Rationale and Objectives
Geometrical distortion is a well-known problem in structural magnetic resonance imaging (MRI), leading to pixel shifts with variations up to several millimeters. Because the main factors of geometrical distortion are proportional to B 0 , MRI spatial encoding distortions tend to increase with higher magnetic field strength. With the increasing prospects of utilizing ultra-high-field MRI (B 0 ≥ 7 Tesla) for neuroimaging and subsequently for image-guided neurosurgical therapy, the evaluation and correction of geometrical distortions occurring in ultra-high-field MRI are essential preconditions for the integration of these data. Hence, we conducted a phantom study to determine hardware-related geometrical distortion in clinically relevant sequences for structural imaging at 7 T MRI and compared the findings to 1.5 T MRI.
Material and Methods
Hardware-related geometrical distortion was evaluated using a MRI phantom (Elekta, Sweden). Both applied scanner systems (Magnetom Avanto 1.5 T and Magnetom 7 T, Siemens Healthcare, Erlangen, Germany) were equipped with similar gradient coils capable of delivering 45 mT/m of maximum amplitude and a slew rate of 220 mT/m/ms. Distortion analysis was performed for various clinically relevant gradient echo and spin echo sequences.
Results
Overall, we found very low mean geometrical distortions at both 7 T and 1.5 T, although single values of up to 1.6 mm were detected. No major differences in mean distortion between the sequences could be found, except significantly higher distortions in turbo spin-echo sequences at 7 T, mainly caused by B 1 inhomogeneities.
Conclusion
Hardware-related geometrical distortions at 7 T MRI are relatively small, which may be acceptable for image coregistration or for direct tissue-targeting procedures. Using a subject-specific correction of object-related distortions, an integration of 7 T MRI data into image-guided applications may be feasible.
The incorporation of magnetic resonance imaging (MRI) data into various neurosurgical procedures has become common clinical routine and is continuously expanding. This accounts in particular for their application in functional and stereotactic neurosurgery. Especially in regard to deep brain stimulation, the growing importance of MRI (eg, for structural targeting) has recently been emphasized . Another important field is image-guided radiosurgery .
Although the application of 3 Tesla (T) field-strength MRI in clinical routine is steadily increasing, higher field-strength systems, specifically 7 T scanners, are still primarily used for scientific or preclinical applications. First experiences from technical studies and single series of volunteers and patients show promising results for both structural and functional imaging . In particular, the delineation of deep brain structures using susceptibility-weighted imaging (SWI) or T2∗-weighted imaging (eg, for deep brain stimulation procedures) has already been shown to be improved at 7 T field strength. The high-resolution structural depiction possible at higher field strengths within reasonable scan time may be valuable for neuronavigated surgery as well. Several exemplary images can be found in Figures 1 and 2 .
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Figure 1
(a–d) Exemplary axial slices of a high-resolution susceptibility-weighted imaging (SWI) sequence at 7 T covering the deep-brain nuclei, starting from a caudal slice. Image contrast at 7 T is mainly generated by the differing tissue T2∗ times, resulting in magnitude and phase differences. 1: subthalamic nucleus; 2: red nucleus, 3: superior colliculus; 4: medial geniculate nucleus; 5: globus pallidus, internal segment; 6: globus pallidus, medial medullary lamina; 7: globus pallidus, external segment; 8: striatum; 9: basal vein; 10: substantia nigra; 11: lateral geniculate nucleus; 12: third ventricle; 13: internal capsule; 14: ventral anterior nucleus; 15: ventral lateral nucleus; 16: dorsomedial nucleus; 17: ventral posterior nucleus; 18: pulvinar nucleus; 19: choroid plexus; 20: globus pallidus; 21: internal cerebral veins.
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Figure 2
Exemplary sagittal images of a clivus chordoma from a contrast-enhanced T1-weighted spin echo (SE) sequence using routine clinical parameters at 1.5 T (a,c) and corresponding slices from a contrast-enhanced T1-weighted fast low-angle shot (FLASH) sequence at 7 T (for sequence parameters, see Table 1 ). Note the clearer depiction of the lesion because of the higher spatial resolution and the sharp contrast differences within the clivus, suggestive of tumor infiltration ( white arrow ).
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Materials and methods
Phantom Descriptions
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CT and MRI Measurements
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Table 1
Scan Parameter Details of the Various Sequences Performed at 1.5 and 7 T
1.5 Tesla MPRAGE FLASH T2 TSE SWI 7 Tesla MPRAGE FLASH T2 TSE SWI Repetition time (ms) 2180 4.77 4570 49 3500 5 4520 29 Echo time (ms) 3.09 1.92 146 40 2.49 2.04 112 15.2 Inversion time (ms) 1100 1100 Nominal flip angle (°) 10 12 150 15 10 12 125 25 Field of view (mm 2 ;) 256 256 220 256 256 256 220 256 Acquisition matrix (pixel) 256 × 256 256 × 256 256 × 512 256 × 256 256 × 256 384 × 384 256 × 512 896 × 672 Bandwidth (Hz/pixel) 130 190 70 75 600 725 350 280 Echo train length (n) 1 1 13 1 1 1 13 1 Slices (n) 240 224 80 60 256 256 86 88 Slice thickness (mm) 1 1 1.5 2 1 0.7 1.5 2 Phase-encoding direction AP L-R AP AP AP L-R AP AP Frequency-encoding direction L-R AP L-R L-R L-R AP L-R L-R Slice-selection direction H-F H-F H-F H-F H-F H-F H-F H-F
AP, anteroposterior; FLASH, fast low-angle shot; H-F, head-feet direction; MPRAGE, magnetization prepared rapid gradient echo; R-L, right-left direction; SWI, susceptibility-weighted imaging; T2 TSE, T2-weighted turbo spin echo.
