Partial Volume Effect Correction Method Research Articles

Partial volume effect of SPECT images in PRRT with 177Lu labelled somatostatin analogues: A practical solution

BackgroundAt present activity quantification is one of the most critical step in dosimetry calculation, and Partial Volume Effect (PVE) one of the most important source of error. In recent years models based upon phantoms that incorporate hot spheres have been used to establish recovery models. In this context the goal of this study was to point out the most critical issues related to PVE and to establish a model closer to a biological imaging environment. MethodsTwo different phantoms, filled with a 177Lu solution, were used to obtain the PVE Recovery Coefficients (RCs): a phantom with spherical inserts and a phantom with organ-shaped inserts. Two additional phantoms with inserts of various geometrical shapes and an anthropomorphic phantom were acquired to compare the real activities to predicted values after PVE correction. ResultsThe RCs versus volume of the inserts produced two different curves, one for the spheres and one for the organs. After PVE correction, accuracy on activity quantification averaged over all inserts of three test phantoms passed from −26% to 1.3% (from 26% to 10% for absolute values). ConclusionRCs is a simple method for PVE correction easily applicable in clinical routine. The use of two different models for organs and lesions has permitted to closely mimic the situation in a living subject. A marked improvement in the quantification of activity was observed when PVE correction was adopted, even if further investigations should be performed for more accurate models of PVE corrections.

A novel partial volume effects correction technique integrating deconvolution associated with denoising within an iterative PET image reconstruction.

Partial volume effect (PVE) plays an important role in both qualitative and quantitative PET image accuracy, especially for small structures. A previously proposed voxelwise PVE correction method applied on PET reconstructed images involves the use of Lucy-Richardson deconvolution incorporating wavelet-based denoising to limit the associated propagation of noise. The aim of this study is to incorporate the deconvolution, coupled with the denoising step, directly inside the iterative reconstruction process to further improve PVE correction. The list-mode ordered subset expectation maximization (OSEM) algorithm has been modified accordingly with the application of the Lucy-Richardson deconvolution algorithm to the current estimation of the image, at each reconstruction iteration. Acquisitions of the NEMA NU2-2001 IQ phantom were performed on a GE DRX PET/CT system to study the impact of incorporating the deconvolution inside the reconstruction [with and without the point spread function (PSF) model] in comparison to its application postreconstruction and to standard iterative reconstruction incorporating the PSF model. The impact of the denoising step was also evaluated. Images were semiquantitatively assessed by studying the trade-off between the intensity recovery and the noise level in the background estimated as relative standard deviation. Qualitative assessments of the developed methods were additionally performed on clinical cases. Incorporating the deconvolution without denoising within the reconstruction achieved superior intensity recovery in comparison to both standard OSEM reconstruction integrating a PSF model and application of the deconvolution algorithm in a postreconstruction process. The addition of the denoising step permitted to limit the SNR degradation while preserving the intensity recovery. This study demonstrates the feasibility of incorporating the Lucy-Richardson deconvolution associated with a wavelet-based denoising in the reconstruction process to better correct for PVE. Future work includes further evaluations of the proposed method on clinical datasets and the use of improved PSF models.

Recovery coefficients determination for partial volume effect correction in oncological PET/CT images considering the effect of activity outside the field of view

The partial volume effect (PVE) has a great impact in quantitative PET/CT imaging. Correction methods have been recently proposed by many authors to make the image quantification more accurate. This work presents a methodology for determining the recovery coefficients (RCs) for PVE correction in PET/CT images. It was taken into account the radioactivity outside the field of view (FOV), which is expected in a patient image acquisition. The NEMA image quality phantom and the NEMA scatter phantom were used. The phantoms were filled with (18)F-FDG for different sphere-to-background ratios. The RCs have been determined from image acquisitions in a Siemens Biograph 16 Hi-Rez PET/CT scanner with and without the scatter phantom. The RC values that ranged from 0.38 to 1.00 without the scatter phantom exhibited a wider variation when this latter was taken into account (from 0.27 to 1.02). This more realistic estimation must be considered if one takes into account that an incorrect SUV measure in tumors leads to errors in the evaluation of the response to therapy based on PET/CT images. The activity outside the FOV should be considered in RCs determination to improve the RC-based PVE correction method.

