3D CRANI, a novel MR neurography sequence, can reliable visualise the extraforaminal cranial and occipital nerves

Objectives We aim to validate 3D CRANI, a novel high-field STIR TSE, MR neurography sequence in the visualisation of the extraforaminal cranial and occipital nerve branches on a 3-T system. Furthermore, we wish to evaluate the role of gadolinium administration and calculate nerve benchmark values for future reference. Methods Eleven consecutive patients underwent MR imaging including the 3D CRANI sequence before and immediately after intravenous gadolinium administration. Two observers rated suppression quality and nerve visualisation using Likert scales before and after contrast administration. Extraforaminal cranial and occipital nerves were assessed. Nerve calibers and signal intensities were measured at predefined anatomical landmarks, and apparent signal intensity ratios were calculated. Results The assessed segments of the cranial and occipital nerves could be identified in most cases. The overall intrarater agreement was 79.2% and interrater agreement was 82.7% (intrarater κ = .561, p < .0001; interrater κ = .642, p < .0001). After contrast administration, this significantly improved to an intrarater agreement of 92.7% and interrater agreement of 93.6% (intrarater κ = .688, p < .0001; interrater κ = .727, p < .0001). Contrast administration improved suppression quality and significant changes in nerve caliber and signal intensity measurements. Nerve diameter and signal intensity benchmarking values were obtained. Conclusion 3D CRANI is reliable for the visualization of the extraforaminal cranial and occipital nerves. Intravenous gadolinium significantly improves MR neurography when applying this sequence. Benchmarking data are published to allow future assessment of the 3D CRANI sequence in patients with pathology of the extraforaminal cranial and occipital nerves. Key Points • MR neurography using the 3D CRANI sequence is a reliable method to evaluate the extraforaminal cranial and occipital nerves. • Gadolinium contrast administration significantly improves suppression quality and nerve visualisation. • Benchmarking values including apparent signal intensity ratios and nerve calibers depend on contrast administration and might play an important role in future studies evaluating extraforaminal cranial and occipital neuropathies. Supplementary Information The online version contains supplementary material available at 10.1007/s00330-022-09269-2.


3D
Three dimensional aNMCNR Apparent nerve-muscle contrast-to-noise ratio ANOVA

Introduction
MR neurography (MRN) in the head and neck region is attracting increasing attention in the literature [1]. This novel MRI technique already showed promise to diagnose peripheral and trigeminal neuropathies [2][3][4]. MRN may localize the neuropathy and even grade the severity of these neuropathies [5]. The obtained information can be useful in diagnosing and treatment planning of patients with neuropathies. Given the recent introduction of MRN in the head and neck area, only a limited number of validation studies are available. The studies by Chabbra and by Burian illustrated the feasibility of MR neurography of the mandibular nerve and its terminal branches [6,7]. But no studies are available that validate MR neurography for all extraforaminal cranial or occipital nerves. The purpose of this study was to validate the use of the previously published 3D CRANI (CRAnial Nerve Imaging) [8], a novel high-field STIR TSE, sequence in extraforaminal cranial and occipital nerve visualisation on a 3-T system. Secondary aims were to assess the role of gadolinium administration on imaging quality and to obtain benchmarking values of signal intensities, apparent signalto-noise (aSNR) and apparent nerve-muscle contrast-to-noise ratios (aNMCNR), and nerve diameters for the evaluated nerve branches.

Subjects
This study was conducted according to the Guidelines for Reporting Reliability and Agreement Studies (GRASS) [9], and additionally, we adhered to the STROBE checklist for observational studies [10]. Retrospectively, 3D CRANI sequencing data was retrieved from consecutive patients visiting the radiology department of Bruges, Belgium, and who underwent head and neck MR imaging. Patients were included whenever the senior radiologist (J.C.) could not identify pathology along the extraforaminal cranial and occipital nerve branches and when a 3D CRANI sequence was present before and after gadolinium contrast administration. Thus, no pathology was present along the course of the observed nerve branches on both sides. Moreover, none of the patients received radiotherapy in the head and neck area nor did they receive chemotherapy. The reason for MRI referral is addressed in supplemental Table 1. Ethical committee approval was waived due to the retrospective nature of this study.

MRI Imaging procedure
Imaging was performed on a 3.0-Tesla (T) MRI system (Ingenia; Philips) equipped with 32 channel head coil (INVIVO). A previously published MR neurography sequence, 3D CRANI, was performed [1,8]. 3D CRANI is a 3D TSE STIR sequence that uses a PSS (pseudo-steady state) sweep in combination with MSDE (Motion Sensitized Driven Equilibrium) Pulse. We used STIR in combination with MSDE to ensure the signal from fat, muscle, and blood is suppressed uniformly across the field of view.

