A Method to Optimize the Image Acquisition Protocol of a MicroCT Unit for Preclinical Studies Using Contrast-Enhanced Digital Subtraction

摘要

Contrast-enhanced digital subtraction (CEDS) imaging refers to radiological techniques based on the use of a contrast medium (generally iodinated) to increase the contrast between organs of interest and background anatomy in medical images. In this work, a commercial microCT (voltage range 30-45 kV, not specifically designed for quantitative studies) was characterized for use in preclinical CEDS quantitative studies. The characterization of the system consisted of quantifying and optimizing the performance of the X-ray tube and detector. The optimization was done on 2D images. A protocol to acquire original images (pre-subtraction) based on the characterization was established. The protocol faced the detector limitations, such as lack of stability of the response and non-uniformity in images of homogeneous objects. Radiological techniques (voltage and additional filtering) for studies with CEDS were optimized. Single energy temporal (SET) and dual energy (DE) subtractions modalities were studied. The optimization was done via subtracted images of calibrated iodine phantoms. The optimal technique was defined as the one that maximized a figure of merit given by the ratio of an image quality parameter (the square of the contrast to noise ratio) to the dose delivered. Dose was obtained using TLD-100 and TLD-300 thermoluminescent dosimeters and dose to water was evaluated inside a 2 cm thick acrylic phantom. The energy dependence of the TLD-100 response was estimated to be a function of the beam effective energy measured with TLD-300. After applying the protocol, it was possible to obtain subtracted images whose mean pixel value was a linear function of the iodine concentration in the object. Application of the protocol reduced the detector spatial and temporal variations down to ±10% and 3.7%, respectively. For SET subtraction, the optimal technique was 45 kV; for this, the measured beam effective energy at the central point inside the phantom was 20.2 ± 1.6 keV. For DE, 30 kV and 45 kV + 0.025 mm Mo (external filtering) was the optimal combination, and the measured beam effective energies were 20.0 ± 2.2 keV and 27.4 ± 3.0, respectively. Dose to water in SET and DE were 29 ± 5 and 14 ± 2 mGy, respectively. We have provided a standardized method to be used in the CEDS studies.