Utility of the fast-field-echo resembling a computed tomography using restricted echo-spacing sequence in evaluating ossification of the posterior longitudinal ligament of the cervical spine

Introduction

Ossification of the posterior longitudinal ligament (OPLL) is a degenerative disease of the cervical spine (1). Due to ectopic calcification of the posterior longitudinal ligament, the spinal cord or nerve roots may be compressed, producing neurological symptoms such as paralysis of the trunk or limbs, paresthesia, and other neurological symptoms (2,3). As the disease progresses, bladder and rectal dysfunction, or even paralysis may occur. First reported by Japanese scholar Tsukimato in 1960, OPLL is a common cause of cervical cord compression in Asian populations, with a prevalence of approximately 1.9–4.3% in individuals aged over 30 years in Japan and 1.6–1.8% in China (4).

The imaging evaluation of the OPLL of the cervical spine was traditionally performed by plain radiography. With the development of medical imaging technology, computed tomography (CT) is now considered the “gold standard” for the diagnosis of OPLL. The advancement of multi-plane reconstruction images of CT has made it possible to accurately assess the degree of ossification in the axial and sagittal planes determine the classification of OPLL, and provide a reference for the formulation of surgical plans (5,6). CT has a high sensitivity for lesions of bony structures of the cervical spine; however, its ability to visualize soft-tissue lesions is limited.

Magnetic resonance imaging (MRI) offers optimal soft-tissue contrast without the use of ionizing radiation, and is typically used to reveal the degree of dural sac compression and changes in spinal cord signaling (7). However, the ability of MRI to evaluate ossified ligaments is limited, as such ligaments produce little to no signal due to their physical relaxation properties. The low hydrogen proton content in bone results in extremely short T2 relaxation times in conventional MRI is, and as a result is not effective at detecting the development of bone cortex or calcification foci (8). Recently, studies have explored potential clinical applications for evaluating bony structures using a three-dimensional (3D) fast-field-echo resembling a computed tomography using restricted echo-spacing (FRACTURE) sequence. This technique uses a series of gradient echoes with constant echo spacing and post-processing subtraction to provide CT-like image contrast (9,10).

To date, no standardized guidelines have been established for the treatment of cervical OPLL, including in relation to the selection of surgical methods. Anterior, posterior, or combined approaches can effectively decompress the spinal canal. The choice of approach is variable and generally depends on a variety of radiological parameters, including sagittal alignment, the distribution and type of OPLL in the cervical spine, ossification thickness, canal-occupying ratio, K-line, cervical spine curvature, dural calcification, and T1 slope (4). This study sought to evaluate the utility of the FRACTURE sequence in the detection of OPLL of the cervical spine in comparison with CT radiological parameters. We present this article in accordance with the STARD reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-2032/rc).

Methods

Participants

The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Ethics Committee of the First Affiliated Hospital of Zhengzhou University (No. 2020-KS-HNSR153), and informed consent was obtained from all the patients.

Participants were recruited from October 2022 until September 2023. The inclusion criteria of the study were as follows: (I) the presence of radicular symptoms such as upper extremity pain, numbness, or decreased muscle strength, or symptoms of upper motor neuron injury, such as hypoesthesia below the injury level or increased muscle tone; (II) a diagnosis of OPLL confirmed by cervical spine X-ray or other examinations at primary hospitals; and (III) performance of both CT and MRI of the cervical spine during hospitalization, including the FRACTURE sequence on MRI, with an interval of no more than 48 hours between the two scans. The exclusion criteria of the study were as follows: (I) previous cervical spinal surgery with implants; (II) trauma, cervical spine infection, or cervical spine tumors; and/or (III) poor imaging data or incomplete medical records.

A total of 52 patients met the inclusion criteria. Among them, seven patients were excluded due to prior cervical spinal surgery, two due to cervical spine tumors, and three because their images were considered non-diagnostic due to severe motion artifacts, resulting in insufficient visualization for reliable assessment as determined by consensus between the two radiologists. Thus, ultimately a total of 40 patients (22 men, 18 women; mean age, 56.8±9.5 years; range, 41–78 years) were included in the study.

