Volume 29, Issue 4, April 2002
Index of content:
- PH. D. THESES ABSTRACTS
Multislice helical computed tomography: Image reconstruction algorithms, implementations, and clinical evaluations29(2002); http://dx.doi.org/10.1118/1.1464549View Description Hide Description
The development of multislice computed tomography (MSCT), which incorporates a multirow detector, has been desired for higher longitudinal (z) spatial resolution, longer volume coverage, shorter scan time, and other clinical requirements. Therefore, the purpose of this study was to develop one of the key technologies, image reconstruction algorithms, that cover most scanning applications: (1) step-and-shoot (nonhelical) scan, (2) standard helical reconstruction, (3) higher temporal resolution and dynamic volumetric (ciné) imaging, and (4) better in-plane (trans-axial) spatial resolution.First, applying the fan-beam reconstruction algorithm was validated for the step-and-shoot mode: there is no practical problem in MSCT with narrow cone angle. Second, helical filter interpolation (HFI) with z filtering was proposed for standard helical reconstruction after a unique z-sampling pattern in multislice helical CT was investigated. HFI can increase the helical pitch three times as fast as the single-slice CT while keeping the same image quality. Clinical evaluation demonstrated the usefulness of HFI. This method has been implemented on most of the multislice helical CT scanners available worldwide. Third, helical half-scanning with time-shift (TS) technique was used to overcome the insufficient in-plane temporal resolution of HFI. This improved the image quality of cardiac, lung, and vascular imaging substantially. TS not only allows us to take full advantage of the high temporal resolution but also enables ciné imaging even in helical scanning. Last, two algorithms were proposed for improving the in-plane spatial resolution. They successfully depict fine structures such as the otitis chain in the middle ear.
29(2002); http://dx.doi.org/10.1118/1.1464550View Description Hide Description
On the basis of phase space (PS) data Monte Carlo (MC) beam models for a 6 and 15 MV photon beam have been developed. This PS information was used to develop simple point source beam models. Since these models are lacking in flexibility and show limitations in modeling the beam outside the geometric radiation field a multiple sourcemodel (MSM) was developed. Original PS data and those reproduced by the MSM are then compared in terms of fluences and mean energies, which all agree within 1%. MC-calculated primary photon energy spectra are verified by energy spectra derived from transmission measurements. Comparisons of MC-calculated depth doses and profiles, using original and reproduced PS data, agree within 1% and 1 mm. Deviations from measured doses are within 1.5% and 1 mm. Additionally, the MSM was supplemented with a model of an 80-leaf MLC. Using this supplemented MSM, dose distributions are calculated in water phantoms. Two test fields for the step-and-shoot technique and two realistic fields delivered with dynamic MLC are investigated. MC-calculated dose profiles are in excellent agreement with measured data from film dosimetry. To assess spectral changes caused by the MLC, energy spectra in portal planes are calculated for moving slits of different widths. The investigation shows an increase in mean photon energy of up to 16% for the 0.25 cm slit compared to the open beam for 6 MV and of up to 6% for 15 MV, respectively. The supplemented MSM has proven to be a powerful tool for investigational and benchmarking purposes and for IMRTtreatment planning.