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

Pulse(d) Wave Doppler Sequence
Pulsed Wave Doppler imaging (or PWD imaging) is commonly used to image and monitor the velocity profile of blood flow in blood vessels. The profile image is presented to the user as a swept-over-time image which is called the Doppler spectrum in the jargon of ultrasound imaging. Moreover, the signal is typically sent to an audio amplifier and presented to the clinician via speakers on the system so they can hear the blood velocity profile.

The typical frame rate of a B-mode sequence is not high enough to track the fast moving blood, specially in larger vessels. Therefore a special sequence is required with a high frame rate to achieve PWD imaging. As a solution, the velocity profile is taken along a single scan line. A single scan line can be acquired repeatedly at a high frame rate, form which the velocity information can be derived. The line segment along which the velocity information is collected is called the gate in the jargon. In summary, simplest PWD sequence, often called PWD mono, consists of multiple scanning of the same scan line at a certain Pulse Repetition Period (PRP).

Combining PWD and B-Mode Sequences
In order to give the clinician, the ability to adjust the gate in real-time on the B-mode image the simple PWD sequence needs to be combined with a B-mode sequence. This is the origin of the PWD Duplex sequence. Acquiring B-mode images at the same time as the fast PWD sequence is not trivial. Depending on the level of PRP, two types of sequences are possible.

If the PRP has a long duration, for instance more than 1 ms, it is possible to acquire other data in between the acquisitions of the PWD scan line. The B-mode sequence is thus broken down to small sectors, each of which is fit in between the successive acquisitions of the PWD scan line. This sequence is called non-interrupted PWD Duplex as the acquisition of the PWD scan lines is not interrupted by the B-mode sequence.

However, if the PRP is too short, for instance 100 microseconds, there is not enough time to acquire even a single scan-line from the B-mode sequence in between the successive acquisitions of the PWD scan line. In this case, the acquisition of the PWD needs to be interrupted, for part of a B-mode image to be acquired. Therefore the sequence will consist of the PWD mono sequence and B-mode sequence interleaved. These are called interrupted PWD Duplex sequences.

Also see:
 * Exam Parameters for PWD Sequencing

Color/Power Doppler Sequence
As with the PWD imaging, color and power Doppler imaging are mainly used to image the blood flow. The difference is that in color/power Doppler imaging, an image of the blood flow distribution is generated, for a region of interest (ROI) chosen by the clinician, which anatomically matches the B-mode image and can be overlaid on top of it.

As with PWD, the typical frame rate of a B-mode sequence is not high enough to track the fast moving blood, specially in larger vessels. Therefore a special sequence is required with a high frame rate to achieve color/power Doppler imaging. The solution here is to divide the ROI into smaller sectors, each of which can be scanned at a higher frame rate. Each sector is repeatedly scanned for a set number of times before the sequencer moves to the next sector, until the entire ROI has been scanned. The set of RF data acquired from the ROI are often called the ensembles. To give the clinician live feedback from the tissue anatomy, in addition to the ensembles, a B-mode sequence is often added. Hence, the final sequence consists of the ensembles plus a B-mode sequence.

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