Quantitative single photon emission tomography: verification for sources in an elliptical water phantom

Quantitation in SPECT using an effective model of the scattering

A new method for correcting simultaneously the attenuation, scatter and resolution effects in SPECT has been developed for the case of a homogeneous medium. It is based on an effective model of the scatter process. This model depends on only four parameters which are determined experimentally and remain independent of the geometry and of the dimensions of the scatter medium. The method uses the data from the peak events and does not need additional energy windows on the scattered events. An original filter is proposed to remove the noise due to the poor statistics of clinical data. Tests on phantoms varying in size and activity show that the model allows absolute activity determination with an accuracy of a few per cent.

Parameters influencing volume and activity quantitation in SPECT

Several factors influence the accuracy to which information on the in vivo activity concentration can be obtained from SPECT images. The accuracy in image contrast and the quantitation of volume and activity has here been examined for a SPECT system aimed for absorbed dose determinations in systemic radiation therapy. The influence of source dimensions, object contrast, noise level, energy window width and the reconstruction filter was examined. All the investigated parameters affected the recoveries. Volumes of above 20 cm3 could be quantified to within +/- 20%, for object contrasts of above 96%, and within +/- 40% for object contrasts above 81%, providing limited levels of noise. Both the image contrast and the volume quantification is expected to be improved in situations with low object contrasts, if accurate correction for scattered photons can be developed. Obtained limitations in the quantitation of activity with the present method should be reduced by development of a more accurate background subtraction technique.

Radio-immunotherapy dosimetry with special emphasis on SPECT quantification and extracorporeal immuno-adsorption

Results from therapeutic trials with radiolabelled monoclonal antibodies are difficult to compare, because of lack of accurate macroscopic and microscopic dosimetry for both tumours and normal tissues. Requirements for such a dosimetry are covered in the paper. Accurate in vivo dosimetric measurement techniques for verification of calculated absorbed doses are also needed to verify treatment planning. In the review, important topics related to dosimetry in therapeutic trials in RIT are covered, such as, absorbed-dose calculations and activity-quantification techniques for planar imaging and SPECT. The latter is particularly discussed, including a summary of different correction techniques. Absorbed-dose calculations and treatment-planning techniques are also discussed. Possible ways of enhancing the therapeutic ratio are reviewed, especially the novel technique with extracorporeal immuno-adsorption. The review could form the basis of the development of future treatment-planning protocols and for dosimetry calculations in radio-immunotherapy, considering some of the most important parameters for approaching an accurate in vivo dosimetry.

Small animal imaging with pinhole single-photon emission computed tomography

BACKGROUND

High resolution spatial details of the distribution of activity in three dimensions is required to evaluate the localization and dosimetric properties of radiolabelled monoclonal antibodies in tumors and normal tissues. Planar imaging of small animals with a resolution of 5-10 mm is usually the imaging modality of choice. The authors investigated high resolution single-photon emission computed tomographic (SPECT) imaging, based on a rotating pinhole scintillation camera. Although the sensitivity of the pinhole collimator is low, several radionuclides offer suitable decay properties to perform pinhole SPECT, especially in conjunction with high activity levels used in radioimmunotherapy.

METHODS

Transverse, sagittal, and coronal sections were reconstructed using a three-dimensional cone-beam algorithm, which is a generalization of the two-dimensional fan-beam filtered backprojection algorithm. Before reconstruction, the pinhole projections were corrected for the decay of the radionuclide, geometric and intrinsic efficiency variations of the camera system, and center of rotation shift.

RESULTS

The spatial resolution at 50 mm from the pinhole collimator with 3.3 mm aperture was 3.4 mm, and the sensitivity 7.2 c/s microCi for technetium-99m. With the 2 mm collimator the resolution was 2.2 mm, and the sensitivity was 2.6 c/s/microCi. To show the spatial resolution in vivo, a rat was injected with 185 MBq of technetium-99m-methylene diphosphonate or with 5 mCi technetium-99m-hexamethylpropylene amine oxime. The bone structures were well delineated in the methylene diphosphonate image, and in the hexamethylpropylene amine oxime image, the brain was nicely shown. For comparison a magnetic resonance image for the same section was done.

CONCLUSIONS

High resolution SPECT imaging with the pinhole collimator provides mapping of the activity in three-dimensions, needed for more detailed biodistribution data and to perform more accurate dosimetry.