Ultrasound (US) is a useful diagnostic tool for use in hospitals. It is noninvasive and inexpensive, and causes no radiation exposure. Besides radiologists, many emergency physicians use US to assist in their decision making during critical conditions . With the current improvement in technology, US machines have become more portable and are available with a better resolution. Ziegler et al  reported that a portable device had approximately 90% accuracy compared with high-end devices. US machines such as PRIMEDIC HandyScan, V-scan, and Sonosite are commonly used as portable devices in prehospital settings.
US has been brought to prehospital settings as a result of the recent advances in technology . A prehospital setting is a unique, most likely noisy, and often limited space. Traditionally, diagnostic tools used in prehospital settings are based on history taking and physical examination.
Physical examination alone cannot be sufﬁcient to diagnose certain conditions . In addition, many studies suggested that prehospital US can change the ﬁnal diagnosis and treatment [5,6]. Prehospital US has a variety of applications, such as focused assessment with sonography in trauma (FAST) , assessment of cardiac arrest , lung US (mainly in pneumothorax) [6,8], and others. Countries that have studied prehospital US extensively include Germany, France, Italy, and the United states . Literature was reviewed and discussed in the following sections.
Feasibility of US in a prehospital environment
Because a prehospital space is unique and limited, a US machine should be smaller in size but should have better image quality. Some studies performed US at the scene, and others in a vehicle, such as an ambulance or a helicopter. If performed at the scene, the delivery time to hospital may be prolonged, and if performed in a helicopter or an ambulance, the transporting environment may inﬂuence the scan. There are studies of prehospital US in a ﬁxed wing and helicopter, which showed good results. However, Melanson et al  reported in their study that the lack of sufﬁcient time during helicopter transport and a proper lighting system in the helicopter can compromise the results of FAST examination. Snaith et al  reported that FAST and abdominal aortic aneurysm (AAA) performed in a static and ground ambulance is of good quality due to the availability of sufﬁcient time and is comparable to that performed at the emergency department.
In Taiwan, emergency medical services mainly involve ground ambulances, and most of the ambulance beds are located at the left side; hence, left-hand-based practice may be helpful for performing the scan. Fixation of machines to the frontal areas of ground ambulances may be helpful in reducing shaking.
Educating paramedics about US
Many studies have invested in the learning curve for US, especially in FAST. They concluded that a 1-day course,including lecture and hand-on practice, can generate good accuracy and competency . Heegaard et al  designed a FAST training course, which lasted 7 hours, for emergency nurses and paramedic ﬂight crews; they reported 100% sensitivity and speciﬁcity in nontrauma patients, and 60% sensitivity and 93% speciﬁcity in trauma patients after 1 year of training. Kim et al  also reported that a 4-hour FAST training course for intermediate emergency medical technicians (EMT) resulted in 61% sensitivity and 96.3% speciﬁcity.