CLINICAL STUDY: NEW METHODS
Rapid evaluation of left ventricular volume and mass without breath-holding using real-time interactive cardiac magnetic resonance imaging system
Shuichiro Kaji, MD*,
Philip C. Yang, MD*,
Adam B. Kerr, PhD ,
W. H. Wilson Tang, MD*,
Craig H. Meyer, PhD,
Albert Macovski, PhD,
John M. Pauly, PhD,
Dwight G. Nishimura, PhD and
Bob S. Hu, MD, FACC*
* Division of Cardiovascular Medicine, Department of Medicine, Stanford University, Stanford, California, USA
Magnetic Resonance Systems Research Laboratory, Department of Electrical Engineering, Stanford University, Stanford, California, USA
Manuscript received March 24, 2000;
revised manuscript received January 16, 2001,
accepted April 27, 2001.
Reprint requests and correspondence: Dr. Bob S. Hu, Division of Cardiovascular Medicine, Stanford University School of Medicine, Room H-2157, 300 Pasteur Drive, Stanford, California 94305-5233
hu{at}Isl.stanford.edu
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Abstract
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OBJECTIVES
The purpose of this study was to validate cardiac measurements derived from real-time cardiac magnetic resonance imaging (MRI) as compared with well-validated conventional cine MRI.
BACKGROUND
Although cardiac MRI provides accurate assessment of left ventricular (LV) volume and mass, most techniques have been relatively slow and required electrocardiogram (ECG) gating over many heart beats. A newly developed real-time MRI system allows continuous real-time dynamic acquisition and display without cardiac gating or breath-holding.
METHODS
Fourteen healthy volunteers and nine patients with heart failure underwent real-time and cine MRI in the standard short-axis orientation with a 1.5T MRI scanner. Nonbreath-holding cine MRI was performed with ECG gating and respiratory compensation. Left ventricular end-diastolic volume (LVEDV), left ventricular endsystolic volume (LVESV), ejection fraction (EF) and LV mass calculated from the images obtained by real-time MRI were compared to those obtained by cine MRI.
RESULTS
The total study time including localization for real-time MRI was significantly shorter than cine MRI (8.6 ± 2.3 vs. 24.7 ± 3.5 min, p < 0.001). Both imaging techniques yielded good quality images allowing cardiac measurements. The measurements of LVEDV, LVESV, EF and LV mass obtained with real-time MRI showed close correlation with those obtained with cine MRI (LVEDV: r = 0.985, p < 0.001; LVESV: r = 0.994, p < 0.001; EF: r = 0.975, p < 0.001; LV mass: r = 0.977, p < 0.001).
CONCLUSIONS
Real-time MRI provides accurate measurements of LV volume and mass in a time-efficient manner with respect to image acquisition.
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Abbreviations and Acronyms
| | EDV | = end-diastolic volume | | EF | = ejection fraction | | ESV | = end-systolic volume | | LV | = left ventricle/left ventricular | | MRI | = magnetic resonance imaging |
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Left ventricular (LV) volume and mass are powerful prognostic indicators in a variety of cardiac diseases (1–3). These measurements enable invaluable assessment of therapeutic effect and guide clinical management. An ideal imaging technique offers accurate and reproducible measurements of global and regional function in a time-efficient and noninvasive manner.
Cardiac magnetic resonance imaging (MRI) has been a gold standard in the measurements of LV volume and mass (4–7). It is independent of geometric assumptions, is noninvasive, and is free of exposure to contrast agents or ionizing radiation. Conventional cine MRI techniques yield excellent image contrast to delineate the myocardium from the blood pool while maintaining sufficient temporal and spatial resolution for accurate quantification of LV volume and mass. The accuracy of cine MRI for assessing LV function has been proved in the results of several studies (8–11). Nevertheless, there are two major disadvantages of conventional cine MRI: 1) relatively long acquisition times, and 2) sensitivity to respiratory motion. Both factors hamper routine clinical implementation of cine MRI. These limitations may be overcome by fast MRI techniques like segmented k-space cine sequences (12–14) or echo-planar imaging (15,16). Although these techniques allow rapid visualization of cardiac function, patients have to hold their breath for an acquisition of one anatomical slice. Most patients can comfortably hold their breath for 10 to 20 s, but patients with severely impaired ventricular function or respiratory disease find this difficult, resulting in poorer-quality images.
