There are several different scanning modes in medical and obstetric ultrasound. The standard common obstetric diagnostic mode is 2D scanning. In 3D fetal scanning, however, instead of the sound waves being sent straight down and reflected back, they are sent at different angles. The returning echoes are processed by a sophisticated computer program resulting in a reconstructed three-dimensional volume image of the fetus's surface or internal organs, in much the same way as a CT scan machine constructs a CT scan image from multiple x-rays.
Clinical use of this technology is an area of intense research activity especially in fetal anomaly scanning but there are also popular uses that have been shown to improve fetal-maternal bonding. 4D fetal ultrasounds are similar to 3D scans, with the difference associated with time: 4D allows a 3-dimensional picture in real time, rather than delayed, due to the lag associated with the computer constructed image, as in classic 3-dimensional ultrasound.
If the system is used only in the Obstetrics Application, the ultrasound energy is limited by the manufacturer below FDA limits for obstetrical ultrasound, whether scanning 2, 3 or 4 dimensionally. (The FDA limit for obstetrical ultrasound is 94 mW/cm2.) While there has been no conclusive evidence for harmful effects of 3D ultrasounds on a developing fetus, there still remains controversy over its use in non-medical situations, and generally, the AIUM recommends that 3D ultrasounds should be undertaken with the understanding that a risk may exist.
- 1 Background and Risks
- 2 Regional Anesthesia
- 3 References
- 4 Further reading
- 5 External links
Background and Risks
Although 3D ultrasound technology may be used on any part of the body, elective 3D ultrasound conventionally refers to 3D ultrasounds performed on pregnant women for the sole purpose of the woman to see her unborn baby, what the baby looks like, or to see whether the baby will be a boy or a girl. In the medical literature, these elective 3D ultrasounds are also referred to as keepsake ultrasounds, although this term is rarely used in the lay population and only sometimes used in the media.
Risks Related to "Keepsake" Ultrasounds
The use of ultrasound imaging devices for producing fetal keepsake videos is viewed as an unapproved use by the Food and Drug Administration (FDA). "When ultrasound enters the body, it heats the tissues slightly. In some cases, it can also produce small pockets of gas in body fluids or tissues." Phillips [FDA Spokesman] says the long-term effects of tissue heating and of the formation of partial vacuums in a liquid by high-intensity sound waves (cavitation) are not known. "Performing prenatal ultrasounds without medical oversight may put a mother and her unborn baby at risk," says Phillips [FDA Spokesman]. "The bottom line is: Why take a chance with your baby's health for the sake of a video?" Fetal keepsake videos are viewed as a problem because there is no medical benefit derived from the exposure. Further, there is no control on how long a single imaging session will take or how many sessions will occur. FDA is aware of entrepreneurs that are commercializing ultrasonic imaging of fetuses by making keepsake videos. In some cases, the ultrasound machine may be used for as long as an hour to get a video of the fetus.
For these reasons, medical insurance does not cover keepsake ultrasounds and many doctors will discourage the use.
One challenge that ultrasound operators face is keeping the transducer positioned over the part of the fetus the operator is trying to visualize. When fetuses move away from the stream of high-frequency sound waves, they may be feeling vibrations, heat or both. The FDA warned in 2004, "ultrasound is a form of energy, and even at low levels, laboratory studies have shown it can produce physical effect in tissue, such as mechanical vibrations and a rise in temperature." This is consistent with research conducted in 2001 in which an ultrasound transducer aimed directly at a miniature hydrophone placed in a woman's uterus recorded sound "as loud as a subway train coming into the station." 
Early studies showed that subtle effects of neurological damage linked to ultrasound were implicated by an increased incidence in left-handedness in boys (a marker for brain problems when not hereditary) and speech delays. Then in August 2006, Pasko Rakic, chair of Yale School of Medicine's Department of Neurobiology, announced the results of a study in which pregnant mice underwent various durations of ultrasound. The brains of the offspring showed damage consistent with that found in the brains of people with autism. The research, funded by the National Institute of Neurological Disorders and Stroke, also implicated ultrasound in neurodevelopmental problems in children, such as dyslexia, epilepsy, mental retardation and schizophrenia, and showed that damage to brain cells increased with longer exposures.
Other unintended harm caused by 3D ultrasounds may include the finding of false positives, which can be a source of distress to the mother. Cysts found on the fetus that would have been harmless and may not have been found otherwise may be detected on 3D ultrasound and lead to further more invasive exams as well as cause anxiety to the mother. Artifacts can be seen on 3D ultrasound, particularly by those with less experience, such as duplications of body parts, holes, clefts, and missing body parts. However these outcomes are far less likely when the ultrasound is performed by an accredited, experienced ultrasonographer.
