1
1
X-Ray Computed Tomography Measures
Tissue Properties from Macro to Micro
Michael Andre, Ph.D.Department of Radiology
University of California, San Diego
San Diego VA Healthcare System
Outline
• Perspective on Computed Tomography
(image reconstruction from projections)
• Scientific basis and timeline
• Evolution of medical design
• Principles of image reconstruction
• Current medical CT scanner design and applications
• System performance and image display
• Patient Dose Reduction and Dose Reports
• Artifacts
2
Conventional Radiography: 2D map of 3D object
3
• 2D map of x-ray attenuation (e-µx)
• Superposition
• Distortion due to non-uniform magnification
• Non-uniform exposure
• Widely varying image contrast
Goals of CT• 2D image of 2D
object without
superposition
• Uniform object
contrast
• Uniform contrast
sensitivity
• “Calibrated” image
• Structure and
function
Cross-sectional anatomy was a new challenge to the medical community
5
Transmission Emission
Reconstruction from Projections
X-Ray, microwave,
ultrasound, opticalSPECT, PET, MRI
,
),(,r
dsyxfrp
Interaction map
6
Transmission Computed
Tomography
(“Image Reconstruction”)
I0I
Assumed conditions for x-ray CT:
• Straight-line propagation
• Monoenergetic x-ray beam
2
7μ/ρ = Mass attenuation coefficient (cm2/gm)
Incident
Intensity
I0
Transmitted
Intensity
I
Interaction Map: I = I0 e-µx
X
Select “x”
for desired
resolution
and FOV
8
Equivalent transmitted intensity
Sum attenuation coefficients Solution: We need more views
9
Goals of CT• 2D image of 2D
object without
superposition
• Uniform object
contrast
• Uniform contrast
sensitivity
• “Calibrated” image
• Structure and
function
Cross-sectional anatomy was a new challenge to the medical community
11
First Commercial CT ScannerEMI Limited, 1973
I0
I
Translate
Rotate
• “First Generation” Scanner
• Translate-Rotate design
• 13 min to acquire singe slice
• Head scan only
• Rigid head holder and water path 12
1973 1979
First Application: Head Trauma
48x64 512x512
13 min scan, 13 min
reconstruction per slice!
2nd Generation: 30 sec scan,
1 min reconstruction
3
13
Second Generation Scanner
• Translate-Rotate design
• More detectors so fewer angles needed
• 30 sec to acquire singe slice
• Capable of body imaging with breath hold 14
Third Generation Scanner Geometry
• Tube and detector array rotate together
• Complete detector profile from single x-ray tube pulse
• 0.3 – 5 sec rotation speed to acquire single slice
• All current CT scanners use this design geometry
• Number and size of detector elements limits resolution
• Width of fan beam determines Field of View (FOV)
15
Fourth Generation Scanner
• Tube rotates, detectors stationary
• Allows oversampling in rotation to increase resolution significantly
• 1 sec to acquire singe slice
16
Electron Beam CT
• No mechanical motion
• Single slice in 30-50 msec
17
• Required two adjacent rooms to house system
• Remarkable cardiac images, poor for everything else
• Doomed by the advent of multi-slice helical scanners
EBCT Image Reconstruction Methods in Medicine
I. Analytic Methods
a. Filtered back-projection (Convolution, Radon, Fourier
filtering)
1) Dominant method for decades
2) Very fast reconstruction
b. Two- or three-dimensional Fourier reconstruction
1) MRI
II. Iterative Numerical Methods
a. Slower but more accurate (100-1000X longer than FBP)
b. Significant dose reduction
c. Simultaneous Iterative Reconstruction Technique (SIRT)
d. Algebraic Reconstruction Technique (ART)
e. Iterative Least Squares Technique (ILST)
4
19
Object
View = detector profile
20
Backprojection Concept
Single scan
Detector
signal profile
X-ray tube
Detector
Object Backprojection of two profiles for
rectangular object.
For a given point, reconstructed
density is the sum of all ray
projections that pass through it.
21
Backproject multiple detector
profiles
Superimposing each backprojection
leads to messy artifacts!
