Following assessment of an organ and the surrounding structures, ultrasound imaging is used to guide needle placement into a selected tissue or cavity. A sample of tissue or fluid may be drawn through the needle as an aspirate to analyze the cells or other contents of the sample (cytology and microbiology). A larger needle may be used to retrieve a small piece of solid tissue for a core-biopsy to analyze the architecture of the tissue (histopathology). Ultrasound guidance is commonly used to remove fluid from the chest cavity (thoracocentesis) and abdominal cavity (abdominocentesis).