ULTRASOUND. OB/Gyn (Core) Ultrasound PIEZOELECTRIC EFFECT. Principles of Ultrasound Physics and Instrumentation. Nathan Pinkney, BS, CDOS

Transcription

1 1 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 Material SOUND VELOCITIES Meters per second Air 330 Pure Water 1430 Fat 1450 Soft Tissue 1540 Muscle 1585 Bone 4080 PIEZOELECTRIC EFFECT PIEZOELECTRIC EFFECT Electrical energy TRANSMIT Electrical energy to mechanical energy Piezoelectric element (s) RECEIVE Mechanical energy to electrical energy Transmitted pulse Returning echo Mechanical energy Interface

2 2 ACOUSTIC IMPEDANCE INTERFACE MATERIALS & ECHO STRENGTH (Rayls) Air 400 Fat 1,380,000 Water 1,430,000 Soft Tissue 1,630,000 Muscle 1,700,000 Bone 7,800,000 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 GOOD Closely spaced Closely spaced GOOD POOR AXIAL RESOLUTION LATERAL RESOLUTION SCANNED STRUCTURE DISPLAYED IMAGE SCANNED STRUCTURE DISPLAYED IMAGE

3 RESOLUTION & PENETRATION HIGH-FREQUENCY TRANSDUCERS BETTER RESOLUTION GREATER ATTENUATION POORER PENETRATION RESONANT FREQUENCY The fundamental frequency of a transducer LOW-FREQUENCY TRANSDUCERS POORER RESOLUTION LESS ATTENUATION BETTER PENETRATION PIEZOELECTRIC ELEMENT THICKNESS Increase Decrease RESONANT FREQUENCY Decrease Increase TRANSDUCER FREQUENCIES 2 MHz 2.25 MHz 2.5 MHz 3 MHz 3.5 MHz 4 MHz 5 MHz 7 MHz 7.5 MHz 10 MHz 12 MHz 15 MHz TRANSDUCER FREQUENCY ATTENUATION PENETRATION HALF INTENSITY DEPTH Increase Increase Decrease Decrease Decrease Decrease Increase Increase ATTENUATION ATTENUATION COEFFICIENT: (in tissue) a = db per cm per MHz HALF INTENSITY DEPTH: (in tissue) H.I.D. = 6 divided by frequency 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 cm 2.5 MHz cm 3 MHz cm 3.5 MHz cm 4 MHz cm 5 MHz cm 7 MHz cm 7.5 MHz cm 10 MHz cm 15 MHz cm ULTRASOUND BIOEFFECTS HEAT (Thermal) CAVITATION (Mechanical) Stable Transient 3

4 4 ALARA Pulse-echo Imaging Voltage Sound As low as reasonably achievable TRANSDUCER EXCITATION AND OUTPUT POWER TIMING TRANSMITTER TRANSMIT POWER OUTPUT ACOUSTIC POWER ENERGY OUTPUT PRF The frequency of the sound is not affected. Pulse-repetition frequency is not the same as transducer frequency. RECEIVER TIME GAIN COMPENSATION TGC GAIN MASTER GAIN OVERALL GAIN Receiver controls do not affect the patient

5 5 OBSTETRICAL IMAGE Display Modes B-SCAN 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. Ultrasound Transducers 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.

6 6 FLAT-LINEAR ARRAY (linear array) Labeled L FLAT-LINEAR ARRAY (linear array) FLAT-LINEAR ARRAY (linear array) FLAT-LINEAR ARRAY (linear array) Labeled C EMBRYO

7 7 FIBROID UTERUS EMBRYO AND OVARIAN CYST FETUS

8 8 FETUS ENDOVAGINAL TRANSDUCER SAGITTAL SCAN PLANES TRANSABDOMINAL ENDOVAGINAL ENDOVAGINAL UTERUS PHASED ARRAY (electronically steered) PHASED ARRAY (electronically steered) Labeled P or S FETAL HEAD

9 9 PHASED ARRAY (electronically steered) PHASED ARRAY (electronically steered) FETUS ENDOVAGINAL TRANSDUCER PHASED ARRAY (electronically steered) PHASED / VECTOR ARRAY (trapezoidal array) Labeled V ENDOVAGINAL UTERUS PHASED / VECTOR ARRAY (trapezoidal array) ULTRASOUND ARTIFACTS REVERBERATION ENHANCEMENT SHADOWING FETUS

10 10 Doppler Imaging DOPPLER DOPPLER TRANSDUCER SAME FREQUENCY TRANSDUCER LOWER FREQUENCY During Doppler operation, the reflected sound has the same frequency as the transmitted sound if the blood is stationary. During Doppler operation, the reflected sound has a lower frequency if the blood is moving away from the sound source.

11 11 DOPPLER SPECTRAL & COLOR-FLOW DOPPLER TRANSDUCER HIGHER FREQUENCY During Doppler operation, the reflected sound has a higher frequency if the blood is moving toward the sound source. 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

12 #2 #2 The average sound velocity in soft tissue is: The average sound velocity in soft tissue is: 1540 meters per second Material SOUND VELOCITIES Meters per second Air 330 Pure Water 1430 Fat 1450 Soft Tissue 1540 Muscle 1585 #3 High frequency transducers provide improved resolution and: Bone 4080 #3 High frequency transducers provide improved resolution and: poor penetration RESOLUTION & PENETRATION HIGH-FREQUENCY TRANSDUCERS BETTER RESOLUTION GREATER ATTENUATION POORER PENETRATION LOW-FREQUENCY TRANSDUCERS POORER RESOLUTION LESS ATTENUATION BETTER PENETRATION 12

13 #4 #4 Major categories of ultrasound bioeffects are heat and: Major categories of ultrasound bioeffects are heat and: cavitation ULTRASOUND BIOEFFECTS #5 HEAT (Thermal) Two basic scanning formats are: CAVITATION (Mechanical) Stable Transient #5 Two basic scanning formats are: linear and sector 13

14 14