This approach provides a method for assessing the task-based MTF of a CT system using conventional and iterative reconstructions. Results demonstrated that the object-specific MTF can vary as a function of dose and contrast. The analysis highlighted the paradigm shift for iterative reconstructions when compared to FBP, where iterative reconstructions generally offer superior noise performance but with varying resolution as a function of dose and contrast. The MTF(Task) generated by this method is expected to provide a more comprehensive assessment of image resolution across different reconstruction algorithms and imaging tasks.
For equal image quality as measured by low contrast detectability, the CTDI(VOL) of a FKS head and body exam is roughly 22% and 14% higher than that of a routine single-energy head and body exam, respectively, for the phantom measured.
Purpose: Recently, a fast-kVp switching (FKS) dual-energy method has been presented with clinical and phantom results to demonstrate its efficacy. Patient dose concern has been raised on FKS dual-energy since it involves higher energy acquisition at 140 kVp and slower gantry rotation time (e.g., 0.9-1 s) as opposed to 0.5 s as used in routine single-energy exams. The purpose of our study was to quantitatively compare the CTDI VOL of FKS and routine CT exams under the body and head conditions. Methods: For a fair comparison, we have to overcome the difficulty of unmatched protocols between FKS and routine CT exams. In this paper, we propose to match the low contrast detectability (LCD), a critical image quality metric impacting diagnostic quality, before measuring CTDI VOL . The kVp pair, flux ratio, and optimal monochromatic energy have been carefully optimized for FKS protocols prior to the comparison. Our baseline single-energy protocols were per IEC-61223-3-5 under head and body conditions except for mA, which was iteratively adjusted to match the LCD of FKS. CTDI VOL was measured using either a 16 cm (for head scanning) or a 32 cm (for body scanning) PMMA phantom of at least 14 cm in length. The LCD was measured using the uniform section of Catphan 600. To make the study repeatable, the automated statistical LCD measurement tool available on GE Discovery CT750 scanner was used in this work. A visual LCD phantom and a Gammex tissue characterization phantom were also employed to verify the statistical LCD measurements and to introduce various patient sizes and contrast levels. Results: The mean CTDI VOL for the head and body single-energy acquisitions was 57.5 and 29.2 mGy, respectively. The LCD was measured at 0.45% and 0.42%, respectively. The average CTDI-VOL for FKS head and body scans was 70.4 and 33.4 mGy, respectively. The corresponding LCD was measured at 0.45% and 0.43%, respectively. The results from the visual LCD phantom and Gammex phantom supported the statistical LCD measurements. Conclusions: For equal image quality as measured by low contrast detectability, the CTDI VOL of a FKS head and body exam is roughly 22% and 14% higher than that of a routine single-energy head and body exam, respectively, for the phantom measured.
The performance of high-sensitivity x-ray imagers may be limited by additive instrumentation noise rather than by quantum noise when operated at the low exposure rates used in fluoroscopic procedures. The equipment-invasive instrumentation noise measures (in terms of electrons) are generally difficult to make and are potentially not as helpful in clinical practice as would be a direct radiological representation of such noise that may be determined in the field. In this work, we define a clinically relevant representation for instrumentation noise in terms of noise-equivalent detector entrance exposure, termed the instrumentation noise-equivalent exposure (INEE), which can be determined through experimental measurements of noise-variance or signal-to-noise ratio (SNR). The INEE was measured for various detectors, thus demonstrating its usefulness in terms of providing information about the effective operating range of the various detectors. A simulation study is presented to demonstrate the robustness of this metric against post-processing, and its dependence on inherent detector blur. These studies suggest that the INEE may be a practical gauge to determine and compare the range of quantum-limited performance for clinical x-ray detectors of different design, with the implication that detector performance at exposures below the INEE will be instrumentation-noise limited rather than quantum-noise limited.
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