Ultrasonography is a highly operator dependent imaging modality with a number of knobology variables that are under the control of the operator. Knobology is a terminology that describes the manipulation of ultrasound knobs and system controls in order to obtain the best image possible from diagnostic ultrasound. The inadequate use of knobology variables may impair image quality and can result in misdiagnosis. In abdominal sonography, selecting the appropriate application preset for abdominal examination is first step towards achieving an optimum image. The next step is to select an appropriate transducer frequency which must take the size of the patient into account. Transducer frequency is typically in the range of 3–5 MHz, but a lower frequency may achieve better depth penetration in larger patients. While the output power may improve image quality by increasing the intensity of transmitted sound energy, the impact is usually insignificant. The practice of using high output power should therefore be limited because of the risk of biologic effect. Other essential knobs for better image optimization include controlling the overall gain, time gain compensation, focal zone, dynamic range and tissue harmonic imaging. In the assessment of blood flow in abdominal vessels the regulation of the pulse repetition frequency, Doppler gain, imaging angle, and wall filter improves the sensitivity of color and spectral Doppler.
Part of the book: Essentials of Abdominal Ultrasound