Partial Volume Effect Correction Research Articles

Selective neuronal damage and chronic hemodynamic cerebral ischemia

In atherothrombotic internal carotid artery or middle cerebral artery (MCA) occlusive disease, chronic hemodynamic compromise may increase the risk for cerebral ischemic damage. To determine whether selective neuronal damage demonstrated as a decrease in central benzodiazepine receptor (BZR) in the normal-appearing cerebral cortex is associated with increased oxygen extraction fraction (OEF) (misery perfusion). We measured BZR and OEF using positron emission tomography in 105 nondisabled patients with atherothrombotic internal carotid artery or MCA occlusive disease and no cortical infarction. By using three-dimensional stereotactic surface projections and the stereotactic extraction estimation method, without correction for partial volume effects, the abnormally decreased BZR index [(the extent of the pixels with Z-score more than 2 compared with controls) x (average Z-score in those pixels)] in the cerebral cortex of the MCA distribution with arterial disease was calculated, and it was found to be correlated with the mean hemispheric value of OEF and several clinical variables. All patients had pixels with abnormally decreased BZR, with the extent varying from 0.04 to 60.91%. Multivariate analysis showed that the abnormally decreased BZR index was positively correlated with the value of OEF and the history of stroke, whereas it was negatively correlated with the presence of hypercholesterolemia with statin treatment. Follow-up examinations of 17 patients without ischemic episode showed that a decrease of BZR was associated with an increase of OEF. In atherothrombotic internal carotid artery or MCA occlusive disease, misery perfusion may cause selective neuronal damage, and statins might have beneficial effects against neuronal damage.

Apathy correlates with dopamine uptake in neurodegenerative diseases. a spect study with partial volume effect correction

Apathy correlates with dopamine uptake in neurodegenerative diseases. a spect study with partial volume effect correction

The basal ganglia matching tools package for striatal uptake semi-quantification: description and validation

To design a novel algorithm (BasGan) for automatic segmentation of striatal (123)I-FP-CIT SPECT. The BasGan algorithm is based on a high-definition, three-dimensional (3D) striatal template, derived from Talairach's atlas. A blurred template, obtained by convolving the former with a 3D Gaussian kernel (FWHM = 10 mm), approximates striatal activity distribution. The algorithm performs translations and scale transformation on the bicommissural aligned image to set the striatal templates with standard size in an appropriate initial position. An optimization protocol automatically performs fine adjustments in the positioning of blurred templates to best match the radioactive counts, and locates an occipital ROI for background evaluation. Partial volume effect correction is included in the process of uptake computation of caudate, putamen and background. Experimental validation was carried out by means of six acquisitions of an anthropomorphic striatal phantom. The BasGan software was applied to a first set of patients with Parkinson's disease (PD) versus patients affected by essential tremor. A highly significant correlation was achieved between true binding potential and measured (123)I activity from the phantom. (123)I-FP-CIT uptake was significantly lower in all basal ganglia in the PD group versus controls with both BasGan and a conventional ROI method used for comparison, but particularly with the former. Correlations with the motor UPDRS score were far more significant with the BasGan. The novel BasGan algorithm automatically performs the 3D segmentation of striata. Because co-registered MRI is not needed, it can be used by all nuclear medicine departments, since it is freely available on the Web.

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.

Accuracy of partial volume effect correction in clinical molecular imaging of dopamine transporter using SPECT

