Medical Image Formation
Image Sciences Institute
Computed Tomography (CT)
Literature: Farr & Allisy-Roberts, chapter 4
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
X-Ray Tomography
• Limits of radiography / fluoroscopy
– 3D structures are collapsed into 2D image (obscuring
of details, loss of one dimension)
– Low soft-tissue contrast
– Not quantitative
• Features of X-ray CT
– X-ray imaging modality (same principles of
generation, interaction, detection)
– Generation of a sliced view of body interior (“T”,
Tomography from Greek tomos = slice)
– Computational intensive image reconstruction (“C”)
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
CT Images
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Medical Image Formation
Principle of X-Ray CT
Linear scan
Source
• In one plane,
obtain set of line
integrals for
multiple view
angles
• Reconstruct crosssectional views
Angular scan
Object
Detector
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
Scanner design
• Purpose:
– Obtaining axial (transverse) cross-sectional images
(“tomos”) of the human body from many x-ray
projection images
– Use computer to calculate tomographic images from
projection data
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
Realization of X-Ray CT
• Mathematical basis for computed tomography
by Radon (1917)
• Idea popularized by Allan Cormack at Tufts
Univ. (1963)
• First practical x-ray CT scanner introduced by
Godfrey Hounsfield of EMI Ltd., England
(1972)
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Medical Image Formation
Example of an early design
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
First Generation
• EMI Mark I (Hounsfield),
“pencil beam” or parallel-beam
scanner (highly collimated
source) ⇒ excellent scatter
rejection, now outdated
• 180° - 240° rotation angle in
steps of ~1 °
• Used for the head
• 5-min scan time, 20-min
reconstruction
• Original resolution: 80 × 80
pixels (ea. 3 × 3 mm2), 13-mm
slice
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
Second Generation
• Hybrid system: Fan beam,
linear detector array (~30
detectors)
• Translation and rotation,
however
• Reduced number of view
angles ⇒ scan time ~30 s
• Slightly more complicated
reconstruction algorithms
because of fan-beam projection
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Medical Image Formation
Third Generation
• Wide fan beam covers
entire object
• 500-700 detectors
(ionization chamber or
scintillation detector)
• No translation required ⇒
scan time ~seconds
(reduced dose, motion
artifacts)
• Reconstruction time
~seconds
• Pulsed source (reduces
heat load, radiation dose)
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
Fourth Generation
•
•
•
•
Stationary detector ring (600 – 4800 scintillation detectors)
Rotating x-ray tube (inside or outside detector ring)
Scan time, reconstruction time ~seconds
Source either inside detector ring or outside (rocking, nutating
detectors)
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
Fifth generation
• Multiple X-ray tubes or funnel
shaped X-ray tube in which
an electron beam scans
rapidly around a semicircular
target
• Multiple stationary detector
rings allow multiple slices to
be scanned simultaneously
• Owing to fast acquisition
potential for cardiac CT
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Medical Image Formation
Electron Beam CT:
Scanners for CV Imaging
•
•
•
•
No moving parts
Electromagnetically swept electron beam
50 ms scan time → imaging of beating heart
Developed 1979 at UCSF (Boyd et al.), licensed to
Imatron, Inc.
• Multi slice capability
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
Multi-Slice CT
• Reduction of scanning time / increase in zresolution
• Inferior scatter rejection compared to single-slice
CT
• Use of widened (in z-direction!) x-ray beam and
detector array to acquire multiple (typ. 4, up to
16 slices simultaneously.
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
Spiral CT
• Continuous linear motion
of patient table during
multiple scans
• Increased coverage
volume / rotation
• Pitch: Number of slice
thicknesses the table
moves during one rotation
(typically ~1-2)
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Medical Image Formation
Helical Reconstruction
• Projections for one slice do not lie in one plane
• Interpolation from data outside the slice plane necessary
1st 2nd 3rd
Interpolation:
4th Rotation
1st 2nd 3rd
0°
0°
180°
180°
360°
360°
-1 0 1
360 Degree Linear
4th Rotation
-0.5 0.5
Standard (180 Degree Linear)
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Medical Image Formation
Multi Slice Spiral Scanning
• Interweaving multiple helices ⇒ increased data density
• Allows higher pitch (faster scan speed)
pitch = 4 x single slice pitch
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
X-ray Sources
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Medical Image Formation
X-Ray Tubes
• Bremsstrahlung X-ray tubes
– Fixed anode: oil-cooled
– Rotating Anode
• Collimator assembly used to control beam
(slice) width (~1.0 - 10 mm)
• Power: ~120 keV @ 200-500 mA → spectrum
~30 – 120 keV
• Rotating geometry requires slip rings
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Medical Image Formation
Rotary Gantry
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Medical Image Formation
X-ray Detectors
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Medical Image Formation
Detector Performance
Desired:
• High detection efficiency
• Fast response
• Large dynamic range (ratio of largest to smallest
detectable signal)
• Linearity (proportional to X-ray intensity)
• Insensitive to temperature-voltage variations
• Low cost
• Small in size
• Reliable
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
Gas Ionization Chambers
• Measurement of conductivity induced in a gas volume by the
ionizing effect of X-rays.
