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CT 自動曝露控制宣導及使用說明
黎靜盈
APPLICATION SPECILIST
ROTARY TRADING CO.,LTD.
TOSHIBA MEDICAL EQUIPMENT
EXCLUSIVE DISTRIBUTOR OF TAIWAN
Contents
z
Biologic effects of ionizing radiation
z
CT usage and dose contribution
z
CT imaging techniques (parameters) and dose
z
Technology to reduce CT dose
--- Tube current modulation (Sure Exposure)
z
Pediatric issue
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Practical tips and strategies
Contents
z
Biologic effects of ionizing radiation
z
CT usage and dose contribution
z
CT imaging techniques (parameters) and dose
z
Technology to reduce CT dose
--- Tube current modulation (Sure Exposure)
z
Pediatric issue
z
Practical tips and strategies
Biologic effects of ionizing radiation
人體吸收輻射能量時，細胞和水
分子會首先被游離或激發，造成
DNA雙鏈全斷或只斷單鏈的傷
害（直接傷害）。因為水分佔了
人體約 70 % 的重量，而水分子
被游離後會產生有害的OH自由
基，這些自由基接續產生一連串
化學反應，使得細胞分子受到損
傷（間接傷害）。所幸細胞有自
行修復的能力，大部分的細胞會
恢復正常。 假若細胞嚴重受損
而無法修復或修復有錯誤時，則
其將顯現出健康受損的症狀。
Linear non-threshold theory
輻射對人體的健康效應，通常分為機率效應和確定效應兩大類。當人體
在短時間內接受劑量超過某一程度以上時，因為許多細胞死亡或已無法
修復，而產生疲倦、噁心、嘔吐、皮膚紅斑、脫髮、血液中白血球及淋
巴球顯著減少等症狀。當接受劑量更高時，症狀的嚴重程度加大，甚至
死亡，這種情況稱為確定效應。通常確定效應必須在接受劑量超過一定
程度以上才會發生。
從日本核爆生存者長期調查顯示，接受低劑量（約250毫西弗以
下）者，並無任何臨床症狀，白血病或其他實體癌的發生率都和一般人
相同。但是為了輻射安全的緣故，國際放射防護委員會( ICRP )做了一
個很保守又很重要的假設：人體只要接受到輻射，不管劑量是多少，都
有引發癌症和不良遺傳的機率存在，沒有低限劑量值，而且致癌或不良
遺傳的機率與接受劑量成正比（直線關係），劑量愈高，罹患的機率也
愈大，這種情況稱為機率效應。
Linear non-threshold theory
從臨床的觀點來區分輻射的健康效
應：健康效應發生在受照射本人身上
的，稱為軀體效應，若發生在受照射
者的後代子孫身上的，稱為遺傳效
應。從發生效應的快慢而言，則分為
急性效應和慢性效應。
PNAS November 25, 2003 vol. 100 no. 24 13765
Linear non-threshold theory
The International Commission on Radiological Protection had previously
estimated that a dose of 10 mSv would cause one in 2000 lifetime
cancers. ( ICRP 1991 ; 21 ( 1-31 ) : 1 - 201 )
Contents
z
Biologic effects of ionizing radiation
z
CT usage and dose contribution
z
CT imaging techniques (parameters) and dose
z
Technology to reduce CT dose
--- Tube current modulation (ACE)
z
Pediatric issue
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Practical tips and strategies
CT usage and dose contribution
Nishizawa, Acta Radiol Jap 2004; 64:151-158
CT usage and dose contribution
Cancer risks from diagnostic radiology
E J HALL, DPhil, DSc, FACR, FRCR and D J BRENNER, PhD, DSc
The British Journal of Radiology, 81 (2008), 362–378
Dose in radiology department
http://www.radiologyinfo.org March 30, 2007
Contents
z
Biologic effects of ionizing radiation
z
CT usage and dose contribution
z
CT imaging techniques (parameters) and dose
z
Technology to reduce CT dose
--- Tube current modulation (ACE)
z
Pediatric issue
z
Practical tips and strategies
CT imaging techniques and dose
Dose depends on: scanner geometry, filtration, kV,
collimation, scan mode , mA , pitch , sec …
CT imaging techniques and dose
CTDI depends on: Scan mode
CTDI increases as phantom/patient size decrease
Pediatric CTDI can be underestimated with standard phantom
CT imaging techniques and dose
CTDI depends on: Collimation
Collimation ↑，CTDI ↓
Collimation ↓，CTDI ↑
CT imaging techniques and dose
CTDI depends on: kVp
z
CTDI increases with kVp
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Approx ∝ kVp2
CT imaging techniques and dose
CTDI depends on: Pitch
CT imaging techniques and dose
CTDIw (mGy)
CTDIvol with scan length (mGy‧cm)
EDLP (mSv mGy-1cm-1 )
mSv
CT imaging techniques and dose
AAPM Report NO.96 January 2008
The measurement ,reporting and Management Radiation Dose in CT.
