Does Continuous Have One or Two Crystals Ultrasound

Transducers are the mechanical housing and electrical connections that plug into an echocardiography machine. Each transducer can be engineered for a specific purpose or, as in the case of transesophageal echocardiography probes, multiple functions. The basic mechanical arrangement of a transducer are:

Piezoelectric Crystal Arrangements

Piezoelectric crystals can have the following arrangements:

• Single or Double Crystal
• Linear Phased Array
• Annular Phased Array
• Mechanical

A single or double crystal (Figure 1.4.4and Figure 1.4.5) arrangement is used for A-Mode or M-Mode echocardiography. A single crystal is not capable of continuous transmission and reception of ultrasound signals (continuous wave Doppler) where as the double, linear array (Figure 1.4.6), and annular arrays (Figure 1.4.7) have all Doppler modality capabilities.

Piezoelectric Crystal Arrangements

Figure 1.4.4
Single

Figure 1.4.5
Double

Figure 1.4.6
Linear Array

Figure 1.4.7
Annular Array

A mechanical transducer (Animation 1.4.8 and 1.4.9) will mechanically move the crystals through the planes of the scanning area. The two types of mechanical transducers are rotating and wobbling mechanical transducers. Rotating transducers rotate the crystal 360 degrees. Wobbling mechanical transducers move the crystal through an arc back and forth. Mechanical transducers, by virtue of their moving parts, are prone to mechanical breakdown. Mechanical transducers also have limited Doppler capabilities.

Animation 1.4.8
Wobbling   Mechanical
Transducer

Animation 1.4.9
Rotating   Mechanical
Transducer

Animation 1.4.10
Linear Array

A phased array is a series of piezoelectric crystals that are next to each other. If the phased array is oriented in rows (lines) and columns then it is called a linear array (Figure 1.4.6 and Animation 1.4.10) . There are typically 32, 64, or 128 elements in rows. An annular array (Figure 1.4.7) will have the piezoelectric crystals arranged in a circular pattern or concentric rings.

Figure 1.4.11
3D Pyramid

A linear array will scan in a rectangular-shaped scan plane whereas a sector scanner will sweep the beam across an arc yielding a sector scan. 3D arrays are arranged in 64 elements in 64 rows and provides a sector scan in 3D that has 4096 scan lines. Instead of a 2D image with height x width, a 3D scan will yield a height x width x depth for a pyramidal shaped scan that is populated with voxels rather than pixels found on 2D scans (Figure 1.4.11).

The piezoelectric crystals can function as either a transmitter or a receiver of sound waves, but not both at the same time. If one crystal is used as a receiver and a transmitter, it will transmit a very short signal, called a pulse, and have a relatively long listening time. If two crystals are used, one can continuously transmit while the other can continuously receive a signal. Therefore, piezoelectric crystals can function in a pulse mode or a continuous mode with the continuous mode requiring at least two crystals.

2D Echocardiography uses a change in amplitude to display images. Returned signals that have a higher amplitude are displayed as brighter than returned signals with a smaller amplitude. Doppler uses a change in frequency to display images. A returned signal that has a much higher frequency will be displayed with a higher velocity color, whereas, a signal with the lower frequency will be displayed with a lower velocity color. A display that has both 2D and Doppler images displayed simultaneously is called a duplex image or a duplex scan.

Sector Scanning

Sector scanning, or sweeping the ultrasound wave thru an arc, can be achieved by mechanical or electrical transducers. Sector scanning starts with multiple pulse transmission down a scan line, whose reflected signal is then received by the transducer. The scan lines are then swept across the sector scan area.

Animation 1.4.11
Single Scan Line

Animation 1.4.12
Several Scan Lines

Animation 1.4.13
A Sector Scan

Electrical or phased array transducers use electronic sweeping to scan the arc. A sequential delay of pulses allows the transducer to direct the beam across a sector arc. Electrical or phased arrays have all Doppler modalities available. The beam can also be electronically focused while it is swept across the sector scan. This allows improved resolution at the maximal point of focusing.  The image below shows a pulse that is sequentially delayed.  As the pulse propagates into tissue, the delaying of one side of the pulse, when compared to the other side, causes the beam to be steering towards the delayed side .  The animations below show a  beam without steering, beam steering, and beam focusing. If the beam is steered towards a focal point, beam focusing occurs.  The ability to focus a beam electronically is called dynamic focusing.

Figure 1.4.13
Steering of an Ultrasound Wave
by Electronic Delay

Animation 1.4.15
Beam Without Steering

Animation 1.4.16
Beam Steering

Animation 1.4.17
Beam Focusing

Transducer Properties & Types

A transducer exhibits the following properties:

• Frequency
• Focal Depth
• Bandwidth
• Aperture
• Power Output

Figure 1.4.18
Bandwidth

Figure 1.4.19
Multifrequency Transducer

A transducer can either be a single frequency or a multiple frequency transducer.   A multiple frequency transducer can have its frequency changed to allow for different scanning conditions while using the same transducer.  The focal depth of the transducer is the depth of the focal point of the transducer. The focal point is the point of maximal resolution. The focal point is dependent upon several factors such as the frequency, acoustic lenses, etc and is discussed in greater detail in the next section.  The ultrasound pulse is made up of several frequencies. The range of frequencies is called the bandwidth.  A wider bandwidth can improve axial resolution. If the transducer emitted a single frequency the axial resolution would be a fixed value. With the transmission of multiple frequencies in a bandwidth the larger frequencies have better resolution and the lower frequencies have better penetration.  The aperture or footprint of a transducer is the surface area that is in contact with the patient's tissues. A large aperture transducer will have more difficulty maintaining contact than a small aperture transducer.  The types of transducers can also be categorized by their function. Transducers are used for:

• Transthoracic Echocardiography (TTE)
• Transesophageal Echocardiography (TEE)
• Intravascular Echocardiography (IVE)

For transesophageal and intravascular echocardiography size is an important factor. The size limits the range (and functions) of transducers.

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Source: https://e-echocardiography.com/page/page.php?UID=1429454172

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