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Published online before print March 24, 2004, 10.1148/radiol.2312030451
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(Radiology 2004;231:581-586.)
© RSNA, 2004

Technical Developments

Free-breathing 3D Steady-State Free Precession Coronary MR Angiography with Radial k-Space Sampling: Comparison with Cartesian k-Space Sampling and Cartesian Gradient-Echo Coronary MR Angiography—Pilot Study1

Elmar Spuentrup, MD, Marcus Katoh, MD, Arno Buecker, MD, Warren J. Manning, MD, Tobias Schaeffter, PhD, Trung-Hieu Nguyen, Harald P. Kühl, MD, Matthias Stuber, PhD, Rene M. Botnar, PhD and Rolf W. Günther, MD

1 From the Department of Diagnostic Radiology (E.S., M.K., A.B., T.H.N., R.W.G.) and Medical Clinic I (H.P.K.), University Hospital, Aachen University of Technology, Pauwelsstrasse 30, 52057 Aachen, Germany; Departments of Medicine (Cardiovascular Division) (E.S., M.S., R.M.B., W.J.M.) and Radiology (W.J.M.), Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Mass; Philips Research Laboratories, Hamburg, Germany (T.S.); and Philips Medical Systems, Best, the Netherlands (M.S., R.M.B.). Received March 23, 2003; revision requested May 28; final revision received August 12; accepted September 8. Address correspondence to E.S. (e-mail: spuenti@rad.rwth-aachen.de).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and Methods
Results
Discussion
 REFERENCES
 
The authors compared radial steady-state free precession (SSFP) coronary magnetic resonance (MR) angiography, cartesian k-space sampling SSFP coronary MR angiography, and gradient-echo coronary MR angiography in 16 healthy adults and four pilot study patients. Standard gradient-echo MR imaging with a T2 preparatory pulse and cartesian k-space sampling was the reference technique. Image quality was compared by using subjective motion artifact level and objective contrast-to-noise ratio and vessel sharpness. Radial SSFP, compared with cartesian SSFP and gradient-echo MR angiography, resulted in reduced motion artifacts and superior vessel sharpness. Cartesian SSFP resulted in increased motion artifacts (P < .05). Contrast-to-noise ratio with radial SSFP was lower than that with cartesian SSFP and similar to that with the reference technique. Radial SSFP coronary MR angiography appears preferable because of improved definition of vessel borders.

© RSNA, 2004

Index terms: Coronary angiography, comparative studies, 54.121412, 54.121415, 54.121416, 54.12142, 54.12149 • Coronary vessels, MR, 54.12142 • Magnetic resonance (MR), k-space • Magnetic resonance (MR), motion correction • Magnetic resonance (MR), three-dimensional


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and Methods
Results
Discussion
 REFERENCES
 
Over the past decade, T2-weighted navigator-gated fat-suppressed free-breathing gradient-echo coronary magnetic resonance (MR) angiography has been successfully implemented for characterizing coronary artery integrity (1). Navigator-gated free-breathing steady-state free precession (SSFP) also has been proved an effective technique for bright-blood coronary MR angiography (2–4). Advantages of SSFP sequences include enhanced signal intensity and intrinsically high contrast between the blood pool and the surrounding tissue (2,3,5–8). Currently, coronary MR angiography sequences are typically performed with cartesian k-space sampling. However, other k-space data sampling strategies, including radial sampling, may be advantageous because they provide reduced sensitivity to motion artifacts (9). Thus, the purpose of our study was to compare radial SSFP coronary MR angiography (10), cartesian k-space sampling coronary MR angiography (11), and gradient-echo coronary MR angiography (1) in healthy adults and in pilot study patients.


   Materials and Methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
Discussion
 REFERENCES
 
Subjects
Imaging was performed in 16 healthy adults who were consecutively scheduled for cardiac MR examination. All 16 (nine men and seven women aged 22–34 years; mean age ± SD, 25.7 years ± 2.7) had undergone physical examination and had no history of cardiovascular disease. The number of subjects was chosen according to previous coronary MR angiography comparison studies (12). Subsequently, four patients (mean age, 59 years ± 21; age range, 23–73 years) also were investigated, including three men (mean age, 70 years ± 2.5) and one woman aged 23 years. Two patients had patent bypass grafts; one patient had an anomalous coronary artery origin, with the right coronary artery arising from the left sinus and passing between the aortic root and pulmonary artery; and one patient had radiographic findings indicative of coronary artery disease. The study was approved by the institutional review board, and written informed consent was obtained from all participants.

