Ultrasound in Med. & Biol., Vol. -, No. -, pp. 1–12, 2014Copyright � 2014 World Federation for Ultrasound in Medicine & Biology

Printed in the USA. All rights reserved0301-5629/$ - see front matter

/j.ultrasmedbio.2014.01.021

http://dx.doi.org/10.1016

d Original Contribution

COMBINED VECTOR VELOCITYAND SPECTRAL DOPPLER IMAGING FORIMPROVED IMAGINGOFCOMPLEXBLOODFLOW IN THECAROTIDARTERIES

INGVILD KINN EKROLL,* TORBJØRN DAHL,y HANS TORP,* and LASSE LØVSTAKKEN**Medical Imaging Laboratory and Department of Circulation and Medical Imaging, Norwegian University of Science and

Technology (NTNU), Trondheim, Norway; and ySt. Olav’s University Hospital, Trondheim, Norway

(Received 10 March 2013; revised 21 January 2014; in final form 27 January 2014)

ADepartMedisiE-mail

Abstract—Color flow imaging and pulsed wave (PW) Doppler are important diagnostic tools in the examination ofpatients with carotid artery disease. However, measurement of the true peak systolic velocity is dependent on sam-ple volume placement and the operator’s ability to provide an educated guess of the flow direction. Using planewave transmissions and a duplex imaging scheme, we present an all-in-one modality that provides both vector ve-locity and spectral Doppler imaging from one acquisition, in addition to separate B-mode images of sufficient qual-ity. The vector Doppler information was used to provide automatically calibrated (angle-corrected) PW Dopplerspectra at every image point. It was demonstrated that the combined information can be used to generate spatialmaps of the peak systolic velocity, highlighting regions of high velocity and the extent of the stenotic region, whichcould be used to automate work flow as well as improve the accuracy of measurement of true peak systolic velocity.The modality was tested in a small group (N5 12) of patients with carotid artery disease. PW Doppler, vector ve-locity and B-mode images could successfully be obtained from a single recording for all patients with a body massindex ranging from 21 to 31 and a carotid depth ranging from 16 to 28 mm. (E-mail: ingvild.k.ekroll@ntnu.no oringvild.k.ekroll@ntnu.no) � 2014 World Federation for Ultrasound in Medicine & Biology.

KeyWords:Blood flow imaging, Carotid artery stenosis, Vector Doppler imaging, Pulsedwave Doppler calibration,Plane wave imaging.

INTRODUCTION

Imaging of the carotid arteries is of special interest as theyare responsible for 20% of all strokes of thrombo-embolicorigin (Bamford et al. 1991). Situated at shallow depths,they are also well suited for ultrasound imaging. Throughlocal manifestations of cardiovascular disease, the carotidarteries may not only provide information on developingatherosclerosis in that particular region, but also predictcoronary artery disease events (Inaba et al. 2012). Never-theless, in clinical practice, the use of ultrasound in rela-tion to carotid artery disease is mostly limited to gradingof plaque stenosis. Through the use of B-mode and colorflow imaging (CFI) for navigation and detection, the peaksystolic velocity is measured from the pulsed wave (PW)Doppler spectrogram, and the value is used to assess thedegree of stenosis.

ddress correspondence to: Ingvild Kinn Ekroll, MI Lab andment of Circulation and Medical Imaging, NTNU, PB 8905,nsk Tekninsk Forskningssenter, 7491 Trondheim, Norway.: ingvild.k.ekroll@ntnu.no or ingvild.k.ekroll@ntnu.no

1

Doppler ultrasound is angle dependent, as only thevelocity component parallel to the beam direction ismeasured. This means that although the flow patterns inthe carotid arteries are complex (Reneman et al. 1985),including, for instance, presence of helical flow (Harloffet al. 2009) and reversed flow (Savva et al. 2010), conven-tional Doppler imaging assesses only how fast blood isflowing in the direction of the ultrasound beam—in thefollowing termed the axial direction. The quantitativeuse of CFI, where the mean axial velocity in a region ofinterest is displayed, is therefore limited, and in conven-tional ultrasound systems, the true velocity magnitudecan only be estimated in single sample volumes usingPW Doppler and manual angle correction. Based on themaximum velocity in systole, the degree of stenosis isestimated and is further used as a selection criterion forcarotid endarterectomy (Goldstein et al. 2011). This esti-mate is highly dependent on the placement of the samplevolume and the operator’s ability to perform correct anglecorrection of the velocity spectrum. In complex geome-tries or stenosed regions where the blood path lines differfrom the vessel course, operator-dependent errors may

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result in large deviations from the actual peak velocity(Lui et al. 2005; von Reutern et al. 2012), hamperingpatient follow-up and multicenter studies.

