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Medical ImagingSeeing Through the Human Body
Elsa AngeliniDépartement Traitement du Signal et des Images,Ecole Nationale Supérieure des Télécommunications
Source: uhrad.com.
November 19th 2004
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It all started in 1845 with the first X-ray image
Roentgen’s wife Von Kolliker
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X R
ayU
SM
RI
Nuc
lear
History of Medical Imaging
• 1845: Discovery of X-ray by Wilhelm Conrad Röntgen, german physicist.• 1972: First scanner set up by Allan Mc Cornack et Godfrey N. Hounsfield, Nobel
prize in 1979 for this work.
• 1915: Ultrasound propagation principles for SONAR (SOund NAvigation Ranging).• 1955: First echography by Inge Edler (1911-2001), swedish cardiologist suédois.
• 1945: Discovery of nuclear resonance under a magnetic field by Edward Purcell and Felix Bloch, both nobel prize winners in 1952.
• 1973: First MRI image on an animal by the american chemist Paul Lauterbur.• 1980: Spectroscopy by magnetic resonance.
• 1934: Discovary of natural radioactivity by Henri Becquerel, Pierre et Marie Curie. Discovery of artificial radioactivity by Irène et Frédéric Joliot-Curie
• 1990: Nuclear medicine development with scintigraphy and positron emission tomography.
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Terminology• Transmission: Photons passing through the body• Absorption: Partial or total absorption of energy in the patient• Scatter: Radiation diverted in a new direction
Source: Utrecht University, NL
• Transaxial - plane normal to a vector from head to toe. • Coronal - plane normal to a vector from front to back • Sagittal - plane normal to a vector from left to right. • Oblique – otherwise.
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Imaging Data Types• 2D:
– A single slice. • 3D:
– A series of parallel slices with uniform spacing.
– A series of slices with varying spacing and/or orientation.
– A volumetric data set.
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Digital Display(soft copy)
Films(hard copy)
Imaging Support
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Imaging Modalities
• X-Ray:– Traditional – Computerized Tomography
• Ultrasound• Magnetic Resonance Imaging (MRI)• Nuclear Imaging
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Conventional X-Ray
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Conventional X-Ray
• Accelerating & decelerating electrons generate electromagnetic radiation
• Bombardment of a target material with a beam of fast electrons
• Electrons are emitted thermally from a heated cathode (C) and are accelerated toward the anode target (A) by the applied voltage potential V (~kV).
• Electromagnetic radiation is produced by the quick deceleration of the electrons when hitting the target (bremsstrahlung). – ~99% of energy converted into heat.– Few generated x ray photon with
wavelength set by the amount of energy loss E=hν=E2-E1
+-V = 5-150kV
CA
E1
E2
hν
Nucleus
e-
X-rays E ≈ 5-150keV
Physics Principles
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Conventional X-Ray
• X-ray beam goes though the patient with decrease energy function of the absorption pattern inside the body.
Image Formation
Screen + detector
patient
collimator
Primary X-rays
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Conventional X-Ray
• Image captured on phosphor screen, with conversion to visible light.
• Recording on film (negative image)
• Display on video monitor (positive image)
Image Capture
récepteur
patient
plomb
Rayons primaires
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Conventional X-RaySystem Design
HUILE
VERRE BOMBARDE
CONE PLOMBE
cathode
lead
oil
scatters
Optical field
Optical field
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Conventional X-RaySystem Design
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Conventional X-Ray
• Advantages:– Fast– Not expensive
• Disadvantages:– Ionizing radiation involved.– Projection images.– Low blood vessel and soft-tissue contrasts.
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• Applications:– Lungs, breasts, GI track– Blood vessels (contrast agent)– Bones, joints
Conventional X-Ray
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Conventional X-RayExamples: Bones
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Examples: Lung
Pleural line
Patient with hemo-pneumothorax Patient with pacemaker
Source: www.mypacs.net
Conventional X-Ray
air + fluid
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Examples: Mammography• Low kV can be used (little
attenuation). => sufficient photoelectric absorption to differentiate between normal tissue and pathology
• Dose must be limited• Breast must be compressed
– To avoid motion.– To minimize focus/film distance.– To have equal tissue thickness.
Conventional X-Ray
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Examples: Fluoroscopy• Real-time X-ray imaging.• Direct visualization on screen.• Used for guidance of interventions
(mainly vascular and orthopedic).