MR Imaging and Interpretation
All examinations were performed by using a 1.5-T whole-body MR imaging system (Gyroscan Intera; Philips Medical Systems, Best, the Netherlands) with a maximum gradient strength of 23 mT/m (219-µsec rise time) and a five-element cardiac coil (Synergy; Philips Medical Systems, Best, the Netherlands) for signal reception. Subjects were imaged in the supine position, with anteriorly placed electrocardiographic leads for cardiac triggering.

Scout imaging.—An electrocardiographically triggered fast field-echo–echo-planar cine MR imaging sequence (one signal acquired per R-R interval, echo time [TE] of 9.7 msec, seven echo-planar imaging readouts, 40 heart phases) was applied in the transverse plane at the level of the proximal-to-medial right coronary artery to visually determine the optimal diastolic rest period (13). Subsequently, a previously described navigator-gated three-dimensional (3D) fast SSFP sequence was used to determine the optimal oblique planes for coronary MR angiographic imaging (14).

Gradient-echo coronary MR angiography.—A previously described electrocardiographically triggered segmented-k-space 3D gradient-echo sequence (1,15,16) was used as the reference standard. A T2 preparatory pulse (TE, 50 msec) and fat saturation preparatory pulse were used to enhance the signal-intensity contrast between tissues (15). A repetition time (TR) of 6.5 msec and TE of 2.2 msec were used, with 15 signals acquired per R-R interval, resulting in a 99-msec data acquisition window. Data were acquired by using centric k-space ordering, with priority given to the central k-space profiles. The acquired 3D volume included 12 sections, each with a thickness of 3 mm, interpolated by means of zero filling to produce a volume of 24 contiguous 1.5-mm sections. With a field of view of 360 mm2 and a matrix of 384 x 384 pixels, the in-plane resolution was 0.9 x 0.9 mm2; with reconstruction to a 512 x 512 matrix, it was 0.7 x 0.7 mm2. Acquisition time was 5 minutes 1 second with a heart rate of 80 beats per minute.

SSFP coronary MR angiography.—Both of the 3D SSFP sequences included a T2 preparatory pulse (TE, 50 msec) and fat suppression preparatory pulse for improved signal intensity contrast (3,15). To obtain steady-state conditions, four dummy radiofrequency pulses were applied prior to the imaging portion of the sequence (11,17) (Fig 1a). An {alpha}/2 approach (2,18) was used to bring the magnetization more rapidly to steady-state conditions, thereby minimizing the time delay between the navigator pulse and the imaging portion of the sequence (Fig 1a). (Minimization of this time delay has been shown to be important for respiratory motion artifact suppression, because motion during the time delay results in blurring [19].) Sequence parameters were as follows: TR msec/TE msec, 6.1/3.0; flip angle, 120°; field of view, 360 mm2. A 384 x 384 matrix with cartesian k-space sampling, or 384 radials with radial k-space sampling, yielded an in-plane resolution of approximately 0.9 x 0.9 mm2 (with the reconstructed 512 x 512 matrix, 0.7 x 0.7 mm2). In radial sampling, a "stack-of-stars" approach was used for data acquisition in the x-y direction, and Fourier encoding was used for data acquisition in the z direction (Fig 1b). In both cartesian and radial sampling, the data were acquired by using centric k-space ordering or Fourier encoding. In cartesian k-space sampling, the centric encoding direction was along the y axis, whereas in radial sampling the centric encoding direction was along the z axis. With the stack-of-stars approach, the latter was essential to provide improved contrast due to the T2-preparatory and fat-suppression preparatory pulses. The number of dummy pulses, the flip angle, and the k-space ordering scheme had been optimized with testing in five other subjects prior to this study. Twelve 3-mm sections were reconstructed to 24 contiguous 1.5-mm sections. Sixteen radiofrequency signals acquired per R-R interval resulted in a data acquisition window of 96 msec. Imaging time was 4 minutes 49 seconds with a heart rate of 80 beats per minute.