Vector velocity imaging, where the 2-D or 3-D ve-locity magnitude and direction are estimated, has beeninvestigated for several years (Bohs et al. 2000;Dunmire et al. 2000; Jensen and Munk 1998; Yeunget al. 1998). Such a modality could provide a morequantitative blood flow imaging technique, as the angledependency of color Doppler is overcome. However,until recently, the robustness, frame rate and/or size ofthe image region have been limited, and the method hasnot yet reached clinical practice.

As a consequence of recent technology advances,high frame rates can be maintained while increasing theinformation content per ultrasound image. Through theuse of plane wave transmissions, each pulse covers abroad spatial region, and parallel beamforming (vonRamm et al. 1991) may be used to form multiple imagelines or even a full image for every transmitted ultrasoundpulse. Through the use of these techniques, vector veloc-ity imaging may be realized with improved robustnessand temporal resolution (Ekroll et al. 2013; Hansenet al. 2009; Tanter et al. 2002). Available 2-D velocityinformation could, for instance, be used to locate flowpatterns known to enhance plaque formation (Koskinaset al. 2009) or to assess wall shear stress. In particular,it could be used to improve accuracy and reproducibilityin grading of stenoses by providing automatic estimatesof flow direction, reducing inter-observer variability inmeasurements of peak systolic velocity (Hoskins 1997;Steel et al. 2003).

Given the opportunities enabled by plane waveimaging techniques, we developed and evaluated anall-in-one modality for imaging of blood flow in thecarotid arteries. A duplex acquisition scheme was im-plemented, providing 2-D vector velocity blood flowimages in addition to separate B-mode images of suffi-cient quality. Further, PW Doppler spectra may begenerated from arbitrary image points based on thesame data (Bercoff et al. 2011). Finally, automationof the clinical protocol with respect to measurementsof peak systolic velocity is looked into, using vectorDoppler to automatically calibrate the velocity spectrafor all image points simultaneously.

Because of the limitations in penetration comparedwith transmission of focused ultrasound beams, it is notclear whether the plane wave techniques will workrobustly in a diverse patient group. Both carotid depthand surrounding tissue calcifications will vary from pa-tient to patient, potentially providing an insufficientsignal-to-noise ratio (SNR). To investigate whetherplane wave imaging presents disadvantages that makeimaging of patients with carotid artery disease unfeasi-

ble in practice, the modality was applied in a small(N 5 12) patient population.

METHODS

Plane wave imaging (PWI), in which unfocused ul-trasound beams are transmitted instead of focused beams,may be used to increase the acquisition rate in ultrasoundimaging. As illustrated in Figure 1, a broader region of in-terest is insonified for every pulse transmission, enablingseveral parallel beams to be generated on receive. Theconcept has previously been used both in elastography(Tanter et al. 2002) and in blood flow imaging (Bercoffet al. 2011; Hansen et al. 2009; Udesen et al. 2008).The main disadvantage of plane wave imaging is thereduced penetration caused by lack of focusing, as wellas decreased contrast and lateral resolution. Althoughcoherent compounding, whereby beamformed imagesfrom several plane wave transmit angles are coherentlycombined, has been reported to restore these properties,a relatively large number of transmissions are needed toprovide an image quality comparable to the optimalmultifocus image (Montaldo et al. 2009). In Dopplerimaging, however, the dynamic range is lower than inB-mode, and the side lobe level is less critical. Thus,planewave imaging might be used directly (without com-pounding), to increase the frame rate and ensembles incolor flow imaging. Further, these benefits may be usedto improve robustness of existing vector Dopplertechniques. This was investigated in a previous work(Ekroll et al. 2013). Simulations of complex flow in thecarotid artery bifurcation were performed and used toassess the robustness of plane wave vector Doppler imag-ing. The findings were used in the design of the imagingschemes applied in the study described here.

