背景與目的：以超音波影像量測肌肉骨骼之形態學變化來表示肌肉活性的研究漸漸增加，確認肌肉邊界的判定很重要，然而超音波影像主要由電腦計其音波反射的時間和強度來成像，假影及影像的解讀上仍需注意，核磁共振攝影是人體影像的黃金標準，可確認超音波影像判定，使用超音波觀察大頭後直肌時，在肌肉和椎板間有一條在其他肌肉影像中未發現的高超音波線，本實驗的目的是用超音波及核磁共振影像確認此線為何。方法：研究徵招一位男性受試者接受超音波和核磁共振影像檢查；受試者為有趴姿，超音波探頭角度垂直大頭後直肌的走向，並微調至肌肉邊界最清楚的角度。核磁共振攝影以同樣原則、相似角度拍攝。結果：超音波影像及核磁共振攝影中都發現此高超音波線，此線應為大頭後直肌的下邊界。結論：大頭後直肌到寰椎椎板間的層次由淺到深為大頭後宣肌、肌筋膜、脂肪、骨皮質、椎板。我們認為量測小肌肉或收縮時變化小的肌肉之線性參數時，應從肌肉的上筋膜下緣量測到下筋膜的上緣以避免誤差。

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.