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8/2/2019 Intro to Medical Imaging
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Slide 1
Introduction to Medical Imaging
Eduardo Bartolome
E-bartolome1@ti.com
X-ray, CT, PET, Ultrasound, MRI
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Targets
• Describe the physical principles behindeach imaging modality.
• Overview of the engineering solutions.
• Future trends.
• Take away knowledge for our personalhealth and have some fun.
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Imaging Modalities
X-ray CT PET Ultrasound MRI
Others: OCT, hyperspectral imaging…
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Digital X-rays
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Spectrum
~124eV
~124keV
~511keV
E= h.f = hc
λh = 6.63e-34 J.s= 4.1e-15 eV.s
6-8um
30um
384Mm
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X-ray – Generation
Dose α I
Energy α Voltage
Courtesy of Siemens
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X-ray Machine
Filter/collimator
Anti-scatter grid
D E T E C T O R
ABSORBEDSCATTEREDTRANSMITTED
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X-ray imagers
overview
Courtesy of Hologic
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Indirect
Scintillator
AddressingReadout
ADC
PFL- Aachen, M. Overdick, 11 Sept 2002, IWORID 2002
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AFE-XR0064
operation1. CDS samples offset.2. The panel control turns on
the TFTs of a new columnof pixels.
3. The charge is integrated(needs about 14us).
4. The CDS takes theintegrated values andsubtracts the offsets.
5. We can now RST theintegrators. CDS still holds
the analog values.6. Analog values are muxed to
the ADC inputs.
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Readout time
Scan lines controllinggates of TFT:
• Ron ≅ 1-2MΩ
• Cpixel ≅ 1-2pF
Example: 1536 * 1536 panel
Divide panel on 2 blocks of 768
columns, each with 24 AFEXR0064:
768*27.8us = 21.35ms
FR >30fps
For 128 lines(1 pixel/line):Tmin = 27.8us
130mW
142mW
142mW
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CTComputer Tomography
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The machine
3 revolutions per
second
1000 profiles per
revolution
3KSPS/pixel
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Imaging the heart
Challenges
• @ 60bpm 1 beat/s.• Need 100ms shot at least to resolve 1mm
in diastole (when heart is more still)• Faster shot for other phases of the heart
or better resolution (for plaque, smaller arteries…)
• 12cm long.• Image the heart in one breath hold.• Varying beats: % case with stable heart
beat (courtesy of GE):
• 4 beat: 97%• 5 beat: 92%• 8 beat: 39%• 10 beat: 10%
GE
1s100ms
P Q S T
R
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DAS
Data Acquisition System
Scintillator
Ceramic
PhotodiodesX-rayGE Siemens
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Example
Flux * T * Exray * Yieldscintillator * Ephoton * η * S * 1/T
<1.9Gp/(mm2.s)
# of x-ray photons
Incident energy
100keV
# of photons from scintillator
Visible light energy out from scintillator
Visible light at the photodiode
Charge
Current
>15kphotons/MeV 2.6eV 0.7 0.3 6k
3e5 photons
3e10eV
475M light photons
1.2GeV
40pC
250nA
0.86GeV
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The spec
• Sampling rate: 5 - 10KSPS.
• Maximum charge/current: 150pC/150nA
• Noise: <1fCrms, <1 to 1.5pArms
• Linearity: +/- 1ppm of FSR + % of reading
• “Null” offset drift with temperature• mW’s/channel
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Switched Integrator
PhotodiodeCurrent
A-
+
CINT
B -
+
CINT
ADC
20bits≅ 6KSPS (x2)
≅ 7mW/channel
FS ≅ 100’s pC
DDC232
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PET
Positron Emision Tomography
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The machine
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The detector
I
Example (from Derenzo): NaI(TI) - 3.3 cmLight output: 50k (38k?) photons/MeVPrincipal Decay time: 230 nsIndex of refraction: 1.85
15000 photons atphotocathode
3000 photoelectrons
at first dynode
3.109 electrons atanode
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Centroid
To ADCsPosition
Anger logic
∑
∑=
i
mi
i
mii
E
E x
mX
1
2
3
4
1 2 3 4
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Front-end
AMP
SE DIFF
SE DIFF
SE DIFF
SE DIFF
Time Coincidence(TDC)
∑
∑=
i
mi
i
mii
E
E x
mX
½ of ADS5273
12bit70MSPS
DSP
LVDS
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Radiation
Natural background: 2.4mSv/year Air travel crew: 3mSv/year
Radiation worker federal limit: 50mSv/year Dental radiography: 0.01mSvChest radiography: 0.1mSvMammography: 0.7mSvPET/SPECT : 7mSvChest CT: 8mSvPelvic/abdomen CT: 10mSv
CT Angiography: 15mSv50% of cases die in 30 days: 3Sv
References: http://www.xrayrisk.com/
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Ultrasound
*@?!?
