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AJR 2005; 185:103-109
© American Roentgen Ray Society

Original Research

Comparison of 3D Segmented Gradient-Echo and Steady-State Free Precession Coronary MRI Sequences in Patients with Coronary Artery Disease

Murat Ozgun1, Andreas Hoffmeier1, Marc Kouwenhoven2, René M. Botnar3, Matthias Stuber4, Hans Heinrich Scheld1, Warren J. Manning3, Walter Heindel1 and David Maintz1

1 Department of Clinical Radiology, University of Muenster, Albert-Schweitzer-Strasse 33, Muenster 48129, Germany.
2 Philips Medical Systems, Andover, MA.
3 Department of Radiology, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, MA.
4 Department of Radiology, Johns Hopkins University, Baltimore, MD.

Received July 26, 2004; accepted after revision October 4, 2004.

 
Address correspondence to M. Ozgun.


Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
OBJECTIVE. Our objective was to compare two state-of-the-art coronary MRI (CMRI) sequences with regard to image quality and diagnostic accuracy for the detection of coronary artery disease (CAD).

SUBJECTS AND METHODS. Twenty patients with known CAD were examined with a navigator-gated and corrected free-breathing 3D segmented gradient-echo (turbo field-echo) CMRI sequence and a steady-state free precession sequence (balanced turbo field-echo). CMRI was performed in a transverse plane for the left coronary artery and a double-oblique plane for the right coronary artery system. Subjective image quality (1- to 4-point scale, with 1 indicating excellent quality) and objective image quality parameters were independently determined for both sequences. Sensitivity, specificity, and accuracy for the detection of significant (≥ 50% diameter) coronary artery stenoses were determined as defined in invasive catheter X-ray coronary angiography.

RESULTS. Subjective image quality was superior for the balanced turbo field-echo approach (1.8 ± 0.9 vs 2.3 ± 1.0 for turbo field-echo; p < 0.001). Vessel sharpness, signal-to-noise ratio, and contrast-to-noise ratio were all superior for the balanced turbo field-echo approach (p < 0.01 for signal-to-noise ratio and contrast-to-noise ratio). Of the 103 segments, 18% of turbo field-echo segments and 9% of balanced turbo field-echo segments had to be excluded from disease evaluation because of insufficient image quality. Sensitivity, specificity, and accuracy for the detection of significant coronary artery stenoses in the evaluated segments were 92%, 67%, 85%, respectively, for turbo field-echo and 82%, 82%, 81%, respectively, for balanced turbo field-echo.

CONCLUSION. Balanced turbo field-echo offers improved image quality with significantly fewer nondiagnostic segments when compared with turbo field-echo. For the detection of CAD, both sequences showed comparable accuracy for the visualized segments.


Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
Since the first reports on coronary MRI (CMRI) in the early 1990s [1, 2], a large array of CMRI techniques has been reported. Imaging sequences include segmented gradient-echo (turbo field-echo), steady-state free precession (balanced turbo field-echo), and fast spin-echo [3-7] with data sampling along Cartesian, radial, or spiral k-space trajectories [3, 8-12] and free-breathing navigator gating or prolonged breath-holding. To date, the optimal CMRI strategy remains undetermined [13]. Free-breathing turbo field-echo CMRI has been evaluated in comparison with X-ray angiography in several single-center trials and a multicenter trial [3, 14, 15]. These studies have shown the value of this method for the detection of disease of the left main coronary artery (LM) and of multiple coronary arteries. A major limitation of this method remains the 15-20% of examinations with insufficient image quality. Recently, balanced turbo field-echo CMRI has been shown to be superior to turbo field-echo approaches for subjective image quality, signal-to-noise ratio, and contrast-to-noise ratio within a population of healthy subjects [16]. We sought to compare free-breathing turbo field-echo and balanced turbo field-echo CMRI for subjective and objective image quality parameters and their accuracy for the detection of significant coronary stenoses in a population of patients with coronary artery disease (CAD).


Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Patients
Twenty consecutive patients (14 men and six women; mean age, 61 ± 11 [SD] years; mean body weight, 78 ± 11 kg) with CAD suspected or known on the basis of clinical findings such as pathologic ECG, stable angina, or previous coronary angiography were studied. In all patients, diagnostic X-ray coronary angiography was performed 1 day to 4 weeks before CMRI (mean, 18 ± 8 days). There were no intervening events or change in medication regimen. The results of X-ray coronary angiography were unknown at the time of the CMRI examination. Written informed consent was obtained from all participants. None of the subjects had contraindications to CMRI. All patients were in sinus rhythm.

