Section 9.1: Bioeffects, Safety, and the ALARA Principle

Section 9.1: Bioeffects, Safety, and the ALARA Principle

Diagnostic ultrasound is a safe and widely used imaging modality, but it requires an understanding of acoustic energy and its interactions with biological tissues. Because ultrasound is a form of mechanical energy, it has the potential to induce changes in the media through which it travels. These changes are collectively referred to as bioeffects. To date, there are no documented cases of adverse biological effects in humans caused by diagnostic levels of ultrasound in the absence of microbubble contrast agents. However, research in vitro and in animal models has demonstrated that high levels of acoustic exposure can cause cellular damage. Therefore, sonographers must understand safety parameters, monitor thermal and mechanical indices in real time, and adhere to clinical guidelines.

Thermal Mechanisms and the Thermal Index (TI)

Thermal bioeffects arise from the conversion of acoustic energy into heat as the ultrasound beam propagates through tissue. As sound waves travel, the friction of oscillating particles leads to absorption, transforming mechanical energy into thermal energy. The rate of temperature rise depends on the tissue's absorption coefficient, acoustic output power, transducer frequency, and local vascular perfusion, which acts as a cooling mechanism.

To help sonographers monitor thermal risks, ultrasound systems display the Thermal Index (TI). The TI is defined as the ratio of the total acoustic power transmitted by the transducer to the power required to raise the temperature of the target tissue by 1°C. A TI of 1.0 indicates that the system is operating at a level that could theoretically raise the local tissue temperature by 1°C. There are three formulations of the Thermal Index, tailored to different anatomical environments:

• Thermal Index for Soft Tissue (TIS): Assumes the beam travels through homogeneous soft tissue. This is monitored during early obstetric scans, general abdominal exams (e.g., liver, pancreas, spleen), and superficial imaging.
• Thermal Index for Bone (TIB): Assumes the beam focus lies at or near bone. Bone has a much higher absorption coefficient than soft tissue, meaning it heats rapidly. TIB is monitored during late second- and third-trimester obstetric scans when fetal bone ossification is complete, and during adult scans where the beam path intersects ribs or the spine.
• Thermal Index for Cranial Bone (TIC): Assumes the transducer is close to bone, such as in transcranial Doppler examinations. It models the temperature rise in the skull itself.

Clinical guidelines suggest that a temperature rise of less than 1.5°C is safe for clinical use. An increase of 2.0°C or more above normal body temperature for extended periods is considered potentially hazardous, particularly to developing embryonic and fetal tissues.

Mechanical Mechanisms and the Mechanical Index (MI)

Mechanical (non-thermal) bioeffects result from physical forces exerted by the ultrasound beam. The primary mechanical bioeffect is cavitation, which refers to the interaction of ultrasound waves with microscopic gas bubbles in the tissue. The likelihood of cavitation is represented by the Mechanical Index (MI).

The Mechanical Index is mathematically defined as: $MI = \frac{p_r}{\sqrt{f}}$ where $p_r$ is the peak rarefactional pressure (the negative pressure phase of the sound wave that pulls molecules apart) and $f$ is the center frequency of the transducer. The risk of cavitation is highest when using lower-frequency transducers and high acoustic output settings, as these produce larger pressure amplitudes and longer rarefactional phases.

Cavitation is classified into two distinct forms:

1. Stable Cavitation: At lower acoustic pressures, microbubbles already present in the medium expand and contract (oscillate) in response to the alternating compressions and rarefactions of the sound wave. The bubbles do not burst. This oscillation creates localized fluid movement known as microstreaming, which can generate shear stress on adjacent cell membranes, potentially altering cell function.
2. Transient Cavitation (also known as Inertial or Normal Cavitation): At higher acoustic pressures, the microbubbles expand violently during the rarefaction phase and then implode during the compression phase. The collapse of these bubbles is highly localized but extremely energetic, producing shock waves, microscopic jets of liquid, localized temperatures of several thousand Kelvin, and the release of highly reactive free radicals. Transient cavitation can cause localized cell death and tissue damage.

FDA Output Limits and Acoustic Intensity

The Food and Drug Administration (FDA) regulates the maximum acoustic output of diagnostic ultrasound devices. To standardize safety, the FDA utilizes the Spatial Peak Temporal Average ($I_{SPTA}$) intensity, which measures the maximum intensity in the tissue averaged over the pulse repetition period.

Under current regulatory standards, the maximum permitted $I_{SPTA}$ for general abdominal, obstetric, and pediatric imaging is: $I_{SPTA} \le 720\text{ mW/cm}^2$ For ophthalmic applications, the limit is much lower ($50\text{ mW/cm}^2$).

Different imaging modalities deliver different levels of acoustic energy. The typical hierarchy of acoustic intensities from highest to lowest is:

1. Spectral (PW) Doppler: Emits continuous or long-pulse trains to measure velocities, yielding the highest $I_{SPTA}$ values and the greatest thermal risk.
2. Color Doppler: Uses multiple pulses per scan line to map velocities, presenting intermediate risk.
3. M-Mode: Uses a single line of sight with frequent pulses, presenting moderate risk.
4. B-Mode (2D Grayscale): Uses short pulses swept across a wider area, resulting in the lowest $I_{SPTA}$ and lowest risk.

The ALARA Principle in Clinical Practice

The ALARA (As Low As Reasonably Achievable) principle dictates that sonographers must minimize acoustic exposure to the patient while obtaining diagnostic-quality images. The sonographer acts as the primary gatekeeper of safety by controlling system parameters, scan times, and clinical justification.

When adjusting image brightness, a fundamental rule of ALARA is the distinction between output power and receiver gain:

• Acoustic Output Power: Controls the electrical voltage applied to the transducer crystals, directly increasing the intensity of the sound wave transmitted into the patient. Increasing output power increases both thermal and mechanical indices.
• Receiver Gain (Amplification): Amplifies the returning echoes after they have entered the transducer. Increasing receiver gain brightens the image on the display but does not transmit any additional energy into the patient.

Therefore, if an image is too dark, the sonographer should first increase the receiver gain or adjust the Time Gain Compensation (TGC). If the image is too bright, the sonographer should first decrease the acoustic output power to minimize patient exposure.

Additionally, sonographers must select the correct patient presets, avoid resting the transducer on one spot for prolonged periods (dwell time), and perform examinations only when clinically justified.

Safety Parameter Reference Table