Since the Siemens Somatom Definition Flash CT acquisition,
the medical physics unit performed an important work of CT characterization [11].
In particular,
a preliminary study was performed on two phantoms: a Catphan 600 and a water phantom,
as shown in Fig. 2 .
Both phantoms were acquired with two full-dose protocols (FDPs): Thorax-Abdominal-Dual Energy protocol (TA protocol,
tube A 100kV,
tube B 140kV,
230 mAs Ref-mAs of tube A and reconstruction kernel I30f strength 2) and Head protocol (H protocol,
tube 120kV,
390 mAs Ref-mAs and reconstruction kernel H31s).
For each FDPs two low dose protocols (LDPs) were obtained by dropping off 10% (LDP1) and 20% (LDP2) mAs.
The phantoms were acquired with LDP1 and LDP2 and raw-data were reconstructed using all the available SAFIRE filters at 5 strength levels.
For each image dataset modulation transfer function (MTF),
noise,
noise power spectrum (NPS),
contrast and contrast to noise ratio (CNR) were evaluated and compared with the ones obtained with the FDP in order to find the filter that better match FDP performance.
The best filter for each dose level was found as the one with MTF,
contrast and CNR equal to or higher than FDP ones,
noise lower than FDP one and NPS the most similar to FDP one.
The better filter that matches FDP performance was I30f strength 3 for the TA protocol and the iterative J40s strength 3 for the H protocol.
A validation study was carried out on 30 patients undergoing 3 follow-up exams during a period of 1 year (twice yearly): the first was acquired with FDP,
the second with LDP1 and the third with LDP2. LDP images were compared to FDP acquisitions in terms of dose (volumetric CT dose index,
CTDIvol) and IQ,
defined by the following scores: diagnostic confidence,
qualitative image noise,
artifacts presence and visibility of small anatomic structures (see Fig. 1 ).
A CTDIvol reduction equal to 12% and 18% was obtained for TA protocol LDP1 and LDP2,
respectively,
while maintaining the same IQ (p>0.25 for all the image quality criteria,
Fig. 1 for TA protocol).
Similar reductions,
equal to 10% and 20%,
were found for the LDP H protocols.
Two new protocols were implemented by the validation study: a new Thorax-Abdominal-Dual Energy protocol (tube A 100kV,
tube B 140kV,
180 mAs Ref-mAs of tube A and reconstruction kernel I30f strength 2) and a new Head protocol (tube 120kV,
350 mAs Ref-mAs and reconstruction kernel H31s).
Both protocols were clinically implemented by November 2013.
A retrospective analysis was carried out during the subsequent 2 years in order to estimate the dose reduction,
in terms of average CTDIvol,
due to the introduction of these new protocols.
In particular,
CTDIvol of 100 patients for each year (2013–15) and for each protocol were retrospectively analyzed.
The average CTDIvol related to the pre-optimization protocol was (13.7 ± 1.8) mGy for the TA and (60.8 ± 0.1) mGy for the H protocol.
The TA protocol optimization led to a 30% dose reduction between 2013 and 2014,
resulting in an average CTDIvol of (9.7 ± 1.5) mGy respectively.
Similarly,
we obtained a dose reduction of 10% for the H protocol,
equivalent to an average CTDIvol of (54.7 ± 0.6) mGy in the 2015.