MRI protocol

All the MRI examinations were performed on a 3.0-T scanner (Ingenia Elition X; Philips Medical Systems, Best, The Netherlands) using a 20-channel head and neck coil. In addition to conventional MRI sequences, FRACTURE sequences for dedicated bone imaging were acquired. The following sequences were acquired in sagittal plane with a T1-weighted turbo spin-echo (TSE) sequence, a T2-weighted TSE sequence, and a T2-weighted TSE sequence with spectral pre-saturation with inversion recovery. The 3D FRACTURE sequence was acquired using six in-phase echoes [starting echo time (TE): 2.3 ms, repetition time (TR): 28 ms, echo-spacing: 4.6 ms], with a flip angle 15°, isotropic voxel dimensions of 0.7×0.7×0.7 mm³, and a field of view of 180×169×70 mm³.

To prevent chemical shift artifacts due to signal reduction if the oscillating signal intensities of fat and water were out of phase, all echoes were acquired at 4.6-ms intervals, when fat and water were in-phase. Images obtained at shorter TEs helped to increase the signal-to-noise ratio, while the image from the longest TE, which resembled a T2-weighted image, enhanced the image contrast (10). The total scan time was 5 minutes 11 seconds. Conventional MRI sequences were obtained for routine clinical evaluation and anatomical reference only, and were not used for any quantitative measurements or for the primary comparison between FRACTURE and CT.

CT protocol

All the CT examinations were performed on a 256-slice scanner (Revolution Apex; GE Healthcare, Waukesha, WI, USA). The clinical scan parameters were set according to routine protocols: tube voltage of 120 kV, tube current using SmartmA (100–400 mA), detector configuration of 256 mm × 0.625 mm, pitch factor of 0.992, slice thickness of 0.625 mm, and rotation time of 0.50 seconds. All the images were reconstructed in the sagittal plane with a bone window setting [width 2,000 Hounsfield units (HU), center 500 HU], and processed using 50% adaptive statistical iterative reconstruction-V (ASiR-V) on the Revolution CT scanner.

Quantitative measurements

The MRI and CT images were read by two radiologists with 10 and 12 years of experience in musculoskeletal radiology, respectively. The image analysis was performed using FRACTURE-MRI images alone and CT images alone, respectively, on a picture archiving and communication system (PACS) workstation certified for clinical use (VisionPACS, Intech Hosun). Conventional MRI sequences were not used for the evaluation of the morphologic parameters in this study. For each modality, a separate dataset was generated independently in random order, and the readers were blinded to the clinical information for the image analyses. The FRACTURE images and CT images were interpreted in separate reading sessions with a washout period of at least 4 weeks between the two modalities. During image interpretation, both readers were blinded to all clinical information, the results of the other imaging modality, and each other’s assessments. Image quality was assessed by each radiologist as either diagnostic or non-diagnostic due to artifacts; Patients whose images were defined as non-diagnostic by at least one reader were excluded from the study.

Based on these images, the following features were evaluated: (I) OPLL classification (11) (Figure 1A-1H); (II) distribution of OPLL: recorded as the number of involved vertebral levels, categorized from 1 to 7; (III) K-line: defined as a straight line connecting the midpoints of the spinal canal at C2 and C7 on mid-sagittal images. OPLL was classified as (+) if the ossified mass did not exceed this line and (−) if it extended beyond the line; (IV) dural calcification (Figure 1I,1J); (V) intervertebral vacuum phenomenon (Figure 1K,1L); (VI) ossification thickness: measured on the axial image at the level demonstrating the greatest extent of ossification. Thickness was defined as the maximal perpendicular distance from the posterior vertebral body margin to the most posterior border of the ossified lesion; and (VII) canal-occupying ratio: calculated using the formula: (ossification thickness/spinal canal diameter) × 100%.

Figure 1 Imaging comparison of the FRACTURE sequence and CT scans in patients with OPLL. (A,B) Continuous-type OPLL; (C,D) segmental-type OPLL; (E,F) mixed-type OPLL; (G,H) circumscribed-type OPLL; (I,J) dural calcification; (K,L) intervertebral vacuum phenomenon. For each pair of images, the left side shows the FRACTURE sequence, and the right side shows the CT image. The arrows indicate the ossified lesions, dural calcification, and intervertebral vacuum phenomenon. CT, computed tomography; FRACTURE, fast-field-echo resembling a computed tomography using restricted echo-spacing; OPLL, ossification of the posterior longitudinal ligament.

Quantitative parameters (ossification thickness and the canal-occupying ratio) were recorded as the average of the two readers’ measurements for inter-modality comparison. For categorical data (OPLL classification, distribution of OPLL, K-line, dural calcification, and intervertebral vacuum phenomenon), discrepancies were resolved by consensus after discussion.

