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Technical Developments |
1 From the Cardiovascular Division, Beth Israel Deaconess Medical Center and Harvard Medical School, 330 Brookline Ave, Boston, MA 02215 (M.S., R.M.B., P.G.D., K.V.K., W.J.M.), and Philips Medical Systems, Best, the Netherlands (M.S., R.M.B.). Received August 17, 1998; revision requested September 22; revision received October 19; accepted January 20, 1999. Address reprint requests to M.S. (e-mail: mstuber@caregroup.harvard.edu).
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Abstract |
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Index terms: Coronary vessels, MR, 54.121416, 54.12142 • Heart, MR, 54.12142 • Magnetic resonance (MR), motion correction • Magnetic resonance (MR), vascular studies, 54.121416, 54.12142
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Introduction |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Removal of the time constraints of breath holding would allow 3D imaging and offer the option for improved spatial resolution (7,17–20) and enhanced clinical applicability. A very powerful method for this is the application of MR navigator techniques that allow signal sampling during free breathing (21–25). The real-time navigator motion-correction method has been shown to be useful for 2D techniques (21,26,27), but no 3D approaches have been reported, to our knowledge. Navigators with gating and real-time motion correction result in reduced partial volume effect, increased navigator efficiency, and therefore shortened imaging times when compared to those with gating-only methods (26,27).
The most obvious interface for navigator monitoring of the respiratory level is the lung and the right hemidiaphragm (23–31). However, diaphragmatic and coronary motion may not be linearly related (28). Therefore, a model that incorporates the relationship between diaphragmatic motion and coronary displacement may be used for real-time motion-correction techniques. For coronary MR angiography, positioning of the navigator at the base of the anterior wall of the left ventricle (LV) may therefore be advantageous, given the proximity of this region to the proximal coronary arteries. Although MR navigators that use interceptional planes suffer from the disadvantage that magnetization voids occur in the region of interest (31,32), navigators that use 2D selective-excitation pulses (33–36) allow use of a shallow flip angle without compromise of the magnetization for subsequent imaging. This approach allows a real-time 2D selective navigator to be applied for gating and correction directly at the base of the heart.
In the present study, a navigator for gating and real-time motion correction was extended with a high-spatial-resolution 3D sequence for free-breathing coronary MR angiography. Results obtained with the navigator at the basal LV free wall are compared to those with the more conventional approach, in which the navigator is positioned at the dome of the right hemidiaphragm. Quantitative assessment was performed of the sharpness of the vessel boundaries, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) on the coronary MR angiograms.
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Materials and Methods |
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Sequence for 3D Coronary MR Angiography
Three-dimensional coronary MR angiography with real-time navigator gating and motion correction was implemented on a commercially available 1.5-T whole-body system (ACS-NT Gyroscan; Philips Medical Systems, Best, the Netherlands) equipped with a cardiac research patch software (CPR5) and a prototypic gradient system (PowerTrak 3000 [15 mT/m, 300-µsec rise time]) and six parallel digital receiver channels. For signal acquisition, a prototypic five-element cardiac phased-array coil with three rigid posterior elements and two flexible anterior elements was used. Each coil element is equipped with a separate preamplifier. The signals of the individual coil elements are combined as the square root of the sum of squares. A flow-insensitive T2 preparation sequence was applied (37–40) before the imaging part of the sequence. With this preparation of the magnetization, differences in the natural T2 of arterial blood (50 msec) and myocardial muscle (223 msec) can be used for contrast enhancement (38).
An electrocardiography-gated, fat-suppressed, 3D segmented-k-space gradient-echo sequence (Turbo Field Echo; Philips Medical Systems) with a repetition time of 7.4 msec and an echo time of 2.4 msec (7.4/2.4) was used. In Figure 1, the order of the sequence elements is displayed. The T2 preparation sequence (38) for contrast enhancement (echo time, 50 msec) is followed by the navigator excitation and postprocessing of the navigator data, the spectral-selective (chemical shift, 120° excitation) fat-saturation pulse, and the segmented-k-space gradient-echo sequence. The T2 preparation sequence involves a train of six nonselective composite radio-frequency pulses with corresponding radio-frequency pulse angles of 90°, 180°, 180°, -180°, -180°, and -90° (38). The excitation angle for the fat-saturation pulse was 110° to allow sampling of the center of k space at the zero crossing of the longitudinal magnetization, or Mz, of fat.