Objectives Partial volume effect (PVE) is a major source of bias in brain SPECT imaging of dopamine transporter. Various PVE corrections (PVC) making use of anatomical data have been developed and yield encouraging results. However, their accuracy in clinical data is difficult to demonstrate because the gold standard (GS) is usually unknown. The objective of this study was to assess the accuracy of PVC. Method Twenty-three patients underwent MRI and 123I-FP-CIT SPECT. The binding potential (BP) values were measured in the striata segmented on the MR images after coregistration to SPECT images. These values were calculated without and with an original PVC. In addition, for each patient, a Monte Carlo simulation of the SPECT scan was performed. For these simulations where true simulated BP values were known, percent biases in BP estimates were calculated. For the real data, an evaluation method that simultaneously estimates the GS and a quadratic relationship between the observed and the GS values was used. It yields a surrogate mean square error (sMSE) between the estimated values and the estimated GS values. Results The averaged percent difference between BP measured for real and for simulated patients was 0.7±9.7% without PVC and was −8.5±14.5% with PVC, suggesting that the simulated data reproduced the real data well enough. For the simulated patients, BP was underestimated by 66.6±9.3% on average without PVC and overestimated by 11.3±9.5% with PVC, demonstrating the greatest accuracy of BP estimates with PVC. For the simulated data, sMSE were 27.3 without PVC and 0.90 with PVC, confirming that our sMSE index properly captured the greatest accuracy of BP estimates with PVC. For the real patient data, sMSE was 50.8 without PVC and 3.5 with PVC. These results were consistent with those obtained on the simulated data, suggesting that for clinical data, and despite probable segmentation and registration errors, BP were more accurately estimated with PVC than without. Conclusion PVC was very efficient to greatly reduce the error in BP estimates in clinical imaging of dopamine transporter.

Comparative assessment of statistical brain MR image segmentation algorithms and their impact on partial volume correction in PET

Magnetic resonance imaging (MRI)-guided partial volume effect correction (PVC) in brain positron emission tomography (PET) is now a well-established approach to compensate the large bias in the estimate of regional radioactivity concentration, especially for small structures. The accuracy of the algorithms developed so far is, however, largely dependent on the performance of segmentation methods partitioning MRI brain data into its main classes, namely gray matter (GM), white matter (WM), and cerebrospinal fluid (CSF). A comparative evaluation of three brain MRI segmentation algorithms using simulated and clinical brain MR data was performed, and subsequently their impact on PVC in 18F-FDG and 18F-DOPA brain PET imaging was assessed. Two algorithms, the first is bundled in the Statistical Parametric Mapping (SPM2) package while the other is the Expectation Maximization Segmentation (EMS) algorithm, incorporate a priori probability images derived from MR images of a large number of subjects. The third, here referred to as the HBSA algorithm, is a histogram-based segmentation algorithm incorporating an Expectation Maximization approach to model a four-Gaussian mixture for both global and local histograms. Simulated under different combinations of noise and intensity non-uniformity, MR brain phantoms with known true volumes for the different brain classes were generated. The algorithms' performance was checked by calculating the kappa index assessing similarities with the “ground truth” as well as multiclass type I and type II errors including misclassification rates. The impact of image segmentation algorithms on PVC was then quantified using clinical data. The segmented tissues of patients' brain MRI were given as input to the region of interest (RoI)-based geometric transfer matrix (GTM) PVC algorithm, and quantitative comparisons were made. The results of digital MRI phantom studies suggest that the use of HBSA produces the best performance for WM classification. For GM classification, it is suggested to use the EMS. Segmentation performed on clinical MRI data show quite substantial differences, especially when lesions are present. For the particular case of PVC, SPM2 and EMS algorithms show very similar results and may be used interchangeably. The use of HBSA is not recommended for PVC. The partial volume corrected activities in some regions of the brain show quite large relative differences when performing paired analysis on 2 algorithms, implying a careful choice of the segmentation algorithm for GTM-based PVC.

Antiepileptic Drugs Do Not Affect 5-HT1A Receptor Binding Measured by Positron Emission Topography

Objective To study the effect of antiepileptic drugs on 5-HT 1A receptor binding in patients with temporal lobe epilepsy. Background 5-HT 1A receptor binding is reduced in patients with temporal lobe epilepsy. Animal models show serotonergic effects of antipepileptic drugs including carbamazepine and lamotrigine. Methods We studied 31 patients and 10 normal controls. Patients with structural lesions, progressive neurological disorders, or taking other medications were excluded. None had a seizure for at least two days before PET. [ 18 F]FCWAY PET was performed on a GE Advance scanner with continuous EEG monitoring. Functional images of the distribution volume ( V ) were generated. Anatomic regions of interest were applied to coregistered PET images, after correction for partial volume effect. Results Patients had significantly higher [ 18 F]FCWAY free fraction ( f 1 ) than controls. 5HT1A receptor binding was reduced in temporal lobe epileptic foci after partial volume correction. There were no AED effects on interictal [ 18 F]FCWAY binding after correction for plasma free fraction. [ 18 F]FCWAY V / f 1 reduction in epileptic foci was not affected by AEDs. Conclusions Antiepileptic drugs do not appear to have significant effects on 5HT1A receptor binding. Plasma free fractions should be measured when PET receptor studies are performed in patients with epilepsy.

