The Advantages of Spectrally Calibrated 3D-MSI in Assessments of Symptomatic Spinal Fusion
S Sivaram Kaushik1, Abhishiek Sharma2, Rajeev Mannem1, Cathy Marszalkowski1, Scott Rand1, Dennis Maiman2, and Kevin M Koch1

1Radiology, Medical College of Wisconsin, Milwaukee, WI, United States, 2Neurosurgery, Medical College of Wisconsin, Milwaukee, WI, United States

Synopsis

Failed back surgery syndrome (FBSS) is a commonly encountered clinical condition for which the root cause often remains elusive after lengthy clinical assessment. While MRI provides great potential utility for uncovering the causes of failed back surgery syndrome, it also is confounded by metal-induced artifacts. This study explores the benefits of a prospectively calibrated 3D-MSI metal artifact reduction technique in assessing instrumented FBSS. Residual artifacts and clinical diagnostic impact were assessed on a cohort of symptomatic FBSS subjects. The results of the study indicate diagnostic advantages using calibrated 3D-MSI, even in the presence of lower-susceptibility titanium spinal hardware.

Introduction

Spinal fusion is the most commonly employed surgical procedure for managing conditions of the spine. Unfortunately, these procedures often do not produce expected results. The failure of surgical intervention in the spine to relieve symptoms often is described by the generalized term ‚”Failed Back Surgery Syndrome” (FBSS) [1].

Fusion procedures often implant metallic hardware, which generate MRI artifacts. In practice, spine surgeons often install titanium alloyed hardware components. While titanium implants induce far less artifact than other metallic alloys, easily discernible artifacts remain in conventional MRI. To elucidate the nature of these subtle artifacts, both conventional metal artifact reduction sequences (MARS) and advanced 3D multi-spectral imaging (3D-MSI) techniques were applied and analyzed on a cohort of symptomatic FBSS subjects.

Commercially available 3D-MSI sequences are not yet equipped with prospective capabilities to adjust to different implant scenarios. The lower susceptibility titanium alloys utilized in spinal fusion devices are often imaged with 3D-MSI parameters that are‚ ”overkill”‚ for the problem at hand. Therefore, in this study, a prospective calibration scan (~1 min) was utilized to calibrate each 3D-MSI sequence to the implanted fusion device [2,3]. This spectral calibration provides crucial efficiency improvements to 3D-MSI, which further allows for higher resolution and higher SNR spinal imaging, often with shorter acquisition times than conventional MARS techniques.

Methods

MRI were collected from a GE Healthcare Optima 450w GEM 1.5T system. GEM variable receiver array configurations were utilized for all spine acquisitions. A cohort of symptomatic FBSS subjects (N=21) provided written consent for an imaging research protocol approved by the MCW Institutional Review Board. Subjects were imaged with a routine MARS acquisition protocol as well as a series of T1w and T2w prospectively calibrated MAVRIC SL 3D-MSI images. The most commonly fused segments were cervical and lumbar, with 10 subjects each. One subject underwent evaluation after thoracic.

To quantify artifact reduction, a subset of subjects was identified from the study cohort (N=6, 3 L-Spine, 3 C-Spine) who previously had CT imaging of their spinal fusion. Bone CT images of titanium fusion devices have limited beam scattering artifacts and allow for easy identification of implant interfaces. After affine co-registration of CT/MARS/3D-MSI image volumes, implant or artifact volumes were identified using a combination of manual segmentation (MRI) and seeded threshold-based region growing (CT). The difference of the implant/artifact volumes between the MRI and the CT provided an estimate of residual artifact volume. Multiple-observer measurements were used to estimate the error bars in these volume-estimates. To asses the diagnostic impact of calibrated 3D-MSI, the MARS and 3D-MSI image sets were reviewed and compared with clinical records to independently identify complications in each image set.

Results

Figure 1 provides volumetric renderings of a segmented lumbar spine fusion device. Quantitatively, the subtraction of measured and rendered volumes in this case yielded 3.9±1.6 cm3 of residual artifact in the MAVRIC SL 3D-MSI images and 58.0±12.2cm3 of residual artifact in the MARS images. It is important to note that the dominant volume displacement is occurring in the vertical dimension, which is the slice-selective dimension in this axial MARS acquisition.

Figure 2 shows that for all subjects studied, the measured artifact volume in the MARS image was significantly higher than in the MAVRIC images. The 3D-MSI residual artifact was subtle and within the measurement error. For the subject shown in Figure 1, Figure 3 shows a 3D rendering of the segmented nerve-roots from the MAVRIC and MARS images. It is clear where the expanded artifact volume of the MARS image interferes with the nerve root and can impact clinical assessments.

Clinically, 10 of the subjects exhibited normal post-surgical findings on plain-film radiography, CT, and MRI. Imaging findings identified adjacent segment disease in eight subjects; in which it was at least partially identified on both MARS and 3D-MSI. Of these cases, 3D-MSI uniquely identified foraminal pathology in three subjects, nerve root edema in one subject, epidural fibrosis in one subject, and a malpositioned screw in one subject.

Figure 4 provides sample images from this MARS preliminary clinical analysis. Foraminal pathology adjacent to a pedicle screw is highlighted in the indicated zoomed region. In the MAVRIC SL image, a compression of the nerve root is identified (dashed white arrow). This compression is obscured by artifact (dashed white arrow) in the corresponding MARS image.

Conclusion

A prospectively calibrated 3D-MSI technique provided clear visible and quantifiable improvements and multiple diagnostic benefits on a cohort of symptomatic titanium alloy instrumented FBSS subjects.

Acknowledgements

Advancing a Healthier Wisconsin Research and Education Fund, #5520357

GE Healthcare Technical Development Grant Support

References

[1] F. Shafaie, C. Bundschuh, and J. Jinkins. The post-therapeutic lumbosacral spine. In J. Jinkins, editor, Post-therapeutic neurodiagnostic imaging, pages 223–243. Lippincott-Raven, 1997.

[2] K.M Koch, Metal Implant-Induced Spectral Range Optimization using Rapid 3D-MSI Calibration Scans, In: Proc. ISMRM, 2015, 2511

[3] S. Kaushik, C Marszalkowski, K.M Koch, External Calibration of the Spectral Coverage for 3D Multi-Spectral Magnetic Resonance Imaging, Magn. Reson. Med (in press)

Figures

Figure 1. Hardware/artifact volume render for L-Spine hardware. Note the substantial increase in artifact in the vertical dimension of the MARS volume. This is due to slice distortion in this image acquisition.

Figure 2. Hardware/artifact and nerve root volume renderings as measured on 3D-MSI and MARS MRI. Note the substantial obstruction of the nerve root in the MARS renderings.

Figure 3. In vivo computed 3D-MSI and MARS for six titanium fusion hardware cases at 1.5T. Compiled results at the bottom of the chart demonstrate the substantial and easily detected artifact difference between the 3D-MSI and MARS images.

Figure 4. Clinical impact of calibrated 3D-MSI technology. The dashed white arrow shows a region of compromised image quality in the MARS MRI that is clearly visualized in the 3D-MSI image and demonstrates nerve root compression.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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