Statistical analysis

The inter-observer agreement for the FRACTURE sequence and inter-modality agreement were analyzed. The intraclass correlation coefficient (ICC; model 2,1) and Bland-Altman analysis were used to evaluate the consistency of quantitative measurements (12). For the Bland-Altman analysis, the mean bias and 95% limits of agreement (LOA) with 95% confidence intervals (CIs) were calculated. Proportional bias was assessed using linear regression of the differences on the mean values to detect systematic errors related to measurement magnitude. The Cohen kappa test of agreement was used to determine the agreement for the categorical data. A Cohen kappa value of 0.2 was defined as slight agreement, 0.21–0.40 as fair agreement, 0.41–0.60 as moderate agreement, 0.61–0.80 as strong agreement, and 0.81–1.00 as almost perfect agreement. All the statistical analyses were performed using SPSS software (version 26, Chicago, IL) and MedCalc software (version 15.6.1, Ostend, Belgium). A P value <0.05 was considered statistically significant.

Results

Quantitative data

Inter-observer agreement (FRACTURE sequence)

Two observers measured the ossification thickness and canal-occupying ratio in the OPLL patients based on the FRACTURE sequence. The inter-observer agreement was excellent for both ossification thickness (ICC =0.99, 95% CI: 0.98–0.99) and the canal-occupying ratio (ICC =0.99, 95% CI: 0.98–0.99). As shown in Figures 2,3, the Bland-Altman analysis showed a mean bias of −0.04 mm (ossification thickness) and −0.12% (canal-occupying ratio), with 95% LOA of −0.51 mm (95% CI: −0.64 to −0.38) to 0.43 mm (95% CI: 0.30 to 0.56), and −3.22% (95% CI: −4.09 to −2.35) to 2.98% (95% CI: 2.11 to 3.86), respectively. No significant proportional bias was observed (P=0.47 and P=0.07, respectively).

Figure 2 Inter-observer reproducibility for ossification thickness measurements based on the FRACTURE sequence. Bland-Altman plot of the mean ossification thickness of the two observers (x-axis) against the difference in ossification thickness between the two observers (y-axis). The horizontal solid line represents the mean difference. The 95% LOA are represented as two dashed lines and are defined as the mean difference ± 1.96 standard deviations. FRACTURE, fast-field-echo resembling a computed tomography using restricted echo-spacing; LOA, limits of agreement; OPLL, ossification of the posterior longitudinal ligament; SD, standard deviation.

Figure 3 Inter-observer reproducibility for canal-occupying ratio measurements based on the FRACTURE sequence. Bland-Altman plot of the mean canal-occupying ratio of the two observers (x-axis) against the difference in canal-occupying ratio between the two observers (y-axis). The horizontal solid line represents the mean difference. The 95% LOA are represented as two dashed lines and are defined as the mean difference ± 1.96 standard deviations. FRACTURE, fast-field-echo resembling a computed tomography using restricted echo-spacing; LOA, limits of agreement; OPLL, ossification of the posterior longitudinal ligament; SD, standard deviation.

Inter-modality agreement (FRACTURE vs. CT)

The ossification thickness measurements demonstrated excellent agreement (ICC =0.99, 95% CI: 0.98–0.99) with a mean bias of −0.03 mm and 95% LOA of −0.51 mm (95% CI: −0.66 to −0.36) to 0.56 mm (95% CI: 0.41 to 0.71) (Figure 4). The canal-occupying ratio measurements also showed strong agreement (ICC =0.98, 95% CI: 0.97–0.99) with a mean bias of 0.23% and 95% LOA of −3.52% (95% CI: −4.58 to −2.47) to 3.97% (95% CI: 2.92 to 5.03) (Figure 5). No significant proportional bias was observed for either measurement (P=0.83 and 0.28, respectively).

Figure 4 Inter-modality reproducibility for ossification thickness measurements based on the FRACTURE sequence and CT images. Bland-Altman plot of the mean ossification thickness of the two methods (x-axis) against the difference in ossification thickness between the two methods (y-axis). The horizontal solid line represents the mean difference. The 95% LOA are represented as two dashed lines and are defined as the mean difference ± 1.96 standard deviations. CT, computed tomography; FRACTURE, fast-field-echo resembling a computed tomography using restricted echo-spacing; LOA, limits of agreement; OPLL, ossification of the posterior longitudinal ligament; SD, standard deviation.

