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Technical Developments |
1 From the Department of Radiology, University of Bonn, Sigmund-Freud-Strasse 25, D-53105 Bonn, Germany (W.A.W., M.B., B.S., H. J. Tschampa, C.K., H.U., H. J. Textor, H.H.S.); and Philips Medical Systems, Best, the Netherlands (J.G.). Received June 27, 2002; revision requested August 23; final revision received March 15, 2003; accepted March 20. Address correspondence to W.A.W. (e-mail: willinek@uni-bonn.de).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and Methods Results DiscussionStatistical Consultant...REFERENCES |
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© RSNA, 2003
Index terms: Brain, MR, 17.12142 • Magnetic resonance (MR), vascular studies, 17.12142
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and Methods Results DiscussionStatistical Consultant...REFERENCES |
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Three-dimensional (3D) time-of-flight (TOF) MR angiography remains a first-line diagnostic tool in MR examination of intracranial steno-occlusive disease (6,7), as well as in the noninvasive assessment of intracranial aneurysms (8–15). The purpose of our study was to evaluate standard-resolution and high-spatial-resolution 3D TOF MR angiography at 3.0-T imaging and compare it with standard-resolution TOF MR angiography at 1.5-T imaging.
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Materials and Methods |
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TOPABSTRACTINTRODUCTIONMaterials and Methods Results DiscussionStatistical Consultant...REFERENCES |
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MR Angiography
Studies were performed with 1.5- and 3.0-T MR units (Intera; Philips Medical Systems, Best, the Netherlands) that were both equipped with a strong gradient system (maximal gradient amplitude, 30 mT; slew rate, 150 T/m/sec). Receive and transmit-receive birdcage head coils were used for imaging at 1.5 and 3.0-T imaging, respectively.
Our standard intracranial 1.5-T 3D TOF protocol was performed in all subjects with the following parameters: repetition time of 28 msec and echo time of 6.9 msec, and a tilted optimized nonsaturating excitation pulse was used with a central flip angle of 20°. Section thickness of the 50 partitions was 2 mm (1-mm overlap). Acquisition parameters are listed in Table 1. A field of view of 160 x 160 mm was used with a matrix of 336 x 212, which yielded a voxel size of 0.48 x 0.75 x 2.00 mm (volume, 0.92 mm3). Effective voxel volume was 0.31 x 0.31 x 1.00 mm. Acquisition time was 2 minutes 34 seconds.
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The higher signal intensity with the higher field strength enabled the use of TOF MR angiography with higher spatial resolution at 3.0-T imaging. Acquisition parameters of high-spatial-resolution TOF MR angiography at 3.0-T imaging were repetition time of 26 msec and echo time of 3.4 msec, and a tilted optimized nonsaturating excitation pulse was used with a central flip angle of 20°. Section thickness of the 100 partitions was 1 mm (0.5-mm overlap). A field of view of 250 x 250 mm was used with a matrix of 832 x 571, which yielded a non–zero-filled voxel size of 0.30 x 0.44 x 1.00 mm (volume, 0.13 mm3). Effective voxel volume was 0.24 x 0.24 x 0.50 mm (volume, 0.029 mm3). Acquisition time was 7 minutes 57 seconds (Table 1).
Image Analysis
Intracranial TOF MR angiograms were independently graded by two radiologists (H.J. Textor, H.U.) on coronal, transverse, and sagittal maximum intensity projections according to diagnostic image quality, depiction of small vessel segments, and the presence or absence of artifacts. The readers were blinded to the acquisition technique and the field strength. Scores for the presence or absence of artifacts were the following: 5 = absent, 4 = minimal, 3 = moderate, 2 = interfering with image interpretation, 1 = resulting in nondiagnostic study.
Scores for image quality were the following: 5 = excellent (no artifacts present and all vessels in field of view depicted with same quality, which is close to that at digital subtraction angiography), 4 = more than adequate for diagnosis (minor artifacts present or all vessels in field of view clearly visualized but image quality somewhat reduced compared with that at digital subtraction angiography), 3 = adequate for diagnosis (minor artifacts present and image quality somewhat reduced compared with that at digital subtraction angiography but still sufficient for diagnosis of vascular abnormalities), 2 = questionable for diagnosis (image quality impaired by artifacts and vessel tree poorly visualized so diagnostic value of images is questionable for depiction of vascular abnormalities), 1 = nondiagnostic (image quality heavily impaired by artifacts and readers not able to assess vessels in field of view).
