(MI) The mechanical index is an estimate of the maximum amplitude of the pressure pulse in tissue. It is an indicator of the likelihood of mechanical bioeffects (streaming and cavitation). The mechanical index of the ultrasound beam is the amount of negative acoustic pressure within a ultrasonic field and is used to modulate the output signature of US contrast agents and to incite different microbubble responses.
The mechanical index is defined as the peak rarefactional pressure (negative pressure) divided by the square root of the ultrasound frequency.
The FDA ultrasound regulations allow a mechanical index of up to 1.9 to be used for all applications except ophthalmic (maximum 0.23). The used range varies from 0.05 to 1.9.
At low acoustic power, the acoustic response is considered as linear. At a low MI (less than 0.2), the microbubbles undergo oscillation with compression and rarefaction that are equal in amplitude and no special contrast enhanced signal is created. Microbubbles act as strong scattering objects due to the difference in impedance between air and liquid, and the acoustic response is optimized at the resonant frequency of a microbubble.
At higher acoustic power (MI between 0.2-0.5), nonlinear oscillation occurs preferentially with the bubbles undergoing rarefaction that is greater than compression. Ultrasound waves are created at harmonics of the delivered frequency. The harmonic response frequencies are different from that of the incident wave (fundamental frequency) with subharmonics (half of the fundamental frequency), harmonics (including the second harmonic response at twice the fundamental frequency), and ultra-harmonics obtained at 1.5 or 2.5 times the fundamental frequency. These contrast enhanced ultrasound signals are microbubble-specific.
At high acoustic power (MI greater than 0.5), microbubble destruction begins with emission of high intensity transient signals very rich in nonlinear components. Intermittent imaging becomes needed to allow the capillaries to be refilled with fresh microbubbles. Microbubble destruction occurs to some degree at all mechanical indices. A mechanical index from 0.8 to 1.9 creates high microbubble destruction. The output signal is unique to the contrast agent.

(US) Also called echography, sonography, ultrasonography, echotomography, ultrasonic tomography.
Diagnostic imaging plays a vital role in modern healthcare, allowing medical professionals to visualize internal structures of the body and assist in the diagnosis and treatment of various conditions. Two terms that are commonly used interchangeably but possess distinct meanings in the field of medical imaging are 'ultrasound' and 'sonography.'
Ultrasound is the imaging technique that utilizes sound waves to create real-time images, while sonography encompasses the entire process of performing ultrasound examinations and interpreting the obtained images. Ultrasonography is a synonymous term for sonography, emphasizing the use of ultrasound technology in diagnostic imaging. A sonogram, on the other hand, refers to the resulting image produced during an ultrasound examination.
Ultrasonic waves, generated by a quartz crystal, cause mechanical perturbation of an elastic medium, resulting in rarefaction and compression of the medium particles. These waves are reflected at the interfaces between different tissues due to differences in their mechanical properties. The transmission and reflection of these high-frequency waves are displayed with different types of ultrasound modes.
By utilizing the speed of wave propagation in tissues, the time of reflection information can be converted into distance of reflection information. The use of higher frequencies in medical ultrasound imaging yields better image resolution. However, higher frequencies also lead to increased absorption of the sound beam by the medium, limiting its penetration depth. For instance, higher frequencies (e.g., 7.5 MHz) are employed to provide detailed imaging of superficial organs like the thyroid gland and breast, while lower frequencies (e.g., 3.5 MHz) are used for abdominal examinations.

Ultrasound in medical imaging offers several advantages including:

Diagnostic ultrasound imaging is generally considered safe, with no adverse effects. As medical ultrasound is extensively used in pregnancy and pediatric imaging, it is crucial for practitioners to ensure its safe usage. Ultrasound can cause mechanical and thermal effects in tissue, which are amplified with increased output power. Consequently, guidelines for the safe use of ultrasound have been issued to address the growing use of color flow imaging, pulsed spectral Doppler, and higher demands on B-mode imaging. Furthermore, recent ultrasound safety regulations have shifted more responsibility to the operator to ensure the safe use of ultrasound.

See also Skinline, Pregnancy Ultrasound, Obstetric and Gynecologic Ultrasound, Musculoskeletal and Joint Ultrasound, Ultrasound Elastography and Prostate Ultrasound.

The main advantage of ultrasound is that certain structures can be observed without using radiation. However, ultrasound is energy and there are ultrasound safety regulations, because two bioeffects of ultrasound are heat and cavitation. Ultrasound is a mechanical energy in which a pressure wave travels through tissue. Reflection and scattering back to the transducer are used to form the image. As sound energy is transmitted through the tissue, some energy is reflected and some power is absorbed.
Possible physical effects with ultrasound:
The ultrasound safety is based on two indices, the mechanical index (MI) and the thermal index (TI). The WFUMB guidelines state that ultrasound that produces temperature rises of less than 1.5°C may be used without reservation. They also state that ultrasonic exposure causing temperature rises of greater than 4°C for over 5 min should be considered potentially hazardous. This leaves a wide range of temperature increases which are within the capability of diagnostic ultrasound equipment to produce and for which no time limits are recommended. However, it has not been determined that medical ultrasound causes any adverse reaction or deleterious effect.
The American Institute of Ultrasound in Medicine states that as of 1982, no independently confirmed significant biologic effects had been observed in mammalian tissue below (medical usage) 100mW/cm2.

See also Ultrasound Regulations and Ultrasound Radiation Force.