lowering the dose: ma and kvp - society of computed body ... · •exceptions: screening chest ct...
TRANSCRIPT
![Page 1: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/1.jpg)
Lowering the Dose: mA and kVp
Marilyn J. Siegel, M.D
Mallinckrodt Institute of Radiology
Washington University Medical Center
St. Louis, MO
![Page 2: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/2.jpg)
Disclosure of Commercial Interest
I have a financial relationship with a
commercial organization that may have a direct
or indirect interest in the content as follows:
• Siemens Medical Solutions: consultant,
speakers bureau
![Page 3: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/3.jpg)
Objective
• Describe role of mA and kVp in
lowering CT in an adult population
![Page 4: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/4.jpg)
Major CT Parameters Affecting Dose
• Tube current (mA)
• Tube voltage (kV)
• Pitch
• Collimation
![Page 5: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/5.jpg)
Effect of CT Adaptations on Dose
• 50% decrease in mA = 50% dose decrease
• 80 kVp = 30% to 50% dose decrease
• 50% increase in pitch = 50% dose decrease
• Thicker slices = dose decrease
![Page 6: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/6.jpg)
mA and kV
• Tube milliamperage (mA)
– rate of x-ray production
• Kilovoltage (kV)
–number of x-rays produced
![Page 7: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/7.jpg)
mA
Tube Current Reduction
![Page 8: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/8.jpg)
Best Approach for mA Reduction
• Automated technology introduced in 1990s
• Superior to technique charts (no guess work)
• Exceptions: screening chest CT
• Other exception is head CT where beam
attenuation comes from bone formation and
this process is age dependent so mA is age
dependent-use fixed tube current
Graser AJR 2006; 659
Herzog et al. AJR 2008, 1232
Mulkens Radiology 2005; 213
![Page 9: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/9.jpg)
Automated mA Reduction
• mA is adapted to body parts, not patient weight,
thinner parts need less radiation
• mA based on attenuation data from scout image
• Dose modulation done in x, y, z planes
Tube
current
Z-position
![Page 10: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/10.jpg)
Ex: shoulders to pelvis Ref value 150 mA
![Page 11: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/11.jpg)
Automated mA Modulation Mean Dose Reduction
• Stone disease 64%
– Mulkens AJR 2007; 188: 553
• Cardiac CT 60%
– Herzog, AJR 2008; 190:1232
• Colonography 35%
– Graser, AJR 2006; 187:695
![Page 12: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/12.jpg)
Lung Cancer Screening
• Fixed tube current
• mA of 20 to 40 rather than standard 150
• Low attenuation background of air-filled
lungs offers a high contrast setting which
can be used favorably to assess lung
abnormalities
![Page 13: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/13.jpg)
Older woman breast cancer low dose vs. standard CT
Similar image quality
150 mA 20 mA
![Page 14: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/14.jpg)
National Lung Cancer Screening Trial
• 26,724 participants
• Scanned with 20-40 mA
• Average effective dose
–2 mSv for low-dose CT vs.
–7-8 mSv for standard chest CT
• Dose >> females than males (2.4/1.6)
Larke et al. AJR Nov 2011; 197:1165-1169
![Page 15: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/15.jpg)
A Tale of Two Techniques
kV Modulation
![Page 16: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/16.jpg)
Major Untapped Potential has been kV
?
