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Clinical Observations |
an1
1 Department of Radiology, Leiden University Medical Center, C2-S, Albinusdreef
2, 2333 ZA Leiden, The Netherlands.
2 Department of Cardiology, Leiden University Medical Center, Leiden, The
Netherlands.
Received April 13, 2005;
accepted after revision July 10, 2005.
Address correspondence to A. de Roos.
Abstract
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CONCLUSION. RV volumes can be accurately assessed using MDCT.
Keywords: cardiac imaging • CT • ECG gating • heart • MDCT • right ventricular function
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In recent years, the critical role of RV function in determining the clinical outcome of patients with acute pulmonary embolism [3, 4] and of those with chronic heart failure has been recognized [5]. In particular, the widespread use of MDCT in patients with suspected pulmonary embolism has raised interest in assessing RV function in this patient category. Interestingly, simple measurements of RV dimensions using non-ECG-gated MDCT images of the heart have shown to be of prognostic significance in patients with acute pulmonary embolism [3, 4]. It is conceivable that more accurate dynamic information could enhance the value of MDCT for the evaluation of RV function.
Currently, 2D echocardiography is the technique of choice for routine clinical evaluation of LV and RV function. Echocardiography estimates LV function using geometric assumptions. The RV has a more complex shape; therefore, it is difficult to accurately assess the volumetrics of the RV when using non-3D methods, such as 2D echocardiography.
MDCT, similar to MRI, is an inherently 3D technique that allows full coverage of the ventricular volumes without the use of geometric assumptions.
Accordingly, the objective of our study was to validate the assessment of RV function using MDCT. For internal validation, the volumetrics of both ventricles were compared. Furthermore, echocardiography was used as an external reference for validating the measurements of LV function using MDCT and thereby also for indirectly validating the measurements of RV function.
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Validation Techniques
Agreement for stroke volume (SV) between the LV and RV obtained from MDCT
data was used for internal validation. The RVSV and LVSV should be identical
because there were no signs of valvular disease or intracardiac shunting on 2D
or color Doppler echocardiography. As external validation, MDCT-derived
systolic LV function was compared with systolic LV function as assessed by 2D
echocardiography using the biplane Simpson's technique as previously shown
[1].
Imaging Protocols
MDCT data acquisition and reconstructions—Contrast-enhanced
ECG-gated cardiac MDCT was performed using a 16-MDCT scanner (Aquilion 16CFX,
Toshiba Medical Systems). Contrast agent (120-140 mL iobitridol [Xenetix 300,
Guerbet]) was administered in an antecubital vein with a flow rate of 4 mL/sec
to ensure enhancement in the ascending aorta while contrast enhancement in the
RV was still present. No ß-blocker preparation was used. To trigger the
start of the acquisition, we used automated threshold enhancement detection in
the aortic root. The rotation time was 0.4 or 0.5 sec, and the pitch factor
varied between 0.20 and 0.30. The optimal combination of rotation time and
pitch factor was chosen automatically by Sure Cardio software (Toshiba Medical
Systems) to provide the best temporal resolution for a given heart rate.
Temporal resolution varied between 50 and 250 msec depending on the heart
rate. Collimation for raw data acquisition was 16 x 0.5 mm; the tube
current was 250 mA and the tube voltage, 120 kV.
Two-millimeter-thick contiguous slices were retrospectively reconstructed in a 512 x 512 matrix using a 240-mm field of view; data for 20 cardiac phases in steps of 5% of the R-R interval (ranging from 0% to 95% for each investigation) were obtained using a segmental reconstruction algorithm. The whole heart from the aortic root to the diaphragm was covered within the reconstructed 60-80 slices per cardiac phase point. The data were stored in DICOM format and transferred to a PC workstation (Dell) running on Linux software (Suse Linux GmbH).
Echocardiographic image acquisition—Patients were imaged in the left lateral decubitus position using a commercially available system (Vingmed System FiVe, GE Healthcare). Images were obtained using a 3.5-MHz transducer at a depth of 16 cm in the parasternal and apical views (standard parasternal long- and sort-axis views and apical two- and four-chamber images). The images were triggered to the QRS complex and saved in cine loop format.