Simple Backprojection Concept
22
2323
BP
Video
First Medical Backprojection
24
Improved Solution: Filtered Backprojection
Single scan
Filtered detector
signal
Detector
X-ray tube Backproject filtered profiles for
rectangular object
(Shepp-Logan filter)
5
25
Points outside the object receive positive and
negative contributions from backprojected views to
cancel out artifacts
Filtered Backprojection is a Fourier Transform Method
26
• Fourier coeffs. of Back-Projected image are equal to the exact Fourier
coeffs F(kx,ky) divided by the frequency
• One can compute Back-projected image, take 2-D F.T. and multiply by │k│
(magn of spatial freq) to reconstruct new image
• Projections p are filtered to obtain p* which are then backprojected
• Ideally this should be exact reconstruction
• Very fast since reconstruction can start the moment the first projections are
acquired
k
kkF
k
kPkkF
yx
yx
),(),(),(ˆ
p x k p x p xk x x
x xdxm
m
R
R
*( ) ( ) ( )sin
2
2 2
p(x) is measured profile, p*(x) is filtered profile, R = max radius of object,
km represents high frequency cut-off, sin2 reduces to 0 or 1, simple program results
27
Shepp-Logan filters are used in FBP for X-Ray CT
Lak works best in absence
of noise (never so in x-ray).
S-L has high freq roll-off.
H has extreme roll-off with
more noise suppression.
Image Reconstruction Methods in Medicine
I. Analytic Methods
a. Filtered back-projection (Convolution, Radon, Fourier
filtering)
1) Dominant method for decades
2) Very fast reconstruction
b. Two- or three-dimensional Fourier reconstruction
1) MRI
II. Iterative Numerical Methods
a. Slower but more accurate (100-1000X longer than FBP)
b. Significant dose reduction
c. Simultaneous Iterative Reconstruction Technique (SIRT)
d. Algebraic Reconstruction Technique (ART)
e. Iterative Least Squares Technique (ILST)
29
Illustration of Iterative Reconstruction (ART)Now in use for lower dose, quantitative multi-energy reconstructions,
artifact suppression
30
Vary: 1) number of samples (rays) in each view
2) number of views (detector profiles) per rotation
1
2
Object
X,Y = rays or samples
View = detector profile
FOV
6
Samples vs. Angles
• Common to set pixel size to ½ the desired spatial resolution of the scanner (i.e., FWHM).
• Following the Nyquist Sampling Theorem, need ≥ 2 samples/pixel
Example: 0.5 mm pixel for FWHM of 1.0 mm.
• Number of measurement angles needs to be /2 times the number of samples per x-ray pulse for a fan beam
Example
Want 25 cm FOV to achieve 1.0 mm FWHM requires
25 cm x 10 mm/cm x 2 samples/mm = 500 samples
The number of angles of view = /2 x 500 = 785 angles
25 cm FOV, 512x512 matrix gives (25/512) ~0.5 mm pixel
• Additional measurements are made to improve image quality, 3.2-2.9 times as many measurements as image pixels 31
Object
X,Y = rays or samples
View = detector profile
FOV
32
Reconstruction Filters
33
Effect of Filter on MTF
Why use “less sharp filter”? To reduce impact of noise!
