Study Cohort
This retrospective, single-institution, two-arm study was approved by the institutional review board. All patients with uterine fibroids that were treated using uterine artery embolization (UAE) at our institution consecutively between May 2014 and April 2015 were retrospectively included in the study.
MR Imaging Technique
All patients underwent baseline MR imaging for treatment planning [12] within 5 months before UAE (mean 71 days, range 9–131), using our institutional protocol, as previously described [13]. For each woman, the sagittal abdominal diameter was measured at the level of uterus on sagittal T1-weighted MR images.
Angiographic Imaging Systems
The procedures were performed in two different angiographic suites: one being equipped with a recently released imaging system (AlluraClarity FD 20, Philips Healthcare, Best, The Netherlands; study group) and the other one with the preceding generation imaging system of the same vendor (AlluraXper FD 20; control group). Both imaging systems share 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. Automatic tube current modulation was enabled on both systems. However, as previously mentioned, the new system used additional filtering (0.1-mm copper and 1-mm aluminum filters) and optimized acquisition protocols (a decreased tube voltage of 75 vs. 78 kVp and a smaller focal spot size of 0.4 vs. 0.7 mm) to lower the radiation exposure, and real-time image processing algorithms to compensate for the loss in image quality due to lower radiation flux (spatial noise reduction for digital fluoroscopy (DF) and spatial noise reduction, temporal averaging and automatic pixel shift for DSA [6]).
Embolization Protocol
An experienced interventional radiologist (K.H., 10 years of experience in abdominal interventions) performed all embolization procedures. A consistent approach according to our standard institutional protocol was used for all patients [14].
Radiation Exposure Measurements and Calculations
The new imaging system (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. 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 system (used for the control group) did not support RDSR; thus, the examination reports generated by the system were used. These examination reports contained the AK and the DAP of the entire procedure as well as the cumulative DAP for DF and DSA runs, respectively. However, the cumulative radiation time was only provided for DF runs, whereas for DSA runs, the number of acquired images was provided. Thus, the DSA radiation time had to be calculated using the number of images acquired during each run and knowing the frame rate used. For example, 15 frames at a frame rate of three frames per second correspond to a radiation time of 5 s. To prove that all these calculations were correct, examination reports of five patients undergoing UAE 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 DAP values were normalized by the corresponding radiation times for both DF and DSA runs. For example, the normalized DAP for 1 s of DSA was calculated as
$${\text{DSA}}\;{\text{Cumulative}}\;{\text{DAP}}\; ( {\text{Gy}}*{\text{cm}}^{2} ) / {\text{DSA}}\;{\text{radiation}}\;{\text{time}}\;({\text{s}}).$$
DF Image Quality Assessment
Objective DF image quality assessment was performed in a blinded fashion on an Osirix workstation (Pixmeo, Bernex, Switzerland) by an interventional radiologist (R.E.S.) with more than 5 years of clinical experience in UAE and corresponding imaging, who did not participate in the UAE procedures. A circular region of interest (ROI) with an area of 3 cm2 was placed on the iliac bone, avoiding gas-filled intestines, and the mean signal intensity and the standard deviation of pixels within the ROI were recorded. A signal-to-noise ratio (SNR) was calculated using the formula
$${\text{SNR}} = {\text{mean/standard}}\;{\text{deviation}}.$$
In addition, the signal intensity of the guidance wire was assessed by placing an elliptic ROI with an area of 3 cm2 on the wire and recording the minimum signal intensity within the ROI, which corresponds to the wire. A contrast ratio (CR) was calculated with the formula
$${\text{CR}} = {\text{mean/guidance}}\;{\text{wire}}.$$
DSA Image Quality Assessment
Qualitative DSA image analysis was performed by two interventional radiologists (R.E.S. and R.D.) each with more than 5 years of clinical experience in UAE and corresponding imaging, who did not participate in the UAE procedures. The DSA images of both uterine arteries of all women were presented in a blinded and randomized fashion on an Osirix workstation. The window/level settings used were maintained to be the default settings in Osirix. Both readers determined independently of each other in separate reading sessions the visibility of the small feeding arteries of the uterine fibroids (parameter 1) as well as the absence of artifacts related to breathing (parameter 2) and small bowel movement (parameter 3) on a binary scale (yes = 1, no = 0). These three image quality parameters were summarized in a four-scale scoring system, where a score of 3 was considered best and a score of 0 was considered worst.
Statistical Analysis
All statistical computations were performed in SPSS Statistics 23 (IBM Corp., Armonk, NY). A p value < 0.05 was considered statistically significant. Descriptive statistics were performed to summarize the data. The distribution of 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, median, count and percentage were used and a Wilcoxon rank-sum test was performed. For the assessment of interobserver agreement, Kendall’s tau coefficient was calculated.