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  		Figure 1a. (a) Diagram of the T2-weighted navigator-gated free-breathing SSFP coronary MR angiography pulse sequence used with radial or cartesian k-space sampling. The real-time navigator pulse receded the four preparatory dummy pulses applied in the {alpha}/2 approach to achieve steady-state conditions. A T2 preparatory pulse and a fat-saturation preparatory pulse with spectral inversion (SPIR) were used for contrast enhancement. The imaging portion of the sequence was triggered by the onset of the R wave on the electrocardiographic tracing. (b) Diagram of k-space sampling with the stack-of-stars approach in radial SSFP coronary MR angiography. Radial sampling was applied in two k-space directions (kx, ky), whereas in the third direction (kz) a Fourier encoding scheme was applied.

 


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  		Figure 1b. (a) Diagram of the T2-weighted navigator-gated free-breathing SSFP coronary MR angiography pulse sequence used with radial or cartesian k-space sampling. The real-time navigator pulse receded the four preparatory dummy pulses applied in the {alpha}/2 approach to achieve steady-state conditions. A T2 preparatory pulse and a fat-saturation preparatory pulse with spectral inversion (SPIR) were used for contrast enhancement. The imaging portion of the sequence was triggered by the onset of the R wave on the electrocardiographic tracing. (b) Diagram of k-space sampling with the stack-of-stars approach in radial SSFP coronary MR angiography. Radial sampling was applied in two k-space directions (kx, ky), whereas in the third direction (kz) a Fourier encoding scheme was applied.

 
Two-dimensional selective navigator pulse.—All sequences included a previously described prospective real-time right hemidiaphragmatic navigator pulse, which preceded the imaging portion of the sequence, for suppression of respiratory motion artifacts (12). In addition to navigator gating, either real-time motion correction in all three spatial dimensions or tracking with a factor of 0.6 in the caudocephalic direction (20) was used to achieve improved respiratory motion artifact suppression (12,21).

Examinations.—On the basis of the anatomic display of the coronary arteries on the transverse 3D SSFP scout image, coronary MR angiography was performed along the major axis of the right coronary artery in eight healthy adults (three men and five women; mean age, 24 years ± 2) and two patients (both men) and along the left anterior descending artery in eight healthy adults (four men and four women; mean age, 24 years ± 3) and two patients (one man and one woman) by using a three-point plan-scan tool (16). Identical section geometry was used for all three investigated MR imaging sequences. Prior to image interpretation, the order of the images was numerically randomized with regard to the coronary artery system measured and the MR imaging sequence used.

Data analysis.—For systematic qualitative image analysis, coronary MR angiographic image data from the healthy subjects were used (22). Data were transferred from the MR imaging unit to a commercially available workstation (Easy Vision 4.0; Philips Medical Systems) for multiplanar 3D reformatting (12,15,16). The longest continuously visible vessel length was semiquantitatively measured by using a previously described interactive software tool (15,16). Image quality was determined by consensus of two observers (A.B. and H.P.K., each with more than 5 years of experience in cardiac and coronary MR angiography) who were blinded to sequence parameters. Subjective image quality was graded on a scale from 1 to 3 for depiction of the right or left coronary artery and for the presence and severity of motion artifacts (1 = good image quality without any motion artifacts and with sharply defined vessel borders; 2 = minor motion artifacts, including ghosting and blurring of the vessel borders; 3 = major motion artifacts, with severe blurring of the vessel borders). Objective quantification of vessel sharpness and contrast-to-noise ratio (CNR) was performed by one of the authors (T.H.N.) using a previously described edge detection tool (15).

Statistical analysis.—All values were calculated as mean ± 1 SD. Quantitative signal intensity measurements (CNR and vessel sharpness) and image quality scores were compared by using a paired bidirectional Student t test. P < .05 was considered to indicate statistical significance.


