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Magnetic Resonance Real Time Imaging for the Evaluation of Left Ventricular Function

Eike Nagel, Uta Vogel, Simon Schalla, Tareq Ibrahim,
*Bernhard Schnackenburg, Axel Bornstedt, Christoph Klein,
Hans B. Lehmkuhl, Eckart Fleckuthor

Department of Internal Medicine/Cardiology, German Heart Institute Berlin & Charité Campus Virchow, Humboldt University, Berlin; Germany and *Philips Medical Systems
Berlin - Hamburg; Germany

Abstract
Introduction
Material and Methods
Results
Discussion
Conclusions

Abstract
New ultrafast gradient systems and hybrid imaging sequences make it possible to acquire a complete image in real-time, without the need for breath holding or ECG triggering.
In 21 patients left ventricular function was assessed by the use of a turbo-gradient echo technique, an echo-planar imaging technique and a new real-time imaging technique. End-diastolic and end-systolic volumes, left ventricular muscle mass and ejection fraction of the ultrafast techniques were compared with the turbo-gradient echo technique. Inter- and intra-observer variability was determined for each technique.
Image quality was sufficient for automated contour detection in all but two patients in whom foldover occurred in the real-time images. Results of the ultrafast imaging techniques were comparable with conventional turbo-gradient echo techniques. There was a tendency to overestimate the end-diastolic volume by 3.9 and 1.3 ml respectively with EPI real-time imaging, the end-systolic volume by 0.9 and 5.0 ml, and the left ventricular mass by 2.6 and 23.8 g. Ejection fraction showed a tendency to be overestimated by 1.1% with EPI and underestimated by 4.5% with real-time imaging. Correlation between EPI real-time imaging and turbo-gradient echo were 0.94 and 0.95 respectively for end-diastolic volumes, 0.98 and 0.96 respectively for end-systolic volumes, and 0.96 and 0.89 respectively for left ventricular mass. Inter- and intra-observer variability was low with all three techniques.
Real-time imaging allows an accurate determination of left ventricular function without ECG triggering. Scan times can be reduced significantly with this new technique. Further studies will have to assess the value of real-time imaging for the detection of wall motion abnormalities and the imaging of patients with atrial fibrillation.

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Introduction: Magnetic resonance (MR) imaging of the heart has been shown to be highly accurate and reproducible for the evaluation of left ventricular function and muscle mass. In contrast to angiography or scintigraphy, no geometric assumptions for the determination of volumes are needed as 3-dimensional data sets are available. The feasibility and accuracy of turbo-gradient echo breath hold imaging for evaluation of left ventricular volume and mass with low inter study variability has been demonstrated. Recently, it has been shown that good accuracy can also be attained with rectilinear echo-planar imaging (EPI) and spiral echo-planar imaging. However, even with these ultrafast breath hold techniques, one of the major limitations of MR tomography when compared with echocardiography is the acquisition of images during several heart beats. Real-time planning or adapting of imaging planes is impossible and breath holding is required to suppress breathing motion artifacts. The development of high-performance gradient systems and optimized hybrid sequences, which combine turbo-gradient echo and EPI, today, enable the acquisition of complete cardiac images in real time with high temporal resolution. In combination with interactive planning tools, real-time planning and adaptation of imaging planes can be performed. However, to reach this acquisition speed spatial resolution needs to be reduced and high EPI factors have to be accepted, which may lead to image distortion and reduce accuracy.

The aim of this study was to analyze the accuracy and reproducibility of real-time imaging and EPI techniques for the evaluation of left ventricular (LV) function, including end-diastolic volume, end-systolic volume and left ventricular ejection fraction, as well as LV muscle mass when compared with ultrafast turbo-gradient echo techniques.