In CT (and other digital imaging modalities), image post-
processing can do all of the following except:
A. Reduce the appearance of noise
B. Enhance the appearance of edges
C. Reduce artifacts
D. Reduce appearance of motion blur
E. Extend dynamic range
34
In CT (and other digital imaging modalities), image post-
processing can do all of the following except:
A. Reduce the appearance of noise
B. Enhance the appearance of edges
C. Reduce artifacts
D. Reduce appearance of motion blur
E. Extend dynamic range
Dynamic range is determined by the acquisition
system. Post-processing cannot extend data that
were never recorded.35 36
The “Sinogram” and Backprojection
7
37Video
Sinogram
A stack of all
measured 1D
profiles that
are then
spatially
filtered and
backprojected
Translation
Rotation
38
Computer
Reconstruction Output:
2D Map of µ
(5120 or more numbers)
For human image perception:
2D Map of Display Brightness
with limited gray scale
39
CT Numbers (HU)
Tissue
Compact bone
Cancellous bone
Liver
Muscle
Kidney
Water
Fat
Lung
Air
CT # (HU)
400 – 1500
200 – 400
40 – 60
10 – 40
30
0
-75
-500
-1024
“CT Numbers” (Hounsfield Unit)
Scale for medical scanners:
-1024 to 4096 HU
1000
w
wtHU
µw = Linear atten coeff of water
µt = Linear atten coeff of tissue
Which is true? In a CT image, the CT number:
A. Of a material is dependent on its linear attenuation
coefficient
B. Increases if the window width is increased
C. Has units of “per cm”
D. Is reduced if the image matrix is changed from
512x512 to 320x320
E. Increases with mAs
40
In a CT image, the CT number:
A. Of a material is dependent on its linear attenuation
coefficient
B. Increases if the window width is increased
C. Has units of “per cm”
D. Is reduced if the image matrix is changed from
512x512 to 320x320
E. Increases with mAs
HU have no dimensional units, they are a ratio. Other
factors have no effect on HU.41
CT Number (HU) Calibration
Highly linear for a modest range
-1500
-1000
-500
0
500
1000
1500
0 1 2 3 4
CT
Nu
mb
er
(HU
)
Relative Attenuation (Density)
42
8
43
Non-linear
look-up
tables are
rarely if
ever used
in CT
Window Level / Window Width
Linear
transformation
256
grey
levels
is
typical
On a CT scanner, if the Hounsfield unit for water is zero
and for air is -1024, the HU for cortical bone, fat, lung
and muscle are typically (in order):
A. 1600, 30, -600, -100
B. -600, 30, 1600, -100
C. 1600, -100, -600, 30
D. -600, -100, 30, 1600
44
On a CT scanner, if the Hounsfield unit for water is zero
and for air is -1024, the HU for cortical bone, fat, lung
and muscle are typically (in order):
A. 1600, 30, -600, -100
B. -600, 30, 1600, -100
C. 1600, -100, -600, 30
D. -600, -100, 30, 1600
Bone is highest HU of any tissue. Fat has slightly lower
density and µ than water while muscle is slightly
higher. Lung has high air content, low density and µ. 45
Innovations have Launched a
CT Revolution in Medicine
• Continuous rotation “slip ring” gantries
• Multi-detector multi-slice scanning
• Dose reduction techniques
• Iterative reconstuctions
• Multi-energy scanners
46
47
Helical CT ScannerContinuous rotation
• Tube and detector array rotate together continuously (slip ring)
• Complete detector profile from single x-ray tube pulse
• 0.27 – 5 sec rotation speed
• All current CT scanners use this design geometry
• Number and size of detector elements determines resolution
• Width of fan beam determines Field of View (FOV)
Video
48
Contiguous slices permit
multi-planar reformattingMay not have isotropic voxels
9
49
Volumetric Rendering
• Assign colors to tissues, segment by HU
• Variable transparency
• Some tissues are difficult to segment uniquely
• Pretty pictures but 3D not widely adopted in
clinical practice until very recently
• First applications used Pixar animation
computer
50
dw
Pitch = d/w
= 1 as depicted
table movement
514- to 320-detector arrays
Multi-slice
Multi-Detector
CT Scanner
MDCT Pitch
• Pitch = Table Speed / (Beam Width = Detector size x # Detectors)
• Increasing pitch reduces dose, increases blurring and partial volume
• What is the Pitch?
• Table Speed = 21.25 mm/rotation
• Detector Width = 0.625 mm
• Number of Detectors = 64
64 x 0.625 mm = 40 mm
Pitch = 21.25 / 40 = 0.531 52
– 16 cm coverage per rotation
– 320 X 0.5mm detector
elements
– 350 msec rotation time
– 650 lb patient couch
Dynamic Volume CT
53
Aquilion ONE
64
0.5mm x 320
16cm of coverage
Toshiba Detector EvolutionScintillator on Photodiode array (99% absorption efficiency)
164
10
Courtesy of, Dr Katada & Dr Anno, Fujita
Cardiac: Whole heart cardiac perfusion
in one beat
LAD Stenosis
56
Video of polyp
“Virtual” CT Colonography
2D
Polyp
3D
57
Computed Tomography is
Quantum Noise Limited
The smallest image contrast difference (ΔCT#)
that can be detected is ~ 0.25%.
