Purpose or Case Report: Ultra-Short Echo Time (UTE) imaging, characterized by acquisition schemes with Echo Times (TE) of < 1 msec, have enabled new applications of pediatric musculoskeletal (MSK) MRI including cortical bone, tendons and ligaments. The protons in the bone collagen matrix have an effective T2* on the order of 20 – 80 µsec. The UTE sequence consists of a 40 µs long non-selective RF pulse followed by transmit/receive switch time and a 100% asymmetric data, i.e. free induction decay, acquisition from the center to the surface of a sphere. In order to achieve the shortest possible TE, data acquisition starts already during ramp-up time of the readout gradient. The aim of our study was to assess the feasibility and compare two UTE sequences, a prototype 3D UTE with radial koosh-ball acquisition and clinically available pointwise encoding time reduction with radial acquisition (PETRA), in pediatric patients referred for clinical MSK studies.
Methods & Materials: This prospective study included patients referred for clinical MR imaging, and underwent UTE imaging using both 3D with radial koosh-ball acquisition (TE = 0.04ms) and PETRA (TE=0.07ms). Scans were done on 3T Skyra (Siemens, USA). Qualitative assessments were performed between conventional sequences and UTE sequences.
Results: A variety of pediatric MSK studies with pathology were compared, located at various anatomic locations, including knees, shoulders, elbows and ankles. There were no differences (p=0.08) in the sequence acquisition time between prototype 3D UTE (mean time, 6 mins.) and PETRA (mean time, 5 mins.) with read-out voxel size ranging 0.5-0.7mm. When compared to routine conventional (TE>1msec) pulse sequences, UTE imaging provided a more direct depiction of the regional osseous anatomy and morphology. Small cortical avulsion fractures are often only conspicuous on UTE imaging when compared to conventional sequences. When compared to PETRA, 3D UTE was less susceptible to signal loss outside of the immediate isocenter of the designated field of view. We will share images showing the importance of using UTE for clinical bone imaging.
Conclusions: UTE is becoming an important diagnostic tool for assessing cortical bone. Our results show UTE images obtained using 3D with radial koosh-ball acquisition are superior as compared to PETRA in terms of image quality.