normal confidence intervals for peak frequency data and spectrum width were established and Doppler spectrum abnormalities were separated into three types (spectral broadening with preserved window, spectral broadening without window, sequence of abnormalities including inverted Doppler spectrum). In a second phase the diagnostic accuracy of these variables was tested on 86 carotid arteries as independently compared with angiography. Whereas both peak frequency data and spectrum distribution findings were invariably abnormal in stenoses greater than or equal to 60%, spectrum distribution findings were more accurate in 30-59% stenosis. Using spectrum distribution criteria, sensitivity and specificity for stenosis greater than or equal to 30% were 94.6% and 90.7% respectively. All variables were insensitive for stenoses less than 30% lumen diameter reduction.

The interpretation of continuous wave ultrasonic Doppler blood velocity signals viewed as a problem in pattern recognition

The process of inferring the state of a patient's circulation from ultrasonic Doppler waveforms may be regarded as a problem in pattern recognition. In this article, each stage in the process, as it might be applied to the analysis of Doppler signals, is examined in turn and it is shown how an automatic on-line system for arterial assessment may one day be practical using such principles.

Limitations in the accuracy of peak frequency measurements in the diagnosis of carotid disease

Peak Doppler frequency is an index of the severity of carotid stenosis. Variability in this measurement is examined through in vitro and clinical studies. In vitro studies, using a carotid flow model, show that observers locate a stenosis and interpret the peak frequency differently, and each observer uses a different probe-vessel angle. Clinical studies support these findings. Comparison of 304 carotid Doppler studies with arteriograms demonstrates 90% overall clinical accuracy. Each observer has a consistent range of peak frequency measurements, yet the description of a discrete percent stenosis is limited by observer variability.

Pulsed Doppler techniques for measuring instantaneous maximum and mean flow velocities in carotid arteries

A 5 MHz pulsed Doppler instrument measuring instantaneous maximum and mean flow velocities is presented. The maximum velocity estimator is based on the principle of frequency variable filtering controlled by a feedback loop to follow the velocity spectrum envelope. Findings by Doppler and bilateral selective carotid arteriography in 216 patients were compared. Extracranial carotid stenoses were identified by the finding of a vessel segment with locally increased flow velocity. Peak Velocity Ratio (PVR) was calculated from maximum velocities measured in the stenosis and in more distal Internal Carotid Artery (ICA) segments. Using PVR, ICA stenoses greater than 20% were detected with sensitivity 96%, specificity 94%, positive accuracy 94% and negative accuracy 96%. Total ICA occlusions were identified with sensitivity 97% and specificity 99%.

Pulsed Doppler spectral analysis of bounded fluid jets

To test the hypothesis that pulsed Doppler ultrasound spectral properties of bounded fluid jets related to orifice size, in vitro examinations were performed using a hydraulic simulator (standpipe, compliant receiving chamber, and variable round orifice). Orifices of 17.3, 7.9, 4.5 and 2.0 mm2 area resulted in flows of 4.0, 1.7, 1.1 and 0.3 L/min respectively. We interrogated the fluid jet at midchamber (MC) and at the chamber will impact site (IS), and the resulting Doppler shifts were displayed on a spectrum analyzer. For each orifice size, 36 independent observations were made at each interrogation site. At site MC, a characteristic low break frequency (K) was observed in an otherwise flat spectrum. The spectrum from site IS were peaked, with the peak described by center frequency (P), peak amplitude (A), and frequency at which the peak merged with the background (X). As orifice size increased from smallest to largest, monotonic changes in K (1020-440 Hz), P(330-750 Hz), A (8-28 db), and X (840-2660 Hz) were observed. Standard deviation about each mean ranged from 9 to 28%. Orifice shape, receiving chamber viscosity, and standpipe pressure significantly influenced the spectra. These data suggest that quantification of jet flow is possible in vitro using spectral analysis and pulsed Doppler ultrasound.

Spectral analysis of Doppler flow velocity signals: assessment of objectives, methods, and interpretation

The objective of this paper is to review the theoretical basis and clinical application of electrical impedance plethysmography in the noninvasive evaluation of peripheral arterial and venous disease. Theoretical, experimental and clinical studies have not demonstrated a direct relationship between electrical impedance changes and limb volume changes. Potential sources of error have also been identified. This has led to the development of clinical tests based on impedance plethysmography for the detection of peripheral arterial disease, venous insufficiency and venous outflow obstruction. Impedance plethysmography, using the method of venous occlusion, is presently the most commonly employed noninvasive method for the detection of deep venous thrombosis.

Ultrasonic doppler spectral broadening in the diagnosis of internal carotid artery stenosis

In a clinical study, 78 carotid bifurcations were examined by pulsed Doppler ultrasonography and X-ray arteriography. Maximum Doppler frequencies greater than or equal to 3 kHz diagnosed stenoses of greater than or equal to 25% diameter reduction with a sensitivity of 73% and specificity of 79%. A quantitative analysis of the degree of spectral broadening was obtained from the ratio of the maximum to mean frequency at peak systole and improved the sensitivity and specificity to 90% and 98% respectively. All vessels greater than 40% stenosis were correctly classified as stenosed and all less than 20% stenosis were classified as normal.

An in vitro model and its application for the study of carotid Doppler spectral broadening

A new in vitro model has been developed for studying the changes in the ultrasound Doppler spectrum that occur in the region of a stenosis. Pulsatile flow in rigid acrylic tubes was produced by means of a modified hemodialysis pump. The Doppler spectral waveforms were measured using a continuous wave Doppler system, a probe of a known field pattern, a real-time high resolution frequency analyzer, and a video display and recording system. The flow velocity waveforms were found to be nearly identical to those seen in the human carotid. Measurements were made to determine the critical stenosis and the results are similar to those reported from in vivo studies. In a preliminary study, the extent of spectral broadening was found to be dependent on the recording site in relation to the stenosis, the severity of the stenosis, and the flow rate. Using qualitative methods it was not possible to determine either the influence of the shape of the stenosis or the phase of the cardiac cycle on spectral broadening.