MRI
All scans were obtained on a 1.5-T Gyroscan Intera MRI scanner (Philips Medical Systems) equipped with a Master gradient system (30 mT/m, 200-µm rise time) using a five-element phased-array cardiac synergy coil and R9 software with the cardiovascular package (Phillips).

For coronary artery localization and for navigator positioning, three scout scans were obtained. The first thoracic scout was an ECG-triggered, free-breathing, multislice 2D segmented gradient-echo survey [3]. For determination of the quiescent diastolic period in the cardiac cycle, a transverse, retrospectively triggered cine turbo field-echo sequence with 60 heart phases per R-R interval was obtained [17, 18]. The third scout was a large-volume ECG-triggered and navigator-gated or -corrected 3D MRI sequence showing extensive portions of the left anterior descending coronary artery (LAD), the left circumflex coronary artery (LCX), and the right coronary artery (RCA), which could be identified and used for planning.

Subsequently, CMRI (turbo field-echo and balanced turbo field-echo) of the left (transverse slices) or right (double-oblique plane) coronary system was performed. The side was chosen randomly by the referring physician. For patients with stents, the referring physician instructed the scanning team to scan the side opposite the stent. The reviewers were not informed about the choice of side. Turbo field-echo and balanced turbo field-echo scans were obtained in random order.

Coronary MRI Sequences
The details of both scans are displayed in Table 1.


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  		TABLE 1 : Comparison of Imaging Parameters of Balanced Turbo Field-Echo and Turbo Field-Echo Sequences.

 

Turbo Field-Echo Sequence
The 3D segmented k-space turbo field-echo sequence consisted of a flow-insensitive T2 prepulse [19] for myocardial suppression, followed by a right hemidiaphragmatic navigator pulse [20] and a spectrally selective fat-saturation pulse.

Balanced Turbo Field-Echo Sequence
The 3D segmented k-space balanced turbo field-echo sequence also consisted of a T2 prepulse and a fat-saturation pulse for contrast enhancement. The T2 prepulse minimized the need for dummy startup echoes in the balanced turbo field-echo acquisition shot.

Catheter X-Ray Coronary Angiography
Conventional X-ray coronary angiography was performed in multiple projections by experienced invasive cardiologists using standard techniques [21]. Each X-ray coronary angiogram was assessed during routine clinical activity, and visual estimation of segments and maximal percentage reductions of luminal diameters were reported.

Data Evaluation
All CMRI data were transferred to a commercially available workstation (EasyVision 5.1, Philips Medical Systems) and a PC equipped with a "soap-bubble" tool for coronary reformatting and quantitative coronary analysis [22].

Subjective image quality (1, excellent; 2, very good; 3, good; 4, poor) of all scans was analyzed on a consensus basis by two experienced observers unaware of the sequence parameters and X-ray coronary angiography results.

For the determination of focal coronary artery stenoses, two observers unaware of the X-ray coronary angiography findings evaluated the data set on the EasyVision workstation and the soap-bubble reconstructions, analyzing all visible segments and specifying the coronary segments with more than 50% stenosis by scrolling through the individual slices from the 3D data set and interpreting marked signal attenuation of the coronary lumen as a significant stenosis. Segmental analysis of the coronary artery tree was performed according to the 20-segment classification of the American Heart Association. The evaluated segments included the LM (segment 11); proximal and middle third of the LAD (segments 12 and 13); proximal and distal LCX (segments 18 and 19); and, depending on the sampled volume, the first diagonal branch of the LAD (segment 15) and the first marginal branch of the LCX (segment 20). In the double-oblique volume, the proximal, middle, and distal segments of the RCA were evaluated (segments 1, 2, and 3, respectively).

For signal quantification, user-specified regions of interest were placed in the aorta at the level of the LM (transverse views) or the RCA (double-oblique views) and in the myocardial septum of the anterior or the septal wall of the ventricle. Signal-to-noise ratio and contrast-to-noise ratio were measured for both the transverse (for the LM and LAD) and the double-angulated (for the RCA) images. Signal-to-noise ratio and contrast-to-noise ratio were as previously defined [19].

For objective quantification of vessel diameter and vessel sharpness for each coronary segment, image reformation was done manually with the previously described soap-bubble tool [22] by an independent observer unaware of the sequence parameters. All quantitative data were expressed as mean ± SD. Comparisons were made using paired t testing (two-tailed). A p value of less than 0.05 was considered significant. The overall diagnostic sensitivity, specificity, accuracy, and positive and negative predictive values of both sequences for the detection of significant stenoses in coronary arteries were determined for CMRI as compared with X-ray coronary angiography.



