Point of Care Ultrasound Made Easy by John McCafferty – James M Forsyth
◾ Understand the basic physics behind medical ultrasound imaging.
◾ Understand the hardware components of a medical ultrasound scanner.
◾ Understand the different imaging modes: B-mode, M-mode and
◾ Know how to acquire an ultrasound image, appreciating the imaging
◾ Know how to optimise your ultrasound image.
◾ Appreciate your choices of point-of-care ultrasound (POCUS) device.
◾ Have an awareness of safety factors in POCUS.
What Is Ultrasound?
Ultrasound waves are mechanical energy that are transmitted through a medium from repetitive periodic oscillations of a transducer. The number of such cycles per second is termed the frequency of the ultrasound signal. Ultrasound refers to sound propagated at frequencies higher than the audible spectrum for humans (>20 KHz) (see Figure 1.1). Typically, medical ultrasound imaging operates in the range 3.5–20 MHz.
Properties of Ultrasound in Tissues
A B-mode ultrasound image is constructed from echoes generated by reflection of ultrasound from boundaries and scattering from small inhomogeneities within the tissue structures. Reflection occurs at the boundaries between two mediums of different acoustic impedance such as those between organs. Here, acoustic impedance is defined as the product of density of the tissue and the speed of sound within the tissue. The amplitude of the ultrasound wave that is reflected back into the tissue/organ or transmitted across a tissue/organ boundary is dependent on the change in acoustic impedance. If the change is small, the majority of the ultrasound will be transmitted and little reflected; if the change is large, the majority will be reflected back and little transmitted. For example, the medium of fluid or water has low impedance and will conduct the ultrasound wave, whereas bone is a poor conductor and will reflect the ultrasound wave back towards its source (see
Small inhomogeneities within tissues/organs result in scattering of the ultrasound beam. This scattering is dependent on the size of the scatterer relative to the frequency/wavelength of the ultrasound beam. However, the scattering from these small structures (including red blood cells) is much smaller than typical reflections from boundaries so that within a B-mode image, boundaries are associated with brighter echoes compared to the less bright echoes generated by scattering within soft tissue.
Ultrasound imaging uses the differences in the acoustic properties of tissues to build a picture of the structures within the body. The ultrasound signal is generated by a transducer that contains an array of tiny piezoelectric crystals which when excited by an electrical signal vibrate at a set frequency and transmit an ultrasound signal. The transducer is also a receiver, detecting the reflected signal via the piezoelectric crystals whilst ‘listening’ between transmission phases. This is referred to as send-receive mode (see Figure 1.3).
The reflected signal is processed so that high intensity signals from reflected ultrasound waves represent brighter areas on screen. Complex signal processing therefore allows the display in real time of the ultrasound image as a grey-scale image on screen (see Figure 1.4).