Data Analysis
The MDCT images were analyzed with dedicated cardiac function analysis
software (CT-MASS, Medical Imaging Systems). By inspection of smooth running
cine movies, images in which the LV volumes were the largest and the smallest
were selected as the end-diastolic and end-systolic phases in the cardiac
cycle, respectively. On every other axial slice, the end-diastolic and
end-systolic RV and LV endocardial border contours were drawn in consensus by
two researchers; the trabeculae and papillary muscles were included in the
ventricular cavity. The entire ventricles were covered including the outflow
tract (Figs. 1A,
1B,
1C, and
1D). End-diastolic and
end-systolic volumes, SVs, and EFs were calculated. Images were reanalyzed to
assess interobserver agreement using a paired Student's t test.
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Statistical Analyses
The data were analyzed with SPSS software (version 11.5, Statistical
Package for the Social Sciences) for Windows (Microsoft) and Excel 2000
(Microsoft). Continuous data were expressed as means ± SD.
Linear regression analysis was performed to estimate the coefficients of the linear equation and interdependence between the RVSV and LVSV, as determined by MDCT, and between the LVEF determined by MDCT and that determined by echocardiography.
Paired Student's t tests were performed to determine concordance between SVs and EFs. Bland-Altman plots were reconstructed to determine the limits of agreement between SVs and between EFs.
For all statistical testing, a p value of less than 0.05 was considered statistically significant.
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We found a small but systematic difference between the SVs of both ventricles with mild underestimation of the RVSV as compared with the LVSV when measured from MDCT data. A possible explanation for this difference is the slight electrophysiologic asynchronicity of ventricular contractions [6] in combination with the suboptimal temporal resolution. In the present study, the LV was systematically chosen to isolate end-systolic time points. The left part of the interventricular septum is the first part of the ventricle to become depolarized, resulting in a physiologic asynchronicity of LV and RV contractions [6].
MRI is widely accepted as a gold standard for measurements of systolic LV function [7] and RV function [8]. A mean difference in SVs of up to 5.8 ± 12.9 mL between the LV and RV has been found when using MRI for internal validation [8]. The mean differences in SVs and the SDs observed in the present MDCT study were even smaller, indicating the reliability of MDCT measurements. The smaller variance in MDCT volumetric measurements may also be a consequence of the single breath-hold approach for MDCT as opposed to the multiple breath-hold MRI technique. With the single breath-hold technique, all consecutive slices originate from a single 3D volume.
However, the temporal resolution of current MDCT technology is still limited. Echocardiography and MRI allow image acquisition at a high frame rate, thereby allowing accurate isolation of end-diastolic and end-systolic time points.
It has been reported that a temporal resolution of less than 100 msec is needed for obtaining accurate cardiac volumetrics [9, 10]. The current MDCT technology allows a reconstruction of any phase point over the cardiac cycle, although the actual temporal resolution in cardiac MDCT is not defined by the selected number of phase points, but by the amount of view sharing within images representing consecutive phases. The amount of view sharing depends on the pitch factor and the heart rate.
The difference in the mean LVEF measured with MDCT and that measured with echocardiography was only 3.3% in the present study, as compared with the 8.5% difference between MRI and echocardiography [11]. The reproducibility of LVEF measurements is therefore similar between MDCT and MRI.
ECG-gated MDCT allows assessment of cardiac function in a reliable manner. New indications for MDCT could include cardiac function assessment in patients with contraindications for MRI (e.g., pacemakers and other metallic implants) and RV function assessment in patients with pulmonary embolism and those with pulmonary hypertension. Additional clinical studies are needed to show the clinical impact of RV function assessment using MDCT.
Acknowledgments
We thank Berend C. Stoel, Annette van den Berg-Huysmans, Koos Geleijns, and
Joost J. H. Roelofs for technical assistance and Bart Mertens (Department of
Medical Statistics) for statistical support.
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) in stroke volumes (SVs)
between left ventricle (LV) and right ventricle (RV) is plotted against
average value of SVs of both ventricles (solid line = mean value of
differences, dotted lines = mean value of differences ± 2 SD).
Difference in SVs does not vary in any systematic way over range of
measurements. Mean difference to prejudice RVSV is 3.6 ± 4.0 mL (range,
-4.4 to 11.6 mL) as compared with LVSV.
