Doppler techniques are dependent on the transducers used. The transducer operating in continuous wave mode utilizes one half of the elements and is continuously sending sound energy while the other half is continuously receiving the reflected signals. If the transducer is being used in a pulsed wave mode, the whole transducer is used to send and then receive the returning signals.
Pulsed wave techniques allow the accurate measurement of blood flow at a specific area in the heart and the detection of both velocity and direction. Measurement is performed by timing the reception of the returning signals giving a view of flows at specific depths. The region where flow velocities are measured is called the sample volume. Errors in the accuracy of the information arise if the velocities exceed a certain speed. The highest velocity accurately measured is called the Nyquist limit.

See also Doppler Velocity Signal and Doppler Effect.

(F) The number of cycles of a periodic process per unit time. Frequency and wavelength are inversely related. The higher the frequency the smaller the wavelength. The frequency of ultrasound is expressed in units of hertz (Hz), where 1 Hz = 1 cycle per second.
The effect of different frequencies on tissue penetration:
The higher the frequency the less the penetration, the lower the frequency the greater the penetration. As frequency increases, resolution improves but the imaging depth or penetration decreases. The lower the axial resolution, the more detail can be seen.
Usual frequencies for pediatric ultrasound: 5.0mHz to 7.5mHz and 10mHz.
Usual frequencies for adult ultrasound: 2.0mHz to 3.0mHz.

See also Doppler Interrogation Frequency, Multi-frequency Probe, and Huygens Principle.

Standard scanners allow visualizing microbubbles on conventional gray scale imaging in large vascular spaces. In the periphery, more sensitive techniques such as Doppler or non-linear gray scale modes must be used because of the dilution of the microbubbles in the blood pool. Harmonic power Doppler (HPD) is one of the most sensitive techniques for detecting ultrasound contrast agents.
Commonly microbubbles are encapsulated or otherwise stabilized to prolong their lifetime after injection. These bubbles can be altered by exposure to ultrasound pulses. Depending on the contrast agent and the insonating pulse, the changes include deformation or breakage of the encapsulating or stabilizing material, generation of free gas bubbles, reshaping or resizing of gas volumes.
High acoustic pressure amplitudes and long pulses increase the changes. However, safety considerations limit the pressure amplitude and long pulses decrease spatial resolution. In addition, lowering the pulse frequency increases destruction of contrast bubbles. However, at low insonation power levels, contrast agent particles resist insonation without detectable changes. Newer agents are more reflective and will usually allow gray scale imaging to be used with the advantages of better spatial resolution, fewer artifacts and faster frame rates.

Feasible imaging methods with advantages in specific acoustic microbubble properties:
Resonating microbubbles emit harmonic signals at double their resonance frequency. If a scanner is modified to select only these harmonic signals, this non-linear mode produces a clear image or trace. The effect depends on the fact that it is easier to expand a bubble than to compress it so that it responds asymmetrically to a symmetrical ultrasound wave. A special array design allows to perform third or fourth harmonic imaging. This probe type is called a dual frequency phased array transducer.

See also Bubble Specific Imaging.

Higher frequencies are attenuated by tissue more than lower frequencies. This means that the higher the frequency the lower the depth of penetration but the greater the resolution.
Harmonic imaging allows the use of a lower frequency pulse to be picked up and sampled at the second harmonic (higher frequency) where the low frequency allows greater penetration and high frequency provides better resolution.

See also Skinline.

Ultrasound technology has evolved significantly, providing sonographers with a wide range of ultrasound machines. As technology has advanced, portable ultrasound equipment, including handheld ultrasound systems, have emerged in the field of medical imaging. However, these devices may have limited imaging capabilities and reduced image quality compared to larger systems.
Types of ultrasound systems compiled according to their portability:

In summary, pros and cons of portable ultrasound machines:

See also Ultrasound Accessories and Supplies, Environmental Protection, Sonographer, Ultrasound Technology and Equipment Preparation.