But we almost never operate the scanner at this setting
due to excessive patient dose.
All medical x-ray imaging should be quantum noise
limited using the minimum dose required to address the
medical question.
CT phantom low contrast module imaged under two different
noise conditions on the same scanner with:
58
Therefore, if the mAs is reduced by 1/3, then noise should
increase by √ 3 =1.73 (73% increase).
240 mAs 80 mAs
DosemAsNoiseQuantum
11
Contrast Factors (Noise)
• X-Ray flux (dose, mAs)
• Slice thickness
• X-ray scatter rejection at the detector
• Computational noise
• Low-frequency response of convolution filter
• Reconstruction interpolation
• Machine round-off error
Noise
Standard deviation (HU)σ% = -------------------------------- x 100
1000
Example
σ% = ±5 on ±1000 HU scale
σ% = 5 x 100/1000 = ±0.5%
59
X-ray Exposure (mAs or Dose) and Noise
60
10 mA 40 mA 160 mA 640 mA
10 mA
640 mA
Mean: 185.3 HU
SD: 18.9 HUMean: 136.0 HU
SD: 3.1 HU
DosemAsNoiseQuantum
11
√64 = 8
11
61 62Assumes we wish to maintain same noise value per pixel
Public Concern with Patient Doses in CT
63
October 15, 2009: Radiation Overdoses Point Up Dangers of
CT Scans
“For reasons not yet fully understood, the X-ray technologist . . .
activated the CT scan 151 times on the same area.”
December 8, 2009: More Radiation Overdoses Reported
“The number of hospitals where suspected stroke patients were
over-radiated while undergoing CT scans has risen to three in
California, with an unconfirmed case at a fourth hospital in
Alabama, . . .”
Patient Doses in CT
64
Alabama Brain Perfusion Patient
Threshold for epilation – 3 Sv
Typical CT Perfusion today – 12 mSv EDE
Natural background – 3 mSv/yr
• Sievert is the unit of effective dose, which accounts for the
tissue absorbed dose (Gy), type of radiation, relative
sensitivity of the organs exposed (conversion factor k)
• Natural background radiation from all sources: 3 mSv/year
• The effective dose of a single abdomen and pelvis CT scan
is greater than three times that of a year of background
radiation
• The most radiosensitive organs are red bone marrow,
lungs, breast, stomach, colon
• Medical radiation is associated with increased risk of
malignancy and is much higher in infants and children
Radiation
65
Average Natural Background Radiation
Source Level
Radon 2 mSv/year
Cosmic Rays 0.3 mSv/year
Gamma Rays (Soil Radionuclides) 0.3 mSv/year
Internal Radionuclides (40K, 14C) 0.4 mSv/year
Total ~3 mSv/year
Transcontinental Flight 0.03 mSv
Radiation in Nature
Example of Non-Medical Radiation
EPA Radon Map of US
(red is 2X yellow)
1. Centers for Disease Control and Prevention, Healthy Housing Reference Manual, Chapter 5
http://www.cdc.gov/nceh/publications/books/housing/figure_cha05.htm#5_5
2. Walter Huda, Review of Radiological Physics, Chapter 8, Radiation Protection66
12
Patient Doses from CT
67
Average Annual Effective Dose to the US Population
Early 1980s 2006
MedicalMedical
BackgroundBackground
Average Annual Effective Dose from All Sources = 6 mSv
68
Medical = 50%
A major reason for
this increase is
greater utilization
of CT imaging.
High priority effort
in Radiology to
reduce all patient
doses has been
successful
especially in
pediatrics.
CT
Medical Sources of Radiation
Adapted from ICRP Publication 102: Managing Patient Dose in Multi-Detector Computed Tomography (MDCT), 102 Annals of the ICRP Volume 37/1, Chapter 4 (2007)
The effective dose of a single abdomen and pelvis CT scan is greater than three times that of a year of background radiation (what is that number again?)
