Study cohort
This prospective, single-institution, two-arm study was conducted in compliance with the Health Insurance Portability and Accountability Act and was approved by the institutional review board. In January and February 2014, 26 consecutive patients with primary or secondary liver cancer treated with TACE at our institution using the C-arm system (Allura Xper FD20, Philips Healthcare, Best, the Netherlands) available at that time were included (control group). The imaging platform was then upgraded to the new platform (AlluraClarity) and a second cohort of 52 consecutive patients between March and June 2014 with liver cancer treated with TACE (study group) was included. Thus the final population included a total of 78 patients.
MR imaging technique
All patients underwent baseline MR imaging approximately 2 weeks before intra-arterial treatment (mean 16 days, range 0–40) using a 1.5-T MRI unit (Magnetom Avanto, Siemens Medical Solutions, Forchheim, Germany), using our institutional protocol as previously described [15]. For each patient, the sagittal abdominal diameter at the level of the portal vein bifurcation was measured on axial T1-weighted MR images in the portal-venous phase.
C-arm imaging system
The differences between the two platforms are new acquisition software protocols for DF and DSA that lower the X-ray flux, reducing the dose at the cost of decreased image quality, and a workstation with real-time image processing algorithms to recover for the loss in image quality. In detail, the new acquisition parameters consisted of a decreased tube voltage (75 vs 78 kVp), use of the smaller focal spot size (0.4 vs. 0.7 mm), additional 0.1 mm copper and 1 mm aluminium filters. Automatic tube current modulation was enabled on both imaging platforms. The real-time image processing algorithms were spatial noise reduction for DF and spatial noise reduction, temporal averaging and automatic pixel shift for DSA [12].
Besides the new acquisition protocols and the real-time post processing, the two platforms are identical, using a dynamic 14-bit flat panel digital detector with an image matrix of 2480 × 1920 pixels, a pixel pitch of 154 × 154 μm and a maximum field of view of 30 × 38 cm.
The dose-saving features apply only to 2D imaging and so for CBCT, the acquisition parameters and the 3D reconstruction were identical for both C-arm imaging platforms. The acquisition parameters for CBCT were 120 kVp tube voltage, 5 ms exposure and 50 mA tube current, the last of these being modulated automatically during the acquisition. During 5.2 s of exposure, 312 projection images (60 frames/s) were acquired with the motorized C-arm covering a 240° clockwise arc at a rotation speed of up to 55°/s.
TACE protocol
The indication for treatment and the choice of treatment modality were discussed at our multidisciplinary liver tumor board on a case-by-case basis. All TACE procedures were performed by the same interventional radiologist (JFG) with 18 years of experience in hepatic interventions. A standardized approach according to our institutional protocol was used [16].
A dual-phase C-arm contrast-enhanced CBCT was acquired with the microcatheter placed in the hepatic artery branch considered to be the main tumour feeding vessel to verify correct positioning. After drug delivery, a single CBCT acquisition was performed to confirm technical success of embolization. Additional details of the CBCT protocols for identification of feedings arteries and assessment of treatment success have been reported previously [14, 15, 17, 18]. CBCTs were not acquired for a few patients who were unable to hold their breath since acquiring CBCTs with breathing motion would result in insufficient image quality and would have been of no clinical value.
Radiation exposure measurements and calculations
The new imaging platform (used for the study group) supported Digital Imaging and Communications in Medicine (DICOM) Radiation Dose Structured Reports (RDSR). RDSR contains detailed log information of every X-ray event, including radiation time, air kerma (AK), dose area product (DAP) and number of images acquired (see Appendix 1). A dedicated workstation was set up with DoseUtility (PixelMed Publishing, Bangor, PA) to receive, store and evaluate the RDSRs of the study group patients.
