When a sound wave, such as an ultrasound wave, strikes a human bone, it interacts with the bone's structure in complex ways due to the unique composition and layered structure of the bone. The bone is composed of several layers, each with different densities and acoustic properties. From the outside in, these layers are the periosteum, the cortical bone (compact bone), the trabecular bone (spongy bone), and the bone marrow.
- Periosteum: This is a dense layer of vascular connective tissue enveloping the bones except at the surfaces of the joints. It is relatively thin compared to the other layers.
- Cortical bone: This is the hard outer layer of bones. It is dense, strong, and durable, and it comprises about 80% of the total bone mass in an adult human body.
- Trabecular bone: Also known as cancellous or spongy bone, this is located inside the cortical bone and is much less dense, with a honeycomb-like structure.
- Bone marrow: This is the soft, spongy tissue in the center of most bones, where new blood cells are produced.
When a sound wave strikes the bone, several things happen:
- Reflection: Due to the significant difference in acoustic impedance between soft tissue and bone, a large portion of the sound wave is reflected back when it hits the periosteum. This is why bones appear bright white on an ultrasound image.
- Scattering: As the sound wave enters the cortical bone, some of the wave's energy is scattered in various directions due to the dense, irregular structure of the bone. This scattering increases as the wave encounters the less dense trabecular bone, with its many small, irregular cavities.
- Absorption: Some of the sound wave's energy is absorbed by the bone and converted into heat. This absorption is greater in the denser cortical bone than in the trabecular bone or bone marrow.
- Transmission: A small portion of the sound wave's energy continues to travel through the bone, being further reflected, scattered, and absorbed as it encounters the different layers.
The combined effects of reflection, scattering, and absorption result in significant attenuation of the sound wave as it travels through the bone. This is why it is challenging to image structures deep to bones using ultrasound.
- Szabo, T. L. (2014). Diagnostic ultrasound imaging: inside out. Academic Press.
- Duck, F. A. (1990). Physical properties of tissue: a comprehensive reference book. Academic Press.