dAcquisition setting optimization and quantitative imaging for 124I studies with the Inveon microPET-CT system

International audienceABSTRACT: BACKGROUND: Noninvasive multimodality imaging is essential for preclinical evaluation of the biodistribution and pharmacokinetics of radionuclide therapy and for monitoring tumor response. Imaging with nonstandard positron-emission tomography [PET] isotopes such as 124I is promising in that context but requires accurate activity quantification. The decay scheme of 124I implies an optimization of both acquisition settings and correction processing. The PET scanner investigated in this study was the Inveon PET/CT system dedicated to small animal imaging. METHODS: The noise equivalent count rate [NECR], the scatter fraction [SF], and the gamma-prompt fraction [GF] were used to determine the best acquisition parameters for mouse- and rat-sized phantoms filled with 124I. An image-quality phantom as specified by the National Electrical Manufacturers Association NU 4-2008 protocol was acquired and reconstructed with two-dimensional filtered back projection, 2D ordered-subset expectation maximization [2DOSEM], and 3DOSEM with maximum a posteriori [3DOSEM/MAP] algorithms, with and without attenuation correction, scatter correction, and gamma-prompt correction (weighted uniform distribution subtraction). RESULTS: Optimal energy windows were established for the rat phantom (390 to 550 keV) and the mouse phantom (400 to 590 keV) by combining the NECR, SF, and GF results. The coincidence time window had no significant impact regarding the NECR curve variation. Activity concentration of 124I measured in the uniform region of an image-quality phantom was underestimated by 9.9% for the 3DOSEM/MAP algorithm with attenuation and scatter corrections, and by 23% with the gamma-prompt correction. Attenuation, scatter, and gamma-prompt corrections decreased the residual signal in the cold insert. CONCLUSIONS: The optimal energy windows were chosen with the NECR, SF, and GF evaluation. Nevertheless, an image quality and an activity quantification assessment were required to establish the most suitable reconstruction algorithm and corrections for 124I small animal imaging

Reduced administered activity, reduced acquisition time, and preserved image quality for the new CZT camera.

BACKGROUND: For a 1-day myocardial perfusion SPECT (MPS) the recommendations for administered activity stated in the EANM guidelines results in an effective dose of up to 16 mSv per patient. Recently, a gamma camera system, based on cadmium zinc telluride (CZT) technology, was introduced. This technique has the potential to reduce the effective dose and scan time compared to the conventional NaI gamma camera. The aim of this study was to investigate if the effective dose can be reduced with a preserved image quality using CZT technology in MPS. METHODS: In total, 150 patients were included in the study. All underwent a 1-day (99m)Tc-tetrofosmin stress-rest protocol and were divided into three subgroups (n = 50 in each group) with 4, 3, and 2.5 MBq/kg body weight of administered activity in the stress examination, respectively. The acquisition time was increased in proportion to the decrease in administered activity. All examinations were analyzed for image quality by visual grading on a 4-point scale (1 = poor, 2 = adequate, 3 = good, 4 = excellent), by two expert readers. RESULTS: The total effective dose (stress + rest) decreased from 9.3 to 5.8 mSv comparing 4 to 2.5 MBq/kg body weight. For the patients undergoing stress examination only (35%) the effective dose, administrating 2.5 MBq/kg, was 1.4 mSv. The image acquisition times for 2.5 MBq/kg body weight were 475 and 300 seconds (stress and rest) compared to 900 seconds for each when using conventional MPS. The average image quality was 3.7 ± 0.5, 3.8 ± 0.5, and 3.8 ± 0.4 for the stress images and 3.5 ± 0.6, 3.6 ± 0.6, and 3.5 ± 0.6 for the rest images and showed no statistically significant difference (P = .62) among the 4, 3, and 2.5 MBq/kg groups. CONCLUSIONS: The new CZT technology can be used to considerably decrease the effective dose and acquisition time for MPS with preserved high image quality

Internal dosimetry. Macroscopic, small-scale and microscopic perspectives

Internal dosimetry deals with the assessment of absorbed dose for radionuclides distributed inside the body. The absorbed dose is in its turn used for correlation with the biological effect caused by the irradiation. In radioimmunotherapy is however the correlation not easily found and factors influencing this are evaluated and discussed in this work. The internal dosimetry could be subdivided into three levels; macroscopic, small-scale, and microscopic dosimetry. Macroscopic dosimetry: The MIRD S formalism is used to assess the mean absorbed dose to normal organs and tissues. The activity distribution is assumed to be uniform and the calculated mean absorbed dose serves as a good representation of the absorbed dose since the volumes are large compared to the range of emitted particles. The mean absorbed dose to normal organs and tumours was determined for B-cell lymphoma patients undergoing radioimmunotherapy with 90Y-hLL2 (Paper I). The absorbed dose to bone marrow, which is the most radiation sensitive tissue in the body, could be calculated via a method based on the activity in blood samples. The ratio of the activity concentration in bone marrow to the activity concentration in blood was, however, found not to be constant over time. A method for taking this into account in the calculations was proposed (Paper II). Lymphomas are in general radiation sensitive, fast-responding tumours. A decrease in the mass of a tumour during the course of radioimmunotherapy could have a strong influence on the calculated absorbed dose and a method for correction due to this effect was developed (Paper III). Small-scale dosimetry: The MIRD formalism is used, but as the volume is smaller, the mean absorbed dose serves as a poorer representation of the absorbed dose. A model of the anatomy of a mouse was developed and the influence on the S values (absorbed dose per decay) for the choice of organ mass, shape of the organs and distances between the organs was investigated (Paper IV). The average number of atoms per tumour cell was determined from blood samples from a patient having a B cell lymphoma. The MIRD cellular S values were used for calculation of the mean absorbed dose to a cell (Paper V). Internal microdosimetry: The absorbed dose is the expectation value of the specific energy, which is a quantity that takes stochastic effects of the energy depositions into account. The smaller a volume, the larger stochastic effects are seen. Lymphoma patients could have a leukaemic spread of their disease and as 90Y often is used for therapy, the treatment to the single tumour cells is not optimized. Theoretical calculations were performed based on experimental data for an evaluation (Paper VI)

