1

An Brief Introduction to

4D CT Scanning

Steve B. Jiang, Ph.D.Dept of Radiation Onco logyUniv of California San Diego

AcknowledgementsPaul Keall, George Chen,Dan Low, Gig Mageras,

Ken Foster

2

Outline

� Problems with free breathing 3D scanning

� What is 4D CT? How does it work?

3

Three Types of Motio n Ar tifacts

� If CT scannin g speed << tumor motion speed,→→→→ smeared tumor imag e

� If CT scannin g speed >> tumor motion speed,→→→→ tumo r positio n and shape captured at anarbitra ry breath ing phase

� If CT scannin g speed ~ tumor motion speed,→→→→ tumo r positio n and shape heavily distor ted

4

CT Art ifacts in Free Breathi ng 3D Scan

Courtesy of Eike Rietzel and George T.Y. Chen

Helica l lig ht breathing scan 4DCT – 1 phase

Tumo r

2

5

Interpla y: Tumor Motion and Couch Motion

Courtesy of Kevin Beaudette andGeorge T.Y. Chen

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Moving Objects dur ing CT – A Phantom Study

Video courtesy of George T.Y. Chen

Chen, Kung, Beaudette, Artifacts in computed tomography scanning of moving objects, Semin Radiat Oncol. 14(1):19-26, 2004.

7

Motion Artifac ts

Photo Static Moving / HS mode HQCourtesy of George Chen

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Motion Induce d Ar tifac ts

� Respirato ry org an motion can cause severe

geometrical distortion in free breathing CT

scanni ng

� Dis tort ions alon g the axis of motion cou ld eith er

lengthen or sho rten the target, kind of random

� In additi on to shape distortion, the center of the

imaged target can be dis plac ed by as much as

the amplitu de of the motion

3

9

Considerations of Organ Motion in CT Sim

� Breath -hold CT scan� Volun tary breath hold

� Acti ve breathing control

� Combine inhale and exhale GTVs to get ITV

� Slow CT san� 4 seconds per slice in axial mode

� Gated CT scan� imag es at onl y 1 phase, acquisition times 4-5x longer

� 4D CT scan� 3D scans at multiple phases

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Basic Idea of 4D CT Scan

� Over-samp lin g image s at every

position of in terest along the

patient ’s lon g axis

� Each image is tagged with

breathing signals

� Images are sorted

retro spectively based on the

correspon ding breathing signals

� Many 3-D CT sets are obtaine d, each corresponding to a parti cul arbreathing phase

� Together, they cons titute a 4-D CT set tha t cove rs the ent irebreathing cycle

Photo Courtesy of Gig Mageras

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Resorting 500-1500 CT Slices

Courtesy of Eike Rietzel andGeorge T.Y. Chen

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Acquisition of 4D CT Images

� Image acquisition can be performed in either

� Ciné mode: repeated image acquisiti on at each couchposition, or

� Helical mode: with small pitch

� Breathing signals can be obtained using either

� A monitor such as RPM, an abdo minal bel t, or

� variation of the patient anterio r surf ace in the CTimages

4

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Sorting of 4D CT Images

� Retrospective sorting can be done in either

� Sinogram space (before reconstruction), or

� Image space (after reconstruction)

� Images can be sorted using either

� Breath ing phase , or

� Amplitude

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0:01

00.0 cm

4D Image Acquisition in Cine Mode

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

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0:02

00.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

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0:03

00.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

5

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0:04

00.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

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0:05

00.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

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0:06

01.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

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0:07

01.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

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21

0:08

01.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

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0:09

01.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

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0:10

01.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

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0:11

01.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

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0:12

02.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

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0:13

02.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

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0:14

02.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

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0:15

02.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

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0:16

02.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

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0:17

02.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

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0:18

03.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

32

0:19

03.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

9

33

0:20

03.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

34

0:21

03.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

35

0:22

03.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

36

0:23

03.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

10

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0:24

04.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

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0:25

04.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

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0:26

04.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

40

0:27

04.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

11

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0:28

04.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

42

0:29

04.0 cm

4 x 2.5 mm Multi Slice(10 mm total coverage)

X-ray Tube

Table location

Acq. Time

Courtesy of Ken Foster

4D Image Acquisition in Cine Mode

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BreathingSignal

X-ray on Signal

First couch position Second couch position Third couch position

• Respir atory tracking wit h Varian RPMoptical monitor

4D Image Acquisition in Cine Mode

Courtesy of KenFoster

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Courtesy of Gig Mageras

Sorted Slic es

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4DCT Scan of A Phantom

Courtesy of Eike Rietzel andGeorge T.Y. Chen

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4DCT Scan of A Lung Tumo r

Courtesy of Eike Rietzel and George T.Y. Chen

47Patient 5618

4DCT Scan of A Liver Tumor

Courtesy of Eike Rietzel and George T.Y. Chen

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Summary

� Respirato ry org an motion can cause severegeometrical distortion in free breathing CTscanni ng

� Dis tort ions alon g patient long axis could eitherlengthen or sho rten the target

� The center of the imaged target can be disp lacedby as much as the amplitud e of the motion

� 4D CT has been deve lope d to

� reduce motion artifacts

� give the tumor/organ motion information

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What kind of artifacts can be caused by respiratorymotion in free breathing helical CT scanning?

78%

10%

2%

6%

4%

1. Lengthen the target along the patient’s long axis

2. Shorten the target along the patient’s long axis

3. Displace the target center along the patient’s longaxis by as much as the motion amplitude

4. None of the above

5. All of the above

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Answer: 5 – All of the above

References

Chen G T, Kung J H and Beaudette K P 2004Artifacts in computed tomography scanning of

moving objectsSemin Radiat Oncol 14(1): 19-26.

51

4D CT scan can be used to:

96%

1%

0%

2%

2%

1. Reduce motion artifact

2. Estimate motion range

3. Reconstruct tumor motion trajectory

4. Design ITV

5. All of the above

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Answer: 5 – All of the above

References

Keall P 20044-dimensional computed tomography imaging and

treatment planningSemin Radiat Oncol 14(1): 81-90.