Fluoroscopic guidance with umbilical catheter injecting contrast agent in the small bowel
Source: www.mypacs.net
Conventional X-Ray
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Source: Utrecht University, NL
C-arm
patient
cathode
Examples: FluoroscopyConventional X-Ray
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Computerized Tomography(CT)
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CTPhysics Principles
• Same as X-ray.
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CT
• Same as X-ray.Image Formation
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CTImage Capture
• Generation of a sliced view of body interior (“T”) (cf. tomos = slice in Greek).
• Computational intensive image reconstruction (“C”)
µ(x,y) ?
X-ray source
Measure = integral attenuation
Patient’s body
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CTI(ϕ
,ξ)
X-ra
y so
urce
I(ϕ,ξ)
X-ray source
Image Capture
• Translation and rotation of integral measure of absorption.
•Backprojection
I = Ioexp − µ(x, y)dlL∫
=> µtotal = µ(x,y)dlL∫ = −ln I
I0
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I = Ioexp − µ(x, y)dlL∫
g ≡ ln IoI
(Signal)
X-ra
y sou
rce
The Radon transform g(s,θ) of a function µ(x,y) is defined as its line integral along a line inclined at an angle θ from
the y-axis and at a distance s from the origin.
x’
x
y
θs
g(s,θ)
µ(x,y)unknown
absorption
y’
g(s,θ ) = µ(x, y)dlL∫
CTImage Capture: Backprojection
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g(s,θ ) = µ(x, y)dlL∫
• The Radon transform maps the spatial domain (x,y) to the domain (s,θ).• Each point in the (s,θ) space corresponds to a line in the spatial domain (x,y).
x’
x
y
θs
g(s,θ)
µ(x,y)unknownabsorption
y’
g(s,θ ) = µ(x, y)δ (x cosθ + y sinθ − s)dxdy∫∫ = µ(s cosθ − y' sinθ ,s sinθ + y' cosθ)dy'∫
CTImage Capture: Backprojection
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2D-space domain of µ(x,y)
2D-frequency domain of µ(x,y)
x
y
θ
g(s,θ)
µ(x,y)ωx
ωy
θ
s1D-Fourier transform F(g(s))
Radon Transform & Projection Theorem (Center slice theorem)
CTImage Capture: Backprojection
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HU = 1000 •µ − µw
µw
• Relates the linear attenuation coefficient of a local region µ to the linear attenuation coefficient of water, µW @ 70 keV (=Eeff).
• Based on measurements with the original EMI scanner invented by Hounsfield.
• Assign calibrated values to gray scale of CT images.
CTImage Capture: Hounsfield Units
CT
• 1st generation: Original resolution: 80 × 80 pixels (ea. 3 × 3 mm2), 13-mm slice.5-min scan time, 20-min reconstruction.
• 2nd generation: reduced number of view angles ⇒ scan time ~30 s.
• 3rd generation: scan time ~seconds (reduced dose, motion artifacts). Reconstruction time ~seconds.
System DesignTranslation & Rotation
Translation & Rotation
only Rotation
• 4th generation: scan time, reconstruction time ~sec.
• Spiral: Continuous linear motion of patient table during multiple scans. Increased coverage volume per rotation.
only Rotation of sources
CTSystem Design
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CT
20001975
128x128 pixels, 1-4 hours acquisition, 1-5 days computation
512x512pixels, 0.35 sec aquisition, <1sec computation
Example: Image quality
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CTExample: Lungs
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CT of right lung with active pulmonary tuberculosis: shows centrilobular lesions (nodules or branching linear structures 2-4 mm in diameter) in and around the small airways.
Source: www.mypacs.net
CTExample: Lungs
35Diagnosis of appendicitis
CTExamples: Abdomen
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TRA CTScout CT Coronal MPR
Virtual endoscopy
Source: www.mypacs.net
CTExample: Abdomen
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CTExample: Blood vessels
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CTExample: Blood Vessels
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Ultrasound
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UltrasoundPhysics Principle
• Traveling pressure waves through a medium.
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20 Hz 20 kHz 1 MHz 20 MHz
Frequency f (Hz)101 102 103 104 105 106 107
infra-sound
audible sound ULTRASOUND
DiagnosticUltrasound
flight ~ 1015
fxray ~ 1018
UltrasoundPhysics Principle
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Ultrasound
• Sound waves propagate through the body.
• At each interface, part is reflected, part is transmitted.
• Reflected waves is recorded back.