DEXA scan 0 .004
70
Patient Dose Reports in CT
Dose Indices in CT:
• CT Dose Index (volumetric)
• Dose-Length Product
• CTDIVOL and DLP indicate
measured doses to 100 mm ion
chamber in a cylindrical plastic
phantom
• CT head phantom, 16 cm
• CT body phantom, 32 cm
• Not a measured dose to the
patient
71 72
13
CT Dose Report Definitions
73
Scanner Display
• CTDIvol (mGy)
Weighted avg measurement in
phantom w/pencil chamber
• DLP (mGy•cm)
CTDIvol x scan z-axis length
• Dose Efficiency (%)
Measure of z-axis beam usage
• Projected Series DLP
Based on tabulated measures
• Accumulated Exam DLP
Sum of series DLPs
Images CTDIvol
mGy
DLP
mGy•
cm
Dose Eff
%
Phantom
cm
1-8 10.44 20.88 92.70 Head 16
9-977 4.01 249.59 92.70 Body 32
978-
1025
4.65 27.32 92.70 Body 32
1026-
1217
18.98 113.94 92.70 Body 32
Projected series DLP:
Accumulated exam DLP:
532.03
1561.23
mGy•cm
mGy•cm
Methods to Reduce CT Dose
74
CT Exam Reference
CTDIvol
Notification
CTDIvol
Typical
Dose Equiv
Head 75 mGy 80 mGy 2 mSv
Adult Abdomen 25 mGy 50 mGy 8 mSv
Pediatric Abdomen (5 yo) 20 mGy 25 mGy 2 mSv
Brain Perfusion 500 mGy 600 mGy 12 mSv
Background Radiation Dose 3.1 mSv/yr
• Reduce mA
• Dose is proportional to mA
• Use adaptive mA (less for thinner pats or body part)
• Reduce kVp for thinner patients/pediatrics
•Dose increases exponentially with kVp
• Increase pitch, gantry speed or detector aperture
• Use iterative reconstruction
• Reduce number of series -- Do you really need pre-contrast?
• Angle gantry to avoid direct exposure of eyes, breast, gonads
Results
in more
noise
Automatic Tube Current Modulation
Fixed mA technique uses constant dose: too low or too high.
Auto-mA modulation may be vulnerable to user error.• Four adjustments per rotation: Sectors 1-4
• Noise index and slice thickness determine mA and thus dose.
• Reducing slice thickness without adjusting the noise index can cause
enormous patient doses.
• Positioning is critical. 75
2
1
4
3
3D Automatic Tube Current Modulation
http://www.gehealthcare.com/usen/education/tip_app/docs/AutomA-SmartmA%20Theory.pdf
76
De
cre
asin
g D
ose
Iterative Reconstruction to Reduce Dose and Noise
77
Image pairs were generated from the same raw CT data
87 mAs
2.6 mSv
20 mAs
.68 mSv
Original FBP Interative
X-Ray System Optics
FBP ignores the geometry of the focal spot and detector
• Assumed to be a point beam
• Interaction of beam and detector cell assumed to take place at geometric
center (pencil-beam)
Adaptive Statistical Iterative Reconstruction (ASIR)
1. http://ctug.org.uk/pipermail/ctusers_ctug.org.uk/attachments/20091029/cac95ee6/attachment.obj2. Thibault JB, Sauer KD et al. A three-dimensional statistical approach to improved image quality for multislice CT. Med. Phys. 2007.
34(11) 4526-4544.
78
14
System Optics
Most time consuming portion
(>100 - 1000x FBP)
Improved spatial resolution
Adaptive Statistical Iterative Reconstruction (ASIR)
System Statistics
Affects noise of resulting image
1. http://ctug.org.uk/pipermail/ctusers_ctug.org.uk/attachments/20091029/cac95ee6/attachment.obj
2. Thibault JB, Sauer KD et al. A three-dimensional statistical approach to improved image quality for multislice CT. Med. Phys. 2007. 34(11) 4526-4544.