SU-D-500-03: A Correction of Partial Volume Effect for Using PET to Define Metabolically Active Tumor Volume for Pancreatic Cancer

Purpose: Accurate delineation of tumor volume based on PET is affected by partial volume effect (PVE) that has been shown to Result in underestimations of the standardized uptake value (SUV), leading to decreased metabolically‐active tumor volume (MATV). Here we assess the impact of a PVE correction on the SUV and the PET defined MATV in patients with pancreatic cancer. Methods: A voxel‐by‐voxel PVE correction method utilizing a recovery coefficient (RC) approach is implemented for correction in the region of interest (ROI). A profile‐analysis study was used to define the ideal‐cutoff‐values (IDCVs) to threshold PET. The RC and the IDCVs were calculated using NEMA phantom. The pancreas tumor volumes were delineated on the T1 with contrast and diffusion‐weighted MRI (DWI), and on the PET using four different cutoff values: 2.5‐SUV, tumor‐to‐background (T/B) ratio, 42% of SUVmax, and IDCVs. The PVE‐corrected PET and MATV were compared with the original PET, and the resulted tumor sizes were compared with those from contrast‐enhanced CT and MRI. Results: For all patients studied, the mean RC was 0.72 (ranging 0.58 – 0.81) based on tumor diameter on the CT (ranging 1.9 – 5.8) cm, and tumor‐to‐background ratio (ranging 2.4 – 8.7). The mean SUVmax values in the ROI were 8.2 (ranging 3.9 – 13.1) and 10.6 (ranging 5.57 – 16.37) g/ml, respectively, before and after PVE correction. Generally, all of the 2.5‐SUV, T/B ratio, and 42% SUVmax showed an increase of up to 34% in the MATV after PVE correction except for the IDCVs MATV was reduced by 21%. The differences between tumor sizes from CT, T1, and DWI and those from PET with PVE‐correction were reduced compared to those without correction. Conclusion: The partial volume effect correction considerably impacts PET SUV and improves the PET‐defined metabolic ‐ ally‐active tumor volume, particularly for small pancreatic tumors.

Quantification of opioid receptor availability following spontaneous epileptic seizures: Correction of [11C]diprenorphine PET data for the partial-volume effect

Previous positron emission tomography (PET) studies in refractory temporal lobe epilepsy (TLE) using the non-selective opioid receptor antagonist [(11)C]diprenorphine (DPN) did not detect any changes in mesial temporal structures, despite known involvement of the hippocampus in seizure generation. Normal binding in smaller hippocampi is suggestive of increased receptor concentration in the remaining grey matter. Correction for partial-volume effect (PVE) has not been used in previous DPN PET studies. Here, we present PVE-corrected DPN-PET data quantifying post-ictal and interictal opioid receptor availability in humans with mTLE. Eight paired datasets of post-ictal and interictal DPN PET scans and eleven test/retest control datasets were available from a previously published study on opioid receptor changes in TLE following seizures (Hammers et al., 2007a). Five of the eight participants with TLE had documented hippocampal sclerosis. Data were re-analyzed using regions of interest and a novel PVE correction method (structural functional synergistic-resolution recovery (SFS-RR); (Shidahara et al., 2012)). Data were denoised, followed by application of SFS-RR, with anatomical information derived via precise anatomical segmentation of the participants' MRI (MAPER; (Heckemann et al., 2010)). [(11)C]diprenorphine volume-of-distribution (VT) was quantified in six regions of interest. Post-ictal increases were observed in the ipsilateral fusiform gyri and lateral temporal pole. A novel finding was a post-ictal increase in [(11)C]DPN VT relative to the interictal state in the ipsilateral parahippocampal gyrus, not observed in uncorrected datasets. As for voxel-based (SPM) analyses, correction for global VT values was essential in order to demonstrate focal post-ictal increases in [(11)C]DPN VT. This study provides further direct human in vivo evidence for changes in opioid receptor availability in TLE following seizures, including changes that were not evident without PVE correction. Denoising, resolution recovery and precise anatomical segmentation can extract valuable information from PET studies that would be missed with conventional post-processing procedures.