Image Formation
Near Field
Far Field
Source: The essential physics of Medical Imaging
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TISSUE
Ultrasound Pulse
D = c*Time/2c = 1.54 mm/µs
Echo
D
Echo
transducerUltrasoundImage Formation
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UltrasoundImage Capture
• Recorded signals:– Specular reflection, Diffuse
scattering, Rayleigh scattering• Frame rate =number of times
per second a sweep of the ultrasound beam is performed.– The higher the frame rate, the
better the temporal resolution. – Limited by:
• Time delay from maximum depth.• Speed of sound.
Tline = 2D/c
Tframe = N Tline = 2ND/c
FRmax = 1/ Tframe
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Piezoelectric Element
Matching Layer
Acoustic Insulator
Backing Material
ElectricConnector
UltrasoundSystem Design
Phased array transducer
Display
Transducer
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Ultrasound• Advantages:
– Portable– Inexpensive– Good spatial resolution (~1mm)– High temporal resolution
• Disadvantages:– Poor image quality– High level of noise
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A-modeamplitude display
B-modeBrightness display(Gray-scale)
UltrasoundSystem Design
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UltrasoundSystem Design
A-mode B-mode
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• Obstetric• Heart• GI track (liver, gall bladder, pancreas),• Urinary track (bladder, kidneys) • Genital organs (prostate, testicles,
ovaries, uterus). • Blood vessels (Doppler).
UltrasoundApplications
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– Establish number of fetus.– Check position of placenta. – Determine fetal age and development.– Check congenital defects.– Check fetus position.
Example: Blood vesselsUltrasound
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1961: First image of fetus 1990: First 3D visualization of fetus
Example: ObstetricUltrasound
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- Valve diseases.- Cardiac function (pumping efficiency).- Blood clots in cardiac chamber.
Example: HeartUltrasound
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• Biopsy guidance.• Diagnosis of abdominal pains, kidney stones, inflamed appendicitis. kidney
liverappendicitis
Examples: AbdomenUltrasound
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• Artery blockage from blood clots.• Arteriosclerosis (plaques).• Stent position
Example: Blood vesselsUltrasound
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Ultrasound
• DopplerExample: Blood vessels
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Magnetic Resonance Imaging(MRI)
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MRIPhysics Principle
No magnetic field Magnetic field
B0
z
Source: Hielscher, Columbia Univ.
• Magnetic field makes nuclei precess along its
direction.• The precession rateis the Larmor frequency.
• fL = γ B0 = 43•1.5 = 64 MHz for Hydrogen.
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MRIImage Formation
Y
Z
J or µ or M
X
B0
|B0|•γ•t
• For every 1,000,000 nuclei in upper state (mz=+1/2) there are 1,000,001 nuclei in lower state (mz=-1/2).
• This difference is enough to result in macroscopic net magnetization of material.
En erg
y
|B0|
mz = +1/2
mz = -1/2
Spin flip transition
∆Ε = γΒ0
B0
z
• The nucleus has a magnetic dipole moment, µ, and angular momentum, S.•The net magnetization, M, is the density of nuclear magnetic dipole moments.
•If you tip M away from B0 it will precess, at frequency γB0, producing a measurable RF magnetic field.
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• As the magnetization precesses it creates its own RF magnetic field.
• This field is much smaller than the exciting RF field.
• It can be detected with a standard radio receiver (e.g. rf coils used to tip M away from B0)
• The resulting signal from precession is called the Free Induction Decay or FID.
Y
Z
J or µ or M
X
B0
|B0|•γ•t
Lab Frame
Image CaptureMRI
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MRISystem Design• Main Magnet
– High, constant,Uniform Field, B0.
• Gradient Coils– Produce pulsed, linear gradients in
this field: Gx, Gy, & Gz
• RF coils– Transmit: B1 Excites NMR signal (
Free Induction Decay or FID).– Receive: Senses FID.
B0
B0
B0
B1
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Magnet + shield + cooling system
gantry
MRISystem Design
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MRI• Advantages:
– High image quality– Good spatial resolution (~1mm)– All tissues (except lung)
• Disadvantages:– Very expensive scanners– Some patients cannot be imaged
(pacemaker, clostrophobia).
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• Brain (Multiple sclerosis, Alzheimer’s disease, epilepsy)
• Spine• Muscles, tendons. • Blood vessels: angiographie (brain
seizures)• Heart• soft tissues
MRIApplications
64IRM
Example: HeartMRI
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Angiography
Example: Blood vesselsMRI
66Aorta
Example: Blood vesselsMRI
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Example: BrainMRI
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• Diagnosis of Alzheimer’s and Parkinson’s disease.Example: Brain
MRI
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Angiographie par IRM
Example: Blood vesselsMRI
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Nuclear Imaging
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Nuclear ImagingPhysics Principle: PET • Nuclear
imaging is the use of radioactive materials for imaging structures and functioninside body.
