As far as ultrasound is concerned, 4D ultrasound (also referred to as live 3D ultrasound or 4B-mode) is the latest ultrasound technology - the fourth dimension means length, width, and depth over time. 4D Ultrasound takes 3D ultrasound images and adds the element of time to the progress so that a moving three-dimensional image is seen on the monitor. A 4D scan takes the same amounts of time as a 2D or 3D scan; the difference is the ultrasound equipment being used. One advantage of a 4D fetal ultrasound to a 2D-mode is that parents can see how their baby will generally look like. However, there are different opinions over the medical advantages.
To scan a 3D ultrasound image, the probe is swept over the maternal abdomen. A computer takes multiple images and renders the 3D picture. With 4D imaging, the computer takes the images as multiple pictures while the probe is hold still and a 3D image is simultaneously rendered in real time on a monitor.
In most cases, the standard 2D ultrasound is taken, and then the 3D/4D scan capability is added if an abnormality is detected or suspected. The 3D/4D sonogram is then focused on a specific area, to provide the details needed to assess and diagnose a suspected problem. A quick 4D scan of the face of the fetus may be performed at the end of a routine exam, providing the parents with a photo.

Sound pressure is reported on a logarithmic scale called sound-pressure level, expressed in decibel (dB) referenced to the weakest audible 1 000 Hz sound pressure of 2*10-5 Pascal (20 mP). Sound level meters contain filters that simulate the ear's frequency response. The most commonly used filter provides what is called 'A' weighting, with the letter 'A' appended to the dB units, i.e. dBA.

An ultrasound (US) scanning gel has the same conductivity as the human body and is applied between the transducer and the skin surface. Air is a bad conductor of US, so this acoustic gel is used to conducts the sound beam and allows the ultrasound probe to pass smoothly over the skin.
The gel will be removed after the examination, and it will not stain skin or clothing. The basic dermatological requirement of a scanning gel is that it be free of skin irritants or sensitizers. In addition, effective preservatives with low incidence of skin reaction are required to prevent microbiological degradation of the gel. The broad range of patients imaged with ultrasound, from pregnant women and infants to the infirm or elderly dictates that the risk of skin reaction must be minimized.
The effect of small bubbles in the ultrasound couplant under the transducer is to disperse the ultrasound which results in clouding of the image. This effect is most clearly seen on anechoic regions of the image which becomes cloudy. Air bubbles, regardless of their size, degrade the performance of ultrasound in all medical applications including imaging, Lithotripsy and physical therapy.
There are some chemicals, including mineral oil, silicone oil, alcohol, surfactants, and fragrances that can degrade the acoustic lens, destroy bonding, or change the acoustic properties of the lens. The use of scanning gels or lotions in diagnostic ultrasound containing these chemicals should be avoided. In therapeutic ultrasound, ultrasound transmission gels and lotions commonly contain oils and other chemicals not intended for use with diagnostic imaging transducers.

See also Ultrasound Therapy and Ultrasound Physics.

An echo is defined as the repetition of a sound by reflection of sound waves from a surface.
Echo types used in ultrasound imaging:
Specular echoes are created from relatively large, regularly shaped objects with smooth surfaces. Specular echoes are relatively intense and angle dependent.
Scattered echoes are created from relatively small, weakly reflective, irregularly shaped objects. Scattered echoes are less angle dependant and less intense.

See also Specular Echo, and Scattered Echo.

Fetal ultrasound is a safe and non-invasive imaging technique used to visualize and monitor the development of a fetus during pregnancy. It employs high-frequency sound waves to create detailed images of the baby, the placenta, and the uterus. Fetal ultrasound provides valuable information about the baby's growth, organ development, and overall well-being. It is commonly used to determine gestational age, assess fetal anatomy, detect abnormalities, and monitor fetal movements and heart rate. This essential tool enables healthcare professionals to ensure the optimal health of both the mother and the baby throughout the pregnancy.
The FDA (Food and Drug Administration) has established regulations governing ultrasound usage, including specific guidelines for fetal ultrasound examinations. These regulations permit an eight-fold increase in ultrasound intensity for fetal scans. They place considerably responsibility on the user to understand the output measurements, the mechanical index (MI), the thermal index (TI) and to use them in their scanning. The primary safety concern in prenatal diagnostic imaging is temperature rise. It is known that hyperthermia is teratogenic. The efforts of investigators have concentrated on defining the temperature increases and exposure times which may give rise to biological effects and on determining the ultrasound levels which might, in turn, lead to those temperature rises.
In fetal ultrasound, the highest temperature increase would be expected to occur at bone and the thermal index with bone at/near the focus (TIB) would give the 'worst case' conditions. The mechanical index and thermal index must be displayed if the ultrasound system is capable of exceeding an index of 1. The displayed indices are based on the manufacturer's experimental and modeled data. However, an independent study has demonstrated significant discrepancies over declared spatial peak time averaged intensity (I-SPTA) output of up to 400%.

See also ALARA Principle, Pregnancy Ultrasound and Doppler Fluximetry in Pregnancy.