Radioembolization in patients with hepatocellular carcinoma: a series of 53 cases.

OBJECTIVE: To contribute our results to increase the scientific evidence about the use of radioembolization in the management of patients with hepatocellular carcinoma. MATERIAL AND METHODS: This retrospective review included 53 patients with hepatocellular carcinoma treated with radioembolization at our center. Patients were classified according to the BCLC algorithm in detail according to their Child-Pugh functional status. We analyzed survival using the Kaplan-Meier method. We used Cox regression analysis to determine clinically significant parameters, including the doses administered in the parameters studied. RESULTS: Patients ranged in age from 28 to 86 years (mean, 60 years). A total of 61 procedures were done. The mean activity administered was 2.8GBq (0.7-6.4GBq), with a mean dose of 229.9Gy (74-425.9Gy) administered in the tumor. Progression-free survival was 6.7 months and overall survival was 12.8 months. Differences in disease-free survival according to BCLC and Child-Pugh classification were not significant (p=0.848 and p=0.252, respectively). The clinical parameters that were significantly different with respect to overall survival were bilirubin levels (p<0.001), pretreatment transaminase levels (AST) (p=0.022), Child-Pugh subclassification (p=0.003), and dose administered in the tumor (p=0.001). Only one patient had a severe adverse reaction, developing posttreatment liver failure resulting in death. CONCLUSIONS: Radioembolization is safe and efficacious in the treatment of patients with hepatocellular carcinoma. Liver function and the doses received by the tumor are key parameters for the efficacy of treatment. The increase in the scientific evidence supports the inclusion of this technique in treatment guidelines.

Radioembolización en hepatocarcinoma: a propósito de 53 casos / Radioembolization in patients with hepatocellular carcinoma: a series of 53 cases

Objetivo Contribuir con la presentación de los resultados de nuestro estudio a ampliar la evidencia científica sobre el empleo de la radioembolización en el manejo de pacientes con hepatocarcinoma. Material y método Se trata de una revisión retrospectiva realizada en nuestro centro que incluye a 53 pacientes con hepatocarcinoma tratados con radioembolización. Los pacientes fueron clasificados según el algoritmo del BCLC (Barcelona Clinic Liver Cancer) y de forma pormenorizada por su estado funcional siguiendo la clasificación de Child-Pugh. Se realizó un estudio de supervivencia siguiendo la metodología de Kaplan-Meier. Se empleó el método de regresión de Cox para la determinación de parámetros clínicos significativos, incluyendo dosis administradas en los parámetros estudiados. Resultados La serie evaluada comprende a pacientes con una media de edad de 60 años (rango 28-86). Se llevaron a cabo un total de 61 procedimientos. La actividad media administrada fue de 2,8 GBq (0,7-6,4 GBq), administrando una dosis media en tumor de 229,9 Gy (74-425,9 Gy). El tiempo libre de progresión fue de 6,7 meses desde el momento del tratamiento y la supervivencia global fue de 12,8 meses. La clasificación de los pacientes según BCLC (p=0,848) y Child-Pugh (p=0,252) no resultó significativa respecto al tiempo libre de enfermedad. Los parámetros clínicos que resultaron con diferencias significativas en cuanto a supervivencia global fueron los niveles de bilirrubina (p<0.001) y las cifras de transaminasas (GOT) pretratamiento (p=0.022), la subclasificación Child-Pugh (p=0.003) y la dosis recibida por el tumor (p=0,001). Tan solo uno de los pacientes tratados presentó un efecto adverso grave, con fallo hepático posterapia y resultado de muerte... (AU)