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Principle
t
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Physics (I)
43.10-50.0013331 Air
4-106.121.73600Compact bone
21.631.041568Muscle
0.51.420.971470Fat
Absorption[dB/MHz.cm]
Z
[105 Rayl]ρ [g/cm3]c [m/s]Substance
Imaging Systems for Medical Diagnostics - Siemens
PositionFrame rate
Reflections Depth
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Physics (II)
f.2.x.α = 100dBα = 1dB/(MHz.cm) θ cos.
2w
r f c R Lateral =
f
cc
FBW R dB Axial
22.26
∝==−
τ λ
0.150.43.30.115
0.20.650.16100.351.2100.315
0.83250.782
Axial resolution
[mm]
Lateral resolution
[mm]
Penetration
depth [cm]
Wavelength
[mm]
Frequency
[MHz]
c = 1560m/s
Imaging Systems for Medical Diagnostics - Siemens
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Scan
Courtesy of GE
Mechanical
Mechanical
Electrical
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The machine
Top level
B-mode
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Ultrasound System
Tx Buffer Amp
ADC
DAC5652OPA695
VCA2615/7VCA8613/7
VCA8500/10
AFE5805 ADS527x/8x
ADS1605/06/25/26
Color Doppler (PW)Processing(F-Mode)
Image &Motion
Processing(B-Mode)
SpectralDoppler
Processing(D-Mode)
CW AnalogBeam Former
Transducer
HV Mux/Demux
T/RSwitches
Transducer
Transducer
Transducer
Transducer
Transducer
Transducer
Transducer
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Tx beamformer
Transducer
array
1
2
A
3
4
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Rx beamformer
Transducer
array
1
2
A
3
4
Σ
Delay
Apodization
R i B f i
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Receive Beamforming
Transducerarray
1
2
A1
3
4
B1 C1 L i n e 1
L i n e 2
A D C s
4 3
2 1
5ns
A1
A2 B2 C2
ADCsamples Interpolated
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Signal chain analysis
10uV-1V
0.85nV/rtHz15mm
TI AFE5805
LNA VCA ADC
12bit40MSPS
2-3polefilter
CW
MUX
P0_A
N0_A
P1_A P2_A
N2_AN1_A
+HV1 +HV2
-HV2-HV1GND
GND
9mm
20dB <35dB
PG=30dB; Clamp in VCA disabled; Filter setting = 15MHz
0.00
5.00
10.00
15.00
20.00
25.00
30.00
0.2 0.4 0.6 0.8 1 1.2
Vcontrol (V)
I R
N o i s e ( n V / r t ( H z )
x8
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MRIMagnetic Resonance Imaging
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The machine
Liquid Nitrogen
Magnet Coils
RF Volume Coils
Liquid Helium
BoY
X
Z
www.e-mri.org
Surface Coils
NMR B i Ph i
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NMR Basic Physics
No magnetic field
www.e-mri.org
Hidrogen AtomicNucleus
Matter
Spin/magneticmoment
NMR B i Ph i
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www.e-mri.org
Larmor frequency:ω
0 = 42.57[MHz/T].B0
NMR Basic Physics
With magnetic field
M
Magnetizationvector
Nuclear Excitation
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Nuclear Excitation
and Relaxation
Bo
X
Y
Z
M
RF @fo
90
180
Decay Time
Excitation Relaxation
Depends on B0 Depends on material
G di t fi ld
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Gradient field
Bo
1. Slice: Gradient on excitation. Only the nucleuses with a Larmor frequencyequal to the RF pulse frequency will be excited.
2. Frequency encoding: Gradient on relaxation. Looking at the spectrum of the received RF, the spatial position of the nucleuses can be identified.
RF Signal
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RF Signal
Characteristics
• Carrier depending on Larmor frequency (depending on fixmagnet).
• BW below 1MHz (depending on gradient coil).
• RF emissions from the human body have a decay timewhich depends on the nature of the tissue.
• Decay time can vary between 5ms and 200ms.
• The emissions SNR is dependent upon: – Field strength and homogeneity
– RF coil design/tuning
– Distance of coil to the sample – The sample itself
• For medical applications the typical dynamic range is ~80dB.
Signal amplitude can range from µV’s to several mV’s.
Si l Ch i
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Signal Chain
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Slide 44
OTHER ASPECTS OF MEDICAL IMAGING
Volumes/Prices
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Volumes/Prices
• Digital X-ray: 25Kunits/year ~$300k• CT: 5Kunits/year $1-$2.5M• PET/CT: 0.8kunits/year <$5M
• Ultrasound: 70Kunits/year $10K-$250k• MRI: 3.5Kunits/year <$5M
S
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Summary
X-ray CT PET Ultrasound MRI
Quality Radiation Cost Quality Cost
Radiation
Others: OCT, hyperspectral imaging…
I s s
u e s
References
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References
• www.e-mri.org
• http://www.cis.rit.edu/htbooks/mri/
• http://www.nlm.nih.gov/medlineplus/tutorials/mri/htm/index.htm
• http://www.mri-tutorial.com/
• http://users.fmrib.ox.ac.uk/~stuart/thesis/chapter_ 2/contents.html
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