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DOI:10.2214/AJR.05.1012
AJR 2006; 186:1669-1675
© American Roentgen Ray Society

Original Research

SENSE or k-MAG to Accelerate Free Breathing Navigator-Guided Coronary MR Angiography

Raja Muthupillai1,2, Jouke Smink2, Steve Hong1, Roshan Ravindran1, Vei Vei Lee3 and Scott D. Flamm1,4

1 Department of Radiology, St. Luke's Episcopal Hospital and Texas Heart Institute and Baylor College of Medicine, 6720 Bertner Ave., MC 2-256, Houston, TX 77030.
2 Philips Medical Systems, Cleveland, OH.
3 Department of Biostatistics, Texas Heart Institute, Houston, TX 77030.
4 Department of Cardiology, St. Luke's Episcopal Hospital and Texas Heart Institute, Houston, TX 77030.

Received June 13, 2005; accepted after revision October 25, 2005.

 
Address correspondence to R. Muthupillai (raja.muthupillai{at}philips.com).


Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
OBJECTIVE. The purpose of this study was to assess the relative merits of reducing the scanning time of navigator-guided (NAV) coronary MR angiography by including, both independently and in combination, two time-saving strategies: k-space weighted motion-adapted gating (k-MAG) and sensitivity encoding (SENSE, factor = 2).

SUBJECTS AND METHODS. Coronary arteries of 21 healthy subjects were imaged with four NAV MR angiography sequences: conventional NAV sequence, NAV with the addition of SENSE, NAV with the addition of k-MAG, and NAV with a combination of SENSE and k-MAG. All imaging parameters including the magnetization preparation schemes, prescribed spatial resolution, and acquisition duration per R-R interval were identical for all techniques. The total scanning time, navigator efficiency, visible length of the coronary artery, and subjective image quality were used as metrics for evaluating the performance of the techniques.

RESULTS. The results show that the addition of k-MAG to NAV coronary MR angiography (with or without SENSE) improved scan efficiency and decreased scanning time by an average of 17% without compromising the length of coronary artery visible or the image quality. The addition of SENSE to the NAV technique (with or without k-MAG) reduces the scanning time by an average of 50%.

CONCLUSION. While the average image quality of coronary arteries was unaffected by the addition of k-MAG to navigator techniques, there was a slight reduction in image quality scores for the navigator sequence with SENSE. Identification of the proximal coronary arteries was not hampered by the addition of k-MAG, SENSE, or both to the NAV coronary MR angiography sequence.

Keywords: cardiac imaging • cardiovascular disease • coronary MR angiography • MR • MR technique


Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
MRI of coronary arteries is challenging primarily for two reasons. First, coronary arteries are small and tortuous, requiring both high spatial resolution and large coverage. Second, data collection is limited to a small fraction of the cardiac cycle (about 100 msec or so) when the motion of the coronary arteries is minimal and the coronary blood flow is maximal (i.e., mid-diastole for the left coronary arteries and systole for the right coronary artery). In practice, attempting to fulfill both these desirable requirements increases the scanning time to several hundreds of heartbeats, prolonging the acquisition time beyond the capability of patients to perform a breath-hold [1, 2].

The latter consideration has prompted the development of free-breathing approaches to combat the deleterious effect of respiratory motion in coronary artery imaging [3-10]. Recent reports have shown that navigator echo-based coronary MR angiography techniques are effective in addressing respiratory motion-induced artifacts [5, 6]. In their simplest implementation, these prospective gating techniques monitor the respiratory position of the diaphragm or the heart using a pencil-beam radiofrequency excitation (the navigator) during each heartbeat before collecting imaging data during diastole. If the acquired imaging data fall within a user-prescribed margin for the position of the diaphragm as determined by the navigator (referred to as the acceptance window), for example, 2 mm about the end expiratory phase, then the imaging data are accepted. Otherwise the imaging data are discarded and reacquired. This process continues until all the imaging data necessary for a high-resolution 3D image are acquired.