Distribution of scans in clinical practice
![Page 17: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/17.jpg)
Why use low kV---Rationale
• Lowers radiation dose
• Improves contrast
• Greatest benefit in contrast CT exams
Siegel MJ Radiology 2004; 233:515
Funama, et al., Radiology 2005
Nakayama, et al., Radiology 2005
Huda, et al., Med Phys 2004
Nakayama, et al. AJR 2006 80kV 120kV
![Page 18: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/18.jpg)
Low kV-Effect on Dose: Phantom Study
Siegel MJ: Radiology 2004; 233: 515
0
5
10
15
20
25
30
35
40
45
0 4 8 12 16 20 24 28 32 36
Phantom diameter, cm
Do
se,
mG
y
140 kV
120 kV
100 kV
80 kV
•Reducing kV reduces dose
![Page 19: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/19.jpg)
0
50
100
150
200
250
300
350
400
0 4 8 12 16 20 24 28 32 36
Phantom diameter, cm
Iod
ine
co
ntr
as
t(H
U)
80 kV
100 kV
120 kV
140 kV
Low kVp: Effect on Contrast
•Reducing kV increases Iodine contrast (HU)
HU
Siegel MJ: Radiology 2004; 233: 515
![Page 20: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/20.jpg)
Low kVp
• Decreases dose & increases contrast
• Rationale: K-edge of iodine 32 keV
• Mean photon energy
– 80 kVp 44 keV
–100 kVp 52 keV
–120 kVp 57 keV
–140 kVp 62 keV
Huda W, et al. Radiology 2000; 217:430
![Page 21: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/21.jpg)
Low kVp: Effect on Image Noise
0
10
20
30
40
50
60
0 4 8 12 16 20 24 28 32 36
Phantom diameter, cm
No
ise
(HU
) 80 kV
100 kV
120 kV
140 kV
•Reducing kV increases image noise in large phantoms
Noise
![Page 22: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/22.jpg)
kV Modulation-Key Point
• The use of lower-kV is highly dependent
on patient size and diagnostic task
• Increases in contrast are seen ONLY
with high atomic number substances
–Iodine
–NOT water, soft tissue, calcium
• Benefit greatest in children and small
sized adults and contrast examinations
![Page 23: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/23.jpg)
Approaches for kV Reduction?
• Conventionally done using technique charts
• Recently automatic selection tools have
become available (2011)
• Most experience with kV reduction is in PE
and cardiac CTA
![Page 24: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/24.jpg)
Low kV Coronary CTA
• 100 patients (≤ 85 kg)
–Dual Source 64 CT
–Retrospective gating
–120 kVp / 330 mAs: 12 mSv
–100 kVp / 330 mAs: < 8 mSv
• 39% decrease in radiation exposure
Pflederer T, et al. AJR 2009; 192:1045–1050
![Page 25: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/25.jpg)
Low kVp and High Pitch CT
• Dose < 1 mSv
• Total scan time 0.25-0.27s
100 kV and 0.8 mSv
Achenbach et al Eur Heart J. 2010;31:340-6
Leschka et al. Eur Radiol. 2009;19:2896-903
![Page 26: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/26.jpg)
Low kV Pulmonary Embolism CTA
• Park et al. (Korean J Radiol 2009; 10:235)
–dose reduction of 24%
–10.1 to 7.8 mSv (120 to 100 kV)
–Smaller patients (BMI < 25)
• Matsuoka et al. (AJR 2009; 1651-1656)
–40 to 50% dose reduction (120 to 100 kV)
![Page 27: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/27.jpg)
Low kV—Pulmonary Embolism CTA
• Lower dose, better contrast
• Mean enhancement > with 100 kVp (492 vs. 345 HU)
• Sensitivity 100% for detecting PE
120 kV 80 kV
Szucs-Farkas AJR 2011: 197:852
![Page 28: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/28.jpg)
Automated kV Selection Technology
• A tool which automatically adjusts kV for body size
determined from topogram and the exam type
• Up to 60% dose reduction
![Page 29: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/29.jpg)
Auto kV User Interface
• With auto kV off: choose exam type as you would routinely do, screen shows mA, kV & dose -
• Turn auto kV on: take scout CT-program selects best kV to lower dose and maintain image quality
Small adult
Contrast
chest CT
![Page 30: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/30.jpg)
Auto kV
• Used in conjunction with auto mA
• kV decreases AND mA increases
• But overall dose is lower than with fixed kV
![Page 31: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/31.jpg)
Auto kV contrast abdomen, pelvis
• 15 year old boy
• REF: 120kV / 75mA, CTDIvol = 5.77 mGy
• 80 kV / 177mA CTDIvol = 3.2 mGy
![Page 32: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/32.jpg)
Adults--Automated kV 2011
Automated kV selection based on the attenuation
profile of the topogram is feasible, provides a
diagnostic image quality for body CTA, and reduces
overall radiation dose by 25% as compared with a
standard protocol with 120 kV
Investigative Radiology 2011
![Page 33: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/33.jpg)
Auto kV vs. Image Quality
Investigative Radiology 2011
![Page 34: Lowering the Dose: mA and kVp - Society of Computed Body ... · •Exceptions: screening chest CT •Other exception is head CT where beam attenuation comes from bone formation and](https://reader033.vdocument.in/reader033/viewer/2022042912/5f472174cf257e76ea22904f/html5/thumbnails/34.jpg)
Summary
• Beautiful pictures do NOT need to come
at the cost of higher radiation dose
• Adjusting mA and kV can dial down the
dose and drive up imaging quality-ALARA