Challenge by the murine brain: multi-pinhole SPECT of 123I-labelled pharmaceuticals

Reducing multiplexing artifacts in multi-pinhole SPECT with a stacked silicon-germanium system: a simulation study

In pinhole single photon emission computed tomography (SPECT), multi-pinhole collimators can increase sensitivity but may lead to projection overlap, or multiplexing, which can cause image artifacts. In this work, we explore whether a stacked-detector configuration with a germanium and a silicon detector, used with 123I (27-32, 159 keV), where little multiplexing occurs in the Si projections, can reduce image artifacts caused by highly-multiplexed Ge projections. Simulations are first used to determine a reconstruction method that combines the Si and Ge projections to maximize image quality. Next, simulations of different pinhole configurations (varying projection multiplexing) in conjunction with digital phantoms are used to examine whether additional Si projections mitigate artifacts from the multiplexing in the Ge projections. Reconstructed images using both Si and Ge data are compared to those using Ge data alone. Normalized mean-square error and normalized standard deviation provide a quantitative evaluation of reconstructed images' error and noise, respectively, and are used to evaluate the impact of the additional nonmultiplexed data on image quality. For a qualitative comparison, the differential point response function is used to examine multiplexing artifacts. Results show that in cases of highly-multiplexed Ge projections, the addition of low-multiplexed Si projections helps to reduce image artifacts both quantitatively and qualitatively.

Advanced brain dopamine transporter imaging in mice using small-animal SPECT/CT

Background

Iodine-123-β-CIT, a single-photon emission computed tomography (SPECT) ligand for dopamine transporters (DATs), has been used for in vivo studies in humans, monkeys, and rats but has not yet been used extensively in mice. To validate the imaging and analysis methods for preclinical DAT imaging, wild-type healthy mice were scanned using ¹²³I-β-CIT.

Methods

The pharmacokinetics and reliability of ¹²³I-β-CIT in mice (n = 8) were studied with a multipinhole SPECT/CT camera after intravenous injection of ¹²³I-β-CIT (38 ± 3 MBq). Kinetic imaging of three mice was continued for 7 h postinjection to obtain the time-activity curves in the striatum and cerebellum volumes. Five mice had repeated measures 4 h post-¹²³I-β-CIT injection to provide an indication of test-retest reliability. The same five mice served as a basis for a healthy mean SPECT template.

Results

Specific binding of ¹²³I-β-CIT within the mouse striatum could be clearly visualized with SPECT. The kinetics of ¹²³I-β-CIT was similar to that in previously published autoradiography studies. Binding potential mean values of the test-retest studies were 6.6 ± 15.7% and 6.6 ± 4.6%, respectively, and the variability was 9%. The SPECT template was aggregated from the first and second imaging of the test-retest animals. No significant difference between the templates (P > 0.05) was found. From the test template, a striatal volume of 22.3 mm³ was defined.

Conclusions

This study demonstrates that high-resolution SPECT/CT is capable of accurate, repeatable, and semiquantitative measurement of ¹²³I-β-CIT DAT binding in the mouse brain. This methodology will enable further studies on DAT density and neuroprotective properties of drugs in mice.

Three-dimensional quantitation of regional cerebral blood flow in mice using a high-resolution pinhole SPECT system and 123I-iodoamphetamine

INTRODUCTION

This study is intended to evaluate the feasibility of using a high-resolution pinhole SPECT system and iodine-123-N-isopropyl-4-iodoamphetamine ((123)I-IMP) for three-dimensional (3D) absolute quantitation of regional cerebral blood flow (rCBF) in mice.

METHODS

The pinhole SPECT system consists of a rotating stage and a pinhole collimator attached to a clinical gamma camera. The collimator's focal length is 251 mm. Phantom studies were performed to evaluate sensitivity and full-width half-maximum (FWHM) spatial resolution. The aperture-to-object distance was 15 mm. Six mice were studied. Cerebral infarctions were induced by ligating and disconnecting the distal portion of the left middle cerebral artery. Ex vivo SPECT studies were performed using harvested brains and skulls. The CBF volumetric image was computed using the standardized input function.

