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DOI:10.2214/AJR.07.2297
AJR 2008; 190:389-395
© American Roentgen Ray Society

Original Research

3-T Contrast-Enhanced MR Angiography in Evaluation of Suspected Intracranial Aneurysm: Comparison with MDCT Angiography

Kambiz Nael1, J. Pablo Villablanca1, Léonard Mossaz1, Whitney Pope1, Alex Juncosa1, Gerhard Laub2 and J. Paul Finn1

1 Department of Radiological Sciences, David Geffen School of Medicine at University of California Los Angeles, 10945 Le Conte Ave., Suite 3371, Los Angeles, CA 90095-7206.
2 Siemens Medical Solutions, Malvern, PA.

Received October 15, 2006; accepted after revision September 9, 2007.

 
Address correspondence to K. Nael.


Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
OBJECTIVE. The purpose of this study was to prospectively evaluate a high-spatial-resolution contrast-enhanced 3-T MR angiography protocol for detection and characterization of intracranial aneurysms and to compare the results with those of MDCT angiography.

SUBJECTS AND METHODS. Forty-one patients with suspected intracranial aneurysm underwent high-spatial-resolution 3D contrast-enhanced MR angiography and CT angiography (CTA). With a generalized autocalibrating partially parallel acquisition algorithm with an acceleration factor of 4 at 3 T, contrast-enhanced MR angiographic images were acquired over 20 seconds with a spatial-resolution of 0.7 x 0.7 x 0.8 mm. CTA images were acquired with a spatial resolution of 0.35 x 0.35 x 0.8 mm on a 16-MDCT scanner in 17 seconds. The images from the two studies were evaluated independently by two neuroradiologists for image quality, presence of aneurysm, and characterization of aneurysm. The dimensions of the aneurysm were measured independently with both techniques.

RESULTS. A total of 25 aneurysms were identified with both contrast-enhanced MR angiography and CTA. A comparative analysis of detection and depiction of aneurysms showed excellent interobserver agreement for both contrast-enhanced MR angiography ({kappa} = 0.81) and CTA ({kappa} = 0.91) images. There was significant correlation between the techniques for both qualitative assessment of aneurysm depiction ({rho} = 0.92; 95% CI, 0.88–0.95) and quantitative dimensional measurement of aneurysm size (r = 0.94; 95% CI, 0.92–0.97).

CONCLUSION. Contrast-enhanced MR angiography at 3 T is reliable for evaluation and characterization of intracranial aneurysms. The results are comparable with those of MDCTA.

Keywords: 3 T • comparison studies • CT angiography • intracranial aneurysms • MR angiography • high magnetic field strength • parallel acquisition


Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
Intracranial aneurysms are reported to be present in 0.2–8% of the general population [1–4], and the annual risk of rupture is estimated to be 1–2% [5, 6]. The outcome among patients with intracranial aneurysms treated before rupture is markedly better than that among those treated after aneurysm rupture [3, 7]. Although screening the general population to identify and manage unruptured aneurysms is not recommended [8], screening may be beneficial in certain populations at higher risk of harboring an intracranial aneurysm, such as those with a family history of intracranial aneurysm, persons with autosomal dominant polycystic kidney disease [9], persons with fibromuscular dysplasia [10], first-degree relatives of persons with subarachnoid hemorrhage [11], and patients with relevant clinical symptoms.

Conventional catheter angiography is the reference standard technique for diagnosis of intracranial aneurysms. However, the invasive nature, associated radiation, and small risk of associated neurologic complications [12, 13] make conventional angiography less than ideal for screening and repeated follow-up studies. With the introduction of and advances in minimally invasive cross-sectional techniques such as CT angiography (CTA) and MR angiography, the clinical approach to suspected cerebral aneurysm has changed substantially. CTA has been used successfully for evaluation of intracranial aneurysms [14–16]. The success of CTA is in large part due to its speed and high spatial resolution, and advances in MDCT technology have raised the bar for other alternative noninvasive imaging techniques. The use of contrast-enhanced MR angiography also has been explored for evaluation of intracranial aneurysms [14–17]. However, the competing requirements for adequate coverage and acquisition speed often force a compromise in the spatial resolution of MR angiography relative to that of CTA.

