George DavidAssociate Professor

Medical College of GeorgiaAugusta, Georgia

Resident Physics LecturesResident Physics Lectures

03:03:

AttenuationAttenuation

George DavidAssociate Professor

Medical College of GeorgiaAugusta, Georgia

AttenuationAttenuation

• Reduction in amplitude & intensity as sound travels through medium

• Causes absorption

» sound energy converted to heat

» dominant influence in soft tissue

reflection scattering

AbsorptionAbsorption

• Unitsdecibels (dB)decibels (dB)

• dB indicates signal gain“+” indicates signal gets larger

“-” indicates signal gets smaller ultrasound absorption is always negative dB

» sound always loses intensity» negative sometimes implied

dB indicates fraction of intensity lost

George DavidAssociate Professor

Medical College of GeorgiaAugusta, Georgia

Logarithm ReviewLogarithm Review

x = log10(y) means

10 to what power = y ?

or

10x = y

Logarithms ReviewLogarithms Review

log 1 = 0

log 10 = 1

log 100 = 2

log 10n = n

log (1/10) = 10-1 = -1

log (1/100) = 10-2 = -2

log (1/1000) = 10-3 = -3

Gain & DecibelsGain & Decibels

• decibel definition

dB =10 X log10 [power out / power in]

Tissue(attenuation)

Power In Power Out

Power Ratio = Power Out / Power In

dB =10 X log10 [power ratio]

Gain & DecibelsGain & Decibels

• Power Ratio > 1 Amplifier Power Out > Power In Log [Power ratio] >0

Tissue(attenuation)

Power In Power Out

Power Ratio = Power Out / Power In

dB =10 X log10 [power ratio]

• Power Ratio < 1 Absorber / Attenuator Power Out < Power In Log [Power ratio] <0

                

Power RatioPower Ratio

logarithmslog 1 = 0

log 10 = 1

log 100 = 2

log 10n = n

log (1/10) = 10-1 = -1

log (1/100) = 10-2 = -2

log (1/1000) = 10-3 = -3

Decibel calculationPower ratio dB

1 0

10 10

100 20

1/100 -20

10 n n X 10

2 3

Power Ratio = Power Out / Power In

dB =10 X log10 [power ratio]

George DavidAssociate Professor

Medical College of GeorgiaAugusta, Georgia

dB AttenuationdB Attenuation

• dB / 10 indicates # of powers of ten attenuation• Every increase of 10 dB indicates another

factor of 10 attenuation

dB: Try AgaindB: Try Again

10 dB = 1 power of 10 = 1020 dB = 2 powers of 10 = 10060 dB = 6 powers of 10 = 1,000,000

10 dB: 1 factor of 10 or 10 you

morons

20 dB: 2 factors of 10 or 10 X 10

or 100

60 dB: 6 factors of 10 or

1,000,000, nyuk, nyuk, nyuk

Attenuation & FrequencyAttenuation & Frequency

• Attenuation affected by medium frequency

• As frequency increases, so does attenuation bass sound carries farther than treble high frequency = poorer penetration

Attenuation In Soft Tissue Rule of Thumb

Attenuation In Soft Tissue Rule of Thumb

• 0.5 dB / cm attenuation for each MHz frequency “cm” refers to distance of sound travel other texts may say 1 dB / cm depth / MHz

» 1 cm depth equivalent to 2 cm sound travel

Rule of Thumb0.5 dB/cm/MHzRule of Thumb0.5 dB/cm/MHz

• To calculate attenuation (dB) simply multiply rule of thumb by round trip distance & by frequency

• 5 MHz sound; 10 cm sound travel attenuation = 0.5 dB/cm/MHz X 10 cm X 5 MHz = 25 dB

• 3.5 MHz sound; 4 cm sound travel attenuation = 0.5 dB/cm/MHz X 4 cm X 3.5 MHz = 7 dB

Attenuation CoefficientAttenuation Coefficient

• Attenuation Coefficient = 0.5 * Freq.(dB/cm) (dB/cm/MHz) * (MHz)

• indicates fraction of beamintensity lost per unit distanceof sound traval

Attenuation CoefficientAttenuation Coefficient• Attenuation Coefficient = 0.5 * Freq.

(dB/cm) (dB/cm/MHz) * (MHz)

Frequency (MHz)

Attenuation Coefficient

(dB/cm)1 0.5

2 1.0

5 2.5

10 5

Attenuation CoefficientComments

Attenuation CoefficientComments

• Attenuation Coefficient = 0.5 * Freq.(dB/cm) (dB/cm/MHz) (MHz)

• Longer path results in increased attenuation

• Higher frequency results in increased attenuation coefficient

• Higher attenuation coefficient results in more attenuation

dB vs. Intensity RatiodB vs. Intensity Ratio

dB Intensity Fraction

atten. Ratio atten.1 .79 .21

2 .63 .37

3 .50 .50

4 .40 .60

5 .32 .68

10 .1 .90

20 .01 .99

30 .001 .999

dB attenuation =10 X log10 [intensity ratio]

Fraction attenuated = 1 - intensity ratio

Soft Tissue Attenuation Calculation

Soft Tissue Attenuation Calculation

Attenuation = Attenuation Coefficient X Path Length

Freq. Atten Coef. Atten(dB). % Int. Red. Atten(dB) % Int. Red. dB / cm 1 cm 1 cm 10 cm 10 cm

2.0 1.0 1 21 10 90 3.5 1.8 1.8 34 18 98 5.0 2.5 2.5 44 25 99.7 7.5 3.8 3.8 58 38 99.9810.0 5.0 5.0 68 50 99.999

AttenuationAttenuation

• Why dB? dB’s can be added together

• Rule of thumb doesn’t always work Attenuation higher in lung & bone

than in soft tissue Attenuation in lung and bone not

proportional to frequency

Class during lecture on attenuation

Attenuation CoefficientsAttenuation Coefficients

• Fat 0.6

• Brain 0.6

• Liver 0.5

• Kidney 0.9

• Muscle 1.0

• Heart 1.1

Tissue Attenuation Coefficient (dB/cm/MHz)

0.5 dB/cm/MHz is soft tissue average assumed by scanner

Half Intensity DepthHalf Intensity Depth

• Decreases with increasing Decreases with increasing frequencyfrequency

HID = 3 dB / Attenuation Coefficient

HID = 3 dB / Freq (MHz) * 2

Frequency Atten Coef. HID(MHz) dB/cm cm------------------------------------------------------ 1 0.5 6.0 2 1.0 3.0 5 2.5 1.210 5.0 0.6

AttenuationAttenuation

• half intensity depth (HID)half intensity depth (HID) depth where intensity = 50% of original corresponds to 3dB attenuation

180150100

6639

HID

Practical Implications of Attenuation

Practical Implications of Attenuation

• limits maximum imaging depth

• higher frequencies result in increased attenuation decreased imaging depth improved axial resolution