| dc.description.abstract |
<p>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.</p>
<p>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.</p>
<p>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 (CTDI<sub>CB</sub>) 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.</p>
<p>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,
CTDI<sub>CB</sub>, dose profile integral (DPI), cone beam dose length product (DLP<sub>CB</sub>),
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.</p>
<p>The CTDI<sub>CB</sub> of the old protocols were found to be 84 and 45 mGy for standard
dose, head and body protocols. The CTDI<sub>CB</sub> 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.</p>
<p>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.</p>
<p>The MC method successfully estimated the CTDI<sub>CB</sub>, 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.</p>
|
|
