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OB/Gyn (Core) Ultrasound

Principles of Ultrasound

Physics and Instrumentation

Nathan Pinkney, BS, CDOS

Philadelphia College of Osteopathic Medicine

2016 ULTRASOUND

CATEGORIES OF SOUND

• INFRASOUND = below 20 Hz

• AUDIBLE SOUND = 20 Hz to 20 kHz

• ULTRASOUND = above 20 kHz

o Medical Diagnostic Ultrasound = above 1 MHz

SOUND VELOCITIES

Material Meters per second

Air 330

Pure Water 1430

Fat 1450

Soft Tissue 1540

Muscle 1585

Bone 4080

•TRANSMIT – Electrical energy to mechanical energy

•RECEIVE – Mechanical energy to electrical energy

PIEZOELECTRIC EFFECT

PIEZOELECTRIC EFFECT

Transmitted pulse

Piezoelectric element (s)

Interface

Mechanical

energy

Electrical

energy

Returning echo

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ACOUSTIC IMPEDANCE

(Rayls)

Air 400

Fat 1,380,000

Water 1,430,000

Soft Tissue 1,630,000

Muscle 1,700,000

Bone 7,800,000

INTERFACE MATERIALS & ECHO STRENGTH

Soft Tissue to Muscle - Weak (1%)

Fat to Soft Tissue - Weak (1%)

Soft Tissue to Bone - Strong (50%)

Blood to Plaque - Strong (50%)

Soft Tissue to Air - Very Strong (100%)

GRAY SCALE ASSIGNMENT RESOLUTION

Interfaces not closely spaced

Closely spaced

GOOD

GOOD POOR

Closely spaced

AXIAL RESOLUTION

SCANNED STRUCTURE DISPLAYED IMAGE

LATERAL RESOLUTION

SCANNED STRUCTURE DISPLAYED IMAGE

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HIGH-FREQUENCY TRANSDUCERS

BETTER RESOLUTION

GREATER ATTENUATION

POORER PENETRATION

LOW-FREQUENCY TRANSDUCERS

POORER RESOLUTION

LESS ATTENUATION

BETTER PENETRATION

RESOLUTION & PENETRATION

The fundamental frequency of a transducer

RESONANT FREQUENCY

PIEZOELECTRIC

ELEMENT THICKNESS RESONANT FREQUENCY

Increase Decrease

Decrease Increase

2 MHz 2.25 MHz 2.5 MHz

TRANSDUCER FREQUENCIES

5 MHz 7 MHz 7.5 MHz

10 MHz 12 MHz 15 MHz

3 MHz 3.5 MHz 4 MHz

TRANSDUCER FREQUENCY ATTENUATION PENETRATION HALF INTENSITY DEPTH

Increase Increase Decrease Decrease

Decrease Decrease Increase Increase

ATTENUATION COEFFICIENT:

(in tissue)

a = - 0.5 dB per cm per MHz

HALF INTENSITY DEPTH:

(in tissue)

H.I.D. = 6 divided by frequency

ATTENUATION

IN TISSUE:

Attenuation = 0.5 dB per cm per MHz

H.I.D. = 6 ÷ Frequency

Frequency -dB per cm Half-Intensity-Depth

2 MHz 1 3 cm

2.25 MHz 1.125 2.67 cm

2.5 MHz 1.25 2.4 cm

3 MHz 1.5 2 cm

3.5 MHz 1.75 1.71 cm

4 MHz 2 1.5 cm

5 MHz 2.5 1.2 cm

7 MHz 3.5 0.86 cm

7.5 MHz 3.75 0.8 cm

10 MHz 5 0.6 cm

15 MHz 7.5 0.4 cm

HEAT (Thermal)

CAVITATION (Mechanical)

Stable

Transient

ULTRASOUND BIOEFFECTS

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ALARA

“As low as reasonably achievable”

Pulse-echo Imaging

Voltage

Sound

TRANSDUCER EXCITATION

AND OUTPUT POWER

TRANSMITTER

TRANSMIT POWER

OUTPUT

ACOUSTIC POWER

ENERGY OUTPUT

The frequency of the sound is not affected.

TIMING

PRF

Pulse-repetition frequency is not

the same as transducer frequency.

