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Cardiac Imaging |
1 From the Departments of Radiology (Y.W., E.V.) and Medicine (G.W.B.), Joan and Sanford I. Weill Medical College, Cornell University, 1300 York Ave, New York, NY 10021. From the 1998 RSNA scientific assembly. Received September 21, 1998; revision requested November 10; final revision received March 5, 1999; accepted April 29. Supported in part by research grants from the Whitaker Foundation and EPIX Medical. Address reprint requests to Y.W. (e-mail: yiwang@mail.med.cornell.edu).
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Abstract |
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MATERIALS AND METHODS: Motion of coronary arteries was measured in 13 patients by using breath-hold, biplane, conventional angiography, with frontal and lateral projections of the left and right coronary arteries acquired at 30 frames per second. The time courses of the coordinates of bifurcations of proximal parts of the coronary arteries were measured, from which the rest period (motion < 1 mm in orthogonal axes), velocity, displacement range, motion correlation, and reproducibility from heartbeat to heartbeat were estimated.
RESULTS: Both the motion pattern and the amplitude varied substantially from patient to patient. The rest period varied from 66 to 333 msec (mean, 161 msec) for the left coronary artery and from 66 to 200 msec (mean, 120 msec) for the right coronary artery.
CONCLUSION: The rest period for coronary arteries in the cardiac cycle varies substantially from patient to patient, which may cause quality to be inconsistent in current coronary MR angiography. A cardiac motion image prior to coronary data acquisition (preimage) may be used to estimate the optimal duration and timing in the cardiac cycle for coronary MR angiography.
Index terms: Coronary angiography, 54.1244 • Coronary arteries, MR, 54.12142 • Heart, flow dynamics, 54.12142, 54.1244, 54.761 • Magnetic resonance (MR), artifact, 54.12142
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Introduction |
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However, the quality and accuracy of coronary MR angiography remain variable (4,7,16,17,25–27). An important cause of image-quality variability may be the motion due to cardiac contraction. In reported coronary MR angiographic studies, data were acquired in a 100–160-msec window within the cardiac cycle. Considerable coronary motion due to cardiac contraction may exist in this period. Reducing such cardiac motion effects in coronary MR angiography requires quantitative information on the motion of the coronary arteries throughout the cardiac cycle. Quantitative information on coronary motion during the cardiac cycle is important not only for coronary MR angiography, but also for coronary flow quantification (28).
Although cardiac motion occurs in both systole and diastole, it is said to be minimal at middiastole. Thus, coronary image data have been acquired at middiastole (1–24). However, there is little quantitative information on this so-called rest period. In-plane coronary motion has been measured by using respiratory-triggered cine MR imaging (28). Motion depicted in cine MR imaging is not in real-time but is averaged over many heartbeats. Such measurements were performed in a fixed plane, so the "motion" observed was not the motion of a fixed point on the coronary artery but the appearance of different points along the coronary artery as the vessel moved through the fixed plane. Furthermore, cine MR imaging still lacks adequate spatial resolution and image quality for precise measurement of coronary motion.
More precise data could be derived from biplane, cine, conventional angiography (29,30). Motion of bifurcation points on the left coronary artery has been used to evaluate ventricular wall motion (31–34). We performed this study to measure the duration of the rest period in the cardiac cycle, a parameter vital to data acquisition in coronary MR angiography. This article is a report of our study findings on the motion of coronary arteries by using conventional x-ray angiography. Motion parameters important for coronary MR angiography, such as the rest period, velocity, and spatial correlation, were measured. Details are reported in the following sections.
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MATERIALS AND METHODS |
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Easily recognizable points on the proximal parts of the coronary arteries (defined by major branching points, such as the left main bifurcation, the circumflex obtuse marginal artery origin, the left anterior descending diagonal artery origin, the right acute marginal artery origin, and the posterior descending artery origin) were used as anatomic landmarks for measuring displacement of the coronary arteries during the cardiac cycle. Three landmarks were usually used in defining motion for the LCA: the left anterior descending and circumflex branches and the left main bifurcation (Fig 1a). Two landmarks were used in the motion measurements of the RCA: the RCA acute marginal origin and the posterior descending artery origin (Fig 1b). The motion of these landmarks was measured by using the spatial coordinates of each point in a series of consecutive frames from each projection (craniocaudal or superoinferior and right-left in the frontal projection, superoinferior and anteroposterior in the lateral projection). Fifty frames were used in 11 patients, while 40 and 60 frames were used in the two remaining patients.
