Attenuation - Radiology - Lecture Slides, Slides of Radiology

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Attenuation

Attenuation

• Reduction in amplitude &

intensity as sound

travels through medium

• Causes

 absorption » sound energy converted to heat » dominant influence in soft tissue  reflection  scattering

Logarithm Review

x = log 10 (y) means

10 to what power = y?

or

10 x^ = y

Logarithms Review

log 1 = 0

log 10 = 1

log 100 = 2

log 10n^ = n

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

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

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

Gain & Decibels

• Power Ratio > 1

Amplifier Power Out > Power In Log [Power ratio] >

Tissue (attenuation)

Power In Power Out

Power Ratio = Power Out / Power In

dB =10 X log 10 [power ratio]

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

Power Ratio

logarithms

log 1 = 0

log 10 = 1

log 100 = 2

log 10n^ = n

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

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

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

Decibel calculation Power ratio dB 1 0 10 10 100 20 1/100 - 10 n^ n X 10 2 3

Power Ratio = Power Out / Power In

dB =10 X log 10 [power ratio]

dB: Try Again

10 dB = 1 power of 10 = 10

20 dB = 2 powers of 10 = 100

60 dB = 6 powers of 10 = 1,000,

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 & Frequency

• Attenuation affected by

medium frequency

• As frequency increases, so does

attenuation bass sound carries farther than treble high frequency = poorer penetration

Rule of Thumb

0.5 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 Coefficient

• Attenuation Coefficient = 0.5 * Freq.

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

• indicates fraction of beam

intensity lost per unit distance of sound traval

Attenuation Coefficient

Comments

• 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 Ratio

dB Intensity Fraction

atten. Ratio atten.

dB attenuation =10 X log 10 [intensity ratio] Fraction attenuated = 1 - intensity ratio

Attenuation

• 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 docsity.com

Attenuation Coefficients

• Fat 0.
• Brain 0.
• Liver 0.
• Kidney 0.
• Muscle 1.
• Heart 1.

Tissue Attenuation Coefficient (dB/cm/MHz)

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