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Original Research |
1 Department of Diagnostic Radiology, RWTH Aachen University Hospital,
Pauwelsstraβe 30, D 52057 Aachen, Germany.
2 Department of Cardiology, Sarawak General Hospital, Sarawak, Malaysia.
3 Siemens Medical Solutions, Erlangen, Germany.
Received August 14, 2007;
accepted after revision October 19, 2007.
Address correspondence to C. Plumhans
(plumhans{at}rad.rwth-aachen.de).
Abstract
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SUBJECTS AND METHODS. Thirty-eight patients (25 men, 13 women; mean age ± SD, 55.0 ± 8.8 years) with suspected coronary artery disease underwent contrast-enhanced 64-MDCT (64 x 0.6 mm, 120 kV, 770 mAseff) and 1.5-T MRI (balanced fast-field echo; TR/TE, 3.3/1.6; flip angle, 60°; 50 phases). Double oblique short-axis MDCT and MR images were used for further analysis. End-diastolic volume (EDV), end-systolic volume (ESV), stroke volume (SV), and ejection fraction (EF) were computed from manually drawn endocardial contours of the right ventricle. For statistical analysis, repeated-measures analysis of variance and Pearson's correlation coefficients were calculated. Bland-Altman plots were computed.
RESULTS. In general, RV volumes calculated from 64-MDCT agreed well
with those calculated from MRI. The mean EF (± SD) calculated from MDCT
and MRI was 51.0% ± 7.8% and 51.4% ± 7.3%, respectively. An
excellent correlation was observed for EDV (r = 0.99), ESV
(r = 0.98), SV (r = 0.98), and EF (r = 0.97).
Bland-Altman plots showed no systematic variation between MDCT and MRI data.
No statistically significant differences (p 
0.05) between the
techniques were found.
CONCLUSION. Although contrast injection is optimized for visualization of the coronary arteries, retrospectively ECG-gated 64-MDCT permits reliable assessment of global RV function.
Keywords: cardiac imaging • heart disease • MDCT • MRI • right ventricular function
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Potential advantages of 64-MDCT in cardiac function imaging compared with 16-MDCT are better temporal resolution, shorter breath-hold period, different contrast enhancement, and lack of β-blockers. In consideration of these advantages, our study differs from a previous similar study with 16-MDCT.
The aim of this study was to compare the assessment of global RV function using contrast-enhanced ECG-gated cardiac 64-MDCT with 1.5-T cine MRI.
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Patients with a resting heart rate above 70 beats per minute (bpm) received a β-blocker (50–100 mg of metoprolol) orally (n = 4) to minimize motion artifacts in MDCT coronary angiography. The CT examinations were per formed on a 64-MDCT scanner (Somatom Sensa tion 64 Cardiac, Siemens Medical Solutions). A standard ized scanning pro tocol with a collimation of 64 x 0.6 mm, gantry rotation time of 330 milliseconds, and table feed of 3.8 mm per ro tation, resulting in a pitch of 0.2, was used. A tube voltage of 120 kV with an effective tube current–time product of 770 mAseff was applied. No ECG-dependent tube current modulation was applied. Temporal resolution ranged between 83 and 165 milliseconds depending on each individual patient's heart rate. Radiation exposure doses were calculated (CT Expo version 1.5, Micrsoft Excel) [20].
For contrast enhancement, a nonionic contrast agent with an iodine concentration of 370 mg I/mL (iopromide [Ultravist 370, Bayer HealthCare]) was administered via an 18-gauge venous access placed in an antecubital vein. For optimized contrast timing, the test-bolus technique was used: 10 mL of contrast medium was injected at a flow rate of 5 mL/s followed by a 50-mL saline chaser injected at the same flow rate. The time between contrast injection and maximum enhancement in the ascending aorta was used as the scanning delay for optimal contrast in the coronary arteries. The total volume of contrast medium used was calculated as the scanning time multiplied by the flow rate of 5 mL/s plus an additional 8 mL, resulting in a mean total contrast volume of 75 ± 2 mL.
From the CT raw data, a stack of images was calculated every 5% of the cardiac cycle (0–95%). Double oblique short-axis reformations of the center on the right ventricle were computed on an external workstation (Leonardo, Siemens Medical Solutions). The slice thickness of the short-axis reformations was 5 mm without an interslice gap. For further data analysis, a dedicated software tool (Argus, Siemens) was used. End-systole was defined as the cardiac phase with the smallest RV volume, whereas end-diastole was defined as the cardiac phase with the largest RV volume (Fig. 1A, 1B, 1C, 1D). Endocardial contours were drawn manually using end-systolic and end-diastolic images. Slices from the apex to the tricuspid valve were included in the analysis, according to standard conventions defined by Pennell [21]. The RV outflow to the pulmonary valve was included in the RV volume. Papillary muscles were considered to be part of the RV cavity. For drawing endocardial contours, the window settings were individually adapted according to the half-contour principle.
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End-diastolic volume (EDV), end-systolic volume (ESV), stroke volume (SV), and ejection fraction (EF) were calculated using Simpson's method.