Objective To contribute our results to increase the scientific evidence about the use of radioembolization in the management of patients with hepatocellular carcinoma. Material and methods This retrospective review included 53 patients with hepatocellular carcinoma treated with radioembolization at our center. Patients were classified according to the BCLC algorithm in detail according to their Child-Pugh functional status. We analyzed survival using the Kaplan-Meier method. We used Cox regression analysis to determine clinically significant parameters, including the doses administered in the parameters studied. Results Patients ranged in age from 28 to 86 years (mean, 60 years). A total of 61 procedures were done. The mean activity administered was 2.8 GBq (0.7-6.4 GBq), with a mean dose of 229.9Gy (74-425.9Gy) administered in the tumor. Progression-free survival was 6.7 months and overall survival was 12.8 months. Differences in disease-free survival according to BCLC and Child-Pugh classification were not significant (p=0.848 and p=0.252, respectively). The clinical parameters that were significantly different with respect to overall survival were bilirubin levels (p<0.001), pretreatment transaminase levels (AST) (p=0.022), Child-Pugh subclassification (p=0.003), and dose administered in the tumor (p=0.001). Only one patient had a severe adverse reaction, developing posttreatment liver failure resulting in death. Conclusions Radioembolization is safe and efficacious in the treatment of patients with hepatocellular carcinoma. Liver function and the doses received by the tumor are key parameters for the efficacy of treatment. The increase in the scientific evidence supports the inclusion of this technique in treatment guidelines. (AU)

Radioembolization in patients with hepatocellular carcinoma: a series of 53 cases. / Radioembolización de hepatocarcinomas: a propósito de 53 casos.

OBJECTIVE: To contribute our results to increase the scientific evidence about the use of radioembolization in the management of patients with hepatocellular carcinoma. MATERIAL AND METHODS: This retrospective review included 53 patients with hepatocellular carcinoma treated with radioembolization at our center. Patients were classified according to the BCLC algorithm in detail according to their Child-Pugh functional status. We analyzed survival using the Kaplan-Meier method. We used Cox regression analysis to determine clinically significant parameters, including the doses administered in the parameters studied. RESULTS: Patients ranged in age from 28 to 86 years (mean, 60 years). A total of 61 procedures were done. The mean activity administered was 2.8 GBq (0.7-6.4 GBq), with a mean dose of 229.9Gy (74-425.9Gy) administered in the tumor. Progression-free survival was 6.7 months and overall survival was 12.8 months. Differences in disease-free survival according to BCLC and Child-Pugh classification were not significant (p=0.848 and p=0.252, respectively). The clinical parameters that were significantly different with respect to overall survival were bilirubin levels (p<0.001), pretreatment transaminase levels (AST) (p=0.022), Child-Pugh subclassification (p=0.003), and dose administered in the tumor (p=0.001). Only one patient had a severe adverse reaction, developing posttreatment liver failure resulting in death. CONCLUSIONS: Radioembolization is safe and efficacious in the treatment of patients with hepatocellular carcinoma. Liver function and the doses received by the tumor are key parameters for the efficacy of treatment. The increase in the scientific evidence supports the inclusion of this technique in treatment guidelines.

Significance of the impact of motion compensation on the variability of PET image features.

In lung cancer, quantification by positron emission tomography/computed tomography (PET/CT) imaging presents challenges due to respiratory movement. Our primary aim was to study the impact of motion compensation implied by retrospectively gated (4D)-PET/CT on the variability of PET quantitative parameters. Its significance was evaluated by comparison with the variability due to (i) the voxel size in image reconstruction and (ii) the voxel size in image post-resampling. The method employed for feature extraction was chosen based on the analysis of (i) the effect of discretization of the standardized uptake value (SUV) on complementarity between texture features (TF) and conventional indices, (ii) the impact of the segmentation method on the variability of image features, and (iii) the variability of image features across the time-frame of 4D-PET. Thirty-one PET-features were involved. Three SUV discretization methods were applied: a constant width (SUV resolution) of the resampling bin (method RW), a constant number of bins (method RN) and RN on the image obtained after histogram equalization (method EqRN). The segmentation approaches evaluated were 40[Formula: see text] of SUVmax and the contrast oriented algorithm (COA). Parameters derived from 4D-PET images were compared with values derived from the PET image obtained for (i) the static protocol used in our clinical routine (3D) and (ii) the 3D image post-resampled to the voxel size of the 4D image and PET image derived after modifying the reconstruction of the 3D image to comprise the voxel size of the 4D image. Results showed that TF complementarity with conventional indices was sensitive to the SUV discretization method. In the comparison of COA and 40[Formula: see text] contours, despite the values not being interchangeable, all image features showed strong linear correlations (r > 0.91, [Formula: see text]). Across the time-frames of 4D-PET, all image features followed a normal distribution in most patients. For our patient cohort, the compensation of tumor motion did not have a significant impact on the quantitative PET parameters. The variability of PET parameters due to voxel size in image reconstruction was more significant than variability due to voxel size in image post-resampling. In conclusion, most of the parameters (apart from the contrast of neighborhood matrix) were robust to the motion compensation implied by 4D-PET/CT. The impact on parameter variability due to the voxel size in image reconstruction and in image post-resampling could not be assumed to be equivalent.