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Nuclear ImagingPhysics Principle: SPECT • Nuclear
imaging is the use of radioactive materials for imaging structures and functioninside body.
• In unstable nucleus, (more protons than neutrons) proton converts to neutron under emission of positron.
•Positron travels limited distance in tissue.
• Positron cab combine with electron of nearby atom and emits two 511 keV X-ray photons (γ rays) that travel in opposite direction.
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Nuclear ImagingImage Capture: PET
Anger Camera
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Nuclear ImagingImage Capture: PET
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Nuclear ImagingImage Capture: SPECT
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Nuclear ImagingImage Capture: SPECT
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Nuclear ImagingSystem Design: PET
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• Coincidence occurs with simultaneous detection of 2 opposite detectors.
• Number of counts at the detectors gives the line integral
• Use standard tomography reconstruction algorithm or statistical approaches (EM).
Nuclear ImagingSystem Design: SPECT
SPECT Measurements
α(x,y) ,µ(x,y)
unknownactivity & absorption
cross-section
g(s,θ)= α(x, y)exp− µ(x,ydl
L1
Lmax
∫
d
Lmin
Lmax
∫ l
I(σ,θ
)
L1Lmax
problem is ill-posed! ⇒ different combinations of α and
µcan yield same measurement
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Nuclear Imaging
• Advantages– Functional imaging
• Disadvantages– Poor spatial resolution (~8mm)– High level of noise– Expensive hardware– Need of cyclotron
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• Cœur• Poumons• Reins• Vessie• Organes digestifs• Système endocrinien (hormones)• Système immunitaire (globules blancs).
Nuclear ImagingApplications
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Example: LungNuclear Imaging
SPECT scan for lung perfusion (pulmonary embolism)
Source: Brigham & women’s Hospital82
PET with FDG tracer for diagnosis of brain tumor
Source: V. Cunningham, GlaxoSmithKline.
Example: LungNuclear Imaging
83Source: V. Cunningham, GlaxoSmithKline.
Example: BrainNuclear Imaging
PET for in-vivo testing of new drugs
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Research in Medical Imaging
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Research in Medical Imaging• Topics
– Improve image quality.– Extraction of organ contours.– Quantitative measures on organs (volume, cardiac
function, deformations, contractility,…).– Modelization of organs.– Validation of image information for diagnosis.– Tumor detection.– Functional analysis, neuroscience.
• Tools:– Computers, mathematics, programming, biology,
physics, clinical studies.
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Original Gaussian filtering
Adaptive Wavelet Thresholding
Wavelet Thresholding
Denoising Application
Example: Denoising of 3DRX spine study
Research in Medical Imaging
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Example: Segmentation of sub-cortical structures
Research in Medical Imaging
Lateral ventricles
Third ventricle
Caudate nucleus
Thalamus
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Example: Registration of MRI and PET abdomen images
Research in Medical Imaging
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Example: Segmentation of cortical brain structures
Research in Medical Imaging
90
Example: Diffusion Tensor MRI for Brain White Matter Fibers
Research in Medical Imaging
91
Motor control of fingers
Example: Functional Brain MRIResearch in Medical Imaging
92Motor control of right hand
Example: Functional MRIResearch in Medical Imaging
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Books•Foundation of Medical Imaging, Z.H. Cho, J.P. Jones, M. Singh, John Wiley &Sons, Inc., New York 1993, ISBN 0-471-54573-2.
•Principles of Medical Imaging, K.K. Shung, M.B. Smith, B. Tsui, Academic Press, San Diego 1992, ISBN 0-12-640970-6.
•The Essential Physics of Medical Imaging (2nd Edition) ,Jerrold T. Bushberg, J. Anthony Seibert, Edwin M. LeidholdtJr., John M. Boone, Lippincott Williams & Wilkins; 2nd edition, ISBN 0683301187.
•Image Reconstruction in Radiology, J.A. Parker, CRS Press, Boca Raton, FL 1990, ISBN 0-8493-0150-5/90.
•Principles of Magnetic Resonance Imaging, Zhi-Pei Liang, P.C. Lauterbur IEEE Press, New York, NY 2000, ISBN 0-7803-4723-4.