How to reduce CT dose
ALARA
as low as reasonably achievable
How to reduce CT dose
Guidelines or regulation
Contents
z
Biologic effects of ionizing radiation
z
CT usage and dose contribution
z
CT imaging techniques (parameters) and dose
z
Technology to reduce CT dose
--- Tube current modulation (AEC)
z
Pediatric issue
z
Practical tips and strategies
Tube current modulation (ACE)
z
Unlike screen-film radiographs, images acquired with computed
tomography (CT) never look overexposed
Noise
The
Thehigher
higherthe
thedose,
dose,
the
thelower
lowerthe
thenoise.
noise.
high
1
noise
dose
low
dose
low
high
Tube current modulation (ACE)
DoseX
X/2
X/4
X/8
X/16
X/64
Tube current modulation (ACE)
DoseX
X/2
X/4
X/8
X/16
X/64
z
z
z
z
Patients size AEC
Z-axis AEC
Rotational AEC
Combination
Tube current modulation (ACE)
z
z
A) patient size AEC: higher mA
is used for larger patient
c) rotational AEC: the degree of
modulation depends on asymmetry
at each z-axis position
z
b) z-axis AEC: higher mA used at
more attenuating z-axis positions
z
d) combined effects of using all
three levels of AEC
Tube current modulation (ACE)
Tube current modulation (ACE)
Benefit with AEC
z
z
Consistent image quality
Reduction in photon starvation artifact
High attenuation
LOW attenuation
Constant mAs
199mAs,
noise = 12.9HU
Tube mA modulation
189mAs,
noise = 9.4HU
Tube current modulation (ACE)
Benefit with AEC
z
z
Potential for dose reduction through exposure
optimization
Reduced tube loading (extended scan runs)
Dose saved
Tube current modulation (ACE)
Summary
Sure
Adjusts mA to achieve your
desired level of Image
quality.
Ensures image quality for all
patient sizes.
Can reduce dose up to 40%.
Exposure
Scanogram
• The first Scanogram performed is used in the
calculation of Sure Exposure
•The quality, i.e. adquate penetration, of the
Scanogram directly affects the calculations
(same KV)
•Positioning within the isocentre can affect the
exposure calculations
Scanogram
3 Dimensional Sure Exposure tube current modulation
AP scanogram will be used to calculate the AP / PA exposure (Y-axis). The
Lateral scanogram will be used to calculate the lateral exposure (X-axis). Z axis
modulation still applies.
AP Modulation Graph
mA graph display
toggle switch
LAT Modulation Graph
Scanogram Dose
• Scanogram Dose is low…..Very low.
• The dose display on the image is a total mAs.
Scanogram Dose
Eff. mAs = mA x Slice thickness
Table Speed
Slice Thickness
Table Speed
Eff. mAs =
= 2mm
= 100mm/sec
mA x 2
mm
100 mm/s
Eff. mAs = mA x 0.02 sec
Scanogram Dose
Eff. mAs = mA x 0.02 sec
For a 50 mA scanogram
Eff. mAs = 50 x 0.02sec
Eff. mAs = 1 mAs
Water Equivalent Thickness
The attenuation values from the scanogram are converted
into a “Water equivalent thickness.
