Sequencing

Sequencing is the process through which a sequence of ultrasound pulses are transmitted into the tissue and the RF data are collected. 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 small 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.

The ultrasound propagates through most soft tissue at a speed of 1540 m/s. Therefore to image deeper points in the tissue, the ultrasound system has to wait long enough for the echoes to come back. For instance to acquire RF data to a depth of 80 mm, 80e-3*2/1540 = 103 microseconds is required. To acquire 128 lines of RF data, 128*103e-6 = 13.3 milliseconds is required. Hence, the physical limitation of ultrasound speed together with the imaging sequence determines the maximum frame rate at which the imaging can be performed. In this case the maximum frame rate would be 1/13.3 = 75 Hz.

In practice, more complex sequences are usually used for B-Mode imaging. The Sonix platforms provide great flexibility for the design of complex sequences. Here, some of the more important complex sequences are described.

Multiple Focal Depths
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. Given the same depth of imaging as the previous example the frame rate will drop to 37.6 Hz. As a general rule, the more the number of focal depths, the lower the frame rate.

Pulse Inversion Harmonics
Tissue mechanical behavior is nonlinear in nature. Although a large portion of the transmitted energy to the tissue in the form of ultrasound waves has a linear interaction with the tissue, a small amount of nonlinear interaction is always present. This interaction is useful for imaging special features of tissue.

Because of the nonlinear interaction, the RF data not only contains a signal centered at the transmission pulse frequency, but also has signals around multiples of the transmission frequency, or its harmonics. One method of imaging the harmonics is to scan each location with two pulses which have an identical shape but reverse polarities. When the RF data coming back from tissue for these two pulses are added together, the signals centered at the transmission frequency, as well as its odd harmonics will be cancelled out. However the signals around the even numbered harmonics will be added together. This method is called Pulse Inversion Harmonics (PIH) imaging.

For information on how to use the Sonix platforms for custom sequencing see:
 * Texo Sequencing
 * Texo functions and parameters for sequencing
 * Exam parameters for sequencing
 * Through B-GEOM
 * Through B-FOCUS

Color/Power Doppler Sequence
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