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Assessment of left ventricular volumes using simplified 3-D echocardiography and computed tomography – a phantom and clinical study
© Mårtensson et al; licensee BioMed Central Ltd. 2008
Received: 28 April 2008
Accepted: 04 June 2008
Published: 04 June 2008
To compare the accuracy of simplified 3-dimensional (3-D) echocardiography vs. multi-slice computed tomography (MSCT) software for the quantification of left ventricular (LV) volumes.
Three-D echocardiography (3-planes approach) and MSCT-CardIQ software were calibrated by measuring known volumes of 10 phantoms designed to closely mimic blood-endocardium interface. Subsequently, LV volumes were measured with both the methods in 9 patients referred routinely for coronary angiography and the agreement between the measurements was evaluated.
Simplified 3D-echocardiography provided higher degree of agreement between the measured and true phantom volumes (mean difference 0 ± 1 ml, variation range +4 to -4 ml) than MSCT software (mean difference 6 ± 5 ml; variation range +22 to -10 ml). The agreement between LV measurements in the patients was considerably poorer, with significantly larger volumes produced by MSCT (mean difference -23 ± 40 ml, variation between +93 and -138 ml).
Simplified 3-D echocardiography provides more accurate assessment of phantom volumes than MSCT-CardIQ software. The discrepancy between the results of LV measurements with the two methods is even greater and does not warrant their interchangeable diagnostic use.
Echocardiography and x-ray based computed tomography provides possibilities of detailed evaluation of cardiac morphology and function and the introduction of these techniques constitutes without any doubt an important landmark in the history of diagnostic cardiology. Today, both the methods not only form a backbone of diagnostic cardiac procedures but also are much appreciated as sensitive research instruments. Recently, the diagnostic capacity of echocardiography has been further improved by the addition of 3-dimensional cardiac imaging.
An important link in the process of cardiac diagnostics is accurate estimation of left ventricular volume. In this respect, MSCT has been shown to be both feasible and accurate when compared with left ventriculography , magnetic resonance techniques [2–4], and 2-dimensional echocardiography [4–7]. At the same time, the diagnostic performance of 3-D echocardiography has been demonstrated to be superior to that provided by 2-dimensional echocardiographic imaging [8–13]and a strong correlation was observed between the results obtained with 3-D echocardiographic technique and magnetic resonance imaging [9, 11, 14–17].
However, the results of a recently published study indicate that even though both 3-D echocardiography and MSCT measurements correlate highly with magnetic resonance imaging, 3-D echocardiography compares more favourably in this respect than MSCT that tend to overestimate the magnetic resonance values . Therefore, the aim of this study was to evaluate further the accuracy of both methods in the assessment of LV volumes in clinical subjects and to verify the results of volume measurements in in vitro setting using phantoms.
True phantom volumes and the volumes obtained with simplified 3-D echocardiography and MSCT-CardIQ software.
(mean ± SD; n = 5)
(mean ± SD; n = 5)
36.6 ± 0.55
40.4 ± 1.14
99.4 ± 0.55
99.8 ± 1.10
333.8 ± 0.84
348.4 ± 5.64
291.6 ± 1.14
296.6 ± 0.55
128.0 ± 1.23
131.6 ± 1.68
51.6 ± 0.55
54.6 ± 1.52
202.2 ± 0.84
209.2 ± 1.64
246.0 ± 1.23
251.8 ± 1.48
68.0 ± 1.30
72.6 ± 0.55
158.6 ± 3.44
172.6 ± 0.55
The speed of ultrasound in the phantom material was measured in two pieces of the phantom material, one with and the other without glass powder, using piezoelectric crystals (one working as a transmitter and the other as a receiver) applied to the opposite sides of the test pieces. An electrical circuit and an oscilloscope connected to the crystals measured the time for a pulse of ultrasound to travel the distance between the crystals. The velocities of ultrasound within the phantoms were found to be 1470 m/s in the outer section with the glass powder and 1540 m/s in the inner section with iodixanol.
Nine patients (5 men), aged 64 (range 51–82) years were selected consecutively from a larger, prospective study evaluating efficiency of coronary MSCT for the detection of coronary artery disease in patients referred routinely for coronary angiography due to known or suspected coronary artery or valvular diseases. In all the selected patients, 3-D echocardiography was performed in addition to MSCT. Both 3-D echocardiography and MSCT data were evaluated by independent trained interpreters. The study was approved by the local ethics committee of Karolinska University Hospital, Stockholm, Sweden.
