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European Journal of Echocardiography Advance Access published online on November 6, 2008
European Journal of Echocardiography, doi:10.1093/ejechocard/jen298
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Published on behalf of the European Society of Cardiology. All rights reserved. © The Author 2008. For permissions please email: journals.permissions@oxfordjournals.org

Comparison between colour-coded and spectral tissue Doppler measurements of systolic and diastolic myocardial velocities: effect of temporal filtering and offline gain setting

Aristomenis Manouras1,*, Kambiz Shahgaldi1, Reidar Winter1, Jacek Nowak1 and Lars-Åke Brodin2

1 Department of Clinical Physiology, Karolinska University Hospital Huddinge, SE-141 86 Stockholm, Sweden
2 School of Technology and Health, Royal Institute of Technology, Flemingsberg, Stockholm, Sweden

Received 11 July 2008; accepted after revision 5 October 2008.

* Corresponding author. Tel: +46 8 5858 8679; fax: +46 8 774 8082. E-mail address: aristomenis.manouras{at}karolinska.se


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 Funding
 References
 
Aims: Colour tissue Doppler (TD) has been reported to underestimate the longitudinal myocardial motion velocities measured with spectral TD. This study evaluates the effect of temporal smoothing and offline gain settings on the results of velocity measurements with these two methods and the difference between them.

Methods and results: In 57 patients, 2D data and left ventricular velocity profiles were acquired using spectral and colour TD for a subsequent offline analysis. Longitudinal myocardial velocities were measured at unsaturated, 50%-saturated and fully saturated gain, and before and after temporal smoothing using 30, 50, and 70 ms filters, respectively. Gain level and filter width altered significantly the measured velocities. Peak systolic and early diastolic velocities were significantly higher (P < 0.001) and E/E' ratio was significantly lower (P < 0.001) with spectral TD than with colour TD, although there was a good correlation between the results of both TD modalities. The differences between the methods increased at increasing filter width and gain level.

Conclusion: Despite good correlation of the results, spectral TD produces significantly higher myocardial velocity values and lower E/E' ratio than colour TD modality. Increasing gain and temporal smoothing alter significantly the results of velocity measurements and accentuate the difference between the two TD methods.

Keywords: Spectral Doppler; Colour Doppler; Temporal filtering; Offline gain; Myocardial velocity


   Introduction
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 Abstract
 Introduction
Methods
 Results
 Discussion
 Funding
 References
 
Introduction of the echocardiographic analysis of myocardial wall movements into clinical practice1,2 constitutes undoubtedly an important landmark in the history of echocardiography and its value as a diagnostic tool in cardiology is now well established.3 The possibility of measurement of myocardial tissue motion has created basis for detailed evaluation of systolic and diastolic cardiac function not only in terms of velocity, but also in terms of acceleration, displacement, and deformation variables, and two echocardiographic approaches are generally used today, i.e. spectral tissue Doppler in pulsed mode1 (spectral TD) and colour tissue Doppler2 (colour TD). However, there are some essential differences between the two TD methods. Spectral TD detects the frequency shift between the emitted and returning ultrasound signal that is subjected to Fast Fourier analysis and the method provides peak velocity at a given myocardial location. On the other hand, colour TD appreciates Doppler shift indirectly using phase shift analysis with autocorrelation technique and the method provides mean myocardial velocity value for a chosen myocardial location.

Both TD methods have been validated in several studies4–12 and their value in the evaluation of systolic and diastolic myocardial function in different cardiac disorders has been well documented.13–21 However, despite rapid methodological and technical development in recent years, TD approach does not always provide sufficiently noise-free tissue velocity signal since even modern echocardiographic equipment can still produce a random noise. In addition, the high data acquisition frequency required for optimal temporal resolution and high-fidelity rendering of myocardial velocity curves12 will inevitably result in an increased presence of noise altering the tissue Doppler signal. Therefore, in order to extract optimal velocity information from the acquired Doppler data, a modulation of the tissue velocity signal may be needed. This may be achieved by smoothing of the colour TD signal by temporal filtering or by amplification or reduction of spectral TD signal intensity using different gain settings.

It has been reported earlier that due to inherent differences between the methods, colour TD measurements significantly underestimate myocardial velocities obtained with pulsed TD.10,11 However, the effect of temporal filters and gain settings that may be used during data acquisition and analysis on the outcomes of the measurements with the two TD methods has not yet been systematically studied, although we reported previously on the effect of temporal filtering on the results of myocardial tissue velocity measurements with colour TD.22 Therefore, the aim of this study was to compare the results of myocardial velocity measurements obtained with colour and spectral TD methods, and to find out whether, and if so, to what extent different filter and gain settings alter the results and influence the difference between measurements with these two TD modalities.