Imaging schemesA duplex acquisition scheme was chosen (Fig. 2),

where parameters for Doppler and B-mode imagingcould be optimized separately. Patients were imagedusing a research system (Sonix MDP, Ultrasonix, Rich-mond, BC, Canada), where the all-in-one acquisitionwas implemented (beamforming and display were doneoffline), and a high-end ultrasound system (Vivid E9,GE Vingmed, Horten, Norway). The systems were equip-ped with linear probes of center frequencies 5 and 6MHz,respectively. One image per transmitted pulse could beacquired using the research system, whereas the high-end system had more limited parallel receive capability(16 parallel receive lines in our setup). The research sys-tem was, however, limited to acquiring only one data setper patient because of long data transfer times and thelimited memory of the data acquisition system. The

Fig. 1. Difference between focused transmission and planewave transmission.With the use of focused transmission, onlyone receive beam is normally generated per transmit beam, whereas multiple receive beams may be generated in the plane

wave case.

Vector velocity 1 spectral Doppler in blood flow imaging d I. K. EKROLL et al. 3

high-end system was therefore included to enable a com-parison between regular and plane wave imaging.

Research system schemesTwo imaging schemes were implemented in the

Sonix MDP research ultrasound system: one using planewave transmissions for both B-mode and Doppler (planewave scheme) and one using focused B-mode interleavedwith plane wave Doppler (combined scheme). The acqui-sition setups were implemented using the development

Fig. 2. Imaging scheme, from acquisition of B-mode and Doppthe end of the processing chain, B-mode and vector Doppler im

Doppler spectra in every image point. IQ 5 in-

toolkit Texo, which allows control of the transmit delaysfor individual elements and thereby allows custom(including plane wave) transmit sequences to be created.A Sonix DAQ (Ultrasonix) was used for channel dataacquisition.

The plane wave scheme consisted of a plane wavecoherent compounding sequence for B-mode and a vectorDoppler sequence using plane waves angled at q5610�.The angle switched between each transmission to avoidartifacts in the vector Doppler image caused by a time

ler channel data through the different processing steps. Atages are available, in addition to calibrated pulsed wavephase quadrature, VD 5 vector Doppler.

4 Ultrasound in Medicine and Biology Volume -, Number -, 2014

lag between the (axial) velocity estimates from the twoinsonation angles. Forty-one angles in the interval –7.4�

# a# 7.4� were used to construct the B-mode image. Af-ter coherent compounding, this selection of transmit an-gles provided a lateral resolution in the imagecomparable to that obtained by using focused transmitbeams with an F-number (F#) of 4 (Montaldo et al.2009). The plane wave scheme resulted in 41 1 23 50transmit events per duplex frame, yielding a frame rateof 63 Hz with the given imaging pulse repetition fre-quencies (PRFs).

The combined scheme consisted of focused B-modeimaging with four interleave groups, each of 32 imagelines using an F# 5 2, interrupted by 2 3 50 plane waveDoppler transmissions using q 5 610�. In this scheme,the frame rate of vector Doppler was 66 Hz, whereas theB-mode frame rate was 16 Hz, because of the interleavedacquisition. In both schemes, the channel radiofrequencydata were IQ-demodulated and low-pass filtered to reducethe noise bandwidth, and beamformed using a Hammingwindow over the active receive aperture.

The transmit schemes applied were within the guide-lines from the U.S. Food and Drug Administration (FDA),and acoustical and thermalmeasurements performed usingmaximal PRF for a given setup are summarized in Table 1.Whereas the limits for mechanical index and Ispta are wellbelow FDA limits, the maximum PRF applicable for acertain pulse shape is limited by surface heating.

HIGH-END ALTERNATIVE SCHEMES

The Vivid E9 system was set up (following FDAguidelines) to include imaging schemes using planewave transmission and 16 parallel receive beams. Thelimited number of parallel receive beams meant thatseveral transmissions were needed to cover a full imagesector. An ensemble length of 12 was therefore used inthe color flow applications, as larger ensembles wouldsignificantly lower the frame rate and thus limit one of

Table 1. Safety measurements, research system

Setting 1 2 3 FDA limit

Pulse center frequency, f0 (MHz) 5 6.7 6.7Number of elements 128 128 26Cycles at f0 2.5 1.5 1Focal depth (mm) N N 19Pulse repetition frequency (kHz) 12 15 12Drive voltage (Vpp) 56 68 83AcousticsMechanical index 0.4 0.3 0.8 1.9Ispta (mW/cm2) 124 63 451 720

Temperature (�C)DTair 23 18.3 1.8 27DTphantom 6.5 9.1 0.8 10

FDA 5 Food and Drug Administration.

the main advantages of plane wave imaging. Imagingwas performed using (i) conventional B-mode and con-ventional CFI; (ii) conventional B-mode and planewave CFI; and (iii) plane wave B-mode and plane waveCFI. The alternative imaging schemes included in theVivid E9 enabled direct comparison of different acquisi-tion schemes from one imaging plane in the same patient.

