AJR InPractice
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Stafford, R. B.
Right arrow Articles by Frayne, R.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Stafford, R. B.
Right arrow Articles by Frayne, R.
Social Bookmarking
Add to CiteULike   Add to Complore   Add to Connotea   Add to Del.icio.us   Add to Digg   Add to Reddit   Add to Technorati  
What's this?
Hotlight (NEW!)
Right arrow
What's Hotlight?
DOI:10.2214/AJR.07.3076
AJR 2008; 191:243-246
© American Roentgen Ray Society

Technical Innovation

Unenhanced MR Angiography of the Renal Arteries with Balanced Steady-State Free Precession Dixon Method

Randall B. Stafford1,2, Mohammad Sabati2,3,4, Michael J. Haakstad2, Houman Mahallati2,3 and Richard Frayne1,2,3,4

1 Department of Physics and Astronomy, University of Calgary, Calgary, AB, Canada.
2 Seaman Family MR Research Centre, 1403 29th St. NW, Calgary, AB T2N 2T9, Canada.
3 Department of Radiology, Hotchkiss Brain Institute, University of Calgary, Calgary, AB, Canada.
4 Department of Clinical Neurosciences, Hotchkiss Brain Institute, University of Calgary, Calgary, AB, Canada.

Received August 29, 2007; accepted after revision January 10, 2008.

 
Address correspondence to R. Frayne (rfrayne{at}ucalgary.ca).

Partially funded by the Heart and Stroke Foundation of Canada.

R. Frayne is a Canada Research Chair and an Alberta Heritage Foundation for Medical Research Senior Medical Scholar.


Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
OBJECTIVE. The purpose of this study was to evaluate the feasibility of a novel technique for fat–water separation to image the renal arteries without using a contrast agent.

CONCLUSION. Five healthy volunteers were imaged on a 3-T clinical MR scanner using the balanced steady-state free precession (SSFP) Dixon method. We were able to image the proximal renal arteries with high conspicuity within a 3-minute overall scanning time. The balanced-SSFP Dixon method shows potential for unenhanced MR angiography of the proximal renal arteries.

Keywords: balanced steady-state free precession • Dixon method • MRI • renal arteries • unenhanced MR angiography


Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Atherosclerotic renal artery stenosis typically occurs at, or within a few centimeters of, the ostium [1] and can result in hypertension and contribute to decreased renal function. Recent studies have shown that patients with renal insufficiency, particularly those on hemodialysis, are at increased risk of nephrogenic systemic fibrosis from gadolinium-based contrast agents in MR angiography [2], making an unenhanced approach a potentially important clinical option for this patient population.

The most common methods for unenhanced MR angiography are phase-contrast angiography [3] and time-of-flight (TOF) angiography [4], which both use the differing signal characteristics between nuclear spins in stationary tissue and flowing blood [5]. Phase-contrast angiography encodes motion into the MR signal [3] but requires longer scanning times. TOF angiography continuously saturates the stationary tissue while unsaturated blood enters the image volume [4], although it is susceptible to signal loss in areas of decreased or in-plane (to the image slice) blood flow [6].

Steady-state free precession (SSFP) pulse sequences produce images with a high signal-to-noise ratio (SNR) using a short TR [7]. These sequences have also shown promise for unenhanced MR angiography of the renal [8, 9] and peripheral [10] arteries. SSFP sequences characterized by an alternating flip angle between neighboring excitations (+{alpha}, –{alpha}, +{alpha}, –{alpha}, etc.) and a TE equal to half of the TR are said to be balanced. Modified balanced-SSFP acquisitions can produce uniform fat–water separation without the use of magnetization preparation or fat saturation [10, 11]. The balanced-SSFP Dixon method uses a TR that is an odd half-multiple of the cycle time between fat and water (2.3 milliseconds at 3 T) [10, 11]. Two images are produced in which fat and water are in phase and opposed phase, respectively, by adjusting the center frequency. Water-only images (i.e., fat-suppressed images) are generated by the addition of the in-phase and opposed-phase images. Conversely, fat-only images (i.e., water-suppressed images) are generated by the subtraction of the in-phase and opposed-phase images. The image contrast in the water-only images is based on the T2/T1 ratio, as per balanced-SSFP images [12]. The arteries appear brighter than venous blood and muscle because arterial blood has a higher T2/T1 ratio than either of the other two tissue types. Furthermore, Huang et al. [11] showed that this method produces better fat suppression in the abdomen than standard fat-saturation techniques. Thus, the balanced-SSFP Dixon method has the potential to provide high-quality images of the renal arteries without the use of a contrast agent and with a short overall scanning time.