The introduction of 3-T MRI systems, which have a higher available signal-to-noise ratio (SNR) than earlier systems, combined with advances in parallel imaging techniques [18–20] has greatly improved the performance of MR angiography in terms of spatial resolution, speed, and coverage for the evaluation of cerebrovascular diseases [21–24]. MRI at 3 T has the potential to enhance the performance of contrast-enhanced MR angiography compared with CTA without the disadvantages of radiation exposure, risk of nephrotoxicity from IV contrast agents, and image degradation arising from vascular calcifications and the bony calvarium. The purpose of this study was to prospectively evaluate a high-spatial-resolution contrast-enhanced 3-T MR angiography protocol for visualization and characterization of intracranial aneurysms and to compare the results with those of MDCTA.


Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Patients
Forty-one adult patients with suspected intracranial aneurysms (15 men, 26 women; age range, 22–64 years) prospectively underwent high-spatial-resolution contrast-enhanced 3-T MR angiography and CTA on a 16-MDCT scanner. The mean delay time between the examinations was 2 weeks (1–23 days).

Twenty-three patients had signs or symptoms suggestive of intracranial aneurysm, including persistent headache (n = 12), oculomotor nerve palsy (n = 5), and visual disturbance (n = 6). Eighteen patients underwent MR angiography for screening purposes because of a variety of associated risk factors, including family history of intracranial aneurysm or history of subarachnoid hemorrhage in a first- or second-degree relative (n = 15), history of fibromuscular dysplasia (n = 2), and history of autosomal dominant polycystic kidney disease (n = 1). Exclusion criteria included all standard contraindications to MRI (e.g., pacemaker, claustrophobia, contrast reaction, implanted metallic devices). All studies were performed in accordance with institutional review board guidelines under an approved protocol.

Imaging Technique MR angiography
MR angiography was performed with a 3-T whole-body MRI system (Magnetom Trio, Siemens Medical Solutions) equipped with 32 receiver channels and a fast gradient system (peak gradient amplitude, 45 mT/m; maximum slew rate, 200 mT/m/ms).

Patients were placed supine on the MRI table and advanced head-first into the magnet bore. For signal reception, a combination of a 16-element array coil (head, n = 12; neck, n = 4) was used. A total dose of 0.15 mmol/kg of gadopentetate dimeglumine (Magnevist, Bayer HealthCare) was injected at 1.5 mL/s with an electronic power injector (Spectris, Medrad). A 2-mL timing bolus (1.5 mL/s) was used to measure the contrast transit time from the arm vein to the carotid arteries. High-spatial-resolution contrast-enhanced MR angiography was performed in the coronal plane with a fast spoiled gradient-recalled echo sequence (TR/TE, 3/1.2; flip angle, 20°; bandwidth, 720 Hz/pixel; field of view, 360 x 240 mm; matrix size, 576 x 324). An asymmetric k-space sampling scheme (partial Fourier, 80%) and zero interpolation were applied in all three planes to minimize the TE and the acquisition time. Parallel imaging was performed with a generalized autocalibrating partially parallel acquisition algorithm based on autocalibration of simultaneous acquisition of spatial harmonics and parallel acquisition [20]. An acceleration factor of four was used with 24 reference k-space lines for calibration in the left-to-right phase-encoding direction. With selection of 120 partitions with a thickness of 0.8 mm, contrast-enhanced MR angiography of the entire supraaortic circulation was performed in a 20-second acquisition with acquired voxel dimensions of 0.7 x 0.7 x 0.8 mm.

CT angiography
CTA examinations were performed within 1–21 days after contrast-enhanced MR angiography on a 16-MDCT scanner (Somatom Sensation 16, Siemens Medical Solutions). Patients were placed in the supine position with the head tilted to avoid inclusion of dental hardware in the field of view. Patients were instructed to breathe quietly without swallowing during the scanning period. Helical data were acquired with 0.8-mm slice collimation and a table speed of 8.6 mm/s for 17 seconds starting at T4 and proceeding to the cranial vertex. A field of view of 180 mm and reconstruction interval of 0.5 mm allowed spatial-resolution of 0.35 x 0.35 x 0.8 mm. Total average coverage was 240 mm for a total of 640 reconstructed transverse sections. Helical acquisition was initiated after the start of bolus administration of contrast medium, which was determined with a test of circulation time (CARE bolus, Siemens Medical Solutions) with a trigger point of 150 H within the common carotid artery. Contrast administration involved IV injection of 90 mL of nonionic contrast medium (iohexol, Omnipaque 350, GE Healthcare) at 3 mL/s.

