Browsing by Subject "cone beam"

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    Characterization of Image Quality for 3D Scatter Corrected Breast CT Images.
    (2012) Pachon, Jan Harwin
    The goal of this study was to characterize the image quality of our dedicated, quasi-monochromatic spectrum, cone beam breast imaging system under scatter corrected and non-scatter corrected conditions for a variety of breast compositions. CT projections were acquired of a breast phantom containing two concentric sets of acrylic spheres that varied in size (1-8mm) based on their polar position. The breast phantom was filled with 3 different concentrations of methanol and water, simulating a range of breast densities (0.79-1.0g/cc); acrylic yarn was sometimes included to simulate connective tissue of a breast. For each phantom condition, 2D scatter was measured for all projection angles. Scatter-corrected and uncorrected projections were then reconstructed with an iterative ordered subsets convex algorithm. Reconstructed image quality was characterized using SNR and contrast analysis, and followed by a human observer detection task for the spheres in the different concentric rings. Results show that scatter correction effectively reduces the cupping artifact and improves image contrast and SNR. Results from the observer study indicate that there was no statistical difference in the number or sizes of lesions observed in the scatter versus non-scatter corrected images for all densities. Nonetheless, applying scatter correction for differing breast conditions improves overall image quality.
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    CONE BEAM COMPUTED TOMOGRAPHY (CBCT) DOSIMETRY: MEASUREMENTS AND MONTE CARLO SIMULATIONS
    (2010) Kim, Sangroh

    Cone beam computed tomography (CBCT) is a 3D x-ray imaging technique in which the x-ray beam is transmitted to an object with wide beam geometry producing a 2D image per projection. Due to its faster image acquisition time, wide coverage length per scan, and fewer motion artifacts, the CBCT system is rapidly replacing the conventional CT system and becoming popular in diagnostic and therapeutic radiology. However, there are few studies performed in CBCT dosimetry because of the absence of a standard dosimetric protocol for CBCT. Computed tomography dose index (CTDI), a standardized metric in conventional CT dosimetry, or direct organ dose measurements have been limitedly used in the CBCT dosimetry.

    This dissertation investigated the CBCT dosimetry from the CTDI method to the organ, effective dose, risk estimations with physical measurements and Monte Carlo (MC) simulations.

    An On-Board Imager (OBI, Varian Medical Systems, Palo Alto, CA) was used to perform old and new CBCT scan protocols. The new CBCT protocols introduced both partial and full angle scan modes while the old CBCT protocols only used the full angle mode. A metal-oxide-semiconductor-field-effect transistor (MOSFET) and an ion chamber were employed to measure the cone beam CTDI (CTDICB) in CT phantoms and organ dose in a 5-year-old pediatric anthropomorphic phantom. Radiochromic film was also employed to measure the axial dose profiles. A point dose method was used in the CTDI estimation.

    The BEAMnrc/EGSnrc MC system was used to simulate the CBCT scans; the MC model of the OBI x-ray tube was built into the system and validated by measurements characterizing the cone beam quality in the aspects of the x-ray spectrum, half value layer (HVL) and dose profiles for both full-fan and half-fan modes. Using the validated MC model, CTDICB, dose profile integral (DPI), cone beam dose length product (DLPCB), and organ doses were calculated with voxelized MC CT phantoms or anthropomorphic phantoms. Effective dose and radiation risks were estimated from the organ dose results.

    The CTDICB of the old protocols were found to be 84 and 45 mGy for standard dose, head and body protocols. The CTDICB of the new protocols were found to be 6.0, 3.2, 29.0, 25.4, 23.8, and 7.7 mGy for the standard dose head, low dose head, high quality head, pelvis, pelvis spotlight, and low dose thorax protocols respectively. The new scan protocols were found to be advantageous in reducing the patient dose while offering acceptable image quality.

    The mean effective dose (ED) was found to be 37.8 ±0.7 mSv for the standard head and 8.1±0.2 mSv for the low dose head protocols (old) in the 5-year-old phantom. The lifetime attributable risk (LAR) of cancer incidence ranged from 23 to 144 cases per 100,000 exposed persons for the standard-dose mode and from five to 31 cases per 100,000 exposed persons for the low-dose mode. The relative risk (RR) of cancer incidence ranged from 1.003 to 1.054 for the standard-dose mode and from 1.001 to 1.012 for the low-dose mode.

    The MC method successfully estimated the CTDICB, organ and effective dose despite the heavy calculation time. The point dose method was found to be capable of estimating the CBCT dose with reasonable accuracy in the clinical environment.