Transcript Biomedical Imaging I

Biomedical Imaging I
Class 3 – X-Ray CT Instrumentation
9/28/04
BMI I FS05 – Class 3 “CT Instrumentation” Slide 1
X-ray computed 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
Computed reconstruction of images
Good soft-tissue contrast
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Examples of CT images
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Principle of x-ray CT
In one plane, obtain set of line integrals for multiple view angles
Reconstruct cross-sectional views
Linear scan
Source
Angular scan
Object
Detector
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Scanner Design
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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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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collimated source)  excellent scatter rejection, now outdated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
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Second generation
Hybrid system: Fan beam, linear detector array (~30 detectors)
Translation and rotation
Reduced number of view angles  scan time ~30 s
Slightly more complicated reconstruction algorithms because of fan-beam
projection
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Third generation
Wide fan beam covers entire object
500-700 detectors (ionization chamber or scintillation detector)
No translation required  scan time ~seconds (reduced dose, fewer motion
artifacts)
Reconstruction time ~seconds
Pulsed source (reduces heat load & radiation dose)
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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)
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Comparison of 3rd and 4th generation
Both designs currently employed, neither can be considered superior
3rd Generation (GE, Siemens):
 Fewer detectors (better match, cheaper)
 Good scatter rejection with focused septa
 Cumulative detector drift
4th Generation (Picker, Toshiba):
 Less moving parts
 Detectors calibrated twice per rotation
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X-ray CT sources
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X-ray tubes
Bremsstrahlung x-ray tubes
Fixed anode: oil-cooled
Rotating Anode
Two focal spot sizes (~0.5 mm  1.5 mm and ~1.0 mm  2.5 mm)
Collimator assembly used to control beam (slice) width (~1.0 - 10 mm)
Power: ~120 kV @ 200-500 mA  spectrum ~30 – 120 keV
High frequency generators (5-50 kHz)
Rotating geometry requires slip rings
High voltage slip rings (~120 kV) if generator stationary
Lower voltage slip rings (480 V) if generator on rotary gantry
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Rotary Gantry
X-ray tube
Picker International, Inc.
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Slip rings
Picker
International, Inc.
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X-ray Detectors
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Detector Performance
Desired:
High overall efficiency to minimize patient radiation dose (typ. 0.45…0.85) =
product of
Geometric efficiency: fraction of detector area sensitive to radiation
Quantum efficiency: fraction of radiation energy deposited
Conversion efficiency: fraction of absorbed radiation contributing to
electrical signal
Large dynamic range (ratio of largest to smallest detectable signal)
Stable in time (low drift)
Insensitive to temperature variations
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Gas ionization chambers I
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
+
- - + + +
+
-
Ampmeter
Cathode
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Gas ionization chambers II
Usually filled with Xenon (high Z) under pressure (up to 30 atm) to optimize
efficiency
 Cheap
 Excellent stability
 Large dynamic range
 High spatial resolution
 Low efficiency
BMI I FS05 – Class 3 “CT Instrumentation” Slide 19
Scintillation detectors
Scintillating material (phosphor) converts x-ray energy into flashes of visible
light
Light is measured using photomultiplier tube (PMT) or photo diode (PD)
Scintillation materials:
For PMT: NaI(Tl), BGO
For PD: CdWO4, CsI, rare earth oxides
Scintillation material thick enough to provide quantum efficiency ~ 100%
Scintillator
PD / PMT
Electric signal
Collimator
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Photomultiplier tubes (PMT)
External photoelectric effect converts light
intensity into current of free electrons
Electrostatic acceleration of secondary
electrons
Cascade of secondary electron emission
and multiplication on dynodes
Signal amplification G =  N typ. ~106
(N: no. of dynodes, : gain per dynode ~4)
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Photodiode
Photons create electrons-hole pairs in semiconductor (photoelectric effect)
Direct conversion of visible photons into electric energy
Generation of photocurrent (~0.5 A / 1 Wopt) requires precision amplifier
Packaging in x-Ray CT Detector
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Advanced Applications
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5th Generation scanners
Exploring the temporal dimension
Especially important in cardiovascular (CV) imaging because of fast moving
structures
Fast slice acquisition
Triggering on cardiac cycle
High repetition rate
BMI I FS05 – Class 3 “CT Instrumentation” Slide 24
Imatron
No moving parts
Electromagnetically swept electron beam
50 ms (single slice) or 100 ms (multi-slice) scan time  imaging of beating
heart
Developed 1979 at UCSF (Boyd et al.), licensed to Imatron, Inc.
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Imatron front view
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Imatron as marketed by GE
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Single slice sequence (100 ms)
Continuous volume scanning (CVS)
Step volume scanning (SVS)
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Multi slice sequence (50 ms)
8 cm axial coverage
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Triggered acquisition
RCA moves at velocities of ~25 – 100 mm/s
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Imaging examples I
Aortic stent
Colon w/ 7-mm polyp
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Imaging examples II
Cardiac wall motion
"Sharp, motion-free 50 ms images of the heart throughout one entire heart cycle aid physicians
in determining and specifying wall motion anomalies."
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Axial Scans
Obtaining Volumetric (3D) Information
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Volumetric imaging
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Slice Sensitivity Profile SSP
Defined by variation of relative sensitivity along z in the slice center
Ideally rectangular (stop-and-shoot profile)
-1.5
-1
-0.5
0
0.5
1
1.5
distance to slice center
beam with
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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)
pitch
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Helical reconstruction
Projections for one slice do not lie in one plane
Interpolation from data outside the slice plane necessary
1st 2nd 3rd
4th Rotation
0
1st 2nd 3rd
4th Rotation
0
direct data
180
180
complementary
data
360
Interpolation:
-1 0 1
360 Degree Linear
360
-0.5 0.5
Standard (180 Degree Linear)
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Complementary data
Data sets for view angles 180º apart are identical:
Detector array
=
=
Detector array
180º
360º
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Spiral CT SSP
Because of interpolation, SSP deviates from square profile
Depending on pitch
Full width at half maximum (FWHM) ~ nominal slice width
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Multi slice spiral scanning I
Interweaving multiple helices  increased data density
Allows higher pitch (faster scan speed)
pitch = 4 x single slice pitch
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Variable Slice Thickness
Detector elements (~ 1000 scintillator/PD) are multiplexed to vary slice
number and thickness
Scan time ~ 0.5 s per rotation
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Hounsfield units
Assign calibrated values to gray scale of CT images
Based on measurements with the original EMI scanner invented by
Hounsfield
Relates the linear attenuation coefficient of a local region  to the linear
attenuation coefficient of water, W (Eeff = 70 keV)
HU original  500
  W
W

HU  2HU original
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