Comparison of radiation dose and image quality between flat panel computed tomography and multidetector computed tomography in a hybrid CT‐angiography suite

Abstract The purpose of this study was to compare, using the same radiation dose and image quality metrics, flat panel computed tomography (FPCT) to multidetector CT (MDCT) in interventional radiology. A single robotic angiography system with FPCT was compared to a single MDCT system, both installed in a hybrid CT‐angiography laboratory and both operating under automatic exposure control. Radiation dose was measured on the central axis (Dc) of a CT dosimetry phantom 30 cm in diameter and 60 cm in length using default protocols for FPCT and MDCT with the imaged length in MDCT matched to the field of view of FPCT. The noise power spectrum (NPS), modulation transfer function (MTF), and z‐axis resolution were measured using the same phantom. Iodine contrast to noise ratio (CNR) was also measured. Radiation dose (Dc) was 41%–69% lower in MDCT compared to FPCT when default protocols and automatic exposure control were used. While spatial resolution could generally be matched with appropriate choice of kernel in MDCT, MTF dropped more quickly at higher spatial frequency for MDCT than FPCT. Image noise was 49%–120% higher for MDCT compared to FPCT for comparable in‐plane spatial resolution. Z‐axis resolution was slightly better for MDCT than FPCT, while iodine CNR depended on protocol selection. Radiation dose was much lower for MDCT compared to FPCT, but image noise was much higher. Matching image noise in MDCT to FPCT would result in similar radiation doses. Iodine contrast depended on dose modulation settings for MDCT.


2.A | Radiation dose
The default 6sDCT Body and 5sDCT CARE Body FPCT organ programs on the angiography system were used to image the phantom, which was positioned at isocenter. The acquisition parameters for these programs are listed in Table 1, and the fluoroscope acquired projection images using AEC, as is standard during clinical operation. The default Abdomen Routine CT protocol was used to measure radiation dose (D c ) for MDCT in the same fashion. The prototype ICRU phantom was positioned at isocenter and a topogram of the phantom was acquired. The Abdomen Routine protocol used both tube current (CareDose 4D) and kV modulation (CarekV, Siemens Healthineers, Malvern, PA). The CarekV algorithm is task-specific, 11 and for the current study Slider Position 7 (soft tissue contrast) and Slider Position 9 (midway between soft tissue contrast and vascular) were evaluated. The length of the scan was set to provide the same imaged length in MDCT as was acquired using FPCT. The acquisition parameters for the MDCT scan are listed in Table 2.
These methods allowed for comparison of radiation dose on an interval scale.

2.B | Image quality
Image quality was assessed using the American College of Radiology (ACR) CT accreditation phantom and a multienergy CT quality control (QC) phantom (CT ACR 464 and Multi-Energy CT Phantom, Sun Nuclear Corporation, Melbourne, FL). The software developed by Friedman et al. 12 was used to calculate modulation transfer functions (MTF) and noise power spectra (NPS) using images of the ACR CT accreditation phantom. It was not possible to use the outer phantom contour to calculate the MTF as described by Friedman et al., as the field of view (FOV) for FPCT was too small. Instead, the air object within the phantom was used to calculate MTFs for both FPCT and MDCT. NPS and standard deviations were measured in the uniform section of the phantom, NPS using the methods of Friedman et al. 12 and standard deviations using 15 cm 2 regions of interest (ROI).
These methods allowed for comparison of both high contrast spatial resolution and noise on interval scales.
Two objects in the multienergy CT QC phantom simulating different mixtures of iodine contrast and blood, with iodine concentrations of 2 mg/cc and 4 mg/cc, were used to measure iodine contrast and contrast-to-noise ratio (CNR). The phantom was scanned twice for each scenario, and the 2 mg/cc object was moved between the T A B L E 1 Acquisition parameters for FPCT.

| RESULTS
Results are summarized in Tables 4 and 5 (Table 4). There were some differences in the shape of the MTF curves, including evidence of less apodization (i.e., higher MTF at higher spatial frequencies) in FPCT compared to MDCT (Table 4 and Fig. 1).
In general, FPCT was characterized by lower noise than MDCT when technical factors were selected automatically by the imaging systems [ Fig. 2 D c , increasing the radiation dose from MDCT to a level that is approximately equal to that from FPCT. Differences in contrast and CNR between FPCT and MDCT resulted from differences in kV and scatter-to-primary ratio (SPR), and, as expected, contrast and CNR were better at the periphery than the center of FPCT images.
The results of this study align most closely with those of Stuewe et al., 7 indicating that, when using clinical modes of acquisition, radiation doses from FPCT are 100-200% higher than those from MDCT. Kwok et al. found that radiation doses during abdominal imaging using fixed techniques were about 50% higher in FPCT compared to MDCT. 9 The results of this study are in contrast to those of Bai et al., who found that when using fixed techniques radiation doses from MDCT were 20% higher than FPCT. 8 Measured MTF values in this study (0.5 MTF and 0.1 MTF, Table 4 to FBP depending on the task, 17,18 and model-based reconstruction may offer dose reduction from 47-89% compared to FPB, depending on the task. 19 While iterative reconstruction and model-based reconstruction have spatial resolution performance that is similar to FBP for high contrast tasks, their performance is inferior to FPB for low-contrast tasks at reduced doses. 17,19 Finally, only a single model of FPCT and MDCT, from a single manufacturer, was studied.

| CONCLUSION
When a single robotic angiographic C-arm with FPCT capability and a FPCT had slightly higher spatial resolution at higher spatial frequencies.
Contrast and CNR were similar between the two modalities. Z-axis resolution was slightly better for MDCT compared to FPCT. In light of these results, it is reasonable to consider that other differences between MDCT and FPCT, such as the larger FOV, faster data acquisition time, and increased projection sampling in MDCT compared to FPCT, may be more important than the differences in fundamental image quality and radiation dose metrics between the modalities. It is not clear to what extent these results can be generalized, as the FPCT and MDCT systems studied were from a single manufacturer and were operated under automatic exposure control.

CONF LICTS OF INTEREST
This work was funded in part by a contract with Siemens Medical Systems, Inc. which is a declared Conflict of Interest for A. Kyle Jones. Bruno C. Odisio has a contract with Siemens Medical Systems, Inc. that is not related to the present work. as all modern CT scanners are, in fact, cone beam CT systems, using a wide detector array along the z axis, and therefore a large cone angle, for imaging. The term cone beam CT (CBCT) is no longer specific to FPCT technology implemented on an interventional C-arm.