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(Radiology. 2001;218:580-585.)
© RSNA, 2001

Technical Developments

Coronary MR Angiography: Selection of Acquisition Window of Minimal Cardiac Motion with Electrocardiography-triggered Navigator Cardiac Motion Prescanning—Initial Results1

Yi Wang, PhD, Richard Watts, PhD, Ian R. Mitchell, PhD, Thanh D. Nguyen, PhD, Jeffrey W. Bezanson, Geoffrey W. Bergman, MD and Martin R. Prince, MD, PhD

1 From the Departments of Radiology (Y.W., R.W., I.R.M., T.D.N., J.W.B., M.R.P.) and Medicine (G.W.B.), Cornell University, Joan and Sanford I. Weill Medical College, 515 E 71st St, Suite S120, New York, NY 10021. Received March 17, 2000; revision requested May 2; revision received May 31; accepted July 11. Supported in part by research grants from the Whitaker Foundation and the American Heart Association and National Institutes of Health grant R01HL60879. Address correspondence to Y.W. (e-mail: yiwang@med.cornell.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and Methods
Results
 Discussion
 REFERENCES
 
The authors developed an electrocardiography-triggered M-mode navigator-echo technique to help monitor cardiac motion and identify the period of minimal cardiac motion in the cardiac cycle. Coronary magnetic resonance angiography was performed in eight healthy adult volunteers and one patient with heart disease. To minimize cardiac motion effects, trigger delays were estimated with the navigator-echo technique and two empirical formulas. The quality of images obtained with the different delay times was compared for clarity of depiction of the coronary arteries. Image quality was best with the delay calculated with the navigator-echo technique.

Index terms: Coronary vessels, MR, 54.12142 • Heart, MR, 54.12142 • Magnetic resonance (MR), motion correction, 54.12142 • Magnetic resonance (MR), vascular studies, 54.12142


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and Methods
Results
 Discussion
 REFERENCES
 
The major challenge for coronary magnetic resonance (MR) angiography is the cardiac and respiratory motion of coronary arteries (1–8). Breath-hold and navigator techniques have been developed to suppress respiratory motion effects (1,2,9–23). Clinical data from respiratory-motion–suppressed three-dimensional coronary MR angiography suggest that further improvement in coronary MR angiography requires more effective means to suppress both cardiac and respiratory motion effects (24–26). For reducing cardiac motion effects, current coronary MR angiographic techniques limit data acquisition to a portion of diastole when cardiac motion is expected to be minimal. A delay from the electrocardiographic trigger is used in every patient to start data acquisition in the diastole. Typically used delays are 500 msec with plus or minus 100-msec adjustment according to heart rate (1–26).

Recently, empirical estimations for middiastole have also been reported (20,27–29). However, these empirical delay equations were estimated from data such as heart sound recording, carotid arterial pulse tracing, and electrocardiography (27,28). They are not based on knowledge of the actual cardiac motion that is occurring during data acquisition. To estimate the motion of coronary arteries, breath-hold cine segmented fast gradient-echo MR imaging with multiple cardiac phases may be performed at a fixed location (29). However, it is difficult to extract an optimal electrocardiographic trigger delay from such a set of multiphase MR angiograms automatically and independent of the operator. In addition, the operator-estimated cardiac motions are averaged over many heartbeats (~24 heartbeats) and may be contaminated by imperfect breath holding.

The purpose of this study was to develop a cardiac navigator-echo technique that can be used to monitor cardiac motion directly and help identify the window of minimal cardiac motion in each patient before data acquisition at coronary MR angiography.


   Materials and Methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
 REFERENCES
 
Cardiac Motion Prescanning
On the basis of two-dimensional radio-frequency excitation (30–32), an electrocardiography-triggered pencil-beam gradient navigator echo was implemented with a 1.5-T MR imager (CVi; GE Medical Systems, Milwaukee, Wis; maximum gradient strength, 40 mT/m; slew rate, 150 mT/m/msec). Data collection parameters were 19-msec repetition time, 5-msec radio-frequency excitation, 16-kHz receiver bandwidth, craniocaudal axis, and 240-mm field of view. Imaging was positioned on the apical left ventricle and the anterior right ventricle (identified on breath-hold scout images obtained with the real-time workstation [CARDTOOL; GE Medical Systems]). The two-dimensional excitation had a Gaussian profile with 10-mm full-width at half-maximum.

