Effect of the static magnetic field of the MR-scanner on ERPs: evaluation of visual, cognitive and motor potentials.

OBJECTIVE: This work investigates the influence of the static magnetic field of the MR-scanner on ERPs extracted from simultaneous EEG-fMRI recordings. The quality of the ERPs after BallistoCardioGraphic (BCG) artifact removal, as well as the reproducibility of the waveforms in different environments is investigated. METHODS: We consider a Detection, a Go-Nogo and a Motor task, eliciting peaks that differ in amplitude, latency and scalp topography, repeated in two situations: outside the scanner room (0T) and inside the MR-scanner but without gradients (3T). The BCG artifact is removed by means of three techniques: the Average Artifact Subtraction (AAS) method, the Optimal Basis Set (OBS) method and the Canonical Correlation Analysis (CCA) approach. RESULTS: The performance of the three methods depends on the amount of averaged trials. Moreover, differences are found on both amplitude and latency of ERP components recorded in two environments (0T vs 3T). CONCLUSIONS: We showed that, while ERPs can be extracted from simultaneous EEG-fMRI data at 3T, the static magnetic field might affect the physiological processes under investigation. SIGNIFICANCE: The reproducibility of the ERPs in different recording environments (0T vs 3T) is a relevant issue that deserves further investigation to clarify the equivalence of cognitive processes in both behavioral and imaging studies.

Automated identification of ERP peaks through Dynamic Time Warping: an application to developmental dyslexia.

OBJECTIVE: This article proposes a method to automatically identify and label event-related potential (ERP) components with high accuracy and precision. METHODS: We present a framework, referred to as peak-picking Dynamic Time Warping (ppDTW), where a priori knowledge about the ERPs under investigation is used to define a reference signal. We developed a combination of peak-picking and Dynamic Time Warping (DTW) that makes the temporal intervals for peak-picking adaptive on the basis of the morphology of the data. We tested the procedure on experimental data recorded from a control group and from children diagnosed with developmental dyslexia. RESULTS: We compared our results with the traditional peak-picking. We demonstrated that our method achieves better performance than peak-picking, with an overall precision, recall and F-score of 93%, 86% and 89%, respectively, versus 93%, 80% and 85% achieved by peak-picking. CONCLUSION: We showed that our hybrid method outperforms peak-picking, when dealing with data involving several peaks of interest. SIGNIFICANCE: The proposed method can reliably identify and label ERP components in challenging event-related recordings, thus assisting the clinician in an objective assessment of amplitudes and latencies of peaks of clinical interest.

Removing muscle and eye artifacts using blind source separation techniques in ictal EEG source imaging.

OBJECTIVE: The contamination of muscle and eye artifacts during an ictal period of the EEG significantly distorts source estimation algorithms. Recent blind source separation (BSS) techniques based on canonical correlation (BSS-CCA) and independent component analysis with spatial constraints (SCICA) have shown much promise in the removal of these artifacts. In this study we want to use BSS-CCA and SCICA as a preprocessing step before the source estimation during the ictal period. METHODS: Both the contaminated and cleaned ictal EEG were subjected to the RAP-MUSIC algorithm. This is a multiple dipole source estimation technique based on the separation of the EEG in signal and noise subspace. The source estimates were compared with the subtracted ictal SPECT (iSPECT) coregistered to magnetic resonance imaging (SISCOM) by means of the euclidean distance between the iSPECT activations and the dipole location estimates. SISCOM results in an image denoting the ictal onset zone with a propagation. RESULTS: We applied the artifact removal and the source estimation on 8 patients. Qualitatively, we can see that 5 out of 8 patients show an improvement of the dipoles. The dipoles are nearer to or have tighter clusters near the iSPECT activation. From the median of the distance measure, we could appreciate that 5 out of 8 patients show improvement. CONCLUSIONS: The results show that BSS-CCA and SCICA can be applied to remove artifacts, but the results should be interpreted with care. The results of the source estimation can be misleading due to excessive noise or modeling errors. Therefore, the accuracy of the source estimation can be increased by preprocessing the ictal EEG segment by BSS-CCA and SCICA. SIGNIFICANCE: This is a pilot study where EEG source localization in the presurgical evaluation can be made more reliable, if preprocessing techniques such as BSS-CCA and SCICA are used prior to EEG source analysis on ictal episodes.

