Purpose or Case Report: Anatomic changes during bone growth and skeletal maturation are under-studied and incompletely understood, partially attributed to the sparing use of ionizing radiation in children. Recently, a clinically-feasible UTE (ultra-short echo time) MRI technique has emerged, enabling the direct visualization of bone. Although validated in adults, its spatial resolution and reproducibility have not been fully investigated in children. Therefore, the purpose of this study is 1) to determine the feasibility of UTE MRI to quantify changes in vertebral porosity and disc hydration along the thoracolumbar spine and 2) to investigate inter-rater reproducibility.
Methods & Materials: This is a HIPPA-compliant MRI examination performed for sequence optimization and for feasibility determination. Dual-echo UTE images were acquired on a 3-Tesla MRI scanner (Skyra, Siemens), using a clinically available spine multi-channel surface coil. Quantitative analysis was performed using the calculated porosity index (TE_long / TE_short) within manually selected regions of interests (ROIs). ROIs were drawn around six consecutive vertebral bodies and six intervertebral disc levels by 2 research assistants, independently and under the supervision of a board-certified, fellowship-trained pediatric musculoskeletal radiologist. Chi-square test was used to compare porosity index between different vertebral levels and hydration index between different disc levels. Intra-class correlation was used to determine reproducibility.
Results: High-resolution UTE MRI required approximately 6-10 minutes to produce 3D bone images with a readout voxel size ranging from 0.47mm to 0.81mm. The thoracolumbar spine from a volunteer (19 year-old female) was imaged in both axial and sagittal planes. Using the sagittal images (voxel size: 0.63mm), 19 ROIs were drawn around each of the 6 vertebral and disc levels to produce a total sample size of 114 vertebral body ROIs and 114 disc ROIs. Vertebral porosity (p<0.0001) and disc hydration (p=0.004) increased in the craniocaudal direction. Intra-class correlation coefficients were excellent for all 6 vertebral levels (range: 0.970-0.998).
Conclusions: High-resolution UTE MRI can reliably quantify changes in vertebral porosity and disc hydration between levels, which reflect location-dependent changes in tissue composition and microstructure. Further investigation is warranted to determine age- and gender-dependent changes in health and disease.