Imaging techniques for guidance of radionuclide therapy

Abstract

The aim of this thesis is to investigate imaging techniques for guidance of radionuclide therapy. The first part describes the development of instrumentation for guidance of radionuclide therapy.

To date, no real-time hybrid imaging modalities for interventional purposes have been developed that combine simultaneously acquired nuclear and anatomic images. Real-time functional imaging in concert with anatomic imaging would provide the physician with valuable information during the procedure, thereby improving therapeutic efficiency. Procedures that can potentially benefit from real-time simultaneous hybrid imaging include liver radioembolization, biopsies. Measurements with our hybrid imaging prototype device have shown that simultaneous fluoroscopic and nuclear imaging of the same field of view is feasible.

Accurate determination of the system parameters that describe the position of the x-ray tube, x-ray detector, gamma camera and collimators is crucial to optimize image quality. A calibration method was presented that improves the resolution and co-registration of simultaneously acquired hybrid fluoroscopic and nuclear images by estimating the geometric parameter set.

The second part of this thesis concerns the quality of Single Photon Emission Computed Tomography (SPECT) images of high-energy photon-emitting isotopes. Methods to improve the quality of high-energy SPECT images are introduced, and the implications of reduced image quality in clinical practice are discussed.

In SPECT using high-energy photon-emitting isotopes, such as 131I, parallel-hole collimators with thick septa are required to limit septal penetration, at the cost of sensitivity and resolution. To this end, we propose a collimator design (the parallel-cone (PC) collimator) consisting of a repetitive grid of parallel cones, capable of improving the image quality of high-energy SPECT. High-energy SPECT imaging with a single slice prototype of the proposed PC collimator has shown the potential for significantly improved image quality in comparison with standard parallel hole collimators.

Radioiodine therapy with 131I is used for treatment of suspected recurrence of differentiated thyroid carcinoma. Pretherapeutic 124I Positron Emission Tomography/Computed Tomography (PET/CT) with a low activity (∼1% of 131I activity) can be performed to determine whether uptake of 131I, and thereby the desired therapeutic effect, may be expected. However, false-negative 124I PET/CT results as compared with posttherapeutic 131I SPECT/CT have been reported by several groups. Phantom measurements showed that the reported discrepancies may be ascribed to a difference in lesion detectability between 124I PET/CT and 131I SPECT/CT and, hence, higher 124I activities may be warranted to obtain equal detectability.

Quantitative SPECT imaging of high-energy isotopes such as 131I remains a challenge, because of scatter and collimator penetration. The quality of SPECT images can be improved by incorporating scatter and collimator-detector-response (CDR) models in the reconstruction. By performing phantom measurements, it was shown that the reconstruction method incorporating Monte Carlo based scatter correction and simulated CDR modelling produced intrinsically quantitative 131I SPECT images with contrast recovery coefficient similar to the Triple Energy Window scatter correction method, without the use of experimentally determined weighting factors.