Improved Coronary Artery Definition With T2-Weighted, Free-Breathing, Three-Dimensional Coronary MRA
Background—Three-dimensional (3D) navigator-gated and prospectively corrected free-breathing coronary magnetic resonance angiography (MRA) allows for submillimeter image resolution but suffers from poor contrast between coronary blood and myocardium. Data collected over >100 ms/heart beat are also susceptible to bulk cardiac and respiratory motion. To address these problems, we examined the effect of a T2 preparation prepulse (T2prep) for myocardial suppression and a shortened acquisition window on coronary definition.
Methods and Results—Eight healthy adult subjects and 5 patients with confirmed coronary artery disease (CAD) underwent free-breathing 3D MRA with and without T2prep and with 120- and 60-ms data-acquisition windows. The T2prep resulted in a 123% (P<0.001) increase in contrast-to-noise ratio (CNR). Coronary edge definition was improved by 33% (P<0.001). Acquisition window shortening from 120 to 60 ms resulted in better vessel definition (11%; P<0.001). Among patients with CAD, there was a good correspondence with disease.
Conclusions—Free-breathing, T2prep, 3D coronary MRA with a shorter acquisition window resulted in improved CNR and better coronary artery definition, allowing the assessment of coronary disease. This approach offers the potential for free-breathing, noninvasive assessment of the major coronary arteries.
Coronary MR angiography (MRA) is able to depict the major coronary arteries.1 2 3 4 5 6 Several impediments, however, limit its clinical utility. These obstacles include compensation for cardiac and respiratory motion, need for submillimeter spatial resolution, and suppression of signal from adjacent epicardial fat and myocardium.
Initial approaches used 2-dimensional (2D) gradient-echo strategies1 2 3 5 7 8 to take advantage of unsaturated blood inflow and breath holding to minimize respiratory motion. In contrast to breath holding, respiratory bellows9 or MR navigator echoes that assess diaphragmatic or cardiac position can be used for respiratory gating with or without slice correction to allow for free-breathing coronary MRA.9 10 11 12 13 14 15 16 By eliminating the time constraint of the breath hold, navigators allow for submillimeter spatial resolution and minimize registration errors. Three-dimensional (3D) MRA techniques offer favorable signal-to-noise ratios (SNR) and are also well suited for navigator approaches.13 17 A disadvantage of 3D versus 2D MRA is lower contrast between coronary blood and myocardium. To overcome this problem, contrast agents18 or prepulses4 may be useful.
We chose to examine the impact of a T2 preparation prepulse (T2prep), which has been demonstrated to enhance contrast in 2D MRA applications.7 Because image quality may also be affected by respiratory motion during the data-acquisition window, we also varied the acquisition window between 60 and 120 ms. To avoid bias in image assessment, we developed an objective tool to determine vessel border sharpness. We hypothesized that the combination of 3D MRA with T2prep and a shorter acquisition window would allow for both improved contrast-to-noise ratio (CNR) and coronary edge definition.
Eight healthy adults (mean age, 24 years; range, 18 to 37; 5 men) without a history of cardiovascular disease and 5 patients (mean age, 58 years; range, 42 to 71; 4 men) with radiographically confirmed coronary disease (CAD) were studied. No breath holds or respiratory coaching were performed. Written informed consent was obtained from all participants, and the protocol was approved by the hospital’s Committee on Clinical Investigations.
All subjects were examined supine with a commercial, 1.5-T Gyroscan ACS-NT (Philips Medical Systems) scanner with PowerTrak 3000 gradients (15 mT/m, 50 mT · m−1 · ms−1 slew rate), a cardiac software patch, and a 5-element phased-array research cardiac coil.
Two scout scans were performed to localize the left main (LM) coronary artery and to position the cardiac navigator. An ECG-triggered multislice, multishot 2D turbo field echo scan with thoracic transverse, sagittal, and coronal images was used to identify the heart, followed by a coronal scout with left ventricular (LV) navigator gating and prospective slice correction.14 The coronal scout images were used to localize the LM and position the LV navigator (Figure 1⇓).