PET/TAC: bases físicas, instrumentación y avances / PET/CT: underlying physics, instrumentation, and advances

Desde su introducción, el objetivo principal de un PET/TAC fue proporcionar estudios de alta calidad clínica tanto de PET como de TAC, y presentarlos a los médicos nucleares y radiólogos como una imagen fusionada y perfectamente alineada. El uso de las imágenes de PET y TAC fusionadas se convirtió en muy poco tiempo en rutina clínica, lo que demuestra el gran potencial que tienen estos equipos híbridos. Gracias a este éxito, los fabricantes se han centrado en diseñar sistemas PET y TAC de alto rendimiento y prestaciones, en lugar de ver al TAC en una función más reducida como la de mero corrector de la atenuación para PET. Desde el primer sistema PET/TAC comercial en 2001, tanto la componente PET como la TAC han mejorado enormemente. En el caso del PET, cristales centelleadores más rápidos y con alto poder de frenado como el LYSO han posibilitado la construcción de dispositivos con una mayor sensibilidad, con reducción en el número de coincidencias no deseadas y el uso de la técnica de tiempo de vuelo (TOF, Time of Flight). Todos estos avances llevan a un aumento de la detección de lesiones, especialmente en situaciones con fondo muy ruidoso. Los métodos de reconstrucción iterativos junto con las correcciones implementadas durante la reconstrucción y el uso de la función de dispersión de punto han proporcionado mejoras en la calidad de la imagen. Paralelamente, se han producido mejoras significativas en la instrumentación del TAC, y se puede considerar que los TAC de 64 y 128 filas de detectores han sido incorporados a los PET/TAC actuales. Con ello se pueden obtener imágenes anatómicas de alta calidad diagnóstica en unos pocos segundos y estas se usan tanto para la corrección de atenuación del PET como para proporcionar información diagnóstica. Además, en la actualidad casi todos los escáneres PET/TAC cuentan con su sistema de modulación de la dosis en función de la región escaneada que se imparte al paciente por estudio de TAC. En este artículo se revisarán las bases físicas del PET y del TAC por separado, se describirán las modificaciones realizadas en la instrumentación y los protocolos estándar de un sistema conjunto PET/TAC y se finalizará destacando los avances más importantes de este sistema híbrido (AU)

Since it was first introduced, the main goal of PET/CT has been to provide both PET and CT images with high clinical quality and to present them to radiologists and specialists in nuclear medicine as a fused, perfectly aligned image. The use of fused PET and CT images quickly became routine in clinical practice, showing the great potential of these hybrid scanners. Thanks to this success, manufacturers have gone beyond considering CT as a mere attenuation corrector for PET, concentrating instead on design high performance PET and CT scanners with more interesting features. Since the first commercial PET/CT scanner became available in 2001, both the PET component and the CT component have improved immensely. In the case of PET, faster scintillation crystals with high stopping power such as LYSO crystals have enabled more sensitive devices to be built, making it possible to reduce the number of undesired coincidence events and to use time of flight (TOF) techniques. All these advances have improved lesion detection, especially in situations with very noisy backgrounds. Iterative reconstruction methods, together with the corrections carried out during the reconstruction and the use of the point-spread function, have improved image quality. In parallel, CT instrumentation has also improved significantly, and 64- and 128-row detectors have been incorporated into the most modern PET/CT scanners. This makes it possible to obtain high quality diagnostic anatomic images in a few seconds that both enable the correction of PET attenuation and provide information for diagnosis. Furthermore, nowadays nearly all PET/CT scanners have a system that modulates the dose of radiation that the patient is exposed to in the CT study in function of the region scanned. This article reviews the underlying physics of PET and CT imaging separately, describes the changes in the instrumentation and standard protocols in a combined PET/CT system, and finally points out the most important advances in this hybrid imaging modality (AU)

