A Composite High-Frame-Rate System for Clinical ...orion.bme.columbia.edu/ueil/documents/article/2008-wang...the left ventricle and the abdominal aorta of healthy human subjects was

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Abstract –High frame-rate ultrasound RF data acquisition has been proved to be critical for novel cardiovascular imag-ing techniques, such as high-precision myocardial elastography, pulse wave imaging (PWI), and electromechanical wave imag-ing (EWI). To overcome the frame-rate limitations on stan-dard clinical ultrasound systems, we developed an automated method for multi-sector ultrasound imaging through retrospec-tive electrocardiogram (ECG) gating on a clinically used open-architecture system. The method achieved both high spatial (64 beam density) and high temporal resolution (frame rate of 481 Hz) at an imaging depth up to 11 cm and a 100% field of view in a single breath-hold duration. Full-view imaging of the left ventricle and the abdominal aorta of healthy human subjects was performed using the proposed technique in vivo. ECG and ultrasound RF signals were simultaneously acquired on a personal computer (PC). Composite, full-view frames both in RF- and B-mode were reconstructed through retrospective combination of seven small (20%) juxtaposed sectors using an ECG-gating technique. The axial displacement of the left ven-tricle, in both long-axis and short-axis views, and that of the abdominal aorta, in a long-axis view, were estimated using a RF-based speckle tracking technique. The electromechanical wave and the pulse wave propagation were imaged in a ciné-loop using the proposed imaging technique. Abnormal pat-terns of such wave propagation can serve as indicators of early cardiovascular disease. This clinical system could thus expand the range of applications in cardiovascular elasticity imaging for quantitative, noninvasive diagnosis of myocardial ischemia or infarction, arrhythmia, abdominal aortic aneurysms, and early-stage atherosclerosis.

I. Introduction

cardiovascular disease is a growing problem world-wide [1], [2]. In 2006, almost 5 million americans were

diagnosed having heart disease and another 550,000 are diagnosed annually [3]. Echocardiography has become the predominant noninvasive imaging modality in diagnostic cardiology owing to its real-time and high temporal resolu-tion capability. It is known that the heart is an electrically driven organ. The electrical excitation, which induces con-traction and relaxation of the cardiac muscle, results in a localized contraction along its path yielding the so-called electromechanical wave [5]. The velocities of those waves

typically range between 0.5 and 7 m/s [6]. The fast wave propagation and the resulting transient small motion of the heart wall can only be captured using high frame-rate imaging techniques. In cardiology, higher temporal resolu-tion, typically smaller than 5 ms [4], is required to observe the detailed myocardial events and the fast electrical con-duction waves for early detection of cardiac disease. The abnormal pattern of those wave propagation and myocar-dial wall motion should be an indicator of cardiovascular disease. High frame-rate (> 200 Hz), full-view ultrasound imaging has been an emerging topic for small animal im-aging, where high heart rates are typically encountered, but, to our knowledge, has not been widely used in hu-mans as of yet. several alternative methods have been developed to increase the ultrasound frame rate such as coded-excitation ultrasound imaging [7]–[13] and parallel processing techniques [14]–[16]. others, such as Konofa-gou et al. [17] and d’hooge et al. [18], increased the frame rate of the rF images to above 200 Hz by reducing the size of the field of view and the total number of rF imag-ing lines. Kanai et al. [19], [20] used a sparse sector-scan format, in which the number of ultrasound beams was decreased to about 16 to achieve the peak frame rate of 450 Hz. The onset of “pulsive waves” in the heart wall at end-systole and the propagation of spontaneous vibrations was estimated using a phased-tracking method in human subjects [6], [19], [21], [22].

The aforementioned methods sacrificed the field of view, or the ultrasound beam density, to increase the frame rate. The echocardiogram (EcG)-gating technique in ultrasound imaging has been introduced to combine the individual sectors into a large field-of-view (FoV) at high beam density while at the same time attaining high frame rates [4], [23]–[25]. This technique has been successfully applied in 3-d echocardiography reconstruction [23], volu-metric measurements [26]–[28], and high frame-rate small animal imaging (e.g., mice) [4], [29]. EcG gating entails the acquisition of ultrasound rF signals on a multi-sector basis at different angles over several cardiac cycles [30]. The rF lines of each sector from separate cycles are then combined into a full-view 2-d or 3-d image. cherin et al. [31] transferred the EcG signals to an arbitrary wave-form generator and used the EcG r-wave as a trigger to control the transmitted pulses with the synchronization implemented in hardware for mouse imaging.

despite the advances in small animal imaging, there is still a lack of high frame-rate clinical systems for hu-man cardiovascular studies. In this paper, we applied a

A Composite High-Frame-Rate System for Clinical Cardiovascular Imaging

shougang Wang, Member, IEEE, Wei-ning lee, Student Member, IEEE, Jean Provost, Jianwen luo, Member, IEEE, and Elisa E. Konofagou, Member, IEEE

Manuscript received January 7, 2008; accepted May 15, 2008. This work was supported by the Wallace H. coulter Foundation (WHcF cU02650301), the american Heart association (sdG0435444T), and the national Institutes of Health (r01Eb006042-01).

s. Wang, W.-n. lee, J. Provost, J. luo, and E. Konofagou are with the department of biomedical Engineering, columbia University, new york (e-mail: [email protected]).

