Bedside ultrasound (US) is a rapid and noninvasive method of diagnosing patients in the emergency department. US works on the principle that dense materials reflect more sound waves back to the transmitter than non-dense materials. Therefore, as US waves cross from regions of low density into regions of high density, more waves will be reflected back to the transmitter than if the US waves were crossing from a region of low density to a region of even lower density. The computer then calculates the distance between the probe and the material by relating the time delay between initial wave transmission and subsequent detection of the echo (assuming constant wave velocity in tissue) and creates a 2D visualization of the area. This is also known as using the US machine in B (brightness) mode. B mode was used to visualize the cysts of a pregnant female patient who presented with polycystic kidneys. A polycystic kidney can be differentiated from a healthy kidney because on the US machine, a polycystic kidney will have a large number of dark spots (cysts).
An US machine can also be used in M (moving) mode. In M mode, the US machine only images along one ultrasound line. The benefits of M mode over B mode is that M mode is more sensitive and able to detect rapid motions within the patient. This is why M mode was selected to measure fetal heart rate in the aforementioned patient.
Figure: B-mode US on top, M-mode US on bottom