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  		Fig. 1A —56-year-old woman with 80% stenosis (arrows) of mid segment of right coronary artery. Stenosis is seen on X-ray coronary angiography (A). Visualization of stenosis is slightly better by balanced turbo field-echo CMRI (B) than by turbo field-echo CMRI (C), and diameter of vessel appears larger using turbo field-echo sequence.

 



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  		Fig. 1B —56-year-old woman with 80% stenosis (arrows) of mid segment of right coronary artery. Stenosis is seen on X-ray coronary angiography (A). Visualization of stenosis is slightly better by balanced turbo field-echo CMRI (B) than by turbo field-echo CMRI (C), and diameter of vessel appears larger using turbo field-echo sequence.

 



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  		Fig. 1C —56-year-old woman with 80% stenosis (arrows) of mid segment of right coronary artery. Stenosis is seen on X-ray coronary angiography (A). Visualization of stenosis is slightly better by balanced turbo field-echo CMRI (B) than by turbo field-echo CMRI (C), and diameter of vessel appears larger using turbo field-echo sequence.

 

Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
All subjects completed scanning without any complications. In all cases, the intended coronary artery system could be imaged with both sequences (left coronary system, n =15; right coronary system, n = 5). Representative images are shown in Figures 1A, 1B, 1C, 2A, 2B, 2C, 3A, 3B, 3C, and 3D.



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  		Fig. 2A —73-year-old man with subtotal stenosis (arrows) of left anterior descending coronary artery. Stenosis is seen on X-ray angiography (A). Both turbo field-echo CMRI sequence (B) and balanced turbo field-echo CMRI (C) sequences show loss of luminal signal in area of stenosis.

 


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  		Fig. 2B —73-year-old man with subtotal stenosis (arrows) of left anterior descending coronary artery. Stenosis is seen on X-ray angiography (A). Both turbo field-echo CMRI sequence (B) and balanced turbo field-echo CMRI (C) sequences show loss of luminal signal in area of stenosis.

 


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  		Fig. 2C —73-year-old man with subtotal stenosis (arrows) of left anterior descending coronary artery. Stenosis is seen on X-ray angiography (A). Both turbo field-echo CMRI sequence (B) and balanced turbo field-echo CMRI (C) sequences show loss of luminal signal in area of stenosis.

 


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  		Fig. 3A —69-year-old man with two areas of coronary artery stenosis. X-ray angiography shows 75% ostial stenosis (arrow) of right coronary artery (A) and diffuse atherosclerosis of left anterior descending coronary artery (B).

 


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  		Fig. 3B —69-year-old man with two areas of coronary artery stenosis. X-ray angiography shows 75% ostial stenosis (arrow) of right coronary artery (A) and diffuse atherosclerosis of left anterior descending coronary artery (B).

 


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  		Fig. 3C —69-year-old man with two areas of coronary artery stenosis. Both turbo field-echo (C) and balanced turbo field-echo (D) sequences depict right coronary artery stenosis (arrows) well and show diffuse luminal irregularities of left anterior descending coronary artery.

 


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  		Fig. 3D —69-year-old man with two areas of coronary artery stenosis. Both turbo field-echo (C) and balanced turbo field-echo (D) sequences depict right coronary artery stenosis (arrows) well and show diffuse luminal irregularities of left anterior descending coronary artery.

 
The mean scanning time was 7 min 47 sec ± 2 min 22 sec for the balanced turbo field-echo and 9 min 28 sec ± 2 min 46 sec for the turbo field-echo sequence (p < 0.05), with an overall navigator efficiency of 48% ± 12%. The navigator efficiency is defined as the percentage of accepted data during the respiration cycle. Both sequences included 103 segments in their scan volumes, with 28 clinically significant lesions (≥ 50% reduction in diameter in X-ray coronary angiography). Of the 103 segments, 9% (one RCA, three LAD, and three LCX segments) could not be assessed with the balanced turbo field-echo and 18% (nine LAD and eight LCX segments) with the turbo field-echo sequence because of poor image quality (grade 4) or stent artifacts (two segments in each sequence). Of those with poor image quality with balanced turbo field-echo (seven segments), two segments with no stenosis were accurately depicted by turbo field-echo. In four of the remaining five segments, the uninterpretable segment was situated after a high-grade stenosis (> 75% according to X-ray coronary angiography results). All were mid or distal segments of coronary branches. Of those uninterpretable on the turbo field-echo sequence (17 segments), five segments should have displayed a relevant occlusion and four were directly behind a high-grade stenosis.

