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Technical Developments |
1 From the Departments of Radiology (D.L., J.C.C., S.M.S., J.Z., V.S.D., J.P.F.) and Biomedical Engineering (D.L., S.M.S., V.S.D.), Northwestern University, 448 E Ontario St, Suite 700, Chicago, IL 60611; and the Department of Radiology, Daniel den Hoed Kliniek, Rotterdam, the Netherlands (P.A.W.). From the 1999 RSNA scientific assembly. Received March 22, 2000; revision requested May 12; revision received August 8; accepted August 30. Supported in part by National Institutes of Health grant HL 38698 and by Bracco Imaging, Milan, Italy and Siemens Medical Systems, Erlangen, Germany. Address correspondence to D.L. (e-mail: d-li2@northwestern.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and MethodsResultsDiscussionREFERENCES |
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Index terms: Coronary vessels, MR, 54.12142, 54.12143 • Magnetic resonance (MR), contrast enhancement, 54.12142, 54.12143 • Magnetic resonance (MR), vascular studies, 54.12142, 54.12143
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionREFERENCES |
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Coronary images acquired with an electrocardiographically triggered nonenhanced non–magnetization-prepared 3D sequence is largely proton-density weighted because of the magnetization recovery during the trigger delay time and the limited inflow-related blood signal intensity enhancement. Magnetization transfer (1,2) and T2 preparations (3,4) have been used to suppress the myocardial signal intensity surrounding coronary arteries. However, both schemes reduce the blood signal intensity as well.
The use of T1-shortening contrast agents has revolutionized MR angiography of the entire body. It dramatically improved blood SNR and permitted the use of short repetition times with high-performance gradient systems. In addition, the blood signal intensity becomes largely flow independent, which is critically important for the depiction of slow-flowing blood and the reliable detection of vascular diseases.
Goldfarb and Edelman (5) demonstrated the improvements in coronary SNR and CNR with the administration of an extracellular contrast agent by using a 3D breath-hold imaging method. However, only one slab was acquired for each subject with an injection of 30–40 mL of contrast agent to cover one side of the coronary arterial system. A steady-state magnetization preparation scheme was used to suppress the myocardial signal intensity, which also suppressed the blood signal intensity and resulted in suboptimal CNR. Finally, transverse images were obtained because of the limitation of the gradient system. As a result, only short segments of coronary arteries were depicted. Authors of subsequent studies (6–9) have made various improvements to the method. However, some of the problems mentioned earlier remain, and the consistent depiction of both the left coronary artery and the right coronary artery (RCA) in the same subject has not been demonstrated with 3D breath-hold imaging and injections of extracellular agents.
The purpose of this study was to optimize a magnetization-prepared volume-targeted approach to improve the SNR, CNR, coverage, and definition of coronary arteries with 3D breath-hold MR imaging and injections of extracellular contrast agents.
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Materials and Methods |
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TOPABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionREFERENCES |
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For each subject, a 3D segmented echo-planar imaging volume localizer was first acquired to cover the major portion of the heart around the origin of the ascending aorta. The images were then reformatted with multiplanar reconstruction to find the double-oblique angles of proximal portions of the left anterior descending coronary artery (LAD) and RCA. Breath-hold images were acquired before the injection of contrast agent along the vessel axes as determined by means of multiplanar reconstruction. Contrast agent was then injected, and enhanced 3D breath-hold images were obtained with the same imaging volumes. All images were obtained with a 1.5-T whole-body imaging system (Sonata; Siemens Medical Systems, Erlangen, Germany) with a high-performance gradient subsystem (maximum gradient strength, 40 mT/m; shortest gradient rise time from 0 to maximum strength, 200 µsec).
Segmented Echo-planar Imaging Volume Localizer
There were six readouts after each radio-frequency pulse. The duration of each readout was 768 µsec, and the spacing between radio-frequency pulses was 6.2 msec. Twenty radio-frequency pulses were applied during each heartbeat. The slab thickness was 110 mm, and the number of partitions was 40. The field of view was 165 x 220 mm2, and the data acquisition matrix was 96 x 256 (phase encoding by readout) with half-Fourier reconstruction in the phase-encoding direction (1). Twenty magnetization transfer pulses and a spectral selective saturation pulse were applied before data acquisition to suppress myocardial and fat signal intensities, respectively. Two regional saturation bands were placed to reduce image wraparound artifacts in the phase-encoding direction. The image orientation was transverse. The total imaging time was 17 heartbeats, and subjects were instructed to hold their breath at the end of expiration.