Typically, 256 navigator echoes were prescribed, and 64 navigator echoes were acquired continuously after the electrocardiographic trigger. If necessary, the next trigger was skipped and extended to the following heartbeat. The cardiac navigator-echo acquisition lasted for four heartbeats (eight heartbeats if a trigger was skipped), with breath hold at the normal expiration.

We used a correlation algorithm to estimate the optimal acquisition window for coronary MR angiography. A matrix was generated by correlating every navigator-echo profile with all the others. For a given acquisition duration per cardiac cycle, n (defined as the given acquisition window divided by the navigator-echo repetition time, or nine for 160-msec duration) contiguous echoes that had the largest average correlation value were identified as the optimal acquisition window in the cardiac cycle. The optimal delay corresponded to the delay of the first echo in the optimal window. After the data were acquired, the delay was determined by using a C-based program on the real-time workstation.

Data Acquisition
Our institutional research board approved this study, and all participants gave their informed consent. Study participants included eight healthy adult volunteers (one woman and seven men; age range, 28–37 years; mean age, 31 years) with no history of heart disease and one 59-year-old female patient with minimal heart disease (normal ventricular function and a luminal-diameter stenosis of less than 40% in a major epicardial coronary artery).

The pulse sequence and algorithms were tested in the first three volunteers. In the last five volunteers, breath-hold two-dimensional coronary MR angiography (1) of the right coronary artery was performed at different delay times to evaluate delay times identified at navigator-echo cardiac motion prescanning and delay times calculated with the Weissler (27) or Stuber (20) empirical formulas. MR angiography (repetition time of 10 msec, echo time of 2 msec, 16 echoes per cardiac cycle, 160-msec acquisition window, centric phase encoding, 25° flip angle, 24–28-cm field of view, 16-kHz receiver bandwidth, 256 x 160 matrix) was performed with a four-element phased-array surface coil (cardiac coil) as the receiver. The right coronary artery was selected as the target because its motion amplitude is larger than that of the left coronary artery (33).

The MR angiographic planes were localized with use of the real-time interactive imaging tool on the MR imager. Five contiguous 5-mm-thick sections with 2-mm overlap were acquired to cover a coronary artery. Imaging was repeated with five to seven different electrocardiographic trigger delays (varied at 50–100-msec steps and acquired in random order). With one breath hold for each section, two-dimensional coronary MR angiography lasted 25–35 breath holds. As a consequence, a short breath hold (12 heartbeats) was used to allow the volunteers to complete all required breath holds without difficulty. For the patient, both the right and left coronary arteries were imaged with an 80-msec acquisition window and 22-heartbeat breath hold.

Data Analysis
MR angiograms obtained with different delay times were compared for the sharpness of their depiction of the coronary arteries. All sections containing the targeted coronary artery were included in image analysis. Two investigators (Y.W., M.R.P.), who were blinded to the delay times, reviewed and scored the MR angiograms independently. The score scale was as follows: 0, poor, the main coronary arteries were blurred and not well depicted; 1, good, the main coronary arteries were clearly depicted; 2, excellent, the main coronary artery and its branches were clearly depicted. To estimate reader agreement, {kappa} statistics were calculated to test overall scores (image qualities at all delay times for all subjects). To calculate the delay corresponding to the best image quality (Eq [1]), a consensus score was derived from a joint session by resolving the score difference.

Then a delay corresponding to the best image quality (DI) was estimated from the center of mass of the image quality versus delay curve:

where Q is the image quality score, D is the trigger delay used to acquire images, and SD is the delay step size. Note that with Q (0, 1, or 2), the effects of asymmetric sampling of the quality versus delay curve are eliminated.

DI was compared with the optimal delay estimated from navigator-echo prescanning (DN), delay derived from the Weissler empirical formula (Dw) (27),

where "RR" is the RR interval, and delay derived from the Stuber empirical formula ((DS) (20),

Dw is based on the length of systole (L = 550 msec - 2 x 60 x [1 sec/RR]) (27) with the following relationship: M = L + 0.5 x (RR - L), where M is the delay to middiastole.