Specific absorption rate (SAR) limitations make rapid imaging challenging in the abdomen at high magnetic field strength (SAR is proportional to B 20), particularly when trying to image within a comfortable breath-hold. Non-breath-hold imaging introduces motion artifacts, which can degrade image quality, especially in balanced-SSFP images. SAR restrictions, however, can be overcome through sequence optimization and design [13]. Spectral–spatial saturation suppresses the fat signal and satisfies SAR constraints because of increased pulse duration [14]. However, presaturation inversion recovery has been shown to perform better than spectrospatial saturation for angiography [15]. The balanced-SSFP Dixon method produces better fat suppression than inversion recovery [11] and only requires a reduced flip angle to meet SAR restrictions. Using a breath-hold during data acquisition minimizes breathing-related motion artifacts, but this means data acquisition must occur in a window of ideally less than 20 seconds [16]. This challenge can be overcome by using an elliptic centric k-space acquisition, allowing for an abbreviated breath-hold of only 10–15 seconds while the central portion of k-space is acquired, followed by normal breathing during the acquisition of the high-frequency information [17].

Herborn et al. [8] and Wyttenbach et al. [9] showed that SSFP sequences can be used to perform 3D unenhanced renal MR angiography using fat-saturation at 1.5 T. Huang et al. [11] showed that the balanced-SSFP Dixon method is capable of producing non-fat-saturated water-only images in the abdomen in 2D at 3 T. We have successfully implemented the Huang et al. method in 3D coronal volumes to achieve unenhanced MR angiography of the lower extremities using a sequential phase-encode ordering [10].

In this study, we hypothesize that the balanced-SSFP Dixon method is capable of performing 3D unenhanced MR angiography of the renal arteries using elliptic centric phase-encode ordering [17]. This method differs from methods used in previous SSFP renal studies [8, 9] because it does not use any fatsaturation preparation but rather uses fat–water separation to suppress the fat signal [10, 11]. Data acquisition in our proposed method would need two 15-second breath-holds and an overall scanning time of less than 3 minutes; however, it would still satisfy all SAR requirements. Here, we evaluate the feasibility of this method on a 3-T MR scanner using healthy volunteers.


Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Images were collected in five healthy volunteers using a modified 3D balanced-SSFP pulse sequence. The volunteers were scanned on a 3-T MR scanner (Signa VH/i, GE Healthcare) using the built-in transmit–receive body coil (40 mT/m gradient strength, 268 microseconds rise time, and 150 T/m/s slew rate). Informed written consent was obtained from all volunteers before imaging. A 30-second localization image was first acquired to identify the locations of the kidneys and the descending aorta. The balanced-SSFP sequence parameters were TR/TE, 3.4/1.7 and flip angle, 25°. Each slice in the 3D acquisition was collected in a 256 x 256 matrix, with a 2-mm slice thickness [10]. The number of slices, field-of-view, and voxel size for each volunteer are given in Table 1.


View this table:
[in this window]
[in a new window]

  		TABLE 1: Acquisition Parameters for All Five Volunteers

 

High-order shimming was used to help ensure field homogeneity near the kidneys and vessels. The sequence collected images with –100-Hz and +100-Hz center frequency offsets to generate opposed-phase and in-phase images, respectively [10]. For each of these two elliptic centric acquisitions, the volunteers were instructed to perform a 15-second end-expiration breath-hold followed by normal breathing. Water-only images were obtained by the complex addition of real and imaginary data from the in-phase and opposed-phase images. Using standard clinical software available on the scanner console, the images were processed to isolate the renal arteries. Maximum-intensity-projection (MIP) images were calculated at various orientations for each image volume, as well as reformatted thin-slice images oriented obliquely along the renal arteries. A fellowship-trained radiologist with 5 years of cardiovascular MR experience inspected the images for vascular conspicuity to confirm that the proximal renal arteries, at least to the first branch points, were visible and could be subjectively assessed for patency and caliber. Signal-to-noise ratio (SNR), image contrast, and contrast-to-noise ratio (CNR) measurements were made using region-of-interest (ROI) analysis to quantitatively verify that the arterial signal was higher than the signal from the surrounding tissue.