3D Coronary MRA With T2prep
The coronal scout position of the LM (Figure 1⇑) was used as the “center slice” position of the 3D MRA scan. The MR pulse sequence consisted of 4 blocks (Figure 2⇓) repeated with every heart cycle, with a temporal relationship chosen to minimize the time interval between the navigator and the imaging sequence.
Flow-independent coronary MRA with T2prep was first described by Brittain et al.7 It uses a 90° radiofrequency (RF) pulse to flip the Mz magnetization vector into the Mxy plane and is most beneficial in sequences with low inflow contrast. T2prep allows suppression of tissues with short T2 relaxation times, eg, cardiac muscle (T2=50 ms),19 cardiac veins with deoxygenated blood (20% O2 saturation, T2=35 ms),20 and epicardial fat. Tissues with long T2 relaxation times, eg, arterial blood (T2=250 ms), are minimally influenced. The CNR between arterial blood and myocardium can be adjusted by varying the time between the 90° and the tip-up pulse.
Flow sensitivity of T2prep can be minimized by nonselective RF pulses and use of an even number of 180° refocusing pulses. We applied 4 refocusing pulses7 and used an MLEV (Malcom-Levitt)–weighted T2prep as well as composite 180° RF pulses (90x180y90x) to correct for pulse imperfections due to B0 and B1 field inhomogeneities.
A Bloch equation21 simulation, taking into account arterial blood and myocardium T1 and T2 (at 1.5 T) of 1200 and 250 ms and 850 and 50 ms,19 22 respectively, was used. This demonstrated maximal CNR with 90-ms T90–90 spacing. However, SNR drops as T90–90 increases (Figure 3⇓). On the basis of in vivo experiments, the threshold for SNR of arterial blood was set to 8, corresponding to a T90–90 of 50 ms, a reasonable compromise between SNR and CNR (Figure 3⇓).
Navigator-Guided Prospective Slice Correction
A cranial-caudal–oriented 2D selective navigator through the basal LV was used to compensate for respiratory motion.16 23 Navigator gating with prospective slice correction was used with a 3-mm gating window and slice correction in the craniocaudal direction (principal respiratory motion). For motion correction in slice-selection direction, the measured displacement was translated in a frequency offset of the RF excitation pulse of the imaging sequence. No scaling of the measured displacement was performed. Scan efficiency was defined as accepted shots/total heart beats.
A frequency-selective fat-suppression pulse to suppress signal from epicardial fat preceded the imaging acquisition. The field-of-view was 360×305 mm with an image matrix of 512×304 (in-plane resolution, 0.7×1.0 mm). The 3D slab had 10 slices with a 3 mm slice thickness, interpolated to 20 slices with a 1.5 mm thickness. Each 3D volume slice was acquired in 38 shots by use of a centric k-space ordering scheme with a 7.4-ms repetition time and 2.5-ms time echo. To examine the impact of the data-acquisition window, imaging was performed with 120-ms (16 echoes/shot) and 60-ms (8 echoes/shot) acquisition windows. All data were acquired at mid diastole.
Three coronary scans were performed in all healthy subjects. To investigate the impact of the T2prep on image quality, scans with and without T2prep were performed in random order. In all other aspects, imaging parameters were maintained, including an acquisition window of 60 ms. The third scan used T2prep with a 120-ms acquisition window.
SNR and CNR Evaluation
SNR of coronary blood was determined from a region-of-interest in the ascending aorta at the level of the LM origin. Mean blood signal (Sblood) and SD (Nblood) were calculated, with SNR defined as Sblood/Nblood. Contrast between coronary blood and cardiac muscle was calculated as
Mean muscle signal (Smuscle) was determined at the anterobasal LV. CNR was also calculated between proximal left anterior descending coronary artery (LAD; arterial) and great cardiac vein (venous) blood.
Curved-Plane Vessel Reconstruction
All data were analyzed on an EasyVision workstation (Philips Medical Systems). The user navigates through the 3D data set and interactively marks the vessel of interest. The course of the LAD and left circumflex artery (LCx) are then reconstructed (Figure 4⇓) and displayed in a single image plane, from which the contiguous length of the LAD and LCx was then determined.
