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S Akoka

Publications and source records attributed to S Akoka.

At least 19 recordsLinked to original sources
PubMed1998

Effects of neonatal focal cerebral hypoxia-ischemia on sleep-waking pattern, ECoG power spectra and locomotor activity in the adult rat.

The aim of this study was to determine the influence of neonatal focal cerebral hypoxia-ischemia (HI) on sleep-waking pattern, electrocorticogram (ECoG) power spectra and locomotor activity (LA) in adult Wistar rats. Seven-day old pups were subjected to permanent unilateral ligation of the common carotid artery and transient hypoxia (8% O2). At 10 weeks of age, the extent of brain damages was evaluated by magnetic resonance imaging (MRI) and homogenous injured animals were selected before chronic implantation of radiotelemetry device. Using a single ECoG recording channel method, waking (W), paradoxical sleep (PS) and slow wave sleep (SWS) were continuously recorded for 72 h and they were semi-automatically analyzed off-line. We observed that neonatal HI triggers a cascade of events leading, in adult rats, to brain dysfunction characterized by an increase in SWS (55.0 vs. 40.2% in sham-operated rats, p<0.05) and a marked decrease in W phases duration (43.4 vs. 51.5%, p<0.05) while PS was almost suppressed in HI rats (1.6 vs. 8.3%, p<0.05). In addition, power spectral analysis of ECoG revealed significant (p<0.05) alteration in PS power density with a shift of the dominant frequency peak (5.0 to 7.5 Hz for HI and sham-operated rats, respectively). During the light period, we found that HI induced a pronounced reduction of LA (-30%, p<0.05). These results indicate that Wistar rats exposed to a neonatal unilateral cerebral HI present significant ECoG activity, sleep-waking pattern and behavioral disturbances when adults. However, it remains to establish whether such alterations can be prevented by neuroprotective agents.

Animals↗
PubMed1998

[Comparison of three fat suppression sequences for the detection of vertebral detection. Turbo STIR, phase contrast gradient-echo, and MISTEC-Chopper after gadolinium injection].

OBJECTIVES: Assess three fat suppression sequences used to search for spinal metastases: TurboSTIR, phase contrast gradient-echo, and MISTEC-Chopper after gadolinium injection. MATERIAL AND METHODS: A prospective study was conducted in 10 patients with primary neoplasia. MIR sequences acquired (1 Tesla) were TurboSTIR, T1 spin-echo with and without gadolinium injection, phase contrast gradient-echo and M-Chop after gadolinium injection. Signal intensity in normal bone marrow, metastatic tissue, and subcutaneous fat as well as background noise was measured. Signal-to-noise (S/N) ratio was determined. Lesion borders, artefacts, and extent of detected lesions were determined quantitatively. Bone marrow signal intensity was also recorded. RESULTS: S/N ratio was best with gradient-echo which identified well the borders of lesions within the hemopoietic marrow. For lesions located in high-fat marrow (as in post-radiation marrow), the high intensity signal of the lesion confounded with the fat signal. TurboSTIR gave effective fat signal suppression and was particularly useful for yellow marrow, less so for red marrow. This technique confounded cell proliferation with perilesional edema (enlarging lesion extention). In one case, this sequence did not detect a small lesion visible with the two other sequences. This sequence was sensitive to artefacts (especially vascular artefacts) which can produce false nodular images. M-Chop gave good suppression of vertebral fat tissue (better for yellow marrow) but subjective detection of lesions was more difficult. CONCLUSION: The phase contrast gradient-echo sequence after gadlinium injection appeared to be the best sequence excepting cases of post-trauma (radiotherapy or chemotherapy) fat transformation of the marrow where the TurboSTIR sequence could be preferred.

Adipose Tissue↗
PubMed1997

A new method for absolute quantitation of MRS metabolites.

A new method for absolute quantitation of MRS spectra is presented. This method is not based on a reference peak, derived from a real NMR signal, but rather on a synthesized NMR reference produced by an electronic device, transmitted by a broad-band antenna to avoid quality factor variations. This signal is therefore received at the same time as the sample signal. The reference line produced is stable in time (maximum variation lower than 2%) and allows precise and accurate measurement of absolute concentrations (mean error lower than 3%) in vitro and in vivo.

Humans↗
PubMed1997

Three-point Dixon method with a MISSTEC sequence.

We propose an adaptation of the MISSTEC sequence (simultaneous acquisition of a spin echo and several stimulated echoes) for performing fat suppression with a three-point Dixon method. In vivo measurements were performed on healthy volunteers using a sequence of three echoes (one spin echo and two stimulated echoes) within a third of the acquisition time taken by the regular three-point method, with the same spatial resolution and with half the signal-to-noise ratio (SNR). Homogeneity of fat suppression was greatly increased in comparison with the two-point method.

