Ultrasound sono-micrometry is the measurement of distances within soft tissue using the principle of the "time-of-flight" of ultrasound (ie sound waves) between transmitting and receiving transducers. Because of past instrument limitations, the size of these transducers have been from 5 to 10 mm in diameter. However, since 1994 (the introduction of products made by Sonometrics) transducers with sizes ranging from 0.75 to 2.3 mm in diameter have replaced those larger transducers.

These crystal transducers define the points between which distances are measured, which leads to necessity that the crystals must be physically attached or imbedded in the myocardium. The crystals can be used acutely or can be left in place for weeks or months. This method has been used for several decades to determine cardiac function on large research animals (dogs, pigs, sheep, monkeys, etc) and now such measurements on mice and rats are being performed. Wires leading from the crystals (typically 1 to 2 meters long) are connected to the sonomicrometer circuits, and the measurements of distances between crystals are used in relatively simple formulas to calculate ventricular volume. For chronic long-term implants, the leads from the crystals can be attached to connectors known as "skin buttons". These are low-profile titanium connectors which are sewn into the skin (typically at the back of the neck of the animal). They allow for easy connectivity of the animal during periods of data recording (while the animal is concious and relaxed, or even during activity such as on a treadmill).

It has been well established in the literature that that the left ventricle can be approximated (geometrically speaking) as an ellipsoid with either 2 or 3 uniquely defined, orthogonal axis. This means that a pair of crystals placed on the base and apex would define the long axis, a second pair (placed on the equatorial posterior and anterior LV free wall) would define a second axis. This second axis could be squared (assuming a circular equatorial cross section) or a third pair of crystals (free wall to septum) could be used to model an elliptical cross sectional area. The multiplication of these axial diameters with the constant pi/6 results directly in a measurement of LV volume with units of cubic mm. Going further, a subtraction-shell model can be used to measure RV volume accurately via sonomicrometry.