Introduction to medical imaging

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Introduction to medical imaging

• BEST Lecture
• (SIMILAR-Louvain-la-Neuve-Summer 2004)

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Objectives

• Medical imaging modalities
• x-ray
• computerized tomography
• magnetic resonance imaging
• Ultrasound
• nuclear medicine
• Medical image processing applications
• Diagnosis
• Image guided surgery
• Drug discovery and functionnal imaging

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What is medical imaging ?

• Creation of images of structures with the human
  body
• Applicable to non medical objects
• Elements water (tissue, fluids), bone, air
• Contrast change in signal intensity due to
  difference in physical properties
• Attenuation, relaxation, refractive index
• Provide information on anatomy and function

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Examples
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Generic imaging system
Rf coil, phosphor, scintillation counter, antennae
Detector
Reconstruction Image/signal processing
Source
Storage (PACS)
Nuclear (source is inside)
Xray, rf, ultrasound (energy)
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Systems framework

• Medical imaging systems
• Non linear
• Space variant
• Approximations to linear, space invariant system
• Advantages
• Linear system theory
• Y(jw) H(jw)X(jw)
• H(jw) is the impulse response
• Space is more relevant in imaging than time

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Space invariance
System T
Impulse response
impulse

• space shift in input gt space shift in response
  but no change i.e. h(x2, y2 e,? ) h(x2- e ,
  y2-?)
• s(x,y) ?? s(e,?) d(x- e , y-?) de d?
• I(x,y) Ts(x,y) ?? s(e,?) Td(x- e , y-?)
  de d?
• ?? s(e,?) h(x- e , y-?) de d? since Td(x)
  h(x) by defn.
• I Sh or FT(I)
  FT(S)FT(h)

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Contrast, resolution, SNR

• Contrast
• Ability of distinguish an object from background
  (DI/I)
• Resolution
• The small distance between two points that can be
  imaged
• Signal to noise ratio (SNR)
• S/sI, (S measured signal s noise standard
  deviation)
• Similarly CNR DI/sI

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Contrast, resolution, SNR
Resolution phantom
SNR phantom (dilution study)
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Probe characteristics

• Wavelength must be short enough for adequate
  resolution.
• bone fractures, small vessels lt 1 mm
• large lesions lt 1 cm
• Body should be semi-transparent to the probe.
• transmission gt 10-1 poor contrast
• transmission lt 10 -3 poor signal (SNR)
• wavelength lt 1 cm resolution
• Standard X-rays .01 Å lt ? lt .5 Å
• 25 Kev to 1.0 Mev per photon

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EM transmission
Infrared, optical, UV
Graph Medical Imaging Systems Macovski, 1983
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EM probes

• Probes
• Xray (Mammography, CT) XR 0-30s CT 70s
• Magnetic field (MRI) 80s-90s
• Sound waves (Ultrasound) 60s
• Photon/Positrons (PET, SPECT) 80s
• Infrared, optical current research
• Differing characteristics
• Xray-gt no diffraction US -gt diffraction
• Internal vs. external source
• Toxic vs. non toxic
• Invasive vs. non invasive

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Xrays
Wilhelm Conrad Roentgen 1895 noticed crystals
glowing across the room Did not patent for
widespread dissemination
xray source (Crookes tube) object/patient
phosphor Mrs Rs hand
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Xrays

• Cheap, portable
• Collapsed image (projection) gt loss of depth
  info
• Distortion due to point source, magnification
  etc.

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Xray developments (early 20th c)
Use of intravenous agents (e.g. I, Ba) to enhance
contrast Subtraction of non contrast with
contrast (digital subtraction angiography) Real
time imaging or fluoroscopy
Barium (enema) fluoro
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Tomography (Gr tomos section)
First generation CT scanner Record signal for
each beam and translate Rotate the
source-detector setup and repeat Back projection
to get m(x,y)
Fourth generation CT scanner Fan beam Detector
rings
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CT (contd)
Heart scan Brain trauma Liver scan
(tumor)

• Good contrast, depth resolution
• Axial slices only, Radiation exposure

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• Ultrasound uses the transmission and reflection
  of acoustic energy.