Pattern of cerebral hypoperfusion in Alzheimer's disease and mild cognitive impairment measured with arterial spin-labeling MR imaging: Initial experience

PurposeTo prospectively determine if pulsed arterial spin-labeling perfusion magnetic resonance (MR) imaging depicts regional cerebral hypoperfusion in subjects with Alzheimer's disease (AD) and mild cognitive impairment (MCI), compared with perfusion in cognitively normal (CN) subjects, that is consistent with results of fluorodeoxyglucose (FDG) positron emission tomography (PET) and hexamethylpropyleneamine oxime (HMPAO) single photon emission computed tomography (SPECT) studies of similar populations. Materials and methodsInstitutional review board approval and informed consent were obtained. 20subjects with AD (13men, 7women; mean age=72.9years), 18 with MCI (9men, 9women; mean age=73.3years), and 23CN subjects (10men, 13women; mean age=72.9years) underwent arterial spin-labeling and volumetric T1-weighted structural MR imaging. Perfusion images were coregistered to structural images, corrected for partial volume effects (PVEs) with information from the structural image to determine tissue content of perfusion voxels, and normalized to a study-specific template. Analyses of perfusion differences between groups, with and without corrections for PVEs, were performed on a voxel-by-voxel basis with a one-tailed fixed-effects analysis of covariance model adjusted for age. In addition, tests were performed with and without accounting for global perfusion. ResultsThe AD group showed significant regional hypoperfusion, compared with the CN group, in the right inferior parietal cortex extending into the bilateral posterior cingulate gyri (P<0.001), bilateral superior and middle frontal gyri (P<0.001), and left inferior parietal lobe (P=0.007). When PVEs from underlying cortical gray matter atrophy were accounted for, the AD group still showed hypoperfusion in the right inferior parietal lobe extending into the bilateral posterior cingulate gyri (P<0.001) and left (P=0.003) and right (P=0.012) middle frontal gyri. With a more liberal voxel-level threshold of P<0.01, the MCI group showed significant regional hypoperfusion relative to the CN group in the inferior right parietal lobe (P=0.046), similar to the region of greatest significance in the AD group. ConclusionArterial spin-labeling MR imaging showed regional hypoperfusion with AD, in brain regions similar to those seen in FDG PET and HMPAO SPECT studies of similar populations; this hypoperfusion persists after accounting for underlying cortical gray matter atrophy.

Study of relative quantification of Tc-99 m with partial volume effect and spillover correction for SPECT oncology imaging

The apparent concentration of activity in structures in nuclear medicine images depends on their size relative to the system spatial resolution. This dependence is called the partial volume effect (PVE). Spillover (SO) or the blurring into a structure of counts originating in nearby structures also alters the apparent concentration of activity. In combination these effects impact the detection of lesions and quantification of activity within structures in the slices. The increased accessibility of dual-modality imaging systems makes available high-resolution anatomical information which is registered with the emission slices and can be used in correcting for the PVE and SO. In this study we investigated the use of the template projection-reconstruction method for correction of the PVE and SO. We examined the impact of correction on visual image quality and the quantification of activity in simulated spheres of varying contrast relative to a uniform background distribution of activity. Our enhancements to the template projection-reconstruction methodology included both an improvement in the matching of the blurring in the reconstructed templates of structures to the actual blurring in the reconstructed slices, and accounting for the fractional presence of structures in SPECT voxels. We determined that such corrections for the PVE and SO can dramatically improve both the visualization and quantification of activity within the source distributions we investigated.