Figure 5 Inter-modality reproducibility for canal-occupying ratio measurements based on the FRACTURE sequence and CT images. Bland-Altman plot of the mean canal-occupying ratio of the two methods (x-axis) against the difference in canal-occupying ratio between the two methods (y-axis). The horizontal solid line represents the mean difference. The 95% LOA are represented as two dashed lines and are defined as the mean difference ± 1.96 standard deviations. CT, computed tomography; FRACTURE, fast-field-echo resembling a computed tomography using restricted echo-spacing; LOA, limits of agreement; OPLL, ossification of the posterior longitudinal ligament; SD, standard deviation.

Categorical data

As shown in Table 1, the inter-observer/inter-modality agreement was strong to perfect for the OPLL classification, distribution of OPLL, K-line (+) or (−), and dural calcification evaluation. However, the consistency in detecting intervertebral vacuum phenomenon was notably lower, with values of 0.541 (0.186–0.896) and 0.483 (0.130–0.836), respectively.

Discussion

Due to minimal calcification, patients with OPLL often do not exhibit typical neurological symptoms in the early stage; however, in some patients, the ossification gradually thickens at a rate of 0.6 mm per year (13). Prolonged compression of the spinal cord and nerve roots can lead to a progressive deterioration of neurological symptoms in the short term, necessitating surgery to improve the patient’s quality of life.

Surgical treatment for OPLL includes anterior, posterior, and combined approaches. The choice of approach is guided by radiological parameters, including OPLL classification, K-line status, and canal occupancy (14,15). Posterior surgery is often preferred for multi-level, K-line (+) OPLL, while anterior or combined approaches are considered for K-line (−) patients (16). Therefore, the accurate imaging evaluation of these morphological features is critical for surgical decision-making.

The evaluation of radiological parameters for OPLL is frequently based on lateral X-ray images, axial, and sagittal CT scans. For example, Tsuyama’s OPLL classification (11) relies on lateral X-ray images. However, research (17) has revealed that sagittal CT images demonstrate higher interobserver reliability compared to X-rays. Consequently, the OPLL classification in this study was based on sagittal CT reconstructions.

Due to ionizing radiation, radiographic examinations are limited in specific populations, including pregnant women, children, and patients with hyperthyroidism or multiple myeloma. Conversely, MRI uses non-ionizing radiation, providing superior soft-tissue contrast. However, its bone imaging capability is limited by low bone tissue proton density and abbreviated T2/T2* relaxation times. Recent research has shown that MRI holds promise for bone imaging, as evidenced by studies investigating ultrashort echo time (UTE) (18), zero echo time (ZTE) (19), and black bone (20) techniques. These three techniques require advanced hardware and software capabilities, and are presently only available at a select few academic institutions and hospitals.

Conversely, the FRACTURE sequence places comparatively lower demands on pulse/gradient effects. It relies on high-resolution 3D gradient echo technology, employing multiple echoes with uniform echo-spacing and subsequent subtraction processing to deliver CT-like image contrast, effectively mitigating background signals. It provides high contrast for visualizing the bone cortex and trabecular bone, making it a valuable diagnostic tool.

Deininger-Czermak et al. (21) conducted a comparative analysis of the FRACTURE and UTE sequences, using CT as a reference. Their study aimed to assess the consistency of these sequences in evaluating cervical spine degeneration. The findings demonstrated a strong level of consistency between both sequences and CT in the evaluation of cervical spine degeneration. In terms of image quality, it was observed that FRACTURE outperformed UTE, achieving superior contrast between bone and soft-tissue structures. It should be noted that conventional MRI in this study served only as a routine clinical reference, and was not included in the primary quantitative comparison between the FRACTURE sequence and CT.

In the present study, with the exception of the assessment of intervertebral vacuum phenomenon, the imaging parameters measured using the FRACTURE sequence demonstrated strong interobserver consistency and exhibited a high level of agreement with corresponding measurements obtained from CT scans. The results suggest that under specific conditions, the FRACTURE sequence has the potential to serve as an alternative examination method to CT. The findings of several recent studies support this conclusion (10,22,23).

In a study of 27 samples, Deininger-Czermak et al. (23) observed strong consistency between the FRACTURE sequence and CT scans in evaluating aspects such as vertebral morphological changes (ICC =0.88, CI: 0.83) and joint degeneration scores (ICC =0.78, CI: 0.69–0.84). Additionally, Feuerriegel et al. (24) reported robust agreement between the measurements obtained using the FRACTURE sequence and those from CT scans when assessing glenoid bone loss in shoulder joint Bankart lesions. They noted that this sequence could serve as a valuable alternative to traditional CT scans in cases of acute shoulder injuries.

