Monoclonal antibodies for radioimmunotargeting are often tested in tumour bearing nude mice. In vivo determination of the uptake of the monoclonal antibody in the tumour requires quantitative scintigraphy, and this in turn requires an adequate method for subtraction of radiation from the normal tissue. For this reason, two different methods for background subtraction were evaluated, a contralateral background region of interest or an irregular one, surrounding the tumour. A pinhole collimator was used for the scintigraphy and the monoclonal antibodies were labelled with 125I. Furthermore, a method was developed for estimation of the mean absorbed dose in the tumour from these repeated quantitative scintigraphic measurements. This requires that the tumour mass can be accurately estimated in vivo. Finally, the results were compared with in vitro measurements of the uptake.

A method for attenuation and scatter correction of brain single photon emission computed tomography (SPECT) is described where computed tomography (CT) images of the brain are used for the calculation of attenuation maps. The method is evaluated for the substance 99mTc hexamethylpropylene amine oxime. A transmission dependent scatter correction is utilized and is based on ray sums calculated through the attenuation map. A method based on external markers is used to align the SPECT and CT image volumes. The markers need only to be present during the SPECT acquisition since the corresponding landmarks can be found without markers on the CT images. The mismatching has been investigated for five patients who have undergone both a CT examination and a SPECT examination with markers. Twelve individuals from the staff have pointed out the landmarks on the CT images, with an average standard deviation of 3.4 mm. Reconstructions with an attenuation map shifted the corresponding 95% confidence interval have been performed to obtain an estimation of the quantitative uncertainty caused by the mismatching, and quantitative errors of up to 6.3% have been measured. At present the method is probably most useful when groups of patients are studied.

Transmission-dependent convolution subtraction has been shown to be useful when correcting for malpositioned scattered events in single photon emission computed tomography (SPECT). The method is based on convolution subtraction but includes a matrix of scatter fractions instead of a global scatter fraction. In this study, this method is evaluated for regional cerebral blood flow SPECT with 99mTc-hexamethyl propylene-amine oxime (HMPAO) by using Monte Carlo simulations. Different geometries for generating the scatter fractions as a function of the attenuation path length are studied and compared. The most optimal value of the exponential describing the falloff of the monoexponential scatter kernel is determined for each geometry. The method is also compared with convolution subtraction with a global scatter fraction. It is shown that the most optimal of the tested geometries is a homogeneous activity distribution. A scatter kernel with an exponential of 0.15 pixel-1 is most optimal for this geometry. A comparison with convolution subtraction shows that transmission-dependent convolution subtraction can give more accurate results if used with optimal parameters.

In single photon emission computed tomography (SPECT), transmission-dependent convolution subtraction has been shown to be useful when correcting for scattered events. The method is based on convolution subtraction, but includes a matrix of scatter fractions instead of a global scatter fraction. The method can be extended to iteratively improve the scatter estimate, but in this note we show that this requires a modification of the theory to use scatter-to-total scatter fractions for the first iteration only and scatter-to-primary fractions thereafter. To demonstrate this, scatter correction is performed on a Monte Carlo simulated image of a point source of activity in water. The modification of the theory is compared to corrections where the scatter fractions are based on the scatter-to-total ratio, using one and ten iterations. The resulting ratios of subtracted to original counts are compared to the true scatter-to-total ratio of the simulation and the most accurate result is found for our modification of the theory.

To use conventional ray tracing methods in Monte Carlo simulation of the collimator in a scintillation camera system can be time consuming. It is however necessary to take collimator interactions into account when simulating radionuclides emitting high-energy photons that can penetrate the septa in the collimator. In this work a statistical collimator algorithm, based on the Delta-Scattering method, is evaluated using 123I. The evaluation is performed by comparing results from Monte Carlo simulations and measurements for a scintillation camera system, using point sources and a nonhomogeneous brain phantom. A good agreement can be seen for both images and energy spectra.