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
Discussion
 REFERENCES
 
The cardiac navigator echoes were successfully obtained in all subjects, and the delay was identical for two navigator echoes acquired from the apical left ventricle and the anterior right ventricle. An example of the cardiac navigator-echo record is illustrated in Figure 1a. The correlation analysis identified the optimal acquisition window, which corresponded to a high plateau in the correlation graph (Fig 1b) or a region of steady signal intensity in the navigator-echo record (Fig 1a). DN was 735 msec for this case, and corresponding coronary MR angiograms acquired at different delay times are illustrated in Figure 2. As expected, the delay (750 msec) closest to the identified optimal delay provided the sharpest depiction of the right coronary artery branches. The branches of the right coronary artery disappeared when the delay was too early (Fig 2a, 2b) or too late (Fig 2e). The orientation of the right coronary artery changed with delay, and adjacent sections were included in assessing the overall image quality. DI was 721 msec, Dw was 765 msec, and DS was 572 msec. With DI as the standard, corresponding errors for DN, Dw, and DS were 14, 44, and -49 msec. DN was the most accurate for estimating the best delay.



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  		Figure 1a. (a) Navigator-echo record. The vertical axis is the craniocaudal direction and the horizontal axis is time. (b) Corresponding correlation graph. The gray bar indicates the optimal acquisition window (160-msec duration) for coronary MR angiography.

 


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  		Figure 1b. (a) Navigator-echo record. The vertical axis is the craniocaudal direction and the horizontal axis is time. (b) Corresponding correlation graph. The gray bar indicates the optimal acquisition window (160-msec duration) for coronary MR angiography.

 


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  		Figure 2a. MR angiograms of the right coronary arteries were acquired at different delay times: (a) 450, (b) 550, (c) 650, (d) 750, (e) 850, and (f) 950 msec. Corresponding overall quality scores for the right coronary artery are 1 (poor) for a, 2 (good) for b, 3 (excellent) for c, 3 for d, 3 for e, and 1 for f. Note that the right coronary artery appears the sharpest in d, as indicated by its branches (arrow). This is close to the optimal delay of 735 msec determined with the navigator echo.

 


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  		Figure 2b. MR angiograms of the right coronary arteries were acquired at different delay times: (a) 450, (b) 550, (c) 650, (d) 750, (e) 850, and (f) 950 msec. Corresponding overall quality scores for the right coronary artery are 1 (poor) for a, 2 (good) for b, 3 (excellent) for c, 3 for d, 3 for e, and 1 for f. Note that the right coronary artery appears the sharpest in d, as indicated by its branches (arrow). This is close to the optimal delay of 735 msec determined with the navigator echo.

 


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  		Figure 2c. MR angiograms of the right coronary arteries were acquired at different delay times: (a) 450, (b) 550, (c) 650, (d) 750, (e) 850, and (f) 950 msec. Corresponding overall quality scores for the right coronary artery are 1 (poor) for a, 2 (good) for b, 3 (excellent) for c, 3 for d, 3 for e, and 1 for f. Note that the right coronary artery appears the sharpest in d, as indicated by its branches (arrow). This is close to the optimal delay of 735 msec determined with the navigator echo.

 


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  		Figure 2d. MR angiograms of the right coronary arteries were acquired at different delay times: (a) 450, (b) 550, (c) 650, (d) 750, (e) 850, and (f) 950 msec. Corresponding overall quality scores for the right coronary artery are 1 (poor) for a, 2 (good) for b, 3 (excellent) for c, 3 for d, 3 for e, and 1 for f. Note that the right coronary artery appears the sharpest in d, as indicated by its branches (arrow). This is close to the optimal delay of 735 msec determined with the navigator echo.

 


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  		Figure 2e. MR angiograms of the right coronary arteries were acquired at different delay times: (a) 450, (b) 550, (c) 650, (d) 750, (e) 850, and (f) 950 msec. Corresponding overall quality scores for the right coronary artery are 1 (poor) for a, 2 (good) for b, 3 (excellent) for c, 3 for d, 3 for e, and 1 for f. Note that the right coronary artery appears the sharpest in d, as indicated by its branches (arrow). This is close to the optimal delay of 735 msec determined with the navigator echo.