RESULTS

Excellent spatial resolution of 0.9-mm FWHM and uniform sensitivity throughout the 3D volume were demonstrated in the phantom experiments. The CBF images showed a defect in the infarcted areas and a reduction of CBF values in the infarcted region as compared with the control region.

CONCLUSIONS

This study demonstrated the feasibility of the 3D quantitation of rCBF in mice using a high-resolution pinhole SPECT system and (123)I-IMP.

Neuroimaging in psychiatric disorders

In psychiatry, neuroimaging facilitates the diagnosis of psychiatric disorders and the development of new medications. It is used to detect structural lesions causing psychosis and to differentiate depression from neurodegenerative disorders or brain tumors. Functional neuroimaging, mostly in the form of molecular neuroimaging with positron emission tomography or single photon emission tomography, facilitates the identification of therapeutic targets, the determination of the dose of a new drug needed to occupy its target in the brain, and the selection of patients for clinical trials.

Systems biology through mouse imaging centers: experience and new directions

The completed sequencing of genomes has forced upon us the challenge of understanding how the detailed information in the genome gives rise to the specific characteristics--phenotype--of the individual. This is crucial for understanding not only normal development but also, from a medical perspective, the genetic basis of disease. Much of the mammalian genome-to-phenotype relationship will be worked out in the mouse, for which powerful genetic-manipulation tools are available. Mouse imaging combined with powerful statistical methods has a unique and growing role to play in phenotyping genetically modified mice. This review outlines the challenges for image-based phenotyping, summarizes the current state of three-dimensional imaging technologies for the mouse, and highlights new opportunities in systems biology that are opened by imaging mice.

Targeting murine heart and brain: visualisation conditions for multi-pinhole SPECT with (99m)Tc- and (123)I-labelled probes

PURPOSE

The study serves to optimise conditions for multi-pinhole SPECT small animal imaging of (123)I- and (99m)Tc-labelled radiopharmaceuticals with different distributions in murine heart and brain and to investigate detection and dose range thresholds for verification of differences in tracer uptake.

METHODS

A Triad 88/Trionix system with three 6-pinhole collimators was used for investigation of dose requirements for imaging of the dopamine D(2) receptor ligand [(123)I]IBZM and the cerebral perfusion tracer [(99m)Tc]HMPAO (1.2-0.4 MBq/g body weight) in healthy mice. The fatty acid [(123)I]IPPA (0.94 +/- 0.05 MBq/g body weight) and the perfusion tracer [(99m)Tc]sestamibi (3.8 +/- 0.45 MBq/g body weight) were applied to cardiomyopathic mice overexpressing the prostaglandin EP(3) receptor.

RESULTS

In vivo imaging and in vitro data revealed 45 kBq total cerebral uptake and 201 kBq cardiac uptake as thresholds for visualisation of striatal [(123)I]IBZM and of cardiac [(99m)Tc]sestamibi using 100 and 150 s acquisition time, respectively. Alterations of maximal cerebral uptake of [(123)I]IBZM by >20% (116 kBq) were verified with the prerequisite of 50% striatal of total uptake. The labelling with [(99m)Tc]sestamibi revealed a 30% lower uptake in cardiomyopathic hearts compared to wild types. [(123)I]IPPA uptake could be visualised at activity doses of 0.8 MBq/g body weight.

CONCLUSION

Multi-pinhole SPECT enables detection of alterations of the cerebral uptake of (123)I- and (99m)Tc-labelled tracers in an appropriate dose range in murine models targeting physiological processes in brain and heart. The thresholds of detection for differences in the tracer uptake determined under the conditions of our experiments well reflect distinctions in molar activity and uptake characteristics of the tracers.