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The sequence was delayed with respect to the R wave of the electrocardiogram. The trigger delay (Td) was adjusted to sample the low-ky profiles in middle diastole and was defined according to the following equations (heart rate is measured in beats per minute): tRR = (1,000 x 60)/heart rate, and Td = [(tRR - 350) x 0.3] + 350, where tRR refers to the time delay between two subsequent R waves.
This definition takes advantage of the relatively constant duration of the systolic part of the cardiac cycle (~350 msec) (41).
Navigator Technique
The navigator used a 2D selective excitation in which 256 data points were sampled along the longitudinal axis of the navigator beam with a spatial resolution of 1 mm. After the readout, the sampled signal was Fourier transformed and cross correlated with a reference navigator profile that was acquired at end expiration during the preparation steps of the sequence. A user-selected portion of this reference navigator (kernel) (Fig 2) was used for cross correlation. The peak position of the cross correlation helps localize the actual position of the detected interface (17). If this location is within a user-defined range or gating window (Fig 3), the subsequently acquired profiles are accepted. Otherwise, they are rejected, and the same set of profiles are sampled again.
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For navigator definition of the desired interface, the diameter of the navigator has to be relatively small with respect to the interface surface. Therefore, a navigator with a freely adjustable diameter was implemented on the system. The current implementation allowed a diameter adjustment of 4–100 mm. The real-time navigator for gating and motion correction was applied at two locations—at the dome of the right hemidiaphragm and at the basal anterior wall of the LV (Fig 2, A [coronal] and B and C [transverse]). For both navigator locations, a navigator excitation angle of 30° was applied. Signal acquisition for the navigator readout was performed with use of the body coil.
Right hemidiaphragm navigator.—For the localized vertical right hemidiaphragm navigator, a diameter of 15 mm with a kernel length of 80 mm was used (Fig 2, A [coronal]). The gating window for the diaphragm was 5 mm, and a relationship of 0.6 (correction factor) was used between the superoinferior position of the diaphragm and the superoinferior position of the proximal coronary arteries (28).
Anterior LV navigator.—For the vertically oriented LV navigator, a 6-mm diameter was used (Fig 2, A [coronal] and B [transverse]). Since the anatomic location of the detected heart-lung interface is close to the proximal coronary arteries, a correction factor of 1.0 was used for motion correction. At the heart, a reduced maximal superoinferior displacement is expected when compared to that with the diaphragm (28). Therefore, the kernel length was reduced to 50 mm for the LV navigator, and a gating window of 3 mm was used.
For both navigator locations, the reference kernel was acquired at an end-expiratory position. For all measurements, the imaging efficiency, defined as the number of excitations accepted divided by the total number of R waves detected during the measurement, was recorded. For example, an imaging efficiency of 50% refers to an acquisition in which on average, the ky profiles of every other heartbeat are accepted.
Protocol
All subjects were examined in the supine position with electrocardiography leads on the anterior left hemithorax. All images (including the scout images) were acquired during free breathing. To localize the coronary arteries and position the navigator, two scout images were obtained. The first scout image (11/1.8) was an electrocardiography-triggered, non–fat-suppressed, multisection 2D segmented-k-space gradient-echo image with 10 transverse, 10 coronal, and 10 sagittal sections of the thorax, which allowed definition of the basal LV borders. Transverse, sagittal, and coronal sections were interleaved during acquisition of the same image. The field of view was 450 mm with a matrix of 256 data points. On these images, the navigator position at the base of the LV was planned for the second scout image.