1
79
High “ASIR level”
decreases noise,
and moderately
degrades spatial
resolution
(increases blur)
ASIR 10%
ASIR 100%
ASIR
80
81
Illustration of Iterative Reconstruction (ART)Now in use for lower dose, quantitative multi-energy reconstructions,
artifact suppression
Recall speed and
accuracy depend on
quality of the “starting
estimate”
Latest Advance: Dual-Source, Dual-Energy
82
• Very fast scans (75 msec temporal resolution)
• Very low dose possible (<1 mSv)
• Accurate measures of attenuation coefficient
• Physics-based Iterative reconstruction to reduce artifacts
120-140 kV80 kV
X-ray beam
filter
Dual-Energy CT
83
80 kVp 120 kVp
80 kV
120 kV
overlap
120 kV
Why Dual Source, Dual-Energy?
84
Remarkable Applications!• Chemical tissue analysis
• Quantitative lung perfusion
• Bone removal, bone mineral
• Vascular imaging
• Aortic dissection w/ stent
• Fast trauma diagnosis
15
Dual Energy, Multi-Spectral Reconstruction
85
CT Performance Summary
• Spatial Resolution
– ~0.25 mm maximum (isotropic) in smallest FOV
– Micro CT capable of 0.025 mm resolution
• Contrast Detectability
– 0.25% (ΔHU)
– Quantum noise limited (dose), iterative methods help
– May require iv contrast media (Iodine, Xenon) for many soft
tissues
• 0.25 sec minimum rotation speed
– EKG gating allows shorter time intervals for image formation
– 75 msec temporal resolution with Dual E-Dual Source
• Patient Dose: <1 to 500+ mSv
– Potential for high doses
– (1 year background radiation ~ 3 mSv)86
CT Image Artifacts
• Originate in Patient
– Motion
– “Missing rays” due to radio-opaque metal
– Partial volume
– Edge “ringing”
• Originate in Scanner
– Detector failure or out of calibration (“detector ring”)
– Beam hardening
87 88
Motion artifact due to peristalsis of a very high
contrast interface (air-tissue)
Other
stationary
high contrast
edges do not
show this
effect
89
X-Ray Beam is not MonoenergeticLower energy x-rays preferentially absorbed/scattered
“Beam hardening”
New clinical tool: Iterative reconstruction and/or dual- or multi-energy CT for
tissue-specific imaging and quantitative tissue characterization
Energy spectrum
90
Beam hardening is greater with higher attenuating materials
Small motions cause blur, large motions may produce “ghosts”
16
91
Streak artifact from
biopsy needle outside
the FOV during CT-
guided abdominal biopsy
Shading from missing
data in shadow of biopsy
needle
Iterative reconstruction techniques
reduce metal artifacts
Iterative
reconstruction,
Needle tip visible
92
Partial Volume Artifact• In this example it produces “phantom lesion” in liver
• More pronounced with thicker slices
• May vary with location in the FOV (center vs. edge)
93 94
95
“Ring” artifact
due to defective detector that
interrupted x-ray detection
“Ring-like” artifact
due to x-ray arc that interrupted
x-ray output
Artifacts due to Equipment FailureVideo
96
17
97
For which CT exam would the following settings
be appropriate: 80 kVp, 0.5 sec, 3 mm slice,
pitch = 1.3?
A. Large adult abdomen
B. Small pediatric abdomen
C.Adult head
D.Pediatric head
E. High resolution chest
98
For which CT exam would the following settings be
appropriate: 80 kVp, 0.5 sec, 3 mm slice, pitch =
1.3?
A. Large adult abdomen
B.Small pediatric abdomen
C.Adult head
D.Pediatric head
E. High resolution chest
Low kVp is used with small pediatric patients to reduce dose.
Pediatric risk of cancer may increase 25% with CT. Low kVp
is inappropriate for larger adult patients due to excessive
quantum noise. Typical adult abdomen = 120 kVp, 300 mAs.
Chest requires ~1 mm.
99
Sievert is a unit of:
A. Exposure
B. Effective dose
C.Absorbed dose
D.Dose rate
E. Energy
100
Sievert is a unit of:
A. Exposure
B.Effective dose (or effective dose
equivalent)
C.Absorbed dose
D.Dose rate
E. Energy
101
Which of the following situations does not reduce the dose to the patient?