RECEIVER

TGC

GAIN

MASTER GAIN

OVERALL GAIN

Receiver controls do not affect the patient

TIME GAIN COMPENSATION

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OBSTETRICAL IMAGE

B-SCAN

Display Modes

B-SCAN (2-D) M-MODE

PATIENT-ORIENTED B-SCAN PLANES OBSTETRICAL IMAGES

2D 3D/4D

3D/4D

Freehand 3D, often called manual 3D uses a

standard 2D transducer and produces a static

volumetric image after the transducer is slowly

moved along a scan plane.

Automatic 3D requires a dedicated transducer and

can produce a volumetric image from a fixed

transducer position.

4D also requires a dedicated transducer, but the

volumetric image is displayed in real-time.

Ultrasound Transducers

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FLAT-LINEAR ARRAY

(linear array)

Labeled “L”

FLAT-LINEAR ARRAY

(linear array)

FLAT-LINEAR ARRAY

(linear array)

EMBRYO

FLAT-LINEAR ARRAY

(linear array) CURVED-LINEAR ARRAY

(convex array)

Labeled “C”

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CURVED-LINEAR ARRAY

(convex array)

CURVED-LINEAR ARRAY

(convex array)

CURVED-LINEAR ARRAY

(convex array)

FIBROID UTERUS

CURVED-LINEAR ARRAY

(convex array)

EMBRYO AND OVARIAN CYST

CURVED-LINEAR ARRAY

(convex array)

FETUS

CURVED-LINEAR ARRAY

(convex array)

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FETUS

CURVED-LINEAR ARRAY

(convex array)

ENDOVAGINAL TRANSDUCER

CURVED-LINEAR ARRAY

(convex array)

ENDOVAGINAL TRANSABDOMINAL

SAGITTAL SCAN PLANES

ENDOVAGINAL UTERUS

CURVED-LINEAR ARRAY

(convex array)

PHASED ARRAY

(electronically steered)

Labeled “P” or “S”

FETAL HEAD

PHASED ARRAY

(electronically steered)

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FETUS

PHASED ARRAY

(electronically steered)

ENDOVAGINAL

TRANSDUCER

PHASED ARRAY

(electronically steered)

ENDOVAGINAL UTERUS

PHASED ARRAY

(electronically steered)

PHASED / VECTOR ARRAY

(trapezoidal array)

Labeled “V”

FETUS

PHASED / VECTOR ARRAY

(trapezoidal array) ULTRASOUND ARTIFACTS

REVERBERATION

ENHANCEMENT

SHADOWING

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Doppler Imaging

DOPPLER

SAME

FREQUENCY

TRANSDUCER

During Doppler operation, the reflected sound has the same frequency as the

transmitted sound if the blood is stationary.

DOPPLER

LOWER

FREQUENCY

TRANSDUCER

During Doppler operation, the reflected sound has a lower frequency if the

blood is moving away from the sound source.

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DOPPLER

HIGHER

FREQUENCY

TRANSDUCER

During Doppler operation, the reflected sound has a higher frequency if the

blood is moving toward the sound source.

SPECTRAL & COLOR-FLOW DOPPLER

KEY ITEMS

FOR REVIEW

#1

Frequencies used for medical

diagnostic ultrasound are:

#1

Frequencies used for medical

diagnostic ultrasound are:

above 1 MHz

CATEGORIES OF SOUND

• INFRASOUND = below 20 Hz

• AUDIBLE SOUND = 20 Hz to 20 kHz

• ULTRASOUND = above 20 kHz

o Medical Diagnostic Ultrasound = above 1 MHz

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#2

The average sound velocity in

soft tissue is:

#2

The average sound velocity in

soft tissue is:

1540 meters per second

SOUND VELOCITIES

Material Meters per second

Air 330

Pure Water 1430

Fat 1450

Soft Tissue 1540

Muscle 1585

Bone 4080

#3

High frequency transducers

provide improved resolution and:

#3

High frequency transducers

provide improved resolution and:

poor penetration

HIGH-FREQUENCY TRANSDUCERS

BETTER RESOLUTION

GREATER ATTENUATION

POORER PENETRATION

LOW-FREQUENCY TRANSDUCERS

POORER RESOLUTION

LESS ATTENUATION

BETTER PENETRATION

RESOLUTION & PENETRATION

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#4

Major categories of ultrasound

bioeffects are heat and:

#4

Major categories of ultrasound

bioeffects are heat and:

cavitation

HEAT (Thermal)

CAVITATION (Mechanical)

Stable

Transient

ULTRASOUND BIOEFFECTS #5

Two basic scanning formats are:

#5

Two basic scanning formats are:

linear and sector

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