 


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  		Figure 2f. MR angiograms of the right coronary arteries were acquired at different delay times: (a) 450, (b) 550, (c) 650, (d) 750, (e) 850, and (f) 950 msec. Corresponding overall quality scores for the right coronary artery are 1 (poor) for a, 2 (good) for b, 3 (excellent) for c, 3 for d, 3 for e, and 1 for f. Note that the right coronary artery appears the sharpest in d, as indicated by its branches (arrow). This is close to the optimal delay of 735 msec determined with the navigator echo.

 
The overall image quality versus delay curves are summarized in Figure 3. The independent scores from the two readers had good agreement ({kappa} = 0.85). In all cases, the peak center of the quality versus delay curve, the location of DI, was close to DN (Fig 3a). However, Dw consistently overestimated the delay (the peak center precedes Dw in Fig 3b), and DS consistently underestimated the delay (the peak center is behind DS in Fig 3c). This phenomenon can also be observed in Figure 4, which summarizes all data for the five volunteer cases. With DI as the standard, the average errors for DN, Dw, and DS were 6, 62, and -104 msec, respectively. Consistently over the five cases, DN values were closest to DI values.



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  		Figure 3a. Quality versus delay curves. Graphs depict overall image quality (0, poor; 1, good; and 2, excellent) of coronary arteries for the five volunteers ({square}, {lozenge}, {circ}, {triangleup}, x) at different delay times: (a) DN, (b) Dw, and (c) DS. DN corresponds to the center of the curve, which indicates good timing, whereas Dw tends toward the tail (too late) and DS tends toward the head (too early).

 


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  		Figure 3b. Quality versus delay curves. Graphs depict overall image quality (0, poor; 1, good; and 2, excellent) of coronary arteries for the five volunteers ({square}, {lozenge}, {circ}, {triangleup}, x) at different delay times: (a) DN, (b) Dw, and (c) DS. DN corresponds to the center of the curve, which indicates good timing, whereas Dw tends toward the tail (too late) and DS tends toward the head (too early).

 


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  		Figure 3c. Quality versus delay curves. Graphs depict overall image quality (0, poor; 1, good; and 2, excellent) of coronary arteries for the five volunteers ({square}, {lozenge}, {circ}, {triangleup}, x) at different delay times: (a) DN, (b) Dw, and (c) DS. DN corresponds to the center of the curve, which indicates good timing, whereas Dw tends toward the tail (too late) and DS tends toward the head (too early).

 


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  		Figure 4. Scatterplot summarizes results in the five volunteers for DI (delay_image), DN (delay_navigator), Dw (delay_weissler), and DS (delay _stuber). Compared with DI, DN provides the closest approximation, whereas Dw is consistently higher and DS is consistently lower.

 

   Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
REFERENCES
 
The results of this study demonstrate that an electrocardiography-triggered navigator-echo technique can be used to identify an optimal acquisition window to minimize cardiac motion effects in coronary MR angiography. The cardiac navigator echo is acquired in a brief breath hold before coronary MR angiography is performed. We recommend that navigator-echo cardiac motion prescanning be performed routinely for all coronary MR angiographic studies.

Navigator-echo cardiac motion prescanning can be used to select an optimal electrocardiographic delay for either two- or three-dimensional coronary MR angiography, with a long or short acquisition window. The criterion to validate DN is that the optimal delay corresponds to the peak of the image quality versus delay curve at coronary MR angiography. In practice, it is impossible to obtain a quality versus delay curve for a three-dimensional technique because of the long scanning time.

In this study, we obtained the quality versus delay curve for breath-hold two-dimensional coronary MR angiography. To shorten the required breath hold and allow the volunteers to complete the MR angiographic study, we used a 12-heartbeat breath hold, a 160-msec acquisition window, and a 50–100-msec delay step. As a consequence of this coarse sampling of the quality versus delay curve, the absolute errors in DN, DW, and DS were difficult to obtain.

Use of a shorter acquisition window and a finer variation in delay step might provide a better quality versus delay curve. This requires an increase in the length of breath hold and in the number of breath holds, however, which makes the study difficult for a subject to complete. Furthermore, digitization error in the quantification of image quality fundamentally limits the goodness of the image quality versus delay curve. On the other hand, the quality versus delay curves obtained in this study were good enough to allow validation.

Figures 3 and 4 demonstrate that the accuracy of estimation of the optimal delay for coronary MR angiography to minimize cardiac motion effects was substantially improved with the navigator DN in comparison with the empirical DW and DS. Owing to the coarse sampling and digitization error, we used the well-defined center of the quality versus delay curve (Eq [1]) instead of the ambiguous peak of the curve to mark the best delay.

Measurement of the quality versus delay curve may also be affected by inconsistency and imperfection in breath holding (34,35). To minimize any imperfection, we performed imaging with a short breath hold (12 heartbeats). All volunteers reported no problem with performing such breath holds as many times as required. In two of the volunteers, imaging with the first delay time was repeated at the end of the study, and there was no noticeable difference in image quality.

To minimize the effects of inconsistency in breath holding, we used 2-mm overlaps among five contiguous sections, and all five sections were included in image analysis with use of a criterion focused on the sharpness of the coronary artery. We believe the coarse sampling and digitization error rather than the inconsistency and imperfection of breath holding were the major problems.

Although the empirical Dw consistently overestimated the delay and DS consistently underestimated the delay in our five cases, extrapolation of this observation to a patient population remains to be verified. Results from conventional x-ray angiography have demonstrated that the coronary rest period in the cardiac cycle varies substantially from patient to patient even with the same heart rate (33). This unpredictability of the optimal delay necessitates the use of cardiac motion prescanning to minimize cardiac motion effects at coronary MR angiography.

We found a brief rest period in systole after the QRS complex (Fig 1b) that can also be used for coronary MR angiography. This finding is consistent with a previous report about coronary MR angiography during peak systole (36). This systolic window for data acquisition is short, however, and the longest possible acquisition window must be used for coronary MR angiography to shorten imaging time.

There may be several rest periods in the cardiac cycle in patients with coronary artery diseases, and the longest rest period may not be in middiastole (33,37–39). The value of navigator-echo cardiac motion prescanning is that it can be used reliably to identify the longest rest periods in the cardiac cycle.

The M-mode pencil-beam navigator echo has been applied previously in the heart to monitor cardiac motion (40,41). In our study, the use of a state-of-the-art fast gradient system shortened the excitation and echo times, which led to a more robust cardiac signal. Navigator echoes have also been used to acquire velocity profiles of the heart for wireless cardiac triggering (42,43). Such profiles depict primarily the motion of blood in the chamber, not the motion of the cardiac chamber wall, and therefore do not provide a good indication for the motion of coronary arteries.

There are some limitations of the navigator-echo technique used in this study. The motion of chamber blood during diastole can affect the profile of the navigator echo and, consequently, identification of the optimal delay. This effect can be minimized if the navigator echo is positioned away from the center of the heart, such as at the apex. A better solution may be to use a spin-echo type of navigator echo, with which the blood signal is minimal, or to acquire only the fat signal surrounding the coronary arteries.

Another improvement over the navigator-echo method used in this study is the quantification of cardiac motion. Quantification is needed to address the following important question for coronary MR angiography: What is the optimal acquisition window for a given cardiac motion tolerance range? The answer to this question is beyond the scope of this article. Although the pencil-beam navigator-echo signal is sensitive to three-dimensional motion of the heart, quantification of all components of cardiac contraction is more complicated. Coronary motion may be measured quantitatively from the myocardium or fat signals by using multiple navigator echoes or sensitive orbital navigator echoes (44).

In summary, an electrocardiography-triggered cardiac navigator-echo technique was developed for estimating cardiac motion of coronary arteries and identifying the optimal delay for the coronary MR angiographic data acquisition window in the cardiac cycle.


   FOOTNOTES
 
Abbreviations: DI = delay corresponding to the best image quality, DN = optimal delay at navigator-echo prescanning, DS = delay derived from the Stuber empirical formula, DW = delay derived from the Weissler empirical formula

Author contributions: Guarantor of integrity of entire study, Y.W.; study concepts and design, Y.W.; definition of intellectual content, Y.W.; literature research, Y.W.; clinical studies, Y.W., M.R.P., G.W.B.; data acquisition, Y.W., M.R.P., R.W., I.R.M., T.D.N., J.W.B.; data analysis, Y.W., R.W.; statistical analysis, Y.W.; manuscript preparation, Y.W., M.R.P., R.W.; manuscript editing, Y.W., M.R.P., R.W.; manuscript review, Y.W.


   REFERENCES
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 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
 REFERENCES
 

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