When we listen to music or have a conversation, sound waves generated by the instruments, the singer or the people we are talking to penetrate our ears to eventually reach our brain and become a song we know or words we understand. Sound is a succession of positive and negative pressures. The pitch of the sound is defined by its frequency, that is the number of vibrations per second, expressed in hertz (Hz).

For instance, the sound of a cello is at about 65Hz and the sound of a flute around 2000Hz. The human ear can perceive frequencies from about 20 to 20000Hz. Ultrasound is a form of sound but at much higher frequencies. For instance, obstetrics ultrasound, in general, ranges from 2 million Hz (or megahertz, MHz), to about 10MHz, so way beyond anything we, humans, or even most animals can hear. Higher frequencies are used in other branches of medicine. There are other properties of any sound wave such as the amplitude or how loud the sound is, but this is less relevant in ultrasound.

The first descriptions of ultrasound as an imaging mode date from the 19th century. The French engineer Paul Langevin designed an ultrasound machine with which he attempted to use to detect enemy submarines through echo location (hence the later coined term SONAR: Sound Navigation And Ranging) during World War I.

Ultrasound does not travel well through air, hence, in medicine, the need to put some gel on the skin for better contact. When a probe (wand) is applied to the skin, the ultrasound beam penetrates the body and whenever the waves encounter a structure, a portion bounces back (echoes) and these echoes are transformed into dots on a screen. When the object encountered in the body is dense, such as a bone, strong echoes are produced, and this creates white dots on the screen. Because of various degrees of “hardness," many shades of grey appear on the screen and create the image. The frequency of the ultrasound waves is very important. Lower frequencies can penetrate deep in the body but have lower resolution ( “larger pixels”) than higher frequencies which allow better resolution (“smaller pixels”) but can only penetrate the body for a shallower distance.

Ultrasound has many uses: from cleaning jewels to distance measuring, repelling noxious animals and use in phone industry with non-audible ringing (to older adults), also called “ultraring” or “antiadult ringing” or “teen phones” because high frequencies become less to non-audible above age 25, to name a few. Of course, medicine is a major field which uses ultrasound.

A few examples include detecting stones in the gallbladder, examining the liver or the kidneys, imaging the lungs, for instance looking for specific signs of COVID-19, evaluating pelvic organs (uterus and ovaries) and assessing a pregnancy, perhaps the use best known to the general public. In pregnancy, ultrasound is invaluable in determining the number of fetuses, exactly how far the pregnancy is, verifying whether the fetus is growing normally and its internal organs do not show any problems (such as cardiac anomaly, for instance). In addition, the location of the placenta (the afterbirth) can be determined to make sure the baby will be able to be born naturally, with no need for surgery (cesarean section).

Various forms of ultrasound may be used in medicine: two-dimensional or 2D (the most common), Doppler (to visualize blood vessels for instance or calculate how fast a structure such as the heart valves or blood in vessels circulate), three/four-dimensional or 3D/4D, where many successive 2D planes are acquired and assembled together by the computer to show, for instance, nice pictures of a baby’s face. Ultrasound machines used to be big and not movable. With time, size has slowly shrunk and there are, nowadays, machines the size of tablets or cell phones. This is particularly important in a relatively new application: point of care ultrasound, or POCUS. This is the use of ultrasound at the bedside, on the battlefield, in the ambulance or the emergency room to obtain immediate diagnostic images.

A question that is often asked is: is ultrasound safe? The short answer is yes. There is, however, information that both the patient and the ultrasound user must be aware of, particularly when imaging a pregnancy. Ultrasound is a form of energy and each time it penetrates a tissue, two effects occur: the acoustic energy has an indirect effect where some of it is transformed into heat and a direct effect, resulting from the alternating positive and negative pressures. Two numbers appear on the machine screen to allow the user to assess the risk of these effects: the thermal index and the mechanical index. As a rule, if these are kept below 1, there does not appear to be any risk. Because of this, ultrasound is one of the safest and most useful procedures used in medicine. 

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