Vector Doppler estimationVector Doppler imaging based on autocorrelation

was chosen to achieve the 2-D vector velocity fieldbecause of its low computational demands and becauseautocorrelation is a robust and well-established estima-tion technique in conventional color flow imaging. Thevelocity vector components were obtained in the wholeimage region simultaneously by combining overlappingDoppler measurements from two directions as given inthe equation (Kripfgans et al. 2006)

vx 52c

4f0 sinðqÞ�bf 12bf 2�

vz 52c

4f0 cosðqÞ�bf 21bf 1�

(1)

where the separation angle between the two transmit/receive directions is 2q, c is the speed of sound, f0 is thepulse center frequency and bf 1 and bf 2 are the Dopplerfrequencies estimated from the IQ data based on thetemporal autocorrelation function with lag 1:ð:Rð1ÞÞ=2p3PRF (Namekawa et al. 1983; Kasai et al.1985). Spatial averaging of the autocorrelation estimateover an area of approximately 1 3 1 mm was used tofurther improve the estimates. The theoretical spatialresolution in the image before averaging wasapproximately 0.4 3 0.4 mm, but a somewhat poorerspatial resolution was accepted to achieve robustvelocity estimates (Torp et al. 1994).

Blood pool segmentation was done based on thesignal power after wall filtering, velocity and the B-mode image intensity. Through the use of ensembles of50, a 3.-order polynomial regression filter was used, giv-ing a –3-dB cutoff at 1.5 cm/s, approximately 5% of theNyquist velocity (nnyq). Similar stop-band characteristicsachieved for smaller ensembles of length 16 gave a –3-dBcutoff at 3.3 cm/s, approximately 10% of nnyq.

Based on user preference, color flow images orvelocity magnitude could be shown in supplement tothe vector velocity arrows. Examples of both visualiza-tion techniques are provided.

Spectral Doppler estimationAcquiring Doppler ensembles of length 50 enabled

velocity spectra to be calculated using the conventionalperiodogram approach. Because of the simultaneous

Vector velocity 1 spectral Doppler in blood flow imaging d I. K. EKROLL et al. 5

acquisition of large ensembles in every image point,spatial spectral profiles indicating the variation of thevelocity within a region of interest at a certain timemay also be produced (Bercoff et al. 2011). The periodo-gram has the following estimate of the power at theDoppler frequency u:

bPðuÞ5 1

RB

XR21

r5 0

XB21

b5 0

�����XN21

n5 0

wðnÞxr;bðnÞe2iun

�����2

: (2)

xr,b(n) is the IQ (in-phase quadrature) signal from a(range, beam) pair in slow time, w(n) is a smooth windowfunction, R is the number of range samples, B is the num-ber of receive beams and N is the ensemble length. In thiswork, windowing was performed using Hammingweights. The spectral estimates were averaged using anumber of receive beams and range samples correspond-ing to a 1 3 1-mm region of interest.

An adaptive spectral estimation technique was alsoused.Advantages of adaptive techniques include increasedvelocity resolution for equal observation time andincreased suppression of noise. Additionally, the velocityresolution can be kept comparable to that of the conven-tional approach for shorter observation times (Ekrollet al. 2012; Gran et al. 2009). This latter advantage canbe used to produce multiple velocity spectra per Dopplerensemble, by subdividing ensembles in shorter observ-ation windows and estimating, for example, twice thenumber of time points as provided by the conventionalapproach.

To estimate the power at the Doppler frequency u,the adaptive Capon (1969) technique uses

bPCapon uð Þ5 1

aH uð ÞbR21

x a uð Þ; (3)

where a(u)5 [1eiu. ei(N – 1)u]T, and bRx is an estimate ofthe signal covariance matrix, here produced from an aver-aging area of 1 3 1 mm.

Spectral Doppler calibrationPulsed wave Doppler spectrograms were calibrated

automatically by shifting the baseline and correctingthe velocity scale according to the angle estimated fromvector Doppler at the specific location. Shift of the base-line was performed by selecting the one resulting inlowest standard deviation as calculated from thebaseline-shifted spectrum. A simple algorithm was cho-sen to trace the spectral envelope, based on the meanand standard deviation of the baseline-shifted frequencypower spectrum above a certain threshold value(–25 dB). That is,

ftrace 5 fmean63stdðf Þ; (4)

where ftrace is the trace of the spectral envelope, fmean andstd (f) are the mean frequency and standard deviation ofthe baseline-shifted frequency spectrum respectivelyand the sign (6) is dependent on the direction of the base-line shift.

