Intro to Medical Imaging

8/2/2019 Intro to Medical Imaging

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Slide 1

Introduction to Medical Imaging

Eduardo Bartolome

[email protected]

X-ray, CT, PET, Ultrasound, MRI



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.



Imaging Modalities

X-ray CT PET Ultrasound MRI

Others: OCT, hyperspectral imaging…



Digital X-rays



Spectrum

~124eV

~124keV

~511keV

E= h.f = hc

λh = 6.63e-34 J.s= 4.1e-15 eV.s

6-8um

30um

384Mm



X-ray – Generation

Dose α I

Energy α Voltage

Courtesy of Siemens



X-ray Machine

Filter/collimator

Anti-scatter grid

D E T E C T O R

ABSORBEDSCATTEREDTRANSMITTED



X-ray imagers

overview

Courtesy of Hologic



Indirect

Scintillator

AddressingReadout

ADC

PFL- Aachen, M. Overdick, 11 Sept 2002, IWORID 2002



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.



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



CTComputer Tomography



The machine

3 revolutions per

second

1000 profiles per

revolution

3KSPS/pixel



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



DAS

Data Acquisition System

Scintillator

Ceramic

PhotodiodesX-rayGE Siemens



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



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



Switched Integrator

PhotodiodeCurrent

A-

+

CINT

B -

+

CINT

ADC

20bits≅ 6KSPS (x2)

≅ 7mW/channel

FS ≅ 100’s pC

DDC232



PET

Positron Emision Tomography



The machine



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



Centroid

To ADCsPosition

Anger logic

∑

∑=

i

mi

i

mii

E

E x

mX

1

2

3

4

1 2 3 4



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



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/



Ultrasound

*@?!?



Principle

t



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



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



Scan

Courtesy of GE

Mechanical

Mechanical

Electrical



The machine

Top level

B-mode



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



Tx beamformer

Transducer

array

1

2

A

3

4



Rx beamformer

Transducer

array

1

2

A

3

4

Σ

Delay

Apodization

R i B f i



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



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



MRIMagnetic Resonance Imaging



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



NMR Basic Physics

No magnetic field

www.e-mri.org

Hidrogen AtomicNucleus

Matter

Spin/magneticmoment

NMR B i Ph i



www.e-mri.org

Larmor frequency:ω

0 = 42.57[MHz/T].B0

NMR Basic Physics

With magnetic field

M

Magnetizationvector

Nuclear Excitation



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



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



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



Signal Chain



Slide 44

OTHER ASPECTS OF MEDICAL IMAGING

Volumes/Prices



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



Summary

X-ray CT PET Ultrasound MRI

Quality Radiation Cost Quality Cost

Radiation

Others: OCT, hyperspectral imaging…

I s s

u e s

References



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

Documents

Intro to Medical Imaging