Geometric models in dosimetry of thyroid remnant mass.

AIM: Absorbed dose to thyroid remnant tissue after 131I ablation becomes mass/size-dependent. This is a direct consequence of the small remnant size and radiation escape starts to be relevant. The self-absorbed fraction becomes mass/size-dependent. We have used Monte Carlo simulations to investigate the influence of the thyroid remnant shape upon the absorbed fraction calculation. METHODS: Thyroid residue was modeled using spherical, cylindrical and elliptical shapes. Uniform beta activity distribution and unit density medium (water) within a remnant was assumed. For each of the geometrical models beta self-absorbed fraction (varphi(beta)) was calculated using Monte Carlo codes, while the mean absorbed dose per unit cumulated activity (S(beta)) was calculated using MIRD formalism. RESULTS: For spherical objects varphi(mono) for mean beta energy (E = 0.182 MeV) of 131I is always greater than varphi(beta) calculated for the complete beta spectrum. For spheres having diameters 2-6 mm and assumption varphi(beta) = 1, S(beta) is over-estimated by 11-37%. For cylinder and prolate spheroid of the same length and thickness, S(beta) for cylinder is 30% smaller because of the greater mass. Similarly, elliptical cylinder and general ellipsoid of the same length and the same perpendicular dimensions (width and breadth), have similar varphi(beta), while S(beta) for elliptical cylinder is correspondingly smaller. CONCLUSION: For accurate dosimetry of thyroid remnants having masses <1 g and chordal lengths <1 cm it is necessary to calculate varphi(beta) for the full beta spectrum, or S(beta) will be overestimated. The shape of the remnant may also be important since elongated non-spherical objects may also have varphi(beta) < 1.

Triple-head gamma camera PET: system overview and performance characteristics.

Positron emission tomography (PET) is currently performed using either a dedicated PET scanner or scintillation gamma camera equipped with electronic circuitry for coincidence detection of 511 keV annihilation quanta (gamma camera PET system). Although the resolution limits of these two instruments are comparable, the sensitivity and count rate performance of the gamma camera PET system are several times lower than that of the PET scanner. Most gamma camera PET systems are manufactured as dual-detector systems capable of performing dual-head coincidence imaging. One possible step towards the improvement of the sensitivity of the gamma camera PET system is to add another detector head. This work investigates the characteristics of one such triple-head gamma camera PET system capable of performing triple-head coincidence imaging. The following performance characteristics of the system were assessed: spatial resolution, sensitivity, count rate performance. The spatial resolution, expressed as the full width at half-maximum (FWHM), at 1 cm radius is 5.9 mm; at 10 cm radius, the transverse radial resolution is 5.3 mm, whilst the transverse tangential and axial resolutions are 8.9 mm and 13.3 mm, respectively. The sensitivity for a standard cylindrical phantom is 255 counts.s(-1).MBq*(-1)), using a 30% width photopeak energy window. An increase of 35% in the PET sensitivity is achievable by opening an additional 30% width energy window in the Compton region. The count rate in coincidence mode, at the upper limit of the systems optimal performance, is 45 kc.s(-1) (kc=kilocounts) using the photopeak energy window only, and increases to 60 kc.s(-1) using the photopeak + Compton windows. Sensitivity results are compared with published data for a similar dual-head detector system.

Determination of small objects from pinhole scintigrams.

This study assessed the possibility of measuring the linear dimensions of small structures using pinhole scintigraphy. A number of glass objects were made with a spherical, cylindrical or conical shape. Their maximum dimensions (diameters and heights) were 3.5-22.5 mm. These glass objects were filled with 131I, placed inside a plastic neck phantom and imaged using a gamma camera equipped with a pinhole collimator. The source-to-collimator distance was varied from 2 to 12 cm. An algorithm for image segmentation (threshold selection) was used to divide the image into object and background. On the segmented image, the number of non-zero pixels in the direction of the principal axes was multiplied by the appropriate calibration factor to obtain the linear dimensions of the object. Spatial resolution of the pinhole collimator, expressed as the full-width at half-maximum (FWHM), varied from 8 to 10 mm for the range of source-to-collimator distances examined. We found that, for dimensions up to 1.5 x FWHM, finite spatial resolution affects the accuracy of measurement. Non-linear correlation between true and calculated dimensions was used to take the latter into account. Our results are now being used to improve quantitation of remnant thyroid tissue masses for the calculation of radioiodine ablation doses.