Physics 101: The Basics II

Keywords:
Continous Wave
Pulsed Ultrasound
Pulse Repitition Frequency
Pulse Repitition Period
Duty Factor
Spatial Pulse Length
Bandwidth
Fractional Bandwidth
Quality factor

Continuous wave (CW) is described where cycles are being repeated indefinitely thereby defined as a single frequency. On the other hand, pulsed wave or pulsed ultrasound (PW) is described as an ultrasound pulse of a few cycles seperated by gaps in time. The Pulse Repitition Frequency (PRF) is the number of pulses occuring in one second, and the units are in kilohertz (kHz). The Pulse Repition Period is the time measured from the beginning of one pulse to the beginning of the next, and is reciprocal to PRF. The Pulse Duration (PD) is the time it takes for one pulse to occur, and is proportional or equal to the period times the number of pulses or cycles in the pulse (n). The Pulse Duration (PD) is also reciprocal to the frequency, that is, if the frequency increases, then PD decreases thus a shorter pulse is produced. Shorter pulses are necessary to improve the quality of sonographic images.

In CW, the ultrasound is on 100% all the time, whereas in PW, the ultrasound is not on all the time. The time it takes for ultrasound to be on depends upon the Duty Factor (DF), which is the "fraction" of time that pulsed ultrasound is on. In other words, DF represents the amount of time that the sound is on. The Duty Factor (DF) is proportional to PRF and PD and reciprocal to PRP; that is, as PRF increases then DF increases and PRP decreases. In relation to frequency, the acutal number of cycles in PW depends upon DF as well. For example, a 5-MHz frequency CW ultrasound has 5-million cycles occuring per second, whereas in a 5-MHz frequency PW ultrasound has 50,000 cycles occuring per second or 50kHz. This is due to DF being only 1.0%, which makes the ultrasound being on only one hundreth of the time.

A pulse can be measured by acquiring the Spatial Pulse Length, which is the length of the pulse from front to back. The Spatial Pulse Length is proportional to the number of cycles in a pulse and the wavelength of the pulse. Units for SPL are in millimeters. Since wavelength increases with decreasing frequency, SPL increases with decreasing frequency. Therefore, if frequency increases then SPL and wavelength decreases and a shorter pulse is produced, which improves sonographic image resolution.

As frequency increases, shorter pulses are generated, which will increase the bandwith (more broader) and decrease the wavelength as well as SPL. In other words, the shorter the pulse (the fewer number of cycles), the more frequencies present in it (broader bandwidth). Bandwidth is defined as the range of frequencies contained in a pulse. Therefore, shorter pulses have broader bandwidths and fractional bandwidths, which is the bandwidth divided by the operating frequency and is reciprocal to the Quality factor (Q). In this case, Q would be low and thus signify a shorter pulse with a broad bandwidth at higher frequencies.