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BMI 1 FS05 – Class 8, “US Instrumentation” Slide 1
Biomedical Imaging IBiomedical Imaging I
Class 8 – Ultrasound Imaging II: Instrumentation and Applications
11/02/05
BMI 1 FS05 – Class 8, “US Instrumentation” Slide 2
Generation and Detection of Ultrasound
Generation and Detection of Ultrasound
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 3
Piezoelectric effect IPiezoelectric effect I
Conversion of electric energy into mechanical energy and vice versa in materials with intrinsic el. dipole moments (structural anisotropy).
Electric field (~100 V) causes re-orientation of dipoles deformation
Deformation causes shift of dipoles induces Voltage
Examples of piezoelectric Materials:
Crystalline (quartz), Polycrystalline ceramic (PZT, lead zirconium titanate), Polymers (PVDF)
Crystalline: Quartz (SiO2)
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 4
Piezoelectric effect IIPiezoelectric effect II
Polycrystalline (e.g., ferroelectric, PZT) Polymers (PVDF)
“ phase” (not p.e. active)
“ phase” (p.e. active)
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 5
Transducer Q-factorTransducer Q-factor
Disc of piezoelectric material (usually PZT) shows mechanical resonance frequencies fres
Resonance curve (Q-factor
High Q: strong resonance(narrow curve)
Low Q: strongly damped, weak resonance (broad curve)
Tradeoff of high Q:
+ Efficient at fres (high signal-to-noise ratio)
– Pulse distortion
flo-Q
A (fres) = 0 dB
- 3 dB
fhi-Q
Amplitude
Frequency
0
3dB
fQ
f
020log
3 2
AdB A
db
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 6
Transducer resonancesTransducer resonances
Transducer (disc) has mechanical resonances at frequencies
Lowest (fundamental) resonance frequency (standing wave):
Crystal
Len
gth
of
crys
tal,
L c
or 1,2,3,...2 2res c
c
ncf L n n
L
(c: speed of sound, : wavelength)
time
Transducer ends have 180 phase difference (= = /2)
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 7
Transducer backingTransducer backing
Backing of transducer with impedance-matched, absorbing material reduces reflections from back damping of resonance
Reduces efficiency
Increases Bandwidth (lowers Q)
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 8
Transducer–tissue mismatchTransducer–tissue mismatch
Impedance mismatch causes reflection, inefficient coupling of acoustical energy from transducer into tissue:
ZT 30 MRaylZL 1.5 MRayl It/Ii = 0.18
Solution: Matching layer(s)
increases coupling efficiency
damps crystal oscillations, increases bandwidth (reduces efficiency)
2
4t T l
i T l
I Z Z
I Z Z
Transducer Load (tissue)
ItIi
Ir
ZT
ZL
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 9
Matching layersMatching layers
A layer between transducer and tissue with ZT > Zl > ZL creates stepwise transition
Ideally, 100 % coupling efficiency across a matching layer is possible because of destructive interference of back reflections if
layer thickness = /4
Zl chosen so that Ir,1 = Ir,2 :
Problems: Finding material with exact Zl value (~6.7 MRayl)
Dual-layer:
Mat
chin
gLa
yer
Loa
d (T
arge
t)
Tra
nsdu
cer
ZT
Zl
ZL
ItIiIt,LIr,1
Ir,lIr,2
l T LZ Z Z
= /4
= /23/ 4 1/ 4 1/ 4 3/ 4
,1 ,2;T L T Ll lZ Z Z Z Z Z
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 10
Pulsed vs. C.W. modePulsed vs. C.W. mode
Low bandwidth:
No backing, matching possible
High efficiency (SNR)
High-Q
Strong “Pulse ringing” c.w. applications
Large Bandwidth:
Pulsed applications
Backing, matching
Low-Q
Lowered efficiency
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 11
Axial beam profileAxial beam profile
Piston source: Oscillations of axial pressure in near-field (e.g. z0= (1 mm)2/0.3mm = 3 mm)
Caused by superposition of point wave sources across transducer (Huygens’ principle)
Function, see Webb Eq. (3.30)
2
0 NFB
rz z
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 12
Lateral beam profile Lateral beam profile
Determined by Fraunhofer diffraction in the far field.
Given by Fourier Transform of the aperture function
Lateral resolution is defined by width of first lobe (angle of fist zero) in diffraction pattern
For slit (width a):
For disc (radius r, piston source):
sin 0.61 arcsin 0.61r r
0
sin sinc
Minima at: sin
aI I
na
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 13
Axial and lateral resolutionAxial and lateral resolution
Axial resolution = 0.5c, determined by spatial pulse length (= pulse duration). Pulse length determined by location of -3 dB point.