More sophisticated variations of the basic navigator approach have also been proposed in the literature [5, 9, 10]. In one variant, the navigator information is used in two ways. First, it is used to determine if the imaging data would fall within the acceptance window. Second, by determining the positional offset of the navigator from the center of the acceptance window, the excitation slab is moved by adjusting the center frequency of the radiofrequency excitation before collecting imaging data, to essentially "freeze" the imaging volume from an acquisition point of view. This approach has been referred to as prospective respiratory gating with real-time tracking [9, 10] and is the technique used in the current investigation.


Figure 1
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  		Fig. 1 —Graph shows k-space modulation used to smoothly vary acceptance window from 4 mm at central 25% of k-space to 8 mm (26-100% of k-space). Modulation function is given by w = {alpha} +(ß - {alpha})exp(-{eta}/|k|))3, in which {alpha} is starting window width; ß is ending gate width; ß is central window width; and w is resulting acceptance window width as function of k-space value, k.

 
Although such navigator-based approaches have been effective in delineating proximal coronary arteries, their main drawback is the long acquisition time, typically approximately 12-15 min of acquisition time per coronary artery segment [5]. Several strategies have been proposed in the literature to reduce the scanning time of these navigator-based approaches [11-16]. One such approach is k-space weighted motion-adapted gating (k-MAG), wherein the width of the acceptance window is stringent when acquiring low spatial frequencies (low ky-kz values) and is progressively relaxed when acquiring higher spatial frequencies. This improves scan efficiency at the cost of accepting some potential loss of spatial resolution caused by motion-induced blurring [12]. An alternative approach is to reduce the acquisition time drastically, for example, by a factor of 2 or more, by using a parallel acquisition strategy such as sensitivity encoding (SENSE) [17] or simultaneous acquisition of spatial harmonics (SMASH) [18]. The scanning time reduction in these parallel imaging approaches entails a concomitant reduction in signal-to-noise ratio (SNR) without compromising the acquired spatial resolution. The purpose of this study is to quantitatively evaluate the relative merits of using k-MAG- and SENSE-based parallel imaging techniques, independently and in combination, to reduce the acquisition time of navigator-based coronary MR angiography.


Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Subjects
Twenty-one healthy volunteers were recruited for this study composed of 11 men (mean age, 41 ± 10 years) and 10 women (mean age, 51 ± 11 years) with an age range of 27-76 years (mean, 45 ± 11 years) without history of heart disease, in sinus rhythm (mean heart rate, 71 ± 10 beats per minute [bpm]; range, 60-85 bpm), and without contraindications to MRI. The local institutional review board approved the protocol, and all subjects gave written informed consent before entry.

MRI Protocol
All imaging was performed on a 1.5-T MRI scanner (NT Intera, Philips Medical Systems) using a commercially available 5-element phased-array cardiac coil for signal reception. The MRI protocol consisted of four 3D, prospective navigator-based coronary MR angiography sequences with the same prescribed spatial resolution, coverage, and acquisition time per cardiac cycle. The four sequences were conventional 3D prospective navigator-guided (NAV) technique with real-time slice tracking, NAV with the addition of SENSE, NAV with the addition of k-MAG, and NAV with the addition of SENSE and k-MAG. The following acquisition parameters were kept constant for all four sequences: TR/TE, 7.1/2.5; flip angle, 30°; field of view, 360-400 x 270-300; matrix size, 512 x 384; number of slices, 10; slice thickness, 3 mm. The true acquired voxel size was 0.74 x 0.74 x 3 mm and was reconstructed as 0.74 x 0.74 x 1.5 mm by zero padding in the slice-select direction. The data acquisition module collected 12 phase-encoding steps per R-R interval (85 msec/R-R interval). A composite radiofrequency pulse for muscle signal suppression, a fat suppression pulse for suppressing epicardial fat, and a navigator echo for measuring diaphragmatic motion in real time preceded each data acquisition module. More specific information about magnetization preparation pulses has been described in detail previously [9, 10]. A localized shim volume covering the heart was used to facilitate effective fat suppression. The data acquisition was timed to occur in mid-diastole using vector-cardiographic (VCG) triggering. The acceptance window used for the NAV technique was 4 mm and was kept constant throughout the acquisition. The parameters that varied from the standard NAV technique are as follows. When k-MAG was used (i.e., with and without SENSE), the acceptance window was progressively and smoothly increased from 4 mm (central 25% of k-space) to 8 mm (26-100% of k-space). The function used for the variation is shown in Figure 1. For the two SENSE acquisitions (i.e., with and without k-MAG), the in-plane phase-encoding steps were reduced by a factor of 2.