The following vessel segments were evaluated: (a) M1 segment of middle cerebral artery (MCA) (defined as vessel segment from intracranial internal carotid artery bifurcation to main division of the MCA), (b) M2 segment of MCA (defined as vessel segment from main division to circular sulcus of insula), (c) M3 segment of MCA (defined as vessel segment from circular sulcus of insula to opercular turn of MCA branches), (d) lenticulostriate arteries of MCA, (e) A1 segment of anterior cerebral artery (defined as vessel segment from intracranial internal carotid artery bifurcation to anterior communicating artery), (f) A2 segment (defined as vessel segment from anterior communicating artery to junction of rostrum and genu of corpus callosum, (g) A3 segment (defined as vessel segment around genu of corpus callosum), (h) P1 segment of posterior cerebral artery (defined as vessel segment from basilar artery bifurcation to posterior communicating artery), (i) P2 segment (defined as vessel segment from posterior communicating artery to back of midbrain), and (j) P3 segment (defined from back of midbrain to division into posterior temporal, calcarine, and parieto-occipital arteries).
Scores for depiction of small vessel segments (first-, second-, and third-order segments of the anterior, middle, and posterior cerebral arteries) were the following: 3 = excellent depiction of vessel segment (small vessel segment clearly visualized and vessel-tissue contrast appears high), 2 = vessel segment visible (visualization of small vessel segment adequate for diagnosis but vessel-tissue contrast somewhat reduced), 1 = vessel segment scarcely visible (vessel depiction not adequate for diagnosis), and 0 = vessel segment not visible (small vessel segments not visualized).
After independent interpretations were performed, discrepancies between the two readers in the scoring of the vessel depiction were resolved by consensus to establish a final score. In addition, the same two readers reviewed by consensus the MR angiograms in the eight patients with regard to vascular abnormalities. In a direct comparison, visualization of vascular disease was rated from 0 to 3: 3 = excellent depiction of vascular disease, full level of confidence; 2 = vascular disease visible, adequate for diagnosis; 1 = vascular disease scarcely visible, questionable for diagnosis; 0 = vascular disease not visible.
For quantitative evaluation, the SNR and contrast-to-noise ratio (CNR) were calculated from the source images for each patient by using the largest region of interest at the basilar artery and the same region-of-interest size at the brainstem. To ensure consistency, all regions of interest were placed by the same author (B.S.). The SNR was determined as the mean value of signal intensity in the enhanced arterial lumen divided by the SD of the signal intensity in the noise or ghosting free background. The CNR was defined as the mean difference between the signal intensity in the arterial lumen and that in the surrounding soft tissue (brainstem) divided by the SD of the signal intensity in the background. Mean SNR and CNR of the specific pulse sequences in relation to the field strength were calculated.
Statistical Analysis
For statistical evaluation, scores for image quality and presence or absence of artifacts were compared by means of a Wilcoxon matched pairs test. A general linear model approach was performed on the data to test for statistical independence wtih different vessel segments in the same patient as "repeated measurements." To account for the fact that vessel segments are nested in patients, a multivariate analysis was performed. In this analysis, the TOF MR angiography method was used as the fixed factor and the patients as covariates. A two-sided paired t test was calculated with the SNR and CNR data. To evaluate the level of interobserver agreement of scores of image quality and image degradation by artifacts, a Kendall W test was performed. Values between 0.5 and 0.8 were considered to suggest good agreement, and values greater than 0.8 were considered to indicate excellent agreement. A P value less than .05 was considered to indicate a statistically significant difference (SPSS, version 10.0; SPSS, Chicago, Ill).
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Results |
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TOPABSTRACTINTRODUCTIONMaterials and Methods Results DiscussionStatistical Consultant...REFERENCES |
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Vascular Abnormalities
In five of the eight patients, five vascular abnormalities were detected: MCA aneurysm in one patient and vessel obstruction or stenosis in four patients. High-spatial-resolution 3.0-T imaging provided the best delineation of vascular disease (score of 3 for all abnormalities), followed by standard 3.0-T imaging (score of 3 for one abnormality, score of 2 for three abnormalities, and score of 0 for one abnormality), and standard 1.5-T imaging (score of 2 for two abnormalities, score of 1 for one abnormality, and score of 0 for two abnormalities), as listed in Table 3.
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Discussion |
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TOPABSTRACTINTRODUCTIONMaterials and Methods Results DiscussionStatistical Consultant...REFERENCES |
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Spatial resolution at 3D TOF MR angiography is still limited at 1.5 T, however, and peripheral segments of the main cerebral arteries, which are potential causes of infarction or potential locations of intracranial aneurysms, can often not be sufficiently visualized. Therefore, noninvasive 3D TOF MR angiography with standard spatial resolution may not be an alternative to intraarterial digital subtraction angiography in many cases.