Figure 1
View larger version (62K):
[in this window]
[in a new window]
[as a PowerPoint slide]
  		Fig. 1A —Single-slice, balanced steady-state free precession (SSFP) Dixon method water-only, axial images collected from 25-year-old healthy volunteer. Left renal artery (arrow, A) and right renal artery (arrow, B) can be seen. Descending aorta (Ao), inferior vena cava (IVC), and superior mesenteric artery (SMA) are also easily identifiable in both images.

 


Figure 2
View larger version (65K):
[in this window]
[in a new window]
[as a PowerPoint slide]
  		Fig. 1B —Single-slice, balanced steady-state free precession (SSFP) Dixon method water-only, axial images collected from 25-year-old healthy volunteer. Left renal artery (arrow, A) and right renal artery (arrow, B) can be seen. Descending aorta (Ao), inferior vena cava (IVC), and superior mesenteric artery (SMA) are also easily identifiable in both images.

 

Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
All volunteers tolerated the breath-holds. Figure 1A, 1B shows two single-slice water-only images collected from one healthy volunteer showing the left and right renal arteries. The descending aorta and inferior vena cava are clearly visible in both slices. Because arterial blood has a higher T2/T1 ratio than the veins and surrounding tissues [12], the arteries appear brighter than these surrounding tissues in the water-only images, as expected.

Figure 2A, 2B shows two processed MIP images calculated from axial and coronal orientations for a different healthy volunteer. Because of the aforementioned signal differences and good spatial resolution (voxel size: mean, 2.83 mm3; SD, 0.49 mm3), the venous portions could be easily removed. The renal arteries and the descending aorta are clearly visible in both of these images. Similar results showing the renal vasculature were obtained for all five healthy volunteers. The thin-slice reformatted images better show the smaller vessels compared with the MIP images [18] (Fig. 3A, 3B). The total scanning time for the localization scan and the two balanced-SSFP Dixon method scans was less than 2.5 minutes for all volunteers (Table 1). The radiologist confirmed that the proximal renal arteries were conspicuous in all volunteers. Table 2 summarizes the results of the SNR, image contrast, and CNR analysis. The arterial SNR and contrast were higher than those for venous and background tissue in all five volunteers. Furthermore, the CNR analysis confirmed that the renal arteries had a higher signal than the surrounding parenchyma for all volunteers.


Figure 3
View larger version (25K):
[in this window]
[in a new window]
[as a PowerPoint slide]
  		Fig. 2A —Maximum-intensity-projection (MIP) images calculated from image volume in 24-year-old healthy volunteer. Axial (A) and coronal (B) processed MIP images have same window width and level settings. Because of signal differences and spatial resolution, venous portions of image volumes were easily removed.

 

Figure 4
View larger version (29K):
[in this window]
[in a new window]
[as a PowerPoint slide]
  		Fig. 2B —Maximum-intensity-projection (MIP) images calculated from image volume in 24-year-old healthy volunteer. Axial (A) and coronal (B) processed MIP images have same window width and level settings. Because of signal differences and spatial resolution, venous portions of image volumes were easily removed.

 

Figure 5
View larger version (52K):
[in this window]
[in a new window]
[as a PowerPoint slide]
  		Fig. 3A —24-year-old healthy volunteer (same patient as in Fig. 2A, 2B). Processed maximum-intensity-projection (MIP) image of left renal artery (A) and reformatted thin-slice image of left renal artery (B) generated from data set collected from same healthy volunteer as in Figure 2A, 2B. Thin-slice reformatted image (B) was calculated by orienting oblique thin slice along direction of vessel. Although separate reformatted image in different orientation is required to achieve similar results for right renal artery, reformatted image shows better vessel conspicuity, both in main renal artery and in smaller distal vessels (arrow, B) compared with standard coronal projection through MIP image volume [11].

