Identification of Rectus Capitis Posterior Major with Ultrasonography-A Case Study Verified with MRI

Background and Purpose: The use of ultrasonographic image to measure morphological change of deep muscular-skeletal tissue is increasing gradually. The methodology to identify the border of muscle is important. The ultrasonographic image is formed by the calculation of the reflection of the sound wave. The interpretation of detail structure should be careful to avoid artifact. Magnetic resonance image (MRI) is a golden standard to clarify the unidentified structures. While examining the rectus capitis posterior major (RCP Major) by ultrasonography (US), an unknown line were observed between the interface of muscle and bone. The purpose of this study was to examine the lines under the RCP Major by comparing US with MRI. Methods: A subject was assessed over suboccipital area by US and MRI in the same day with 30 minutes interval. The probe of US was placed perpendicular to the alignment of the muscle belly and tilted to obtain the clear image of the muscle border. The MRI was scanned following the similar principles. Results: A thin soft-tissue layer between the RCP Major and lamina of the atlas was shown in both US and MRI. The unknown line in US was not artifact, but the inferior RCP Major fascia. Conclusions: The order of layers from the RCP Major to lamina is proposed to be muscle, inferior fascia, fat, periosteum and bone. The distance between inner sides of superior and inferior muscle fascia could be considered as thickness of a muscle. Identification of the muscle border is important in clinic, especially in muscles with small inear dimension or small change of muscle thickness during muscle contraction.