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Dose Reduction and Modulation Tools 李偉群

CT Application Specialist友信行

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DoseX X/2 X/4

X/8 X/16 X/64

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DoseX X/2 X/4

X/8 X/16 X/64

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mA is adjusted automatically in order to keep dose and image noise constant

CTDI depends on: Effective mAs (mAs / slice)

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CTDI depends on: Effective mAs (mAs / slice)

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Dose (CTDIw)-Image quality(Noise)

• CTDI α mAs

• CTDI α kVδ (δ between 2.6 and 3)

• CTDI depends on the collimation

• Noise α 1/mAs1/2

• Noise α 1/kVδ/2

• Noise α 1/SW1/2

Dose Increase Factor

80 kV / 120 kV 3.3

2.3

1.7

90 kV / 120 kV

100 kV / 120 kV

1.5120 kV / 140 kV

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Dose (CTDIw)-Image quality(Noise)

• CTDI α mAs

• CTDI α kVδ (δ between 2.6 and 3)

• CTDI depends on the collimation

• Noise α 1/mAs1/2

• Noise α 1/kVδ/2

• Noise α 1/SW1/2

Noise α 1/CTDI1/2

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Dose (CTDIw)-Image quality(Noise)

• CTDI α mAs

• CTDI α kVδ (δ between 2.6 and 3)

• CTDI depends on the collimation

• Noise α 1/mAs1/2

• Noise α 1/kVδ/2

• Noise α 1/SW1/2

Noise = β/CTDI1/2

β depends onPatient size

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• Unlike screen-film radiographs, images acquiredwith computed tomography (CT) never look overexposed

Noise

high

low

highlow

Rule of thumb:The higher the dose,the lower the noise.

dose

noisedose

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~ 4cm

1 2

1 2

Water Half-Value Layer (HVL) ~ 4 cm @ 120 kVp

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Dose (CTDIw)-Image quality(Noise)

influence of patient size

Noise factor = 1 HVL ~ 4 cm

(in a water phantom @ 120 kVp)Exponential Relationship

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Dose (CTDIw)-Image quality(Noise)

influence of patient size

Noise factor = 1 HVL ~ 4 cm(in a water phantom @ 120 kVp)

Exponential Relationship

e referencefwreference

nmAsmAs )( Φ−Φ⋅= μ

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DoseRight Tools

• Automatic Current Selection (ACS)

• Dose Modulation

Longitudinal Modulation Z-DOM

Angular Modulation D-DOM

ECG gated modulation (Cardiac DoseRight)

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Patient Size: Modeling Body Attenuation Through a Single Pilot Scan

water

Φ(Ζ)

Φ is the water equivalent diameter

a

bwater

μwa and μwb are the line integrals measured and calculated from a single Surview.

Scanned body

Z

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DoseRight v1.0 – ACS : Example

5

10

15

20

25

30

35

40

45

25 30 35 40 45Weighted Diameter (cm)

Noi

se (S

TD)

DR OffDR On

Abdomen Protocol: 40 patients

140 mAs 44 mAs

Reference

Acquired on IDT 16-slice: DoseRight v1.0; noise factor = 1.0

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DoseRight v1.0 – ACS : Example

140 mAs 44 mAs

Reference

Acquired on IDT 16-slice: DoseRight v1.0; noise factor = 1.0

Noise Factor (nf) = 1 not adequate SmallPatient Size : nf = 0.52 (7cm) Large Patient Size : nf = 0.73 (5cm)

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Scan region

Surview

Scan region defined

Per scan:

Upper 2% clipping, Max size

Max size is compared to a reference size

mAs profile is derived from scan Max size

and reference size

Φmax

ACS/Zdom algorithm

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DoseRight – ACS : Example

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DoseRight – ACS : Example

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DoseRight – ACS : Example

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Surview: influence of Voltage

80 kVp 140 kVp

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80 kVp 140 kVp

HeadNeck

Shoulders

Thorax

Liver

Surview Voltage

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Surview Direction

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Scan Region Scan Region

Surview Direction

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Lower Position Mid Position Upper Position

Surview: Table Height

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Mid Scan RegionSurview: Table Height

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Example: Head-neck-shoulder Scan

Scan direction5 10 15 20 25 30 35

Body attenuation profile along Z

-50 0 50 100 150 200 250 300 350 40060

80

100

120

140

160

180

200

220ZDOM mAs profiles

Bed position (mm)

mA

s

128*0.62564*0.62516*0.625

headTeethNeck

Shoulders

As the collimation is getting wider, the modulation is less noticeable, since narrow absorbing part of the patient, like the teeth, are seen by the large collimation for a longer distance along Z and prevent change in the current.

Collimation

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Scan Region Collimation

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Scan Region Collimation

32 x 0.625 mm

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Scan Region Collimation

32 x 0.625 mm

64 x 0.625 mm

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Scan Region Collimation

128 x 0.625 mm

32 x 0.625 mm

64 x 0.625 mm

In this example: DLP64 = 1.06 DLP32DLP128 = 1.13 DLP32

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Scan Region

128x0.625

64x0.625

Example: TAP

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Scan Region

128x0.625

64x0.625

Example: TAP

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No D-DOM

• Angular Modulation (D-DOM)– High frequency modulation– Modulates the tube current according to the organ eccentricity

(attenuation changes along the rotation angle). – Modulation is calculated on line. Each rotation is base on previous

rotation data

Shoulders ThoraxNeck

With D-DOMmAs

ShouldersNeck Thorax

z

mAs

z

DoseRight - Dose Modulation: D-DOM

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ShoulderLungs,head

ba

Scanned body*

* Equivalent body dimensions according to attenuation

DoseRight - Dose Modulation: D-DOM

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Dose Reduction = 33.5% - Improved IQ

D-DOM Off 200mAs

SD = 21.4

D-DOM On 133mAs

SD = 19.3

DoseRight - Dose Modulation: D-DOM

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DoseRight Cardiac – ECG Gated Dose Modulation

• ECG gated Modulation (Cardiac Dose Right)

- Tube current modulation according to reconstructed phases

- Beneficial in spiral acquisition with retrospective gating

- Maximum current during selected phases

- Current reduced to 20% elsewhere

- Dose savings depend on heart rate

(fixed delay for mAs modulation ⇒ more savings at low heart rate)

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Collimator = ( )

With Eclipse

Without Eclipse

Using The Same 80mm Collimation

Technology to reduce CT doseHelical dynamic collimator

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