A portable ultrasound imaging system employs a mechanically focused multi-element circular annular transducer that is mechanically scanned using a motor. Received echoes are processed to form two dimensional gray scale B mode images or two dimensional color tissue flow images which are displayed on a display unit. In case of color flow imaging, a high pulse repetition frequency imaging sequence is employed for a reasonable frame rate and special down-sampling techniques are applied to achieve an effective low pulse repetition frequency for flow estimation with enough signal to noise ratio. The system also includes a docking subsystem which charges a system battery and transfers patient and image data between a PACS system, workstation or other information system and the portable ultrasound imaging system.