The old imaging platform (used for the control group) did not support RDSR, thus the examination reports generated by the system were used (see Appendix 2). These examination reports contained the AK and the DAP of the whole procedure as well as the cumulative DAP and radiation time of all DF runs. However, the DAP of all DSA and CBCT runs was combined into a single “Cumulative DAP (exposure)” on these examination reports (see Appendix 2). To separate DAP of DSA and CBCT, the DAP shown on-screen during the treatment before and after each CBCT scan was manually recorded. The DAP of each CBCT was calculated using the following formula:
$$ \mathrm{DAP}\left(\mathrm{CBCT}\right) = \mathrm{DAP}\left(\mathrm{p}\mathrm{ost}\kern0.5em \mathrm{CBCT}\right)-\mathrm{DAP}\left(\mathrm{p}\mathrm{r}\mathrm{e}\kern0.5em \mathrm{CBCT}\right) $$
The cumulative DAP of all DSA runs was calculated using the formula:
$$ \mathrm{Cumulative}\ \mathrm{DAP}\left(\mathrm{D}\mathrm{S}\mathrm{A}\right) = \mathrm{Cumulative}\ \mathrm{DAP}\left(\mathrm{exposure}\right)-\mathrm{Cumulative}\ \mathrm{DAP}\left(\mathrm{CBCT}\right) $$
In addition, the examination reports provided only the number of images acquired and not the radiation time for DSA. Thus, the latter had to be calculated using the number of images acquired during each run and knowing the frame rate used. For example, 30 frames at a frame rate of 3 frames per second correspond to a radiation time of 10 s. To prove that all these calculations were correct, examination reports of five patients undergoing TACE on the new system were also collected, and the calculated values were found to be the same as the values recorded using RDSR.
To compensate for the differences in procedure complexity and thus in radiation time between the patients, the recorded and calculated DAP values were normalized to time by dividing by the corresponding radiation times for both DF and DSA runs. For example, the normalized DAP for 1 min of DF was calculated as DF Cumulative DAP(Gy cm2)/DF radiation time(min).
DF image quality assessment
Objective DF image quality assessment was performed on an Osirix workstation (Pixmeo, Bernex, Switzerland) by an interventional radiologist (RES) with more than 4 years of clinical experience in liver imaging and TACE, who did not participate in the TACE procedures. For the assessment, a region of interest (ROI) with an area of 3 cm2 was placed in the abdomen, avoiding gas-filled intestines and bones, and the mean signal intensity and the standard deviation in Hounsfield units (HU) were recorded. A signal-to-noise ratio (SNR) was calculated using the formula SNR = mean HU/standard deviation HU. In addition, the signal intensity of the guidance wire in HU was recorded and a contrast ratio (CR) was calculated with the formula CR = mean HU/guidance wire HU.
DSA image quality assessment
Qualitative DSA image analysis was performed by two interventional radiologists (RES and RD), each with more than 4 years of clinical experience in liver imaging and TACE, who did not participate in the TACE procedures. The DSA images of the celiac run of all patients were presented in a blinded and randomized fashion on an Osirix workstation. The readers were blinded to the imaging C-arm system used and the imaging parameters. The window/level settings used were maintained as the default settings in Osirix. Both readers determined independently in separate reading sessions the visibility of the hepatic arteries using a four-point grading score (Table 1).
Table 1 Four-point grading score for the subjective assessment of DSA image quality
Statistical analysis
All statistical computations were performed in SPSS Statistics 22 (IBM Corp., Armonk, NY). A p value less than 0.05 was considered statistically significant. Descriptive statistics were performed to summarize the data. The distribution of all scale variables was assessed with the Shapiro–Wilk test. For scale variables with normal distribution, mean, standard deviation and range were used and an unpaired t test was performed. For scale variables with non-Gaussian distribution, median, interquartile range and range were used and a Wilcoxon rank-sum test was performed. For ordinal variables, count and percentage were used and a Wilcoxon rank-sum test was performed. Cohen’s kappa was used to calculate inter-rater reliability.