Image Analysis
After data acquisition, image postprocessing, and analysis were independently performed on a 3D workstation (Vitrea version 3.6, Vital Images) by two neuroradiologists with 10 and 8 years of experience. The readers were informed of each patient's clinical history, but they were blinded to patient name, medical record number, and results of other imaging studies. Separate image reading sessions were organized for both readers by the study coordinator, who attended all reading sessions. Evaluation sessions for CTA and MR angiography were performed at least 4 weeks apart to avoid possible recall bias. Three-dimensional volume-rendered images, multiplanar gray-scale 2D single-section images, and thick-slab 2D multiplanar reformation images were evaluated.

The presence of an arterial aneurysm and its characterization in terms of location, orientation, shape, and size were noted by each observer for the MR angiographic and CTA data sets. The presence of possible arterial incorporations into the aneurysm sac or neck also was noted. An aneurysm was defined as a saccular or fusiform outpouching from a parent artery with a clearly definable sac and neck. The observers were asked to grade the quality of aneurysm depiction and their diagnostic confidence using the following 3-point scale: grade 3, presence of an aneurysm diagnosed with full confidence, excellent depiction of the lesion with good analysis of the relations between the aneurysm and the parent vessel; grade 2, good depiction of an aneurysm but relations to parent vessel incompletely assessed; and grade 1, lesion scarcely visible, images insufficient for aneurysm characterization. Single-section multiplanar reformatted images from the MR angiographic and CTA data sets were used for quantitation of all three orthogonal dimensions (transverse, anteroposterior, and craniocaudal) of the aneurysm sac and to obtain aneurysm neck dimensions in two planes. All quantitation was performed by one observer with 6 years of experience in 3D image processing. This observer used an internal digital caliper with an accuracy of ± 10% for measurements less than 2 mm, ± 5% for measurements greater than 2 mm and less than 10 mm, and ± 2% for measurements greater than 10 mm.

For statistical analysis, the degree of interobserver agreement and agreement between techniques for depiction of aneurysms were determined by calculation of the kappa value. The relation between contrast-enhanced MR angiography and CTA in terms of depiction of aneurysm was analyzed with Spearman's rank correlation coefficient ({rho}) and determination of the 95% CI. A paired Student's t test was used to evaluate the significance of the dimensional measurement differences between the two techniques (p < 0.05 indicated a statistically significant difference). Bland-Altman plots were prepared, and the correlation coefficient (r) and 95% CI were calculated for the measurements of aneurysm dimensions between contrast-enhanced MR angiography and CTA [25].


Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
Qualitative Analysis
In the evaluation of contrast-enhanced MR angiography, 25 aneurysms were detected in 18 of 41 patients. A single aneurysm was detected in 13 patients, three patients had two aneurysms, and two patients had three aneurysms. The qualitative analysis for aneurysm detection and depiction showed good interobserver agreement ({kappa} = 0.81) and significant correlation ({rho} = 0.89; 95% CI, 0.79–0.94) between the two readers in the evaluation of contrast-enhanced MR angiography.

In the evaluation of CTA, 25 and 24 aneurysms were detected by readers 1 and 2, respectively. One aneurysm was not identified on CTA by reader 2 during the first reading session. This lesion was a 2.4-mm right cavernous internal carotid artery aneurysm. Subsequent analysis of the data set revealed that the aneurysm was clearly present on the 2D and 3D CT angiograms but was overlooked during the initial reading (Fig. 1A, 1B, 1C, 1D). The probable reasons were its proximity to bone and its pronounced medial projection. The qualitative analysis of aneurysm detection and depiction showed excellent interobserver agreement ({kappa} = 0.91) and significant correlation ({rho} = 0.94; 95% CI, 0.89–0.97) between the two readers in evaluation of CTA images. There was a good agreement between contrast-enhanced MR angiography and CTA in qualitative assessment of aneurysm depiction ({kappa} = 0.78) with significant correlation ({rho} = 0.92; 95% CI, 0.88–0.95) (Figs. 2A, 2B, 2C, 2D, 2E and 3A, 3B, 3C, 3D). Arterial incorporations occurred in only 12% of aneurysms (three of 25 aneurysms) and were detected by both observers and with both imaging techniques (Fig. 4A, 4B).


