Synopsis

Direct visualization of solid cortical bone structures with MRI is gaining increased interest. “Bright-bone” techniques based on ultrashort echo time (UTE) and Zero TE (ZTE) sequences enable obtaining signals from the cortical bone and are thus desirable for visualizing cortical bone. This work demonstrated that high quality cortical bone imaging of the human skull and c-spine can be achieved on a clinical 3T scanner in a reasonable acquisition time (<6 minutes). This can potentially widen the usage of MRI for C-spine imaging.

Introduction

Materials and methods

Imaging experiments were performed on a PRISMA 3.0T scanner (Siemens Healthineers, Erlangen, Germany) using a customized Zero TE sequence. A 64-ch head-neck coil was used for imaging. IRB approval was obtained for all healthy human studies. Common acquisition parameters included FOV/BW/flip angle/TR = 24cm / 390Hz/pixel / 2°/ 1.76 ms. The equivalent acquisition matrix was 320 × 320 ×320 and the spatial resolution was 0.75 mm isotropic. The acquisition time was 5’54’’.

Image reconstruction was carried out offline. The k-space data were first re-sampled onto Cartesian grids . Fast Fourier transform was then used to generate the magnitude images. These images were processed in Matlab (Mathworks, Natick, MA) to obtain the bright bone images. Due to the array coil configuration, significant signal inhomogeneity was seen across the FOV for C-spine imaging. Correction for signal inhomogeneity was therefore critical. This was achieved in two steps. First, signal intensity correction was performed based on the sparseness property of the gradient probability distribution ⁴. Second, a multi-resolution ROI based intensity correction was applied to further correct for residual signal intensity variations in soft tissues while keeping the signal level in cortical bone and air low, in a way similar to that proposed previously ⁵. The signal intensity in the resultant images was then inverted and the logarithm was taken. Finally, a mask was generated based on the histogram of the images and applied to mask out air and obtain the final images.

Results and discussion

Fig. 1 shows two representative sections of a healthy volunteer. The uncorrected magnitude images (Fig. 1a,b) show proton-density weighted contrast and the signal intensities are increasingly lower towards the center due to the coil sensitivity variation in the axial image (Fig. 1a). The severity of inhomogeneity is apparent in the sagittal image (Fig. 1b). After signal intensity correction, the images in Fig. 1c and Fig. 1d show homogeneous intensities across the FOV. The short T2 tissues such as the teeth and the cervical spine are well depicted in the bright bone images (Fig. 1e,f). The volume-rendered images in Fig. 2 show excellent depiction of the skull and teeth. The C-spine is also well visualized.The artifacts in Fig. 2d and Fig. 2f in the mouth were due to metal dental work.

Due to the configuration of the head coil used, the image signal-to-noise ratio (SNR) in the head was noticeably better than that in the C-spine. Further improvement in C-spine visualization can therefore be achieved by using appropriate coil adapted to the shape of the neck, e.g., flexible array coils.

Conclusion

Acknowledgements

References