For this second scout image (6.4/1.6), the data were acquired in diastole (2). Ten contiguous coronal 2D sections were acquired during free breathing by using a navigator-gated and motion-corrected, electrocardiography-triggered, non–fat-suppressed, segmented-k-space gradient-echo sequence. The field of view was 350 mm, and the image matrix included 256 data points in the measurement direction. Image data were acquired in the partial Fourier mode. The navigator for this second scout image was positioned at the anterior free wall of the LV. Navigator parameters included a 30° 2D selective excitation angle, a navigator diameter of 6 mm, a kernel length of 50 mm, a navigator window of 5.0 mm, and a superoinferior correction factor of 1.0 (28). In the second scout images, the dome of the right hemidiaphragm, the lateral free wall of the LV, and the left main coronary artery (LM) can be identified at an end-expiratory position (Fig 2, A [coronal]). The position of the LV navigator, the right hemidiaphragm navigator, and the location of the 3D volume for high-spatial-resolution coronary MR angiography were planned on the second scout images, with the center of the 3D volume positioned at the superoinferior position of the LM.
Coronary MR angiography of both right and left coronary arteries were acquired with (a) a navigator positioned at the dome of the right hemidiaphragm and (b) a navigator at the base of the LV free wall. For both navigator positions, transverse 3D volumes including the LM, the proximal and middle left anterior descending coronary artery (LAD), left circumflex coronary artery (LCX), and the proximal part of the right coronary artery (RCA) were imaged. For imaging of more distal parts of the RCA, another 3D image with a single oblique sagittal orientation was obtained for both navigator locations. This image was planned on the basis of findings on the transverse coronary MR angiograms. To avoid bias, the four images were obtained in random order.
Evaluation
For a quantitative assessment of image quality and contrast between the coronary segments and myocardial muscle or fat, an algorithm for edge detection as proposed by Deriche (42) was implemented on a personal computer (Red Hat 4.2; Linux, Durham, NC).
In a first step, the LAD, LCX, and RCA were manually identified by one author (M.S.) by drawing a centerline on the vessels (Fig 4, dashed line in A) of the unfiltered x4 zoomed anatomic images. No adaptation of window or level was performed. In a second step, the Deriche algorithm, which combines low-pass filtering for noise reduction and high-pass filtering for edge detection, was applied to the zoomed images. On these edge-enhanced filtered images, maximum values near the centerline (constrained by full width at half maximum, or FWHM, of the raw images) were then automatically determined with the algorithm (Fig 4, B). These maximum values delineate the vessel borders and can be used to identify the vessel diameter (Fig 4). In addition, these values represent the sharpness of the vessel boundaries with respect to their environment. A sharpness of 100% refers to a maximum change in signal intensity in two adjacent pixels (full dynamic range on the image). This evaluation procedure was applied to all the acquired 3D data sets. Equal segments of the LAD, LCX, proximal RCA, and distal RCA were evaluated and compared for both navigator locations.
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At the same level, the SNR was also quantified in the ascending aorta. The SNR of the muscle signal intensity was determined in the muscle of the LV anterolateral wall at the level of the proximal RCA. The CNR between blood and muscle was defined as (SIblood - SImuscle)/ [0.5(SDmuscle + SDblood)], where all values are means.
For 3D reconstruction of the coronary arteries, the images were transferred to a commercially available workstation (EasyVision; Philips Medical Systems), on which three orthogonal sections through the 3D data set can be displayed. Interactively, the user navigates on these orthogonal planes through the entire 3D data set. The LM, LAD, LCX, and RCA were manually identified in all three displayed planes. The 3D pathway of the coronary arteries was then reformatted and depicted in one plane.
Statistical Analysis
All values are presented as the mean plus or minus SEM. Comparisons were made by means of a paired Student t test. In all cases, a two-tailed test was performed. P values of .05 or less were considered statistically significant.
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Results |
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The vessel diameters for the LV navigator and the right hemidiaphragm navigator are shown in the Table. Navigator position resulted in no significant difference in diameter measurements for the proximal LAD, proximal LAD, distal RCA, or LCX.