PET/CT: underlying physics, instrumentation, and advances. / PET/TAC: bases físicas, instrumentación y avances.

Since it was first introduced, the main goal of PET/CT has been to provide both PET and CT images with high clinical quality and to present them to radiologists and specialists in nuclear medicine as a fused, perfectly aligned image. The use of fused PET and CT images quickly became routine in clinical practice, showing the great potential of these hybrid scanners. Thanks to this success, manufacturers have gone beyond considering CT as a mere attenuation corrector for PET, concentrating instead on design high performance PET and CT scanners with more interesting features. Since the first commercial PET/CT scanner became available in 2001, both the PET component and the CT component have improved immensely. In the case of PET, faster scintillation crystals with high stopping power such as LYSO crystals have enabled more sensitive devices to be built, making it possible to reduce the number of undesired coincidence events and to use time of flight (TOF) techniques. All these advances have improved lesion detection, especially in situations with very noisy backgrounds. Iterative reconstruction methods, together with the corrections carried out during the reconstruction and the use of the point-spread function, have improved image quality. In parallel, CT instrumentation has also improved significantly, and 64- and 128-row detectors have been incorporated into the most modern PET/CT scanners. This makes it possible to obtain high quality diagnostic anatomic images in a few seconds that both enable the correction of PET attenuation and provide information for diagnosis. Furthermore, nowadays nearly all PET/CT scanners have a system that modulates the dose of radiation that the patient is exposed to in the CT study in function of the region scanned. This article reviews the underlying physics of PET and CT imaging separately, describes the changes in the instrumentation and standard protocols in a combined PET/CT system, and finally points out the most important advances in this hybrid imaging modality.

Evaluation of PET texture features with heterogeneous phantoms: complementarity and effect of motion and segmentation method.

A major source of error in quantitative PET/CT scans of lung cancer tumors is respiratory motion. Regarding the variability of PET texture features (TF), the impact of respiratory motion has not been properly studied with experimental phantoms. The primary aim of this work was to evaluate the current use of PET texture analysis for heterogeneity characterization in lesions affected by respiratory motion. Twenty-eight heterogeneous lesions were simulated by a mixture of alginate and 18 F-fluoro-2-deoxy-D-glucose (FDG). Sixteen respiratory patterns were applied. Firstly, the TF response for different heterogeneous phantoms and its robustness with respect to the segmentation method were calculated. Secondly, the variability for TF derived from PET image with (gated, G-) and without (ungated, U-) motion compensation was analyzed. Finally, TF complementarity was assessed. In the comparison of TF derived from the ideal contour with respect to TF derived from 40%-threshold and adaptive-threshold PET contours, 7/8 TF showed strong linear correlation (LC) (p < 0.001, r > 0.75), despite a significant volume underestimation. Independence of lesion movement (LC in 100% of the combined pairs of movements, p < 0.05) was obtained for 1/8 TF with U-image (width of the volume-activity histogram, WH) and 4/8 TF with G-image (WH and energy (ENG), local-homogeneity (LH) and entropy (ENT), derived from the co-ocurrence matrix). Their variability in terms of the coefficient of variance ([Formula: see text]) resulted in [Formula: see text](WH) = 0.18 on the U-image and [Formula: see text](WH) = 0.24, [Formula: see text](ENG) = 0.15, [Formula: see text](LH) = 0.07 and [Formula: see text](ENT) = 0.06 on the G-image. Apart from WH (r > 0.9, p < 0.001), not one of these TF has shown LC with C max. Complementarity was observed for the TF pairs: ENG-LH, CONT (contrast)-ENT and LH-ENT. In conclusion, the effect of respiratory motion should be taken into account when the heterogeneity of lung cancer is quantified on PET/CT images. Despite inaccurate volume delineation, TF derived from 40% and COA contours could be reliable for their prognostic use. The TF that exhibited simultaneous added value and independence of lesion movement were ENG and ENT computed from the G-image. Their use is therefore recommended for heterogeneity quantification of lesions affected by respiratory motion.