E. Konofagou is also with the department of radiology, columbia University, new york.

digital object Identifier 10.1109/TUFFc.921

Authorized licensed use limited to: Columbia University. Downloaded on March 18, 2009 at 12:07 from IEEE Xplore. Restrictions apply.

multi-thread software technique for simultaneous EcG and rF data acquisition, which significantly lowered the system cost while retaining the high temporal resolution for cardiovascular imaging. However, most commercially available ultrasound systems operate at 60 to 120 Hz for b-mode imaging depending on the applications. Impor-tant information such as frequency and phase carried by the rF echo signals is typically discarded during data con-version and compression to avoid large data storage and to minimize the processing time. For researchers, there is a special need to have access to the rF data streams for further development and improvement of their methods. The sonix rP system (Ultrasonix Medical corporation, richmond, bc, canada) is such an open-architecture sys-tem, which provides maximal flexibility for researchers to acquire the raw rF signals directly as well as control the beam density, sector size, and other system parameters. an application programming interface (aPI) was also provided, which is essential for the connection between commercially available modalities and scientific research developments. The capability of direct access to the rF data could eventually help scientific researchers to develop other rF-signal based imaging protocols on the sonix rP system.

In this paper, we proposed an EcG-gated compos-ite imaging method at a frame rate (481 Hz) five times higher than the traditional ultrasound system (~90 Hz) based on the clinically used sonix rP system. In sec-tions II-a and II-b, a method is described in which the region of interest (roI) was initially decreased to a 20% sector size to achieve such a high frame rate. digitized rF and EcG signals for each sector were simultaneously acquired through 2 threads in a computer. signals from seven separate sectors at different angles were thus auto-matically and sequentially acquired to cover the (100%) full-view ciné-loop. an EcG-gating technique was then applied to use the rF signals acquired during multiple cardiac cycles and retrospectively reconstruct small roIs into a complete 100% field-of-view. sixty-four lines were acquired in a (100%) full view to preserve the high lateral resolution. In section II-c, regional displacements along the beam axis were estimated offline using a cross-cor-relation technique on the acquired rF signals. The fast electromechanical wave propagation in the left ventricle and the pulse wave traveling along the abdominal aorta were clearly depicted in the resulting sequences of 2-d displacement images in section III. The method could be implemented on other clinically used ultrasound systems given access to the rF signal and system control is avail-able, and could be widely used as a hospital standard module once further developed.

II. systems and Methods

A. Data Acquisition

The sonix rP system equipped with a clinical phased-array transducer (Model Pa4–2/20) operating at 3.3

MHz was used in all experiments. a separate EcG mod-ule (Mcc Gesellschaft für in Medizin und Technik mbH & co. KG, Karlsruhe, Germany) was connected to the sonix rP system through an rs232 serial interface. The EcG signals were digitized at a sampling frequency of 300 Hz and transmitted to the host Pc at a 9600 baud rate. because the EcG and ultrasound rF signals were acquired through two separate modules, synchronization was critical for the EcG gating and multi-sector combi-nation. Thus, customized automated rF and EcG sig-nal acquisition software was developed in c++ based on the Ulterius software development kit (sdK) (Ultrasonix Medical corporation, richmond, bc, canada). Each EcG sample and each rF frame were time-stamped for synchronization purposes. Fig. 1 shows the block diagram of the custom-designed program illustrating the EcG and the ultrasound rF signal acquisition. Two separate threads—for rF and EcG data acquisition, [Fig. 1(a) and (b)]—operated simultaneously on a Pc computer. both threads started to store signals when the record flag was set to TrUE. Each individual rF frame along with its occurrence time was then transferred from the Ultrasonix rP hardware to the predefined user buffer as shown in Fig. 1(c) for post processing. The EcG acquisition was implemented on a thread operating in parallel with the main program retrieving the digitized EcG from the serial port; see Fig. 1(b) and (d).

B. Composite Processing Through ECG R-Wave Gating

seven (20%) sectors were employed to cover the (100%) full-view image, which contained 12 ultrasound beams in each sector. Using these settings, a 25% overlap was used between neighboring sectors. The overlap between neigh-boring sectors ensured that the composite full-view image covered the entire left ventricle. Therefore, 3 overlapping lines were removed from each sector before constructing the composite image, except in the case of the first sector, in which only 2 lines were removed. The total line density of the composite full-view image thus remained equal to 64, which is the original beam density for a 90-degree full-view image acquisition.

