Which of the following is ultrasonic and least useful in diagnostic imaging?

• A. 30 kHz
• B. 8 MHz
• C. 8,000 Hz
• D. 15 Hz

Answer: (A)

Diagnostic ultrasound relies on frequencies in the megahertz range because higher frequency waves give finer spatial detail, though they don’t penetrate as deeply. Lower frequencies travel farther but produce blurrier images. Among the options, frequencies of 15 Hz and 8,000 Hz are not ultrasonic at all. An 8 MHz frequency is commonly used in clinical imaging, providing a good balance of resolution and penetration. A frequency of 30 kilohertz is indeed ultrasonic, but it’s far too low for diagnostic imaging—the long wavelength would yield extremely poor resolution and unusable images. Therefore, the ultrasonic frequency that is least useful for diagnostic imaging is 30 kilohertz.