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Introduction to medical imaging

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Wilhelm Conrad Roentgen. 1895: noticed crystals glowing across the room ... Roentgen (1901, Xrays) Bragg(s) (1915, Xray energy levels) Hounsfield, Cormack (1979, CT) ... – PowerPoint PPT presentation

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Title: Introduction to medical imaging


1
Introduction to medical imaging
  • BEST Lecture
  • (SIMILAR-Louvain-la-Neuve-Summer 2004)

2
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

3
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

4
Examples
5
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)
6
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

7
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)

8
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

9
Contrast, resolution, SNR
Resolution phantom
SNR phantom (dilution study)
10
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

11
EM transmission
Infrared, optical, UV
Graph Medical Imaging Systems Macovski, 1983
12
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

13
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
14
Xrays
  • Cheap, portable
  • Collapsed image (projection) gt loss of depth
    info
  • Distortion due to point source, magnification
    etc.

15
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
16
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
17
CT (contd)
Heart scan Brain trauma Liver scan
(tumor)
  • Good contrast, depth resolution
  • Axial slices only, Radiation exposure

18
  • Ultrasound uses the transmission and reflection
    of acoustic energy.

? prenatal ultrasound image ?clinical
ultrasound system
19
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.

20
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.

21
Ultrasound
Gallstone
Colour doppler of portal vein
Heart scan using US
22
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)

23
Gamma camera
Thyroid SPECT
24
Nuclear medicine
Brain PET (FDG) with tumour
Bone scan (Tc-Phosphate)
Thyroid SPECT
25
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
26
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
27
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

28
B1 excitation
z
z
Bo
Bo
? gB1t
M
?
M
y
y
B1
x
x
Image courtesy Physics of Diagnostic Radiology,
Christensen
29
MRI systems
  • 0.3 T permanent magnet

3 T superconducting magnet
30
MRI
Brain tumor
Spine herniated disc (C4)
Long axis movie of heart
Knee with torn ACL/hematoma
31
Comparison
32
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

33
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)
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