SD level is set in Exam Plan
Parameters of the
reconstruction, are considered
for the resultant SD
Water equivalent thickness
mA is calculated to get
same image SD @ All
positions.
The operator predetermines the SD. Sure Exposure
calculates the mA required to achieve the same noise
level (SD) for each position.
Standard Deviation
• Standard Deviation (SD) is a measurement of image noise.
•It indicates the CT value deviation around the mean value.
•The higher the value the greater the noise level
ROI
Water Phantom
Standard Deviation
Low noise image
High noise image
ROI
ROI
Mean= 0HU
SD= +/- 2HU
Mean= 0HU
SD= +/- 30HU
SUREExposure
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SUREExposure
3D – Preset
presets now include target slice thickness and
reconstruction parameter settings.
SUREExposure
z
3D – Preset
Head Parameters apply to exam plans with Head body region selected.
Head Region
SUREExposure
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3D – Preset
Neck Parameters apply to exam plans with Neck body region selected.
Neck Region
SUREExposure
z
3D – Preset
Body Parameters apply to exam plans with all other body regions
selected.
Body Region
SUREExposure
z
3D – Preset
Set a target SD and SUREIQ parameter for EACH preset
SUREExposure
z
3D – Preset
Set a minimum and maximum mA for EACH preset
SUREExposure
z
3D – Preset
XY modulation can be set ON/OFF for EACH preset
SUREExposure
z
z
3D – Preset
Set a target slice thickness for EACH preset
Note: Have the same target slice thickness for each body region
SUREExposure
z
z
SUREExposure
3D – Scanning
settings menu does not need target SD settings.
Easy quality selection is by preset name only.
SUREExposure
z
3D – Scanning
Manual settings have been hidden from the main menu.
SUREExposure
z
3D – Scanning
Manual settings require input of target slice thickness and SUREIQ
parameters.
New Default
SUREExposure
Settings
Contents
z
Biologic effects of ionizing radiation
z
CT usage and dose contribution
z
CT imaging techniques (parameters) and dose
z
Technology to reduce CT dose
--- Tube current modulation (ACE)
z
Pediatric issue
z
Practical tips and strategies
Pediatric issue
David J. Brenner1 Carl, D. Elliston1,Eric J. Hall1, Walter E. Berdon2 ;AJR:176, February 2001
Pediatric issue
z
High risk
David J. Brenner1 Carl, D. Elliston1,Eric J. Hall1, Walter E. Berdon2 ;AJR:176, February 2001
Pediatric issue
z
The unique consideration in children
• increased radiosensitivity of certain tissues,
particularly in infancy
• longer lifetime for radiation-related cancer to occur(慢
性效應)
• The thyroid gland, breast tissue, and gonads are
structures that have an increased sensitivity to
radiation in growing children.
Pediatric issue
AAPM Report NO.96 January 2008
The measurement ,reporting and Management Radiation Dose in CT.
Pediatric issue
Physical Statistics
Age (Y)
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1.5
1
0.75
0.5
0.25
0
Weight (kg)
63.1
61.8
58.8
54.5
48.8
42.2
38.3
35.2
32.6
29.9
27.3
24.5
21.9
19.4
16.5
14.6
12.6
11.4
10.0
9.1
7.6
5.7
3.4
Hipwidth (cm)
28.0
27.8
27.4
26.7
25.8
24.6
23.5
22.6
22.0
21.4
20.7
19.9
19.1
18.3
16.9
15.8
14.4
13.7
13.0
12.3
11.6
10.6
8.1
Chestcircumf (cm)
83.4
82.5
80.7
78.0
74.5
70.3
67.8
65.9
63.9
61.8
59.8
57.8
56.1
54.5
53.7
52.4
50.8
49.5
48.0
46.0
43.7
40.6
33.2
Documenta Geigy Wissenschaftliche Tabellen
Pediatric issue
Age does not tell you all...