A. Reducing mA, with no changes to kVp and pitch
B. Increasing the mA by 10% and increasing the pitch by 20%
C. Reducing kVp, use Auto-mA to maintain noise, no change to pitch
D. Increasing the kVp, with no changes to mA and pitch
E. Increasing the pitch in helical scanning, with no changes to kVp and mA
F. Increasing the gantry rotation speed with no changes to kVp and mA
102
18
Which of the following situations does not reduce the dose to the patient?
A. Reducing mA, with no changes to kVp and pitch
B. Increasing the mA by 10% and increasing the pitch by 20%
C. Reducing kVp, use Auto-mA to maintain noise, no change to pitch
D. Increasing the kVp, with no changes to mA and pitch
E. Increasing the pitch in helical scanning, with no changes to kVp and mA
F. Increasing the gantry rotation speed with no changes to kVp and mA
103
Approximate CTDIvol values for adult head and abdomen scans are:
A. 5 rad head, 1 rad abdomen
B. 5 mGy head, 1 mGy abdomen
C. 50 mGy head, 10 mGy abdomen
D. 500 mGy head, 100 mGy abdomen
104
Approximate CTDIvol values for adult head and abdomen scans are:
A. 5 rad head, 1 rad abdomen (CTDIvol is defined only for mGy)
B. 5 mGy head, 1 mGy abdomen
C. 50 mGy head, 10 mGy abdomen
D. 500 mGy head, 100 mGy abdomen
105
Occupational Limit
For a Radiation Worker, such as a Radiologist,
Nuclear Medicine Physician or technologist, the
maximum permissible effective dose is _______
mSv/year.
A. 5
B. 10
C.50
D.100
Occupational Limit
For a Radiation Worker, such as a Radiologist,
Nuclear Medicine Physician or technologist, the
maximum permissible effective dose is _______
mSv/year.
A. 5
B. 10
C.50
D.100
Reducing the slice thickness in CT with no change in technique (i.e.,
same kVp and mAs) will:
A. Improve spatial resolution
B. Increase the signal to noise ratio
C. Increase the dose from primary radiation to tissue in the slice
D. Improve contrast resolution
E. Change CT number calibration
108
19
Reducing the slice thickness in CT with no change in technique (i.e.,
same kVp and mAs) will:
A. Improve spatial resolution
B. Increase the signal to noise ratio
C. Increase the dose from primary radiation to tissue in the slice
D. Improve contrast resolution
E. Change CT number calibration
109
Reducing mA by one-half in CT with no other changes will:
A. Improve contrast resolution
B. Increase spatial resolution
C. Decrease patient dose by one-half
D. Increase tube heat loading
E. Decrease image noise by about 40%
110
Reducing mA by one-half in CT with no other changes will:
A. Improve contrast resolution
B. Increase spatial resolution
C. Decrease patient dose by one-half
D. Increase tube heat loading
E. Decrease image noise by about 40%
111
Fini
Any ??
112
113
Acute StrokeThis case shows a complete acute stroke evaluation acquired on 320-slice
scanner in 60 seconds. Using a dynamic volume CT scan sequence this technique
provides a fast and comprehensive neurological assessment when time to
diagnosis is critical.
Note the abnormal cerebral blood flow (CBF), cerebral blood volume (CBV)
and mean transit time (MTT) within the left temporal region. There is an additional
perfusion deficit along the vertex of the brain which unfortunately could have been
missed using a conventional MDCT perfusion protocol.http://medical.toshiba.com/Products/CT/DynamicVolume/Clinical-Studies/Acute-Stroke.aspx
Clinical
applications
http://medical.
toshiba.com
114Video
Digital radiograph: Scout
Metal artifact
20
115
• Very long acquisitions – 5 min or more
• Mouse must be anesthetized or euthanized
• Very high doses
• Fantastic for early drug discovery
Micro CT of Mouse25 micron resolution
Micro CT Scanner
116
2D CCD
Cone beam
• Geometric magnification of mouse on detector
• Feldkamp cone beam reconstruction algorithm
• Dual-energy methods available
• Mouse squeezed into small cylinder
Results are Worth the Long Wait
117
Neonatal mouse Mouse femoral head
3D Rendered Airways
118
Lungs Implanted tumor
Vascular Imaging
119
Quantitative with Extra EffortLung Tumor Model
Living and breathing
(but motion artifact)Euthanized
(but totally cooperative)
Methods to Reduce CT Dose
120
• Reduce mA• Dose is proportional to mAs
• Use Auto-mA (less dose for thinner patient or body part)
• Accept more quantum noise in the image
• Reduce kVp when appropriate• Dose increases exponentially with kVp for same mAs
• Best when used with iodinated contrast media to
increase object contrast exploiting the 33 keV k-edge
• Use Iterative Reconstruction with lower mA
• Increase pitch
• Reduce number of series • Do you really need pre-contrast?
