B-Mode Ultrasound

B-mode is one of the most commonly used operation modesof a clinical ultrasound scanner. As explained earlier,ultrasound is a reflection or scattering based imagingmodality, and the sophisticated generation of a sound waveallows the focusing of the sound to a specific location. Eachtransmission yields one scan line around the targeted focalpoint. If only one focal point is selected, one scan lineextends over the total depth range, which is user definedin the current imaging settings. In order to record a fullimage frame using a linear array, the imaging software ofthe scanner electronically moves the active aperture of thearray across the physical aperture to transmit and receive at a given line density.

Typically, hundreds of scan linesare generated this way and displayed on a monitor.Figure 12 shows the cross-sectional sagittal (front to back,vertical slice) image of a fetus in utero. She is sucking onher thumb, as real-time video reveals. On the left of Fig. 12one can see the head and the strong reflection of the skullbone. The very left side of the image is black, an artifactthat could be due to maternal bowel gases that scatter thesound away from the transducer. The remainder of theskull is clearly visible from the forehead to the chin andfrom the back of the head to the neck area. Bones reflectsound waves well and result in a bright signal in the image.The black surroundings of the fetus are regions of amnioticfluid, which does not scatter sound due to the homogeneousnature of the fluid. .

In front of the mouth, one can see the hand of the fetus.Once again the bones of the hand, namely, the knuckles,are pronounced since they scatter more ultrasound thanthe soft tissue of the hand. In the same fashion one can seethe reflections of the spine.