Figure 1
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  		Fig. 1A —27-year-old woman with exacerbated chronic headache. Coronal oblique 2D multiplanar reformation and 3D volume-rendered projections from contrast-enhanced MR angiography (A and C) and CT angiography (B and D) show small 2.3 x 2.1 x 2.4 mm (transverse x anteroposterior x craniocaudal) aneurysm arising from cavernous portion of right internal carotid artery. Aneurysm was detected by both readers on contrast-enhanced MR angiography but was not identified initially on CT angiograms by reader 2. Subsequent analysis of data set revealed that aneurysm (arrow) was clearly present on CT angiograms but was overlooked during initial reading.

 

Figure 2
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  		Fig. 1B —27-year-old woman with exacerbated chronic headache. Coronal oblique 2D multiplanar reformation and 3D volume-rendered projections from contrast-enhanced MR angiography (A and C) and CT angiography (B and D) show small 2.3 x 2.1 x 2.4 mm (transverse x anteroposterior x craniocaudal) aneurysm arising from cavernous portion of right internal carotid artery. Aneurysm was detected by both readers on contrast-enhanced MR angiography but was not identified initially on CT angiograms by reader 2. Subsequent analysis of data set revealed that aneurysm (arrow) was clearly present on CT angiograms but was overlooked during initial reading.

 

Figure 3
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  		Fig. 1C —27-year-old woman with exacerbated chronic headache. Coronal oblique 2D multiplanar reformation and 3D volume-rendered projections from contrast-enhanced MR angiography (A and C) and CT angiography (B and D) show small 2.3 x 2.1 x 2.4 mm (transverse x anteroposterior x craniocaudal) aneurysm arising from cavernous portion of right internal carotid artery. Aneurysm was detected by both readers on contrast-enhanced MR angiography but was not identified initially on CT angiograms by reader 2. Subsequent analysis of data set revealed that aneurysm (arrow) was clearly present on CT angiograms but was overlooked during initial reading.

 

Figure 4
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  		Fig. 1D —27-year-old woman with exacerbated chronic headache. Coronal oblique 2D multiplanar reformation and 3D volume-rendered projections from contrast-enhanced MR angiography (A and C) and CT angiography (B and D) show small 2.3 x 2.1 x 2.4 mm (transverse x anteroposterior x craniocaudal) aneurysm arising from cavernous portion of right internal carotid artery. Aneurysm was detected by both readers on contrast-enhanced MR angiography but was not identified initially on CT angiograms by reader 2. Subsequent analysis of data set revealed that aneurysm (arrow) was clearly present on CT angiograms but was overlooked during initial reading.

 

Figure 5
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  		Fig. 2A —35 year-old woman with history of fibromuscular dysplasia. Coronal maximum-intensity-projection image obtained in 20 seconds at contrast-enhanced MR angiography shows all supraaortic arteries.

 

Figure 6
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  		Fig. 2B —35 year-old woman with history of fibromuscular dysplasia. Coronal (B and D) and sagittal (C and E) maximum-intensity-projection images from contrast-enhanced MR angiography (B and C) and CT angiography (D and E) show three aneurysms including one saccular aneurysm (arrowhead, B and D) at distal right middle cerebral artery (M2 segment), one saccular aneurysm (large arrow, B and D) at left anterior cerebral artery (A2 segment), and one fusiform aneurysm (small arrow, C and E) at tip of basilar artery. CT angiography was performed with 0.35 x 0.35 x 0.8 mm voxels in 17 seconds. MR angiography was performed with 0.7 x 0.7 x 0.8 mm voxels in 20 seconds.

 

Figure 7
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  		Fig. 2C —35 year-old woman with history of fibromuscular dysplasia. Coronal (B and D) and sagittal (C and E) maximum-intensity-projection images from contrast-enhanced MR angiography (B and C) and CT angiography (D and E) show three aneurysms including one saccular aneurysm (arrowhead, B and D) at distal right middle cerebral artery (M2 segment), one saccular aneurysm (large arrow, B and D) at left anterior cerebral artery (A2 segment), and one fusiform aneurysm (small arrow, C and E) at tip of basilar artery. CT angiography was performed with 0.35 x 0.35 x 0.8 mm voxels in 17 seconds. MR angiography was performed with 0.7 x 0.7 x 0.8 mm voxels in 20 seconds.