Further increases in spatial resolution at 1.5-T imaging would be associated with lower SNR and would simultaneously impair image quality; therefore TOF MR angiography protocols have to compromise the diverging demands of high spatial resolution and limited SNR. The hypothesis of this study was that the better SNR at higher magnetic field strengths might be an important benefit to further increase spatial resolution at 3D TOF MR angiography and would improve visualization of small vessel segments and vascular disease.
To prove this, we performed an intraindividual comparative study. First, we evaluated the effect of higher magnetic field strength at 3D TOF MR angiography with a 3.0-T protocol that was based on our standard 1.5-T pulse sequence for 3D TOF MR angiography, with identical spatial resolution. Second, we investigated the use of high magnetic field strength for high-spatial-resolution 3D TOF MR angiography. In the high-spatial-resolution protocol at 3.0-T imaging, image matrix was 832 x 571 to achieve a true voxel size of 0.30 x 0.44 x 1.00 mm (volume, 0.13 mm3).
The 3.0-T studies were successfully completed in all cases (15 subjects), and diagnostic image quality was obtained at standard 1.5-T imaging and standard and high-spatial-resolution 3.0-T imaging. For the standard 3.0-T imaging, SNR was almost double that at standard 1.5-T imaging. These results are consistent with phantom and in vivo measurements (1). By increasing spatial resolution by a factor of 7.1 at high-spatial-resolution 3.0-T imaging (increasing voxel size from 0.92 to 0.13 mm3), the absolute signal gain at 3.0-T imaging was partially compensated. Comparison of the mean SNR at standard 3.0-T imaging with that at high-spatial-resolution 3.0-T imaging yielded 7.3 as the factor of difference. Similar results were observed for the mean CNR. The absolute mean CNR at 3.0-T imaging was partially compensated by the smaller voxel size at high-spatial-resolution 3.0-T imaging compared with that at standard 3.0-T imaging. The mean CNR at high-spatial-resolution 3.0-T imaging compared with that at standard 3.0-T imaging was somewhat higher than expected and differed by a factor of 4.2 instead of 7.3. This finding might be explained in part by partial volume effects that have occurred as a result of measuring the contrast on the source images with the larger voxels.
Like the SNR, the CNR at standard 3.0-T imaging was approximately twice that at standard 1.5-T imaging (a factor of 2.1). For direct CNR and SNR comparison, it is important to take into account that T1 is prolonged at the higher magnetic field strength (by approximately 30%, from 1.5- to 3.0-T imaging). Theoretically, this should result in a higher effective CNR at 3.0-T imaging than is expected by doubling the field strength. This was observed in our study: When the CNR at standard 3.0-T imaging was compared with that at standard 1.5-T imaging, the values differed by a factor of 2.1 (instead of 2.0). The 15% (0.3/2.0) higher mean CNR might be attributable to the superior vessel-tissue contrast at 3.0-T imaging as a result of the prolonged T1 compared with that at 1.5-T imaging.
Consequently, the image quality at standard 3.0-T imaging was graded as significantly better than that at standard 1.5-T imaging. In a comparison of the 3.0-T protocols, image quality at high-spatial-resolution 3.0-T imaging significantly exceeded that at standard 3.0-T imaging. This improvement in image quality is consistent with initial experiences reported by Bernstein et al (1) and Al-Kwifi et al (4). Theoretically, artifacts, especially flow artifacts, can be pronounced at higher magnetic field strengths (1) mainly as a result of the increase in artifact-to-noise ratio, which corresponds to the increase in SNR, and susceptibility. However, we did not experience substantial artifacts at standard and high-spatial-resolution 3.0-T imaging. The reason for this might be the shorter out-of-phase echo times that were used at 3.0-T imaging as a result of doubling of the water-fat chemical shift.
The better image quality and higher vessel-tissue contrast at standard and high-spatial-resolution 3.0-T imaging allowed better depiction of small vessel segments, including M3, P3, A2/3, and lenticulostriate arteries. However, not all arteries, especially perforating arteries, that are usually visible at digital subtraction angiography were visualized at 3D TOF MR angiography. With respect to vessel depiction, the underlying assumption for the statistical analysis was that the segments are independent. The null hypothesis of independence was not rejected by using a linear model approach for all vessel segments (ie, M1, M2, M3; P1, P2, P3; A1, A2/3) except the lenticulostriate arteries. This might be explained by the fact that these very small lenticulostriate arteries can only be visualized if a certain degree of image quality is provided that inherently allows better visualization of other vessel segments.