Precontrast Targeted Imaging
Two precontrast targeted acquisitions were obtained to image the LAD and RCA separately. The imaging technique was an electrocardiographically triggered 3D segmented fast low-angle shot, or FLASH, sequence. During each heartbeat, 25 or 31 in-plane phase-encoding steps were performed, depending on the heart rate of the subject. With a repetition time of 3.8 msec, the data acquisition duration per heartbeat was 95 or 118 msec. Three heartbeats were required to cover each kx-ky plane of the 3D k space in an interleaved manner. Other imaging parameters included the following: echo time, 1.9 msec; flip angle, 15°; in-plane resolution, 1.4–2.0 x 1.0–1.2 mm2; slab thickness, 24–32 mm; number of partitions, eight; section thickness, 3–4 mm; and imaging time, 24 heartbeats.
Data were acquired with a centric order in the phase-encoding direction and a linear order in the partition-encoding direction. The images were interpolated with a sinc function to 16 sections to facilitate image display and reformation. Partial k space was covered in the readout (75%) and phase-encoding (80%) directions, and the echo center occurred at the 64th and 77th points, respectively, in a 256 x 256 matrix in the two directions. These measures were used to shorten repetition time and to reduce the number of phase-encoding lines to be collected. Phase- and partition-encoding gradients were rewound after data readout, and radio-frequency spoiling was used to eliminate transverse magnetization coherence. A two-channel body phased-array coil was placed on the chest of the subject for signal receiving. To reduce image wraparound with a small field of view, no posterior receivers were used.
Magnetization-prepared Contrast-enhanced Volume-targeted Imaging
Two groups of volunteers were examined with the same contrast agent infusion time but different injection rates. A total of 40 mL of gadoteridol (Prohance; Bracco, Milan, Italy) was administered for each volunteer. In the first group (n = 6), 40 mL of contrast agent was administered as a single injection, and a targeted 3D acquisition was obtained to cover either the left coronary artery or the RCA. In the second group (n = 10), two separate 20-mL injections of contrast agent were administered, and after each injection a targeted acquisition was obtained. The two acquisitions covered the left coronary artery and RCA separately. The interval between the two injections was approximately 2–3 minutes.
The contrast agent was injected uniformly over 20 seconds by using an MR injector (Spectris; Medrad, Indianola, Pa), with injection rates of 2 mL/sec and 1 mL/sec in the first group and second group, respectively. On the basis of our past experience, data collection started approximately 25 seconds after the initiation of the contrast agent injection. The sequence parameters and orientations in images obtained after the injection of contrast agent were the same as those in precontrast images, except an inversion-recovery preparation was used prior to data acquisition (10,11) and a larger flip angle of 22° was used. The nonselective inversion pulse was a hyperbolic secant function.
Computer simulations were performed to choose the inversion time for maximal contrast between blood and myocardium. As shown in Figure 1, an inversion time of longer than 300 msec allows almost full recovery of blood magnetization for T1s less than 100 msec, which are the blood T1 values expected during the first pass of 20- or 40-mL injections of contrast agent. On the other hand, the magnetization of myocardium with T1s of 400–600 msec, which are the myocardial T1 values expected during the first pass of injections of contrast agent, is nulled with inversion times of 250–300 msec. Therefore, an inversion time of 300 msec was chosen for all volunteer studies.
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Image reformatting.—To facilitate coronary arterial depiction, maximum intensity projection images were obtained for each volunteer study. Background tissues overlapping with coronary arteries were removed manually from individual sections before maximum intensity projection images were obtained by using the public domain image-processing software.
SNR and CNR measurements.—Blood signal intensities were measured at a cross-sectional plane of the ascending aorta at the level of either the left main artery or RCA origin in pre- and postcontrast images acquired in both groups of volunteers. The SD of noise in the image was estimated by dividing the mean signal intensity measured in background air by a factor of 1.25 (12). Myocardial signal intensity was measured at the anterior portion of the left ventricular wall inferior to the LAD. All regions of interest were carefully placed by an author (V.S.D.) to avoid apparent motion or flow artifacts. The blood SNR and blood-myocardium CNR were then calculated as follows: SNR = 1.25 · blood signal intensity/signal intensity of background air, and CNR = 1.25 · signal intensity difference between blood and myocardium/signal intensity of background air.