Dosimetría individualizada para pacientes de cáncer diferenciado de tiroides basada en tasa de dosis externa. Optimización del número de medidas / Individualised dosimetry in patients with differentiated thyroid cancer based on external dose-rate. Optimisation of the number of measurements

Objetivos. Comparar los resultados de la dosimetría individual en pacientes de cáncer diferenciado de tiroides tratados con 131I en nuestro centro con los límites dosimétricos establecidos y con resultados obtenidos en estudios publicados. Analizar el número óptimo de medidas a realizar para reducir el impacto de la dosimetría en el bienestar del paciente y, en segundo término, en la carga de trabajo del personal sanitario. Material y métodos. Se realiza la dosimetría a 29 pacientes del Servicio de Medicina Nuclear del Hospital Universitario y Politécnico La Fe, afectados de cáncer diferenciado de tiroides y tratados con actividades de entre 1,02 y 5,51 GBq (promedio de 2,68 GBq) de 131I. Para ello se utiliza el protocolo de la Sociedad Española de Física Médica (SEFM) basado en medidas de tasa de dosis externa ajustadas a una curva biexponencial, de acuerdo con un modelo de dos compartimentos. Para cada paciente se realizan distintas dosimetrías tomando diferentes selecciones de las medidas disponibles a fin de buscar el número óptimo. Resultados. Los resultados están muy por debajo de los límites dosimétricos y son coherentes con los obtenidos en otros centros. El número de medidas puede reducirse de 5, como propone el protocolo de la SEFM, a 4 sin pérdida de precisión significativa. Una mayor reducción de medidas puede justificarse en casos particulares. Conclusiones. Los valores obtenidos para las magnitudes dosimétricas quedan muy por debajo de los límites establecidos. Se puede asumir la reducción de medidas a costa de un aumento de la incertidumbre moderado beneficiando del paciente (AU)

Objectives. To compare the results of individual dosimetry in differentiated thyroid cancer patients treated with 131I at our centre with the established limits and dosimetry results of published studies. Analysis of the optimal number of measurements necessary to reduce the impact of dosimetry for the comfort of the patient and, secondly, on the workload of health workers. Material and methods. Dosimetry was performed in the Nuclear Medicine Department of the University and Polytechnic Hospital La Fe, on 29 patients suffering from differentiated thyroid cancer and treated with activities between 1.02 and 5.51 GBq (mean 2.68 GBq) of 131I. The Spanish Society of Medical Physics (SEFM) protocol was used, based on measurements of external dose rate adjusted to a bi-exponential curve according to a two compartment model. Different dosimetries were performed on each patient, taking different selections of the available measurements in order to find the optimal number. Results. Results are well below the dosimetry limits, and are consistent with those obtained in other centres. The number of measurements can be reduced from 5, as proposed in the SEFM protocol, to 4 without significant loss of accuracy. Further reducing measures may be justified in individual cases. Conclusions. The values obtained for the dosimetry quantities are significantly below the established limits. A reduction in measurements can be assumed at the cost of a moderate increase in uncertainty, benefiting the patient (AU)

Individualised dosimetry in patients with differentiated thyroid cancer based on external dose-rate. Optimisation of the number of measurements.