Imaging analysis
Three orthogonal planes, as well as a plane following the course of the mandibular nerve using multiplanar reformation (MPR) and maximum intensity projection (MIP), were reconstructed using the Philips Volume post-processing package. A reformatted slab thickness of 5 mm and gap of −0.5 mm allowed for the best demonstration of the nerve trajectory. The images were analyzed by two trained observers (F.V.D.C. with 5 years of experience in head and neck imaging, F.V. with 5 years of radiology experience, and 2 years in head and neck imaging). After a calibration session, initial evaluations were made independently and blinded from each other using a scoring form ( Table 1). The observers first scored the suppression quality for arteries, veins, fat, and lymph nodes before and after contrast administration on the 3D CRANI sequence. Next, all cranial nerves were assessed and scored for visualization before and after contrast administration. The following nerves were evaluated on both sides: trigeminal nerve branches, facial nerve, glossopharyngeal nerve, vagus and accessory nerve, hypoglossal nerve, and the greater and lesser occipital nerves. We defined a midpoint for each cranial nerve resulting in a proximal and distal segment (Table 1). Both observers were asked if they could identify each nerve before and after contrast administration. Next, a nerve visualisation score was adopted using a 5-point scale (4, excellent: both proximal and distal portion identified; 3, good: both portions identified but not continuous; 2, fair: only proximal portion identified; 1, poor: only proximal portion identified but not continuous; 0, nerve could not be identified) [11]. If the nerve was not located in the field of view, this could also be indicated. The observers were allowed to consider the proximal portion of cranial nerves IX-X-XI as one and the same given their close anatomical location and in accordance with a previously published study [11]. The measurements were repeated after one month by both observers and after randomizing all cases. After this qualitative analysis, each nerve was analyzed quantitatively to obtain benchmark values before and after contrast administration during the first observation session. Both observers measured signal intensities of the cranial nerves by placing a circular region of interests (ROI) within the identified cranial nerves (iROI) at the predefined landmarks. Similarly, a 1 cm 2 ROI was drawn within the masseter muscle (mROI) and in air (aROI) (Fig. 1). The apparent signal-to-noise ratio (aSNR), the apparent nerve-muscle contrast-to-noise ratio (aNMCNR) and nerve diameter were measured for each cranial nerve. aSNR and aNMCNR were calculated by normalising with the standard deviation of air (SD air ) [4]. Equations used to calculate aSNR and aNMCNR:

Statistical analysis
All statistical analyses were done by a certified statistician (FVDC) with RStudio Team (2020)  quality scores and Fleiss' kappa statistics to assess inter-and intrarater agreement on the ordinal outcome measures (nerve identification and suppression quality). Group differences between continuous measurements were compared using a Student's T-test or ANOVA test in the case of multiple groups. Intraclass correlation coefficients were calculated to determine agreement on the quantitative continuous measurements. A p value of less than 0.05 was considered significant. There was no missing data in the final dataset.

Nerve identification and visualisation score
Data from eleven patients were included in this study between January and September 2020 (Supplemental table 1): six males and five females with an average age of 47 (range: 14-83).
Most extraforaminal cranial nerve branches could be identified in all subjects by both observers after administration of gadolinium contrast agent, except for the lesser occipital and ophthalmic division of the trigeminal nerve where detection rates were considerably lower ( Table 2). The use of gadolinium contrast significantly improved nerve detection rates on the 3D CRANI sequence when comparing combined detection rates before and after contrast administration (p < 0.001). 3D CRANI allowed us to obtain high spatial resolution (Figs. 2, 3, 4, and 5). The ophthalmic trigeminal branch and the occipital nerve branches were the most difficult to distinguish as illustrated by lower identification scores. A similar pattern was seen when nerve visualisation scores were evaluated (Fig. 6). On average, the visualisation of most cranial nerve branches was scored as good to excellent, except for the glossopharyngeal and vagus nerves and the smaller nerve branches such as the deep temporal and ophthalmic nerves which still received a fair score meaning the proximal portion of these branches could be identified. Nerve identification before contrast administration showed an overall intrarater agreement of 79.2% and interrater agreement of 82.7% (intrarater κ = .561, p < .0001; interrater κ = .642, p < .0001). After contrast administration, this improved to an overall intrarater agreement of 92.7% and interrater agreement of 93.6% (intrarater κ = .688, p < .0001; interrater κ = .727, p < .0001).

Suppression quality of surrounding structures
The arterial and fat suppression quality was moderate to excellent both before and after contrast administration. Venous and lymph node suppression quality was scored non-suppressed to excellently suppressed, with an improvement in suppression quality after contrast administration (Table 3, Fig. 5). Excellent agreement was seen for arterial and fat suppression. Venous and lymph node suppression quality scores showed varying agreement between and within observers. Kappa statistics varied from poor to moderate (Supplemental table 2).

