Synopsis

We proposed a new technique for the cervical spine MR bone imaging based on broadband inversion recovery prepared segmented multispoke UTE sequence (3D BoneVIEW) for assessment of ossification of the posterior longitudinal ligament (OPLL). 3D BoneVIEW provided robust spinal bone imaging with sufficient background suppression. This sequence might be useful for assessment of the OPLL as an alternative to CT bone imaging.

Purpose

Ossification of the posterior longitudinal ligament (OPLL) is a multifactorial condition caused by ectopic hyperostosis and calcification of the posterior longitudinal ligament¹. To diagnose OPLL, both magnetic resonance imaging (MRI) and computed tomography (CT) examinations are critical². MRI better delineates the extent of soft tissue abnormalities, whereas CT more readily identifies the foci of frank ossification². CT analysis must be performed to differentiate spondylotic myelopathy, which is essential when considering operative intervention³.

Recently, MR bone imaging, using ultrashort echo-time (UTE) or zero echo-time (ZTE) sequence, has gained more attention for detection and assessment of bone pathology as an alternative to computed tomography (CT) bone imaging^4-8. Challenge of MR bone imaging for clinical application in the cervical spine is to increase the robustness of image quality. Bone weighted imaging based on simultaneous long- and short T₂ suppression inversion-recovery (IR) technique^9,10 may suffer from insufficient fat and long-T₂ suppression due to B₀ inhomogeneities caused by the complex anatomical structure of the cervical area.

To overcome this problem, we proposed a new technique based on broadband IR prepared segmented (multispoke) UTE sequence (3D BoneVIEW). The purpose of this study was to evaluate the feasibility of 3D BoneVIEW technique in the cervical spine for assessment of OPLL.

Methods

3D BoneVIEW is based on inversion recovery prepared segmented multispoke UTE 3D stack-of-stars radial sequence⁹ (IR-prepared turbo field echo, IR-TFE) [Fig.1a]. Adiabatic inversion pulses selectively invert long-T₂ species and fat simultaneously¹⁰. For robustness against B₁ and B₀ sensitivity, we applied a broadband offset independent trapezoid (OIT) pulse, which has already been used for brachial plexus MR neurography^11,12. To suppress the background signals sufficiently, we used a long duration OIT inversion pulse (≥20ms). By using such a longer pulse duration, long T₂ species are inverted whereas short T₂ species are saturated⁹ because the T₂ relaxation time of bone is significantly shorter than the duration of the RF pulse. The TFE shot interval (otherwise repetition time, TR) and inversion delay (TI) are chosen such that optimal nulling of the signal from both muscle and fat¹³ is achieved [Fig.1b]. This provides high contrast morphological imaging of bone.

To prevent the blurring of the bone due to T₂* decay during the readout period, the literature¹⁴ suggests that readout duration should be less than 0.81xT₂*. Since the T₂ value of cortical bone is on the order 400us¹⁵, we kept the actual readout duration within 400us. For the TFE (multispoke) excitation, a shot duration of 250ms was applied because the T₁ relaxation time of bone is around 250ms. In addition, we set the constant shot length (actual TRxTFE factor) around 32ms to prevent the recovery of fat signals during TFE shot.

A total of seven volunteers and two patients with OPLL were examined with a 3.0T whole-body clinical system (Ingenia CX, Philips Healthcare). The study was approved by the local IRB, and written informed consent was obtained from all subjects.

Imaging parameters for BoneVIEW were; 3D stack-of-stars radial UTE-TFE, Sagittal or Coronal acquisition, voxel size=1.4*1.4*2.5mm, FOV=400*400mm, TFE shot interval=250ms, flip angle=16°, turbo factor=11, TR/TE=2.6/0.09ms, TI=84ms, and total acquisition time=7 to 10minutes (depend on the body habitus).

Results and Discussion

Figure 2 demonstrates the effect of broadband inversion pulse. The OIT pulse was more robust to B₁ and B₀ inhomogeneities compare to conventional adiabatic pulse in the complex anatomical structure of the cervical area.

Representative coronal and sagittal BoneVIEW images with partial MIP are shown in Figure 3. BoneVIEW clearly depicted the cortical bone structure of the cervical spine while suppressing background signals sufficiently.

Figure 4 shows representative clinical case in a patient with OPLL comparison with CT scan. BoneVIEW clearly showed the existence of ectopic hyperostosis as well as CT scan. Although further clinical investigation is needed, this technique can provide comparable information with CT imaging.

Conclusion

Acknowledgements

References

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