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four or more understanding the basics of multidetector row ct1

Four or More: Understanding the Basics of Multidetector-Row CT

6th Annual International Symposium on Multidetector-Row CT

San Francisco, CA

June 23-26, 2004

Sandy Napel, Ph.D.

Department of Radiology

Stanford University School of Medicine

Stanford, California USA

http://www-radiology.stanford.edu/

outline
Outline
  • Detector and data acquisition basics
  • Helical mode
    • Pitch
    • Image reconstruction

Goal: Provide insight into how to use MDCT

outline1
Outline
  • Detector and data acquisition basics
  • Helical mode
    • Pitch
    • Image reconstruction
detector1

SDCT

10 mm

z

20+ mm

Detector

MDCT

detector2

z

Detector

How to slice it?

Uniform or Matrix Array

N elements

Length = Nd

d

detector3

z

Detector

How to slice it?

Uniform or Matrix Array

N elements

Length = Nd

d

Non-uniform or Adaptive Array

detector4

z

Detector

How to slice it?

Uniform or Matrix Array

N elements

Length = Nd

d

Non-uniform or Adaptive Array

Hybrid Array

{

thinner inner elements

detector5

16 x 1.25 mm

Detector

4-Slice Scanners

20 mm

5

2.5

1.5

1

1

1.5

2.5

5

32 mm

4 x 0.5 mm

4 slice scanners collect 4 simultaneous channels of data
4-Slice Scanners Collect 4 Simultaneous Channels of Data

How do we use all

16 elements?

Detector

16 elements

4 DAS channels

4 x 1 25 mm detector configuration
4 x 1.25 mm Detector Configuration

X-ray Tube Focal Spot

X-ray Beam Collimator

Detector

Switching Array

4 x 2 5 mm detector configuration
4 x 2.5 mm Detector Configuration

X-ray Tube Focal Spot

X-ray Beam Collimator

Detector

Switching Array

4 x 3 75 mm detector configuration
4 x 3.75 mm Detector Configuration

X-ray Tube Focal Spot

X-ray Beam Collimator

Detector

Switching Array

4 x 5 0 mm detector configuration
4 x 5.0 mm Detector Configuration

X-ray Tube Focal Spot

X-ray Beam Collimator

Detector

Switching Array

detector6

5

2.5

1.5

1

1

1.5

2.5

5

Detector

Do I get unequally-sized slices from an MDCT scanner with a non-uniform detector array?

NO!

detector7

5

2.5

1.5

1

1

1.5

2.5

5

Detector

X-ray

source

collimation

2 x 0.5 mm slices

detector8
Detector

X-ray

source

collimation

4 x 1 mm slices

5

2.5

1.5

1

1

1.5

2.5

5

detector9
Detector

X-ray

source

collimation

4 x 2.5 mm slices

5

2.5

1.5

1

1

1.5

2.5

5

detector10
Detector

X-ray

source

collimation

4 x 5 mm slices

5

2.5

1.5

1

1

1.5

2.5

5

8 slice scanners collect 8 simultaneous channels of data
8-Slice Scanners Collect 8 Simultaneous Channels of Data

How do we use all

16 elements?

Detector

16 elements

8 DAS channels

8 x 1 25 mm detector configuration
8 x 1.25 mm Detector Configuration

X-ray Tube Focal Spot

X-ray Beam Collimator

Detector

Switching Array

8 x 2 5 mm detector configuration
8 x 2.5 mm Detector Configuration

X-ray Tube Focal Spot

X-ray Beam Collimator

Detector

Switching Array

detector11
Detector

16-Slice Scanners

3 basic detector designs

detector12

16 x 0.625 mm

4 x 1.25 mm

4 x 1.25 mm

20 mm

Detector

16-Slice Scanners

detector13

16 x 0.625 mm

4 x 1.25 mm

4 x 1.25 mm

20 mm

16 x 0.75 mm

4 x 1.5 mm

24 mm

4 x 1.5 mm

Detector

16-Slice Scanners

detector14

16 x 0.625 mm

4 x 1.25 mm

4 x 1.25 mm

20 mm

16 x 0.75 mm

4 x 1.5 mm

24 mm

4 x 1.5 mm

16 x 0.5 mm

12 x 1 mm

12 x 1 mm

32 mm

Detector

16-Slice Scanners

detector15
Detector

c. 2004 Multislice Scanners

4 basic detector designs

detector16

64 x 0.625 mm

40 mm

Detector

c. 2004 Multislice Scanners

A:

  • 64 slice scanner:
    • 64 0.625 mm slices over 40 mm
    • Thicker slices available via software
detector17

40 mm

40 x 0.625 mm

6 x 1.25

mm

6 x 1.25

mm

Detector

c. 2004 Multislice Scanners

B:

  • 40 slice scanner:
    • 40 0.625 mm slices over 25 mm
    • 32 1.25 mm slices over 40 mm
    • Thicker slices available via software
detector18

32 mm

Detector

c. 2004 Multislice Scanners

C:

64 x 0.5 mm

  • 32 slice scanner:
    • 32 0.5 mm slices over 16 mm
    • 32 1.0 mm slices over 32 mm
    • Thicker slices available via software
detector19

4 x 1.2

mm

4 x 1.2

mm

Detector

c. 2004 Multislice Scanners

D:

32 x 0.6 mm

29 mm

  • 64 slice scanner:
    • 32 0.6 mm slices over 19 mm
    • 24 1.2 mm slices over 29 mm
  • 64 0.6 mm slices over 19 mm via focal spot shift
detector20
Detector

c. 2004 Multislice Scanners

D:

.

detector21
Detector

c. 2004 Multislice Scanners

D:

.

detector22
Detector

c. 2004 Multislice Scanners

D:

~ 0.3 mm at isocenter

(~ 50% overlap)

.

detector23

4 x 1.2

mm

4 x 1.2

mm

Detector

c. 2004 Multislice Scanners

D:

32 x 0.6 mm

29 mm

  • 64 slice scanner:
    • 32 0.6 mm slices over 19 mm
    • 24 1.2 mm slices over 29 mm
    • 64 0.6 mm slices over 19 mm via focal spot shift
    • Thicker slices available via software
detector24
Detector

When M ≠ N,

there may be multiple ways to make slices of a given thickness

detector25

least PVA

4 x 1.2

mm

4 x 1.2

mm

Detector

Example: Make 3.6 mm thick slices with this detector

D:

32 x 0.6 mm

29 mm

  • Sum 6 0.6 mm slices:
    • 0.6 mm slices available
    • Coverage/rotation = 19 mm
  • Sum 3 1.2 mm slices:
    • 0.6 mm slices NOT available
    • Coverage/rotation = 29 mm
outline2
Outline
  • Detector and data acquisition basics
  • Helical mode
    • Pitch
    • Image reconstruction
helical pitch

table travel per rotation (mm)

table travel per rotation (mm)

Pitch =

Pitch =

slice thickness (mm)

beam width (mm)

Helical Pitch
  • Single-Slice Helical CT:

slice thickness = x-ray ( beam ) width

helical pitch1

slice thickness <=> D = detector channel width

table travel per rotation (mm)

Pitch =

M • D (mm)

where M = no. of simultaneous slices

Helical Pitch
  • Multi-Slice Helical CT:

1.25 mm

}

helical pitch2

slice thickness <=> D = detector channel width

table travel per rotation (mm)

Pitch =

M • D (mm)

where M = no. of simultaneous slices

Helical Pitch
  • Multi-Slice Helical CT:

}

2.5 mm

helical pitch3

slice thickness <=> D = detector channel width

table travel per rotation (mm)

Pitch =

M • D (mm)

where M = no. of simultaneous slices

Helical Pitch
  • Multi-Slice Helical CT:

}

3.75 mm

helical pitch4

slice thickness <=> D = detector channel width

table travel per rotation (mm)

Pitch =

M • D (mm)

where M = no. of simultaneous slices

Helical Pitch
  • Multi-Slice Helical CT:

}

5 mm

helical pitch5

table travel per rotation (mm)

Pitch =

M • D (mm)

Helical Pitch

With this definition, intuition regarding artifacts and dose as a function of pitch are independent of M.

helical pitch6

table travel per rotation (mm)

Pitch =

M • D (mm)

Helical Pitch

On a 4-slice scanner (M=4):

At pitch = 0.75, table travel per rotation is:

= 0.75 • 4 • D

= 3 • D

At pitch = 1.5, table travel per rotation is:

= 1.5 • 4 • D

= 6 • D

helical pitch7

table travel per rotation (mm)

Pitch =

M • D (mm)

!! (D=.625mm 6 cm/rot)

Helical Pitch

On a 4-slice scanner (M=4):

At pitch = 0.75, table travel per rotation is:

= 0.75 • 4 • D

= 3 • D

On an 64-slice scanner (M=64):

At pitch = 1.5, table travel per rotation is:

= 1.5 • 64 • D

= 96 • D

outline3
Outline
  • Detector and data acquisition basics
  • Helical mode
    • Pitch
    • Image reconstruction
reconstruction approaches
Reconstruction Approaches
  • z-interpolation
  • Single-slice rebinning (ASSR)
  • Cone-beam approximations (Feldkamp)
the need for cone beam reconstruction
The Need for Cone Beam Reconstruction

X-ray Tube Focal Spot

X-ray Beam Collimator

z

isocenter

Detector

the need for cone beam reconstruction1
The Need for Cone Beam Reconstruction

Detector

z

isocenter

X-ray Beam Collimator

X-ray Tube Focal Spot

the need for cone beam reconstruction2
The Need for Cone Beam Reconstruction

X-ray Tube Focal Spot

X-ray Beam Collimator

z

isocenter

Detector

the need for cone beam reconstruction3
The Need for Cone Beam Reconstruction

Detector

z

isocenter

X-ray Beam Collimator

X-ray Tube Focal Spot

Inconsistent rays result in artifacts

Affects non-helical and helical mode

reconstruction approaches1
Reconstruction Approaches
  • z-interpolation
  • Single-slice rebinning (ASSR)
  • Cone-beam approximations (Feldkamp)
helical single slice mode
Helical Single-slice Mode

helical trajectory

translation

z (mm)

t (s)

Interpolation using samples of single detector ring

helical multi slice mode
Helical Multi-slice Mode

M helical trajectories

translation

z

t

Interpolation using samples of ALL detector rings

interpolation in helical ct

180-360

Interpolation in Helical CT

Single-slice CT: Pitch 1

View angle (o)

0-180

180

z (mm)

0

interpolation in helical ct1

x

x

x

x

desired slice location

Interpolation in Helical CT

Single-slice CT: Pitch 1

View angle (o)

0-180

180-360

360-540

540-720

180

z (mm)

0

interpolation in helical ct2

x

x

x

x

desired slice location

Interpolation in Helical CT

Single-slice CT: Pitch 2

View angle (o)

0-180

180-360

360-540

180

z (mm)

0

increasing table speed in helical ct

1

1

180

1DCT:

3x table speed

q

z (mm)

0

Increasing Table Speed in Helical CT

180

1DCT:

q

z (mm)

0

slide61

Pitch =

table travel per rotation (mm)

1

2

3

4

M • D (mm)

3 • D (mm)

1

2

3

4

=

= 0.75

4 • D (mm)

z

Using

a 4-slice

MDCT

scanner…

increasing table speed in helical ct1

2

3

4

4DCT

Increasing Table Speed in Helical CT

180

1DCT:

q

z (mm)

0

1

1

180

1DCT:

3x table speed

q

z (mm)

0

At P=0.75, sampling density restored by additional detectors

increasing table speed in helical ct2

180

q

z (mm)

0

Increasing Table Speed in Helical CT

1

2

3

4,1

180

4DCT:

3x table speed

q

z (mm)

0

1

2

3

4

1

2

3

4

4DCT:

6x table speed

slice thickness in helical multi slice mode
Slice Thickness in Helical Multi-slice Mode
  • Reconstruction kernel (lung, bone, etc.):
    • In-plane spatial filter
    • Trade off in-plane resolution, noise, and artifact
  • Z-filter:
    • Makes “thicker” slices than basic detector configuration would.
    • Trade off longitudinal resolution, noise and artifacts.
slice thickness in helical multi slice mode2

1

2

1

2

3

4

thinnest slice

x

x

1

2

3

4

Slice Thickness in Helical Multi-slice Mode

Example: Pitch 0.75

slice thickness in helical multi slice mode4

1

2

1

2

3

4

1

2

3

4

Slice Thickness in Helical Multi-slice Mode
  • Z-filter:
    • Makes “thicker” slices than basic detector configuration would.
    • Trade off longitudinal resolution, noise and artifacts.

even thicker slice

x

x

x

x

x

x

helical multi slice mode1
Helical Multi-slice Mode

Slice thickness is a reconstruction parameter.

noise as a function of pitch in mdct

x

x

x

x

x

x

x

180

x

x

x

x

x

q

z (mm)

0

Noise as a Function of Pitch in MDCT

1

2

3

4,1

180

4DCT:

P=0.75

q

z (mm)

0

1

2

3

4

1

2

3

4

4DCT:

P=1.5

slide71

Noise increases with pitch

in MDCT!

  • But be careful…
  • Manufacturers may automatically increase mA to compensate.
  • Therefore, an assumption that increasing pitch reduces dose (as in SDCT) may not be true….
summary conclusions
Summary/Conclusions
  • Detector and data acquisition basics
  • Helical mode
    • Pitch
    • Image reconstruction
      • z-interpolation (M<16)
      • Other approaches necessary for M>16
summary conclusions1
Summary/Conclusions
  • Moving forward
    • More detector elements ??
    • More simultaneous slices ??
    • Faster at constant image quality
summary conclusions2
Summary/Conclusions
  • Moving forward
    • When M=N, there are more slice width options for a given scan

to computer

summary conclusions3
Summary/Conclusions
  • Moving forward
    • When M=N, there are more slice width options for a given scan

to computer

• Slice thickness will become a viewing parameter.

references and acknowledgements
References and Acknowledgements
  • H Hu and Y Shen, Helical CT reconstruction with longitudinal filtration, Med Phys 25(11)2130-8; 1998.
  • H Hu, Multi-slice helical CT: scan and reconstruction, Med Phys 26(1):5-18; 1999.
  • WA Kalender, Computed Tomography, Publicis MCD Verlag, Munich Germany, 2000.
  • S. Napel, Basic Principles of MDCT, In: Multidetector CT, Principles, Techniques, and Clinical Applications, E. Fishman and R.B. Jeffrey Jr., Eds., Lippincott Williams and Wilkins, Philadelphia, PA, pp. 3-13, 2004.
  • Stanley Fox, David He, Sholom Ackelsberg, Willi Kalender
outline4
Outline
  • Detector and data acquisition basics
  • Axial multislice mode
  • Helical mode
    • Pitch
    • Image reconstruction
axial multi slice mode
Axial Multi-Slice Mode

Example of a 4 slice scanner

With a 16 element uniform detector

axial multi slice mode1

5 mm

4 x 1.25, 2 x 2.5, 1 x 5.0

5 mm

4 x 2.5, 2 x 5.0, 1 x 10

10 mm

15 mm

20 mm

4 x 3.75, 2 x 7.5

4 x 5, 2 x 10

Axial Multi-Slice Mode

Detector Configuration Slice Thicknesses (mm)

axial multi slice mode2
Axial Multi-Slice Mode

How do I decide?

axial multi slice mode 1 25 mm slices

5 mm

4 x 1.25, 2 x 2.5, 1 x 5.0

4 x 2.5, 2 x 5.0, 1 x 10

4 x 3.75, 2 x 7.5

4 x 5, 2 x 10

Axial Multi-Slice Mode: 1.25 mm Slices

Detector Configuration Slice Thicknesses (mm)