Finally, the peak systolic velocity was found auto-matically from the traces using an algorithm finding themaximum (or minimum) value after the sharpest rise invelocity.

Patient populationTwelve patients were recruited from the outpatient

vascular clinic (median age 5 61, min/max 5 42/77;7 men, 5 women), and imaging using the new ultrasoundmodality was performed after the regular patient exami-nation. The protocol was approved by the NorwegianRegional Committee for Medical and Health ResearchEthics, and written informed consent was obtained fromall participants.

RESULTS

Patient materialIn Figure 3 are duplex images acquired during

patient examination using a high-end ultrasound system(Vivid E9, GE Vingmed Ultrasound, Horten, Norway).The three previously described setups are compared inthe figure. A loss in contrast and resolution can be seenin the downmost B-mode image, where plane wave trans-mission was used. However, plane wave imaging resultedin a marked increase in frame rate, illustrated by the flowvelocity curves in the right part of Figure 3, providingnew opportunities to follow rapid flow changes.

As described in the following, successful vectorvelocity images at a frame rate of 63–66 fps could beobtained using the research scanner in patients with abody mass index (BMI) ranging from 21 to 31 (carotiddepth ranging from 16 to 28 mm). The BMI and carotiddepth ranges indicate that a variety of different patientsmay be imaged using plane wave transmissions.

Flow visualization and quantificationIn Figure 4 are color flow images and vector veloc-

ity estimates obtained using conventional and large-ensemble length imaging. The figure illustrates theincreased information content obtained by plane wavevector Doppler imaging with large ensemble lengths.As illustrated in the color flow images from conven-tional (top panel) and large (middle panel) ensembles,clear benefits were obtained in the large-ensemblecase. In diastole, the Doppler shifts from both transmis-sion angles were very low because of near-transverseand partly out-of-plane flow. However, because of theimproved wall filter properties, there were no dropouts

Fig. 3. Images obtained with the Vivid E9 scanner. Top: Conventional carotid application, using focused B-mode imaging(two transmit foci) and focused color flow imaging (CFI). Middle: Single-focus B-mode and plane wave color flow im-ages. Bottom: Plane wave B-mode and plane wave color flow images. A marked decrease in resolution and contrast is

observed in the plane wave B-mode image. PWE 5 plane wave emission.

6 Ultrasound in Medicine and Biology Volume -, Number -, 2014

in the large-ensemble case. On the other hand, when anensemble length of 16 was used, the blood pool area wasreduced, containing dropouts both in the middle of thevessel and in the near-wall region.

As further seen in the lower panel of Figure 4, theaddition of vector velocity information to the color flowimage resulted in a better understanding of the bloodflow in the imaging plane. Even though this patient hadno visible plaques, disturbed (non-laminar) flow wasclearly present in early diastole.

Figure 5 provides another example of complexflow that could not be perceived using regular color

flow imaging. This patient had stenoses in both theinternal carotid artery and external carotid artery.With vector Doppler, complex secondary flow couldbe observed after the external carotid artery stenosis.The abrupt change from a large pre-stenotic velocitymagnitude to the small mid-stenotic velocity magnitudewas also clearly seen from the vector velocity informa-tion, indicating that the flow direction in the mid-stenotic region was in the out-of-plane direction.

In Figure 6 are images obtained from the combinedacquisition, using velocity magnitude instead of the reg-ular color flow image to indicate blood flow. As seen in

Fig. 4. Increased velocity range and improved visualization ob-tained by using large ensembles and vector Doppler imaging.The dot in the velocity trace represents the time of the depicted

images.

Fig. 5. Vector Doppler image of the carotid bifurcation of a pa-tient with stenoses in both the internal carotid artery (ICA) andexternal carotid artery (ECA). After the stenosed region in theECA, complex flow is present. The blood seems to flow out ofthe plane in the stenotic region, as the apparent velocities are low.

Fig. 6. Vector Doppler estimates overlying the velocity magni-tude from peak systole (top) and diastole (bottom) in one pa-tient. The yellow dashed lines and black square indicate thespatial position of the spectral profiles provided in Figure 7.The B-mode image in the background is based on focused trans-mit beams in four interleave groups, whereas plane waves were

used to acquire Doppler data.

