Sequencing

Sequencing is the process through which a sequence of ultrasound pulses are transmitted to the tissue and the RF data are collected from tissue. Typically, the sequence depends on the type of the ultrasound transducer used, and the type of the image to be acquired.

Ultrasonix systems use 1D transducers arrays, in which the single transducer elements are arranged in a one-dimensional array. For this type of transducer, the sequences used are described here.

B-Mode Sequence
Traditional B-Mode images are 2D cross-sectional images of tissue acquired using 1D transducer arrays. When transducer arrays are used, through electronic transmit beam forming, the ultrasound beam can be narrowed down to scan a narrow column of tissue. The center of this beam and its direction can be chosen by changing the elements involved in forming the beam, and their respective time delays (see here).

In the simplest B-Mode sequence, the beam is moved across the face of the transducer array one line at a time. Typically 96, 128, or 256 pulses are transmitted along the lateral direction of the array and RF data are collected for each transmission. The number of scan lines is called the Line Density in the jargon of ultrasound imaging.

In the simple B-Mode sequence, the beam can only be focused at a single depth for each scan-line. The resulting B-mode image has a higher resolution and focus at this single focal depth than other depths. To get a good resolution and image quality at different depths, multiple transmissions can be used for the same physical location, but with differing focal depths. For example with a Line Density of 128 and two focal depths at 40 mm and 60 mm, first 128 scan lines are acquired with a transmit focus of 40 mm, and then 128 more scan lines are acquired from the same locations but with a transmit focus of 60 mm. Therefore, the total sequence in this case consists of 256 lines. As a result the frame rate will drop. As a general rule, the more the number of focal depths, the lower the frame rate.

More complex sequences can also be programmed. For instance the same location can be scanned a few times with different temporal pulse shapes. The Sonix platforms provide a great flexibility for the design of complex sequences.

See:
 * Texo parameters for programming sequences
 * Exam parameters for sequences