CT Dose Reduction Strategies
Experience with Care (Auto) kV
Marilyn J. Siegel, M.D
Mallinckrodt Institute of Radiology
Washington University Medical Center
St. Louis, MO
Background—
Why the concern about radiation exposure-• Use of CT has grown dramatically
• >10% increase per year over last 15 years
• CT now accounts for
–about 17% of all imaging exams AND
–nearly 50% of per capita exposure
Mettler FA. Radiology 2009; 253:520-531
Mettler FA. Health Phys 2008; 95:502
NRCP Report 160, March 2009
www.cancer.gov/cancertopics/
The Issue is Cancer--BEIR V (1990)
Risk vs. Age
Dose Reduction: The Basic Pillars
• Appropriate utilization
• Study optimization - low dose technology
Dose Reduction
Technology
1. Appropriate Utilization
• Justify the study
• Avoid repetitive studies
• Tailor the exam to the patient
• Use ultrasound or MRI when they
offer comparable or more information
–BUT not always possible
• Benefit needs to outweigh risk
2. Use Low Dose Technology
•
•
•
•
•
Major advances are-Automated mA (Care Dose 4D)
Increased spiral pitch (Flash)
Automated tube voltage (Care kV)
Used in combination, not in isolation
Automatic mA (current) Exposure
Care Dose 4D
• Real time dose modulation in x, y, z planes
• mA is adapted to thickness, not patient weight
• Basis: linear relationship mAs & dose
– 50% mAs decrease = 50% dose decrease
Example: 6 year old boy
• Scanned with adult reference standard,
but using automated mA dose reduction
• Reference eff. mAs =165
• Actual eff. mA = 37
Graser AJR 2006; 659
Herzog et al. AJR 2008, 1232
Mulkens Radiology 2005; 213
Courtesy B. Schmidt
Spiral Pitch Modulation
• Single source CT limited to low pitch
• Dual source CT enables high pitch, thus,
faster scan
• Linear relationship between pitch & dose
–50% increase pitch = 50% dose decrease
High Pitch (=3) CTA
6 day old: rule out
aortic coarctation
Baby crying-not sedated,
vessels well seen with
no motion
• kV-80; mA-76
• Scan time 0.24 sec
E dose = 0.35 mSv
• (DLP 24 x 0.015)
Images courtesy of Johns Hopkins
The Story Goes On--NEWEST TECHNOLOGY
Automated kV Modulation
Major Untapped Potential has been kV
?
Technology needs to go beyond the manual charts
Why use low kV---Rationale
• Lower radiation dose
• Improved contrast
• Greatest benefit in contrast CT exams
80kV
120kV
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
Background Phantom Work
Dose, mGy
kV vs. Dose
45
140 kV
40
120 kV
35
100 kV
30
80 kV
25
20
15
10
5
0
0
4
8
12
16
20
24
28
32
36
Phantom diameter, cm
Siegel MJ: Radiology 2004; 233: 515
Background Low kVp: Contrast
400
350
300
250
HU
200
80 kV
100 kV
120 kV
140 kV
150
100
50
0
0
4
8
12
16
20
24
28
Phantom diameter, c
32
36
Low kVp
• 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
Lower kV: Clinical Experience
• CT angiography
– 91-94% increase in signal w/ 80 kVp
– Equivalent image quality
– Radiation dose reduction of 25-50%
• Coronary CTA
– Retrospective gating
– 40% dose reduction from 120 to 100 kV
Kalva, et al. JCAT 2005; 30:391-397
Pflederer T, et al. (Univ. of Erlangen). SCCT, 2007
Lower kV
Contrast & Dose
120 kV, CTDIvol=5.18 mGy
100 kV, CTDIvol=3.98 mGy
Case courtesy C. McCollough
BUT the Risk is Increased Noise in
Larger Patients
60
80 kV
50
100 kV
40
120 kV
Noise 30
140 kV
20
10
0
0
4
8
12
16
20
24
28
Phantom diameter, c
32
36
Automated kV (Care kV)
What is it?
• A tool that automatically selects the optimal
kV setting for each patient, based on– user selected reference kV and mA for
specific exam type (CTA, contrast, noncontrast)
– body habitus determined by topogram
• Goal is to reduce dose and optimize contrastto-noise ratio
CARE kV: Step 1
• CARE kV off: enter user reference mA / kV for
the exam type: CTA, contrast or non-contrast
chest and/or abdomen
Input: Default protocol
CARE kV: Step 2
• Turn on CARE kV, move slider
• Reference mA/kV are updated automatically
based on topogram to maintain image quality
• Optimal kV and mA are shown along with dose
Input: Default protocol
Output: Optimized protocol
CARE kV-CARE Dose4D
• mA modulated along with kV
• mA default doses have been designed for children
• Brain and HeadAngio--average
• All other Organs--very weak Æ like the
recommendations (e.g. Image Gently)
Automated kV-CARE kV
• And we can do 70 kV imaging
Phantom Work on Auto kV
• 5 kVs-70, 80, 100, 120, 140
• 3 phantoms
– 10 cm
– 25 cm
– 40 cm
• Iodine insert
• Simulate CTA and contrast
enhanced abdomen CT
Effect of kV on Dose
Dose for Different Cross Sections
(same mAs)
70kV
80kV
100kV
120kV
140kV
60,0
CTDI / mGy
50,0
40,0
30,0
20,0
10,0
constant mAs (165)
0,0
small
medium
large
Phantom size
Dose reduction greatest in small phantoms
70%, 58%, and 44%
Effect of kV on Iodine Contrast
Contrast increases with lower kV
Phantom Assessment of Auto kV
Contrast vs. kV
70kV
100kV
Δ 1125 HU
Δ 525 HU
80kV
120kV
Δ 998 HU
Δ 427 HU
10cm phantom:
ƒiodine insert
ƒSignal increase
with lower kV
kV vs. Image Noise
Noise increases in larger phantoms
Auto KV-Clinical Tests
• Tested in > 150 young patients day 1-22 yrs
• 30 had prior scans on 16 row
• Results
– 64 AS, mean dose 3.7 mGy
–16 row, mean dose 5.9 mGy
–Mean dose reduction= 2.1 mGy
»Dose range 0.1 to 6.4 mGy
–Great quality images
Auto kV Chest CTA
• 3 year old girl
• 70kV / 158mA CTDIvol = 1.6 mGy
Auto kV contrast Abdomen, pelvis
• 7year old boy
• 80 kV / 177mA CTDIvol = 3.2 mGy
Summary
• Used optimally, CT is one of our most
valuable tools
• The goal is to dial down the dose and drive
up imaging quality-ALARA
• The challenge is to define body habitus
specific doses
Pediatric CT
• A challenge but dose reduction is possible
using optimized, automated kV, ma, and pitch