Vector velocity 1 spectral Doppler in blood flow imaging d I. K. EKROLL et al. 7

the bottom panel, slow and circulatory flow was presentin diastole, following a small stenosis. A trace of thevelocity magnitude and two spatial spectral profiles areprovided in Figure 7: one parabolic from systole, andone indicating directional change with depth from dias-tole (dashed lines in Fig. 6).

Temporal resolutionThe flow velocity curves in Figure 3 illustrated that

compared with focused flow imaging, plane wave trans-mission enables near-instantaneous images of the flowin a large image region. With the research system, high

Fig. 7. Quantitative information for the patient described inFigure 6. A trace of the velocity magnitude and two spatial spec-tral profiles in systole and diastole, indicated by the blue dots in

the velocity trace, are seen.

Fig. 8. Blood flow in a carotid artery bifurcation during systole(top) and diastole (bottom). In diastole, blood flow was reversedand blood flowed from the external to the internal carotid artery.

Velocity magnitude

8 Ultrasound in Medicine and Biology Volume -, Number -, 2014

frame rates may be achieved with an increased velocityrange (because of large ensemble lengths) and also robustvector Doppler information.

The high temporal resolution obtained in the newmodality is illustrated in Figures 8 and 9. From the vectorDoppler information, it was observed that reversed flowwas present in a small part of the cardiac cycle, andthat during this time window, the blood flowed from theexternal to the internal carotid artery. As more clearlyseen in Figure 9, the flow in the external carotid arteryexperienced a near-150� turn in diastole (region of inter-est indicated by a small rectangle in Fig. 8).

0

0.1

0.2

0.3

0.4

0.4 0.5 0.6 0.7 0.8

200

250

300Velocity direction

0.5

150

[s]

[m/s

][d

eg]

Fig. 9. Velocity magnitude and velocity direction curvesthroughout the cardiac cycle for the indicated region of interestin the patient recording illustrated in Figure 8. The blue dotsrepresent the time points for the frames depicted in Figure 8.

Automatic PW Doppler calibration and tracingThe top panel of Figure 10 is the adaptive spectrogram

from the indicated rectangular region in Figure 8. Based onthe vector Doppler estimates, the spectrogram could beangle corrected automatically. Also in Figure 10 are the cali-brated traces of mean and maximum velocity. After anglecorrection, the spectrogram gave an estimate of the peaksystolic velocity at approximately 0.6 m/s.

As seen in the middle panel of Figure 10, regularspectral estimation could also be used to generatecalibrated velocity spectra, althoughwith reduced contrast(lower SNR). Thewider spectral main lobe, clearly seen inthe bottom panel of Figure 10, also leads to highermaximum velocity estimates, here at approximately0.85 m/s.

In Figure 11 we compare a conventional PWDoppler recording (left) with regular spectral Dopplerfrom the investigated modality (right). The conventional

Fig. 10. Pulsed wave (PW) spectra from –10� transmit/receivecalibrated with angle from vector Doppler in peak systole (seeFig. 9). The black line is the autocorrelation estimate, whereasthe white, solid line is a trace of the peak velocity. The yellowdots mark peak systolic velocity. Color bars are in decibels. Inthe bottom panel are line spectra from peak systole, illustratingthe difference in main lobe width between the spectral estima-tion techniques. PSVcap 5 peak systolic velocity calculatedwith Capon (1969) technique, PSVconv 5 peak systolic velocity

calculated with the conventional technique.

Vector velocity 1 spectral Doppler in blood flow imaging d I. K. EKROLL et al. 9

recording was done in the stenotic region of a patient withinternal carotid artery stenosis, and a correspondingregion of interest was extracted from the plane waverecording. Whereas the conventional spectrogram wasmanually corrected during the examination using theassumed direction of flow, the other was automaticallycorrected with the direction estimated from vectorDoppler. In this case, where the spectrograms originatefrom the mid-stenotic region, the two corrections corre-spond well. The spectral quality is also comparable,though with somewhat lower contrast and temporal reso-lution in the unfocused, retrospective case.

Applications of 2-D spectral informationAs spectrograms and information on velocity

direction are available in every point of the image,

angle-corrected spectral estimation could be per-formed in all image points simultaneously. In regularcolor flow imaging, this is not an option, as the flowangle is generally unknown, and different regions ofinterest must be assessed separately, with the operatorproviding an educated guess of the flow direction.