Partial volume effect correction in SPECT for striatal uptake measurements in patients with neurodegenerative diseases: impact upon patient classification

In single-photon emission computed tomography (SPECT) of the dopaminergic system, measurements of striatal uptake are useful for diagnosis and patient follow-up but are strongly biased by the partial volume effect (PVE). We studied whether PVE correction might improve patient classification based on binding potential (BP) measurements. Patients with a probable diagnosis of dementia with Lewy bodies (DLB, 10 patients) or Alzheimer's disease (AD, 13 patients) were studied by( 123)I-FP-CIT SPECT. SPECT images were reconstructed with and without PVE correction. Each patient SPECT scan was also simulated to obtain SPECT data whose characteristics were fully known. In addition, 17 SPECT scans were simulated with striatal uptake values mimicking pre-symptomatic cases of DLB. Without PVE correction, mean putamen BP values were 2.9+/-0.4 and 0.9+/-0.2 for AD and DLB patients respectively, while with PVE correction, they were 8.6+/-1.5 and 1.9+/-0.5 respectively. All patients were properly identified as having AD or DLB when considering mean putamen BP measured on their real or simulated SPECT scan, with and without PVE correction. All 30 simulations mimicking pre-symptomatic DLB and AD patients were accurately classified with PVE correction, but without PVE correction 15 mean putamen BP values were in a range where AD and DLB could not be distinguished. We conclude that putamen BP values measured without PVE correction can be used to differentiate probable DLB and AD due to the already severe reduction in dopamine transporter levels. PVE correction appeared useful for accurate differential diagnosis between AD and pre-symptomatic DLB.

A multiresolution image based approach for correction of partial volume effects in emission tomography

Partial volume effects (PVEs) are consequences of the limited spatial resolution in emission tomography. They lead to a loss of signal in tissues of size similar to the point spread function and induce activity spillover between regions. Although PVE can be corrected for by using algorithms that provide the correct radioactivity concentration in a series of regions of interest (ROIs), so far little attention has been given to the possibility of creating improved images as a result of PVE correction. Potential advantages of PVE-corrected images include the ability to accurately delineate functional volumes as well as improving tumour-to-background ratio, resulting in an associated improvement in the analysis of response to therapy studies and diagnostic examinations, respectively. The objective of our study was therefore to develop a methodology for PVE correction not only to enable the accurate recuperation of activity concentrations, but also to generate PVE-corrected images. In the multiresolution analysis that we define here, details of a high-resolution image H (MRI or CT) are extracted, transformed and integrated in a low-resolution image L (PET or SPECT). A discrete wavelet transform of both H and L images is performed by using the ‘à trous’ algorithm, which allows the spatial frequencies (details, edges, textures) to be obtained easily at a level of resolution common to H and L. A model is then inferred to build the lacking details of L from the high-frequency details in H. The process was successfully tested on synthetic and simulated data, proving the ability to obtain accurately corrected images. Quantitative PVE correction was found to be comparable with a method considered as a reference but limited to ROI analyses. Visual improvement and quantitative correction were also obtained in two examples of clinical images, the first using a combined PET/CT scanner with a lymphoma patient and the second using a FDG brain PET and corresponding T1-weighted MRI in an epileptic patient.

The Effect of Antiepileptic Drugs on 5‐HT1A–Receptor Binding Measured by Positron Emission Tomography

To study the effect of antiepileptic drugs (AEDs) on 5-HT(1A)-receptor binding in patients with temporal lobe epilepsy. 5-HT(1A)-receptor binding, measured by positron emission tomography, is reduced in patients with temporal lobe epilepsy. Antiepileptic drugs may act on the serotonergic system, as shown in animal models, and thus affect receptor-binding measurements. We analyzed the effect of AEDs on 5-HT(1A)-receptor binding in 31 patients and 10 normal controls. Patients with structural lesions, progressive neurologic disorders, or taking other medications were excluded. None had a seizure for >or=2 days before positron emission tomography (PET). [(18)F]FCWAY PET was performed on a GE Advance scanner with continuous EEG monitoring. Functional images of the distribution volume (V) were generated. Anatomic regions of interest were applied to co-registered PET images, after correction for partial-volume effect. Patients had significantly higher [(18)F]FCWAY free fraction (f(1)) than did controls. No AED effects were observed on interictal [(18)F]FCWAY binding after correction for plasma free fraction. [(18)F]FCWAY V/f1 reduction in epileptic foci was not affected by AEDs. 5-HT(1A)-receptor binding is reduced in temporal lobe epileptic foci after partial-volume correction. AED plasma free fractions should be measured when PET receptor studies are performed in patients with epilepsy.