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
Discussion
 REFERENCES
 
Healthy Subjects
All three coronary MR angiography sequences were applied in all subjects. Multiplanar reformatted images of the right or left coronary artery were obtained in all subjects, and subjective and objective image quality parameters, including image quality score, vessel sharpness, vessel length, and CNR, were assessed. Quantitative measurements are summarized in the Table. Mean navigator efficiency (12) was 52.8% ± 10.1% and did not vary significantly between the three investigated sequences.


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  		Quantitative Measurements for the Three Investigated Coronary MR Angiography Sequences

 
Representative coronary MR angiographic images obtained in planes parallel to the main axes of the left anterior descending artery and the right coronary artery are displayed in Figures 2 and 3. Images from all three coronary MR angiography sequences depicted the coronary artery lumen with high signal intensity, while the signal intensities of the surrounding fat and myocardium were suppressed. The CNR with SSFP imaging was superior to that with conventional T2-weighted gradient-echo imaging, and, as a result, a longer vessel segment was depicted (Fig 2, Table). Compared with radial SSFP, cartesian SSFP produced more-pronounced motion artifacts, especially in the phase encoding direction. The increased phase errors on cartesian SSFP images resulted in a lower image quality score with regard to motion artifact suppression (2.75 ± 0.75), compared with that of T2-weighted gradient-echo images (1.94 ± 0.43, P < .05) and radial SSFP images (1.13 ± 0.33, P < .05). The best motion artifact suppression and, consequently, the highest mean image quality score were found for radial SSFP. Objective vessel sharpness analysis yielded a superior score for radial SSFP in the definition of vessel borders, whereas the lowest definition of vessel borders was observed for T2-weighted gradient-echo imaging (Table). However, the CNR found for radial SSFP was lower than that for cartesian SSFP and similar to that for the T2-weighted gradient-echo sequence (Table).



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  		Figure 2. A, double-oblique coronal multiplanar reformatted images of the left anterior descending coronary artery (LAD), and, B, double-oblique sagittal multiplanar reformatted images of the right coronary artery (RCA), obtained in two healthy adults (top row, bottom row) with navigator-gated free-breathing 3D coronary MR angiography performed with standard T2-weighted gradient-echo imaging with cartesian k-space sampling (T2-prep GE), T2-weighted SSFP with cartesian k-space sampling (T2-prep cart SSFP), and T2-weighted SSFP with radial k-space sampling (T2-prep rad SSFP). Images obtained with cartesian SSFP show more-severe motion artifacts (dashed arrows) than do those obtained with radial SSFP. Moderate motion artifacts also are visible on the T2-weighted gradient-echo images (dashed arrows). Images obtained with radial SSFP show better vessel border delineation and less myocardial suppression (star) than do those obtained with cartesian SSFP. Note that longer vessel segments were visualized with the SSFP sequences than with the gradient-echo sequence.

 


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  		Figure 3. Double-oblique sagittal multiplanar reformatted image of the right coronary artery (RCA) (left) and double-oblique coronal multiplanar reformatted image of the left anterior descending coronary artery (LAD) (right) obtained in a healthy adult subject with T2-weighted navigator-gated free-breathing 3D SSFP coronary MR angiography by using radial k-space sampling and higher magnification than in Figure 2. Note the depiction of a diagonal branch of the left anterior descending artery (arrowhead) and the improved definition of vessel borders.

 
Patients
As in our study in healthy volunteers, our pilot study in patients demonstrated increased phase errors with cartesian SSFP coronary MR angiography that impeded clinically important findings such as patent anastomosis between the bypass graft and the native vessel. In contrast, these conditions were better visualized by using radial SSFP (Figs 4, 5).



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  		Figure 4a. Double-oblique sagittal multiplanar reformatted images obtained with T2-weighted navigator-gated free-breathing 3D SSFP coronary MR angiography in a 73-year-old man with coronary vessel disease and multiple coronary artery bypass grafts. (a) Image obtained with cartesian k-space sampling (T2-prep cart SSFP) shows severe motion artifacts (dotted arrows) caused by phase errors that obscure the anastomosis (arrowhead) of a bypass graft (CABG) and the distal part of the right coronary artery (RCA), whereas both the anastomosis and the artery are clearly depicted on the radial SSFP image (T2-prep rad SSFP). (b) Image obtained with radial k-space sampling shows a patent bypass graft (CABG) to a branch of the left circumflex artery (arrowhead).

