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In general, ultrasound is defined as an acoustic vibration with frequencies greater than 20,000 Hz – in short, beyond our ability to hear, and is usually created by ultrasound machines utilizing ferromagnetic lead-zirconate-fitanate (PZT) ceramics that convert electricity into sound (DeLisa, 2005). Its uses are either diagnostic (under which the well-known fetal ultrasound falls under) or therapeutic, which uses such acoustic energy to produce various thermal and non-thermal effects in tissue. Ultrasonic signals are usually generated by the reverse piezoelectric effect, and lose some energy as they travel through tissue – attenuation. This comes as the result of 3 mechanisms: absorption, in which the ultrasonic energy is absorbed by tissue and then converted into heat; beam divergence, the amount by which a beam spreads out from the transducer; and deflection, which in turn is broken down into reflection, refraction and scattering (Braddom, 2007).
Despite the successive technological breakthroughs where ultrasound is concerned, 2D ultrasound is still the most common variant seen in most clinics and hospitals (GPSMedical, 2007), and, by necessity, is still conducted before 3D or 4D ultrasound (Benaceraff et al, 2005). Since the 3D and 4D variants are still relatively new, 2D will still be the default for a while longer. In fact, because of this, even newer ultrasound machines have a 2D setting.
What differs it from its more advanced cousins is that 2D ultrasound allows one to visualize something in planes or layers, as opposed to 3D or 4D which only shows the exterior (Expectant Mother’s Guide Online Edition, accessed 10/30/09). For pregnant women, it is usually best to have this kind of ultrasound after at least 18-20 weeks of pregnancy, and with a full bladder (Stork Vision, 2009) – this is also the period when doctors check
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