 

Figure 6
View larger version (62K):
[in this window]
[in a new window]
[as a PowerPoint slide]
  		Fig. 3B —24-year-old healthy volunteer (same patient as in Fig. 2A, 2B). Processed maximum-intensity-projection (MIP) image of left renal artery (A) and reformatted thin-slice image of left renal artery (B) generated from data set collected from same healthy volunteer as in Figure 2A, 2B. Thin-slice reformatted image (B) was calculated by orienting oblique thin slice along direction of vessel. Although separate reformatted image in different orientation is required to achieve similar results for right renal artery, reformatted image shows better vessel conspicuity, both in main renal artery and in smaller distal vessels (arrow, B) compared with standard coronal projection through MIP image volume [11].

 

View this table:
[in this window]
[in a new window]

  		TABLE 2: Summary of Signal-to-Noise Ratio, Image Contrast, and Contrast-to-Noise Ratio for Five Volunteers

 


Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
These results have successfully shown the feasibility of collecting 3D image volumes of the renal arteries at 3 T with strong vessel conspicuity using the balanced-SSFP Dixon method along with elliptic centric phase encoding and a short breath-hold. As expected, the arterial signal is higher than that of the surrounding background tissue. Although this method does require the collection of two separate image volumes, the short overall scanning time for each volume helps to ensure that patient motion between contiguous center-frequency-offset acquisitions is minimal. Also, collecting the high-energy (low-frequency) k-space data (as per elliptic centric phase encoding) during end-expiration breath-holds further ensures proper anatomic registration between offset volumes.

The short acquisition time, high SNR efficiency, and flow independence make this technique more advantageous than either phase-contrast or TOF angiography. As previously shown by Huang et al. [11], the balanced-SSFP Dixon method provides better fat suppression in the abdomen than standard fat-saturation SSFP techniques, such as those used by Herborn et al. [8] and Wyttenbach et al. [9], and overcomes partial volume averaging arising from a voxel containing a mix of fat and water.

The focus of this work was to show the feasibility of the balanced-SSFP Dixon technique for the specific purpose of unenhanced renal MR angiography. A rigorous analysis of image quality or comparison with an established technique such as contrast-enhanced MR or CT angiography was not performed. Furthermore, only the conspicuity of the proximal renal arteries was assessed, given that the majority of stenotic lesions occur in the proximal portion of the vessels [1]. The conspicuity of smaller branch vessels was not assessed in this study.

This technique was used to image the vessels of healthy volunteers, and further clinical evaluation is needed to determine whether this balanced-SSFP Dixon method can reliably show stenosis or occlusion in patients with abnormalities. Further technical refinements to image acquisition, postprocessing, or both, could ensure that longer segments and smaller branches of the renal arterial system are better visualized. For example, producing oblique reformatted thin-slice images oriented along the renal arteries has been shown to produce better vessel conspicuity, both in the main renal arteries and in smaller distal vessels, than standard MIP images [18] (Fig. 3A, 3B). Also, using a multichannel torso phased-array coil and cardiac gating [9] could result in better SNR and vessel conspicuity. We have not tested the balanced-SSFP method on other field strengths, although the theory regarding the underlying physics suggests it would be possible, keeping in mind that the cycle time is inversely related to the field strength.

We conclude that the balanced-SSFP Dixon method has great potential to offer fast, unenhanced MR angiography of the renal arteries and even at this early stage, if clinically validated to show proximal abnormality, can be a clinically useful option for MR angiography in patients at risk of nephrogenic systemic fibrosis.


Acknowledgments
 
The authors thank the volunteers for their time.