   Methods
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 Abstract
 Introduction
 Methods
Results
 Discussion
 Funding
 References
 
Of all, 57 patients referred for transthoracic echocardiography on clinical grounds were examined. The study population comprised 31 women and 26 men (50 ± 12 years). All the subjects were in sinus rhythm. They were examined with conventional echocardiography and with tissue Doppler imaging, and all recordings were performed by the same experienced sonographer. The study was approved by the Ethics Committee of Karolinska University Hospital, Stockholm, Sweden.

Conventional echocardiography
Echocardiography was performed using commercially available equipment (Vivid 7, GE Vingmed, Horten, Norway) with a standard phased array 2.5 MHz multi-frequency transducer. The images were acquired from apical four-chamber (4CH) and two-chamber views with the patient in left lateral position, at the end of expiration. In addition to 2D imaging, early mitral inflow velocity (E) was measured using pulsed wave Doppler, by positioning the 5 mm sample volume at the level of the tips of mitral leaflets in the apical 4CH views.

Tissue velocity echocardiography
Colour tissue Doppler
Cineloops of three to six heartbeats were acquired in each case with a high temporal resolution (100–160 frames/s), i.e. in 19 cases with 100 frames/s, in 22 cases with 104–105 frames/s, in 15 cases with 154–162 frames/s, and in 1 case with 184 frames/s. The formatted raw data containing both grey scale and colour Doppler tissue velocity information were stored digitally for a subsequent offline analysis using Echopac software (Echopac, Version 6.0.0, GE Vingmed Ultrasound, Norway). The analysis of longitudinal myocardial velocities was performed from an optimal measuring position (sampling volume 2 mm) set at the basal segment of each LV wall (septal, lateral, inferior, and anterior), depending on image characteristics. The following variables were measured in the left ventricular colour TD-derived myocardial longitudinal velocity curve: (i) peak systolic velocity (PSVm), i.e. maximal velocity on the myocardial velocity curve during systolic ejection; and (ii) peak early diastolic velocity (E'), i.e. maximal velocity on the myocardial velocity curve during early diastolic filling (diastolic E-wave).

Initially, both variables were measured without any filtering. While keeping the myocardial location of sampling point unaltered, the selected cineloops were then subjected to gradual temporal filtering starting at 30 ms followed by 50 ms, and finally by 70 ms filter. The measurements of PSVm and E' were repeated at each filter setting. All measurements were performed on three cardiac cycles and the results were averaged. E/E' ratio was calculated at all filter widths.

Spectral tissue Doppler
Spectral TD velocity data were acquired from an optimal measuring position at the basal segment of septal, lateral, inferior, and anterior wall of LV using a 2 mm sample volume. Myocardial longitudinal velocities during three consecutive cardiac cycles were recorded while care was taken to keep the data sampling point placed at the ventricular myocardium and the angle of insonation as parallel as possible to the long axis of the myocardial movements throughout each cardiac cycle. Nyquist’s limit was adjusted during each recording. Standard gain settings set by manufacturer were used, with a receive gain of 3 dB. The raw data containing both grey scale and TD information were stored for offline analyses on a computer equipped with Echopac software (Version 6.0.0, GE Vingmed Ultrasound).

The measured variables were myocardial longitudinal velocities during (i) peak systole (PSVm), and (ii) early diastole (E'), which were averaged for three cardiac cycles. The initial measurements were performed without any change in the default offline gain setting at 50% saturation of the spectral signal. Subsequently, the PSVm and E' measurements were repeated, first with the offline gain level set to 100% saturation and then with the offline gain reduced to 0% saturation so that the spectral registration was barely visible. E/E' ratio was calculated at all the chosen gain settings.

Statistical analysis
All data are presented as mean ± SD. The statistical significance level was set at P < 0.05. Group comparisons of continuous variables were made using analysis of variance (ANOVA) followed by post hoc Scheffe’s tests. Pearson’s correlation coefficient was used for the analysis of linear correlation between the evaluated methods. Student’s t-test was used when suitable for comparisons of paired data. The analyses were carried out using standard statistical software (SPSS version 11.0.0). Methodological error (Err) in a single measurement estimated from double measurements was calculated according to formula Err = (SDdiff x 100%)/total mean x {surd}2, where SDdiff is the SD of the difference between the measurements.