Not only the depiction of small vessel segments was better at standard and high-spatial-resolution 3.0-T imaging. In addition, detection of disease was more reliable with higher spatial resolution: Faint opacification of the right MCA in a patient with moyamoya disease was misinterpreted as totally obstructed at both standard 1.5-T imaging and standard 3.0-T imaging but not at high-spatial-resolution 3.0-T imaging. The latter also provided the clearest depiction and sharpest margins of other vascular disease, such as MCA stenosis and a 6-mm MCA aneurysm; thus, it was the most effective technique for clinical 3D TOF MR angiography in our study population.
Findings in various studies show the effectiveness of 3D TOF MR angiography for the detection of intracranial aneurysms. It is the detection of small asymptomatic aneurysms between 3 and 5 mm that represents a real clinical concern. As previously mentioned, we had only one case of an intracranial aneurysm in our small study group, so no final conclusions can be drawn in terms of sensitivity. Delineation of this aneurysm at standard and high-spatial-resolution 3.0-T imaging, however, noticeably exceeded that at standard 1.5-T imaging. Further studies are needed to prove whether high-spatial-resolution 3D TOF MR angiography at 3.0-T imaging will yield improved sensitivity, specificity, and diagnostic accuracy for the detection of intracranial cerebrovascular disease.
In recent studies, voxel sizes for 3D TOF MR angiography range between 0.31 and 1.15 mm3, not including zero filling (1,4,5,16). In one volunteer study, voxel volume of 0.14 mm3 is reported (4). Spatial resolution for high-spatial-resolution 3.0-T imaging in the current study was 0.30 x 0.44 x 1.00 mm (volume, 0.13 mm3) without zero filling. Section thickness was 0.50 mm with overlapping sections, which yielded an effective voxel volume of 0.24 x 0.24 x 0.50 mm (volume, 0.029 mm3). To our knowledge, clinical evaluation of intracranial 3D TOF MR angiography at this high spatial resolution has not been reported previously. We speculate that with the high-spatial-resolution protocol used in the current study to depict aneurysms with critical size (<5 mm) and the diagnostic accuracy at high-spatial-resolution 3.0-T imaging will be superior to that at standard 1.5-T and standard 3.0-T imaging.
Total acquisition time at high-spatial-resolution 3D TOF MR angiography at 3.0-T imaging was approximately 8 minutes, which was considered the limit for tolerable acquisition time. We did not further increase the spatial resolution at high-spatial-resolution 3.0-T imaging; in our experience, longer acquisition times are associated with poor image quality because of the increasing risk of patient movements. In the future, high-spatial-resolution 3.0-T imaging will certainly benefit from the use of parallel imaging techniques such as sensitivity encoding to reduce the acquisition time while maintaining the high spatial resolution (17).
These initial results at intracranial 3D TOF MR angiography at 3.0-T imaging are promising. The higher spatial resolution allowed better visualization of small vessel segments and vascular disease, including an MCA aneurysm and intracranial stenoses and obstruction. Even more important for clinical evaluation might be the finding that an MCA stenosis in the patient with moyamoya disease was misinterpreted at both standard 1.5-T imaging and standard 3.0-T imaging but was correctly diagnosed at high-spatial-resolution 3.0-T imaging. In our experience, 3D TOF MR angiography of the circle of Willis at high magnetic field strength offers significantly improved image quality. The signal gain at 3.0-T imaging allows use of higher spatial resolution at 3D TOF MR angiography, which yields a substantial diagnostic improvement in studies of cerebrovascular disease. We conclude that intracranial high-spatial-resolution 3D TOF MR angiography at 3.0-T imaging may well substitute for standard 3D TOF MR angiography at 1.5-T imaging in clinical routine and may further reduce the need for invasive diagnostic studies.
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Statistical Consultant Commentary |
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TOPABSTRACTINTRODUCTIONMaterials and Methods Results DiscussionStatistical Consultant...REFERENCES |
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Author contributions: Guarantors of integrity of entire study, W.A.W., H. J. Textor, H.H.S.; study concepts, W.A.W., J.G., H. J. Textor, H.H.S.; study design, W.A.W., H. J. Textor, H.U.; literature research, W.A.W., B.S.; clinical studies, W.A.W., M.B., H. J. Tschampa, C.K.; data acquisition, W.A.W., M.B., H. J. Tschampa, C.K.; data analysis/interpretation, W.A.W., B.S., H.U., H. J. Textor; statistical analysis, W.A.W.; manuscript preparation, W.A.W.; manuscript definition of intellectual content, W.A.W., J.G., H.U., H. J. Textor; manuscript editing, W.A.W., M.B., C.K., H.U.; manuscript revision/review, W.A.W., J.G., H. J. Tschampa, H. J. Textor, H.H.S.; manuscript final version approval, all authors
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TOPABSTRACTINTRODUCTIONMaterials and Methods Results DiscussionStatistical Consultant...REFERENCES |
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