Compression and reconstruction of sorted PET listmode data.

BACKGROUND: In nuclear medicine data can be stored in histogram or listmode format. The most popular histogram format is the planar projection format. Due to the increase in detector blocks, the improved energy resolution and the trends towards time of flight, dynamic and gated imaging, it can be more appropriate to store the data in listmode format. The size of the storage in this format increases linearly with the number of properties (positions, energy, time info) while the histogram format increases exponentially. However, the datasize of listmode data also increases linearly with the number of coincidences. Due to the high number of counts in 3D PET this will lead to very large datasets. Therefore a good compression algorithm for listmode data is very important. METHODS: A sorting and compression method is proposed to reduce the amount of space needed to store the listmode dataset. One event is represented by one number without any information loss compared to the original listmode file. The next step is to sort all events into an array of increasing numbers. These data are compressed by the gzip routine. One of the advantages of 3D PET listmode reconstructions is that they result in a more uniform resolution across the field of view (FOV), which is not always true for other reconstruction algorithms. This improved resolution is shown for the listmode data of a gamma camera operating in PET mode. RESULTS: First the effect of positional accuracy in the listmode dataset is evaluated by comparing resolution in the reconstructions. It is shown that the highest accuracy is not necessary and a significant reduction in the size of the dataset can be obtained prior to lossless compression. A further reduction can be obtained by using the proposed sorting and compression techniques. It is shown that the storage space decreases linearly with the logarithm of the number of coincidences. The compression obtained by different acquisition matrices was compared. Finally it is shown that the 3D listmode reconstruction of sorted listmode data is faster because of improved cache behaviour. The method can be applied to any kind of listmode data. The compression factors will improve when the ratio of measured events to possible events increases.

Cerebral blood flow and oxygen metabolism in symptomatic internal carotid artery occlusion by (traumatic) cervical artery dissection.

The present study investigates whether cerebral infarction resulting from internal carotid artery occlusion by cervical dissection is due to emboli, released from a superimposed luminal thrombus, or is due to haemodynamic failure and hypoperfusion. Ten patients with a history of stroke and with a visible cerebral infarct on computed tomographic scan, due to cervical dissection and thrombosis of the internal carotid artery, were studied with positron emission tomography in order to assess the regional cerebral blood flow (rCBF), the regional cerebral metabolic rate of oxygen (rCMRO2) and the regional oxygen extraction fraction (rOEF) in different regions of the brain. rCBF and rCMRO2 were only decreased in the infarct area but not in the peri-infarct zone or elsewhere in the brain. As rOEF was not increased in the affected cerebral hemisphere, the present study suggests artery-to-artery embolism rather than a haemodynamic event as the cause of the stroke. Use of anticoagulants thus appears to be the appropriate treatment in the acute stage.

Cerebral bloodflow and oxygen metabolism in borderzone and territorial infarcts due to symptomatic carotid artery occlusion.

It remains controversial whether borderzone infarcts are due to compromised cerebral perfusion and whether territorial infarcts are caused by artery-to-artery emboli in case of occlusion of the internal carotid artery. The present positron emission tomography study compares with normal controls, the average regional cerebral bloodflow (rCBF), regional oxygen extraction fraction (rOEF) and regional cerebral metabolic rate for oxygen (rCMRO(2)) in the infarct area, the peri-infarct zone, the remaining homolateral hemisphere and in the contralateral hemisphere of 10 patients with borderzone and 17 patients with territorial infarcts, due to internal carotid artery occlusion by atherosclerosis and by cervical dissection. The steady-state technique with oxygen-15 was used. A nearly significant increase of rOEF with lowered rCBF and rCMRO(2) was observed in the peri-infarct zone of patients with territorial infarcts. In patients with borderzone infarcts rCMRO(2) was decreased in the peri-infarct zone, in the remaining homolateral hemisphere and in the contralateral hemisphere without changes in rCBF and rOEF. The present study finds no arguments that impaired cerebral perfusion is a more frequent cause of borderzone than of territorial infarcts.