Quantitative analysis: benchmarking values and reliability
Nerve benchmarking values were calculated before and after contrast administration (Supplemental table 3  . Nerve branches as small as 0.5 millimeters could be identified. A significant decrease in nerve diameter measurements and aSNR was observed after contrast administration (p < .05). aNMCNR did not significantly differ before and after contrast administration. The intraclass correlation coefficients (ICC) showed high concordance for all measurements with decreasing ICC values from proximal to distal (Table 4).

Discussion
This study confirms that the novel MR neurography sequence, also denoted as 3D CRANI [8], is a reliable and reproducible MR neurography technique for the visualisation of the extraforaminal cranial and occipital nerves. Previous studies  already evaluated the feasibility of heavily T2-weighted MR imaging for nerve-specific visualization of the mandibular nerve [2,12] but this is the first study to expand on this topic and evaluate the reliability of MRN in cranial and occipital nerve evaluation. Reliable imaging techniques are necessary when dealing with cranial nerve disorders, as electrophysiological and sensory examinations in the head and neck area have their own limitations [13]. Some already described the advantageous role of MRN in diagnosing trigeminal nerve injuries and impact on clinical management [2,5]. Within other domains such as brachial plexus imaging, MRN established its role and showed substantial therapeutic impact in over one third of patients [14]. This is the first study to assess the role of contrast administration in MR neurography. We illustrated improved suppression quality of surrounding structures as well as improved nerve visualisation after gadolinium administration. This  probably results from a short-lasting change in susceptibility of the contrast-filled vessels resulting in faster blood dephasing and thus a better suppression quality.
A significant decrease in signal intensities and nerve diameters immediately after contrast administration was noticed. A possible explanation could be the improved suppression of the surrounding tissues and vasa nervorum. As a result, true MR neurography is achieved. This further implies that benchmarking of signal intensity, but also spatial dimensions, depends on contrast administration. Current literature does not allow unequivocal comparison of benchmarking values as each study applies its own MR sequences, and relative signal calculations, with or without contrast administration [4][5][6]. One study by Burian et al evaluating the lingual and inferior alveolar nerves did produce similar nerve diameters [6]. However, aSNR and aMNCNR do not seem to correspond. Perhaps because different formulas for signal calculation were applied. Publishing all relevant data may overcome this hurdle for future comparison. Furthermore, future studies could compare pathological nerve thickening found on MRN with surgical findings, as exemplified by the work of Zuniga et al [3].
A signal intensity drop moving from proximal to distal along the nerve trajectory was seen. And, as one would expect, the nerve diameter also decreased in the distal direction. This is an important fact if we want to be able to make statements about pathological abnormalities in cranial and occipital neuropathy in the future. Others found similar signal changes in both healthy volunteers and neuropathy cases [4]. In the case of traumatic neuropathies, an increase in focal signal intensity and caliber correlates with histological changes such as endoneural edema, vascular congestion, onset of endoneural fibrosis, and demyelination [15]. Bendszus and colleagues further identified temporal MR changes in the weeks following sciatic nerve lesions in a rat model that correlated with electrophysiological findings [15].
This study had some limitations including its retrospective nature, a small sample size, and limited number of observers. However, a wide age distribution and near-equal female-male ratio was achieved. Both observers anticipated a calibration session to limit method bias. Future studies should confirm these findings on a larger cohort. A large number of measurements could have resulted in measurement errors. Automatic segmentation and signal intensity calculation would be a next step forward in determining benchmarking values for any anatomical location, limiting this bias. The occipital nerves showed a surprisingly low overall visualisation score, probably this was related due to patient positioning resulting in suboptimal suppression quality in the occipital area and not due to inherent flaws in the MRN technique; however, this must be verified in a future study. Suppression quality scores showed varying results both between and within observers. This could be due to several factors such as the use of a limited 4-point Likert scale to score suppression quality and small sample size. Finally, a case-control study will be needed to address the reliability of 3D CRANI in patients with cranial or occipital nerve disorders. Gd, gadolinium; ICC, intraclass correlation coefficient; aSNR, apparent signal-to-noise ratio; aNMCNR, apparent nerve-muscle contrast-to-noise ratio

Conclusion
This study confirms the reliability of the novel 3D CRANI sequence for MR neurography of the extraforaminal cranial and occipital nerves in healthy subjects. Intravenous gadolinium administration improves suppression quality and nerve visualisation but alters signal intensities and nerve calibers. Quantitative measurements are reproducible and may serve as benchmarking for future case-control studies on cranial nerve disorders.
Funding The authors state that this work has not received any funding.

Declarations
Guarantor The scientific guarantor of this publication is Fréderic Van der Cruyssen.

Conflict of interest
The authors of this manuscript declare no relationships with any companies whose products or services may be related to the subject matter of the article.
Statistics and biometry One of the authors has significant statistical expertise.
Informed consent Written informed consent was waived by the Institutional Review Board.
Ethical approval Institutional Review Board approval was not required because of the retrospective nature of this study.

Methodology
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