The mechanism that allows transmission and reception of ultrasonic waves is the piezoelectric crystal. The piezoelectric crystal can transmit ultrasound waves by being stimulated by electrical pulses. When the electrical pulse stops, the crystal continues to emit a signal for a short period while it stops vibrating. The period from the electrical pulse ending to the end of the vibration of the crystal is called the "ring down" period. Once the pulsed signal is reflected back to the piezoelectric crystal, the crystal then generates an electrical signal which can be processed and displayed. The piezoelectric crystal is housed in a transducer which also contains a damping material, an acoustic lens and an electrical cable.

 

 

Piezoelectric crystals are quartz or titanate crystals that emit an ultrasound wave when stimulated by an alternating electrical current. Piezoelectric crystals can also generate an electrical current when stimulated by an ultrasound wave. Therefore, a piezoelectric crystal can function as both a transmitter and a receiver. The frequency of the transmitted ultrasound waves depends upon the type of crystal and its thickness.

Animation 1.3.1
 Transmitting Piezoelectric Crystal

In Animation 1.3.1 a piezoelectric crystal is stimulated by an alternating current which causes the alignment and expansion of the crystal. The expansion results in ultrasound wave transmission. In Animation 1.3.2 the piezoelectric crystal is stimulated by an ultrasound wave. The resultant contraction/expansion of the crystal generates an electrical current.

 

Current 2D TEE probes will have 64 elements (piezoelectric crystals) arranged in an array. Newer 3D TEE probes will have 2500 elements arranged in an array whereas the 3D TTE probes have over 3000 elements.

 

The active crystals generate heat which can burn the patient (auto cool/auto shutdown safeguards are present) which can limit the time spent in color and/or 3D mode. Some of the newer crystals are called pure wave crystals where the crystals produce less heat and use less power. The pure wave crystals have a wider bandwidth which improves/increases penetration, resolution, and contrast.

When a piezoelectric crystal is stimulated by an alternating electrical current, the current is sent in a burst or pulse method. A pulse of electricity is sent to the piezoelectric crystal and the crystal generates the ultrasound wave. The crystal is not stimulated during the next few milliseconds (i.e. 1-6 milliseconds). During this "quiet" time, ultrasound waves that are reflected back to the transducer are received by the piezoelectric crystal and an electrical current is generated. The received signal can be processed and displayed. Short pulse length improves axial resolution. If a set pulse length is 8 cycles, as in Image 1.3.1, 1.3.2 and 1.3.3, higher frequencies will have shorter pulse lengths because of the shorter wavelengths.

 

 Figure 1.3.4 Long and Short Pulse Lengths

Animation 1.3.3
  Long Pulse Duration

Animation 1.3.4
  Short Pulse Duration

Figure 1.3.1 Low Frequency

Figure 1.3.2 Mid Frequency

Figure 1.3.3 High Frequency

The pulse duration is proportional to the number of cycles and the wavelength (and inversely proportional to the frequency). A shorter pulse duration has better axial (along the beam) resolution than a longer pulse duration. Therefore, for better resolution, higher frequencies (shorter wavelength) and a smaller number of cycles will improve axial resolution.

 

 

The time from the start of one pulse to the start of the next pulse is the pulse repetition period. The transmission time and the listening time is the pulse repetition period. While the pulse length could be changed by changing frequencies or adjusting the number of cycles in the pulse duration, the pulse repetition period is mainly determined by the listening time. The listening time is the time that the transducer can receive reflected signals from the transmitted beam. If the depth of the ultrasound scan is increased, a longer listening time is required to be able to receive reflected signals. Therefore, the depth of the scan mainly determines the pulse repetition period.

Figure 1.3.5 Pulse Repetition Period

 

Pulse Repetition Frequency (PRF) is the number of pulses an ultrasound transmits per second. Since the pulse repetition frequency is dependent upon the pulse repetition period, pulse repetition frequency is dependent upon the depth of a scan. A deeper scan requires a slower pulse repetition frequency to allow the deeper reflected signals to be received during the listening time.

