Dottorato in MORFOGENESI E INGEGNERIA TISSUTALE

Titolo della tesi: VARIABLE TILT-ANGLE, PARALLEL-HOLE COLLIMATION SYSTEM FOR HIGH-RESOLUTION MOLECULAR IMAGING GAMMA TOMOSYNTHESIS

The single-photon emission computed tomography (SPECT) is a nuclear medicine technique based on photon detection emitted by a radiotracer injected into the patient’s body. Imaging techniques based on nuclear medicine, which are able to show tissue function, are very often used by medical offices to detect small lesions. However, conventional SPECT systems are not suitable for detecting sub-millimeter lesions (often harbor early stage cancer), as they are large, bulky and designed for general-purpose imaging. To overcome the limitations of commonly used devices, a useful strategy is to perform imaging reducing the patient-to-collimator distance. The present study investigates a novel gamma tomosynthesis (GT) based on a variable tilt angle, parallel-hole collimator (VAPHC) which, mounting to a conventional gamma, is able to perform high-resolution three-dimensional imaging. Rather than rotate the gamma camera around the patient, the VAPHC allows limited angle, tomographic acquisition while the detector remains stationary and flush against the organ to be imaged. This design minimizes the object-to-detector distance for high spatial resolution/sensitivity trade-off. The detection device is located in a fixed position, at the minimum distance from the patient, improving spatial resolution capabilities especially in detection of small lesions. The VAPHC has the remarkable feature to be modular, consisting of independent collimation elements able to slant according to variable angles (from -45° to +45°). The proposed device is capable to acquire planar projection images at different angles, which are then arranged together through the Shift And Add (SAA) method in order to obtain the three-dimensional reconstruction of the studied object. The aim of the project is to demonstrate that the VAPHC allows to capture 3D molecular images at higher sensitivity and spatial resolution than current SPECT scans and that it could provide better contrast of tumors in phantom studies. To this purpose, the technical characteristics of the presented devices were studied and a theoretical analysis on collimator spatial resolution and sensitivity, including Depth Of Interaction (DOI) effects and magnification, were performed. The theoretical results were compared to Monte Carlo simulation and to experimental results obtained with laboratory equipment, showing a good agreement. Moreover, sensitivity, spatial resolution and imaging potentials of the VAPHC with a clinical gamma camera were evaluated. Spatial resolutions were measured in reconstructed GT images using a point source at different source to- collimator distances, while sensitivity was evaluated over the range of slant angles using a disk-source. Image contrast (IC) and contrast-to-noise-ratio (CNR) of sub-centimeters tumors were evaluated using a breast phantom containing a background activity and spheres filled with 99mTc to simulate lesions at two depths. Breast phantom GT images were compared with planar and circular-orbit SPECT acquisitions of equal scan-time. The imaging performances and the ability of GT in preserving axial resolution were also tested by using the Mini Defrise phantom. Planar spatial resolutions range from 9 to 14 mm over a depth range of 6-10 cm; spatial resolution in depth dimension becomes two times greater than those in the other dimensions. The measured sensitivity decreases from 9 cps/microCi to 6 cps/microCi varying the slant angle from 5° to 45°. The measured IC and CNR of GT reconstructed phantom images demonstrated that it was possible to improve the spatial resolution/sensitivity trade-off. The experiment with the Mini Defrise phantom demonstrated that the reconstruction is suitable in preserving axial resolution. The measures carried out and the employment conditions offer a broad view of the potential of this new apparatus: it demonstrated the potential for superior spatial resolution, IC and CNR compared to planar and SPECT acquisitions. This could result in a substantial improvement of diagnostic ability in particular for lesions placed in the vicinity of the patient’s skin. A conventional gamma camera equipped with the VAPHC could be located at the minimum distance from the patient, thus improving detection, localization and characterization of sub-centimeter lesions. Our experience indicates that the presented collimation system could represent in the near future an important advance in developing devices dedicated to imaging of small lesions, without the need to rotate the system around the patient’s body but adapting the acquisition to clinical needs.