Database of normal human cerebral blood flow measured by SPECT: II. Quantification of I-123-IMP studies with ARG method and effects of partial volume correction

The limited spatial resolution of SPECT causes a partial volume effect (PVE) and can lead to the significant underestimation of regional tracer concentration in the small structures surrounded by a low tracer concentration, such as the cortical gray matter of an atrophied brain. The aim of the present study was to determine, using 123I-IMP and SPECT, normal CBF of elderly subjects with and without PVE correction (PVC), and to determine regional differences in the effect of PVC and their association with the regional tissue fraction of the brain. Quantitative CBF SPECT using 123I-IMP was performed in 33 healthy elderly subjects (18 males, 15 females, 54-74 years old) using the autoradiographic method. We corrected CBF for PVE using segmented MR images, and analyzed quantitative CBF and regional differences in the effect of PVC using tissue fractions of gray matter (GM) and white matter (WM) in regions of interest (ROIs) placed on the cortical and subcortical GM regions and deep WM regions. The mean CBF in GM-ROIs were 31.7 +/- 6.6 and 41.0 +/- 8.1 ml/100 g/min for males and females, and in WM-ROIs, 18.2 +/- 0.7 and 22.9 +/- 0.8 ml/100 g/min for males and females, respectively. The mean CBF in GM-ROIs after PVC were 50.9 +/- 12.8 and 65.8 +/- 16.1 ml/100 g/min for males and females, respectively. There were statistically significant differences in the effect of PVC among ROIs, but not between genders. The effect of PVC was small in the cerebellum and parahippocampal gyrus, and it was large in the superior frontal gyrus, superior parietal lobule and precentral gyrus. Quantitative CBF in GM recovered significantly, but did not reach values as high as those obtained by invasive methods or in the H2(15)O PET study that used PVC. There were significant regional differences in the effect of PVC, which were considered to result from regional differences in GM tissue fraction, which is more reduced in the frontoparietal regions in the atrophied brain of the elderly.

Fusion of the MR image to SPECT with possible correction for partial volume effects

Low spatial resolution and the related partial volume effects limit the diagnostic potential of brain single photon emission computed tomography (SPECT) imaging. As a possible remedy for this problem we propose a technique for the fusion of SPECT and MR images, which requires for a given patient the SPECT data and the T1-weighted MR image. Basically, after the reconstruction and coregistration steps, the high-frequency part of the MR, which would be unrecoverable by the set SPECT acquisition system + reconstruction algorithm, is extracted and added to the SPECT image. The tuning of the weight of the MR on the resulting fused image can be performed very quickly, any iterative reconstruction algorithm can be used and, in the case that the SPECT projections are not available, the proposed technique can also be applied directly to the SPECT image, provided that the performance of the scanner is known. The procedure has the potential of increasing the diagnostic value of a SPECT image. Even in the locations of SPECT-MR mismatch it does not significantly affect quantitation over regions of interest (ROIs) whose dimensions are decidedly larger than the SPECT resolution distance. On the other hand, appreciable corrections for partial volume effects are expected in the locations where the contrast in the structural MR matches the corresponding contrast in functional activity.

Increasing the accuracy of in vivo quantification of dopamine-D2/D3 receptor binding in the human brain by partial volume effect correction

Increasing the accuracy of in vivo quantification of dopamine-D2/D3 receptor binding in the human brain by partial volume effect correction

Patient-Specific Three Dimensional (3D) Dosimetry for 90Y Ibritumomab Tiutexan (Zevalin®) in Patients with Follicular Lymphoma.