For both navigator locations, the imaging efficiency of completed images exceeded 50% (Table) and was not significantly different. Average total imaging duration for both navigator locations was 16 minutes. In two cases, imaging was stopped due to the efficiency constraints given by the protocol. In the first case, with the navigator at the right hemidiaphragm, the imaging efficiency was below 20% for more than 3 minutes. In the second case, with the LV navigator, the patient moved and the detected interface between the lung and heart was no longer within the required window. In all completed images, there was excellent suppression of respiratory motion (Table, SNR in front of chest), and no evidence of image artifacts in the region of the LV navigator was seen on the images.
The SNR of muscle was markedly reduced when compared with that of the myocardium (CNR, ~5). This is a consequence of the T2 preparation sequence for contrast enhancement that additionally suppresses deoxygenated venous blood. No navigator-position–dependent change in SNR or CNR was found (Table).
In Figure 5, representative samples are shown from different transverse anatomic levels in different volunteers. The images in the top row were acquired with the LV navigator and in the bottom row with the right hemidiaphragm navigator. Consistent with the quantitative results, different parts of the coronary arteries were delineated equally well with both navigator locations. No navigator-position–dependent change in SNR or CNR was seen on the images, and respiratory artifacts were suppressed equally well.
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Discussion |
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The use of a shallow flip angle and 2D selective navigator excitations is a distinct advantage, since they enabled positioning of the navigator at any arbitrary position. No magnetization in the region of interest was compromised, even though the navigator was positioned at the level of the imaged volume.
For both navigator locations, the objective measure for coronary artery sharpness exceeded 50%, indicating an excellent depiction of the coronary arteries in relation to myocardial muscle or epicardial fat. The relatively low SEM demonstrates a robust and reproducible image quality with either navigator location. This agrees with previously published results in 2D cases (43). In the case of the LV navigator position, geometric overlapping of the navigator and the imaged volume did not seem to affect the reliability of the navigator algorithm for suppression of respiratory motion. Imaging efficiency and, hence, imaging time were also not compromised by positioning the navigator at either location. It is important to note that use of the T2 preparation sequence (to suppress the signal of myocardial muscle and deoxygenated blood) (37,38) did not preclude use of this navigator technology.
Although navigator position close to the coronary arteries is strategically appealing (no model function for correction has to be used), we found only minor quantitative differences in vessel sharpness, vessel diameter, CNR, SNR, or imaging time when comparing the right hemidiaphragm with the LV navigator positions. For the right hemidiaphragm navigator, vessel sharpness of the LAD and distal parts of the RCA was even slightly enhanced. The enhanced sharpness of the distal RCA may be related to the proximity of the right hemidiaphragm navigator to the distal part of the RCA. A basal LV navigator may not appropriately reflect breathing-induced motion of the distal RCA.
Considering the facilitated setup of the right hemidiaphragm navigator (navigator localization), our data suggest that a diaphragmatic navigator is preferable. These findings also confirm previously published results that suggest use of a constant correction factor of 0.6 for real-time motion correction (28). Even though this factor might be individually dependent, on average it seems to be a reliable value.
Future improvements in the LV navigator method may allow full advantage of the proximity to the coronary vessels. In the present study, a single vertically oriented navigator was applied, and only superoinferior correction of the imaged volume was performed. The breathing-induced anteroposterior, or left-right, displacement of the heart was neglected. Even though this displacement has been reported to be minor (28), further improvements may be expected with use of an oblique orientation of the cardiac navigator or multiple navigators that account for the breathing-induced 3D motion of the heart. But this remains to be studied. The SNR of the navigator signal is related to the diameter of the pencil beam that is excited. In the case of the LV navigator, the diameter of the beam needs to be small and, hence, a reduced SNR of the navigator signal results. This may have affected LV navigator performance.
In preparation for the present study, the sequences were optimized at imaging in a large number of healthy volunteers. It has turned out to be evident that plan imaging to position the navigator and the location of the imaged volume has to be performed under exactly the same conditions as the high-spatial-resolution 3D imaging; that is, plan imaging during free breathing with a consistent trigger delay and end-expiratory signal sampling is crucial. For the first scout image, no navigator gating or correction is possible since no image data for the localization of the lung-diaphragm or lung-heart interfaces is present. Thus, this first scout image needs to be obtained during free breathing. Despite some blurring of the lung-heart interface on the images, image quality in all cases was sufficient for appropriate localization of the navigator at the subsequent navigator-gated and motion-corrected coronal scout imaging.