By automatic calibration and tracing of the velocityspectra in every point in the image, the result may, forinstance, be displayed as a spatial map of the peak sys-tolic velocities. In Figure 12, such a map is depicted,generated from a patient with a stenosis in the internalcarotid artery. Examples of velocity spectra, velocitytraces and peak systolic velocity estimates are found ontop, depicting the velocity content in the pre-stenotic,mid-stenotic and post-stenotic regions.

DISCUSSION

We have presented results from an all-in-onemodality using plane wave imaging. The modality pro-vides simultaneous vector Doppler and B-mode images,as well as PW Doppler spectra from all image points.Tested in a population of patients with carotid arterydisease, the modality could provide successful vectorDoppler images and retrospective velocity spectra frompatients within a BMI range of 21 to 31. One limitationof the study is that none of the patients had a large degreeof calcifications; another is that only the carotid arterieswere imaged. Calcifications or, for instance, the increaseddepth of the vertebral arteries would increase the imagingchallenge, and it still remains to be fully establishedwhether the plane wave approach would be successfulunder such circumstances.

A duplex scheme using separate B-mode setup hin-ders a continuous acquisition and processing of flowimages and also limits the overall frame rate. Separateacquisition is, however, necessary to obtain high-qualityB-mode images, which is considered clinically impor-tant. Still, the overall frame rate was significantly higher(.3 times) than typically available in current high-endsystems and sufficiently high to capture fast variationsin the flow.

Duplex acquisition also enables focused B-modeimaging, with advantages including increased penetra-tion, resolution and contrast. By use of an interleavedacquisition scheme, these advantages can be achievedwhile keeping a high Doppler frame rate. The schemeapplied in this work had four interleave groups, resultingin a B-mode frame rate that was only one-fourth of theflow frame rate. However, when applied to carotid imag-ing, where the tissue movement is small compared withblood flow or myocardial contractions, such an inter-leaved approach is justified.

Fig. 11. Left: Conventional pulsed wave (PW) Doppler recording, manually calibrated using the assumed flow direction.Right: Corresponding retrospective PW Doppler spectrum from the plane wave Doppler acquisition, automatically cali-

brated using the directional information from vector Doppler.

10 Ultrasound in Medicine and Biology Volume -, Number -, 2014

In the planewave scheme, in which unfocused trans-missions were used for both flow and tissue imaging, acommon frame rate was used for the B-mode and flowimages. However, a subject for further investigation couldbe finding an optimal number of imaging angles used forcoherent compounding, as the span in angles chosenin our application corresponds to a transmit F# 5 4.Reducing the number of transmitted plane waves andincreasing the angle span would increase the frame rateand lateral resolution respectively, although with a reduc-tion in contrast and SNR.

An approach to obtain both color flow and spectralDoppler simultaneously has been described by Bercoffet al. (2011), where coherent compounding from 3–16 an-gles was used to improve resolution and contrast in thecolor Doppler image. Compounding has clear benefits

Fig. 12. Peak systolic velocity map obtained for a patient withand direction. Based on the flow angle from vector Doppler andpeak systolic velocity was found automatically in every image pvelocities, highlighting the stenotic region. Top: Examples of

in regions of slow flow, where the Doppler PRF can bereduced without risking aliasing artifacts, and as a meansto increase the SNR from single plane wave imaging.However, compounding in fast-flow situations mightinduce artifacts from movement and would also compro-mise the Nyquist (maximum) velocity.

To avoid excessive transducer surface heating,a compromise between penetration depth and pulserepetition frequency was encountered in our approach,thus also limiting the Nyquist velocity. Superficial ves-sels generally lie parallel to the skin surface and willhave relatively large beam-to-flow angles. As a result,most of the patients imaged had Doppler shifts sup-ported by the given velocity range. However, inseverely stenosed regions and in branches, aliasingcould occur.

carotid artery stenosis. Bottom left: Velocity magnitudeavailable spectral information, a calibrated estimate of theoint. Bottom right: Resulting spatial map of peak systolicspectrograms generated from different image locations.

Vector velocity 1 spectral Doppler in blood flow imaging d I. K. EKROLL et al. 11

The Doppler transmissions were in this work inter-leaved, which means that the transmit angle changedbetween each pulse. However, if a full Doppler ensemblewere acquired before changing transmit direction, themaximum velocity would be twice as high. On the otherhand, this may lead to a significant time lag between themean velocity estimates from the two directions.