? prenatal ultrasound image ?clinical
ultrasound system
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Ultrasound

• A pulse is propagated and its reflection is
  received,
• both by the transducer.
• Key assumption
• - Sound waves have a nearly constant velocity
• of 1500 m/s in H2O.
• - Sound wave velocity in H2O is similar to that
  in soft tissue.
• Thus, echo time maps to depth.

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US resolution vs. attenuation

• -
• ?higher frequency ?shorter wavelength
  ? higher attenuation
• Power loss 1db/cm MHz
• Typical Ultrasound Frequencies
• Deep Body 1.5 to 3.0 MHz
• Superficial Structures 5.0 to 10.0 MHz
• e.g. 15 cm depth, 2 MHz, 60 dB round trip
• Why not use a very strong pulse?
• Ultrasound at high energy can be used to ablate
  (kill) tissue.
• Cavitation (bubble formation)
• Temperature increase is limited to 1º C for
  safety.

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Ultrasound
Gallstone
Colour doppler of portal vein
Heart scan using US
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Nuclear medicine

• Uses radio-pharmaceuticals (radioisotopes) like
  Technetium or labelled Glucose which are g
  emitters
• Injected or inhaled or ingested
• Information about function as these isotopes
  enter the metabolic pathways e.g. thyroid takes
  up Iodine or bones take up Strontium
• Detector is a gamma camera (Anger camera) uses CT
  methods
• SPECT (Single Photon Emission CT)
• PET (Positron emission tomography)

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Gamma camera
Thyroid SPECT
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Nuclear medicine
Brain PET (FDG) with tumour
Bone scan (Tc-Phosphate)
Thyroid SPECT
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Magnetic Resonance

• Any particle with a net spin possesses a magnetic
  moment vector (acts like a tiny magnet)
• Magnetic moment in an external magnetic field B0
  is like a top in a gravitational field
• Force applied to spinning objects results in a
  net perpendicular force precession
• Precession (Larmor) Frequency is gBo 63MHz for
  1H

Bo
m
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Spin in B0
m
N

S
Anti-parallel (high)
DE hgB0
Energy
Bo
Parallel (low)
Bo
When a spin is placed in a magnetic field it
assumes a parallel or anti-parallel configuration
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Ensemble of spins in Bo

• Slight excess of parallel over anti-parallel
  spins resulting in a net magnetic moment M
  oriented along B0
• Application of another perpendicular magnetic
  field B1 results in tipping of M due to
  precession of M about (B0 B1)
• Removal of B1 will cause slow recovery to
  equilibrium while still precessing about B0
• A loop of wire (coil) will pick up a voltage due
  to the varying magnetic field

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B1 excitation
z
z
Bo
Bo
? gB1t
M
?
M
y
y
B1
x
x
Image courtesy Physics of Diagnostic Radiology,
Christensen
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MRI systems

• 0.3 T permanent magnet

3 T superconducting magnet
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MRI
Brain tumor
Spine herniated disc (C4)
Long axis movie of heart
Knee with torn ACL/hematoma
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Comparison
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Costs

• US 100K 250K
• CT 400K 1.5 million (helical scanner)
• MR 350K (0.3 T knee) - 3.0M (3T brain)
• PET 3.0 M
• Other issues
• MR requires rf shielded room (siting costs 1M)
• PET requires in situ radioisotope production
  (cyclotron)
• Service Maintenance costs, parts
• Staff Scans performed by technologists

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Nobel Prizes

• Roentgen (1901, Xrays)
• Bragg(s) (1915, Xray energy levels)
• Hounsfield, Cormack (1979, CT)
• Rabi (1944, MR)
• Bloch et al. (1952, MR)
• Ernst (1991, NMR)
• Wuthrich (2002, NMR)
• Lauterbur, Mansfield (2003, MRI)