Geometrical importance sampling and pulse height tallies in GATE.

Monte Carlo simulations are widely used to study the behavior and detection of gamma photons in medical imaging devices. Such simulations are computationally expensive. This is why geometrical importance sampling, a variance reduction technique, was recently incorporated into the GEANT4 Monte Carlo code. In order to use this technique for single photon emission computed tomography (SPECT) imaging, it needed to be made compatible with pulse height tallies. These tallies correspond to the number of detected pulses in distinct energy bins, covering an energy spectrum relevant to SPECT. Since each pulse is the combination of different detector hits, the tally bin is not known until the end of an event. In an analog simulation (without variance reduction) this poses no problems as each detected hit can be stored and the pulse can be calculated at the end of each event. Geometrical importance sampling combined with Russian Roulette however introduces branches into the particle history, which results in a much more complicated pulse calculation. This work describes how pulse height tallies are adjusted to geometrical importance sampling and Russian Roulette within GATE, a medical imaging and simulation application based on GEANT4. The validation of this technique is done through SPECT simulations comparing the analog result with the new method.

Detection of spike and wave discharges in the cortical EEG of genetic absence epilepsy rats from Strasbourg.

Genetic absence epilepsy rats from Strasbourg (GAERS) are a strain of Wistar rats in which all animals present spontaneous occurrence of spike and wave discharges (SWD) in the cortical electroencephalogram (EEG). In this paper, we present a method for the detection of SWD, based on the key observation that SWD are quasi-periodic signals. A spectral-comb based analysis method is used to extract the fundamental frequency and the percentage of energy explained by the harmonic spectral components is subsequently used as a detection parameter. It is shown that a maximum sensitivity and specificity of up to 96 per cent can be achieved. We also compared the performance of this method with the methods presented in the literature and conclude that the surplus value of the novel detection method lies in the higher specificity that can be obtained in the analysis of long-term EEG fragments, which are contaminated by artefacts and contain large portions of slow-wave sleep.

Positron emission tomographic study of post-ischaemic-hypoxic amnesia.

BACKGROUND: Despite extensive research, it still remains controversial as to what the precise location of the critical lesions underlying amnesia actually is. The amnesic syndrome is believed to be heterogeneous and due to several distinct functional deficits. PATIENTS AND METHODS: Two patients, a 45-year-old woman and a 56-year-old man, with sudden cardiopulmonary arrest and successful resuscitation, were left with a clear amnesic syndrome as main neurological sequela. During their revalidation period, they underwent a positron emission tomographic (PET) examination, utilizing the (13)NH(3) bolus technique at rest and after intravenous acetazolamide administration. RESULTS: Both PET studies showed more or less similar features with a decrease in regional cerebral blood flow (rCBF) in the frontal, temporal and parietal lobes. In addition, the rCBF was increased in both thalami of the 45-year-old woman and in the striata of the 56-year-old man. Acetazolamide vasoreactivity was most lost in the frontal lobes. CONCLUSIONS: In the present PET study, we demonstrated that destruction of the inhibitory pathways to the thalamus and basal ganglia by ischaemic-hypoxic frontal lesions could be one of the mechanisms leading to amnesia.

Cobalt-55 positron emission tomography in vascular dementia: significance of white matter changes.