References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

  1. Rieumont MJ, Kaufman JA, Geller SC, et al. Evaluation of renal artery stenosis with dynamic gadolinium-enhanced MR angiography. AJR 1997; 169:39 –44[Abstract/Free Full Text]
  2. Thomsen HS. Nephrogenic systemic fibrosis: a serious late adverse reaction to gadodiamide. Eur Radiol 2006;16 :2619 –2621[CrossRef][Medline]
  3. Dumoulin CL, Souza SP, Walker MF. Three-dimensional phase contrast angiography. Magn Reson Med 1989;9 : 139–149[Medline]
  4. Dumoulin CL, Cline HE, Souza SP. Three-dimensional time-of-flight magnetic resonance angiography using spin saturation. Magn Reson Med 1989; 11:35 –46[Medline]
  5. Singer JR. Blood flow rates by nuclear resonance measurements. Science 1959; 130:1652 –1653[Abstract/Free Full Text]
  6. Urchuck SN, Plewes DB. Mechanisms of flow-induced signal loss in MR angiography. J Magn Reson Imaging 1992;2 : 453–462[Medline]
  7. Oppelt A, Graumann R, Barfuss H, Fischer H, Hartl W, Shajor W. FISP: a new fast MRI sequence [English ed.]. Electromedica 1986;54 : 15–18
  8. Herborn CU, Matkins DM, Runge VM, Gendron JM, Montgomery ML, Naul LG. Renal arteries: comparison of steady-state free precession MR angiography and contrast-enhanced MR angiography. Radiology2006; 239:263 –268[Abstract/Free Full Text]
  9. Wyttenbach R, Braghetti A, Wyss M, et al. Renal artery assessment with nonenhanced steady-state free precession versus contrast-enhanced MR angiography. Radiology 2007;245 : 186–195[Abstract/Free Full Text]
  10. Stafford RB, Sabati M, Mahallati H, Frayne R. 3D non-contrast enhanced MR angiography with balanced steady-state free precession Dixon method. Magn Reson Med 2008;59 : 430–433[CrossRef][Medline]
  11. Huang TY, Chung HW, Wang FN, Ko CW, Chen CY. Fat and water separation in balanced steady-state free precession using the Dixon method. Magn Reson Med 2004;51 : 243–247[CrossRef][Medline]
  12. Scheffler K, Lehnhardt S. Principles and applications of balanced SSFP techniques. Eur Radiol 2003;13 :2409 –2418[CrossRef][Medline]
  13. Lauzon ML, Mahallati H, Frayne R. Time-efficient breath-hold abdominal MRI at 3.0 T. AJR 2006;187 : 649-657[Abstract/Free Full Text]
  14. Morrell G, Macovski A. Three-dimensional spectral-spatial excitation. Magn Reson Med 1997;37 : 378–386[Medline]
  15. Börnert P, Stuber M, Bontar RM, Kissinger KV, Manning WJ. Comparison of fat suppression strategies in 3D spiral coronary magnetic resonance angiography. J Magn Reson Imaging2002; 15:462 –466[CrossRef][Medline]
  16. Dumoulin CL, Yucel EK, Vock P, et al. Two- and three-dimensional phase contrast MR angiography of the abdomen. J Comput Assit Tomogr 1990; 14:779 –784[Medline]
  17. Wilman AH, Riederer SJ, Huston J 3rd, Wald JT, Debbins JP. Arterial phase carotid and vertebral artery imaging in 3D contrast-enhanced MR angiography by combining fluoroscopic triggering with an elliptical centric acquisition order. Magn Reson Med 1998;40 : 24–35[Medline]
  18. Davis CP, Hany TF, Wildermuth S, Schmidt M, Debatin JF. Postprocessing techniques for gadolinium-enhanced three-dimensional MR angiography. RadioGraphics 1997;17 :1061 –1077[Abstract]

Add to CiteULike CiteULike   Add to Complore Complore   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us   Add to Digg Digg   Add to Reddit Reddit   Add to Technorati Technorati    What's this?



This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Stafford, R. B.
Right arrow Articles by Frayne, R.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Stafford, R. B.
Right arrow Articles by Frayne, R.
Social Bookmarking
Add to CiteULike   Add to Complore   Add to Connotea   Add to Del.icio.us   Add to Digg   Add to Reddit   Add to Technorati  
What's this?
Hotlight (NEW!)
Right arrow
What's Hotlight?

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS