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DOI:10.2214/AJR.07.2997
AJR 2008; 190:902-906
© American Roentgen Ray Society

Original Research

Unenhanced MR Angiography of the Thoracic Aorta: Initial Clinical Evaluation

Christopher J. François1,2, David Tuite1, Vibhas Deshpande3, Renate Jerecic3, Peter Weale3 and James C. Carr1

1 Department of Radiology, Northwestern University, Chicago, IL.
2 Present address: Department of Radiology, University of Wisconsin-Madison, 600 Highland Ave., Madison, WI 53792.
3 MR Research and Development, Siemens Medical Solutions, Inc., Malvern, PA.

Received August 8, 2007; accepted after revision October 6, 2007.

 
Address correspondence to C. J. François (cfrancois{at}uwhealth.org).


Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
OBJECTIVE. In patients with difficult IV access or renal insufficiency, or in those who are pregnant, we hypothesized than an unenhanced 3D segmented steady-state free precession (SSFP) MR angiography (MRA) technique would be an alternative to contrast-enhanced MR angiography (CE-MRA) for the evaluation of vasculature.

MATERIALS AND METHODS. MRA examinations of the thoracic aorta were retrospectively reviewed in 23 patients in whom both CE-MRA and 3D SSFP were performed. CE-MRA was performed using an ECG-gated gradient-echo FLASH sequence. Three-dimensional SSFP MRA was performed during free breathing using a motion-adaptive navigator technique. Quantitative assessment of the 3D SSFP and CE-MRA image sets was performed by comparing the aortic lumen diameter. The quality of the images of the aortic root (scale of 1–5) and the presence of cardiovascular and noncardiovascular pathology were independently determined for both techniques by two reviewers. Bland-Altman and Wilcoxon's signed-rank analyses were performed.

RESULTS. The difference in orthogonal measurements of the aortic diameter between those made on images from the 3D SSFP and those made from the CE-MRA sequences was –0.042 cm. The aortic root was better visualized with 3D SSFP: score of 3.78 (of 5) for CE-MRA versus score of 4.65 (of 5) for 3D SSFP (p < 0.05).

CONCLUSION. In patients in whom contrast material is contraindicated, unenhanced MRA using a 3D SSFP technique can be performed.

Keywords: angiography • aorta • MRI • steady-state free precession • thoracic aorta


Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
MR angiography (MRA) has become an established technique for the evaluation of a variety of aortic diseases [1–15], including aneurysms [6–8], acute aortic syndromes (intramural hematomas, penetrating ulcerative plaques, or aortic dissections) [9–11], vasculitis [12, 13], and congenital abnormalities [14, 15]. Over the past several years, contrast-enhanced MR angiography (CE-MRA) techniques [16–19] have largely replaced unenhanced MRA techniques for the evaluation of the thoracic aorta because of their high spatial resolution and reliability. However, in patients who are pregnant, who have extremely poor renal function, or who have inadequate IV access, the use of gadolinium-based contrast material may be contraindicated or not feasible. In these patients, MRA techniques that do not require contrast material but are of equal quality and reliability as CE-MRA techniques, would be desirable.

Several unenhanced MRA techniques, including fast spin-echo [20, 21] and gradient-echo sequences [22, 23], have been evaluated for imaging the aorta. Balanced steady-state with free precession (SSFP) techniques are desirable because of their inherently high signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) [24]. Single-shot techniques are rapid and are useful for the urgent evaluation of patients with suspected acute aortic syndromes or patients incapable of long breath-holding [23]. Three-dimensional SSFP techniques have also been developed to evaluate the heart and the coronary arteries [25, 26].

The purpose of this study was to evaluate a whole-chest, unenhanced, free-breathing, T2-prepared, segmented 3D SSFP imaging sequence for the assessment of thoracic aorta disorders and to compare it with ECG-gated 3D CE-MRA.


Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Patient Selection
Approval for this study was obtained from our institutional review board, and our study was HIPAA-compliant. Between July 2006 and November 2006, 23 consecutive patients (12 men and 11 women, 17–80 years old) who were referred for 3D CE-MRA of the thoracic aorta were imaged with whole-chest 3D SSFP and with conventional 3D CE-MRA. No patients were excluded from the study. Indications for the MR examinations included suspected or known ascending aortic aneurysm (n = 8), postoperative follow-up of repaired aneurysms (n = 1), suspected or known congenital abnormality of the aorta (n = 8), suspected vasculitis (n = 2), assessment of aortic involvement of vascular tumor (n = 1), and other indications (n = 4).

MRI Techniques
All patients underwent imaging with a 1.5-T whole-body MRI system (Magnetom Avanto, Siemens Medical Solutions). All patients had ECG leads placed in a standard fashion, and ECG triggering was performed with standard software. Prospective and retrospective ECG triggering was used for the whole-chest 3D SSFP and CE-MRA sequences, respectively. All patients underwent a standard cardiac MRI examination in addition to the whole-chest 3D SSFP and CE-MRA sequences.

Unenhanced, free-breathing, T2-prepared, segmented 3D SSFP MRA (Fig. 1) of the whole chest was performed in all patients [27]. To enable large field-of-view imaging with uniform blood signal, nonselective radiofrequency excitation was used to shorten the TR and thus reduce the sensitivity of the sequence to field inhomogeneities. Navigator pulses were used to detect respiratory motion, and slice following was implemented to correct for motion that occurred during data acquisition. Motion-adaptive navigator gating was implemented to correct for respiratory drifts that may occur during the relatively long imaging time of a free-breathing scan. Imaging parameters for the 3D SSFP sequence were as follows: TR/TE, 2.3/1.0; flip angle, 90°; readout bandwidth, 980 Hz/pixel; T2 preparation time, 40 milliseconds; field of view, 400 x 400 mm; matrix, 256 x 256; slice thickness, 3 mm interpolated to 1.5 mm; number of lines per heartbeat, 51–77 depending on the heart rate; number of partitions, 88–128; and parallel imaging acceleration factor, 2.


Figure 1
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  		Fig. 1 —Schematic of ECG-triggered, navigator-gated, T2-prepared, segmented, steady-state free precession sequence. TRui (TR shown on user interface) includes T2 preparation time (T2 prep), fat-suppression time (FS), linear flip angle preparation (preps), data acquisition, and spoiler. TRui does not include time for navigator pulses (Nav).

 
The technique used for the high-resolution ECG-gated CE-MRA has recently been described [28]. Briefly, gadopentetate dimeglumine (Magnevist, Bayer HealthCare, formerly Berlex Laboratories) was administered in an antecubital vein by an MR-compatible power injector (Spectris, Medrad). A standard test-bolus acquisition, using 2-mL of contrast material, was performed to determine the contrast arrival time in the ascending thoracic aorta. High-resolution 3D CE-MRA was then performed under suspended respiration after a 0.2 mmol/kg injection of contrast agent (flow rate, 2 mL/s). Before acquisition of the CE-MRA data set, an unenhanced "mask" 3D gradient-echo FLASH image set was acquired and subtracted from the contrast-enhanced 3D FLASH data set to eliminate background signals. A 3D FLASH pulse sequence was used for high-resolution MRA [28]. An asymmetric k-space scanning scheme in all three gradient axes was used; the remainder of the k-space was filled in with zero padding. The following scanning parameters were used: 2.8/1.4; flip angle, 25°; and voxel size, 1.4 x 0.8 x 1.3 mm. ECG gating was used so that each partition was acquired during end-diastole. The approximate acquisition time was 20 seconds.

Qualitative Analysis
The CE-MRA and 3D SSFP examinations for each of the 23 patients were qualitatively reviewed independently by two authors who were blinded to patient diagnoses. After all image sets (CE-MRA and 3D SSFP) were randomized, the two observers evaluated the images for the presence of disorders and scored the image quality of each set by assessing the aortic root because this is the part of the aorta that is most often blurred on MRA due to cardiac motion [28]. Qualitative assessments were scored on the following 5-point Likert scale: 1, nondiagnostic image quality; 2, poor image quality; 3, fair quality; 4, good quality; and 5, excellent quality.

Quantitative Analysis
The image acquisition time required for the 3D SSFP image data set was recorded for each patient. On an image postprocessing workstation (syngo MultiModality Workplace [MMWP], Siemens Medical Solutions), multiplanar reformations of the thoracic aorta were constructed on both the CE-MRA and 3D SSFP examinations. For each examination on every patient, the orthogonal pairs of aortic lumen diameter were measured by one of the authors at seven distinct locations (Figs. 2A and 2B): the annulus, sinuses of Valsalva, sinotubular junction, mid ascending aorta (at the level of the main pulmonary artery bifurcation), proximal aortic arch (at the level of the innominate artery), distal aortic arch (distal to the left subclavian artery), and the hiatus. The orthogonal dimensions for the CE-MRA and 3D SSFP examinations were obtained at different points in time without knowledge of the corresponding measurements using the other technique. In total, 318 individual measurements of aortic diameter were made from the 23 patients imaged with each technique. Four measurements could not be performed in two patients (two measurements in each patient) on the CE-MRA images because that part of the aorta was inadvertently incompletely included in the field of view for that sequence (annulus in one patient and mid ascending aorta in the second patient).


Figure 2
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  		Fig. 2A —23-year-old man with history of Ross procedure and dilated aortic root. Volume-rendered image from 3D steady-state free precession (SSFP) data set indicates seven locations at which measurements were made: 1, annulus; 2, sinuses of Valsalva; 3, sinotubular junction; 4, mid ascending aorta; 5, proximal arch; 6, distal arch; and 7, descending aorta.

 

Figure 3
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  		Fig. 2B —23-year-old man with history of Ross procedure and dilated aortic root. Multiplanar reformatted images from 3D SSFP (left) and contrast-enhanced MR angiography (right) data sets at annulus, sinuses of Valsalva, sinotubular junction, mid ascending aorta, proximal arch, distal arch, and descending aorta.

 
Statistical Analysis
Bland-Altman analysis was used to compare the aortic dimensions measured from the CE-MRA and 3D SSFP sequences. The qualitative scores achieved with CE-MRA and 3D SSFP were compared using a Wilcoxon's signed ranked test. For all tests, statistical significance was defined at the p < 0.05 level.


Results
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 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
CE-MRA and 3D SSFP MRA were successful in all patients. The average acquisition time for the 3D SSFP sequence was 9.3 ± 4.3 minutes. There was agreement in the imaging diagnosis in all patients between CE-MRA and 3D SSFP. Of the 23 patients included in this study, three studies were diagnosed as normal. In the remaining 20 patients, 12 were diagnosed with a dilated or aneurysmal aortic root, five with a dilated or aneurysmal ascending aorta (Fig. 3), two with aortic coarctation (Fig. 4), one with ascending aortic hypoplasia (Fig. 5), two with aortitis, one with aortic sarcoma, and one with pulmonary sequestration. One study was performed in a patient who was being monitored after ascending aortic aneurysm repair.


Figure 4
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  		Fig. 3 —26-year-old woman with congenital bicuspid aortic valve and ascending aortic aneurysm. Coronal 3D steady-state free precession image also shows evidence of aortic insufficiency (arrow).

 

Figure 5
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  		Fig. 4 —55-year-old man with aortic coarctation. Volume-rendered image from 3D steady-state free precession data set confirms aortic coarctation (arrow).

 

Figure 6
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  		Fig. 5 —25-year-old woman with congenital ascending aortic hypoplasia. Coronal 3D steady-state free precession image shows severe narrowing of entire ascending aorta (circle).

 

Qualitative assessment of the two techniques by the two observers showed a significant difference between the techniques in terms of conspicuity of the aortic root (p < 0.05), with an average observer score of 3.78 (of 5) for CE-MRA and 4.65 (of 5) for 3D SSFP (Fig. 6).


Figure 7
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  		Fig. 6 —26-year-old woman with congenital bicuspid aortic valve and ascending aortic aneurysm. Multiplanar reformatted images from 3D steady-state free precession (left) and contrast-enhanced MR angiography (CE-MRA) (right) data sets reveal greater blurring on CE-MRA images at annulus, sinuses of Valsalva, and sinotubular junction.

 
The mean orthogonal diameters at the seven locations on the 3D SSFP and CE-MRA data sets were, respectively, 2.5 and 2.4 cm (annulus), 3.4 and 3.3 cm (sinuses of Valsalva), 2.8 and 2.7 cm (sinotubular junction), 3.0 and 3.1 cm (mid ascending aorta), 2.7 and 2.8 cm (proximal aortic arch), 2.3 and 2.2 cm (distal aortic arch), and 1.9 and 1.8 cm (distal descending aorta). No statistically significant difference in measurements between the two techniques was found (p < 0.05). Bland-Altman analysis revealed a mean measured difference (3D SSFP minus CE-MRA) in aortic diameter between the two techniques of –0.042 ± 0.173 cm (Fig. 7).


Figure 8
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  		Fig. 7 —Bland-Altman plot shows no minimal significant difference in measurements made with 3D steady-state free precession and contrast-enhanced MR angiography data sets.

 

Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
This study compared an unenhanced, whole-chest MRA using a free-breathing 3D SSFP sequence with an ECG-gated, 3D CE-MRA technique for the evaluation of the thoracic aorta in 23 patients. Qualitatively, the images obtained with the 3D SSFP sequence were sharper, with less blurring, at the aortic root than those acquired with the CE-MRA technique. Image data sets obtained with both sequences were also evaluated on a computer workstation with multiplanar reconstruction capabilities for quantitative comparison at seven levels in the aortic root and thoracic aorta. No significant difference was observed in orthogonal measurements made with the 3D SSFP images and those made from the CE-MRA images.

Digital subtraction angiography and CT angiography (CTA) are both rapid, high-resolution imaging methods frequently used to image the thoracic aorta. However, both tech niques suffer significant drawbacks, including ionizing radiation [29, 30] and nephrotoxic contrast agents [31]. In addition, digital subtraction angiography is a relatively invasive procedure that may require hospital admission. Because patients with aortic disorders return for follow-up examinations at frequent short-term intervals, MRA is increasingly being used in these patients.

Advances in MR scanners and sequences have made MRA an excellent noninvasive method of evaluating the vasculature. MRA is particularly suited for evaluating the thoracic aorta, especially when using parallel imaging and ECG gating to offset the effects of cardiac motion [28]. MRA, combined with other nonangiographic MRI techniques, can be used as the primary or sole imaging technique in the evaluation of patients with aortic valvular, aortic root, or thoracic aorta disorders [8]. In addition, time-resolved CE-MRA sequences [18, 19] can be used to assess the hemodynamics of aortic disorders such as aortic dissections and coarctations.

Situations exist in which an MRA technique that does not require the administration of gadolinium-containing contrast agents is desirable because IV contrast material is either not possible or contraindicated. High-resolution CE-MRA is usually performed with a power injector at injection rates of 2–3 mL/s, which requires a large-bore (i.e., 18-gauge) IV catheter. Occasionally, in patients who are obese or who have poor veins, it may not be possible to obtain adequate IV access in a timely manner. IV gadolinium-containing contrast agents have been shown to be teratogenic in animal studies and are therefore usually contraindicated during pregnancy. Recently, the U.S. Food and Drug Administration (FDA) issued revised recommendations regarding the use of gadolinium-containing contrast agents in patients with advanced renal failure [32] because of their association with nephrogenic systemic fibrosis (NSF) [32–34]. This is of particular concern for patients undergoing CE-MRA because higher doses of contrast material are frequently used to perform this examination [16], and patients referred for MRA of the aorta often have decreased renal function as well [8]. In these subsets of patients being referred for MRA, an unenhanced MRA technique may be necessary.

Several unenhanced MRA techniques, including time-of-flight, spin-echo and gradient-echo dark-blood, and phase-contrast sequences, have been used for imaging of the thoracic aorta. Time-of-flight, spin-echo, and phase-contrast sequences have several limitations for routine use as an MRA technique in the thorax, including longer acquisition times that increase the likelihood of motion artifact and poor image quality with spin-echo and time-of-flight sequences in areas of slow or nonlaminar flow (which are likely to occur in aneurysms).

SSFP sequences, because of the inherent high contrast between blood and background tissues, are an attractive alternative for unenhanced vascular imaging. Previous studies have shown the feasibility of a 3D SSFP technique for imaging the coronary arteries [26] and heart in patients with congenital heart disease [25]. In these studies, 3D SSFP was generally implemented with slice selectivity and fat saturation. For both of these applications, the imaging volume was focused on a relatively small anatomic area. In this study, we used a non–slice-selective whole-chest approach for 3D SSFP imaging, which shortens the TR and reduces the sensitivity of this sequence to field inhomogeneities. Because the acquired 3D volume had near-isotropic spatial resolution, it was possible to postprocess the data in arbitrary multiplanar orientations. Recently, time-resolved 3D phase-contrast imaging has been performed with respiratory gating in the thoracic aorta and could be another alternative for unenhanced MRA in the thorax [35].

A limitation of the 3D SSFP sequence used in this study is the relatively long acquisition time. Because respiratory gating and navigator echo were used, the overall acquisition time averaged 10 minutes, depending on the breathing pattern. Although this could potentially also cause problems with the navi gator tracking, a motion-adaptive gating algorithm [27] was used to adapt the acceptance window throughout the course of the acquisition. Consequently, image quality was preserved, even in patients who required longer than average times to image. Another limitation of this study is that no comparison was made with other angiographic techniques such as digital subtraction angiography or CTA. This is mainly because the institution at which the study was performed primarily uses MRA as a first-line study for the investigation of the thoracic aorta in nonemergent situations.

In conclusion, this study showed the feasibility of using an unenhanced 3D SSFP MRA technique for the evaluation of the thoracic aorta. We found no significant difference in aortic measurements made from the 3D SSFP images and those made from the ECG-gated CE-MRA images. Therefore, in patients who have a contraindication to gadolinium-containing contrast agents, 3D SSFP MRA can be used to evaluate the thoracic aorta.


References
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 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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