The origin of the left anterior descending artery (LAD) was identified using a low-resolution scout scan. The center of the transverse 3D volume was positioned at the origin of the LAD and was tilted by 5° in the right-left and anteroposterior directions to maximize coverage of the LAD. The same offsets and angulations were used for all four acquisitions in each patient, and the order of the acquisitions was randomized for each patient study to minimize bias.

Reference Scan Acquisition
As described by Pruessmann et al. [17], the SENSE reconstruction requires information about the coil-sensitivity profiles. Because receiver coil sensitivities vary smoothly and slowly over the spatial domain, a coarse sampling of the coil sensitivities is sufficient [19, 20]. Complex radiofrequency receiver coil sensitivity within the entire imaging volume was calculated from a low-resolution fast-field echo acquisition with a voxel size of 9 x 9 x 9 mm. Coil sensitivities were estimated by interleaving a body coil acquisition with a phased-array coil acquisition for each TR. Further, to minimize a potential source of misregistration between the reference scan and the actual acquisition because of differences in breathing positions, the low-resolution reference scan was averaged with eight acquisitions [19, 20]. The total acquisition time of the reference scan was 1 min, and it was performed at the beginning of imaging for each subject.


Figure 2
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  		Fig. 2 —Reformatted left main and left anterior descending coronary MR angiography obtained in four volunteers using conventional navigator-guided coronary artery imaging technique (NAV), NAV + k-space weighted motion-adapted gating (k-MAG), NAV + sensitivity encoding (SENSE), and NAV+SENSE+k-MAG (from left to right). Notice slight increase in noise in two right-most images due to incorporation of SENSE. Curved black protrusions into right ventricular outflow tract seen on images in row 3 and row 4 are due to "flattening" effect of displaying curvilinear reformation of left main and left anterior descending coronary artery as single image.

 
Data Analysis
Quantitative analysis—The number of total and accepted R-R intervals was recorded for each scan. The acquisition time was measured directly from the total number of R-R intervals used for each scan and the heart rate of the patient during the study. Scan efficiency was defined as the ratio of the number of R-R intervals that fell within the acceptance window to the total number of R-R intervals for the acquisition.

The mean and SD of the signal intensity in regions of interest (ROIs) (> 20 pixels) at the aortic root, myocardium, and air were computed for each of the four techniques. The blood SNR was estimated as the ratio of the mean blood signal intensity to the SD of the noise ({sigma}), and the blood-to-myocardium contrast-to-noise ratio (CNR) was estimated to be the ratio of the signal difference between the mean signal intensity of blood and myocardium to {sigma}. It should be noted that the SNR and CNR analyses were performed in August 2005 at the request of one of the reviewers of this manuscript. (The study was completed in 2002.) At the time of analysis, because the data from five subjects were corrupted and could not be reloaded into the postprocessing workstation, the SNR and CNR analyses reflect results from a sample size of 15 subjects.

All the images were transferred to a commercially available postprocessing workstation (EasyVision, software release 4; Philips) for further analysis. For each volunteer, the 3D volumetric data from all four sequences were reformatted using curved reformation. The curved length of the coronary artery visualized was measured by tracing a path from the origin of the left main coronary ostium to the most distal portion of the LAD visible in the reformatted image.

Qualitative analysis—The reformatted coronary segments for each patient using the four different sequences were filmed side by side (in random order) and were qualitatively assessed by an experienced cardiac MR imager who was blinded to the technique that was used to acquire the data. The image window and level were adjusted individually for comparable image settings. The image quality was ranked using the following scale previously reported by Kim et al. [5]: 1 = poor/uninterpretable (coronary artery visible with markedly blurred borders or edges), 2 = good (coronary artery visible with moderately blurred borders or edges), 3 = very good (coronary artery visible with mildly blurred borders or edges), and 4 = excellent (coronary artery visible with sharply defined borders or edges).