Lateral resolution determined by beam width (-3 dB beam width or - 6 dB width)
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 14
Focusing of ultrasoundFocusing of ultrasound
Increased spatial resolution at specific depth
Self-focusing radiator or acoustic lens
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 15
Transducer arraysTransducer arrays
Linear sequential array lateral scan
Linear phased array for beam steering, focusing
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 16
Array typesArray types
a) Linear Sequential (switched) ~1 cm 10-15 cm, up to 512 elements
b) Curvilinearsimilar to (a), wider field of view
c) Linear Phasedup to 128 elements, small footprint cardiac imaging
d) 1.5D Array3-9 elements in elevation allow for focusing
e) 2D PhasedFocusing, steering in both dimensions
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 17
Array resolutionArray resolution
Lateral resolution determined by width of main lobe according to
Larger array dimension increased resolution
Side lobes (“grating lobes”) reduce resolution and appear at
sinw
wa
g
sin 1,2,3,...g
nn
g
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 18
Radiation patternRadiation pattern
Contributions of different terms to pattern:
Example for:
a =
g = 2
w = 32
a g w
BMI 1 FS05 – Class 8, “US Instrumentation” Slide 19
Ultrasound ImagingUltrasound Imaging
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 20
A-mode (amplitude mode) IA-mode (amplitude mode) I
Oldest, simplest type
Display of the envelope of pulse-echoes vs. time, depth d = ct/2Pulse repetition rate ~ kHz (limited by penetration depth, c 1.5 mm/s 20 cm 270 s, plus additional wait time for reverberation and echoes)
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 21
A-mode IIA-mode II
Frequencies: 2-5 MHz for abdominal, cardiac, brain; 5-15 MHz for ophthalmology, pediatrics, peripheral blood vessels
Applications: ophthalmology (eye length, tumors), localization of brain midline, liver cirrhosis, myocardium infarction
Logarithmic compression of echo amplitude (dynamic range of 70-80 dB)
Logarithmic compression of signals
Time-Gain Compensation
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 22
M-mode (“motion mode”)M-mode (“motion mode”)
Recording of variation in A scan over time
Cardiac imaging: wall thickness, valve function
see Fig. 3.17
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 23
M-mode clinical exampleM-mode clinical example
B-Mode / M-Mode image of mitral valve
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 24
B-mode (“brightness mode”)B-mode (“brightness mode”)
Lateral scan across tissue surface
Grayscale representation of echo amplitude
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 25
Real-time B scannersReal-time B scanners
Frame rate Rf ~30 Hz:
Mechanical scan: Rocking or rotating transducer + no side lobes - mechanical action, motion artifacts
Linear switched array
12
2acq f
d ct N R t
c d N d: depth
N: no. of lines
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 26
Linear switchedLinear switched
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 27
CW DopplerCW Doppler
Doppler shift in detected frequency
Separate transmitter and receiver
Bandpass- filtering of Doppler signal:
Clutter (Doppler signal from slow-moving tissue, mainly vessel walls) @ f<1 kHz
LF (1/f) noise
Blood flow signal @f < 15 kHz
CW Doppler bears no depth information
2 cosshift
vf f
c
v: blood flow velocityc: speed of sound: angle between direction of blood flow and US beam
Frequency Counter
SpectrumAnalyzer
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 28
CW Doppler clinical imagesCW Doppler clinical images
CW ultrasonic flowmeter measurement (radial artery)
Spectrasonogram:
Time-variation of Doppler Spectrum
t
f
t [0.2 s]
v [10cm/s]
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 29
CW Doppler exampleCW Doppler example
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 30
Pulsed Doppler – single volumePulsed Doppler – single volume
Time gate (range gate) is used to define depth location
Sample volume ~mm2
Center or carrier frequency 2-10 Mhz
Pulse repetition rate 1/T~ kHz
Red Box?
Demodulation of signal, see Webb, pp.138
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 31
Duplex ImagingDuplex Imaging
Combines real-time B-scan with US Doppler flowmetry
B-Scan: linear or sector
Doppler: C.W. or pulsed (fc = 2-5 MHz)
Duplex Mode:
Interlaced B-scan and color encoded Doppler images limits acquisition rate to 2 kHz (freezing of B-scan image possible)
Variation of depth window (delay) allows 2D mapping (4-18 pulses per volume)
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 32
Modern US instrumentModern US instrument
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 33
Duplex imaging example (c.w.)Duplex imaging example (c.w.)
www.medical.philips.com
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 34
Duplex imaging (Pulsed Doppler)Duplex imaging (Pulsed Doppler)
BMI 1 FS05 – Class 8 “US Instrumentation” Slide 35
US imaging example (4D)US imaging example (4D)