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The duration of the cardiac cycle was obtained from the periodicity in the time course of the landmarks. The rest period of a coronary artery was defined such that the displacement of the coronary artery during the rest period was less than 1 mm in any orthogonal (superoinferior, anteroposterior, and right-left) direction. The maximal displacement in any direction during such a defined rest period is less than 
3 mm.
The duration of the rest period of coronary arteries was estimated in a four-step process (Table): (a) The consecutive frames during which the coordinate value changed less than 1 mm were identified for both coordinates of all landmarks in both projections (coordinate rest period). (b) The intersection (common frames) of the two coordinate rest periods corresponding to the two coordinates of that landmark in that projection were then derived for all landmarks in both projections (landmark rest period). (c) The intersection of all landmark rest periods corresponding to all landmarks in that projection were derived for both projections (projection rest period). (d) The intersection of both projection rest periods was derived to give the rest period of a coronary artery.
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A general relationship between duration and motion occurred, in that duration was also measured from each landmark's time courses. We implemented the following computer program to find the duration for a given allowed motion range. The middle point in the rest period (defined in two paragraphs before this) was used as a starting point to search for the duration, and the search was performed forward and backward in time. The search was repeated over all possible relative positions of the seed in the motion range to ensure that the identified duration corresponded to the maximal number of contiguous frames.
The collected data were further analyzed to estimate the motion ranges and velocities for both transverse and longitudinal directions for each landmark in each projection. Correlation coefficients among measured time courses were calculated to determine the extent of correlation between different landmarks and the relationship between correlation coefficient and spatial separation. The coordinates of landmarks at rest periods in the two to three cycles measured were compared to check if the coronary arteries return to the same location from heartbeat to heartbeat.
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RESULTS |
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)/{1 + exp[(t - tc)/
)] + f}, where f0 is the initial value, f is the final value, t is time, tc is the transition point, and is the transition interval. At heart rates faster than 65 beats per minute, the rest periods for the LCA and the RCA approach 66 msec asymptotically. At heart rates slower than 45 beats per minute, the rest period reaches 333 msec for the LCA and 200 msec for the RCA.
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Figure 6a and 6b show that the longitudinal displacement for both arteries was very similar. However, the transverse displacement for the RCA was more than twice that for the LCA, especially in the lateral projections. There was no obvious relationship between the heart rate and the displacement ranges (Fig 6c).
Correlation coefficients were calculated for each image set and were plotted against the distance between landmarks during the resting frames (Fig 7). Of all the 271 points, 114 (42%) had correlation coefficients above 0.9, while 187 (69%) had correlation coefficients above 0.8. A substantial number of points (84 [31%] of the 271 points), including the points close to each other on the same coronary artery, did not correlate well (r < 0.8).
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DISCUSSION |
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The measurements of the rest period are consistent with known cardiac physiology. The rest period (diastasis of diastole) shortens as the heart rate increases. This may be explained by the current understanding that the duration of active contraction and relaxation of the myocardium (systole) is less variable in the same patient and that among different patients the difference in systolic duration is smaller than the difference in systolic duration. We do not know of any quantitative electrophysiology that explains the Fermi function. (Fermi function fitting is more accurate than the linear, power, exponential, and gaussian fittings. All fittings have little value for the very slow [<45–beats per minute] heart rate, where there is only one point.)
The gradual slowing of motion prior to the rest period observed in most cases may correspond to the filling of ventricles. The abrupt motion observed at the end of the rest period may correspond to the atrial systole at the late part of diastole. The measured motion ranges for both the RCA and the LCA are consistent with those in previous studies (28,32). The phenomenon in which the proximal part of the RCA moves more than twice as much as the LCA and exhibits larger velocities may be explained by the pulling of the cardiac base (relative lack of contracting muscle) toward the apex during cardiac contraction. The lack of spatial correlation of coronary motion may be due to the independence of the many individual myocardial muscles. The heart is normally confined tightly in the pericardial sac; therefore, it returns to the same rest period location in the thoracic trunk when there is no respiration and no change in cardiac output.
These results are highly valuable for developing methods to reduce cardiac motion effects in coronary MR angiography. That the coronary artery returns to the same rest period location makes coronary MR angiography possible. In current coronary MR angiographic techniques, the data acquisition time during the cardiac cycle is typically 100–160 msec. Although this duration is approximately the mean rest duration we measured, certain patients will have actual rest periods shorter than this duration, as we have seen in patients with fast heart rates. Therefore, cardiac motion effects in current coronary MR angiography may be substantial in these patients. This may be the primary source of quality variation reported in respiratory-gated coronary MR angiographic studies (4,7,16,17,25–27). On the other hand, the data acquisition duration in current coronary MR angiography can be lengthened for certain patients, such as those with slow heart rates, to shorten the imaging time and to increase the signal-to-noise ratio and spatial resolution without increasing cardiac motion effects.
To minimize cardiac motion effects, coronary MR angiographic data acquisition should be limited to the rest period. The variability of the rest period indicates that the data acquisition window in the cardiac cycle should be adjusted for each patient. Figure 3 indicates that the acquisition period may be 60, 60–200, or 200 msec (300 msec if imaging only the LCA) for a heart rate above 65 beats per minute, 45–65 beats per minute, or below 45 beats per minute, respectively.
The precise rest period for heart rates of 45–65 beats per minute and the delay from the electrocardiographic trigger (QRS interval) in general may require further estimation for each patient. This may be achieved by using electrocardiographically triggered continuous navigator echoes acquired through the heart (35). The profile of the heart can be sampled at approximately 30 frames per second. From such an M-mode–like navigator-echo record, the motion of the heart and particularly the rest period can be identified. Accordingly, the optimal acquisition duration and the electrocardiographic trigger delay may be quickly estimated from this navigator motion image prior to coronary data acquisition (preimage), and the use of optimal timing in the cardiac cycle for coronary MR angiographic data acquisition may substantially reduce cardiac motion effects in coronary MR angiography.
Limiting data acquisition in coronary MR angiography to the rest period may be important for reducing cardiac motion effects. It may require a long imaging time when the rest period is short. The typical gradual motion prior to the rest period may allow us to extend data acquisition beyond the rest period into the time interval preceding the rest period. The effects due to gradual motion may be minimized by using techniques such as phase reordering (36,37). The lack of uniform spatial correlation of coronary motion in the cardiac cycle may indicate that image artifacts caused by cardiac motion over the entire cardiac cycle can be difficult to reduce; accordingly, data acquisition duration cannot be extended to the entire cardiac cycle.
The electrocardiographic waveform synchronized to coronary conventional angiograms was not available, and the temporal location of the rest period in the cardiac cycle was not determined in this study. The temporal location of the rest period may be easily obtained by using an M-mode navigator-motion preimage, as suggested above. The temporal resolution for all time measurements (rest periods, durations, and heart rates) was 33 msec (frame duration), which may cause large relative error in these time measurements. However, these time measurements still hold value for guiding MR imaging study. Two to three adjacent heartbeats were measured quantitatively, and there was no change in the rest period or the rest position of the coronary arteries. Variation of the rest position and the duration from heartbeat to heartbeat is expected to be minimal when there is no change in cardiac output and respiration is suspended, because the heart is tightly confined to the pericardial sac.
When coronary bifurcations well depicted in the frontal projection were obscured in the lateral projection, immediately adjacent bifurcations were chosen in the lateral projection for motion measurement. This should not affect the estimation of rest period, motion velocity, motion range, or spatial correlation reported in this study. The 4-pixel criterion limited possible motion in the rest period to be less than 1, 1.4, and 1.7 mm in any orthogonal axis, any orthogonal plane, and any direction, respectively. Although the depth-dependent amplification in projection imaging may affect the accuracy of absolute motion measurement, verification with cine angiographic movies ensured that motion was minimal during the identified rest period.
In summary, the motion pattern of coronary arteries and the rest period of the coronary arteries in particular vary from patient to patient. To minimize the effects of cardiac contraction in coronary MR angiography, a cardiac motion preimage may be used to estimate the optimal timing for data acquisition during the cardiac cycle.
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Footnotes |
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Author contributions: Guarantor of integrity of entire study, Y.W.; study concepts and design, Y.W.; definition of intellectual content, Y.W.; literature research, E.V., Y.W.; clinical studies, G.W.B.; data acquisition, G.W.B.; data analysis, E.V., Y.W.; statistical analysis, E.V., Y.W.; manuscript preparation, E.V., Y.W.; manuscript editing, E.V., G.W.B., Y.W.; manuscript review, Y.W.
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