Introduction:
Ultrasound is a medical imaging technique that uses high-frequency sound waves to visualize the internal structures of the human body. It is commonly used to investigate and track various medical problems. However, ultrasound equipment works at specific frequencies, and the correct answer to the given question is option 'C' - higher than 20000Hz.

Explanation:
Ultrasound imaging involves the use of sound waves that are at a frequency higher than what the human ear can detect. These sound waves are emitted by a transducer, which is a handheld device that sends and receives the waves. The transducer contains piezoelectric crystals that vibrate when an electrical current is applied to them.

Working of Ultrasound Equipment:

• Transmission of Sound Waves: When the transducer is placed on the body, it emits a beam of sound waves into the body. These sound waves penetrate the tissues and bounce back (echo) when they encounter different structures such as organs, blood vessels, or tumors.


• Reception of Echoes: The echoes produced by the sound waves bouncing back are detected by the transducer. The piezoelectric crystals in the transducer convert the echoes into electrical signals.


• Signal Processing: The electrical signals are then processed by the ultrasound machine. The machine analyzes the time it takes for the echoes to return and the strength of the returning echoes to create an image of the internal structures.


• Image Formation: The processed signals are converted into a visual representation on the ultrasound machine's display screen. This image provides valuable information about the anatomy and condition of the tissues being examined.

Choice of Frequency:
The frequency of the sound waves used in ultrasound imaging is crucial for obtaining clear and detailed images. The choice of frequency depends on the specific application and the depth of the structures being examined.

Higher Frequency for Detailed Imaging:
Higher frequencies, such as those greater than 20,000Hz (20kHz), are used for imaging superficial structures close to the skin surface. These higher frequencies provide greater resolution and detail, allowing the visualization of small structures and fine anatomical features. They are commonly used for imaging the breast, thyroid, testicles, and superficial soft tissues.

Limitations of Higher Frequencies:
While higher frequencies offer better resolution, they have limited penetration capabilities. As the frequency increases, the sound waves are absorbed and scattered more rapidly by the tissues, limiting their ability to travel deeper into the body. Therefore, higher frequencies are not suitable for imaging deeper structures or obese patients.

Conclusion:
In conclusion, ultrasound equipment for investigating and tracking medical problems works at frequencies higher than 20,000Hz. The choice of frequency depends on the depth of the structures being examined, with higher frequencies providing better resolution for superficial structures. However, it is important to consider the limitations of higher frequencies in terms of penetration depth.