Moreover, recent studies have suggested that under certain conditions, FRACTURE may even demonstrate superior diagnostic performance compared with CT or conventional MRI in the detection of specific soft-tissue–related abnormalities (25,26). Beyond its advantages in reducing radiation exposure and cost, FRACTURE may also provide enhanced tissue contrast and lesion conspicuity relative to CT, thereby, further expanding its potential clinical utility.

FRACTURE is an MRI imaging technique with several advantages: (I) it acquires high-resolution images, enabling multi-planar reformatting; (II) it can easily achieve contrast similar to CT through post-processing; (III) it preserves signals from soft tissues surrounding bone structures; and (IV) it is radiation-free, making it safe for sensitive populations. However, the magnetic resonance (MR)-FRACTURE sequence has a number of limitations, including elevated costs, longer examination times, and the need for good compliance by patients.

Recent developments in artificial intelligence (AI)—based deep learning reconstruction have enabled substantial acceleration of MRI acquisition while maintaining high diagnostic image quality (27). This technology has also been applied to the FRACTURE sequence, resulting in significantly shortened acquisition times and improved robustness compared with the conventional approach (25). Due to hardware and software configuration limitations of our MRI system, we were unable to incorporate AI-based reconstruction in the present study. Nevertheless, existing evidence suggests that the integration of AI algorithms may help mitigate several inherent limitations of FRACTURE, including the prolonged scan time and motion susceptibility, thereby enhancing its potential for broader clinical application in the future.

We also observed that the low proton density of air results in a conspicuously high signal in the FRACTURE sequence. This phenomenon may lead to the misinterpretation of intradiscal air accumulation—a consequence of intervertebral disc degeneration (commonly referred to as the “intervertebral vacuum phenomenon” on CT images)—as calcification (Figure 1K,1L). Consequently, in this study, the agreement observed between the FRACTURE sequence and CT in detecting the intervertebral vacuum phenomenon was moderate (κ=0.483; 95% CI: 0.130–0.836).

It is important to note that our study design involved the independent assessment of FRACTURE images without access to CT or conventional MRI reference standards. This may have contributed to the moderate agreement in evaluating the intervertebral vacuum phenomenon, as gas could occasionally be misinterpreted as calcification. In clinical practice, reviewing FRACTURE images alongside standard MRI or CT would likely reduce such misinterpretations. Deininger-Czermak et al. (28) encountered a similar phenomenon when applying the FRACTURE sequence to skull fractures. In cranial cavity and skull base imaging, the presence of gas led to artifacts that partially hindered the identification of bony structures. These results suggest the importance of exercising caution when using the FRACTURE sequence to identify bony structures containing gas.

This study had a number of limitations. First, it was a single-center, retrospective study with a limited sample size; thus, multi-center, large-scale prospective studies need to be conducted to further confirm the utility of the sequence. Second, the study included patients with OPLL initially diagnosed by X-rays at local hospitals who later sought treatment at our institution, which may have introduced selection bias. Third, this study did not include a healthy control group due to ethical considerations, which may have introduced statistical bias into the results. Fourth, the study exclusively focused on the cervical spine in OPLL cases, and did not explore applications in other anatomical regions or broader disease categories. Finally, although the FRACTURE sequence provides excellent visualization of ossified OPLL lesions, its ability to directly and accurately assess the degree of spinal cord compression is limited. Therefore, at present, FRACTURE alone is not yet sufficient to replace myelography CT for detailed preoperative surgical planning, especially in cases requiring precise evaluation of the relationship between OPLL and spinal cord lesions. Future studies should seek to expand the applicability of the sequence.

In summary, the FRACTURE sequence provides a reliable radiological evaluation of the cervical OPLL, demonstrating strong concordance with CT scans. For radiation-sensitive populations, including pregnant women and children, the FRACTURE sequence holds promise as an alternative diagnostic tool to CT in specific disease diagnoses.

Conclusions

MRI with the FRACTURE sequence may be sufficient for evaluating cervical spine OPLL. Under specific circumstances, the FRACTURE sequence could serve as a valuable alternative to conventional CT scans, potentially lowering costs and decreasing radiation exposure.

Acknowledgments

We thank all the participants for their cooperation in this study.

Footnote

Reporting Checklist: The authors have completed the STARD reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-2032/rc

Data Sharing Statement: Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-2032/dss

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-2032/coif). D.Z. is an employee of Philips Healthcare. The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Ethics Committee of the First Affiliated Hospital of Zhengzhou University (No. 2020-KS-HNSR153) and informed consent was taken from all the patients.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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