Ultrasound scanners generate high frequency soundwaves through a piezoelectric transducer. These soundwaves are in the range of 2 to 10 Megahertz. Different tissues in the body have different ultrasound velocities. For example, ultrasound velocity in air is 330 m/sec and in skull bone it is 4080 m/sec. But the velocity in most of the abdominal soft tissues is close to 1540 m/sec. Ultrasound images depend primarily on the balance between the reflection and transmission of soundwaves.
Various types of transducers including linear, convex and sector are available. For routine abdominal imaging, a 3.5 MHz convex probe is the best option. Sector probe is especially helpful in scanning through the intercostal areas. Linear probe (3.5 MHz) may be used in some obstetric cases. A 5 MHz probe may be used for pediatric ultrasound.
Coupling gel forms a medium between the transducer and the patient's skin surface. The ideal coupling gel is a water soluble coupling gel, usually comprising of EDTA, carbomer, propylene glycol, trolamine and distilled water. Coupling gel should be applied liberally.
In general, examination of the upper abdomen requires the patient to come after about 8 hours of fast. He/she can have water during this period. In infants and small children, fasting for about 3 hours usually suffices. For lower abdominal scans, the patient is required to drink plenty of fluids and hold urine for about 3 hours. If for some reason the patient cannot be kept fasting for too long or cannot hold urine, then one should go ahead with the scanning and gain as much information as possible. Then, if required, the patient can be called again.
Sagittal (and parasagittal), transverse and coronal sections are the main scanning planes. Modified version of these planes with some angulation and obliquity is required in most cases. Apart from the views mentioned in the following chapters (for different organs), the operator should apply his own judgement to modify these views according to the need.