Validation of a computational chain from PET Monte Carlo simulations to reconstructed images

The study aimed to create a pipeline from Monte Carlo simulated projections of a Gate PET system to reconstructed images. The PET system was modelled after the GE Discovery MI (DMI) PET/CT, and the simulated projections were reconstructed with the stand-alone reconstruction software CASToR. Attenuation correction, normalisation calibration, random estimation, and scatter estimation for the simulations were computed with in-house programs. The pipeline was compared in both projection and image space with data acquired on a clinical DMI and reconstructed with GE's off-line PET reconstruction software (PET Toolbox) and CASToR. The simulated and measured data were compared for the number of prompt coincidences, scatter fraction, contrast recovery coefficient (CRC), signal-to-noise ratio (SNR), background variability, residual lung error, and image profiles. A slight discrepancy was noted in the projection space, but good agreements were generally achieved in image space between simulated and measured data. The CRC was found to be 81 % for Gate – CASToR, 84 % for GE – CASToR, and 84 % for GE - PET Toolbox for the largest sphere of the NEMA image quality (IQ) phantom, and the SNR was found to be 98 for Gate – CASToR, 91 for GE – CASToR, and 93 for GE – PET Toolbox. Profiles drawn over the spheres for the NEMA IQ phantom and the Data Spectrum (DS) phantom show a good match between measurement and simulation. The results indicate feasibility to utilise the pipeline as a tool for off-line simulation-based studies. A complete pipeline introduces possibilities to study the impact of single parameters in the whole chain from simulation to reconstructed images

Differences in attenuation pattern in myocardial SPECT between CZT and conventional gamma cameras

Background: In myocardial perfusion imaging (MPI), single-photon emission tomography (SPECT) soft-tissue attenuation by the abdomen, breasts, and lateral chest wall may create artifacts that mimic true perfusion defects. This may cause misdiagnosis of myocardial perfusion. The aim of the present study was to compare the localization, extent, and depth of attenuation artifacts in MPI SPECT for a multi-pinhole cadmium zinc telluride (CZT) camera vs a conventional gamma camera. Methods: Phantom and patient measurements were performed using a CZT camera (GE NM 530c) and a conventional gamma camera (GE Ventri). All images were attenuation corrected with externally acquired low-dose computed tomography. The localization, extent, and depth of the attenuation artifact were quantified by comparing attenuation-corrected and non-attenuation-corrected images. Results: Attenuation artifacts were shifted from the inferolateral wall to the lateral wall using the CZT camera compared to a conventional camera in both the patient and the phantom. The extent of the attenuation artifact was significantly larger for the CZT camera compared to the conventional camera (23 ± 5% vs 15 ± 5%, P < .001) for patients and the result was similar for the phantom (28% vs 19%). Furthermore, the depth of the attenuation artifact (percent of maximum counts) was less pronounced for the CZT camera than for the conventional camera, both for phantom measurements (73% vs 67%) and patients (72 ± 3% vs 68 ± 4%, P < .001). Conclusions: Attenuation artifacts are found in different locations to different extents and depths when using a CZT camera vs a conventional gamma camera for MPI SPECT. This should be taken into consideration when evaluating MPI SPECT studies to avoid misinterpretation of myocardial perfusion distribution

Clinical dosimetry in the treatment of bone tumors: old and new agents

Treatment of multisite, sclerotic bone metastases is successfully performed by radionuclide therapy. Pain palliation is the most common aim for the treatment. Two radiopharmaceuticals are currently approved by the European Medicines Agency (Sm-153-EDTMP and Sr-89-Cl-2) whilst other radiopharmaceuticals are at different stages of development, or are approved in some European countries (Re-186-HEDP, Sn-117(m)-DTPA and Ra-223-Cl-2). The tissues at risk for the treatment are bone marrow and normal bone. A review of the methods applied for dosimetry for these tissues and for tumours is performed, including the calculation of S values (the absorbed dose per decay) and optimal procedures on how to obtain biodistribution data for each radiopharmaceutical. The dosimetry data can be used to individualise and further improve the treatment for each patient. Dosimetry for radionuclide therapy of bone metastases is feasible and can be performed in a routine clinical practice