Posterior Belly Of Digastric Muscle And Its Shift: Imaging Anatomy Study And Its Significance

Objective To study the details of imaging anatomy of posterior belly of digastric muscle (PBDM), and to explore an approach to differentiate between tumors from parotid space and from parapharyngeal space on CT or MR imaging. Methods 2 formalin-fixed cadavers, one male and one female, was used for imaging anatomy study. 3 PBDMs were dissected from the mastoid notch immediately behind the mastoid of the temporal bone to the fibrous loop that attached to the greater horn of hyoid bone. One PBDM kept intact for comparison. 3 pieces of thin tube that was X-ray opacity (for CT scan) or some vitamin E capsules (for MR imaging) were respectively bound to the PBDMs asan imaging marker. Both cadavers were put on the scan table in supine position. Axial computed cross-cut images parallel to the Reid's line were scanned. The coronal and sagittal images were obtained on MRI scan through the corresponding standard anatomical sections. The images along the PBMD was undertaken on both CT and MRI as to reveal those muscles directly and to compare with the VR (volume rendering) and MPR (multiplanar reformatted) CT images along the PBMD. The PBDM was identified by markers that had bound to that muscle before. 2 normal volunteers were imaged on the same conditions as used on cadaver study and the appearance was compared with autopsy's manifestation. 45 patients with tumors originated at the parotid or parapharyngeal space were reviewed retrospectively. The diagnosis was confirmed pathologically in all of the cases. The correlation among tumor and the PBDM and its shifting sign was evaluated on axial CT or MRI scans by two senior radiologists. All of estimations were compared with results of operation. Ensuing, the 26 cases were investigated prospectively. According as the results from the retrospective study, the originate space of tumors were proposed by the shift of the PBDM. The imaging evaluations were compared with the operation observations and pathological diagnosis. Results Imaging anatomy studies of two cadavers and two normal volunteers showed that the computed cross-cut images could accurately identify the PBDM, especially on axial images andcoronal sections just behind the mandibular condyle. The images along the PBDM, either direct scans or reconstructive images, could improve the visibility of that muscle. Review from the 45 cases showed 27 tumors from parotid space were located superficially to the PBDM and push it medially. On the contrary, 18 tumors from parapharyngeal space were situated profoundly to the PBDM and shift it laterally. Of 26 prospective cases, 15 tumors that located superficially to the PBDM and pushed it inward were parotid tumors. On the other hand, 11 tumors situated profoundly to the PBDM and jostled it outward were neurogenic tumors occurred at carotid sheath. Conclusion The PBDM is visible accurately on computed cross-cut images. The location between the tumor and PBDM and its shift sign can be used to distinguish tumor located at the profound parotid space from the counterpart occurred in parapharygeal space, and help draw a correct conclusion.