Medical Ultrasonography: Wave Properties

• Medical ultrasonography uses ultrasound waves to visualize internal body structures like tendons, joints, muscles, and internal organs.
• The visualization is achieved through the propagation of ultrasound waves and echoes.
• Propagation occurs through the vibration of particles around a mean point.
• Propagation involves the cyclic change/oscillation of pressure.
• Zones of compression are high pressure zones.
• Zones of rarefaction are low pressure zones.
• Peak positive and rarefaction/negative pressure areas exist.

Measuring the strength of the ultrasound beam

• "Strength' of the wave uses acoustic output quantities.
• Quantities include pressure in Pascals, power in Watts, and intensity in W/cm².
• Diagnostic ultrasound can reach up to 4 MPa, typically used at 1 MPa whereas atmospheric pressure has 0.1 MPa.
• Pressure variations are termed pressure amplitude (P).
• Pressure amplitude indicates the difference between peak maximum or peak minimum value and the average pressure in a medium.
• Power is the rate of energy transfer into tissue, measured in Watts (W); Ultrasound power is generally ≤ 10mW.
• Intensity refers to the power per unit area, measured in W/cm² (mW/cm²).
• Intensity is directly proportional to the pressure amplitude squared (I ∝ P²), which means doubling P quadruples intensity.

Intensity

• Decibel (dB) is commonly used in ultrasound to measure strength/relative intensity of the US beam.
• Relative intensity uses dB and calculated with the following equation: Relative intensity (dB) = 10 log (I2/I1).
• Relative pressure (dB) = 20 log (P2/P1).
• Decibel describes attenuation, gain, and dynamic range in medical US imaging.
• Ultrasound systems utilize a logarithmic scale efficiently handle wide intensity ranges akin to the human ear.
• Attenuation is gauged in decibels per centimeter (dB/cm); it changes with tissue type and ultrasound wave frequency.
• Gain control (dB) adjusts image brightness by amplifying returning echoes.
• Time Gain Compensation (TGC) sets gain levels for different depths, ensuring uniform brightness.
• Echo intensity range is measured in decibels.
• A higher dynamic range results in more grey shades, enhancing soft tissue differentiation.
• A lower dynamic range gives a more contrasty image.
• A 3 dB decrease means a 50% intensity loss.
• A 6 dB decrease translates to a 75% intensity loss.
• A 10 dB decrease equates to a 90% intensity loss.
• A 20 dB decrease equates to a 99% intensity loss.
• A +3 dB increase doubles the intensity.
• A +6 dB increase quadruples intensity.
• Ultrasound beam intensity is described considering spatial and temporal factors.

Intensity of the U/sound Beam

• Spatial factors are determined by probe type: convex, linear (soft tissue), or transvaginal
• Intensity is not distributed uniformly throughout the ultrasound beam.
• Spatial Average has an average intensity across the insonated area.
• Spatial Peak contains the highest intensity value within the u/sound beam.
• Temporal factors: Pulse duration and Pulse Repetition Period should be considered.
• Pulse duration is the time for one pulse to occur, calculated as period time (time of one cycle) times number of pulses per cycle.
• Pulse repetition frequency is the number of pulses occurring in one second.
• Pulse repetition period measures the time from the start of one pulse to the next, its the equivalent of 1/PRF.
• Duty factor represents the fraction of time the pulse ultrasound is active.
• Three Intensity/power values: Peak Intensity (power) is linked to the time of greatest pressure/highest intensity. Usually at an arbitrarily small time.
• Average intensity encompasses the average within a pulse(Pulse average).
• Average intensity considers the total period (Time average).
• TA (time average) refers to intensity averaged across the probe's on-time, where low duty factors result in generally low TA,
• PA (pulse average) denotes average intensity as transducer emits sound.
• I(TP) - Intensity temporal Peak – a highest intensity measure.
• Spatial Average - Time Average (SATA) refers to the intensity averaged across all time relative surface area of the receiving transducer; exhibiting the lowest intensity.
• Spatial Peak - Time Average Intensity (SPTA) denotes highest measured intensity overall time.
• Spatial Peak - Pulse Average Intensity (SPPA) denotes the highest intensity read during beam generation.
• Spatial Peak Temporal Peak Intensity (SPTP) refers to the greatest possible reading along any point, and the peak intensity delivered.