Vessel Edge Detection
Vessel tracing and edge detection were performed on gray-scale images of the raw 3D data sets with the LAD and LCx slices closest to the LM bifurcation. Edge detection included a centerline and edge-detection procedure (Figure 5⇓; Appendix). Vessel sharpness was defined as the average edge value along the calculated vessel border. Higher edge values correspond to better vessel delineation.
All patients with radiographically confirmed CAD underwent T2prep 3D MRA with the short (60 ms) acquisition window. The 3D MRA data sets were reformatted along the coronary vessel path and compared with the corresponding x-ray angiogram.
All data are expressed as mean±SD. Continuous variables were compared with a 2-tailed paired Student’s t test, with significance as P≤0.05.
Scanning was completed without incident in all subjects. Total imaging time for each acquisition varied from 10 to 20 minutes depending on heart rate and navigator efficiency.
T2prep: Healthy Subjects
In Figure 6⇓, the CNR impact of T2prep is demonstrated between the ventricular cavity blood and the myocardium. In the absence of T2prep (Figure 6A⇓), there is little contrast between the myocardium and blood pool. With T2prep (Figure 6B⇓), there is suppression of myocardium, with improved LAD visualization.
In Figure 7⇓, a similar comparison is shown for the LAD and the LCx. In addition to myocardial suppression, T2prep also suppressed signal from the great cardiac vein (Figure 7B⇓), and fewer respiratory artifacts were observed (Figure 6B⇑ and Figure 7B⇓ and D⇓). Overall, T2prep improved CNR 123% (P<0.001) between coronary blood and myocardium (Table 1⇓). There was a trend toward improved CNR between arterial and venous blood and a trend toward reduced SNR (Table 1⇓). Application of the T2prep did not adversely affect navigator performance (Table 1⇓).
The use of T2prep allowed for a 33% improvement in LAD and LCx vessel definition (P<0.001; Table 2⇓). The objectively measured proximal vessel diameter was not T2prep dependent. The high image quality of all coronary scans allowed visualization of the entire LM and the proximal and mid LAD (Figures 4⇑ and 8⇓, top) in all subjects. The visible length of the LCx (Figure 8⇓, bottom) was longer (121%; P<0.003) with T2prep (Table 2⇓), and there was a trend toward longer visualization of the LAD (Table 2⇓).
Reduction of the acquisition window resulted in a smaller improvement (11%; P<0.001) in LAD and LCx definition (Table 2⇑). Most notably, the LCx was longer (48%; P<0.05) with the shorter (60 ms) acquisition window (Table 2⇑). There was also a trend (P<0.07) toward a longer visible length of the LAD (Table 2⇑) and a better CNR between arterial and venous blood (Table 1⇑). There was no significant difference in CNR, SNR, or measured vessel diameter (Tables 1⇑ and 2⇑).
Good agreement between x-ray angiography and coronary 3D MRA regarding vessel anatomy can be observed in a representative sample shown in Figure 9⇓. This case shows the potential of coronary MRA to assess the more distal parts of the coronary system. Muscle and fat suppression were sufficient for good delineation of the mid LAD from myocardium and epicardial fat. An example from a patient with diffuse coronary disease is shown in Figure 10⇓.
A noninvasive method to evaluate the native coronary arteries would be a major clinical advance. In this objective study comparing free-breathing 3D coronary MRA with and without T2prep, we demonstrated that T2prep improved CNR and definition of the LAD and LCx but made no significant difference in visible LAD length. The latter was primarily influenced by imaging volume. In contrast, visualization of the LCx appeared to be related to its anatomic location. The improved visualization of the LCx with T2prep is likely due to improved suppression of myocardium and veins. As previously discussed by Brittain et al,7 T2prep should suppress deoxygenated venous blood (Figure 7B⇑) owing to its shorter T2. We observed a trend toward an improved CNR between arterial and venous blood. This may be particularly important, because the paths of the great cardiac vein and LCx are quite close. This suppression might also be helpful for individuals in whom the vein crosses the LAD (Figure 7A⇑ and 7B⇑). Coronary diameters were T2prep independent. T2prep also reduced respiratory motion artifacts, likely owing to suppression of chest wall muscle signal.