 

Figure 8
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  		Fig. 2D —35 year-old woman with history of fibromuscular dysplasia. Coronal (B and D) and sagittal (C and E) maximum-intensity-projection images from contrast-enhanced MR angiography (B and C) and CT angiography (D and E) show three aneurysms including one saccular aneurysm (arrowhead, B and D) at distal right middle cerebral artery (M2 segment), one saccular aneurysm (large arrow, B and D) at left anterior cerebral artery (A2 segment), and one fusiform aneurysm (small arrow, C and E) at tip of basilar artery. CT angiography was performed with 0.35 x 0.35 x 0.8 mm voxels in 17 seconds. MR angiography was performed with 0.7 x 0.7 x 0.8 mm voxels in 20 seconds.

 

Figure 9
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  		Fig. 2E —35 year-old woman with history of fibromuscular dysplasia. Coronal (B and D) and sagittal (C and E) maximum-intensity-projection images from contrast-enhanced MR angiography (B and C) and CT angiography (D and E) show three aneurysms including one saccular aneurysm (arrowhead, B and D) at distal right middle cerebral artery (M2 segment), one saccular aneurysm (large arrow, B and D) at left anterior cerebral artery (A2 segment), and one fusiform aneurysm (small arrow, C and E) at tip of basilar artery. CT angiography was performed with 0.35 x 0.35 x 0.8 mm voxels in 17 seconds. MR angiography was performed with 0.7 x 0.7 x 0.8 mm voxels in 20 seconds.

 

Figure 10
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  		Fig. 3A —30-year-old woman with history of headache. Sagittal oblique 2D multiplanar reformation and 3D volume-rendered projections from contrast-enhanced MR angiography (A and C) and CT angiography (CTA) (B and D) show fusiform aneurysm at right posterior Sylvian branches (M3) measuring 4.3 x 4.2 x 9 mm (transverse x anteroposterior x craniocaudal) detected by both reviewers on contrast-enhanced MR angiography and CTA. CTA was performed with 0.35 x 0.35 x 0.8 mm voxels in 17 seconds. MR angiography was acquired with 0.7 x 0.7 x 0.8 mm voxels in 20 seconds.

 

Figure 11
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  		Fig. 3B —30-year-old woman with history of headache. Sagittal oblique 2D multiplanar reformation and 3D volume-rendered projections from contrast-enhanced MR angiography (A and C) and CT angiography (CTA) (B and D) show fusiform aneurysm at right posterior Sylvian branches (M3) measuring 4.3 x 4.2 x 9 mm (transverse x anteroposterior x craniocaudal) detected by both reviewers on contrast-enhanced MR angiography and CTA. CTA was performed with 0.35 x 0.35 x 0.8 mm voxels in 17 seconds. MR angiography was acquired with 0.7 x 0.7 x 0.8 mm voxels in 20 seconds.

 

Figure 12
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  		Fig. 3C —30-year-old woman with history of headache. Sagittal oblique 2D multiplanar reformation and 3D volume-rendered projections from contrast-enhanced MR angiography (A and C) and CT angiography (CTA) (B and D) show fusiform aneurysm at right posterior Sylvian branches (M3) measuring 4.3 x 4.2 x 9 mm (transverse x anteroposterior x craniocaudal) detected by both reviewers on contrast-enhanced MR angiography and CTA. CTA was performed with 0.35 x 0.35 x 0.8 mm voxels in 17 seconds. MR angiography was acquired with 0.7 x 0.7 x 0.8 mm voxels in 20 seconds.

 

Figure 13
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  		Fig. 3D —30-year-old woman with history of headache. Sagittal oblique 2D multiplanar reformation and 3D volume-rendered projections from contrast-enhanced MR angiography (A and C) and CT angiography (CTA) (B and D) show fusiform aneurysm at right posterior Sylvian branches (M3) measuring 4.3 x 4.2 x 9 mm (transverse x anteroposterior x craniocaudal) detected by both reviewers on contrast-enhanced MR angiography and CTA. CTA was performed with 0.35 x 0.35 x 0.8 mm voxels in 17 seconds. MR angiography was acquired with 0.7 x 0.7 x 0.8 mm voxels in 20 seconds.

 

Figure 14
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  		Fig. 4A —60-year-old woman with headache and visual disturbance. Coronal oblique 3D volume-rendered projections from contrast-enhanced MR angiography (A) and CT angiography (B) show small (1.3 x 2.1 mm [anteroposterior x transverse]) saccular aneurysm (arrow) at anterior communicating artery. Three A2 branches are present, one of which arises from aneurysm (arterial incorporation).