Multi-slice computed tomography
The MSCT examinations of both patients and phantoms were performed employing a 64-slice spiral computed tomography scanner (General Electric (GE) LightSpeed VCT, Milwaukee, Wisc., USA). The gantry rotation time was 0.35 seconds, while a collimation of 64 × 0.625 mm and a tube voltage of 120 kV was used. In the patients, dose modulation was used and the tube current was diminished during systole resulting in the effective current of approximately 240–640 mAs and effective dose of 20.0 mSv. All images (both in patients and phantoms) were acquired with slice thickness of 5 millimetres and spacing equal to 4 millimetres. A bolus injection of 85 ml iodixanol (Visipaque, 320 mgI/ml, GE Healthcare, Little Chalfont, UK) was given intravenously to the patients at a rate of 4–6 ml/sec followed by a flush with 50 ml of saline. Using retrospective electrocardiographic gating, reconstruction of images from 10 phases of the cardiac cycle was done for the patients.
MSCT analysis of ventricular function
Univariate relations between echocardiographic and MSCT-CardIQ measured LV volumes as well as their relation to the true phantom volumes were tested with standard regression analysis. Assessment of agreement between 3-D echocardiography and MSCT-CardIQ software in volume measurements was performed using the method of Bland and Altman . Paired data were compared using the Student's t-test. The data are presented as mean ± SD unless otherwise stated.
The poor agreement between 3-D echocardiography and MSCT-CardIQ software is further illustrated in Bland-Altman plot in Figure 3 (lower panel). The consistent overestimation of the phantom volumes by MSCT-CardIQ software resulted in a mean difference between the results obtained with the both methods amounting to -6 ± 5 ml. The limits of agreement defined as mean difference ± 2 SD were thus +4 ml and -16 ml, respectively. The 95% confidence interval for the upper limit of agreement was +9 to -3 ml and the corresponding confidence interval for the lower limit of agreement was -10 to -21 ml. This implies that the difference between the results of 3-D echocardiographic and MSCT-CardIQ software measurements may assume values ranging from +9 ml to -21 ml.
Measurements in patients
Left ventricular volumes obtained in the same patient with 3-D echocardiography and MSCT-CardIQ software
Measured volumes (ml)
Ejection fraction (%)
In the present study, the results of LV volume measurements by simplified 3-D echocardiography were compared with the results of 64-slice spiral computed tomography and the accuracy of the two methods was evaluated in two steps. First, both the methods were calibrated in vitro by comparison of the measured volumes with known volumes using phantoms, and second, the results of LV volume measurements with both the methods in patients were evaluated.
The phantoms used in the present experiments were especially designed to closely mimic the blood-myocardium border and to provide its equally good detection with both of the tested modalities. In order to fulfil these requirements, the speed of ultrasound wave travelling through the phantom should be in the range for which medical ultrasound equipment is calibrated for, i.e. around 1540 m/s, which is the speed of sound in most tissues imaged by echocardiography. The velocity of sound wave propagation through the currently employed phantoms was found to be 1470 m/s in the section with the glass powder, and 1540 m/s in the inner compartment of the phantom mimicking intracavital LV volume. The lower sound propagation velocity within the outer section of the phantoms resulted in a shift of depth coordinates for this section by +0.0475 mm per each mm. With the thickness of the phantom shell of approximately 8 mm, the total shift in its depth positioning would be about 0.4 mm and this would certainly introduce a systematic error if the total phantom volume was measured. However, since the velocity of ultrasound within the inner section of the phantoms was equal to the calibration velocity for the used equipment, imaging of the phantom "cavity" was not distorted and the currently performed calculations of the inner phantom volumes were therefore not biased.
The absolute value of the x-ray attenuation by the phantom structures was not as critical for the accuracy of the measurements as the velocity of sound wave propagation, but a proper automated border detection by the software required occurrence of a significant attenuation difference between the sections mimicking myocardium and LV cavity. In the present experiments, the chosen concentration of iodixanol in the inner section of the phantoms resulted in the same Hounsfield values as those obtained in routine diagnostic images and the delineation of this section was fully adequate in the case of the symmetric phantoms. The delineation of the "cavities" of the asymmetric phantoms was not entirely satisfactory and sharp contour irregularities of the inner section were not detected resulting in a tendency to overestimation of the phantom "cavity" volume that necessitated manual correction. These inadequacies were, however, not caused by insufficient attenuation differences but by software-dependent limitations to handle highly irregular borders.
The results of phantom volume measurements by simplified 3-D echocardiography showed very good agreement with the true phantom volumes resulting in mean difference between the respective data close to zero with a possible overestimation or underestimation of true volumes by 4 ml that is fully acceptable in clinical practice. On the other hand, the results produced by MSCT-CardIQ software were significantly more biased with a possibility of overestimation by 22 ml or underestimation by 10 ml. Despite the fact that there was a strong relationship between the true phantom volumes and the volumes measured with MSCT-CardIQ software, considerable discrepancies might occur between the true and measured values, and consequently, also between the results obtained with the both methods, thus limiting their interchangeable clinical use.
The poor agreement between the results of 3-D echocardiography and those generated by MSCT-CardIQ software was particularly striking when LV volume measurements in patients were compared. Several previous studies in which assessment of LV volumes with MSCT technique and 2-D echocardiography was evaluated produced results showing a good agreement between these two methods [4–7, 19]. In addition, a good correlation was found between MSCT measurements, and between 3-D echocardiography and magnetic resonance imaging [2–4, 8, 9, 11, 14, 17]. Even if the methodological details and the MSCT software used in the above-mentioned studies differed from the present approach, a similar good relationship with acceptable volume assessments could still be expected to exist between MSCT and 3-D echocardiography. However, the volume overestimation by MSCT-CardIQ software in relation to 3-D echocardiography was found to be considerable in clinical situation and the mean difference between the results obtained with two methods increased from moderate -6.2 ml in phantom experiments to -22.6 ml in patient measurements. At the same time the variation of the results increased considerably resulting in unacceptable limits of agreement ranging between +92.9 ml and -138.1 ml, thereby somewhat in disagreement with some of the above-mentioned earlier published data. On the other hand, the present results showing the larger volumes obtained by MSCT than those measured by 3-D echocardiography are in keeping with the results of recently published study of Sugeng et al. , in which MSCT was found to produce significant overestimation of LV volumes measured by magnetic resonance whereas real-time 3-D echocardiography compared more favourably in this respect.
When considering the results of 3-D echocardiographic and MSCT-CardIQ software based measurements of LV volumes, it has to be kept in mind that the methodological error may be greater in clinical setting than in in vitro phantom measurements. The simplified 3-D echocardiographic technique relies on a computation of LV volume by interpolation of manually traced endocardial border from three different 2-D planes. Consequently, any possible local changes in LV geometry between the traced planes will remain undetected, resulting in calculation error. On the other hand, the delineation of LV area contour with MSCT software may underestimate or overestimate the contribution of the contrast filled crevices and small cavities of the LV myocardial trabecular network. Multiplied by slice thickness, these possible area errors would inevitably result in erroneous LV volume estimations. In addition, the relatively low temporal resolution of the method and the nature of the used MSCT software calculating LV volume by integrating data from short-axis slices make the determination of volumes near to mitral annulus uncertain. However, the discrepancy between the 3-D echocardiographic and MSCT-CardIQ software generated LV volumes in the present study was substantial and suggests the existence of other colluding sources of errors as well. Since the MSCT software performed rather unsatisfactory on phantoms as well, it appears reasonable to believe that the observed considerable differences between the two methods might have been caused by additional, possibly MSCT algorithm dependent error. If so, the present results cast doubt on diagnostic applicability of the currently tested MSCT approach.
In this context, it should be remembered that beside the currently reported results of volume measurements, there are other important factors that would favour the diagnostic use of 3-D echocardiography. For example, the effective radiation dose associated with the MSCT procedure employed in the present study was 20.0 mSv that is a dose equivalent to 1000 chest x-rays or nearly 7 years of background radiation . This dose results in an extra risk of long-term cancer development of about 1/1000 exposed individuals and the procedure is at the same time about 3 times more expensive that conventional echocardiography . On the other side, the 3-D echocardiographic approach, in addition to being radiation-free and more cost-effective, does not require the use of nephrotoxic contrast media, and is faster and easier to perform. These facts should be taken into consideration when adopting As Low As Reasonably Achievable (ALARA) philosophy when performing diagnostic cardiac imaging.
The present results demonstrate that simplified 3-D echocardiography provides a reliable and significantly more accurate assessment of phantom volumes than MSCT-CardIQ software. The discrepancy between the results of both methods increase considerably when LV volume is measured and the limits of agreement are not acceptable for interchangeable diagnostic use of the both methods. Judging from its performance on phantoms, simplified 3-D echocardiography can be expected to provide most accurate LV volume assessments in clinical situations.
This study was supported by grants from the Swedish Heart-Lung Foundation.
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