Seeing the fetus too early, under 17 weeks, may cause some harm to the mother as it may actually decrease bonding and give a feeling to the mother that the baby is alien or "not really a baby yet." It is generally advised to perform 3D ultrasounds at or after 17 weeks of gestation for this reason (Kurjak et al., How useful is 3D and 4D ultrasound in perinatal medicine? J. Perinatal Medicine, 2007).
Other incidental harms to elective 3D ultrasounds include falls or back pain when getting on and off a bed not conducive to pregnant women in 3D ultrasound centers; aggressive manipulation of the belly by an inexperienced ultrasound technician; encouragement of the pregnant mother to drink caffeine-containing beverages to stimulate the baby for a "good photo shot". Caffeine, even in small amounts, has been implicated with increased risk of fetal growth restriction. However most health care professionals state that up to 200 mg of caffeine a day is safe.
Substitution of prenatal care
Pregnant women in the United States who can not otherwise afford health care and do not have health insurance may see the payment of an elective 3D ultrasound, which may cost around $100–$200, as a cheaper alternative to the prices of prenatal care from a doctor, clinic or hospital.
Timing of 3D ultrasound
Although medical textbooks state that the sex of the fetus can be seen on ultrasound as early as 12 weeks, practically speaking, most medical facilities and hospitals do not attempt to determine the sex of the fetus until the 20-week ob/gyn visit. 3D/4D ultrasound centers bridge the gap and fulfill the niche market of pregnant women who can not wait to find out the sex of the fetus, women who did not find out the sex of the fetus at their medical visit and the insurance does not pay for any more ultrasounds, and women who simply want to confirm the sex of the fetus.
Some ultrasound centers attempt to determine the sex of the fetus as early as 15 weeks and advertise as such. The customer is required to pay at the first visit, but most of the time, the customer has to return when the fetus is more mature. At 15 weeks, the success rate for being able to see the sex is around 50% at best, and the accuracy at that gestational age is questionable.
At 16 weeks, the ability to find the sex and accuracy rises to 99%. Most private ultrasound centers offer sex scans from 16 weeks. The hospital do not offer it before the 20-week anatomy scan unless there is a medical reason that it is required.
Visualization of the fetus
3D ultrasounds are best done at 24–32 weeks, and ideally between 26 and 30 weeks. Most 3d imaging centers advise[which?] customers to come in between 26 and 28 weeks to get the best images.
After 32 weeks, the fetus has descended into the pelvis and at that this time getting good 3D images of the fetus is still possible based on the expertise of the ultrasound technician. Clear images are possible to the end of the pregnancy depending on the position of the baby, placenta and amount of amniotic present around the face.
Unlike the medical ultrasound visit, customers of 3D ultrasound centers are not advised to hold their urine or have a full bladder before their 3D ultrasound appointment.
In order to get good images, ultrasound centers advise customers to drink adequate amounts of water (32 ounces/day) for the one to two weeks prior to their appointment. This is in order to ensure there is an adequate amount of amniotic fluid around the fetus and that the fluid is clear. Drinking large amounts of water immediately before the 3D ultrasound appointment does not help to have clearer 3D ultrasound images.
3D ultrasounds may[ambiguous] soon be covered by FSA (flexible spending accounts) and may eventually be accepted by some insurance companies, as medical studies begin to show benefits of elective 3D ultrasounds.
State laws are developing to require oversight and restrictions on 3D ultrasound centers, including the requirement of a medical director, requiring verification of prenatal care, and/or requiring certified ultrasound technicians.
3D ultrasounds are already being used to detect fetal anomalies of the heart. 3D ultrasounds may[ambiguous] be used in the near future for actual neurological and behavioral testing of the fetus to help diagnose or rule out cerebral palsy.
A novel technology enabling the use of 3D ultrasound in remote areas has been recently developed. This new technology leverages the ubiquitous mobile phone available even in the most remote corners of the world. By collecting the raw data at the patient location and sending it for processing and expert evaluation to the central processing station, this technology reduces equipment costs and reduces the hand-eye coordination skills normally required from the technician performing the procedure. 
Conversion of the 3D image files into standard CAD/CAM file formats allows the reconstruction of fetal and other images in a variety of materials including a 3d laser etched images in a crystal glass block or a solid cameo effect using a 3D printer.