 


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  		Figure 4b. Double-oblique sagittal multiplanar reformatted images obtained with T2-weighted navigator-gated free-breathing 3D SSFP coronary MR angiography in a 73-year-old man with coronary vessel disease and multiple coronary artery bypass grafts. (a) Image obtained with cartesian k-space sampling (T2-prep cart SSFP) shows severe motion artifacts (dotted arrows) caused by phase errors that obscure the anastomosis (arrowhead) of a bypass graft (CABG) and the distal part of the right coronary artery (RCA), whereas both the anastomosis and the artery are clearly depicted on the radial SSFP image (T2-prep rad SSFP). (b) Image obtained with radial k-space sampling shows a patent bypass graft (CABG) to a branch of the left circumflex artery (arrowhead).

 


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  		Figure 5. Double-oblique sagittal multiplanar reformatted images (right view more proximal than left) obtained with T2-weighted navigator-gated free-breathing 3D radial SSFP coronary MR angiography in a 67-year-old man with diffuse coronary artery disease show native right coronary artery (RCA), anastomosis (arrowhead) to a coronary artery bypass graft (CABG), and fully MR-compatible aortic valve prosthesis (*).

 

   Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
Discussion
REFERENCES
 
In this study comparing cartesian and radial k-space coronary MR angiography with conventional segmented-k-space coronary MR angiography, we found subjective and objective evidence of improved coronary vessel border definition and reduced motion artifacts with radial SSFP coronary MR angiography. This evidence was seen both among healthy adults and among patients in a pilot study.

Recent advances in MR imaging hardware and software have facilitated high-quality coronary MR angiography with the use of SSFP (5,7,23,24). With SSFP, a T2-like contrast is observed, with high signal of the blood pool (2,3,5,6). Therefore, SSFP sequences applied at navigator-gated free-breathing 3D coronary MR angiography have achieved a high signal-to-noise ratio and strong contrast between the signal intensity of the coronary blood pool and that of the surrounding tissue (4). The rationale for radial acquisition is its potentially reduced susceptibility to motion artifacts (9). Motion artifacts from cardiac or respiratory displacement may cause reduced vessel border definition, and they remain a major impediment to the clinical use of coronary MR angiography. Furthermore, phase errors in the phase encoding direction may substantially reduce image quality (25) and, consequently, coronary artery visualization. With radial data sampling, the reprojection strategy used minimizes these artifacts (9), which explains the superior vessel sharpness and reduced motion artifacts on radial SSFP images compared with cartesian SSFP images.

Compared with other studies, in which lower-spatial-resolution cartesian SSFP coronary MR angiography was used (11,26), in our study the cartesian SSFP sequence resulted in more phase errors in the phase encoding direction. This can be explained by the longer TR and TE associated with higher-spatial-resolution imaging (7,23). In this study, we used 6.1/3.0, compared with 3.9/1.8 in previous studies (11). However, these motion artifacts were not observed in our study with radial data acquisition, which allows for improved spatial resolution in SSFP imaging with a longer TR and TE, as well as for an overall reduction in motion artifacts. One reason for the reduced motion sensitivity with the radial SSFP sequence we used was the choice of an appropriate encoding scheme—the "stack-of-stars" approach. Fourier encoding (in the z-axis direction) was performed during a single R-R interval, whereas radial k-space sampling (in the x-y direction) was performed between different R-R intervals. Incomplete motion correction with respect to respiratory motion or arrhythmia mainly affects k-space sampling between R-R intervals. This may explain the reduction of motion artifacts with our radial SSFP sequence. An alternative technique for achieving reduced motion artifacts may be the acquisition of fractional echoes, with consequent reduction in TR and TE (26). However, fractional echoes result in a reduced signal-to-noise ratio, and the asymmetry of the gradients in the readout direction may reduce the CNR.