The cardiac MR imager also noted if the coronary ostia from which the coronary artery emanated could be clearly identified on a binary scale: 1 = clearly identified, 0 = not identified.

Statistics—All data are presented as mean ± SD. Analysis of variance was used to compare the four sequences in terms of the following parameters: scan efficiency, total number of R-R intervals, and the length of the coronary artery segment visualized using a commercially available statistical software program (SPSS). Statistical significance (reflected by a p < 0.05) was assessed using an analysis of variance with Bonferroni correction. For categoric data (subjective image quality), a nonparametric Wilcoxon's rank sum test was used to assess statistical significance.


Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
The study was successfully completed using the four described sequences in 20 of the 21 subjects. Navigator-gating efficiency was too low (< 5%) in one subject as a result of an irregular breathing pattern, and the study was not completed. A total of 80 (20 subjects x 4 volumes per subject) 3D coronary volumes were successfully reformatted in the postprocessing workstation. A set of representative images from the four techniques is shown in Figure 2. The quantitative metrics measured are shown in Table 1.


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  		TABLE 1: Comparison of Techniques

 

Scan Time and Scan Efficiency
The NAV sequence required 892 ± 303 heartbeats (or about 13 min at the average heart rate of 71 bpm), with a scan efficiency of 52% ± 17%. The addition of k-MAG to the conventional NAV sequence improved the mean scan efficiency by 17% (from 52% to 61%), reducing the scanning time to 744 ± 257 beats. The addition of SENSE to the conventional NAV sequence roughly halved the scanning time to 455 ± 210 beats, with a scan efficiency of 53% ± 20%. The addition of SENSE and k-MAG to NAV improved scan efficiency (from 52% to 60%) and yielded a significant reduction in the scanning time (from 892 ± 303 beats to 386 ± 151 beats; p < 0.05). A pairwise comparison of the four methods to assess whether the results of scanning time and scan efficiency are statistically significant is shown in Figure 3A and 3B. (See legend for more details.)


Figure 3
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  		Fig. 3A —Pairwise statistical comparison of four techniques for two quantitative metrics, R-R interval (A) and scan efficiency (B). Filled circular dots (bullet) indicate statistical significance between techniques (p < 0.05). k-space weighted motion-adapted gating (k-MAG) and sensitivity encoding (SENSE) reduce scan time both independently and in combination.

 

Figure 4
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  		Fig. 3B —Pairwise statistical comparison of four techniques for two quantitative metrics, R-R interval (A) and scan efficiency (B). Filled circular dots (bullet) indicate statistical significance between techniques (p < 0.05). scan efficiency of traditional conventional navigator-guided (NAV) coronary artery imaging technique sequence with addition of k-MAG is better than that of NAV sequence as well as that of NAV + SENSE sequence without k-MAG.

 

Length of LAD
The lengths of the LAD coronary artery visualized using the four techniques were NAV, 52.8 ± 10.7 mm; NAV + k-MAG, 54.3 ± 10.3 mm; NAV + SENSE, 52.6 ± 12 mm; and NAV + SENSE + k-MAG, 53.5 ± 10.8 mm. The analysis of variance revealed no statistically significant differences in the measured length among the four sequences (all p =not significant).

SNR and CNR
Compared with the conventional NAV sequence, the mean aortic blood SNR progressively decreased with the addition of k-MAG, SENSE, and SENSE plus k-MAG, respectively (all p = not significant). The aortic blood SNR values for the four techniques were NAV, 27 ± 5.7; NAV + k-MAG, 25.3 ± 5.3; NAV + SENSE, 24.8 ± 7.6; and NAV + SENSE + k-MAG, 24 ± 7.3. Similarly, blood-to-myocardial CNR was the greatest for the NAV sequence and progressively decreased with the addition of k-MAG, SENSE, and SENSE plus k-MAG, respectively (all p = not significant). The blood-to-myocardial CNR values of the four techniques were NAV, 18.1 ± 4.2; NAV + k-MAG, 17.4 ± 3.7; NAV + SENSE, 15.1 ± 5.4; and NAV + SENSE + k-MAG, 15.4 ± 7.0 (Table 1).