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Image Postprocessing
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Distortion Correction
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Results
Analysis of Distortions
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Table 2
Summary of Positional Deviations at 1.5 T (A) and 7 T (B) in all Three Directions (∆ x ∆ y ∆ z )
A ∆x (mm) ∆y (mm) ∆z (mm) ∆ xyz (mm) Acquisition DC Range Mean SD Range Mean SD Range Mean SD Mean SD 1.5 T MPRAGE NA 0–1.3 0.4 ±0.3 0–0.8 0.3 ±0.2 0–0.7 0.2 ±0.2 0.6 ±0.1 MPRAGE 2D 0–1.0 0.4 ±0.3 0–0.8 0.3 ±0.1 0–0.6 0.2 ±0.1 0.6 ±0.1 MPRAGE 3D 0–1.1 0.4 ±0.2 0–0.9 0.3 ±0.2 0–0.6 0.3 ±0.2 0.6 ±0.1 FLASH NA 0–0.9 0.3 ±0.2 0–1.2 0.4 ±0.2 0–0.8 0.3 ±0.3 0.6 ±0.1 FLASH 2D 0–0.6 0.3 ±0.2 0–0.9 0.4 ±0.2 0–0.6 0.2 ±0.2 0.5 ±0.1 FLASH 3D 0–0.7 0.3 ±0.2 0–0.9 0.3 ±0.3 0–0.6 0.2 ±0.1 0.5 ±0.2 T2 NA 0–0.9 0.4 ±0.3 0–0.8 0.3 ±0.2 0–0.6 0.2 ±0.1 0.6 ±0.1 T2 2D 0–0.8 0.4 ±0.2 0–0.8 0.3 ±0.2 0–0.5 0.2 ±0.2 0.6 ±0.1 SWI NA 0–0.7 0.3 ±0.2 0–0.6 0.3 ±0.2 0–0.7 0.3 ±0.2 0.5 ±0.2 SWI 2D 0–0.6 0.3 ±0.1 0–0.5 0.3 ±0.3 0–0.7 0.3 ±0.2 0.5 ±0.1 SWI 3D 0–0.7 0.3 ±0.2 0–0.5 0.3 ±0.2 0–0.5 0.2 ±0.1 0.5 ±0.1
B ∆x (mm) ∆y (mm) ∆z (mm) ∆ xyz (mm) Acquisition DC Range Mean SD Range Mean SD Range Mean SD Mean SD 7 T MPRAGE NA 0–1.3 0.4 ±0.1 0–0.9 0.4 ±0.2 0–1.1 0.2 ±0.1 0.6 ±0.1 MPRAGE 2D 0–1.1 0.4 ±0.2 0–0.8 0.3 ±0.1 0–0.7 0.2 ±0.2 0.6 ±0.1 MPRAGE 3D 0–1.2 0.4 ±0.2 0–0.8 0.3 ±0.1 0–0.7 0.2 ±0.1 0.5 ±0.1 FLASH NA 0–0.5 0.2 ±0.1 0–0.8 0.3 ±0.2 0–0.4 0.1 ±0.1 0.5 ±0.1 FLASH 2D 0–0.5 0.3 ±0.1 0–0.8 0.3 ±0.2 0–0.6 0.2 ±0.2 0.5 ±0.1 FLASH 3D 0–0.5 0.2 ±0.1 0–0.7 0.3 ±0.2 0–0.6 0.2 ±0.1 0.5 ±0.1 T2 NA 0–1.2 0.5 ±0.3 0–1.6 0.6 ±0.4 0.2–1.4 0.6 ±0.3 1.0 ±0.2 T2 2D 0–1.3 0.5 ±0.2 0–1.5 0.6 ±0.3 0–1.4 0.7 ±0.2 1.1 ±0.2 SWI NA 0–0.5 0.2 ±0.1 0–0.6 0.3 ±0.2 0–0.8 0.3 ±0.2 0.5 ±0.1 SWI 2D 0–0.6 0.2 ±0.2 0–0.8 0.2 ±0.2 0–0.6 0.3 ±0.1 0.5 ±0.1 SWI 3D 0–0.4 0.2 ±0.1 0–1.0 0.3 ±0.1 0–0.7 0.3 ±0.2 0.5 ±0.1
∆ x , right-left; ∆ y , anteroposterior; ∆ z , head-feet; 2D, two-dimensional; 3D, three-dimensional; DC, distortion correction; FLASH, fast low-angle shot; MPRAGE, magnetization prepared rapid gradient echo; NA, with manufacturer-provided DC; SD, standard deviation; SWI, susceptibility-weighted imaging.
Sequences were acquired without (NA) or with the manufacturer-provided DC in 2D or 3D, where applicable. Values are given as range and mean ± SD. Overall deviation is given as ∆ x,y,z . All values are given in millimeters, rounded to the first decimal place.
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Discussion
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Conclusion
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