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

Took 28 minutes going very fast….

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.

Stanford University School of Medicine

Stanford, California USA

Outline CT

• Detector and data acquisition basics

• Helical mode

• Pitch

• Image reconstruction

Goal: Provide insight into how to use MDCT

Outline CT

• Detector and data acquisition basics

• Helical mode

• Pitch

• Image reconstruction

SDCT CT

10 mm

z

Detector

SDCT CT

10 mm

z

20+ mm

Detector

MDCT

z CT

Detector

How to slice it?

Uniform or Matrix Array

N elements

Length = Nd

d

z CT

Detector

How to slice it?

Uniform or Matrix Array

N elements

Length = Nd

d

z CT

Detector

How to slice it?

Uniform or Matrix Array

N elements

Length = Nd

d

Hybrid Array

{

thinner inner elements

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

How do we use all

16 elements?

Detector

16 elements

4 DAS channels

4 CTx 1.25 mm Detector Configuration

X-ray Tube Focal Spot

X-ray Beam Collimator

Detector

Switching Array

4 CTx 2.5 mm Detector Configuration

X-ray Tube Focal Spot

X-ray Beam Collimator

Detector

Switching Array

4 CTx 3.75 mm Detector Configuration

X-ray Tube Focal Spot

X-ray Beam Collimator

Detector

Switching Array

4 CTx 5.0 mm Detector Configuration

X-ray Tube Focal Spot

X-ray Beam Collimator

Detector

Switching Array

5 CT

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!

5 CT

2.5

1.5

1

1

1.5

2.5

5

Detector

X-ray

source

collimation

2 x 0.5 mm slices

Detector CT

X-ray

source

collimation

4 x 1 mm slices

5

2.5

1.5

1

1

1.5

2.5

5

Detector CT

X-ray

source

collimation

4 x 2.5 mm slices

5

2.5

1.5

1

1

1.5

2.5

5

Detector CT

X-ray

source

collimation

4 x 5 mm slices

5

2.5

1.5

1

1

1.5

2.5

5

How do we use all

16 elements?

Detector

16 elements

8 DAS channels

8 CTx 1.25 mm Detector Configuration

X-ray Tube Focal Spot

X-ray Beam Collimator

Detector

Switching Array

8 CTx 2.5 mm Detector Configuration

X-ray Tube Focal Spot

X-ray Beam Collimator

Detector

Switching Array

Detector CT

16-Slice Scanners

3 basic detector designs

4 x 1.25 mm

4 x 1.25 mm

20 mm

Detector

16-Slice Scanners

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

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

Detector CT

c. 2004 Multislice Scanners

4 basic detector designs

40 mm

Detector

c. 2004 Multislice Scanners

A:

• 64 slice scanner:

• 64 0.625 mm slices over 40 mm

• Thicker slices available via software

40 mm CT

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

32 mm CT

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

4 x 1.2 CT

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

Detector CT

c. 2004 Multislice Scanners

D:

.

Detector CT

c. 2004 Multislice Scanners

D:

.

Detector CT

c. 2004 Multislice Scanners

D:

~ 0.3 mm at isocenter

(~ 50% overlap)

.

4 x 1.2 CT

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

Detector CT

When M ≠ N,

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

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

Outline CT

• Detector and data acquisition basics

• Helical mode

• Pitch

• Image reconstruction

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

table travel per rotation (mm)

Pitch =

M • D (mm)

where M = no. of simultaneous slices

Helical Pitch

• Multi-Slice Helical CT:

1.25 mm

}

table travel per rotation (mm)

Pitch =

M • D (mm)

where M = no. of simultaneous slices

Helical Pitch

• Multi-Slice Helical CT:

}

2.5 mm

table travel per rotation (mm)

Pitch =

M • D (mm)

where M = no. of simultaneous slices

Helical Pitch

• Multi-Slice Helical CT:

}

3.75 mm

table travel per rotation (mm)

Pitch =

M • D (mm)

where M = no. of simultaneous slices

Helical Pitch

• Multi-Slice Helical CT:

}

5 mm

Pitch =

M • D (mm)

Helical Pitch

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

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

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

Outline CT

• Detector and data acquisition basics

• Helical mode

• Pitch

• Image reconstruction

• z-interpolation

• Single-slice rebinning (ASSR)

• Cone-beam approximations (Feldkamp)

X-ray Tube Focal Spot

X-ray Beam Collimator

z

isocenter

Detector

Detector

z

isocenter

X-ray Beam Collimator

X-ray Tube Focal Spot

X-ray Tube Focal Spot

X-ray Beam Collimator

z

isocenter

Detector

Detector

z

isocenter

X-ray Beam Collimator

X-ray Tube Focal Spot

Inconsistent rays result in artifacts

Affects non-helical and helical mode

• z-interpolation

• Single-slice rebinning (ASSR)

• Cone-beam approximations (Feldkamp)

helical trajectory

translation

z (mm)

t (s)

Interpolation using samples of single detector ring

M helical trajectories

translation

z

t

Interpolation using samples of ALL detector rings

180-360 CT

Interpolation in Helical CT

Single-slice CT: Pitch 1

View angle (o)

0-180

180

z (mm)

0

x CT

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

x CT

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

1 CT

1

180

1DCT:

3x table speed

q

z (mm)

0

Increasing Table Speed in Helical CT

180

1DCT:

q

z (mm)

0

Pitch = CT

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…

2 CT

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

180 CT

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

• 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.

1 CT

2

1

2

3

4

1

2

3

4

Slice Thickness in Helical Multi-slice Mode

Example: Pitch 0.75

1 CT

2

1

2

3

4

thinnest slice

x

x

1

2

3

4

Slice Thickness in Helical Multi-slice Mode

Example: Pitch 0.75

1 CT

2

1

2

3

4

1

2

3

4

Slice Thickness in Helical Multi-slice Mode

thicker slice

x

x

x

x

1 CT

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

Slice thickness is a reconstruction parameter.

x CT

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

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….

• Detector and data acquisition basics

• Helical mode

• Pitch

• Image reconstruction

• z-interpolation (M<16)

• Other approaches necessary for M>16

• Moving forward

• More detector elements ??

• More simultaneous slices ??

• Faster at constant image quality

• Moving forward

• When M=N, there are more slice width options for a given scan

to computer

• Moving forward

• When M=N, there are more slice width options for a given scan

to computer

• Slice thickness will become a viewing parameter.

• 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

Outline CT

• Detector and data acquisition basics

• Axial multislice mode

• Helical mode

• Pitch

• Image reconstruction

Example of a 4 slice scanner

With a 16 element uniform detector

5 mm CT

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)

How do I decide?

5 mm CT

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

5 mm CT

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

5 mm CT

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

LightSpeed 5 mm (2i)

CT/i 5 mm

Jones, et. al, Radiology June 2001

5 mm CT

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

For Homework...

M ≠ N

z

Detector

Multislice or Multidetector??

N Detector Elements

electronics

M Simultaneous Slices

X-ray Tube Focal Spot

X-ray Beam Collimator

Detector

16 DAS channels

40 mm

40 x 0.625 mm

6 x 1.25

mm

6 x 1.25

mm

Detector

c. 2004 Multislice Scanners

A:

B:

32 mm CT

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

1

2

3

4,1

180

4DCT:

3x table speed

q

z (mm)

0

1

2

3

4,1

180

4DCT:

3x table speed

q

z (mm)

0

1 CT

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

180 CT

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

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

Rule of Thumb:

• 1DCT Pitch 1 4DCT Pitch 0.75

• 1DCT Pitch 2 4DCT Pitch 1.5