Background Intraoperative ultrasound (IOUS) has been increasingly used as a guiding tool during neurosurgical procedures.In this study,we aimed to evaluate the potential application of intraoperative ultrasound assisted surgery in the resection of small,deep-seated,or ill-defined lesions.Methods Eighty-six consecutive patients with small,deep-seated,or ill-defined intracerebral lesions were studied prospectively.An improved intraoperative imaging technique and surgical setup were practiced during the surgery.IOUS was performed in three orthogonal imaging planes (horizontal,coronal and sagittal).Results Histopathological diagnoses of these 86 cases included cavomas,metastases,hemangioblastomas,gliomas,and radiation necrosis.Forty-seven of the 86 lesions (54.7％) were small and deep-seated,34/86 (39.5％) were ill-defined,and 5/86 (5.8％) were small,deep-seated,and ill-defined.Sonograms in the horizontal plane were obtained in all 86 cases.Sonograms in the sagittal plane and in the coronal plane were obtained only in 52 cases and in 46 cases,respectively,due to technical limitation.In 13 cases,sonograms in all three orthogonal planes were available.All lesions were successfully identified and localized by IOUS.Total resection was performed in 67 lesions (77.9％) and partial resection was performed in 19 lesions (22.1％).Conclusions We propose IOUS to be performed in three orthogonal planes when surgery is planned for small,deep-seated,or ill-defined brain lesions.By applying this simple,improved technique,surgeons can perform resection of these lesions precisely.