OBJECTIVES: To compare the results of individual dosimetry in differentiated thyroid cancer patients treated with (131)I at our centre with the established limits and dosimetry results of published studies. Analysis of the optimal number of measurements necessary to reduce the impact of dosimetry for the comfort of the patient and, secondly, on the workload of health workers. MATERIAL AND METHODS: Dosimetry was performed in the Nuclear Medicine Department of the University and Polytechnic Hospital La Fe, on 29 patients suffering from differentiated thyroid cancer and treated with activities between 1.02 and 5.51 GBq (mean 2.68 GBq) of (131)I. The Spanish Society of Medical Physics (SEFM) protocol was used, based on measurements of external dose rate adjusted to a bi-exponential curve according to a two compartment model. Different dosimetries were performed on each patient, taking different selections of the available measurements in order to find the optimal number. RESULTS: Results are well below the dosimetry limits, and are consistent with those obtained in other centres. The number of measurements can be reduced from 5, as proposed in the SEFM protocol, to 4 without significant loss of accuracy. Further reducing measures may be justified in individual cases. CONCLUSIONS: The values obtained for the dosimetry quantities are significantly below the established limits. A reduction in measurements can be assumed at the cost of a moderate increase in uncertainty, benefiting the patient.

Evaluation of resistive-plate-chamber-based TOF-PET applied to in-beam particle therapy monitoring.

Particle therapy is a highly conformal radiotherapy technique which reduces the dose deposited to the surrounding normal tissues. In order to fully exploit its advantages, treatment monitoring is necessary to minimize uncertainties related to the dose delivery. Up to now, the only clinically feasible technique for the monitoring of therapeutic irradiation with particle beams is Positron Emission Tomography (PET). In this work we have compared a Resistive Plate Chamber (RPC)-based PET scanner with a scintillation-crystal-based PET scanner for this application. In general, the main advantages of the RPC-PET system are its excellent timing resolution, low cost, and the possibility of building large area systems. We simulated a partial-ring scanner based on an RPC prototype under construction within the Fondazione per Adroterapia Oncologica (TERA). For comparison with the crystal-based PET scanner we have chosen the geometry of a commercially available PET scanner, the Philips Gemini TF. The coincidence time resolution used in the simulations takes into account the current achievable values as well as expected improvements of both technologies. Several scenarios (including patient data) have been simulated to evaluate the performance of different scanners. Initial results have shown that the low sensitivity of the RPC hampers its application to hadron-beam monitoring, which has an intrinsically low positron yield compared to diagnostic PET. In addition, for in-beam PET there is a further data loss due to the partial ring configuration. In order to improve the performance of the RPC-based scanner, an improved version of the RPC detector (modifying the thickness of the gas and glass layers), providing a larger sensitivity, has been simulated and compared with an axially extended version of the crystal-based device. The improved version of the RPC shows better performance than the prototype, but the extended version of the crystal-based PET outperforms all other options.

Noise evaluation of Compton camera imaging for proton therapy.

Compton Cameras emerged as an alternative for real-time dose monitoring techniques for Particle Therapy (PT), based on the detection of prompt-gammas. As a consequence of the Compton scattering process, the gamma origin point can be restricted onto the surface of a cone (Compton cone). Through image reconstruction techniques, the distribution of the gamma emitters can be estimated, using cone-surfaces backprojections of the Compton cones through the image space, along with more sophisticated statistical methods to improve the image quality. To calculate the Compton cone required for image reconstruction, either two interactions, the last being photoelectric absorption, or three scatter interactions are needed. Because of the high energy of the photons in PT the first option might not be adequate, as the photon is not absorbed in general. However, the second option is less efficient. That is the reason to resort to spectral reconstructions, where the incoming γ energy is considered as a variable in the reconstruction inverse problem. Jointly with prompt gamma, secondary neutrons and scattered photons, not strongly correlated with the dose map, can also reach the imaging detector and produce false events. These events deteriorate the image quality. Also, high intensity beams can produce particle accumulation in the camera, which lead to an increase of random coincidences, meaning events which gather measurements from different incoming particles. The noise scenario is expected to be different if double or triple events are used, and consequently, the reconstructed images can be affected differently by spurious data. The aim of the present work is to study the effect of false events in the reconstructed image, evaluating their impact in the determination of the beam particle ranges. A simulation study that includes misidentified events (neutrons and random coincidences) in the final image of a Compton Telescope for PT monitoring is presented. The complete chain of detection, from the beam particle entering a phantom to the event classification, is simulated using FLUKA. The range determination is later estimated from the reconstructed image obtained from a two and three-event algorithm based on Maximum Likelihood Expectation Maximization. The neutron background and random coincidences due to a therapeutic-like time structure are analyzed for mono-energetic proton beams. The time structure of the beam is included in the simulations, which will affect the rate of particles entering the detector.