The real-time interactive MRI system has been developed specifically to address the limitations of conventional cine MRI or fast MRI technique (17). This system allows continuous real-time dynamic acquisition and display of any scan plane at 16 images/s without cardiac gating or breath-holding. These features are ideal for the assessment of LV volume and mass. However, validation of the volume measurements has not been performed. The purpose of this study was to validate cardiac measurements derived from the real-time MRI as compared with the well-validated conventional cine MRI.
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Methods
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Subjects.
Nine consecutive patients with heart failure and 14 normal healthy volunteers with no history of heart disease were included in the study. All patients were selected on the basis of history, physical examination and echocardiographic findings of abnormal dysfunctional ventricles. All subjects underwent real-time MRI and cine MRI in the short-axis orientation (total 23 subjects, 18 men and 5 women aged 24 to 75 years). The heart rate ranged from 38 to 103 beats/min (mean ± SD: 69 ± 16 beats/min). Informed, written consent was obtained from all subjects.
Conventional nonbreath-holding cine MRI.
Conventional nonbreath-holding cine MRI was done with a 1.5T Signa MRI scanner (GE, Milwaukee, Wisconsin) using the body coil. Images were obtained by using a gradient-echo technique with flow and respiratory compensation. The following parameters were employed: an echo-time of 8.0 ms, a repetition time of 18 ms, a flip angle of 30°, a slice thickness of 10 mm with no interslice gap, an acquisition matrix of 256 x 128, two excitations and a 32 cm field of view. After the scout images were obtained, both ventricles were imaged from the base to the apex in 9 to 12 double- angulated short-axis slices.
Real-time interactive MRI.
The real-time interactive MRI system requires a modest upgrade consisting of a workstation and a bus adapter to a conventional 1.5T Signa MRI scanner. This system accomplishes the following: 1) real-time image acquisition, eliminating the need for cardiac or respiratory gating; 2) interactive selection of scan plane, allowing immediate control of the desired view plane; and 3) real-time image reconstruction and display, providing instant image-based feedback. A detailed description of our MRI system has been reported previously (17,18). The following parameters were employed in this study: an echo-time of 4.6 ms, a repetition time of 30 ms, a flip angle of 30°, a slice thickness of 7 mm with a 0.2 mm interslice gap and a 24 cm field of view with 2.7 mm in-plane resolution. To minimize the respiratory misregistration, the patients were told to breathe shallowly.
The real-time MRI was performed on the same day as the cine MRI. The scan was initiated in an axial view. A long-axis view was prescribed by obtaining a single oblique plane from the initial axial view, and the subsequent four-chamber view was obtained by prescribing double oblique plane from the long-axis view. Nine to 14 equally spaced views of at least eight heartbeats were obtained from the apex to the base of the heart. Images were saved as QuickTime video format (Apple, Cupertino, California) for review.
Imaging analysis.
Multislice short-axis MRI was analyzed with the use of the NIHimage (National Institutes of Health, Bethesda, Maryland). The epicardial and endocardial surfaces of the LV were traced manually. The window and level settings of a representative midventricular image were optimized for best image contrast between the myocardium and the ventricle and then appointed to all images. If necessary, the window and level settings were optimized for individual images. For both cine and real-time MRI, the end-systolic and end-diastolic images were selected as the images with the smallest and largest areas of the respective chamber, respectively (13,19,20). One of the advantages of real-time MRI was that we could detect the diaphragm movement. We selected the cardiac cycles of the end-expiration for the LV analysis based on the diaphragm motion in order to match thoracic positions on multiple slices. The endocardial surfaces of both ventricles and the left epicardial surface were traced with a trackball. The papillary muscles were included as LV mass and excluded from LV volume (19,21).
Both the end-systolic volume (ESV) and the end-diastolic volume (EDV) were obtained by integrating the areas and multiplying by the slice interval (slice thickness + slice gap). Stroke volume was the difference between EDV and ESV. Ejection fraction (EF) was calculated as stroke volume divided by EDV and expressed in percent. The LV mass was calculated by multiplying the myocardial volume measured in the end-diastolic phase with the specific myocardial gravity (1.05 g/cm3).
The MRI was independently analyzed once by two observers and twice more by one of them. Both intra- and interobserver variability were calculated as the percentage of the absolute difference between the two measurements divided by the mean of the measurements.
Statistics.