Real-time three-dimensional ultrasound is used during peripheral nerve blockade procedures to identify relevant anatomy and monitor the spread of local anesthetic around the nerve. Peripheral nerve blockades prevent the transmission of pain signals from the site of injury to the brain without deep sedation, which makes them particularly useful for outpatient orthopedic procedures. Real-time 3D ultrasound allows muscles, nerves and vessels to be clearly identified while a needle or catheter is advanced under the skin. 3D ultrasound is able to view the needle regardless of the plane of the image, which is a substantial improvement over 2D ultrasound. Additionally, the image can be rotated or cropped in real time to reveal anatomical structures within a volume of tissue. Physicians at the Mayo Clinic in Jacksonville have been developing techniques using real time 3D ultrasound to guide peripheral nerve blocks for shoulder, knee, and ankle surgery.
- Michailidis GD, Papageorgiou P, Economides DL (March 2002). "Assessment of fetal anatomy in the first trimester using two- and three-dimensional ultrasound". The British journal of radiology (Br J Radiol.) 75 (891): 215–219. PMID 11932213.
- Von Ramm OT, Smith SW. "Three-dimensional imaging system (patent)".
- Benacerraf, BR; Benson, CB; Abuhamad, AZ; Copel, JA; Abramowicz, JS; Devore, GR; Doubilet, PM; Lee, W; Lev-Toaff, AS; Merz, E; Nelson, TR; O'Neill, MJ; Parsons, AK; Platt, LD; Pretorius, DH; Timor-Tritsch, IE (2005). "Three- and 4-dimensional ultrasound in obstetrics and gynecology: proceedings of the american institute of ultrasound in medicine consensus conference". Journal of ultrasound in medicine : official journal of the American Institute of Ultrasound in Medicine 24 (12): 1587–1597. PMID 16301716.
- Benoit B, Chaoui R (2004). "Three-dimensional ultrasound with maximal mode rendering: a novel technique for the diagnosis of bilateral or unilateral absence or hypoplasia of nasal bones in second-trimester screening for Down syndrome". Ultrasound in Obstetrics and Gynecology (Ultrasound in Obstetrics and Gynecology) 25 (1): 19–24. doi:10.1002/uog.1805. PMID 15690554.
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- Ji EK et al. (473-477). "Effects of ultrasound on maternal-fetal bonding: a comparison of two- and three-dimensional imaging". Ultrasound in Obstetrics and Gynecology (Ultrasound in Obstetrics and Gynaecology) 25 (5): 19. doi:10.1002/uog.1896. PMID 15846757.
- Guidance for Industry and FDA Staff - Information for Manufacturers Seeking Marketing Clearance of Diagnostic Ultrasound Systems and Transducers
- Fetal Keepsake Videos, Laboratory studies have shown that diagnostic levels of ultrasound can produce physical effects in tissue, such as mechanical vibrations and rise in temperature.
- Fetuses can hear ultrasound examinations
- Keiler, H., et al. 2001. Sinistrality—a side-effect of prenatal sonography: A comparative study of young men. Epidemiology 12(6): 618–23; Campbell, J.D., et al. 1993
- Salvesen K.A., Vatten L.J., Eik-Nes S.H. et al. (1993) Routine ultrasonography in utero and subsequent handedness and neurological development. B.M.J. 307: 159-64.
- Kieler H., Axelsson O., Haglund B. et al. (1998) Routine ultrasound screening in pregnancy and children’s subsequent handedness Early Hum. Dev. 50: 233-45.
- BMJ 2008;337:a2332. "Maternal caffeine intake during pregnancy and risk of fetal growth restriction: a large prospective observational study".
- Meir A, Rubinsky B, 2009 Distributed Network, Wireless and Cloud Computing Enabled 3-D Ultrasound; a New Medical Technology Paradigm. PLoS ONE 4(11): e7974. doi:10.1371/journal.pone.0007974
- Beattie, Bryan (2011). "The conversion of 3D ultrasound images to CAD/CAM formats".
- Real-Time 3-D Ultrasound Speeds Patient Recovery
- Feinglass NG, et al. "Real-time three-dimensional ultrasound for continuous popliteal blockade: a case report and image description." Anesth Analg. 2007 Jul;105(1):272-4.
- Article: 3D and 4D Ultrasound Scanning
- Article: 4D: What the Medical Literature Says
- The History of Ultrasounds (including 3D ultrasounds)
- Article: Why are Ultrasound Scans Used In Pregnancy
- Article: Ultrasound Technician Schools in United States
- The Endowment for Human Development provides numerous 4D ultrasounds that can be viewed online.
- Scans uncover secrets of the womb BBC News
- Learn how to perform 3D/4D Elective Ultrasounds
- About the discovery of medical ultrasonography
- RadiologyTube - 3D and 4D Ultrasound Videos
- History of medical sonography (ultrasound)
- Safety Concerns Details a number of health concerns over this unregulated practice