A Partial Volume Effect Correction Tailored for18F-FDG-PET Oncological Studies

We have developed, optimized, and validated a method for partial volume effect (PVE) correction of oncological lesions in positron emission tomography (PET) clinical studies, based on recovery coefficients (RC) and on PET measurements of lesion-to-background ratio (L/B m) and of lesion metabolic volume. An operator-independent technique, based on an optimised threshold of the maximum lesion uptake, allows to define an isocontour around the lesion on PET images in order to measure both lesion radioactivity uptake and lesion metabolic volume. RC are experimentally derived from PET measurements of hot spheres in hot background, miming oncological lesions. RC were obtained as a function of PET measured sphere-to-background ratio and PET measured sphere metabolic volume, both resulting from the threshold-isocontour technique. PVE correction of lesions of a diameter ranging from 10 mm to 40 mm and for measured L/B m from 2 to 30 was performed using measured RC curves tailored at answering the need to quantify a large variety of real oncological lesions by means of PET. Validation of the PVE correction method resulted to be accurate (>89%) in clinical realistic conditions for lesion diameter > 1 cm, recovering >76% of radioactivity for lesion diameter < 1 cm. Results from patient studies showed that the proposed PVE correction method is suitable and feasible and has an impact on a clinical environment.

Evaluation of partial volume effect correction methods for brain positron emission tomography: Quantification and reproducibility

Quantitative accuracy of positron emission tomography (PET) is decreased by the partial volume effect (PVE). The PVE correction (PVC) methods proposed by Alfano et al., Rousset et al., Müller-Gärtner et al. and Meltzer et al. were evaluated in the present study to obtain guidelines for selecting among them. For accuracy evaluation, the Hoffman brain phantom was scanned with three PETs of differing spatial resolution in order to measure the effect of PVC on radioactivity distribution. Test-retest data consisting of duplicate dynamic emission recordings of the dopamine D2-receptor ligand [11C] raclopride obtained in eight healthy control subjects were used to test the correction effect in different regions of interest. The PVC method proposed by Alfano et al. gave the best quantification accuracy in the brain gray matter region. When the effect of PVC on reliability was tested with human data, the method of Meltzer et al. proved to be the most reliable. The method by Alfano et al. may be better for group comparison studies and the method by Meltzer et al. for intra-subject drug-effect studies.

11C-flumazenil PET, volumetric MRI, and quantitative pathology in mesial temporal lobe epilepsy.

Using statistical parametric mapping and 11C-flumazenil (FMZ) PET we have previously shown reduction of central benzodiazepine receptor (cBZR) binding restricted to the hippocampus in mesial temporal lobe epilepsy (mTLE) due to hippocampal sclerosis (HS). The limited spatial resolution of PET, however, results in partial-volume averaging that affects quantitative analysis of cBZR density. We determined hippocampal volume loss and reduction in cBZR binding using an MRI-based method for partial-volume effect correction of 11C-FMZ volume of distribution (FMZ-Vd) in 17 patients with refractory mTLE and an MRI diagnosis of HS that was subsequently histologically verified in all cases. Quantitative neuropathology was performed with assessment of neuron density in 14 of the 17 patients. Absolute FMZ-Vd and asymmetry indices (FMZ-AI) were compared before and after partial-volume effect correction with MRI-determined hippocampal volumes (HCV), hippocampal T2 measurements, and, if available, neuronal cell densities. Compared with 15 age-matched healthy volunteers, significant reductions of absolute hippocampal FMZ-Vd were found before correction for partial-volume effects in 11 of 17 patients (65%) and only abnormal FMZ-AI in the other six patients. After partial-volume effects correction all 17 patients (100%) showed both significant unilateral reduction of absolute FMZ-Vd and abnormal FMZ-AI. There was no correlation between corrected absolute FMZ-Vd and HCV or neuronal cell density. After correction for partial-volume effect we found a mean 38% reduction of FMZ-Vd in the sclerosed hippocampus, over and above the reduction of HCV. Correction for partial-volume effect allows absolute quantitation of FMZ-PET and increases its sensitivity for detecting abnormalities in TLE due to HS. The lack of correlation between cBZR binding and neuronal density implies that atrophy with neuron loss is not the sole determinant of reduced cBZR binding in patients with mTLE and hippocampal sclerosis.