The large Doppler ensembles used in this work(50) were motivated by a desire to make robust velocityestimates, to give more flexible wall filtering and anincreased velocity span, but also by the desire to beable to generate spectra of sufficient velocity resolutionin every point of the image. As demonstrated, the planewave packet acquisition scheme could provide powerspectra of sufficient temporal resolution and contrast. Inaddition to the conventional power spectra illustratingthe temporal dynamics of the flow, spatial spectral pro-files could also be generated, useful for investigation ofblood velocity profiles in different regions of the vessel.Both conventional and adaptive spectral estimation tech-niques could be used as precursors for maps of peak sys-tolic velocities, and examples of spectra generated byboth techniques were given. Increased computationaltime is a disadvantage of the adaptive techniques, but asthe spectral estimation is retrospective in our application,the slight increase in computational complexity is toler-able when a higher spectral resolution and contrast areobtained.

Overestimation of peak systolic velocity is a com-mon and well-known phenomenon (Hoskins 1996; Luiet al. 2005; Steinman et al. 2001), and the error hasmany sources, including spectral broadening. As thebeam-to-flow angle changes throughout the course ofthe vessel, spectral broadening will vary, resulting in avariable amount of overestimation in the peak systolicvelocity at different locations. As depicted in Figure 10,reduced spectral broadening and, therefore, a reductionin peak velocity overestimation may be obtained by usingadaptive spectral estimation. However, further workshould investigate the differences between alternativeand conventional spectral estimation techniques in a situ-ation with a known maximum velocity.

The tracing technique applied in this work was sim-ple, chosen as development of more advanced techniqueswas out of the scope of this work. Still, in our limited datamaterial, it worked adequately. More sophisticated tech-niques for estimating the peak systolic velocities havebeen suggested (Mo et al. 1988), and especially if conven-tional spectral estimation is used (with only moderateSNR), the technique developed by Steinman et al.(2000) could be useful.

It was found that vector Doppler estimates could beused to calibrate spectrograms from every image loca-tion, forming a spatial map of the peak systolic velocities.

In this way, regions of high velocities are highlighted, andthe patient examiner can, either during the examination orretrospectively, more easily place a sample volume in thisregion. The location of the spatial maximum of the peaksystolic velocities may also be found automatically. Mea-surements of peak systolic velocity are an important partof the clinical protocol when examining a patient withcarotid artery disease, and the velocity estimate is oneof the selection criteria for carotid endarterectomy. Inthat respect, automatic angle correction of the velocitieswould be beneficial, because it may reduce both intra-and inter-observer variability (Steel et al. 2003). Alsoof importance is the extent of the stenotic region. Thisis visualized better in a map of the peak systolic velocitiesthan when looking at regular color flow images, where thevelocity shown is dependent on beam-to-flow angleswhich change throughout the course of the stenoticregion.

Vector Doppler estimates are required for several pa-rameters that could be of interest in relation to carotid ar-tery disease. Wall shear stress estimation is one, wherethe impact of different flow patterns on the endotheliumcould be quantified to a larger extent than by visualizingflow patterns alone. However, very low velocity flowclose to the vessel wall remains a challenge, as the useof a wall filter also rejects the signal from slow bloodflow. In such applications, the filter design will be crucialto obtain reliable velocity estimates.

Volume flow estimation would also benefit fromvector velocity estimates, and could provide useful diag-nostic information. However, blood flow is three dimen-sional, and some assumptions must be made for thegeometry and the flow profile when 2-D imaging isused. Still, as illustrated here, although the full velocityinformation is not provided by a 2-D representation ofthe flow field, it gives increased understanding of theflow situation compared with the 1-D color flow images,and might improve our understanding of the vascularphysiology, in both healthy and diseased arteries.

CONCLUSIONS

An all-in-one modality for vector velocity, B-modeand spectral Doppler imaging was presented, and clinicalfeasibility was investigated using a small group of patientswith carotid artery disease. The patient study gave prom-ising results, with successful vector and spectral Dopplerimages generated from patients with BMI ranging from21 to 31 and carotid depth ranging from 16 to 28 mm.By combination of the directional information from vectorDoppler and the spectral information obtained using large-ensemble Doppler imaging, spatial maps of the peak sys-tolic velocity could be generated. Automatic calibrationof PW Doppler spectra may reduce inter-observer

12 Ultrasound in Medicine and Biology Volume -, Number -, 2014

variability, and the visualization of peak systolic velocitymaps may also aid the user in placement of the sample vol-ume, as regions of high velocity magnitude and theirspatial extent are quantified and highlighted.

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