CHAPTER3PULSE-ECHO UL-

TRASOUND IN-STRUMENTATION

윤성수

CONTENTS▣ Pulse-Echo Ultrasound □ Pulsing Characteristics and Duty Factor

▣ Instrumentation □ Beam former □ Pulse Transmitter □ Receiver ▣ Ultrasound scanning

▣ Scanners □ Linear array transducers □ Curvilinear array transducers □ Phased array transducers

▣ Frame rate and scanning speed limitations

PULSE-ECHO ULTRASOUND

□ The Range Equation

□ T : Pulse Echo travel time

□ c : speed of ultrasound in target material

2DT

c

PULSE-ECHO ULTRASOUND

□ Pulse Duration Pulse Duration↓ ⇒ Axial resolution↑ □ Pulse repetition period : Wait for Echo pulse PP↑ ⇒ frame rate ↓

□ Duty Factor

INSTRUMENTATION

BEAM FORMER

BEAM FORMER

□ 구현 방식 ▶ Analog vs Digital

□ Beam steering and focusing (pulse delay sequence) ▶ transmit focusing ▶ dynamic focusing of received echo ▶ controls the beam direction

BEAM FORMER

□ 구현 방식 ▶ Analog ① 반사신호를 한 곳에 모으는

시간이 20~40ns 로 편차가 커 선명한 의료영상 구현 어려움

② 인체 한 부위의 초점 위치를 제외한 나머지 부위에 대해선 수신 focusing 을 구현할 수 없음

BEAM FORMER

□ 구현 방식▶ Digital ① Stable : external factor

(ex. temperature)

② programmability

③ wide range of signal fre-quency

BEAM FORMER

□ Beam steering and focusing (pulse delay sequence) ▶ transmit focusing ▶ dynamic focusing of received echo ▶ controls the beam direction

arraytransmitter

focusing

different time delay

Transmit focusing

BEAM FORMER

□ Beam steering and focusing (pulse delay sequence) ▶ transmit focusing ▶ dynamic focusing of received echo ▶ controls the beam direction

Dynamic focusing of received echo

different time delayarraytrans-ducer

same delayedsignal

focused signal

reflector

BEAM FORMER

□ Beam steering and focusing (pulse delay sequence) ▶ transmit focusing ▶ dynamic focusing of received echo ▶ controls the beam direction

Controls the beam direction

PULSE TRANSMITTER

PULSE TRANSMITTER

□ Pulse transmitter ▶ provide electrical signals for exciting transducer

① Transmit power ↑

② Higher intensity sound wave

③ Higher amplitude echo signal

④ Appear brighter and weaker reflectors In the display But acoustical exposure to the patient↑

PULSE TRANSMITTER

□ Pulse transmitter ▶ provide electrical signals for exciting transducer

Low output power (MI is 0.2) High output power (MI is 0.8)

RECEIVER

RECEIVER

□ Procedure of the receiver

AMPLIFY(RECEIVER)

□ PreamplifierBoosts echo signalsprotect the receiver from high-voltage

□ problem : amplify noise

COMPENSATE(RECEIVER)

□ Gain adjustments ▶ Overall gain control : Increase amplification at all depths

High gainLow gain

COMPENSATE(RECEIVER)

□ Gain adjustments ▶ Overall gain control : Increase amplification at all depths

High gainLow gain

COMPENSATE(RECEIVER)

□ Gain adjustments ▶ Swept gain or TGC(time gain compensation) : Attenuation compensation depending depth

COMPENSATE(RECEIVER)

□ Gain adjustments ▶ Swept gain or TGC ▶ Slider bar TGC control

COMPENSATE(RECEIVER)

□ Gain adjustments ▶ 3-Knob TGC control

COMPENSATE(RECEIVER)

□ Gain adjustments ▶ Internal time-varied gain : automatically set swept gain

▶ Lateral gain

COMPENSATE(RECEIVER)

□ Dynamic frequency tuning ▶ Higher frequency sound waves are attenuated more rapidly and penetrate less deeply than lower frequencies.

COMPENSATE(RECEIVER)

□ Dynamic frequency tuning ▶ Higher frequency sound waves are attenuated more rapidly and penetrate less deeply than lower frequencies.

▶ shallow regions higher frequencies deeper regions lower frequencies

DYNAMIC RANGE AND COMPRESSION(RECEIVER)

□ Dynamic range ▶ Limited input signal amplitude range to respond effectively ▶ Threshold < Input signal < Saturation

DYNAMIC RANGE AND COMPRESSION(RECEIVER)

□ Log compression ▶ Receiver : 100~120dB Memory : 40~45dB Monitor(contrast) : 20~30dB

Log Compression

DYNAMIC RANGE AND COMPRESSION(RECEIVER)

□ Log compression

▶ Low dynamic range : High contrast High dynamic range : Low contrast

DEMODULATION(RECEIVER)

□ Demodulation ▶ Convert the amplified echo signal into a single pulse

Input signal → rectification → smoothing

REJECT(RECEIVER)

□ Reject ▶ reject eliminates both low level electronic noise and low level echoes

REJECT(RECEIVER)

□ Reject ▶ Adaptive threshold processing

ULTRASOUND SCANNING

□ A – mode(amplitude mode) ▶ shows echo amplitude versus reflector distance ▶ only presents echo data from a single beam line(limited uses)

ULTRASOUND SCANNING

□ A – mode(amplitude mode) ▶ shows echo amplitude versus reflector distance ▶ only presents echo data from a single beam line(limited uses) ▶ ophthalmological applications

ULTRASOUND SCANNING

□ B – mode(brightness mode) ▶ echo signals are converted to intensity-modulated dots ▶ brightness ∝ echo signal amplitude

ULTRASOUND SCANNING

A - mode

B - mode

ULTRASOUND SCANNING

□ M – mode(motion mode) ▶ by slowly sweeping a B-mode trace across a screen ▶ depth on one axis and time on an orthogonal axis

ULTRASOUND SCANNING

□ M – mode(motion mode) ▶ velocity of a reflector is estimated from

∆t∆d

ULTRASOUND B-MODE SCANNING

□ Image build-up ▶ called 2-D image ▶ echoes are positioned along a line that corresponds to the beam axis

SCANNERS

□ Linear array transducers ▶ small rectangular elements lined up side by side ▶ parallel beam line

SCANNERS

□ Linear array transducers ▶ small rectangular elements lined up side by side

SCANNERS

□ Convex array transducers ▶ same principles as a linear array ▶ beam lines are not parallel, emerge at different angles

SCANNERS

□ Convex array transducers ▶ larger imaged field than linear array

SCANNERS

□ Phased array transducers ▶ electronically steered at various angles ▶ electronic focusing

SCANNERS

□ Phased array transducers ▶ through the intercostal (small entrance window) ▶ cardiac imaging

SCANNERS

□ Mechanical scan ▶ two to four separate transducers positioned in different locations on the rim of rotating wheel

FRAME RATE AND SCANNING SPEED LIMITA-TIONS

□ Maximum frame rate ▶ D : depth C : speed of sound in tissue (1540 m/s) N : the number of lines

213line

DT s D

C

frame lineT N T

max

1 1 77000 /

13 ( )frame

sFR

T s N D N D cm

FRAME RATE AND SCANNING SPEED LIMITA-TIONS

□ Maximum frame rate ▶ D : depth C : speed of sound in tissue (1540 m/s) N : the number of lines

D ↑ ⇒ Frame Rate↓

N↑ ⇒ Frame Rate↓

N↑ ⇒ Frame Rate↓Smaller image sizeResolution 유지