Ultrasound Physics

Ultrasound Image Method
  • A-Mode: A-Mode shows the echo amplitude directly. On A-mode screen, the horizontal axis is depth corresponding to time takes echo to return to transducer. The vertical axis is the amplitude of echo, and the unit is normally in dB level. Ultrasound echo has very large dynamic ranges, with the echo from bone, stone or air can be 120 dB higher than that from soft tissue.  A-Mode does not provide an image and thus it is not really an imaging mode. In A-Mode, transducer locates in one place and does not move, transmitting and receiving continuously in pule-echo mode.  
  • B-Mode: “B” stands for “brightness”.  Move the transducer in A-Mode laterally, i.e., usually along the target surface, a series of A-Scan line will be acquired. If the moving is controlled well, and lateral distance of each move is recorded, then align all these A-scan lines will form a 2D mapping of the ultrasound beam scanned area.

    • Each A-Mode scan line can be modulated by brightness or, gray scale and an image is formed. The image is called B-Mode image. On B-Mode image, usually horizontal direction is the lateral distance, and vertical is depth. B-Mode image can be a sector, rectangular, or a circular shape depends on the transducer scan mode and applications. In the early days, scanning is performed by a single element transducer, driven by rotation or swing of a motor, and image has a sector shape. Phased array also most likely gives a sector shape image. Linear array normally usually generates a rectangle shape image. For intravascular or endoscope, where the transducer is in rotation, giving a circular image.

  • M-Mode: M-Mode scan is similar to A-Mode, with transducer staying in one location and does not move. The difference is the way to arrange the echo lines. After processing of each echo line, instead of re-plot the curve in the same axis like A-Mode, M-Mode image modulates it as B-Mode, and align them one by one. In this way an image is formed, with horizontal axis is time, and vertical axis corresponds to depth. M-Mode is good to capture motion information of the target.
  • C-Mode: C-Mode is constant depth mode. A transducer focused at certain depth and then scans a 2D area. All the echo lines map a 3D volume. By select only a gated time window on each echo line, and then form an image, is the C-Mode image. C-Mode image gives sectional images at certain depth of the target. It requires good focus of the transducer beam to generator good resolution.