The large gap in dose between the prospective and the retrospective ECG gating modes required to analyse separately the CTDIvol and DLP for the two modes.
Mean values are presented in table 2 for each centre.
Table 2.
Mean values of CTDIvol and DLP per centre,
for CCTA performed with prospective and retrospective ECG-gating modes
Centre
|
A
|
B
|
C
|
D
|
E
|
F
|
G
|
H
|
Retrospective mode
|
|
|
|
|
|
|
|
|
n
|
16
|
49
|
49
|
|
49
|
|
49
|
49
|
mean CTDIvol ± SD (mGy)
|
34±13
|
64±9
|
25±7
|
|
38±14
|
|
21±11
|
26±15
|
mean DLP ± SD (mGy.cm)
|
752
±257
|
1144
±168
|
378
±113
|
|
673
±248
|
|
395
±204
|
574
±291
|
Prospective mode
|
|
|
|
|
|
|
|
|
n
|
39
|
49
|
|
49
|
|
32
|
|
30
|
mean CTDIvol ± SD (mGy)
|
17±7
|
25±4
|
|
15±6
|
|
31±15
|
|
14±3
|
mean DLP ± SD (mGy.cm)
|
301
±123
|
369
±66
|
|
250
±100
|
|
412
±211
|
|
240
±66
|
Mean CTDIvol per centre varied by a factor of 4.5 (range of means±SD: 14±6 to 63±9 mGy).
The dispersion of the mean CTDIvol was more important using retrospective compared to prospective gating (21±10 to 63±9 mGy and 14±6 to 31±15 mGy respectively).
As shown in fig.3,
a 15 to 68 % reduction in median CTDIvol has been registered with prospective mode,
as compared to retrospective,
in the centres where both techniques were performed,
in agreement with international studies [10-12].
The distributions of DLP are similar to those of CTDIvol,
due to a rather homogeneous acquisition length (fig.4).
Mean DLP per centre varied by a factor of 4.8 (range of means±SD: 240±66 to 1144±168 mGy),
similar to the range published by Hausleiter et al.
in 2009 [13].
The mean DLP determined for each centre leaded to an average effective dose for CCTA varying from 3.4 to 5.8 mSv in prospective mode and from 5.3 to 16 mSv in retrospective mode.
The results confirmed the interest of prospective ECG-gating in dose reduction in CCTA,
but this technique:
- requires a low and stable cardiac rhythm (60 beats/min or less),
often obtained with beta-blockers;
- investigates one single phase of the cardiac cycle,
with the risk of artefact when cardiac rhythm is unstable.
For these reasons,
some clinicians would rather use the retrospective ECG-gating mode to the prospective one.
Moreover,
the results of this study showed that with retrospective mode,
CTDIvol (and then patient dose) can be reduced to a level similar to that obtained with prospective mode.
The mean CTDIvol estimated with retrospective mode in centres C and G (25±7 and 21±11 mGy) are close to those of centres B and F for prospective mode (25±4 and 31±15 mGy).
This dose reduction with retrospective mode is obtained with simple optimization methods,
such as tube current and high voltage adaptation to BMI [14-17] but requires the clinicians to accept a lower image quality.
Minimum and maximum mean CTDIvol values (14±6 and 63±9 mGy) were collected on CT scanners using IR.
This reconstruction technique that offers the possibility to reduce patient dose appeared to be not sufficient by itself to reach this goal.
This large variability in CTDIvol underlines the need for cardiac acquisition protocols optimization.
Tools such as DRLs would be of great interest in that purpose.
The results of this study allow to suggest DRLs for CCTA,
established according to the European methodology [18] as the 75th percentile of the CTDIvol and PDL distributions,
for both ECG-gating modes (table 3).
The 25th percentiles are also presented in table 3 as secondary optimization tools (“target values”),
as suggested in some European countries [19],
or even required for other CT examinations [20].
Table 3.
DRL proposals for CCTA,
for prospective and retrospective ECG-gating modes (calcium scoring,
evaluation of bypass graft patency and pre-TAVI excluded)
|
Prospective
|
Retrospective
|
CTDIvol (mGy)
|
DRL (75th percentile)
|
26
|
44
|
25th percentile
|
12
|
20
|
DLP (mGy.cm)
|
DRL (75th percentile)
|
370
|
870
|
25th percentile
|
210
|
355
|