4i: 4x faster than 1DCT

axial multi slice mode 2 5 mm slices

5 mm

4 x 1.25, 2 x 2.5, 1 x 5.0

4 x 2.5, 2 x 5.0, 1 x 10

4 x 3.75, 2 x 7.5

4 x 5, 2 x 10

*Thin-slice retros

Axial Multi-Slice Mode: 2.5 mm Slices

Detector Configuration Slice Thicknesses (mm)

2i: 2x faster than 1DCT, less PVA

4i: 4x faster than 1DCT

axial multi slice mode 5 mm slices

5 mm

4 x 1.25, 2 x 2.5, 1 x 5.0

4 x 2.5, 2 x 5.0, 1 x 10

4 x 3.75, 2 x 7.5

4 x 5, 2 x 10

Axial Multi-Slice Mode: 5 mm Slices

Detector Configuration Slice Thicknesses (mm)

1i: Same speed as 1DCT, much less PVA

2i: 2x faster than 1DCT, less PVA

4i: 4x faster than 1DCT

reduced partial volume artifact
Reduced Partial Volume Artifact

LightSpeed 5 mm (2i)

CT/i 5 mm

Jones, et. al, Radiology June 2001

axial multi slice mode 5 mm slices1

5 mm

4 x 1.25, 2 x 2.5, 1 x 5.0

4 x 2.5, 2 x 5.0, 1 x 10

4 x 3.75, 2 x 7.5

4 x 5, 2 x 10

*Thin-slice retros

Axial Multi-Slice Mode: 5 mm Slices

Detector Configuration Slice Thicknesses (mm)

1i: Same speed as 1DCT, much less PVA

2i: 2x faster than 1DCT, less PVA

4i: 4x faster than 1DCT

detector26

in some designs

M ≠ N

z

Detector

Multislice or Multidetector??

N Detector Elements

electronics

M Simultaneous Slices

16 slice scanners collect 16 simultaneous channels of data
16-Slice Scanners Collect 16 Simultaneous Channels of Data

X-ray Tube Focal Spot

X-ray Beam Collimator

Detector

16 DAS channels

detector27

64 x 0.625 mm

40 mm

40 x 0.625 mm

6 x 1.25

mm

6 x 1.25

mm

Detector

c. 2004 Multislice Scanners

A:

B:

detector28

32 mm

64 x 0.5 mm

4 x 1.2

mm

4 x 1.2

mm

29 mm

Detector

c. 2004 Multislice Scanners

C:

D:

32 x 0.6 mm

increasing table speed in helical ct3
Increasing Table Speed in Helical CT

1

2

3

4,1

180

4DCT:

3x table speed

q

z (mm)

0

increasing table speed in helical ct4
Increasing Table Speed in Helical CT

1

2

3

4,1

180

4DCT:

3x table speed

q

z (mm)

0

increasing table speed in helical ct5

1

2

3

4

180

q

z (mm)

0

@3x

@6x

Increasing Table Speed in Helical CT

1

2

3

4,1

180

4DCT:

3x table speed

q

z (mm)

0

1

2

3

4

increasing table speed in helical ct6

180

q

z (mm)

0

Increasing Table Speed in Helical CT

1

2

3

4,1

180

4DCT:

3x table speed

q

z (mm)

0

1

2

3

4

1

2

3

4

4DCT:

6x table speed

helical pitch8

table travel per rotation (mm)

Pitch =

M • D (mm)

Helical Pitch

On a 4-slice scanner (M=4):

At pitch = 0.75, table travel per rotation is:

= 0.75 • 4 • D

= 3 • D

On an 8-slice scanner (M=8):

At pitch = 0.75, table travel per rotation is:

= 0.75 • 8 • D

= 6 • D

helical multi slice mode2
Helical Multi-slice Mode

Rule of Thumb:

• 1DCT Pitch 1 4DCT Pitch 0.75

• 1DCT Pitch 2 4DCT Pitch 1.5