Lengths of continuously depicted coronary arteries.—Three-dimensional images were first reformatted by manually tracing the coronary arteries through adjacent sections by using the curved multiplanar reconstruction program on our imaging system. As a result, the entire course of the depicted coronary artery was displayed in a single image, from which the lengths of the coronary arteries were measured by an author (S.M.S.) using the distance measurement tool on the imaging system.
Lumen diameter and sharpness.—Original images containing proximal portions of coronary arteries were magnified four times by using bilinear interpolation. Signal intensity profiles along a user-selected line (width, 3 pixels) perpendicular to the vessel axes were obtained within the first 2 cm of the LAD and RCA by using the public domain image-processing software on a personal computer. For each side of the intensity profile, the maximal blood signal intensity and the signal intensity of the background next to the blood vessel were first determined. The full width at half between maximum and background was then calculated to estimate the coronary arterial diameter (13).
For evaluating the sharpness of the artery, the 20% and 80% points between the maximal and background signal intensities were first calculated for each side of the intensity profile. The distance in millimeters between the two points was then determined for each side. The inverse of the averaged distance of the two sides was used as a measurement of the coronary arterial sharpness. The greater the sharpness, the better the vessel definition.
Statistical analysis.—A two-tailed t test was used for statistical analysis. A P value of .05 was considered to be statistically significant.
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Results |
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TOPABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionREFERENCES |
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Vessel Diameter and Sharpness
As shown in Table 2, the length and sharpness of the left coronary artery and RCA are substantially greater in postcontrast images. Note also that the measured diameters of the LAD and RCA in postcontrast images are smaller than those in precontrast images.
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Discussion |
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TOPABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionREFERENCES |
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Breath hold is a simple and effective method for eliminating respiratory motion and was used for two-dimensional imaging in the early 1990s (21–23). One of the problems with two-dimensional imaging was the need for a large number of breath holds to cover the vessels of interest. With improved gradient capabilities in recent years, it has become possible to use short repetition times to acquire a 3D slab within a single breath hold. Coronary artery images acquired with a 3D breath-hold sequence generally have limited SNRs because of the constraint on imaging time. In addition, the depiction of coronary arteries varies substantially, depending on the type of surrounding tissue. Coronary arteries adjacent to myocardium are generally poorly delineated compared with those in the vicinity of epicardial fat, which is suppressed by a frequency selective prepulse. Magnetization transfer prepulses (1,2) and T2 preparation (3,4) have been used to suppress the myocardial signal intensity. However, they also further reduce the SNR of coronary arteries. In this study, we demonstrated that with an appropriate imaging technique, both the SNR and CNR of coronary arteries were significantly increased by the injection of conventional extracellular contrast agents. Visible length and definition of coronary arteries were also improved in postcontrast images.
Inversion recovery has been used previously for background suppression in time-of-flight MR angiography of renal (10) and carotid arteries (24), and more recently with contrast-enhanced coronary MR angiography in which a blood pool agent was used (11,25). It allowed marked background suppression with minimal effects on the blood signal intensity when the blood T1 was sufficiently short, thus creating better contrast than other magnetization preparation schemes (1–5,26). The improved CNR was particularly important for the depiction of the left coronary artery and the distal portion of the RCA, which are in proximity to background tissues such as myocardium or liver.
Suppression of fat signal intensity is of critical importance for coronary imaging. Inversion-recovery preparation substantially suppressed the signal intensity of epicardial fat because it has a T1 of approximately 250 msec, much longer than that of postcontrast blood. This is an important benefit of contrast-enhanced imaging because failure to achieve uniform fat saturation is a serious practical problem in certain subjects.
Because of the limited volume coverage per breath-hold acquisition, it is critical to optimize the plane setup to maximize coverage of coronary arteries for each targeted acquisition. The segmented echo-planar imaging localizer covers almost the entire heart within a single breath hold. Although the images are not of diagnostic quality, they permit the interrogation of the coronary arterial system interactively for the best view of the targeted vessel.
