Interaction of Sound and Media continued Interaction of Sound and Media Chapter 6 As sound travels through a media and interacts with normal anatomical structures its intensity weakens through what is called attenuation (more on this latter) A way of measuring these changes is with decibel notation. Logarithms are a mathematical construction in which decibels are based. logarithms Also call log is the number of times a number 10 must be multiplied by itself to recreate the original number. Ex. 10 x 10 = 100 so the log of 100 = 2 If you increase a log by 1 you will have increased the actual number ten fold The Richter scale is an example of a logarithmic scale Logarithms continued A simple way of determining the log of an even number is to count the zeros. So what is the log of 1,000? Answer 3 What is the log of 100,000? Answer 5 Decibels (db) When using diagnostic ultrasound it is important to know the strength of the sound beam created by the transducer as they travel through the body The common method of measuring these types of signals is call db It does not measure absolutes but rather relative changes Decibels (db) continued The comparison of two intensities are needed to use db Decibels are a ratio 1
Positive Decibels (db) Defined as an increase in signal strength A doubling of the waves intensity creates a relative change of +3 db A ten fold increase will create a change of +10 db Negative Decibels (db) Defined as an decrease in signal strength If the signal or waves intensity is reduced in half a relative change of - 3 db occurs When the intensity or wave is reduced by 1/10 a change of -10 db occurs Attenuation We all deal with this every time we scan a patient. It is defined a weakening of the sound wave as it propagates through the medium (body). Attenuation continued Determined by two factors Pulse length The frequency of the sound Attenuation continued Attenuation continued Distance and attenuation are directly related. Frequency and attenuation are directly related. It is reported in db and is a relative change not an absolute one. Three process contribute to attenuation Reflection Scattering Absorption 2
Reflection As the sound wave hits a boundary layer some is of the sound is redirected Some of the sound goes forward and some is reflected These types of reflections take two forms both are created in tissue and are dependant on the surface in which the sound strikes Specular Diffuse Specular Reflection When the sound strike a boundary layer all the reflected sound is directed in one direction If the beam is even slightly off axis the sound will travel in a direction away from the transducer Specular Reflection Diffuse Reflection When the sound beam strikes an irregular surface reflections radiate in multiple directions. Often referred to as backscatter it can diminish the intensity of the returning sound. Advantages disadvantages Diffuse Reflection continued Scattering Defined as the random redirection of sound from the primary path into many directions This occurs when the interface of the sound and medium is smaller than or equal to the wavelength of the primary beam Scattering is directly related to frequency 3
Scattering continued Rayleigh Scattering A redirection of the sound wave equally in all directions and Occurs when the the primary beams wavelength is larger than the structure imaged. Rayleigh Scattering continued Related to frequency Mathematically it is related frequency to the 4th power Simply stated if you double the frequency Rayleigh scattering is 16 times greater Reflection and Scattering Summary Higher frequency produce better image quality and have shorter pulses but less penetration Lower Frequencies have better penetration with longer pulse length but the image quality is not as good the sonographer must make a choice Absorption the most sizeable component of attenuation results in energy conversion such as heat directly related to the frequency used Attenuation Coefficient Is a way of reporting the amount of energy loss due to scatter, absorption, and reflection. It is also reflective of the frequency of the transducer the distance traveled and type of tissue involved. Does not change as travels through tissue 4
Attenuation Coefficient continued Simply put it is the number of dbs of attenuation when a sound wave travels 1cm it is measured in db/cm multiply the attenuation coefficient by the distance the sound wave traveled to find the answer Attenuation Coefficient in Soft tissue Attenuation coefficient and frequency are directly related Attenuation coefficient is 1/2 of the frequency Attenuation in Media other than Soft Tissue Half-Value Layer Thickness Medium Water Fat Blood, urine, bodily fluids Soft tissue Muscle Bone and lung air Attenuation Extremely low Low Low Intermediate Higher Even higher Extremely high The distance sound travels in tissue that reduces the sound to one- half of its original intensity most often measured in cm with clinical values ranging from 0.25 to 1.0 cm synonyms Half-Value Layer Thickness continued Thick HVL results from low frequency sound or media with a low attenuation rate thin HVL results from high frequency sound or media with a high attenuation rate Reflection and Transmission This forms the basis for ultrasound imaging When sound strikes an interface with a different impedance some sound is reflected while the remainder is transmitted 5
Impedance The acoustic resistance to sound in a medium it is characteristic of the media in which it travels the reflections created by two media at a boundary depends on their impedance difference Impedance Measured in in units of rayls (Z) typical biological values range from 1.25 to 1.75 Mrayls synonym characteristic impedance Incidence The angle in which the sound strikes an interface directly affects the returning sound there are 3 basic angles acute less than 90 o right exactly 90 o obtuse greater than 90 o Intensity Reflection Coefficient (IRC) Expressed in a percent of the original intensity clinically > 1% is reflected when the boundary layer is two soft tissues this dramatically changes between bone or air and soft tissue Normal incidence is when the sound strike the boundary at 90 o the angle must be 90 degrees synonyms include Incidence oblique incidence is when the sound strike the boundary at any angle other than 90 o the angle must not equal 90 degrees synonyms include Incidence 6
Incident, Reflected & Transmitted sound The original sound beam (Incident) strikes and interface some of the sound is reflected back toward the transducer and some is transmitted through the media Incident, Reflected & Transmitted sound as the sound hits the boundary layer there is a energy conversion All of these are intensities and expressed in units of w/cm 2 Intensity Transmission Coefficient (ITC) Expressed in a percent of the original intensity clinically 99% or more is transmitted when the boundary layer is two soft tissues this dramatically changes between bone or air and soft tissue Reflection with normal incidence Reflection only occur if there is a impedance difference at the boundary layer Reflection with normal incidence No reflection with identical impedance small reflection with slight difference large reflection with large or substantial difference Transmission with normal incidence No reflection with identical impedance all the sound will be transmitted values range from 0% - 100% in clinical US imaging 99% is transmitted 7
Reflection and transmission with Oblique Incidence Unlike normal incidence oblique incidence is complex unable to predict whether sound will transmit or reflect reflections may occur even when two media are identical Reflection with Oblique Incidence With oblique incidence reflections may or may not occur two physical principles apply to reflection with oblique incidence conversion of energy reflection angle = incident angle Oblique Incidence The law of conservation of energy applies but what is it? the sum of the percent of reflected sound and transmitted sound must equal 100% equally the sum of the Transmitted intensity and the reflected must equal the incident intensity Reflection angle = incident angle Reflection occurs with oblique incidence sound is directed away from the transducer angle of incidence = the angle of reflection angle of incidence angle of reflection Transmission with oblique incidence When the US beam strikes the interface at an oblique angle part of the beam may be transmitted in a straight line. If it is altered from its primary direction it is called refraction Refraction Occurs when two conditions are met oblique incidence the two media have different propagating speeds refraction is transmission with a bend 8
Refraction continued The physics of refraction is defined by Snell s Law Speed Speed 2 = speed 1 Angle of transmission no refraction, transmission angle = incident angle Refraction continued Speed Speed 2 > than speed 1 Angle of transmission transmission angle > incident angle Refraction continued Speed Speed 2 < speed 1 Angle of transmission transmission angle < incident angle Refraction continued 9