  

 

After the piezoelectric crystal has been stimulated with a pulse of electrical current the piezoelectric crystal doesn't immediately stop emitting an ultrasound wave. The piezoelectric crystal continues to emit an ultrasound wave for a short period of time even though the electrical current is off. The time from the end of the electrical pulse to the end of the ultrasound pulse is the ring down time. The piezoelectric crystal slowly decreases its ultrasound wave emissions until if finally stops. The ring down time is determined by the type of crystal and the damping material used.  A shorter pulse length will also have a shorter ring down time when compared to a longer pulse length.

Animation 1.3.5 Ring-Down Time

  

 

The time delay between the transmitted signal and the returned signal will determine the depth of the structure. If the speed of the ultrasound wave is known or assumed (i.e. 1540 m/sec in blood) then the time between the transmitted signal to the received signal will determine the structure's depth. In Animation 1.3.6, an ultrasound pulse travels to a depth and is reflected back to the transducer and displayed on the monitor. If a structure is twice as deep, then the ultrasound pulse takes twice as long to be transmitted and received. Therefore, the structure would be displayed as being twice as deep as the original structure. However, if a structure were able to trap or delay the returning ultrasound pulse to twice it's original time, then that structure would also be displayed at twice the depth of the original structure.

Animation 1.3.6 Time and Depth Relationship

  

 

Aliasing is a concept where of velocity of an object and sampling rate of a view that includes the object results in a video that appears that the object is moving in the opposite direction. (The classic example is the spinning wheels of a wagon in an old western movie are spinning backwards.) In Animation 1.3.7 a ball is bouncing against a wall. There are 40 frames and all of the frames are sampled (played). The motion of the ball appears continuous and the direction of the ball is easily discerned. The sampling rate of the ball is fast enough to capture the ball, at it's given velocity, to display a smooth video of the ball's path. Each frame captured and displayed represents a sample of the motion of the ball.

 

Animation 1.3.7
  All Frames

An echocardiographic machine works in a similar fashion. From the motion of the ball where every frame is captured and displayed, the motion of the bouncing ball appears smooth. To capture a frame, the echocardiographic machine must send out ultrasound pulses along scan lines, wait for pulses to return, scan thru a sector, interpret the returned pulses and display all of the information.  The scanning process will cause the echocardiographic machine to have a maximal frame rate. If the motion occurs faster than the maximal frame (sampling) rate, then the echocardiographic machine is only intermittently capturing images or frames of the real motion.

 

Animation 1.3.8
  Every 5th Frame

Depending upon the velocity of the motion and the actual frame rate, some interesting effects can occur. If the velocity of the ball were to occur at such a rate where only every fifth frame (shown in Animation 1.3.8) of the real motion were sampled then the motion, while appearing jerky, appears to be traveling in the same direction as the real event.

 

Animation 1.3.9
  Every 10th Frame

In Animation 1.3.9, if the velocity were to increase such that only every tenth frame of the true motion were captured, the result would be a jerkier picture, but still, one can deduce the real path of the ball.

 

Animation 1.3.10
  Every 20th Frame

In Animation 1.3.10, upon a further increase in the ball's velocity, the ball will only be sampled at the same position (every 20th frame) and the ball appears either to not be moving or only flashing in one spot. The direction of the ball is not able to be ascertained.  This is the threshold of aliasing.

 

Animation 1.3.11
  Every 30th Frame

In Animation 1.3.11, further increases in the velocity of the motion will result in a sampling (frame rate) of the true motion of the ball such that the ball will appear to be moving in the opposite (negative) direction along the true path because it is sampled at every 30th frame. The negative or opposite direction of the moving ball is called aliasing. The sampling or frame rate has an aliasing threshold at the 20 fps/40 fps frame or 1/2 of the maximal frame(sampling) rate. The speed of sampling has a threshold level for aliasing at the maximal frame rate divided by two. The maximal frame rate is called the Nyquist Limit. The Nyquist limit, on echocardiographic machines, can be calculated by dividing the maximal velocity of sampling by two. Aliasing occurs above the Nyquist Limit.

 

Animation 1.3.4
  Normal Wave

Animation 1.3.5
  Aliasing

 

In echocardiography machines, many factors control the sampling rate.  If the velocity of a wave exceeds the sampling rate's capacity to display the maximum velocity, the wave's velocity is sampled opposite of it's true direction. Therefore, the wave is displayed as a velocity profile on the opposite side of the baseline. Since the velocity is exceeding the maximum velocity of the scale, it is displayed at the top of the opposite side of the baseline.