Pediatric issue
•The x-ray attenuation doubles for every 4 cm increase in diameter,
thus the mAs needs to be doubled in order to keep the same image noise
Diameter
X-ray signal (50mAs)
mAs for constant noise
0
100.00%
50
4 cm
50.00%
100
8 cm
25.00%
200
12 cm
12.50%
400
16 cm
6.25%
800
Pediatric issue
This could mean...
Suppose he needs 50 mAs...
He might need 100 mAs or even more...
體重(公斤)
Pediatric issue
z
Weight - dose table
Tsai IC, Lee WL, Tsao CR, Chang Y, Chen MC, Lee T, Liao WC.
Comprehensive evaluation of ischemic heart disease using multidetector row CT: Part 1- Comprehensive? Why and how?
AJR:191, July 2008
E-mAs
kVp
CTDIVOL
0～3
280
80
6.0
4～6
320
80
6.9
7～10
400
80
8.6
11～15
520
80
11.1
16～20
200
120
14.0
21～25
250
120
17.5
26～30
300
120
21.0
31～35
350
120
24.5
36～40
400
120
28.0
41～45
450
120
31.5
46～50
500
120
35.0
51～55
550
120
38.5
56～60
600
120
42.0
61～65
650
120
45.5
66～70
700
120
49.0
71～75
750
120
52.5
76～80
800
120
56.0
81～85
850
120
59.5
86～90
900
120
63.0
91～100
950
120
65.8
100以上
800
140
83.3
320-row CT minimizes dose in
pediatric abdominal studies
Newborn with aortic hypoplasia and renal artery stenosis. Weight of 1.7 kg; singlerotation 320-detector-row scan at 0.5 sec, 80 kV, 40 mA, and dose of 0.3 mSv.
All images courtesy of Dr. Patrik Rogalla.
Extended Dose display
(1) Result - Dose Tab.
Extended Dose display
(2) Result exposure record output.
Contents
z
Biologic effects of ionizing radiation
z
CT usage and dose contribution
z
CT imaging techniques (parameters) and dose
z
Technology to reduce CT dose
--- Tube current modulation (ACE)
z
Pediatric issue
z
Practical tips and strategies
Practical tips and strategies
Centering the patient
Li et al, AJR 2007; 188:547-522
Practical tips and strategies
Pregnancy
•
•
•
•
•
Identify probability of pregnancy
Minimize dose, consider alternative
Document dose
Low exposure
High exposure – informed decision, awareness of risk
--- Questionnaire, consent form
Practical tips and strategies
Breast Radiation
z
z
z
Especially scans for pulmonary embolism in young women
Dose to breasts may be up to 19 times that from a
mammogram
Strategy for dose reduction: be selective
Practical tips and strategies
Patient shielding
Courtney et al, AJR:190, January 2008
Practical tips and strategies
FDA recommendation
z
z
z
Optimize CT setting
--- reduce tube current
--- Develop and use a chart or table of tube-current
settings based on patient weight or diameter and
anatomical region of interest
--- Increase table increment (axial scanning) or pitch
(helical scanning).
Reduce the number of multiple scans with contrast
material
Eliminate inappropriate referrals for CT
References
E J HALL et al, The British Journal of Radiology, 81 (2008), 362–378
Courtney Coursey et al, AJR:190, January 2008
Brenner DJ et al, N Engl J Med 2007;357:2277–84.
Brenner DJ et al, Proc Natl Acad Sci USA 2003;100:13761–6.
McCollough et al, Radiographics ,Volume 26 ;Number 2:503-513
…….
http://www.impactscan.org/
http://www.ctug.org.uk/index.html
http://www.auntminnie.com/
http://www.arwen.se/radiologi/omnimas_en/OmnimAs-Instruction.htm