An injection of 40-mL of MR contrast agent resulted in a slightly greater blood SNR than did a 20-mL injection of contrast agent, which is consistent with results with theoretic simulations. However, the CNR improvement with 20-mL injections of contrast agent was not significantly lower than that with 40-mL injections of contrast agent. More important, with the same total volume of contrast agent for each subject, the imaging protocol with 20-mL injections allowed two first-pass acquisitions to cover both the left coronary artery and RCA. It should be noted that the same flip angle was used for the two protocols. If the exact blood T1 value was known for each subject, variable flip angles should have been used to optimize the signal intensity. In addition, with the two-injection protocol, part of the contrast agent from the first injection remains in the blood pool and myocardium during the second injection and imaging. Further investigations are necessary to optimize the doses of the two injections and imaging parameters for maximal CNR improvements.
Substantially sharper boundaries and smaller diameters of coronary arteries were found on postcontrast images, which can be attributed to the suppressed signal intensity from the vessel wall, or the "gray zone" (27), on postcontrast images. Similar observations were made in the study by Botnar et al (4) in which T2 preparation was used to improve the CNR of coronary artery images. Since the images do not have isotropic resolution in the three directions, the apparent vessel diameter and sharpness in reformatted images might vary depending on the view angle. Therefore, the vessel diameter and sharpness were measured in original images.
A fixed delay of 25 seconds between the start of contrast agent injection and that of data acquisition resulted in adequate blood signal intensity enhancement in all but one subject in whom data acquisition was too early. A test bolus or automated bolus arrival detection scheme (28,29) would further improve the timing of postcontrast imaging.
One problem with the current imaging protocol is that long segments of the left circumflex artery were not consistently depicted. This was because the left coronary acquisition was targeted along the LAD. The protocol can be further modified to allow for three first-pass acquisitions with injection of either a larger total volume of contrast agent for each subject or less contrast agent for each acquisition while maintaining the total volume at 40 mL. This will become less of a problem when newly developed blood pool agents become clinically available because they allow an extended window of imaging (25,26,30–32).
Another problem with the current study is the relatively low spatial resolution. Further improvement in resolution with 3D breath-hold imaging is possible by using more efficient data acquisition and image reconstruction techniques such as segmented echo-planar imaging (33,34), spiral imaging (35), volume selective imaging (36), parallel imaging (37), and partial Fourier imaging (38).
With further clinical validation, MR angiography of coronary arteries has the potential to become an important part of the "one-stop shopping" evaluation of ischemic heart disease. A technically more challenging task is the noninvasive depiction and characterization of coronary plaque with the use of MR imaging, which could be extremely useful for risk assessment of coronary artery disease. Promising results have been obtained by using a black-blood, fast spin-echo technique to assess the wall thickness of coronary arteries (39). Further technical improvements in resolution, coverage, SNR, and CNR are necessary for characterizing the composition of the coronary plaque.
In conclusion, a magnetization-prepared contrast-enhanced 3D volume-targeted imaging approach was optimized to cover the left coronary artery and RCA in two breath holds. Results from volunteer studies demonstrated markedly improved coronary artery-myocardium CNR, lengths of depiction, and definition of coronary arteries with the injection of a conventional extracellular contrast agent. Comparison of MR angiographic results with conventional angiographic results in patients with coronary artery disease are required to evaluate the clinical utility of the technique.
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FOOTNOTES |
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Author contributions: Guarantor of integrity of entire study, D.L.; study concepts, D.L., J.Z., J.C.C., S.M.S.; study design, all authors; literature research, D.L., J.C.C., S.M.S., J.Z., V.S.D.; clinical studies, D.L., J.C.C., S.M.S., V.S.D., J.P.F.; experimental studies, all authors; data acquisition, D.L., J.C.C., S.M.S., J.Z., V.S.D., P.A.W.; data analysis/interpretation, all authors; statistical analysis, D.L., S.M.S., J.Z., V.S.D.; manuscript preparation, definition of intellectual content, and editing, all authors; manuscript revision/review, D.L., J.C.C., S.M.S., J.Z.; manuscript final version approval, all authors.
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