Objectives: Dosimetry calculations for radioimmunotherapy are usually performed using a two dimensional (2D) protocol based on planar whole body (WB) scintigraphy and fixed organ masses derived from the MIRD anthropomorphic model (Wiseman GA et al., J Nucl Med 2003: 44;465–74). We compared the absorbed dose obtained with this method with an approach involving patient-specific 3D dosimetry based on quantitative SPECT and patient organs mass estimation.Methods: Five patients with follicular lymphoma in partial or complete remission underwent a CT scan, and five successive WB and SPECT scans at day 0 (D0) 1 hour, D0 4 hours, D1, D4 and D6 after injection of 111In ibritumomab tiutexan (185 MBq). WB scans were used to estimate the activity in the liver, spleen and kidneys. SPECT images were reconstructed using a quantitative processing including corrections for scatter, attenuation, and partial volume effect. The organs mass and activity values were estimated in 3D volumes of interest manually drawn on the CT scan and registered to the SPECT scan. Absorbed doses after administration of 15 MBq/kg of 90Y ibritumomab tiutexan were derived from both 111In WB and SPECT data accounting for the fractions of injected activity measured in each organ at the five time points.Results: The mean differences between the organs mass estimated from the patient CT and the MIRD anthropomorphic model were 8%, 105% and 12% for liver, spleen and kidneys respectively. Measurements on an abdominal phantom filled with 111In showed that errors in organ activity estimates ((estimated activity - true activity)/true activity) for WB and SPECT protocols, were 37% and −6% respectively in the liver, 20% and −10% in the spleen, −26% and −23% in the left kidney, and 47% and −9% in the right kidney. Patient liver, spleen and kidney activity values determined from the SPECT scans were on average 35%, 60% and 39% less than those found from the WB scans. The absorbed doses calculated with the 3D patient-specific protocol were less than those calculated with the 2D protocol, by 46±14%, 77±14% and 70±22% in the liver, spleen and kidneys respectively, except in one patient kidney where SPECT dose exceeded WB dose by 2%.Conclusions: Accounting for patient-specific organ mass and using SPECT activity quantification have a great impact on estimated absorbed doses of 90Y ibritumomab tiutexan if compared with the 2D protocol. This method could improve the correlation between dosimetry and clinical consequences of this treatment, especially in view of dose escalation with stem cell rescue.

Partial volume effects on arterial input functions: Shape and amplitude distortions and their correction

For quantification of perfusion values from a bolus-tracking MRI experiment, the measurement of an arterial input function (AIF) is necessary. Gradient-echo (GE) sequences are commonly used for this type of experiment because they offer a high signal-to-noise ratio (SNR) and the potential to quantify the concentration of contrast agent. Measurements of calibration curves for Gd-DTPA in human blood have shown a quadratic relation between the DeltaR(2)* and the concentration of contrast agent, and a linear relationship between phase changes and the concentration of contrast agent. However, for in vivo studies the spatial resolution is usually limited, which leads to partial volume effects. Partial volume effects result in a complex sum of signal arising from the tissue outside the vessel and a contrast agent concentration-dependent blood signal. Ignoring the presence of partial volume effects can lead to an overestimation or underestimation of the contrast agent concentration, depending on the experimental conditions. Correction for partial volume effects is feasible in arteries that are parallel to the main magnetic field by estimation and subtraction of the static signal of the surrounding tissue. Patient studies showed a large variation due to the AIF measurements, but it has also been shown that this influence can be minimized by correction for partial volume effects.

Reliability of uptake estimates in FDG PET as a function of acquisition and processing protocols using the CPET

Standardized uptake values (SUV) are commonly used in FDG PET to characterize suspicious high uptakes. To better understand the reliability and the limits of SUV, we studied the accuracy of uptake estimates as a function of a number of parameters using phantom data acquired on the CPET. Methods: Using the Data Spectrum thorax phantom in which spheres were inserted, we studied the effect of the sphere sizes, of out-of-the-field-of-view activity (OFOVA), of the emission scan duration and of attenuation and partial volume effect (PVE) corrections upon biases in tumor-to-normal tissue uptake ratio (TNR) estimates. Considering a specific acquisition and processing protocol as a reference, we determined the changes in TNR when modifying this protocol. Results: For a true TNR of 8, estimated TNR were strongly dependent on lesion size, but also on the acquisition and processing protocol. Depending on the method used to derive the attenuation map, estimated TNR could change by more than 50% for lung spheres less than 2 cm in diameter. Using PVE correction, TNR increased by a factor greater than 2 for spheres less than 2 cm in diameter. The very method used to estimate sphere activity from the reconstructed images could also change the TNR by more than 50%. Even with CT-based attenuation correction and PVE correction, TNR estimates remained underestimated by more than 20% in lesions less than 2 cm in diameter. Conclusion: Biases in uptake estimates strongly depend on the acquisition and processing protocols. This suggests that comparing SUV between studies make sense only if the scanner and the acquisition and processing protocols are strictly identical.