Subjective image quality was superior for the balanced turbo field-echo sequence (1.8 ± 0.9, vs 2.3 ± 1.0 for turbo field-echo, p < 0.001). Accuracy data for the detection of relevant stenoses and other diagnostic values are displayed in Table 2. Overall accuracy was 81% for balanced turbo field-echo and 85% for turbo field-echo (p = not statistically significant).


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  		TABLE 2 : Diagnostic Accuracy of Coronary MRI for Detection of Coronary Artery Stenoses of 50% or Greater

 

Table 3 summarizes quantitative image parameters including signal-to-noise ratio, contrast-to-noise ratio, image quality, vessel diameter, and vessel sharpness. The balanced turbo field-echo sequence displayed higher signal-to-noise ratio and contrast-to-noise ratio (p < 0.01 for both). No significant differences were found in average vessel sharpness (p = not statistically significant) or in vessel diameters, except for the RCA, which was smaller on balanced turbo field-echo images (p < 0.001).


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  		TABLE 3 : Comparison of Balanced Turbo Field-Echo and Turbo Field-Echo Coronary MRI

 


Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
Recent advances in MRI hardware and software enabled noninvasive visualization of the proximal coronary arteries. Yet, up to 10-20% of coronary segments are not interpretable because of residual cardiac and respiratory motion [14, 15, 23]. CMRI using breath-hold and free-breathing navigator approaches has been investigated, with the free-breathing approaches offering the potential for higher spatial resolution, more reliable data acquisition, and greater patient comfort [24-26]. Studies with different protocols dealing with the depiction of coronary arteries and stenotic lesions have been performed, and the turbo field-echo sequence has been investigated with reliable results for stenosis detection in the proximal and mid coronary arteries [14, 15]. Different sequences have been compared with this gradient-echo sequence, in studies that have concerned image quality and vessel visibility in healthy subjects [8, 10].

In this study, we sought to compare a 3D turbo field-echo sequence with a recently introduced balanced turbo field-echo approach. This balanced turbo field-echo sequence has previously been validated in healthy subjects and found to show side branches better and in a shorter time [16]. Our study investigated the clinical value of this sequence for the detection of relevant CAD. Consistent with these prior reports, we also found superior subjective image quality and vessel sharpness.

The turbo field-echo sequence is the only sequence that has been evaluated in larger patient groups. Kim et al. [14] found relatively high sensitivity (93%) and overall accuracy (72%) for the detection of coronary artery stenoses, but specificity was low (49%) in their artery-based consensus evaluation. Sommer et al. [15] found a sensitivity of 88% and a specificity of 91% in their evaluation only when examinations with very good or good image quality were included. In both of these studies, the relatively high number of nondiagnostic coronary segments (16% and 28%) was a major limitation.

A major finding of our study was that, using the balanced turbo field-echo sequence, significantly fewer segments were uninterpretable because of suboptimal image quality. Our finding of 18% uninterpretable segments on turbo field-echo is consistent with the findings of a prior study [14]. In this context, difficulties were noted especially in the depiction of segments occluded or directly behind a high-grade stenosis (balanced turbo field-echo, 5/7 segments, and turbo field-echo, 9/17 segments). Other important advantages of the balanced turbo field-echo approach are an improved subjective image quality, superior signal-to-noise and contrast-to-noise ratios, and a shorter scanning time. For the included segments, the diagnostic accuracy for the detection of X-ray coronary angiography-identified coronary artery stenoses was insignificantly lower for balanced turbo field-echo but still comparable for both approaches, suggesting that better motion-compensation algorithms are likely to be as important as the imaging sequence.

A limitation of our study is that we used a visual estimate of stenosis based on X-ray coronary angiography findings as the gold standard. With regard to stenosis seen on MRI, we differentiated only between a luminal reduction of more than 50% and a luminal reduction of less than 50%. A more accurate definition of stenosis using CMRI does not seem realistic at this point. Another limitation was the high prevalence of CAD resulting from the recruitment procedure: Only two patients did not have significant coronary artery stenoses. Because CMRI showed difficulties in poststenotic and occluded segments, the average image quality for patients without CAD can be expected to be better.

In conclusion, balanced turbo field-echo CMRI was found to improve signal-to-noise ratio and contrast-to-noise ratio, improve vessel sharpness, and reduce scanning time, compared with turbo field-echo CMRI. These findings translated into significantly more diagnostic segments and into improved specificity for balanced turbo field-echo CMRI but similar accuracy. Our results suggest that balanced turbo field-echo is likely to be the preferred alternative to turbo field-echo CMRI and warrants further investigation and improvements in motion compensation.


References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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