Data are presented as mean value ± SD. The correlation between two variables was tested by linear regression analysis. To examine further the comparison of two methods of clinical measurements, the method of Bland and Altman was used (22). The degree of agreement was determined as the mean difference, SD of the differences, limits of agreement (mean ± 2 SD), standard-error-in the mean difference and 95% confidence interval (CI) of the mean difference. A p value < 0.05 was considered statistically significant.
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Results
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Time efficiency.
The actual acquisition time for real-time MRI was significantly shorter than that for cine MRI (2.2 ± 0.4 vs. 14.0 ± 3.6 min, p < 0.001). In addition, the total study time including localization was significantly shorter than that for cine MRI (8.6 ± 2.3 vs. 24.7 ± 3.5 min, p < 0.001). Both imaging techniques yielded good-quality images, allowing the assessment of ventricular volume and mass. Figure 1 shows the example images of conventional cine MRI and real-time MRI.
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Figure 1 Short-axis magnestic resonance imaging (MRI) of a heart-failure patient obtained with conventional cine MRI (A: end-diastole; B: end-systole) and real-time MRI (C: end-diastole; D: end-systole).
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Left ventricular measurements.
Results of the Bland-Altman analysis of all subjects are shown in Table 1. Clinical findings and MRI-derived data in heart failure patients are shown in Table 2. Figures 2 and 3show the comparison of LV volume, EF and mass between cine and real -time MRI in the patients (n = 9). Measurements of LVEDV, LVESV, EF and LV mass showed a close correlation between these two methods (LVEDV: r = 0.986, p < 0.001; LVESV: r = 0.994, p < 0.001; EF: r = 0.982, p < 0.001; LV mass: r = 0.963, p < 0.001).
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Table 1 Differences Among End-Diastolic Volume, End-Systolic Volume, Ejection Fraction and Left Ventricular Mass as Revealed on Cine MRI and Real-Time MRI
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Figure 2 (A,C) Scattergram shows correlation between conventional cine magnetic resonance (MR) imaging measurements and real-time MR imaging measurements of left ventricular (A) end-diastolic and (C) end-systolic volume in heart-failure patients (n = 9). (B,D) Plots of the average mean versus the differences between cine MR imaging and real-time MR imaging. In A and C, the solid line is the regression line; the dashed line is the line of the identity; in B and D, the solid line is the mean difference; the dashed lines are 2 SD of the mean difference.
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Figure 3 (A,C) Scattergram shows correlation between conventional cine magnetic resonance (MR) imaging measurements and real-time MR imaging measurements of ejection fraction (A) and left ventricular mass (C) in heart-failure patients (n = 9). (B,D) Plots of the average mean versus the differences between cine MR imaging and real-time MR imaging. Solid and dashed lines as in Figure 2.
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Figures 4 and 5also show the comparison of LV volume and mass in normal volunteers (n = 14). Measurements of LVEDV, LVESV, EF and LV mass exhibited a close correlation between these two methods (LVEDV: r = 0.944, p < 0.001; LVESV: r = 0.944, p < 0.001; EF: r = 0.892, p < 0.001; LV mass: r = 0.963, p < 0.001).
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Figure 4 (A,C) Scattergram shows correlation between conventional cine magnetic resonance (MR) imaging measurements and real-time MR imaging measurements of left ventricular (A) end-diastolic and (C) end-systolic volume in healthy volunteers (n = 14). (B,D) Plots of the average mean versus the differences between cine MR imaging and real-time MR imaging. Solid and dashed lines as in Figure 2.
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Figure 5 (A,C) Scattergram shows correlation between conventional cine magnetic resonance (MR) imaging measurements and real-time MR imaging measurements of ejection fraction (A) and left ventricular mass (C) in healthy volunteers (n = 14). (B,D) Plots of the average mean versus the differences between cine MR imaging and real-time MR imaging. Solid and dashed lines as in Figure 2.
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Reproducibility.
The intra- and interobserver variability for LV volume, EF and mass are summarized in Table 3. The mean percentage differences in intra- and interobserver measurements for volume, EF, and mass obtained with real-time MRI were equal to or less than those obtained with conventional cine MRI.
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Discussion
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In this study, we demonstrated that LV volume and mass were accurately and reproducibly quantified in a highly time-efficient manner with respect to image acquisition using a real-time interactive MRI system.
