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