Ultrasound Scatter and Attenuation | Ultrasound Physics | Radiology Physics Course #8

Understanding Ultrasound Interaction with Tissue

Introduction to Scatter and Attenuation

• The focus shifts to how ultrasound interacts with matter, specifically discussing scatter and attenuation, which is the loss of ultrasound intensity as it travels through tissue.

• It is clarified that while intensity decreases, speed, wavelength, and frequency remain unchanged; only amplitude diminishes.

Concept of Scatter

• Scatter occurs when an ultrasound wave encounters objects smaller than its wavelength within a medium. If no small units are present, the wave travels unaffected.

• Most of the ultrasound wave passes through unaffected; however, interactions with small units result in scattered waves emitted in all directions, contributing to attenuation.

Factors Influencing Scatter

• Denser packing of small units increases scatter; higher acoustic impedance also leads to more scattering. Larger unit sizes further enhance this effect.

• Higher frequencies increase the likelihood of interaction with small units in tissue, resulting in greater scatter and affecting echogenicity.

Value of Scatter in Ultrasound Imaging

• Unlike x-rays where scatter is undesirable, in ultrasound imaging scatter provides valuable information about tissue structure (echotecture).

• Specific patterns of scatter from functional units (e.g., kidneys) yield signals that inform about organ characteristics despite being noisy and disorganized.

Echogenicity and Signal Return

• Scattered signals return to the transducer from various angles; some contribute useful data while others do not directly correlate with anatomy.

• Increased scatter results in hyperechoic images (more echoes), whereas less scatter can lead to hypoechoic or anechoic appearances if no signal returns.

Mechanisms for Intensity Loss

• Scatter contributes to beam attenuation by dissipating energy into tissues.

• Heat generation within tissues also causes intensity loss as energy converts into heat during absorption.

Understanding Attenuation Factors

• Attenuation depends on multiple factors: depth traveled through tissue affects intensity loss significantly.

• Higher frequency beams experience more rapid attenuation due to increased contact with tissue compared to lower frequencies.

Real-world Analogy for Frequency Effects

• An analogy compares sound transmission through walls: low-frequency sounds travel better than high-frequency sounds due to lesser attenuation by barriers like walls.

Understanding Ultrasound Attenuation

What is Attenuation in Ultrasound?

• Attenuation refers to the loss of intensity of ultrasound waves as they travel through different tissues, influenced by frequency, depth, and tissue type.

• It is important to note that attenuation signifies a loss in intensity rather than speed, wavelength, or frequency. A 3 dB loss corresponds to halving the intensity.

• The decibel scale is logarithmic; every 10 dB change represents a tenfold increase or decrease in intensity.

Factors Influencing Attenuation

• Different frequencies of ultrasound probes affect attenuation levels; deeper penetration into tissues also alters attenuation.

• The formula for calculating resultant intensity involves incident intensity and specific attenuation coefficients for each tissue at given frequencies.

• Unlike X-ray physics where increased frequency leads to less attenuation, higher ultrasound frequencies result in greater attenuation due to more interactions with tissue.

Tissue-Specific Attenuation Coefficients

• The attenuation coefficient varies significantly among different tissues; soft tissue has an approximate value of 0.5 dB/cm/MHz.

• Bone has a much higher attenuation coefficient (approximately 40 times that of soft tissue), leading to significant reflection and minimal detail beyond bone structures.

Practical Applications of Attenuation Calculations

• For example, using a 2 MHz ultrasound pulse results in an intensity loss of 1 dB per cm traveled through soft tissue.

• This calculation can be used for determining half-value thickness—the distance required for the beam's intensity to reduce by half (3 dB loss).

Example Calculations

• A 2 MHz beam halves its intensity after traveling approximately 3 cm in soft tissue due to cumulative decibel losses over distance.

• Conversely, a 4 MHz beam loses its initial intensity by half after traveling about 1.5 cm within the same medium.

Understanding Ultrasound Attenuation and Its Implications

The Impact of Frequency and Tissue on Ultrasound Intensity

• As ultrasound travels through tissue, intensity decreases logarithmically due to factors like frequency and tissue type.

• A four megahertz ultrasound beam loses 2 decibels per centimeter; thus, over a distance of five centimeters, it loses 10 decibels.

• This 10 decibel decrease results in a tenfold reduction in ultrasound intensity by the time it reaches the tissue boundary.

• Upon returning to the transducer after reflecting off the surface, an additional loss of 10 decibels occurs, totaling a 20 decibel loss or a hundredfold decrease in intensity.

• If only one percent of the initial intensity is reflected back (as with fat-muscle boundaries), this can lead to a staggering 10,000-fold decrease in intensity.

Echo Detection Challenges and Dynamic Range

• The significant loss in echo intensity necessitates highly sensitive ultrasound machines capable of detecting very small echoes.

• The range within which these machines can detect echoes is referred to as dynamic range—a crucial concept for understanding ultrasound performance.

• Different tissues attenuate ultrasound beams differently; higher frequencies result in greater attenuation as depth increases.

Importance of Tissue Interaction Knowledge

• Understanding how ultrasound interacts with various tissues is essential for grasping core concepts related to artifacts observed during imaging.