By applying a short, 60-msec, acquisition window, cardiac-motion–induced blurring of the coronary arteries can be reduced to a minimum (40), the steady-state magnetization is potentially enhanced, and the T2 prepulses are more effective. In addition, a shortened acquisition window keeps the acquired profiles closer to the navigator, and therefore the sequence is less susceptible to residual breathing artifacts. Further improvements in SNR and CNR may be achieved by applying intravascular contrast agents (44), reordering the profiles in k space (45), or using alternative imaging strategies (6,46). Such approaches remain to be investigated further in combination with the present navigator technique.
In prior studies, visual comparisons, visual grading, or length and diameter determination have been applied successfully (7,10,18,31,47,48). However, such visual ratings may be of limited objectivity. The present computer-assisted image analysis allows objective assessment of vessel sharpness, diameter, SNR, and CNR. Thus, observer dependencies may be reduced, and the method is well suited for use in comparative studies in the field of coronary MR angiography.
In conclusion, free-breathing submillimeter 3D coronary MR angiography was successfully extended with real-time-navigator motion correction. The proximal and middle portions of the left and right native coronary artery system can be reliably depicted with both cardiac and diaphragmatic navigator locations, but the diaphragmatic navigator was objectively found to be superior for distal RCA visualization and LAD sharpness. Though an LV navigator may have theoretic advantages, the right hemidiaphragm navigator used for gating and real-time correction is at least as valuable as the LV navigator. Considering the easier setup for the right hemidiaphragm navigator (localization), we conclude that the use of a diaphragmatic navigator is currently preferred.
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Footnotes |
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Author contributions: Guarantor of integrity of entire study, W.J.M.; study concepts and design, M.S.; definition of intellectual content, M.S.; literature search, M.S.; experimental studies, M.S.; data acquisition, K.V.K.; data analysis, R.M.B.; statistical analysis, M.S.; manuscript preparation and editing, M.S.; manuscript review, P.G.D.
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 W. Y. Kim, P. G. Danias, M. Stuber, S. D. Flamm, S. Plein, E. Nagel, S. E. Langerak, O. M. Weber, E. M. Pedersen, M. Schmidt, et al. Coronary Magnetic Resonance Angiography for the Detection of Coronary Stenoses N. Engl. J. Med., December 27, 2001; 345(26): 1863 - 1869. [Abstract] [Full Text] [PDF]
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S. Schroeder, A. F. Kopp, A. Baumbach, C. Meisner, A. Kuettner, C. Georg, B. Ohnesorge, C. Herdeg, C. D. Claussen, and K. R. Karsch Noninvasive detection and evaluation of atherosclerotic coronary plaques with multislice computed tomography J. Am. Coll. Cardiol., April 1, 2001; 37(5): 1430 - 1435. [Abstract] [Full Text] [PDF]
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M. Stuber, R. M. Botnar, K. V. Kissinger, and W. J. Manning Free-Breathing Black-Blood Coronary MR Angiography: Initial Results Radiology, April 1, 2001; 219(1): 278 - 283. [Abstract] [Full Text]
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Y. Wang, R. Watts, I. R. Mitchell, T. D. Nguyen, J. W. Bezanson, G. W. Bergman, and M. R. Prince Coronary MR Angiography: Selection of Acquisition Window of Minimal Cardiac Motion with Electrocardiography-triggered Navigator Cardiac Motion Prescanning--Initial Results Radiology, February 1, 2001; 218(2): 580 - 585. [Abstract] [Full Text]
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E. Spuentrup, A. Ruebben, T. Schaeffter, W. J. Manning, R. W. Gunther, and A. Buecker Magnetic Resonance-Guided Coronary Artery Stent Placement in a Swine Model Circulation, February 19, 2002; 105(7): 874 - 879. [Abstract] [Full Text] [PDF]
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