• Angle gantry to avoid direct exposure of eyes, breasts
or gonads
21
For the same reconstructed slice widths, a 64-slice CT
compared to a 16-slice CT generally:
A. Has better system resolution
B. Will have improved cardiac imaging
C. Has a lower radiation dose
D. Can use a lower concentration of iodine contrast
E. Has a larger diameter field of view (FOV)
121
For the same reconstructed slice widths, a 64-slice CT
compared to a 16-slice CT generally:
A. Has better system resolution
B. Will have improved cardiac imaging
C. Has a lower radiation dose
D. Can use a lower concentration of iodine contrast
E. Has a larger diameter field of view (FOV)
64-slice covers more Z-direction in a single rotation so
less time needed. All other factors are equivalent.
122
In CT, the ROI tool placed over a uniform area of an
image will return values of “mean” and “standard
deviation” of the CT numbers within the ROI. The
standard deviation is most closely associated with the:
A. X-ray attenuation
B. Spatial resolution
C. Image noise
D. Image contrast
E. Atomic number of the tissue in the ROI
123
In CT, the ROI tool placed over a uniform area of an
image will return values of “mean” and “standard
deviation” of the CT numbers within the ROI. The
standard deviation is most closely associated with the:
A. X-ray attenuation
B. Spatial resolution
C. Image noise
D. Image contrast
E. Atomic number of the tissue in the ROI
124
Concerning image artifacts in x-ray CT:
A. Patient movement results in streak artifacts
B. Beam hardening leads to higher CT numbers in
center of the image
C. Ring artifacts are associated with the presence of
metallic objects in the reconstruction circle
D. Detectors made of high-density material may cause
streak artifacts
125
Concerning image artifacts in x-ray CT:
A. Patient movement results in streak artifacts
B. Beam hardening leads to higher CT numbers in
center of the image
C. Ring artifacts are associated with the presence of
metallic objects in the reconstruction circle
D. Detectors made of high-density material may cause
streak artifacts
Beam hardening: reduced CT #’s. Ring artifacts:
detector or x-ray problems. Metal objects in patient.126
22
Radiation Risk
• Cancer mortality of C-A-P CT of 45 yo man is ~0.08%
– Yearly screening x 30 yrs ↑ ~2% chance of cancer
– Cancer incidence in humans ~25-30%
• Example: Nationwide VA performs ~1.5 M CTs/year
– Could induce 800 malignancies/year (fewer are
actually expressed due to elevated age of patients)
• Skin erythema results with dose ~2.0 Gy
• Temporary epilation with dose ~3.0 Gy
• Background 0.3 Gy/year
127
-1500
-1000
-500
0
500
1000
1500
0 1 2 3 4
CT
Nu
mb
er
(HU
)
Density
128
CT Calcium Measurement
Dual Energy CT
Standard CT
Dual Energy CT provides higher accuracy and sensitivity for spine/hip
bone densitometry and aortic/coronary calcium assessment
Calibration
Phantom
CT or DEXA for Body Composition?Dual-Energy X-Ray Absorptiometry: Bone Mineral Density
129
• Total body lean/fat mass, BMD
• 0.7% precision for BMD
• Lumbar scan in 10 sec
• Low cost (<$100)
• Very low dose: 0.004 mSv
(1/5 Chest XR, 1/20 low dose CT)
• Widely available
• Huge clinical base
• 2D projection so superposition
Body Composition: VAT/SAT, BMD
130
CT is more
specific so it may
be required for
some research
DEXA is preferred
for population
studies
131
New clinical tool: Iterative reconstruction
to reduce metal artifact
Slice Sensitivity Profile
132