dose reduction and modulation tools · confidential 17. scan region. surview. scan region defined....
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Dose Reduction and Modulation Tools 李偉群
CT Application Specialist友信行
Confidential
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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