While our results demonstrate superior motion artifact suppression and enhanced vessel border definition achieved with radial SSFP coronary MR angiography, the CNR was reduced, compared with that at cartesian SSFP coronary MR angiography. Thus, the improvement in signal-to-noise ratio with the use of SSFP imaging versus gradient-echo coronary MR angiography was not seen at radial SSFP coronary MR angiography. Theoretically, radial k-space sampling produces a reduced signal-to-noise ratio, compared with the ratio in analogous cartesian acquisitions, because of the inhomogeneous sampling density (27). In addition, undersampling, backfolding (ie, magnetization outside the field of view), or motion during radial k-space sampling may result in small streaking artifacts that mimic noise. In Figure 2, ghostlike artifacts in the phase encoding direction are visible on images acquired with cartesian k-space sampling. These artifacts were not seen on images acquired with radial k-space sampling, because motion artifacts on such images mimic increased noise.

In our radial SSFP approach, all k-space lines in the x-y orientation were equally weighted. Other researchers have proposed radial encoding during the R-R interval (28), which may result in problems with fat suppression. We chose to prioritize the central k-space lines in the z-axis direction for each diastolic data acquisition window so as to preserve the contrast provided by the T2 magnetization and fat saturation preparatory pulses. Nonetheless, we also found reduced CNR between the coronary artery lumen and the myocardium for radial SSFP coronary MR angiography compared with cartesian SSFP coronary MR angiography. Although the CNR found for radial SSFP was similar to that for gradient-echo imaging, longer vessel segments were depicted with radial SSFP. This may be a result of the motion artifact reduction achieved by using radial k-space sampling.

Our results demonstrate the potential of radial SSFP coronary MR angiography for improving visualization of the native coronary artery tree. Furthermore, the combination of this technique with prospective navigator gating and real-time motion correction during free breathing enables a longer data acquisition time than is possible with breath holding (28) and may therefore be advantageous for further improvement in spatial resolution.

Alternative k-space sampling strategies for improving coronary vessel definition include spiral gradient-echo imaging (29). A direct comparison of radial SSFP and spiral SSFP coronary MR angiography has yet to be performed.

In the present study, two SSFP sequences and one gradient-echo sequence were compared. The comparability of these three sequences may be limited by the fact that although parameters such as TR, TE, and flip angle could be kept constant in imaging with both SSFP techniques, signal intensity and contrast in gradient-echo imaging are not based on steady-state conditions, and, hence, these parameters had to be adjusted.

In addition, our investigation was performed in healthy adult subjects and in patients in a small pilot study. Patients often demonstrate more-variable breathing patterns than do healthy subjects. Our results warrant further studies in a larger patient population to define the potential of radial SSFP coronary MR angiography for detection of coronary artery stenosis in a clinical setting.

In conclusion, T2-weighted navigator-gated real-time motion-corrected free-breathing SSFP coronary MR angiography with radial k-space sampling is an approach for achieving improved coronary vessel border definition. Further evaluation is necessary in a larger population of patients with well-defined disease.


   FOOTNOTES
 
See also the editorial by Riederer in this issue.

Abbreviations: CNR = contrast-to-noise ratio, SSFP = steady-state free precession, TE = echo time, TR = repetition time, 3D = three-dimensional

Author contributions: Guarantors of integrity of entire study, E.S., A.B., R.W.G.; study concepts, E.S., W.J.M., M.S., R.M.B.; study design, E.S., T.S., M.S.; literature research, E.S., T.S.; clinical studies, E.S., M.K., T.H.N.; data acquisition, E.S., M.K., T.H.N.; data analysis/interpretation, A.B., H.P.K., T.H.N., T.S., E.S.; statistical analysis, E.S., R.M.B.; manuscript preparation, E.S.; manuscript definition of intellectual content, E.S., A.B., W.J.M., R.W.G.; manuscript editing, A.B., R.M.B., W.J.M.; manuscript revision/review, A.B., W.J.M., R.W.G.; manuscript final version approval, E.S., W.J.M.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
Discussion
 REFERENCES
 

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