– X-rays ionize gas molecules
– Ions are drawn to electrodes by electric field
• Number of ion pairs N produced ∝ X-ray intensity
Collimator
Anode
+
- - + + +
+
-
Ammeter
Cathode
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Medical Image Formation
Gas Ionization Chambers
Characteristics
• To optimize efficiency
usually filled with Xenon
(high Z) under pressure
(up to 30 atm)
& Cheap
& Excellent stability
& Large dynamic range
& High spatial resolution
' Low efficiency
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Medical Image Formation
Scintillation Detectors
• Scintillation material converts X-ray energy into
flashes of visible light
• Light is measured using photomultiplier tube
(PMT) or photo diode (PD)
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
Principles of CT
• Rotating anode emits
beam
• Beam passes collimator
after travelling through
the patient
• Detectors (+/- 1000)
detect signal (logarithm
of radiation intensity)
• During 360 degrees
rotation +/- 300
projection images of 2/3
ms (1 second per
transverse image)
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
CT reconstructie
• Example: 300x1000 measurements to
reconstruct an image of 256x256 pixels
• Method: “Back-projection”
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Medical Image Formation
CT: combining multiple views into
one image
Result: axial (=transverse) slices
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
CT acquisition
source
X-ray source
object
detectors
detectors
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Medical Image Formation
CT reconstruction
backprojection
backprojection
filtered
backprojection
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Medical Image Formation
Phases in the
backprojection process
profiles
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Medical Image Formation
CT Scanner Performance
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
Contrast Mechanism CT
similar to X-Ray
• Projection data given by raysum:


I i = I 0,i exp  −∑ µij ∆x 
 i, j

• Attenuation in diagnostic range
due to photoelectric effect (pe)
and Compton scattering (cs)
µij = µ pe ,ij ( ρ , Z , ne , E ) + µcs ,ij ( ρ , Z , ne , E )
µ pe ∝
ρ ne Z −3.4
E 3.1
µcs ∝ ρ ne f ( E )
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Medical Image Formation
Beam Hardening
• Lower-energy X-rays more heavily attenuated
than higher-energy X-rays
• Effect accentuated for long paths / high-Z
material
• Compensation:
– Equalization of path lengths (water bag)
– Pre-hardening (filtering)
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
Hounsfield Units
• Assign calibrated values to gray scale of CT images
• Based on measurements with the original EMI scanner invented by
Hounsfield
HU original = 500
µ − µW
µW
→
HU = 2 HU original
• Relates the linear attenuation coefficient of a local region µ to the
linear attenuation coefficient of water
• -1000 for air, 0 for water, approx. 1000 for bone
CT # = 1000( µ t − µ w ) / µ w
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
Applications
•
brain images
inner ear
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Medical Image Formation
Thorax
peripheral bronchial carcinoma
emphysema
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Medical Image Formation
Abdominal Imaging
screening
pancreas
gastro-intestinal
kidneys
liver
multi-phase study of the liver
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Medical Image Formation
Spine
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Medical Image Formation
CT angiography
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Medical Image Formation
CTA circle of Willis
maximum intensity projection
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Medical Image Formation
Volume Rendering
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Medical Image Formation
Dynamic cardiac CT
Coronary
arteries on the
heart
Catharijne
07-11-2002
Image Sciences
institute Conferentie
/ University Medical
Center Utrecht
Medical Image Formation
Tomoscan M/EG
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Mobile CT
Medical Image Formation
Philips Tomoscan M
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Medical Image Formation
Application areas of mobile CT
Radiology
Operating Room
Intensive Care
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Medical Image Formation
CT Guided Navigation
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Medical Image Formation
CT Guided Navigation
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Medical Image Formation
Surgical navigation
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Medical Image Formation
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
Advantages Inherent to CT?
Q: Why the immediate popularity of initial CT, although
– low resolution (802 pixels @ 3 mm × 3mm × 13 mm),
– limited application (head only),
– slow (5 min. scan time, 20 min. reconstruction)?
A: CT copes with the following undesirable features of
radiography:
9 3D structures are collapsed into 2D image (obscuring of details,
competing anatomical patterns, loss of one dimension)
9 Not quantitative
Increased Contrast Resolution
9 Low soft-tissue contrast
Decreased Structure Noise
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Medical Image Formation
Image Contrast in Radioscopy
• Signal contrast:
C=
E0
EB − ES
EB
µB
h
µS
D
x
d
Cr = G ( µ S − µ B ) d
E
EB
• Assume linear D(E) curve
of gradient G and small
difference in µ (as for soft
tissue):
ES
EB
ES
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
• Luminance function of display (similar
to E(D)-curve) results in contrast:
CCT =
µ B − µS
µB
Gray Scale
Image Contrast in CT
µ (HU)
CCT =
Gray Scale
• Windowing: Values below lower
window setting are all black
µ B − µS
µB − µL
• ⇒ Enhanced display contrast
(Choice of contrast / latitude)
µL
µB
µ (HU)
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
Comparison
• Comparing contrasts:
CCT = Cr / G ( µ B − µ L ) d
Because (µB − µL) ~ 200 HU
⇒ Radiography contrast better for
large d (~ cm) or strong contrast
• Use of windowing possible because
of large dynamic range of detection /
display in CT
• (Desire for digital detectors in
radiography)
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Medical Image Formation
Something for nothing?
C
• Limiting factor is noise
Blur
Noise
Blur + Noise
d
Image Sciences institute / University Medical Center Utrecht
Medical Image Formation
Noise-Contrast resolution
• Detailed analysis shows:
SNR r
C /N
:= r r ∝ d −1
SNR ct CCT / N CT
• CT superior for smaller structures @ low intrinsic
contrast (soft tissue),
but limited resolution (~0.5 mm)
• Radiography better suited for larger structures and/or
strong intrinsic contrast, able of higher resolution
(Example: microcalcifications in mammography, d ~50
µm)
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