Ultrasound Safety and Bio-effects

• Bio-effects are either thermal or mechanical.
• Thermal effects result from tissue heating by the ultrasound beam, indicated via the Thermal Index (TI).
• Mechanical effects refers to how tissues/cells react to ultrasound, indicated via the Mechanical Index (MI).
• TI and MI displays are standard on ultrasound machines.
• These indexes overestimate bio-effects, working conservatively with any transducer related to spatial and temporal domains of intensity and pressure.

Thermal Bio-Effects

• Ultrasound energy converts to heat upon attenuation by tissue.
• Thermal effects vary based on tissue type, perfusion, acoustic impedance (Z), insonation duration, location, and ultrasound parameter
• Greater Z, gives greater absorption and greater heating.
• Ultrasound parameters that affect thermal impacts: greater frequency means greater tissue attenuation and heating.
• Additionally thermal effects are affected by: power, pulse repetition frequency, pulse duration, and scan mode (B-mode vs Doppler). Another factor is the source dimension.
• Acoustic impedance increase absorption resulting in higher tissue heating.
• Greater perfusion(blood supply) leads to high cooling, lessening tissue heating.
• Embryonic and fetal tissue is very sensitive to ultrasound exposure where teratogenic heat is created
• Heating below 2° is acceptable as there are no biological effects. After 2° effects depend on exposure duration.

Thermal Index

• Thermal Index gauges maximum temperature rise from prolonged exposure.
• Thermal Index categories: Thermal Index Soft tissue soft tissue/TIS, Thermal Index for Bone at the focus(TIB), Thermal Index for bone in the near field (TIC)
• TIS assumes soft exposure only.
• TIB assumes bone is present where temporal intensity is greatest, typical of obstetric ultrasounds.
• TIC assumes bone is close to the probe, common with transcranial ultrasounds.

Mechanical Bio-Effects

• Categorized into radiation force, acoustic streaming, non-bubble effects, and cavitation.
• Radiation Force: torque/pressure applied to tissue from ultrasound. Is useful in elastography.
• Acoustic Streaming refers to the movement of liquid particles away from the probe caused by ultrasound force (Ultrasound Wind)
• Non-Bubble Mechanical Effects involves the movement of red cells in sensitive regions of the body
• Cavitation is the formation, growth, and collapse of bubbles in liquid.
• Cavitation is caused by rapid changes in pressure to a liquid. Acoustic cavitation relates to bubble formation and collapse as sound waves propogate through liquid.
• Two types of cavitation: stable and inertial/transient.
• Stable Cavitation consists of bubble formation when a low peak pressure area of an ultrasonic wave passes a nucleation site.
• The bubbles then pulse with the high-pressure oscillation action by the u/sound pulses.
• Bubble pulsation causes fluid to flow rapidly.
• Stable Cavitation shows no significance in regards to bio-effects.
• Intertial Cavitation, a bubble formation follows the action of Stable Cavitation.
• Intertial uses higher intensity and pressure, expanding quickly and resulting in violent collapse.
• Collapse results in the release of energy and creating great heat, which can exceed 5000°C. Thus is considered a threshold phenomena. This can result in cell lysis and water vapor dissociation.
• Mechanical Index is a rough scale showing the potential for ultrasound to induce adverse bio effects from non-thermal activity, especially cavitation.
• This gauge is calculated from ultrasound parameters and U/S Machine specifications.
• The numbers are constantly updated using the scanner according to setup.
• The readings are approximations.
• Mechanical Index offers a gauge to cavitation, but the readings are not a literal threshold, and it is suggested MI should be below 1.9.
• The readings are not useful for the study of continuous Doppler ultrasound.

U/sound Bio-Effects

• U/sound related bio-effects found in animals cannot be compared and extrapolated to create accurate effects analysis when applied to humans
• The intensity of the U/sound waves are often much higher than those used in most U/S machines.
• Given current knowledge and an absence of data to assume effects are harmful, it can be assumed the devices are innocuous to us and animal life.
• Current understandings are also based on U/S technology from earlier U/S machines that provide less power and output.
• It is important diagnostic ultra sound adheres to guidelines especially when Doppler is used for fetal scans

Guidelines For Usage

• Examinations should be as quick as reasonably possible, and at a minimum output, known as ALARA principle.
• Equipment should be used following manufacturer guidelines.
• Machines should be used in conjunction with output display options to ensure parameters adhere to ALARA protocols.
• Operators should avoid over use on tissues that include air and bone
• Machines are only to be used for medical purposes.
• Operators must be fully trained.
• Scan times are adjusted to minimize exposure.
• Machines should never be used for providing souvenirs only of the internal environment.