BACKGROUND: Vascular dementia (VaD) is still used as a covering term to indicate the relationship between cerebrovascular disease and the progressive cognitive disorder. The contribution of white matter changes (WMCs), seen with computed tomography (CT) and magnetic resonance imaging (MRI) of the brain, to dementia had not been fully elucidated. Cobalt-55 (55Co) positron emission tomography (PET) allows us to distinguish between recent and old infarcts. PURPOSE: The present study investigates whether 55Co PET can detect the lesions responsible for the progression of the cognitive disorder in VaD patients. PATIENTS AND METHODS: 20 consecutive patients with a previous history of repeated strokes occurring more than 6 months before and with multiple cortical infarcts, lacunes and WMCs on CT and 5 age-matched controls were investigated with 55Co PET. The stroke patients were divided in two groups: 8 with and 12 without VaD. Average 55Co counts in cerebral cortex, deep gray nuclei and white matter were compared to the value in the cerebellum used as reference. RESULTS: In the control group, the 55Co uptake was identical in the cerebral cortex and in the cerebellum, but lower in the deep gray nuclei and the cerebral white matter. When comparing the stroke groups with the control, the 55Co uptake was similar for the cerebral cortex and deep gray nuclei, but significantly increased in the cerebral white matter. The 55Co uptake was also more increased in the stroke group with VaD compared to the non-demented group. CONCLUSION: 55Co PET shows that the WMCs are due to the ongoing damage of probably ischaemic origin which is more prominent in stroke patients with progressive cognitive decline.

Resolution recovery for list-mode reconstruction in SPECT.

The purpose of the study was to evaluate the resolution recovery in the list-mode iterative reconstruction algorithm (LMIRA) for SPECT. In this study we compare the performance of the proposed method with other iterative resolution recovery methods for different noise levels. We developed an iterative reconstruction method which uses list-mode data instead of binned data. The new algorithm makes use of a more accurate model of the collimator structure. We compared the SPECT list-mode reconstruction with MLEM, OSEM and RBI, all including resolution recovery. For the evaluation we used Gaussian shaped sources with different FWHM at three different locations and three noise levels. For these distributions we calculated the reconstructed images for a different number of iterations. The absolute error for the reconstructed images was used to evaluate the performance. The performance of all four methods is comparable for the sources located in the centre of the field of view. For the sources located out of the centre, the error of the list-mode method is significantly lower than that of the other methods. Splitting the system model into a separate object-dependent and detector-dependent module gives us a flexible reconstruction method. With this we can very easily adapt the resolution recovery to different collimator types.

Parkinsonism in patients with vascular dementia: clinical, computed- and positron emission-tomographic findings.

PURPOSE: The present study investigates the vascular nature of parkinsonian features in patients with 'probable' vascular dementia. PATIENTS AND METHODS: Forty patients with vascular dementia were studied with positron emission tomography (PET) using the steady state technique with (15)O in order to assess regional cerebral blood flow (rCBF), regional oxygen extraction rate (rOER) and regional metabolic rate for oxygen (rCMRO(2)) in different brain regions. The findings in 10 patients with (VaDP) were compared to 30 without parkinsonism (VaD). RESULTS: The clinical and computed tomographic findings in the parkinsonian patients were similar to those described in the literature as probable vascular parkinsonism. The PET results showed decreased rCBF and rCMRO(2) in the frontal and parietal cortices and in the striatum of the VaDP compared to the VaD group. The decrease of rCBF and rCMRO(2) in frontal and parietal cortices is due to the more severe mental changes in the VaDP group. In the VaDP patients rCBF and rCMRO(2) were more decreased and rOER increased in the striatum contralateral to the most affected parkinsonian side. CONCLUSION: Our PET findings show that local ischemic changes in the striatum contribute to parkinsonism in vascular dementia patients.

Differential regional cerebral uptake of (18)F-fluoro-2-deoxy-D-glucose in Alzheimer's disease and frontotemporal dementia at initial diagnosis.

In this paper, the cerebral uptake of (18)F-fluoro-2-deoxy-D-glucose (FDG) was studied in a relatively small group of patients suffering from either clinically diagnosed Alzheimer's disease (AD) or frontotemporal dementia during the initial differential diagnostic workup. The image analysis was done visually and semiquantitatively using three different reference regions. Visual analysis confirmed earlier literature findings on the distribution of decreased FDG uptake and demonstrated prevalent asymmetric patterns in both groups. Only semiquantitative analysis using the sensorimotor cortex as a reference region confirmed the visual findings. Moreover, there were no differences in medial temporal lobe activities between both groups, and there was no correlation of medial temporal lobe activity with dementia severity. In the AD group, a correlation of parietal FDG uptake with MMSE scores was found.

