1. CT Radiation Dose at Equal Image Contrast-to-noise Ratio using Iodine- and Novel Tantalum-based Contrast Agents: A Large Habitus Phantom Study
Paul FitzGerald
Jack W. Lambert
Peter M. Edic Robert E. Colborn
Andrew S. Torres
Peter J. Bonitatibus
Benjamin M. Yeh
RSNA 2015
PH277-SD-THA6

2. This work was supported by NIH grant R01EB015476.
Disclosures
Employees, GE Global Research
Paul FitzGerald
Peter M. Edic
Robert E. Colborn
Andrew S. Torres Peter J. Bonitatibus
Employees, UCSF Medical Center
Benjamin M. Yeh
Jack W. Lambert
Thank you to my colleagues, and to the NIH for supporting our work.
This work was supported by NIH grant R01EB
The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH.

3. Purpose
Quantitatively compare CT imaging performance of iodine- and tantalum-based contrast agents in phantoms representing small, medium-large, and extra-large patients

4. Methods – key metrics ↑ ↓
C: Image contrast (HU): mean in sample ROI - mean in background ROI
N: Image noise (HU): s.d. in background ROI
Contrast and noise within valid operating regime
DR: Radiation dose (mGy): SSDE w/water-equivalent diameter (DW)
CNR (dimensionless): Contrast-to-noise ratio, = C/N
DCNR (mGy): Dose normalized by CNR2, = DR/(C/N)2
DVP (mGy-HU2): Dose-variance product, = DR*N2
DVP = DCNR at C = 1, i.e. CNR-normalized dose at low contrast
DC: Contrast agent dose (mg element / mL agent)

5. Methods – phantoms and samples
Pseudo-anthropomorphic phantoms model 007TE, CIRS Inc.
Thoracic and abdominal versions
Modified to accept large sample vial
Can be used to measure - Contrast - Noise - Radiation dose

6. Methods – phantom sizes
Label
CIRS model 007TE-*
AP (cm)
LAT (cm)
Girth (inch)
Water-equivalent diameter, DW (cm)
Th.
Ab.
Adult small S x6 22 30 32
24.3
27.1
Adult Medium-Large ML
x7A
28.5
36.5 40
30.8
33.9
Adult Extra-Large XL x9 34 42 47
35.9
39.4
* x = 0 for abdomen
x = 1 for thorax

7. Methods – samples Active Element Atomic number k edge energy (keV)
Compound
Nominal concentration (mg active element/mL solution)
Measured concentration (mg active element/mL solution)
Correction factor for active element concentration to be applied to measured HU I 53 33
C35H44I6N6O15  10
9.73
1.03 Ta 73 67
(Ta2O5)(C7H14NO5Si)2.8
10.30
0.97

8. Results: Initial overview

9. Results – C (image contrast)

10. Results – C (image contrast)
Image contrast curves are coincident for “ML” and “XL” sizes for both tantalum and iodine

11. Results – C (image contrast)
54% higher image contrast with tantalum versus iodine in “XL”-sized patients at 140 kVp

12. Results – DVP (Dose-variance product)

13. Results – DCNR

14. Results – First interpretation
DVP and DCNR guide us toward the “best” kVp and contrast agent material for each patient size
DVP:
Small patients: 140 kVp → ~20% lower DR vs. 80 kVp
Medium and larger patients: small kVp dependence
DCNR:
Small patients: 80 kVp → slightly lower DR vs. 100 kVp
Medium and larger patients: tantalum → much lower DR
50-65% Radiation dose reduction opportunity with tantalum in large, overweight, and obese patients
BUT…

15. Results – DCNR
Up to ~65% dose reduction hypothetically possible, assuming that the clinical task is purely angiography and high power is required (e.g. CCTA).

16. Results – DCNR
Otherwise, we can reallocate the higher intrinsic signal of tantalum agent to:
reduce radiation dose
reduce contrast agent dose
improve CNR

17. Results:
Rotation time = 0.5 s
CNR and NMax: size-dependent

18. Results – with size-dependent CNR and NMax
Using these CNR and NMax targets,

19. Results – with size-dependent CNR and NMax
With DC constant

20. Results – with size-dependent CNR and NMax
Tantalum can provide:
With DC constant
DR ↓ ~20%

21. Results – with size-dependent CNR and NMax
Tantalum can provide:
CNR ↑ %
With DC constant
DR ↓ ~20%

22. Results – size “S”; CNR=15.0; NMax=22.0; ↓DR
Prescription
100 kVp 127 mAs
D = 9.1 mGy
C = 278 HU
N = 18.6 HU
CNR = 15.0
Iodine
Displayed
100 kVp 135 mAs
D = 9.6 mGy
C = 278 HU
N = 18.0 HU
CNR = 15.5
Prescription
100 kVp 104mAs
D = 7.4 mGy
C = 308 HU
N = 20.5 HU
CNR = 15.0
Tantalum
Displayed
100 kVp 110 mAs
D = 7.8 mGy
C = 308 HU
N = 20.0 HU
CNR = 15.4
DR ↓ 19% Contrast ↑, Noise ↑, CNR ↔

23. Results – size “ML”; CNR=12.3; NMax=23.5; ↓DR
Prescription
100 kVp 330 mAs
D = 18.4 mGy
C = 254 HU
N = 20.7 HU
CNR = 12.3
Iodine
Displayed
100 kVp 340 mAs
D = 18.9 mGy
C = 244 HU
N = 20.4 HU
CNR = 12.5
Prescription
100 kVp 256mAs
D = 14.2 mGy
C = 301HU
N = 23.5 HU
CNR = 12.8
Tantalum
Displayed
100 kVp 260 mAs
D = 14.5 mGy
C = 301 HU
N = 23.3 HU
CNR = 12.9
Dose ↓ 23%
Contrast ↑, Noise ↑, CNR ↑ 4%

24. Results – size “XL”; CNR=10.1; NMax=24.8; ↓DR
Prescription
120 kVp 338 mAs
D = 24.7 mGy
C = 195 HU
N = 22.4 HU
CNR = 8.7
Iodine
Displayed
120 kVp 340 mAs
D = 24.8 mGy
C = 195 HU
N = 22.0 HU
CNR = 8.7
Prescription
120 kVp 276 mAs
D = 20.1 mGy
C = 279 HU
N = 24.8HU
CNR = 11.3
Tantalum
Displayed
120 kVp 260 mAs
D = 19.0 mGy
C = 279 HU
N = 26.8 HU
CNR = 10.9
Dose ↓ 19%
Contrast ↑, Noise ↑, CNR ↑ 30%

25. Conclusion We developed a method to
quantitatively compare CT imaging performance of iodine- and tantalum-based contrast agents.
In medium-sized and larger adults, Tantalum predicted to provide
Up to 65% radiation dose reduction in certain applications
- or -
Up to 55% higher image contrast and CNR
Up to 35% contrast agent dose reduction
A combination of the three.
Maximum improvement predicted in obese patients.
Comparable performance in small adult patients.
Iodine predicted to outperform tantalum in smaller patients.