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Material and Methods:  Twenty-one patients (15 males, 6 females, age 60 ± 9 years, heart rate 75 ± 19 beats per minute) were included in the study after informed consent, of which three patients had left ventricular hypertrophy due to long standing arterial hypertension, four patients had myocardial infarction and one patient had dilative cardiomyopathy. Patients with contraindications for MR examinations or high-grade ventricular arrhythmias or atrial flutter/fibrillation were excluded from the study.

All patients were examined in the supine position with a 1.5 Tesla whole body MR scanner (ACS NT, Philips, The Netherlands, CPR6), which was equipped with ultrafast gradients (21 mT m-1 amplitude, 100 T m-1 sec-1 slew rate) that used a 5-element phased array cardiac coil that was placed around the thorax of the patient. To avoid foldover, only the two anterior segments of the coil were used for data acquisition. All images were acquired during breath holding at end expiration. Respiration was checked with a strain gauge, ECG triggering was used for the turbo-gradient and EPI image acquisition. After two rapid surveys to determine the exact axis of the left ventricle, 7–12 continuous short axis planes (slice thickness 8 mm, no gap), which covered the complete left ventricle, were planned. Turbo-field echo imaging (TFE = T1-weighted turbo gradient echo) was performed with a segmented k-space technique over 16 heart beats (figure 1). The details of the sequence are shown in table 1. Echo-planar imaging was performed with the identical geometry that used a segmented k-space technique over 16 heart beats (figure 1). For details see table 1. In two patients no EPI measurements were performed in order to shorten the duration of the session. Real-time imaging was performed with a hybrid turbo-gradient echo – echo-planar imaging sequence (figure 1). A temporal resolution of 62 ms (= 16 images per second) was achieved by reducing spatial resolution to 2.2 ´ 4.4 mm (table 1). Forty consecutive images were acquired to cover at least two complete heart beats. To ensure a similar geometry when compared with the other two imaging techniques, real-time imaging was also performed during breath holding at end expiration, even though this was not required to preserve image quality.

Figure 1: End-diastolic and end-systolic equatorial short axis
view of a turbo-gradient echo image (top row), echo-planar
image (middle row) and real-time image (bottom row) of the
same patient.

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Table 1: Scan parameters. All measurements were performed with flow compensation.

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In the end-diastolic and end-systolic images the endo- and epicardial borders were traced. For the TFE and EPI image series end-diastolic images were chosen as the first phase after triggering of the R-wave, for the real-time scans the frame with the largest LV cavity area. End-systolic images were defined as the images with the smallest cavity area (figure 1). The most basal slice to be included had to cover >50% of the circumference. The volume of each slice was determined from the area within the endocardial tracing (excluding both papillary muscles) multiplied by the slice thickness. End-diastolic volume (EDV) and end-systolic volume (ESV) were calculated by summing the volumes of all short-axis slices (Simpson’s method). Ejection fraction (EF) was calculated as (ESV - EDV)/EDV and LV muscle mass (LVM) at end-diastole by subtracting the EDV from the end-diastolic epicardial volume multiplied by 1.05 g/cm3 (specific myocardial gravity) and adding the mass of the papillary muscles. All scans were analyzed by two independent observers. Analysis of 10 patients was repeated after 4 weeks by one of the examiners without reviewing the first analysis to determine intra-observer variability.

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Results: Image quality was sufficient for automated contour detection with all techniques in all but two patients (figure 2). In these patients foldover occurred in the real-time images due to the small field of view chosen and these patients were excluded from the analysis. Only minimal user interference was needed in any of the images. Contrast between blood and endocardium was good with all scan techniques. There was a correlation of EPI and real-time with the turbo-gradient echo technique for EDV, ESV and EF (figure 3). The correlation factor for the determination of LVM with real-time or turbo-gradient echo techniques was only 0.88. The absolute and relative differences between EPI, real-time and turbo-gradient echo techniques for EDV, ESV, LVM and EF are summarized in table 2. With the EPI technique, the mean differences from the turbo-gradient echo technique for ESV, EDV, LVM and EF were small and showed no systematic over- or underestimation. There was a tendency to overestimate ESV and LVM with the real-time technique when compared with the turbo-gradient echo technique and to underestimate EF. The relative differences in EF were 1.1 ± 5.8% for EPI versus turbo-gradient echo measurements and - 4.5 ± 4.7% for real-time versus turbo-gradient echo measurements. The highest relative differences were found between real-time and turbo-gradient echo measurements for ESV (14.8%) and LVM (15.9%).