 

Figure 15
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  		Fig. 4B —60-year-old woman with headache and visual disturbance. Coronal oblique 3D volume-rendered projections from contrast-enhanced MR angiography (A) and CT angiography (B) show small (1.3 x 2.1 mm [anteroposterior x transverse]) saccular aneurysm (arrow) at anterior communicating artery. Three A2 branches are present, one of which arises from aneurysm (arterial incorporation).

 
Quantitative Analysis
In the evaluation of contrast-enhanced MR angiographic images, the average maximal aneurysm sac and neck diameters were 4.34 mm (range, 1.8–16.2 mm) and 3.33 mm (range, 1.6–7.1 mm), respectively. The average size of three orthogonal diameters of the sac was 4.5 mm (range, 2–16.2 mm) in the transverse, 4.3 mm (range, 1.8–14.1 mm) in the anteroposterior, and 4.6 mm (range, 1.8–12 mm) in the craniocaudal projections. In evaluation of CTA images, the average maximal aneurysm sac and neck diameters were 4.26 mm (range, 1.6–16 mm) and 3.21 mm (range, 1.4–6.8 mm), respectively. The average size of three orthogonal diameters of the sac was 4.1 mm (range, 1.6–16 mm) in the transverse, 4 mm (range, 1.6–13.7 mm) in the anteroposterior, and 4.6 mm (range, 1.7–11.2 mm) in the craniocaudal projections.

There was no statistically significant difference between contrast-enhanced MR angiography and CTA in quantitative measurement of aneurysm size (p = 0.52). The mean aneurysm size difference (bias) between the two techniques was 0.3 mm with a 95% CI of –0.2 to 0.7 mm. Bland-Altman plots showed differences of no more than 1 mm between aneurysm dimension measurements on contrast-enhanced MR angiography and measurements on CTA. There was significant correlation for the dimensional measurements of the neck and all three orthogonal diameters of the sac between CTA and contrast-enhanced MR angiography (r = 0.94; 95% CI, 0.92–0.97) (Figs. 5 and 6). Twelve (48%) of 25 aneurysms were smaller than 4 mm in maximal diameter, 10 (40%) of 25 aneurysms were 4–10 mm in maximal diameter, and only three (12%) of 25 aneurysms were larger than 10 mm in maximal diameter. Seventeen (68%) of 25 aneurysms were supraclinoid, and eight (32%) of 25 aneurysms were infraclinoid as found on both contrast-enhanced MR angiography and CTA. The location and size distribution of the aneurysms are shown in Table 1.


Figure 16
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  		Fig. 5 —Scatterplot shows significant correlation (r = 0.94; 95% CI, 0.92–0.97) for aneurysm dimension measurements between CT angiography and contrast-enhanced MR angiography.

 

Figure 17
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  		Fig. 6 —Bland-Altman plot shows differences of no more than 1 mm between aneurysm dimension measurements on contrast-enhanced MR angiography and measurements on CT angiography.

 

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  		TABLE 1: Aneurysm Size and Location

 


Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
Although digital subtraction angiography is the standard of reference for the evaluation of intracranial aneurysm, rapid evolution of and technical advances in minimally invasive cross-sectional imaging techniques such CTA and MR angiography have changed the diagnostic approach to screening and initial evaluation of patients with suspected intracranial aneurysms. For therapeutic decision making, accurate assessment of the presence of an aneurysm as small as 1–3 mm and comprehensive visualization of the aneurysm location, orientation, size, morphologic features, and relation to the parent vessels are crucial. For these reasons, high diagnostic image quality with high spatial resolution (submillimeter voxel sizes) are needed [26].

With advances in MDCT technology and high-performance CT scanners, CTA has been established as a valid noninvasive imaging technique for screening and characterization of intracranial aneurysms [27–29]. Contrast-enhanced MR angiography also has gained wide acceptance for evaluation of intracranial aneurysms [14–17]. Because of its speed, lack of sensitivity to flow-related artifacts (spin saturation and phase dispersion), and capability of covering a large field of view, contrast-enhanced MR angiography has inherent advantages over time-of-flight MR angiography [30, 31] and is a practical alternative to CTA. The lack of ionizing radiation and use of a contrast agent with lesser potential for nephrotoxicity are desirable attributes of contrast-enhanced MR angiography. However, the competing requirements for extended coverage and fast acquisition speeds have often forced a compromise in spatial resolution relative to that of MDCTA. A relatively short arteriovenous transit time of the intracranial circulation and the need to image the entire carotid circulation at once while preventing venous overlay have further challenged contrast-enhanced MR angiography protocols.