MRI was performed on a 1.5-T whole-body scanner (Gyroscan Intera, Philips Medical Systems) using a 5-channel cardiac coil. For ECG synchronization, prospective triggering was applied, and during breath-holding, 50 phases for one cardiac cycle were acquired at each slice position. A balanced fast-field echo sequence used the following imaging parameters: TR/TE, 3.3/1.6; flip angle, 60°; and slice thickness, 5.0 mm without an interslice gap. The measured voxel size was 1.83 x 1.70 x 5.00 mm with a reconstructed voxel size of 1.48 x 1.48 x 5.0 mm. Short-axis MR images were analyzed in the same manner as 64-MDCT images using the same software.
MDCT and MR images were evaluated independently by two radiologists with 8 and 2 years' experience in cardiac radiology, respec tively. The two observers were blinded to the results of the other technique.
Continuous data are given as means ± SD. For data analysis,
repeated-measures analysis of variance was performed. Pearson's correlation
coefficient was calculated. Statistical analysis was performed using JMP
software (version 6, SAS Institute). Agreement in global RV function
parameters was determined with Bland-Altman plots (MedCalc, version 8.2.1.0,
MedCalc Software) [22]. A
p value of 0.05 was considered statistically significant.
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On MDCT and MRI, respectively, the mean EDV was 78.3 ± 17.7 vs 77.9 ± 17.7 mL; mean ESV, 38.3 ± 10.1 vs 37.7 ± 9.8 mL; mean SV, 40.0 ± 11.2 vs 40.0 ± 11.0 mL; and mean EF, 51.0% ± 7.8% vs 51.4% ± 7.3%. These results showed an excellent correlation; analysis of variance did not reveal statistically significant differences between the imaging techniques (Table 1).
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Bland-Altman plots showed no systematic variation between MDCT and MR data (Fig. 2A, 2B, 2C, 2D).
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Analysis of RV function was feasible using the reported contrast injection protocol and the same contrast bolus as that used for coronary MDCT angiography. In this particular setting, the use of a contrast injection protocol optimized for enhancement of the coronary arteries did not greatly hamper evaluation of global RV function. Nevertheless, improvements in RV contrast enhancement may ease delineation of the RV endocardial border. The latter may be achieved using biphasic contrast injection protocols with or without a delay between both contrast injections or by mixing 10–30% of contrast material in the saline chaser bolus. Thus, gated cardiac scanning can be optimized for analysis of RV function by using bi- or multiphasic contrast injection [24] or by adding contrast medium to the saline chaser so that a certain amount of contrast enhancement in the right ventricle during scanning could be reached without altering of coronary enhancement. In our study, we opted for a single-phase contrast medium injection to make the viability of the study uniform.
Compared with the studies by Koch et al. [12] and Raman et al. [13] in which a mean contrast volume of 100 mL was injected to assess RV function, in this study a mean total contrast volume of 75 ± 2 mL was sufficient to obtain excellent RV function measurements. Reducing the total volume of contrast injected for cardiac MDCT is particularly suitable for patients with clinical diagnoses such as renal insufficiency.
Otherwise, cardiac MDCT can be optimized for visualization of the truncus pulmonalis, especially for analysis of RV function in patients with pulmonary hypertension or pulmonary embolism. In doing so, we increase the risk of beam hardening in the right ventricle, which causes the endocardial contours of the right ventricle to become less identifiable.
In accordance with the results of other studies [23], our results showed that assessment of RV function using MDCT led to a slight overestimation of the mean EDV and ESV. The limited temporal resolution of MDCT compared with MRI is considered the main reason for this phenomenon [12]. In some patients, discrimination of the heart apex from the diaphragm was hampered by low contrast. Low contrast may result in in-accurate contours being drawn, with subsequent overestimation of the RV volumes. Nevertheless, the average deviation from the reference standard is within clinically acceptable ranges. For clinical purposes, these differences can be neglected.
A major shortcoming of this study is the need to administer β-blockers in some patients with elevated heart rates (> 70 bpm). The negative inotropic and chronotropic effects of β-blockers may limit the significance of results by interfering with the true RV volumes. However, because MRI was performed immediately after 64-MDCT, the effect of the β-blockers likely continued during the MR examination. The mean time delay between MDCT and MRI was 54 ± 17 minutes. This assumption is supported by the comparable heart rates during both examinations. Another general limitation of MDCT is radiation and contrast material exposure. Consequently, there is no indication to perform cardiac MDCT solely to assess cardiac function because radiation-free and contrast material–free imaging techniques, such as MRI and echocardiography, are readily available.
In conclusion, retrospectively ECG-gated 64-MDCT of the heart provides reliable RV function measurements when compared with MRI. Cardiac 64-MDCT shows excellent correlation with MRI for EDV, ESV, SV, and EF measurements. RV function measurements with the aid of 64-MDCT offer precious information in patients with pulmonary embolism, patients with pulmonary hypertension, or patients being evaluated after myocardial infarction, in addition to those who have undergone a triple-rule-out examination.
The intrinsic drawbacks of CT, such as radiation exposure and the need for iodinated contrast material, limit the application of MDCT solely for assessing cardiac function. However, in patients with suspected RV dysfunction who undergo coronary CT angiography, assessment of RV function will improve the diagnostic significance of cardiac MDCT.
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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an H, Kroft LJ, Bax JJ, et al. MDCT assessment of right
ventricular function. AJR 2006;186
[6 suppl 2]:S366
–S370
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