   Results
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 Abstract
 Introduction
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Discussion
 Funding
 References
 
Clinical characteristics of the study group are presented in Table 1. There were no significant differences in the proportion of men and women, age distribution, and the occurrence of different pathologies in the subgroup of men and women.


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  		Table 1 Clinical characteristics of patient population

 
The intra-observer variability for PSVm measured with spectral TD at 50% gain saturation was higher (Err = 5.7%) than the corresponding inter-observer variability (Err = 4.1). Similarly, the intra-observer variability of PSVm obtained with unfiltered colour TD was higher than the corresponding inter-observer variability value (Err = 7.9%).

A typical myocardial longitudinal velocity curves obtained from the basal septum using colour TD with different filter widths are presented in Figure 1 (upper panel), whereas Figure 1(lower panel) shows spectral TD-derived curves for the same myocardial segment using different offline gain settings. As can be seen from the figure, the use of temporal filtering with increasing filter width or changing gain setting from unsaturated to saturated resulted in a clear alteration of the retrieved velocity information.


Figure 1
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  		Figure 1 Typical myocardial tissue velocity curves obtained with colour TD at different temporal filter widths (upper panel) and with spectral TD (lower panel) at different gain settings (0 saturation, left-handed curve; 50% saturation, middle curve; 100% saturation, right-handed curve). Note the change in shape of the curves along with increasing filtering and gain.

 
The myocardial longitudinal systolic velocity values obtained with colour TD at increasing filter width in four different left ventricular walls are presented in Table 2, whereas the effect of temporal filtering on corresponding early diastolic velocities is summarized in Table 3. The effect of different filters on average systolic and early diastolic velocity values based on measurements in all four studied myocardial walls is illustrated in Figure 2 (upper panel). As can be seen, increasing filter width to 30, 50, and 70 ms resulted in gradual decrease in the measured velocities (P < 0.001 for each respective filter width) in all four left ventricular walls. At the same time, the variability of the average myocardial velocity values increased (Figure 2). As a consequence of the observed filter-dependent tendency to underestimation and increased variability of the myocardial early diastolic velocity values, the E/E' ratio increased along with the increasing filter width (P < 0.001 for each respective filter width), and the dispersion of the calculated individual values about the assigned value of 100% corresponding to unfiltered data increased as well (Table 4; Figure 2, lower panel).


Figure 2
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  		Figure 2 The distribution of the PSVm, E', and E/E' values measured with colour TD at different filter widths. The individual data points for PSVm and E' represent average values based on measurements in all four studied myocardial walls, whereas for the calculation of E/E' only the E' velocities from the basal septum were used. All values are expressed as a percentage of the corresponding values obtained without any temporal filtering that were considered to represent a value of 100%. The 100% level is indicated by the dotted lines.

 


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  		Table 2 Peak systolic myocardial velocities measured with colour and spectral tissue Doppler using different filter and gain settings

 


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  		Table 3 Peak early diastolic myocardial velocities measured with colour and spectral tissue Doppler using different filter and gain settings

 


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  		Table 4 Peak early transmitral inflow velocity to peak early mitral annulus tissue velocity (E/E') ratio using colour and spectral TD

 
Similar to what was observed with temporal filtering, changing offline gain setting resulted in a significant alteration of the measured longitudinal peak systolic and early diastolic velocity values. By reducing offline gain to 0% saturation level, a significant underestimation of the myocardial velocity values measured with the manufacturer’s gain setting (50% saturation) was obtained (P < 0.001), whereas increasing offline gain to 100% saturation resulted in a significant (P < 0.001) overestimation of these values (Tables 2 and 3; Figure 3, upper panel). E/E' ratio was consequently significantly increased (P < 0.001) with desaturated (0%), but decreased (P < 0.001) when fully saturated (100%) offline gain was used (Figure 3, lower panel).


Figure 3
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  		Figure 3 The distribution of the PSVm, E', and E/E' values measured with spectral TD at different gain settings. The individual data points for PSVm and E' represent average values based on measurements in all four studied myocardial walls, whereas for the calculation of E/E' only the E' velocities from the basal septum were used. All values are expressed as a percentage of the corresponding values obtained with the default gain setting (50% saturation) that were considered to represent a value of 100%. The 100% level is indicated by the dotted lines.

 
With the use of the manufacturer’s gain setting (50% saturation) and no temporal filter, the results of colour TD measurements correlated significantly with those of spectral TD (Figure 4). The correlation was equally strong for PSVm (R = 0.91; P < 0.01) and E' (R = 0.93; P < 0.01), and there was a significant correlation between the colour TD- and spectral TD-based E/E' ratio as well (R = 0.86; P < 0.01). Despite the strong relation between the respective variables measured with the two tested methods, the obtained results did not agree. The myocardial velocities measured with spectral TD were significantly higher (P < 0.001) than those obtained with colour TD (Tables 2 and 3, Figure 4) and the difference increased along with increasing filter width and gain level. On the other hand, E/E' ratio values derived from spectral TD imaging were significantly lower (P < 0.001) than the corresponding colour TD values (Table 4 and Figure 4, lower panel). Also, the difference between the methods increased at increasing filtering and gain saturation.


Figure 4
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  		Figure 4 Relationship between spectral and colour Doppler-measured PSVm, E', and E/E' values using 50% saturated gain and no temporal filtering. The individual data points for PSVm and E' represent average values based on measurements in all four studied myocardial walls. E/E' is based on the E' velocities from the basal septum.

 
The degree of agreement between the results of spectral and colour TD is described by the figures in Table 5. As can be seen from the table, the limits of agreement for the respective variables are rather wide, reflecting considerable variation of the differences between the two methods.


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  		Table 5 Mean differences and limits of agreement (mean ± 2SD) between colour TD without temporal filtering and spectral TD with 50% offline gain saturation (mean values for all four walls)

 

   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
Funding
 References
 
In the present study, myocardial longitudinal peak systolic and early diastolic velocities were measured using spectral and colour TD modality with different temporal filters and offline gain settings. The obtained results not only reveal the existence of a significant difference between the results produced by these two methods, but also clearly show that temporal filtering and different offline gain settings significantly alter velocity measurements.

As far as the accuracy of the present results concerns, it should be kept in mind that the action of temporal filters does not follow a continuous function but is discrete. This behaviour results in a characteristic interplay between filtering features and sampling rate resulting in that the smoothing effects of different filters may overlap and will gradually dissociate at increasing sampling frequency but will not be fully separated until the frame rate of 183.4 Hz is reached.22 Consequently, over a certain range of sampling rate <183.4 Hz, a frame rate-dependent ambiguity of filtering effect of different filters will occur, with some of the filters spanning over the same number of frames, and hence equalized in their actions. In the present study, the sampling rate ranges of 100–105, 154–162, and 184 Hz were used. At the first two of these ranges, there occurs equalization of the effect of 30 and 40 ms, and 50 and 60 ms filters, and then the equalization of 20 and 30 ms, and 40 and 50 ms filters, respectively, but the filtering effects of currently used 30, 50, and 70 ms filters remain fully separated from each other. Accordingly, no uncertainty caused by filter ambiguity has been introduced into the outcomes of the present measurements.

Another factor that should be considered when comparing the results of colour and spectral TD measurements is the angle of insonation. The influence of unfavourable insonation angle may affect both spectral and colour TD measurements resulting in underestimation of the true myocardial velocities. In order to minimize the possibility of undesirable effects of Doppler angle, all myocardial velocity recordings in the present study were performed by one experienced sonographer and a special effort was made to keep the angle of incidence parallel to the long axis of the ventricle, and the sampling volume at the same place within the myocardium. Certainly, minor angle deviations and differences between spectral and colour TD registrations cannot be entirely excluded, but it is unlikely that their influence would be entirely unidirectional and of magnitude sufficient to obscure the differences between the two tested modalities.

Understanding the significance of the subjective adjustments made by sonographer during echocardiographic data acquisition and analysis for the accuracy of the myocardial velocity measurements is of crucial importance. Modern echocardiography equipments provide high frame rates and generate high-fidelity TD signals that contain detailed information not only on rather slow main systolic and diastolic myocardial movements, but also on rapid motions during isovolumic phases. The isovolumic myocardial motion variables are of considerable clinical interest since they appear to be early markers of ischaemia-induced disturbances in myocardial function,23–27 and the sampling frequencies of at least 100 Hz12 are therefore increasingly used for colour TD data acquisition. However, the use of high sampling rates required for analysis of rapid myocardial events increases the amount of signal noise and creates an intuitive call for signal filtering to facilitate the analysis of acquired velocity data. On the other hand, filtering of the colour TD signal may not only reduce noise, but it may also alter the velocity signal itself and thus introduce a significant errors to the velocity measurements as evidenced by the results of the present study showing decreasing velocity values at increasing filter widths. Hence, in order to ensure the optimal signal-to-noise ratio, temporal filtering should be used with caution, governed by the awareness of the interplay between sampling rate and temporal filters. A specific choice of lower frame rate without filtering or higher frame rate with some cautious filtering may need to be considered depending on whether slow or rapid myocardial movements are targeted.

Similar to what was the case with the temporal filtering of colour CD signal, changes in offline gain resulted in a significant alteration of the spectral TD-measured systolic and diastolic velocities. Compared with the default gain setting of 50% saturation, significantly lower PSVm and E' values were obtained with desaturated gain, whereas 100% saturation of gain resulted in significantly higher velocity values. This is not unexpected because increasing gain will produce increasing spectral broadening leading to upward shift of the outer border of the modal velocity towards higher velocity values. The present results are in keeping with those obtained by Lui et al., 28 in which low gain caused a 10% underestimation and high gain resulted in 20% overestimation of true velocity in an in vitro model of pulsatile flow in unstenosed and stenosed tubes. Interestingly, the same authors also identified the Doppler gain as one of the most significant sources of error and variability when measuring flow velocities in this in vitro model. Similarly, changes of gain setting in the present study produced effects that are clinically highly relevant and emphasize the importance of gain level selection for accurate measurement of myocardial velocities and for critical comparison of different measurements in the same, or in different individuals.

In the present study, PSVm and E' values measured with pulsed TD were significantly higher than the corresponding velocities measured by colour TD modality and the difference between the methods increased along with increasing filtering and gain saturation. Consequently, the E/E' ratio based on spectral TD measurements was significantly lower than that calculated from the results of colour TD measurements and increasing gain saturation or filter width as well augmented this difference. Despite a good correlation between the results obtained with both methods, the agreement between the colour and spectral TD-based measurements was rather poor. These findings are in accord with the outcomes of previous studies10,11 and reflect different principles for velocity computation with the two methods. Spectral TD employs fast Fourier transformation and the myocardial velocity is displayed as a spectrum. Maximal velocity is then measured at the outer border of the velocity waveform. On the other hand, colour TD uses autocorrelation technique and provides an average velocity of myocardial movement within a region of interest at a chosen myocardial location. Another factor that might contribute to the observed method-dependent differences is the inherent higher temporal resolution of pulsed TD modality.

The present results have important clinical implications. Accurate measurement of myocardial tissues velocities is of considerable clinical interest since it is now generally accepted that decreased peak systolic and early diastolic velocities indicate failing myocardial function, and E/E' ratio has been shown to provide a good estimation of left ventricular filling pressure.13,16 Along with a highly noise-free and accurate reconstruction of myocardial velocity wave, the knowledge of existing differences between the spectral and colour TD modalities is a prerequisite for an efficient diagnosis of myocardial dysfunction. In the present study, the constant mean differences between the spectral and colour TD velocities were 2.17 ± 0.89 cm/s for PSv and 3.67 ± 1.82 cm/s for E', whereas the mean difference between the methods for E/E' was –3.70 ± 2.00. This implies that velocity measurements by the two methods may differ as much as 0.39 to 3.95 cm/s for PSv, –0.03 to 7.31 cm/s for E' and by –0.30 to –7.70 for the E/E' ratio. Since the magnitude of longitudinal myocardial velocity changes caused by myocardial ischaemia appears to be around or <2 cm/s,29 the currently found limits of agreement between the methods can hardly be acceptable in clinical practice. This means that the two methods should not be employed interchangeably and the obtained results should be compared with the range of normal values specific for each method. Furthermore, the present results clearly demonstrate the need of viewing the measured myocardial velocities not only in close relation to the employed TD modality, but also in relation to the offline image adjustments using temporal filtering or varying gain level.

In conclusion, the present results demonstrate the existence of significant differences between spectral and colour TD-based measurements of systolic and diastolic myocardial velocities, with higher velocity values and lower E/E' ratio produced by spectral TD. Temporal filtering with increasing filter width, as well as increase in gain saturation level significantly alter the measured velocity values resulting in accentuation of differences between the two TD modalities.

Conflict of interest: none declared.


   Funding
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Funding
References
 
Supported by grants from the Swedish Heart-Lung Foundation.


   References
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 Abstract
 Introduction
 Methods
 Results
 Discussion
 Funding
 References
 

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