Image Quality
The results from the assessment of image quality based on the scoring system described in the previous section are shown in Figures 4A and 4B. The image quality of standard NAV and NAV + k-MAG was comparable, and the difference between the two techniques was not statistically significant (p < 0.05). Only the following two comparisons were assessed to be statistically significant: between NAV and NAV + SENSE and between NAV + k-MAG and NAV + SENSE. Note that the difference between either NAV or NAV + k-MAG with NAV + k-MAG + SENSE was not statistically significant (p = not significant). The origin of the left coronary system was clearly identified in all of the four sequences in all subjects completing the study.


Figure 5
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  		Fig. 4A —Subjective image quality of all four techniques: navigator-guided (NAV) coronary MR angiography, k-space weighted motion-adapted gating (k-MAG), and sensitivity encoding (SENSE, factor = 2). Image quality of all four techniques is greater than 2.5, indicating that the overall image quality was between good and very good (see text for description of scale). Mean image scores of techniques without SENSE were rated slightly above 3.0, and mean scores of techniques using SENSE were rated slightly above 2.5.

 

Figure 6
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  		Fig. 4B —Subjective image quality of all four techniques: navigator-guided (NAV) coronary MR angiography, k-space weighted motion-adapted gating (k-MAG), and sensitivity encoding (SENSE, factor = 2). Pairwise statistical comparison of four techniques for image quality assessment. Only comparisons of conventional NAV coronary artery imaging technique and NAV + SENSE, NAV + k-MAG, and NAV + SENSE were statistically significant.

 

Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
The conventional NAV technique provides high-resolution images of the proximal coronary arteries, with an acquisition time of approximately 13 min per coronary segment. Therefore, to evaluate both the right and left coronary arterial systems, the total scanning time would be approximately 40 min without taking into consideration setup, preparation, and localizer imaging time. From a practical perspective, this time may be even longer in light of the recent multicenter coronary MR angiography trial that reported a mean examination time of 70 min [5]. From a routine clinical imaging point of view, this necessitates dedicating approximately 1 hr for the sole evaluation of coronary arteries, which is impractical in most settings. However, if the time required to image coronary arteries could be reduced to the order of 10-15 min, then the evaluation of coronary arteries could become a subset of a more comprehensive MRI evaluation of the heart.

This clinical need for reducing the acquisition time for NAV high-resolution coronary MR angiography has been well recognized, and over the years, investigators have proposed several strategies for improving the scan efficiency of NAV coronary MR angiography techniques [11-16, 21]. In one class of methods, the scan efficiency is improved by making it possible to widen the acceptance window width without introducing significant motion artifacts either by using intelligent phase encode reordering [22] or by prospectively correcting for motion using a patient-adapted affine motion model [21]. Both approaches have shown promise for clinical adaptation in volunteer studies but are yet to be validated in clinical practice. Other approaches have included prolonging the acquisition time per cardiac cycle with some knowledge of subject-specific cardiac motion [16], use of motion-adapted gating methods [21], incorporation of parallel imaging approaches [23], and acquisition of two data sets simultaneously within a single R-R interval with each data set acquired at different portions of the cardiac cycle [24].

In an approach most similar and relevant to the work presented here, Plein et al. [16] evaluated the potential for reducing scanning time of NAV coronary artery imaging by combining k-MAG (using similar weighting and motion tolerance) and patient-optimized acquisition windows. They determined the starting point and the duration of coronary artery quiescence for each subject from a high-temporal-resolution cine acquisition, and the data acquisition period and duration were coincident with this quiescent period for each subject. Although their results showed a dramatic reduction in scanning time, the individual contribution of k-MAG and patient-specific acquisition windows could not be determined. The results from the current study enable us to get specific information about the benefit of using k-MAG in NAV coronary MR angiography. The results also allow direct comparison of the relative merits of using SENSE and k-MAG independently and in combination to reduce acquisition time in coronary MR angiography acquisitions.