PET imaging using gamma cameras.

This paper will review the recent advances and future developments in the field of coincidence imaging of positron emitters with a conventional Anger-type gamma camera. FDG imaging has shown high clinical importance in cardiology, neurology and especially oncology. Since access to full ring PET is mainly limited to university hospitals, there have been new developments allowing PET imaging on the standard Anger gamma camera. First the principles of coincidence imaging on a gamma camera will be reviewed. We will discuss the limitations of this technique, and the techniques used to partly overcome these limitations. The different configurations of the gamma camera operating in coincidence mode are pointed out. Different corrections for image degrading effects and reconstruction methods are evaluated in the final part.

Iterative reconstruction algorithms in nuclear medicine.

Iterative reconstruction algorithms produce accurate images without streak artifacts as in filtered backprojection. They allow improved incorporation of important corrections for image degrading effects, such as attenuation, scatter and depth-dependent resolution. Only some corrections, which are important for accurate reconstruction in positron emission tomography and single photon emission computed tomography, can be applied to the data before filtered backprojection. The main limitation for introducing iterative algorithms in nuclear medicine has been computation time, which is much longer for iterative techniques than for filtered backprojection. Modern algorithms make use of acceleration techniques to speed up the reconstruction. These acceleration techniques and the development in computer processors have introduced iterative reconstruction in daily nuclear medicine routine. We give an overview of the most important iterative techniques and discuss the different corrections that can be incorporated to improve the image quality.

Image-correction techniques in SPECT.

This overview takes a look at different correction techniques for Single Photon Emission Computed Tomography (SPECT). We discuss the influence of the detection system followed by the scatter and attenuation caused by the object of investigation. When possible we describe how the correction methods for the different physical effects can be incorporated in the reconstruction method, being either filtered backprojection or iterative reconstruction.

Image filtering in single-photon emission computed tomography: principles and applications.

Among the factors contributing to optimal image quality in single-photon emission computed tomography (SPECT), image filtering is an important, though mostly subjectively applied, image-processing parameter. We have aimed at giving an overview of current available SPECT filtering options. Current SPECT applications are discussed and practical guidelines to choose optimal filter parameters for a given situation are provided. It is shown that Hann and Butterworth filters allow accurate approximations to most filter types. The authors support the use of limited filter types in an attempt to standardise image-processing approaches, which may lead to better diagnostic compatibility and interpretation of interdepartmental results.

MRI guided segmentation and quantification of SPECT images of the basal ganglia: a phantom study.

Due to the limited resolution of single-photon emission computed tomography (SPECT) imaging devices, tissue interfaces are not well defined in the reconstructed image, even though resolution recovery techniques may be used during reconstruction. Therefore, segmentation of a particular region and quantification of the tracer uptake in that region is critical due to spillover effects, when based on the SPECT image only. In this study, we present two methods for quantification of tracer uptake in a SPECT image, defined by a matched high resolution structural magnetic resonance image. We show preliminary results of both techniques, when applied for quantifying regional uptake in the different compartments of a phantom simulating the basal ganglia. These results indicate that the quantification method, which takes into account the blurring by the SPECT imaging device, promises to be perform better in the presence of background activity.

European Medical Device Directive: impact on nuclear medicine.

The European Council Directive 93/42/EEC concerning medical devices (14 June 1993) assigns new responsibilities and imposes technical requirements both to the manufacturer and user of medical devices. In this paper the general outlines of the directive are discussed with a particular emphasis on the risk classification of products, the compliance and evaluation process and the CE-marking regulations. Furthermore, some practical implications are highlighted for devices and tools relevant to the field of nuclear medicine such as radiation detectors, gamma- and PET-cameras and software.