Figure 2: Real-time image series in an equatorial short axis view
(ES = end-systole, ED = end-diastole).

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Figure 3: Correlation between the EPI and real-time techniques when compared with the turbo-gradient
echo technique.

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Table 2: Mean differences and relative differences according to Bland and Altman (in brackets)
between the ultrafast techniques (EPI and real-time) when compared with the standard
technique (turbo-gradient echo) for the different parameters and scan techniques.

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Intra-observer variability and inter-observer variability were low for all three techniques (table 3).

Table 3: Inter- and intra-observer variability. The difference and correlation
factor between the two observers (two measurements) is given.

tabla3.gif (4829 bytes)

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Discussion:  With ultrafast MR techniques, it was possible to acquire non-ECG triggered high quality images of the beating heart in real time with a temporal resolution of 62 ms. Two patients had to be excluded due to insufficient image quality caused by foldover. Spatial resolution and contrast between intracavitary blood and endocardium were sufficient to allow a quantitative image analysis. With real-time imaging, a tendency to overestimate left ventricular mass and end-systolic volumes was found, which resulted in a tendency to underestimate ejection fraction. All techniques were highly reproducible with a low intra- and inter-observer variability.

Two factors may explain the differences between turbo-gradient echo, EPI and real-time techniques. First, spatial resolution is decreased from 1.3 ´ 2.6 mm for TFE to 2.2 ´ 4.4 mm for real-time imaging, which decreases the accuracy for the delineation of the endo- and epicardial borders. However, zero filling of raw data by a factor of 2 was used, which reduces partial volume effects and leads to a reduction of edge definition artifacts. Secondly, chemical shift artifacts in EPI sequences may be very pronounced and may lead to a superposition of fat signals on parts of the myocardium. These superpositions introduce an error in the determination of LVM as the epicardial border may not be visible.

Temporal resolution was maximal with EPI (20 ms), less so with turbo-gradient echo (50 ms) and minimal with real-time imaging (62 ms). Thus, temporal resolution should be sufficient with EPI and turbo-gradient echo to accurately detect end-diastolic and end-systolic volumes, which remain stable during the isovolumetric phase that lasts approximately 50–80 ms at end-systole. The mild overestimation of end-systolic volumes with the real-time technique may be explained by a mildly insufficient temporal resolution. In addition, further improvements of temporal resolution are needed to assess hemodynamic parameters such as peak filling rate.

Figure 4: Bland-Altman plot for end-diastolic volume (EDV), end-systolic volume (ESV)
and left ventricular muscle mass (LVM). The difference between the fast technique
(EPI respectively real-time) is plotted against the standard technique (TFE). Mean
and one standard deviation (SD) are given.

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Conclusions:  Real-time imaging allows an accurate and reproducible determination of left ventricular function and volumes without ECG- riggering. Results are comparable with conventional ECG triggered turbo-gradient echo and echo-planar imaging techniques. For a reliable assessment of left ventricular muscle mass with real-time imaging further improvements in spatial resolution are needed. Scan time can be reduced significantly with this new technique. Further studies will have to assess the value of real-time imaging for the detection of wall motion abnormalities and the imaging of patients with atrial fibrillation. In addition, the use of contrast agents has to be assessed, which will most probably enhance edge detection quality.

 

Questions, contributions and commentaries to the Authors: send an e-mail message (up to 15 lines, without attachments) to image-pcvc@pcvc.sminter.com.ar , written either in English, Spanish, or Portuguese.

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Update
Oct/31/1999