Reduction of the acquisition window from 120 to 60 ms led to an improvement in LAD delineation. However, compared with T2prep, the absolute magnitude was small and came at the expense of a doubled scan time. This time might be better used to increase the volume coverage or use of averaging.
T2prep in combination with free-breathing coronary MRA resulted in quantitatively better definition of the LAD and LCx. Compared with previous studies,3 4 5 our findings are based on objective quantitative analyses of vessel-wall sharpness. Application of automated vessel-edge algorithms with a standardized vessel sharpness score might allow for more objective comparisons of different imaging strategies.
T2prep 3D coronary MRA showed good agreement with x-ray angiography for assessment of vessel anatomy as well as CAD. Application of T2prep suppressed the signal from the myocardium and therefore allowed for improved blood-to-myocardium contrast. Both diffuse and focal coronary disease could be visualized. The accuracy of the methodology remains to be examined in large, multicenter studies.
With the introduction of intravascular contrast agents,24 25 the question arises whether T2prep would offer a synergistic effect. According to our preliminary experiences, such intravascular contrast agents reduce the differ-ential in T2 between blood and myocardium, thus decreasing the beneficial effect of the T2prep.
Navigator correction was used to compensate for respiratory motion in the foot-to-head direction. The navigator was applied in the vicinity of the LM with a constant correction factor. This study was not intended to examine the impact of the correction factor. It is likely that the correction factor for the more distal coronary segments would be closer to 1.67.26
The combined approach of free-breathing navigator-gated and slice-tracked 3D coronary MRA together with a T2prep and a shorter acquisition window resulted in an improved CNR between coronary blood and myocardium and thereby allowed for better definition of the coronary vessels. The technique has been successfully applied to healthy volunteers and a small patient group. Visually assessed vessel anatomy as well as diffuse and focal coronary artery disease correlated well with x-ray angiographic findings. This approach offers the potential for noninvasive assessment of the major coronary arteries.
Vessel tracing and edge detection were performed on gray-scale images of the 3D raw data sets. The user defines the vessel of interest by drawing points p⃗[k] along the vessel path, whereby k is the index of each point. Each of these points is a vector, with a unity direction d⃗[k] and a unity normal n̂[k] vector defined by Equation 2:
The position of these points is then updated with the center of gravity p⃗c[k], representing the vessel centerline. The center of gravity was calculated along the intensity profile I[k, r⃗], defined by the normal vector n̂[k] and a predefined window, w (Equation 3).
The centerline point p⃗c[k] is then regridded with cubic spline interpolation and equidistant spacing between adjacent points P⃗cs[i] and p⃗cs[i+1]. The index of the regridded centerline points p⃗cs is i. The direction d⃗cs[i], the normal n⃗cs[i] vectors, and the intensity profiles Ics[i, r⃗] are updated (Equations 2, and 4). After the centerline of the vessel has been defined, vessel border detection is performed. The vessel border location is roughly estimated with full-width half-maximum (FWHM) criteria (Equation 5).
The search window Δ is increased until the criterion of Equation 5 is fulfilled. The orientation of the vessel border points p⃗FWHM±1[i] with respect to the centerline is determined by the cross product (Equation 6).
The final vessel-border detection is done with the Deriche edge-detection filter,27 a combination of a low-pass filter for noise reduction and a high-pass filter for edge detection. The filter parameter α was empirically set to 2.5. Applied on a step function, the Deriche filter returns the step size of the function at the position of the step (edge). The vessel border is identified as the maximum value of the edge profile in the interval [0, pFWHM−1[i]] and [0, pFWHM+1[i]]. The edge points are denoted p⃗Deriche±1[i].
Vessel delineation (sharpness) is defined as the average edge value along the entire vessel border (Equation 7). N is the number of edge points. Higher values identify better vessel delineation. The mean difference between the vessel border points p⃗Deriche−1[i] and p⃗Deriche+1[i] is defined as the vessel diameter.
Reprint requests to René Botnar, PhD, Beth Israel Deaconess Medical Center, Cardiovascular Division, Cardiac MR, 330 Brookline Ave, Boston, MA 02215.
- Received December 27, 1998.
- Revision received March 24, 1999.
- Accepted April 9, 1999.
- Copyright © 1999 by American Heart Association
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