Comparison of two methods for correction of partial volume effects in PET

The low spatial resolution of PET scanners results in partial volume (PV) effects which limit the accurate quantification in anatomical structures such as the cerebral cortex and subcortical regions. Several correction methods have been proposed to deal with these effects. In a first study 1, using simulated data, we compared the PV correction of some cortical regions, obtained with i) the correction algorithm implemented at the Research Center Juelich (PVC-J) and ii) the one implemented at the Brain Imaging Centre of Montreal (PVC-M). Here, still using simulated data, the impact of mis-registration between the PET and the anatomical data on accuracy of the correction, as well as the correction of real dynamic PET data are presented. The PVC-J algorithm - based on the work of Muller-Gartner - has a fully 3D implementation. The resulting corrected grey matter (GM) activity images are obtained by dividing voxel-wise the uncorrected GM images (without white matter contribution) by the GM probability map, derived from the convolution of the corresponding MR segmented image by a 3D spatially variant gaussian function, which reproduces the actual PET image resolution. The PVC-M algorithm - based on the work of Rousset - accounts for the mutual PV effects between any possible tissue structure. After tissue classification of the high resolution MR data, cerebral structures of interest are chosen. Each structure is convolved with a spatially invariant 3D gaussian kernel yielding a set of 3D probability maps, from which recovery and cross-contamination factors are computed. Measured time-activity curves are finally corrected using the correction factor set. A PET dynamic acquisition of an adenosin receptor study was first corrected by both methods and furthermore used to build simulated dynamic data 1 which were translated and rotated by 1 & 2 mm and 1 & 2 degrees, respectively, with respect to the simulated MR image. In case of real data, the mean recovery factor over the dynamic data (6–90 min) of GM is 1.360.05 for PVC-M and 1.410.01 for PVC-J, while in the thalamus region it is 1.170.03 for PVC-M and 1.220.02 for PVC-J. For the mismatched data, the relative difference between the corrected activity values, obtained with the two correction algorithms, and the references was compared. In all but the rotation about the z-axis case, the results are highly consistent. For the rotations about the z axis, the values given by PVC-M and PVC-J show a difference up to about 2.0%. Furthermore, some curves show an instability in the values during the last 30 minutes of the dynamic series, independent from region, mismatch, and method. In this time period the receptor data are affected by lower statistics and low signal-to-contrast ratio. As in the first comparison study, the presented data show an overall high consistency of the results obtained from the two different methods concerning the adenosine receptor. Some of the observed discrepancies could result from the ways the time activity curves are computed with both correction algorithms.

An fMRI investigation of memory-related medial temporal lobe activation in mild cognitive impairment prior to dementia

sessed by single photon emission computed tomography (SPECT). Methods: A community sample of 1708 dementia-free individuals aged 65 years and over was examined by neuropsychological testing in an epidemiological survey. 117 subjects underwent magnetic resonance imaging (MRI) and SPECT with Tc-ethylcisteinate dimer. Subjects were classified into four groups (normal and 3 subclasses for MCI) based on the neuropsychological test. After an MRI-based correction for partial volume effect, differences in relative regional cerebral blood flow (rCBF) across groups were evaluated on a voxel-based basis using an analysis of covariance with age and education as covariates. Conclusions: Out of 117 subjects, 84 were classified as normal, 22 as having MCI-amnestic, 7 as MCI-single nonmemory domain, 4 as MCI-multiple domains slightly impaired, respectively. Subjects with MCI-amnestic showed significant relative rCBF reduction in the bilateral temporoparietal cortices, precuneus, cerebellum and right insula as compared with normal controls. Reduced rCBF were found for MCI-multiple domains slightly impaired in the bilateral anterior cingulate, superior and middle frontal gyrus, left temporoparietal cortex and right parahippocampal gyrus. For the MCI-single nonmemory domain, decreased rCBF were also shown in the bilateral temporoparietal regions and right parahippocampal gyrus. These results were thought to be consistent with the previous reports on MCI and early AD. Temporoparietal hypoperfusion were considered to be most common among subclassifications of MCI.