Abstract
Purpose: The scanner-reported CTDIvol for automatic tube current modulation (TCM) has a different physical meaning from the traditional CTDIvol at constant mA, resulting in the dichotomy "CTDI vol of the first and second kinds" for which a physical interpretation is sought in hopes of establishing some commonality between the two. Methods: Rigorous equations are derived to describe the accumulated dose distributions for TCM. A comparison with formulae for scanner-reported CTDI vol clearly identifies the source of their differences. Graphical dose simulations are also provided for a variety of TCM tube current distributions (including constant mA), all having the same scanner-reported CTDIvol. Results: These convolution equations and simulations show that the local dose at z depends only weakly on the local tube current i(z) due to the strong influence of scatter from all other locations along z, and that the "local CTDIvol(z)" does not represent a local dose but rather only a relative i(z) ≡ mA(z). TCM is a shift-variant technique to which the CTDI-paradigm does not apply and its application to TCM leads to a CTDIvol of the second kind which lacks relevance. Conclusions: While the traditional CTDIvol at constant mA conveys useful information (the peak dose at the center of the scan length), CTDIvol of the second kind conveys no useful information about the associated TCM dose distribution it purportedly represents and its physical interpretation remains elusive. On the other hand, the total energy absorbed E ("integral dose") as well as its surrogate DLP remain robust between variable i(z) TCM and constant current i0 techniques, both depending only on the total mAs = it0 = i0 t0 during the beam-on time t0.
| Original language | English (US) |
|---|---|
| Article number | 111920 |
| Journal | Medical Physics |
| Volume | 40 |
| Issue number | 11 |
| DOIs | |
| State | Published - Nov 2013 |
Keywords
- Computed Tomography(CT)
- CT Dosimetry
ASJC Scopus subject areas
- Biophysics
- Radiology Nuclear Medicine and imaging
Cite this
Dose equations for tube current modulation in CT scanning and the interpretation of the associated CTDIvol . / Dixon, Robert L.; Boone, John M.
In: Medical Physics, Vol. 40, No. 11, 111920, 11.2013.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - Dose equations for tube current modulation in CT scanning and the interpretation of the associated CTDIvol
AU - Dixon, Robert L.
AU - Boone, John M
PY - 2013/11
Y1 - 2013/11
N2 - Purpose: The scanner-reported CTDIvol for automatic tube current modulation (TCM) has a different physical meaning from the traditional CTDIvol at constant mA, resulting in the dichotomy "CTDI vol of the first and second kinds" for which a physical interpretation is sought in hopes of establishing some commonality between the two. Methods: Rigorous equations are derived to describe the accumulated dose distributions for TCM. A comparison with formulae for scanner-reported CTDI vol clearly identifies the source of their differences. Graphical dose simulations are also provided for a variety of TCM tube current distributions (including constant mA), all having the same scanner-reported CTDIvol. Results: These convolution equations and simulations show that the local dose at z depends only weakly on the local tube current i(z) due to the strong influence of scatter from all other locations along z, and that the "local CTDIvol(z)" does not represent a local dose but rather only a relative i(z) ≡ mA(z). TCM is a shift-variant technique to which the CTDI-paradigm does not apply and its application to TCM leads to a CTDIvol of the second kind which lacks relevance. Conclusions: While the traditional CTDIvol at constant mA conveys useful information (the peak dose at the center of the scan length), CTDIvol of the second kind conveys no useful information about the associated TCM dose distribution it purportedly represents and its physical interpretation remains elusive. On the other hand, the total energy absorbed E ("integral dose") as well as its surrogate DLP remain robust between variable i(z) TCM and constant current i0 techniques, both depending only on the total mAs = it0 = i0 t0 during the beam-on time t0.
AB - Purpose: The scanner-reported CTDIvol for automatic tube current modulation (TCM) has a different physical meaning from the traditional CTDIvol at constant mA, resulting in the dichotomy "CTDI vol of the first and second kinds" for which a physical interpretation is sought in hopes of establishing some commonality between the two. Methods: Rigorous equations are derived to describe the accumulated dose distributions for TCM. A comparison with formulae for scanner-reported CTDI vol clearly identifies the source of their differences. Graphical dose simulations are also provided for a variety of TCM tube current distributions (including constant mA), all having the same scanner-reported CTDIvol. Results: These convolution equations and simulations show that the local dose at z depends only weakly on the local tube current i(z) due to the strong influence of scatter from all other locations along z, and that the "local CTDIvol(z)" does not represent a local dose but rather only a relative i(z) ≡ mA(z). TCM is a shift-variant technique to which the CTDI-paradigm does not apply and its application to TCM leads to a CTDIvol of the second kind which lacks relevance. Conclusions: While the traditional CTDIvol at constant mA conveys useful information (the peak dose at the center of the scan length), CTDIvol of the second kind conveys no useful information about the associated TCM dose distribution it purportedly represents and its physical interpretation remains elusive. On the other hand, the total energy absorbed E ("integral dose") as well as its surrogate DLP remain robust between variable i(z) TCM and constant current i0 techniques, both depending only on the total mAs = it0 = i0 t0 during the beam-on time t0.
KW - Computed Tomography(CT)
KW - CT Dosimetry
UR - http://www.scopus.com/inward/record.url?scp=84889648899&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=84889648899&partnerID=8YFLogxK
U2 - 10.1118/1.4824918
DO - 10.1118/1.4824918
M3 - Article
C2 - 24320453
AN - SCOPUS:84889648899
VL - 40
JO - Medical Physics
JF - Medical Physics
SN - 0094-2405
IS - 11
M1 - 111920
ER -