The addition of k-MAG (with or without SENSE) improves the scan efficiency of the conventional NAV sequence by an average of 17% without a significant loss of image quality or reduction in the length of the coronary artery segment visualized. This finding is both encouraging and somewhat surprising for the following reasons. The relaxed tolerance for motion when collecting higher spatial frequencies in k-MAG could reasonably be anticipated to generate greater motion-induced blurring thereby reducing image quality. However, this relaxed tolerance for motion describes the worst-case scenario set by the operator and does not necessarily reflect the motion that actually occurs during data acquisition. In other words, when acquiring the higher spatial frequencies at a relaxed tolerance, not all data points are acquired at the most relaxed tolerance, and many in fact may fall at the center of the acceptance window. This may account for the modest improvement in scan efficiency without a noticeable degradation of subjective image quality. These results suggest the clinical need for evaluating alternate forms of motion-adapted gating wherein the choice of phase-encoding step is determined based on the information from the navigator [12].

With SENSE (both with and without k-MAG), there was a slight reduction in image quality, reflecting the effect of the 2 loss in SNR (reflecting the chosen SENSE factor of 2) in coronary MR angiography compared with data collected without SENSE [17]. The quantitative measurements of SNR and CNR showed only a modest reduction in SNR for the SENSE-assisted NAV sequences (about 10% in SNR and about 17% in CNR). Unlike conventional acquisitions in which the noise may be considered "white" and spatially uniform, in SENSE-assisted scans, the noise is spatially nonuniform and accurate estimates of noise are difficult to obtain from conventional ROI analysis. This may contribute to the slightly lower than theoretically predicted reduction in SNR in the SENSE-assisted scans. However, there was no significant loss in length of the coronary artery segment visualized compared with that without using SENSE.

Taking into consideration that gradient-echo coronary MR angiography at 1.5 T remains a relatively "signal-starved" sequence, further signal loss due to SENSE could potentially jeopardize accurate determination of coronary artery diameter and therefore stenoses, though this was not specifically evaluated in this study because it was restricted to healthy volunteers. Accordingly, the use of SENSE to diminish scanning time of NAV coronary MR angiography is likely to have the greatest potential under high SNR circumstances, for example, higher field strength (3.0 T and higher [25]), with administration of intravascular contrast agents [26-29], or better spiral imaging where k-MAG may be applied along the kz direction (e.g., one spiral interleaf/cardiac cycle allows the use of a 90° pulse to yield twice the signal compared with a 30° pulse used in conventional turbo field echo readouts) [30-32].

Both SENSE and k-MAG are attractive choices for reducing scanning time because neither approach requires patient cooperation or additional postprocessing, and therefore they retain the traditional strengths of the NAV sequence. At present, MR evaluation of native coronary arteries has two major clinical applications: evaluation of proximal coronary artery disease [5, 6, 33] and exclusion/evaluation of anomalous coronary artery origins [34-38]. When viewed from the context of adapting NAV coronary MR angiography for clinical applications, knowledge about the relative merits of k-MAG and SENSE in improving scan efficiency may be useful. From a clinical point of view, the dramatic reduction in scanning time compared with conventional NAV (by 57%) makes the NAV + k-MAG + SENSE sequence a preferred choice for rapid evaluation of suspected congenital coronary artery anomalies. In this clinical application, the primary goal is accurate identification of the coronary artery origins and their proximal courses. The results from this study show that the origins of the coronary arteries can be identified using the NAV + k-MAG + SENSE sequence. Therefore, the slight reduction in image quality secondary to SENSE may be tolerated under such circumstances, whereas the application of SENSE may be more readily applicable (with or without k-MAG) in the high SNR scenario discussed previously.

In summary, this study, designed to high-light the individual effects of k-MAG and the SENSE parallel acquisition techniques to reduce scanning time, singly and in combination, on NAV coronary MR angiography showed that the addition of k-MAG decreases acquisition time by improving scan efficiency without compromising the visualized length of the coronary artery or subjective image quality. The addition of SENSE (factor of 2) to conventional NAV coronary MR angiography, although decreasing acquisition time to a significantly greater degree than k-MAG, also entails a slight but significant decline in subjective image quality, which may limit its service except under high SNR conditions or when coronary artery disease determination is less critical.


References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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