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Prehospital ultrasound is the specialized application of ultrasound by emergency responders, such as paramedics, to guide immediate care and first aid procedures. Like conventional ultrasound, it is a device that produces cyclic sound pressure to penetrate a medium (usually flesh, in this instance) and reveal details about the inner structure of the medium.
Differences between prehospital ultrasound and conventional ultrasound 
Many emergency physicians now view screening ultrasound as a tool, and not a procedure or study. It is primarily used to quickly and correctly ascertain a limited set of internal injuries, specifically those injuries where conventional methods of determining them, such as trauma to the torso or heart, would either take too long, require too much time to prepare, or introduce greater risk to the patient.
While conventional ultrasound can be a lengthy process, and is usually conducted with non-mobile units and advanced image filtering and manipulation built into the unit, emergency ultrasound is as simple and quick to operate as possible, and narrowly focused on a small set of criteria.
Indications for the use of prehospital ultrasound are determining the severity of trauma to the midsection, determining immediate trauma or penetration of the heart, and to process sources and extent of internal bleeding. Specific implementations vary, some areas use the German originated FAST scan system which focuses on fluids in the paracolic gutter and the Douglas and Morrison pouch. Others focus more on cardiac ultrasounds. As a part of the FAST exam, it is common for the examiner to quickly image the sliding lung against the chest wall to rule out pneumothorax. If the visceral and parietal lung have separated, the lung sliding will disappear indicating a probable pneumothorax. If lung sliding is seen, there is no significant pneumothorax. This simple addition to the FAST exam can accurately identify lung collapse in only 1 additional minute. This Enhanced FAST exam is often called EFAST. Another useful indication is assessing intravascular volume by looking directly at the inferior vena cava (IVC). Large IVCs that don't collapse during inspiration suggest that there is plenty of blood returning to the heart. Cardiac imaging can give the provider a sense of ejection fraction and heart function. Patients with pulmonary embolism can have a markedly enlarged right ventricle during the event, giving additional clues to the provider for correct diagnosis and treatment. As time goes on, it is likely that there will be dozens of additional indications for field use.
Prehospital ultrasound procedures 
In most cases, prehospital providers will employ the use of a portable ultrasound unit. In every instance, an attempt is made to find the area best suited to an ultrasound and utilize bare skin if possible. Resolution is vastly decreased if soundings must be made through any form of clothing.
There are two main areas of emergency ultrasound. The Focused Assessment with Sonography for Trauma (or fast) focuses on trying to ascertain if there is internal bodily fluid in blunt abdominal trauma, in the areas between organs, which is a sign of severe internal injury. Echocardiography is used to attempt to find valvular disease, clots, cardiomyopathies or penetrations of the heart. Both systems are scanning methodologies, they use identical equipment.
Strengths of prehospital sonography 
- It images muscle, soft tissue, and bone surfaces very well and is particularly useful for delineating the interfaces between solid and fluid-filled spaces, unlike most other methods of trauma diagnosis, which are little more than educated guesses.
- It renders "live" images, where the operator can dynamically select the most useful section for review, and narrows down the problem area, rather than having to wait until the patient is at the hospital.
- It has no known long-term side effects and rarely causes any discomfort to the patient.
Weaknesses of prehospital sonography 
- Sonographic devices have trouble penetrating bone. For example, sonography of the adult brain is very limited. This means that in terms of trauma diagnosis involving concussions, sonography will not work.
- The depth penetration of ultrasound is limited, making it difficult to image structures deep in the body, especially in obese patients.
- The method is operator-dependent. A high level of skill and experience is needed to acquire good-quality images and make accurate diagnoses, which is one more skill that a limited EMS team must develop. Since most EMS teams are small and suffer high turnover, retaining qualified personnel can be difficult.
See also 
- Bonadonna, Peter. "Paramedic Ultrasound". Retrieved 12 May 2011.
- Emergency Ultrasound Made Easy. Justin Bowra, Russell E. McLaughlin. Elsevier Churchill Livingstone, 2006 ISBN 0-443-10150-7, ISBN 978-0-443-10150-2
- Emergency Ultrasound: Principles and Practice. Romolo Joseph Gaspari, J. Christian Fox, Paul R. Sierzenski. Mosby, 2005. ISBN 0-323-03750-X, 9780323037501
- Atlas of Emergency Medicine. Kevin J. Knoop, Lawrence B. Stack, Alan B. Storrow. McGraw-Hill Professional, 2002. ISBN 0-07-135294-5, ISBN 978-0-07-135294-9
- Robert Thomas. Ultrasound evaluation of blunt of blunt abdominal trauma: Program implementation, initial experience, and learning curve. Journal of Medical Trauma. 1997 Vol. 42(3):384-8.
- Introduction To Emergency Ultrasound: A Review Of Justifications, Indications And Significant Findings. Steven A. Godwin M.D. March, 1999. Jacksonville Medicine Journal. http://www.dcmsonline.org/jax-medicine/1999journals/march99/ultrasound.htm
- Ma J, Mateer J, Ogata M. Prospective analysis of a rapid trauma ultrasound examination performed by emergency physicians. Journal of Medical Trauma. 1995 Vol. 38:879-885.