Automatic, three-segment, MR-based attenuation correction for whole-body PET/MR data.

PURPOSE: The combination of positron emission tomography (PET) and magnetic resonance (MR) tomography in a single device is anticipated to be the next step following PET/CT for future molecular imaging application. Compared to CT, the main advantages of MR are versatile soft tissue contrast and its capability to acquire functional information without ionizing radiation. However, MR is not capable of measuring a physical quantity that would allow a direct derivation of the attenuation values for high-energy photons. METHODS: To overcome this problem, we propose a fully automated approach that uses a dedicated T1-weighted MR sequence in combination with a customized image processing technique to derive attenuation maps for whole-body PET. The algorithm automatically identifies the outer contour of the body and the lungs using region-growing techniques in combination with an intensity analysis for automatic threshold estimation. No user interaction is required to generate the attenuation map. RESULTS: The accuracy of the proposed MR-based attenuation correction (AC) approach was evaluated in a clinical study using whole-body PET/CT and MR images of the same patients (n = 15). The segmentation of the body and lung contour (L-R directions) was evaluated via a four-point scale in comparison to the original MR image (mean values >3.8). PET images were reconstructed using elastically registered MR-based and CT-based (segmented and non-segmented) attenuation maps. The MR-based AC showed similar behaviour as CT-based AC and similar accuracy as offered by segmented CT-based AC. Standardized uptake value (SUV) comparisons with reference to CT-based AC using predefined attenuation coefficients showed the largest difference for bone lesions (mean value ± standard variation of SUV(max): -3.0% ± 3.9% for MR; -6.5% ± 4.1% for segmented CT). A blind comparison of PET images corrected with segmented MR-based, CT-based and segmented CT-based AC afforded identical lesion detectability, but slight differences in image quality were found. CONCLUSION: Our MR-based attenuation correction method offers similar correction accuracy as offered by segmented CT. According to the specialists involved in the blind study, these differences do not affect the diagnostic value of the PET images.

Effect of inter-crystal scatter on estimation methods for random coincidences and subsequent correction.

Random coincidences can contribute substantially to the background in positron emission tomography (PET). Several estimation methods are being used for correcting them. The goal of this study was to investigate the validity of techniques for random coincidence estimation, with various low-energy thresholds (LETs). Simulated singles list-mode data of the MADPET-II small animal PET scanner were used as input. The simulations have been performed using the GATE simulation toolkit. Several sources with different geometries have been employed. We evaluated the number of random events using three methods: delayed window (DW), singles rate (SR) and time histogram fitting (TH). Since the GATE simulations allow random and true coincidences to be distinguished, a comparison between the number of random coincidences estimated using the standard methods and the number obtained using GATE was performed. An overestimation in the number of random events was observed using the DW and SR methods. This overestimation decreases for LETs higher than 255 keV. It is additionally reduced when the single events which have undergone a Compton interaction in crystals before being detected are removed from the data. These two observations lead us to infer that the overestimation is due to inter-crystal scatter. The effect of this mismatch in the reconstructed images is important for quantification because it leads to an underestimation of activity. This was shown using a hot-cold-background source with 3.7 MBq total activity in the background region and a 1.59 MBq total activity in the hot region. For both 200 keV and 400 keV LET, an overestimation of random coincidences for the DW and SR methods was observed, resulting in approximately 1.5% or more (at 200 keV LET: 1.7% for DW and 7% for SR) and less than 1% (at 400 keV LET: both methods) underestimation of activity within the background region. In almost all cases, images obtained by compensating for random events in the reconstruction algorithm were better in terms of quantification than the images made with precorrected data.