Recent advances in MRI techniques, such as refinement of k-space trajectories [32–34] and use of parallel acquisition techniques [18–20], have substantially improved the performance of contrast-enhanced MR angiography. Several investigators [35–37] advocate specific k-space trajectories for contrast-enhanced MR angiography, including elliptic–centric reordering and radial k-space acquisition. These techniques facilitate acquisition of the central k-space points during maximal contrast enhancement, resulting in reliable venous suppression.

Parallel imaging [18–20] is a powerful approach to resolving the divergent demands of high spatial resolution and extended coverage in a short acquisition time. With these techniques, the spatial distribution of signals from multiple component coil elements are used in a radiofrequency coil array to substitute for some phase-encoding gradient steps, reducing the minimum imaging time by a factor termed the acceleration factor. Decreases in minimum imaging time can be used to improve spatial and temporal resolution and increase the number of acquired slices, improving coverage. However, increases in acquisition speed are associated with SNR penalty, mainly because of the degree of k-space undersampling and coil array geometry (g-factor) [19]. The SNR penalty generally limits acceleration to a factor of not more than two in clinical applications at 1.5 T.

To mitigate the limitations, various strategies have been used to minimize SNR deterioration. These strategies include raising the baseline SNR by use of stronger magnets [21, 38] and minimizing noise amplification by use of array coils with multiple channels and improved geometric and sensitivity profiles [39, 40]. In this study, we found that a higher SNR boost and a higher sensitivity to injected gadolinium [41] associated with 3-T imaging combined with a multichannel array coil can effectively support highly acceleration parallel acquisition techniques (generalized autocalibrating partially parallel acquisition; acceleration factor, 4), resulting in improved spatial resolution and coverage without prolonging acquisition time. Our results show that high-spatial-resolution contrast-enhanced MR angiography was reliable for evaluation and characterization of intracranial aneurysms. The ability to screen for associated arch and great vessel disease and having a roadmap for intervention are additional advantages to an extended field of view [42]. The intracranial aneurysms in our study population were identified and characterized with a diagnostic performance equal or superior to that of MDCTA. This finding is reflected in our high interobserver agreement and favorable results of qualitative and quantitative comparative analysis. Although one aneurysm of the right cavernous internal carotid artery was not identified on CTA by reader 2 during the first reading session, subsequent analysis of the data revealed the aneurysm was clearly present on the 2D and 3D CT angiograms (Fig. 1A, 1B, 1C, 1D) but was overlooked during the initial reading because of its proximity to bone and because of partial volume averaging, a potential limitation of CTA [43].

Our study was limited by the lack of conventional catheter angiography as the reference standard. We therefore were unable to evaluate the diagnostic accuracy of the described techniques. However, at our institution (as at many others), conventional angiography is no longer routinely performed on patients with suspected intracranial aneurysms. Therefore, the accuracy of noninvasive cross-sectional techniques, such as contrast-enhanced MR angiography and CTA, in a large patient cohort remains undetermined. Other limitations of this study included a comparatively small sample size and possible selection bias introduced by the referring physician, which likely increased the apparent incidence of aneurysms in this population. Furthermore, contrast-enhanced MR angiography at 3 T is more technically demanding than MDCTA. However, as centers gain more experience, the technical challenges will be outweighed by the ability to avoid repeated radiation exposure during longitudinal monitoring with CTA.

Contrast-enhanced MR angiography at 3 T can be used to reliably evaluate intracranial aneurysms with high correlation with the findings on concurrent CTA studies. Integration of highly accelerated parallel acquisition at 3 T has similar spatial resolution, coverage, and speed to those of MDCTA without the known limitations of CT angiography. With attention to detail and quality control, the diagnostic paradigm for detecting and monitoring intracranial aneurysms may shift from CTA to contrast-enhanced MR angiography. In this context, our results suggest that contrast-enhanced MR angiography at 3 T may be the single most accurate and most informative diagnostic study. Further clinical studies and comparison with catheter angiography will be helpful for determining the diagnostic accuracy and clinical applications of the described technique in a broader clinical setting.


Acknowledgments
 
We thank Ali Nael for contributing to the statistical analysis.


References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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