Fig. 2 shows the concept of the EcG-gated composite technique for high frame-rate imaging. For each sector, the corresponding ultrasound rF frames at the frame rate of 481 Hz and the EcG signals at the sampling rate of 300 Hz were acquired with a 2-s duration; see Fig. 2(a) and (b). The rF frame is a 2-d matrix, whose columns repre-sent the signals acquired along the ultrasound beams and the rows represent the number of beams in that sector. The corresponding time reference of the r-wave peaks was used to search for the rF frame, which occurred closest to the r-wave. rF frames between 2 consecutive EcG r-waves were then extracted to represent one cardiac cycle as illustrated in Fig. 2(c). Ideally, the EcG duration of one cycle should be of identical duration to that of others. The ith (i = 1, 2, …) frame of each sector can then be recombined in sequence to obtain the ith frame of the

2222 IEEE TransacTIons on UlTrasonIcs, FErroElEcTrIcs, and FrEqUEncy conTrol, vol. 55, no. 10, ocTobEr 2008


100% view, as illustrated in Fig. 2(d). However, all EcG-gating/triggering imaging methods are challenged by the variability of the heart rate. In general, the duration of the cardiac cycle could vary by up to 10% between cycles [23]–[26] and the total number of frames per cycle for each sector would vary accordingly. In the case of this study, the duration of a full cardiac cycle was 849 ± 31 ms for a female subject, as shown in Fig. 3(a). It should also be noted that the variability of the EcG signals during the systolic phase was lower than that during the diastolic phase. Therefore, a suitable method to account for the EcG discrepancy was to scale only the diastolic phase of the EcGs to the longest duration of all the 7 cycles and achieve the highest correlation [23] as shown in Fig. 3(b). For EcG scaling purposes, the duration of systole (Tes) in humans was empirically assumed to be equal to 0 343. DT s [32], where ΔT is the duration of the entire cardiac cycle. all seven EcG signals during diastole, as denoted by the vertical dashed line (close to end-systole) in Fig. 3(a) were linearly interpolated to the maximum length among these signals. accordingly, all corresponding 2-d rF frames were linearly interpolated to the maximum number of frames among the 7 sectors. The interpolated rF frames were then used for the composite full-view im-aging.

C. Displacement Estimation

after successful combination of the rF frames from 7 sectors through EcG gating, the axial displacement of the

myocardium was estimated offline using an elastographic technique [4], [17], [33], [34]. The axial displacement be-tween pairs of consecutive frames was estimated using a 1-d cross-correlation technique [4], [29], [34]. The 1-d win-dow size was equal to 6.9 mm, and the window overlap was equal to 80%, deemed optimal for retaining enough axial resolution and elastographic signal-to-noise ratio [35]–[37]. In addition, using a previously developed technique by our group, the manually initialized endocardial borders in the left ventricle could be automatically tracked throughout the entire cardiac cycle [38]. The displacements were then color-coded and superimposed onto the gray-scale b-mode images in an overlay-blending mode [34], [39]. The ciné-loop of the axial displacement was generated at a frame rate of 481 Hz per cardiac cycle. In the displacement imag-es, only the estimates in the roI delineated by the endo- and epicardia were shown for better interpretation. all displacement estimation and overlay methods were imple-mented on Matlab 7.1 (MathWorks, natick, Ma).

D. In Vivo Experiments

The long-axis and short-axis views of a normal left ventricle and the long-axis view of an abdominal aorta of 28-year-old healthy male and female subjects were ac-quired using the proposed composite-imaging technique. The scan was performed by an expert in clinical sonogra-phy using a standard clinical ultrasound b-mode scanning procedure. The imaging depth was equal to 11 cm and

2223WanG ET al.: a composite high-frame-rate system for clinical cardiovascular imaging

Fig. 1. Flowchart of the c++ program for (a) rF signal acquisition, (b) EcG signal acquisition, (c) rF frame data and rF time reference buffer, and (d) EcG data and EcG time reference buffer. Two data acquisition threads are running simultaneously on a Pc computer. both threads start to store data when the record flag is set to TrUE (avl. = available, ref. = reference); ei, ti represent the ith EcG sample and its occurrence time, fi, ti’ represent the ith frame of the rF signal and its occurrence time.


the line density was equal to 64 for both the left ventricle and the abdominal aorta. For each sector, EcG and rF signals of one to two cardiac cycles were recorded during a 2-s scan because the heart rate of the subject was approxi-mately 70 beats per minute (i.e., 1.2 beats/s). a total of 21 s was needed for an entire 7-sector scan, including sys-tem settings and data acquisition. because the respiratory motion could affect the heart’s position, breath-holding during the entire scan was required for higher composite imaging performance. The required breath-holding time is of similar duration to that used in MrI scanning [40], [41]. both the patient’s heart rate variability and the op-erator’s free-hand scanning remain possible, albeit small, sources of misregistration between sectors.

III. results

A. Validation of the ECG-Gated Composite Imaging

To evaluate the performance of the proposed method, a long axis view of the left ventricle of a healthy male subject was shown in Fig. 4. Fig. 4(a) shows a b-mode image of the left ventricle at a 100% view and a frame rate of 90 Hz using a standard acquisition protocol, i.e., without composite imaging. Fig. 4(b) shows a 7-sector composite view of the same left ventricle and the same cardiac phase at 481 Hz using the proposed technique. The red1 (or gray) dot on the EcG trace below each im-age shows the cardiac phase at which the image above it was obtained. all b-mode images were reconstructed from the corresponding rF signals using the Hilbert transform. as shown in Fig. 4(a) and (b), the image quality and the structure information are nearly identical. no significant mismatches between sectors were observed. The images of Fig. 4 also depict some of the main structures of the human left ventricle: the interventricular septum (IVs),

the left-ventricular cavity (lVc), the posterior wall (PW), and the aortic root (ao). a continuous ciné-loop ( ) fur-ther demonstrates the smooth transition between sectors for composite full-view b-mode images, which further vali-dates the correct data acquisition and successful method-ology of the EcG-gated composite imaging technique.

B. Electromechanical Wave Imaging in a Long-Axis View

The continuous composite ciné-loop ( ) with displace-ments overlaid onto the b-mode images showed several strong electromechanical waves propagating in the lon-gitudinal direction of the left ventricle (i.e., both along


Fig. 2. an example of the EcG-gated composite technique for high frame-rate, full-view ultrasound imaging. (a) seven sectors at different angles are employed. (b) seven EcGs and 7 rF signals of each sector are acquired in a continuous sequence. (c) one full cardiac cycle of EcG and rF frame signals are extracted. (d) corresponding frames are recombined to generate full view ultrasound images.

Fig. 3. Illustration of temporal EcG interpolation: (a) the original 7 EcG signals and (b) the interpolated EcG signals. The EcG signals during systole remain relatively unchanged. EcG signals during the diastolic phase after the dashed line (close to end-systole) are linearly interpolated to the maximum length of these signals. The corresponding rF frames associated with each EcG signal are also interpolated to the maximum length of the rF frames through 2-d linear interpolation.

1 When color is mentioned in the text, it refers to color in the online version of the figures.


the septal and posterior walls). Fig. 5 shows a sequence of a wave propagating from the base to the apex. Posi-tive displacements (in red) denote the motion toward the transducer whereas negative displacements (in blue) de-note the motion away from the transducer. The wave was initiated at 24.8 ms after the EcG r-wave and reached the leftmost side of the image in 10.4 ms with a speed of approximately 3 m/s. The wave was expected to subse-quently reach the apical side of the heart but could not be depicted because of the limitations of the transthoracic long-axis view in human echocardiography that typically cannot depict the apex. Fig. 6 shows another sequence of the consecutive color-coded axial displacements overlaid onto the gray-scale b-mode images, from 51.8 ms to 82.8 ms, which occurred approximately 17 ms after the wave in Fig. 5 during systole. as shown in Fig. 6, a wave (in red), with its wavefront denoted by the white arrow, was initiated at the apex (left side) and then propagated to-ward the base (right side) along the posterior wall. after this wave reached the basal side, the whole left ventricle started to contract or undergo myocardial thickening ( , frame # 44–150). This revealed that, while the myocardial wall moved along the radial direction, a transverse wave propagated along the longitudinal direction. This trans-verse wave coincided with the electromechanical activa-tion sequence in the human heart, which closely followed the electrical conduction path [42]. during diastole, an-other clear wave (in red) was found propagating from the apical to the basal side along the septum as shown in Fig. 7. The wave was initiated 476 ms after the r-wave, which coincided with the mitral valve opening. It should be not-ed that there were more wave activities, especially during the transition from systolic to diastolic phase, which did not show in this paper but could be found in the ciné-loop ( ). The waves shown in Fig. 5, 6, and 7 were also found in the female subject.

C. Electromechanical Wave Imaging in a Short-Axis View

The same imaging protocol was also applied in a short-axis view of the left ventricle of a healthy female subject. Transient waves were also detected and imaged during propagation, clockwise or counterclockwise depending on the cardiac phase, along the myocardial wall. Those waves could not be detected by standard 2-d echocar-diography, which typically operates at 60 to 90 Hz or 3-d echocardiography (15–20 Hz). Fig. 8 shows an example of a propagating wave in a short-axis view during the inter-val from 23.1 to 49.5 ms following the r-wave. a wave was depicted propagating counterclockwise, i.e., from the ante-rior to the septal wall until it reached the posterior region ( ) as indicated by the white arrow. The wave shown in Fig. 9 shows a short-axis example during the diastolic phase, when the heart is undergoing relaxation. a wave of downward displacement (in blue) was shown propagating counter-clockwise, from the septal to the posterior wall, finally reaching the anterior side ( ).

D. Wave Along the Aortic Wall

a longitudinal view of the abdominal aorta of the same female subject was also imaged using the proposed 7-sector composite technique at a frame rate of 481 Hz ( ). Fig. 10 shows a sequence of the axial displace-ments in color overlaid onto the gray-scale b-mode im-age, starting at 119 ms after the r-wave peak to allow propagation of the wave to reach the aorta. a threshold was applied and displacements outside the ± 0.01 mm range were filtered for optimal depiction of the pulse-wave propagation. Prior to the arrival of the the pulse-wave, the displacements of the aortic wall were found to be negligible. strong axial displacements of the wall were initiated at 119 ms after the r-wave when the pressure


Fig. 4. (a) a standard echocardiogram of a left ventricle of a male subject obtained without the composite technique at a frame rate of 90Hz; (b) the 7-sector composite b-mode image of the same left ventricle in a long-axis view at a frame rate of 481 Hz (lVc-left ventricular cavity, la-left atrium, IVs-interventricular septum, rV-right ventricle, ao- aortic root, PW-posterior wall).


wave reached the right side of the view. The delay was caused by the pressure wave traveling from the root of the valve to the abdominal aorta. This sudden pressure change of the blood bolus traveling through the vessel, known as the arterial pulse-wave, initiated a transverse wave propagating from the right (heart) to the left (circu-lation) side of the images within 36 ms. Fig.11(a) shows a manually defined traveling path of this pressure wave along the aortic wall based on the sequence of pulse-wave images. The wavefront traveling distance versus the trav-eling time was plotted in Fig. 11(b). The group velocity of this pulse wave was thus estimated to be approximately 4.7 m/s and could be captured by this high frame-rate composite imaging technique during the entire propaga-tion duration.

IV. discussion

A. The Composite Imaging Method

composite imaging using retrospective EcG gating is an effective technique for achieving high frame rates while preserving a 100% field-of-view. a similar acquisition tech-nique has been used before in small animal ultrasound imaging (such as mice and rats) using a mechanically

scanned single-element transducer scan on a line-by-line basis [4], [25], [31], [34]. our composite imaging technique uses a phased-array transducer, with which 12 lines can be acquired simultaneously in a single sector and could poten-tially increase the lateral displacement estimation quality. seven sectors were needed to cover the entire 90-degree full-view images of conventional b-mode imaging. Each sector has a 2-s duration for ultrasound and EcG signal acquisition and a 1-s time interval for configuring system settings and data storage. as a result, signal acquisition for 7 sectors can be achieved during a single breath-hold, which is a widely used method in 3-d echocardiography or MrI [40], [41]. one potential drawback of this breath-holding requirement is that the slow and fast motions, e.g., induced by transient respiration or valve opening/closure, respectively, may pose problems in the 2-d spa-tial matching and could induce shifts between the sec-tors on the b-mode and the displacement images. such shifts have been reported in EcG-gated volume stitching for 3-d echocardiography [23]. The shifts were most no-ticeable during the fast closure and opening of the mitral valve. This is because the underlying assumption of EcG gating is that the heart rate does not vary significantly and that the myocardial motion is reproducible from one


Fig. 5. The propagation of an electromechanical wave along the septum of the male subject in a long-axis view during systole; (a)–(f) are composite ultrasound frames acquired every 2.1 ms starting at approximately 24.8 ms after the r-wave. red (positive) and blue (negative) represent axial mo-tion upward and downward in mm, respectively. The white arrows indicate the propagating wavefront (in blue) along the septum. The red (or gray) dot on the EcG curve represents the cardiac phase which corresponds to the frame above it.


cardiac cycle to the next. This assumption is verified dur-ing the systolic phase as shown in Fig. 3. However, the actual EcG duration (r-r interval) during the diastolic phase can vary up to 10% [26] compared with the systolic phase in Fig. 3(a). In our method, the aforementioned ef-fects were minimized by 2 approaches. First, the subject scanned held his or her breath for up to 21 s to reduce respiratory artifacts and preserve optimal consistency be-tween cycles. second, both the rF and EcG signals from 7 sectors were interpolated to the maximal duration out of those 7 cycles, which meant that the beginning of systole and end-diastole were aligned as shown in Fig. 3(b).

after extracting a full cardiac cycle EcG and its cor-responding rF signals for each sector, the first rF frame and the first EcG sample were considered to be initi-ated at the same time as described previously. However, because the signal is acquired digitally, the maximum latency between the rF and EcG data sets was 1 ms, as determined by the half-time interval between 2 con-secutive rF frames. on the other hand, the EcG signals were sampled at 300 Hz, which could incur a maximum error of the sampled r-wave peak relative to the true peak by the half of the EcG sampling period. Therefore, the maximum latency between the acquired and the actual

rF frame corresponding to the r wave was determined as follows:

Lf f

= +æ

èççç

ö

ø÷÷÷÷

12

1 1

ecg rf

where L is the latency (in s) between the rF frame at the recorded r-wave peak and that at the actual r-wave peak, fecg is the EcG sampling rate and frf is the rF frame rate. In this study, L was estimated equal to 2.6 ms at an EcG sampling rate of 300 Hz and an rF frame rate of 481 Hz. This time interval was deemed acceptable considering that the average total length of a full cardiac cycle was roughly equal to 857 ms in this case.

For further development, the sector size can be de-creased to a single transmission line, and the method can be reduced to line-by-line signal acquisition at an even higher frame rate. consecutive multiple breath-holds may be needed in that case. High frame-rate imaging could detect faster electromechanical waves propagating in the heart [5] as well as increase of the elastographic signal-to-noise ratio (snre) [43], [44]. a high frame rate could thus enhance the precision of strain and strain rate imag-ing [43], [44]. a combination of high frame-rate imaging


Fig. 6. The propagation of an electromechanical wave (in red, initiated after the wave shown in Fig. 5) along the posterior wall of a male subject in the long-axis view during systole; (a)–(f) are composite ultrasound frames acquired every 6.2 ms at approximately 51.8 ms after the occurrence of the r-wave in a single cardiac cycle. White arrows indicate the propagating wavefront (in red) along the posterior wall. designations of scale, white arrows, and the dot on the EcG curve are the same as in Fig. 5.


and volume stitching [23] using the EcG-gating technique should be feasible and may be required for 3-d cardiac imaging.

B. The Waves in the Left Ventricle and the Abdominal Aorta

according to the high frame-rate imaging sequence shown in Fig. 5, at approximately 24.8 ms after the r-wave during the systolic phase, the contraction activation was initiated at the interventricular septum (IVs) (i.e., base) and propagated toward the apex along the septum. White arrows in Fig. 5 clearly showed a wavefront to-ward the apex. after this wave, a clear red (i.e., upward-displacement) wave was depicted propagating from the apex to the base at a lower speed, as shown in Fig. 6. These 2 waves followed the well-established path of elec-trical conduction in the heart and occurred before systole started, i.e., during the isovolumic contraction phase. It is thus deemed that those waves resulted from the electrical activation and subsequent mechanical response of the car-diac muscle. This has recently been verified by our group by modifying the pacing origin in animal experiments [5].

It should be noted that these waves cannot be imaged or detected using standard b-mode echocardiography, which typically operates within the 60 to 90 Hz range. The wave in Fig. 6 reveals that, at the posterior wall, the muscle contracts first near the apex followed by the base. a similar activation pattern has also been reported in the left ventricle of an open-chest guinea pig using high-speed digital video-acquisition system and surface markers [45]. It is known that an approximately 15 ms [42] delay was needed for a pure electrical impulse to propagate from the apex to the base along the Purkinje fibers. In this study, the contraction wave propagated along the posterior wall within approximately 31 ms duration as illustrated in Fig. 6. Therefore, as indicated before, the wave shown in Fig. 6 is concluded to be the consequence of the myocardial tis-sue’s response to the electrical impulse but with a delayed response time. The wave shown in Fig. 7 was consistently found in both subjects shown. obviously, waves shown in this paper and other transient waves observed in the ciné-loop ( ) traveling along both the septum and the posterior wall need to be further investigated and could all be of medical interest as they reflected the myocardial viability. a mechanical wave may also propagate in the


Fig. 7. The propagation of an electromechanical wave (in red, moving upward) along the septum in the long-axis view during diastole; (a)–(f) are composite ultrasound frames acquired every 10.4 ms at approximately 476.1 ms after the occurrence of the r-wave in a single cardiac cycle. White arrows indicate the propagating wavefront (in red) along the posterior wall. designations of scale, white arrows, and the dot on the EcG curve are the same as in Fig. 5.


perpendicular direction, but could not be observed in the 2-d configuration. Thus, a 3-d view may be necessary to simultaneously capture all the components of wave propa-gation.

In a short-axis view, more complex deformations, such as torsion, or rotation, may be coupled to the wave propa-gation. The interpretation of those short-axis waves are more complicated than in the long-axis view. Fig. 8 shows a clear red wave (upward motion) propagating counter-clockwise from the anterior wall to the septum during systole at approximately 23 ms after the r wave. an-other strong blue wave (downward motion) in a short-axis view was imaged after its initiation around the septum and then traveling counter-clockwise to the posterior wall while finally ending at the anterior wall, as shown in Fig. 9 during diastole. The exact origin of each wave needs to be further investigated. In the current 2-d framework, the combination of wave propagation in both long-axis and short-axis views is capable of providing a preliminary il-lustration of the 3-d electromechanical myocardial activa-tion without 3-d imaging.

The mechanism of the propagating wave along the aorta is relatively more straightforward than that of the waves occurring in the heart mainly because there is no electri-cal conduction occurring at the level of the aorta. In the latter, the pressure-induced blood flow emanating from the aortic root induces such a propagation. Fig. 10 clearly shows the pulse-wave propagation from the heart (right, not shown) to the renal side (left, not shown). It should be noted that the anterior wall of the aorta has a relatively larger motion compared to that of the posterior wall that lies along the spinal cord and is thus more restricted. The pulse wave velocity estimated in this case is equal to 4.7 m/s, which falls within the reported range for young hu-man subjects in the literature [46]. The advantage of our technique is that the pulse-wave velocity can be estimated and mapped noninvasively in a localized region.

obviously, a thorough understanding regarding the ori-gin of those wave-propagation patterns in the ventricular electromechanics, or aortic mechanics, are essential prior to wide clinical application. This electromechanical acti-vation in the heart is sensitive to any abnormality in the ventricular function such as ischemia or infarction. The


Fig. 8. The propagation of an electromechanical wave in the cardiac short-axis view during the systolic phase of a female subject; (a)–(f) are ultra-sound frames obtained at 23.1, 26.4, 29.7, 33, 42, and 49.5 ms after the r-wave occurrence. designations of scale, white arrows, and the dot on the EcG curve are the same as in Fig. 5.


patterns of these traveling waves may thus yield charac-teristic features of normal performance of the myocardi-um. It is expected that any abnormal type of contraction would be an indicator of disease. The existence of vascular diseases, such as aneurysm or atherosclerosis, could also significantly alter the patterns of pulse-wave propagation [47] and could thus be detected using this imaging tech-nique at the early stages of the disease. The composite imaging technique was also used to calculate the pulse-wave velocity (PWV) and further estimate the vascular stiffness [47], which has been proposed as an early predic-tor of hypertension and all-cause cardiovascular mortality, primary coronary artery events, and fatal stroke [48]–[51]. Finally, hardware implementation of the proposed tech-nique is expected to achieve real-time capability for both sector matching and processing, i.e., it should effectively avoid the current breath-holding requirement.

V. conclusion

a full-view imaging method with a frame rate (481 Hz) 5 times higher than the conventional ultrasound imag-ing systems (~90 Hz) was developed on a clinically used system. a composite technique on multi-sector rF signals

with EcG gating was developed and used to reconstruct echocardiograms in full left-ventricular sonographic views. displacements were estimated using previously developed elastographic techniques. several electromechanical waves propagating along the septum and posterior wall in long-axis views were imaged in humans in vivo. More transient waves propagating counterclockwise from the septal to the posterior wall, or even to the anterior wall, in a short-axis view were also imaged. In addition, pulse-wave propaga-tion along the abdominal aorta was mapped, which may be proven essential for the early detection of aortic aneu-rysms or atherosclerosis. The high frame-rate ultrasound and elasticity imaging system was thus capable of success-fully depicting fast, physiologic wave propagation, nonin-vasively and in a clinical setting. currently, this technique was implemented on a sonix rP system. It can also be implemented on other clinically used ultrasound systems given access to rF signals and control of the system.

acknowledgments

This study was supported in part by nIH-r01-r01Eb006042, aHa-sdG-0435444T and an Early career award by the Wallace H. coulter Foundation. We would


Fig. 9. sequence of images showing the propagation of an electromechanical wave in cardiac short-axis view during the diastolic phase of the female subject; (a)–(f) are ultrasound frames obtained at 452.1, 475.2, 506.9, 541.2, 561, 584.1 ms after the r-wave occurrence. designations of scale, white arrows, and the dot on the EcG curve are the same as in Fig. 5.



Fig. 10. sequence of the images showing the pulse wave propagation along the aortic wall of the same female subject during systolic phase; (a)–(f) are composite frames every 7 ms at around 119 ms after the r wave. designations of scale, white arrows, and the dot on the EcG curve are the same as in Fig. 5.

Fig. 11. abdominal aorta pulse-wave velocity estimation. (a) abdominal aorta b-mode image. For each point along the overlaid white line, the time of arrival of the pulse-wave was measured. (b) Time profiles of incremental displacement along the line shown in (a). The time of arrival was measured at the onset of the wave and a weighted linear regression (white line) was applied to estimate its velocity. designation of scale is the same as in Fig. 5.


also like to thank Kana Fujikura, M.d., Ph.d., in the ultrasound and elasticity imaging laboratory for the ultra-sound operation and Kris dickie (Ultrasonix Medical Inc.) for providing sonix rP system technical support.

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Shougang Wang was born in Heilongjiang Prov-ince, Prc, in 1976. He received his b.s. degree from Tsinghua University in china and his M.s. degree from the chinese academy of sciences in 1998 and 2001, respectively. In 2006, he received his Ph.d. degree from brown University, Provi-dence, rI, majoring in physical chemistry.

His research interests include ultrasonic vibra-tion potential imaging, ultrasound imaging, car-diac elasticity imaging, and ultrasound aided drug delivery through the blood brain barrier opening.

currently he is a postdoctoral research scientist in ultrasound and elas-ticity imaging laboratory at columbia University, new york. dr. Wang is a member of IEEE and the american chemical society (acs).

Wei-Ning Lee was born in 1979 in Taichung, Taiwan. she received her b.s. and M.s. degrees in electrical engineering from national Taiwan Uni-versity, Taipei, Taiwan, in 2001 and 2003, respec-tively.

she worked as a research assistant with the department of Electrical Engineering at national Taiwan University from 2003 to 2004. she is cur-rently a Ph.d. candidate in the department of biomedical Engineering at columbia University, new york.

Her research interests include ultrasound elastography in cardiovas-cular applications as well as medical imaging. she is a student member of the IEEE Ultrasonics, Ferroelectrics and Frequency control society.

Jean Provost was born in 1983 in Montréal, canada. He received his b.s. degree in engineer-ing physics from École Polytechnique Montréal in 2006, his licence degree in physics from Univer-sité Paris XI in 2005, his Master of Engineering degree from École centrale Paris in 2007, and his M.s. degree in biomedical engineering from École Polytechnique Montréal in 2008.

He is currently a Ph.d. candidate in the de-partment of biomedical Engineering, columbia University, new york. His research interests in-

clude ultrasound imaging in cardiovascular applications.

Jianwen Luo (s’02–M’06) was born in Fujian Province, china, in 1978. He received the b.s., M.s., and Ph. d. (summa cum laude) degrees in biomedical engineering from Tsinghua University, beijing, china, in 2000, 2002, and 2005, respec-tively.

His research interests include ultrasound im-aging and biomedical signal processing. He is currently a postdoctoral research scientist in the department of biomedical engineering at colum-bia University, new york, and is researching high-

resolution cardiovascular imaging, including myocardial elastography, electromechanical imaging, and pulse wave imaging. He is a member of the IEEE and sigma Xi.

Elisa E. Konofagou was born in Paris, France, in 1972. she received her b.s. degree in chemical physics from Université de Pierre et Marie curie, Paris VI in Paris, France, and her M.s. degree in biomedical engineering from Imperial college of Physics, Engineering and Medicine in london, UK, in 1992 and 1993, respectively. In 1999, dr. Konofagou received her Ph.d. degree in biomedi-cal engineering from the University of Houston, Houston, TX, for her work on multidimensional elastography for breast cancer diagnosis at the

University of Texas Medical school, Houston, TX, then pursued her postdoctoral work in elasticity-based monitoring of ultrasound therapy at brigham and Women’s Hospital, Harvard Medical school, boston, Ma.

dr. Konofagou is currently an assistant professor of biomedical en-gineering and radiology and director of the Ultrasound and Elastic-ity Imaging laboratory at columbia University, new york. Her main interests are in the development of novel elasticity imaging techniques and therapeutic ultrasound methods, such as myocardial elastography, breast elastography, ligament elastography, harmonic motion imaging, and ultrasound-induced brain drug delivery, with several clinical collabo-rations in the columbia Presbyterian Medicine center, new york. dr. Konofagou is a technical committee member of the acoustical society of america and a technical standards committee member of the american Institute of Ultrasound in Medicine. she has served on peer-review com-mittees for the national Institutes of Health, the national aeronautics and space administration, and the national science Foundation. she also serves as an associate editor for the journal Medical Physics and is recipient of several awards, including from the acoustical society of america, the american Heart association, the american Institute of Ultrasound in Medicine, the national Institutes of Health, the national science Foundation, the radiology society of north america, and the Wallace H. coulter Foundation. she is also a member of the IEEE Ultra-sonics, Ferroelectrics and Frequency control society, the International society of Therapeutic Ultrasound, the acoustical society of america, the american Institute of Ultrasound in Medicine, and the american Heart association.



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A Composite High-Frame-Rate System for Clinical ...orion.bme.columbia.edu/ueil/documents/article/2008-wang...the left ventricle and the abdominal aorta of healthy human subjects was