Many studies have demonstrated the accuracy and reproducibility of MRI for the determination of cardiac volume (8–10,13,19,20,23–25) and mass (13,19,20,25–27). Accurate determination of these important parameters has implications in both patient management and in clinical study design where therapeutic strategies are being tested (7). Nevertheless, quantitative analyses of cardiac volume and mass with cine MRI are still performed relatively infrequently. The lengthy imaging time of cine MRI is one of the major factors preventing widespread clinical use of this technique. Fast-segmented k-space MRI techniques (12–16) have been developed to overcome the limitation of conventional cine MRI. With these fast MRI techniques, a single slice can be acquired in a single breath-hold of about 10 to 20 s. It has been reported that the cardiac measurements can be assessed highly accurately and reproducibly with these fast MRI techniques (13,16,28).
However, in a significant subgroup, patients find it difficult to hold their breath, due either to dyspnea or simply to poor response to breath-hold instructions. This results in reduced image quality, owing to plane-motion errors, and image misregistration. Although real-time acquisition during free-breathing may result in image misregistration, this can be minimized by instructing patients to breathe shallowly and analyzing only cardiac cycles of end-expiration on the basis of diaphragm motion. There are also other theoretical disadvantages to breath-hold imaging, which include the disparity from the physiologic free-breathing subject. It has been reported that prospective navigator-echo gating could be used in the assessment of cardiac function and that it was particularly useful in patients who find it difficult to hold their breath (29). However, the navigator-echo sequence had a problem of reduced numbers of phases possible, due to the time taken up by the navigator-echo gating.
The real-time interactive MRI system specifically addresses the limitations of the conventional MRI system (17). The real-time interactive control of this system allows rapid image acquisition for cardiac measurements. In our study, both the actual acquisition time and the total study time, including localization, were shorter using real-time MRI than cine MRI. With respect to image quality, we previously reported that the images of real-time MRI system were sufficient to assess global and regional LV wall motion accurately (18). In the current study, both the intra- and interobserver variability for LV volume and mass obtained with conventional cine MRI were similar to those in previous reports (13,19,20,25), and the intra- and interobserver variability obtained with real-time MRI were nearly identical to or less than those obtained with conventional cine MRI. Although images obtained with real-time MRI have lower spatial and temporal resolution than conventional MRI, the use of thinner slices and the smaller field of view in real-time imaging probably offset poorer temporal resolution and a coarser matrix. Therefore, we conclude that image quality of real-time MRI is sufficient for the assessment of LV volume and mass as well as conventional cine MRI.
The temporal resolution for each cardiac phase is an important factor in quantitative assessment of cardiac volume (14,26,30). In this study, the temporal resolution of conventional cine MRI was <60 ms, which is adequate for the accurate depiction of ESV. Analyzing the effect of view-sharing in cardiac measurements, Foo et al. (14) reported that the effective temporal resolution of an acquisition, defined as the time interval between each reconstructed temporal-phase image, was important for the accurate evaluation of cardiac volume. In the real-time MRI, images were reconstructed from six spiral interleaf trajectories of k-space with the use of the sliding-window method (17,31). Although the true temporal resolution was 180 ms, the reconstructed effective temporal resolution of the real-time MRI was 60 ms. Considering the good agreements between ESV measured with real-time MRI and those with cine MRI, the current temporal resolution of real-time MRI is acceptable for qualitative analysis of cardiac volume. We expect that higher absolute temporal resolution may further improve measurement fidelity.
There were slight underestimations of EDV and ESV, which resulted in slight overestimation of EF. This may be due to off-resonance–related blurring of this real-time MRI (17,18). Two methods are currently implemented in our system to improve the off-resonance artifacts: 1) periodic acquisition of linear field maps and 2) dynamic shimming of the magnet to maintain a homogeneous magnetic field.
Because cine MRI has high temporal and spatial resolution, it has been used clinically to study disease progression and response to treatment serially (5,6). However, considering the shorter scan time required, real-time MRI may be a more preferable diagnostic modality for serial analysis or follow-up studies than conventional cine MRI.
Conclusions.
Evaluation of LV volume and mass is feasible without breath-holding with the use of the real-time interactive MRI. Compared with the conventional cine MRI, real-time MRI shows markedly reduced acquisition time in the assessment of LV volume and mass. Results of analysis of LV volume and mass also correlate closely between these two methods. We conclude that real-time MRI system is a valuable technique that provides accurate assessment of cardiac volume and mass in a time-efficient manner with respect to image acquisition.
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Acknowledgments
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We thank Dr. Maehara for her technical support.
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Footnotes
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This study was supported by the National Institutes of Health (Bethesda, Maryland) and by General Electric Medical Systems (Milwaukee, Wisconsin).
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Abstract
Methods