Algorithms↗
PubMed1997

T2 relaxation time as a marker of brain myelination: experimental MR study in two neonatal animal models.

The progress of myelination in the brain was evaluated by visualization of grey/white matter differentiation on magnetic resonance (MR) images and quantitative analysis of MR data. In vivo quantitative MR imaging was used to monitor the T2 transverse relaxation time changes associated with cerebral development and myelination. The progress of myelination was evaluated using two neonatal animal models, the monkey and the dog, known to mature at very different rates. Three beagles were studied from birth to 4 months of age and nine baboons from 1 to 30 months of age. The T2 values in the frontal, parietal and occipital white matter were calculated and the changes in these values with age were followed. Brain maturation in both species was found to correspond to decreasing T2 values in both grey and white matter. This decrease was observed both in the dog brain and, despite slower maturation, in the baboon brain, and appeared to fit with the myelination process in these models. Exploiting the physicochemical parameters of water in tissues via T2 determination is a convenient and reliable strategy for the documentation of brain development in both experimental approaches and clinical situations.

Age Factors↗
PubMed1997

[Circulation and cerebral metabolism in neonatal hypoxia-ischemia].

The basic physiological variable in hypoxic-ischaemic brain injury is cerebral oxygen delivery. When oxygen delivery becomes insufficient to meet the cellular demands for oxygen, a sequence of biochemical events will be triggered leading to cell death. High levels of CBF following severe birth asphyxia is now well documented by Doppler ultrasound which has been shown to be a useful prognostic indicator following birth asphyxia. Near infrared spectroscopy (NIRS) is of great potential value since it may be used at the bed-sid and allows to measure the cerebral blood volume and the concentrations of cytochrome aa3. Magnetic resonance spectroscopy (MRS) allows noninvasive assessment of cerebral metabolism in asphyxiated neonates. 31P MRS has demonstrated that birth asphyxia leads to delayed impairment of cerebral energy metabolism and is predictive of later neurodevelopmental outcome. 1H MRS has shown lactate accumulation and a later decline in N-acetyl aspartate concentration.

Asphyxia Neonatorum↗
PubMed1996

Twenty-day cerebral and umbilical Doppler monitoring on a growth retarded and hypoxic fetus.

In one growth retarded and hypoxic fetus, the cerebral and umbilical hemodynamic changes were assessed (by Doppler), daily over 20 days. The fetal brain was investigated by magnetic resonance imaging (MRI) close to the delivery, and because the fetus died at delivery we performed an anatomical study of the fetal brain. The evolution of the fetal hemodynamics (day by day) was interpreted according to the MRI findings and the clinical findings. During the period of observation (under sustained hypoxia) the fetal deterioration was characterized by: (a) the progressive development of the oligohydramnios (190d), (b) the disappearance of the vascular reactivity (eight successive cerebral resistance index (RI) constant at 194d), (c) the occurrence of fetal heart rate decelerations (199d), and finally (d) the increase of the cerebral vascular resistances with reduction of the brain perfusion (204d). The anatomical study of the brain showed a periventricular congestion however the histology revealed hypoxic lesions like gliosis and a marked vasodilation of the anterior and middle cerebral arteries. Finally in addition to single Doppler measurements performed 1 week before delivery (for prediction of fetal outcome), one can suggest to use the 'loss of fluctuation of the cerebral RI' to identify the beginning of the period of very high risk for the fetus. Such hypothesis may have to be confirmed on a larger number of pathological pregnancies.

Adult↗
PubMed1996

Potential of T2 relaxation time measurements for early detection of radiation injury to the brain: experimental study in pigs.

PURPOSE: To investigate the MR T2 relaxation time and histologic changes after a single-fraction 25-Gy dose of radiation to the brain of pigs. METHODS: The right hemisphere of 10 Meishan pigs was irradiated with a single dose of 25 GY at the 90% isodose, using a 12-MeV electron beam. T2 relaxation time was measured within three regions of interest in the brain: those that had received 90%, 70%, and 40% of the total dose, respectively. T2 kinetics over time was compared with histologic studies. RESULTS: Brain T2 values were noted to increase within the irradiated areas. T2 kinetics were analyzed in three phases: an immediate transient phase and two long-lasting phases. These two long-lasting phases were correlated with the detection of ventricular compression and necrosis, respectively. The T2 increase within the 90% region of interest was 19%, 22%, and 26% for phases I, II, and III, respectively. T2 measurements within other regions of interest were not significant. CONCLUSION: Although our results suggest a dose threshold for T2 variations, brain T2 values increased after irradiation at a level at which disease could not be seen on conventional MR images. This illustrates the value of using conventional MR imaging in a quantitative manner to assess molecular tissue abnormalities at earlier stages of developing diseases.

Animals↗
PubMed1996

[Use of a stimulated echo sequence in the MRI study of the brain and spine].

We describe in this paper how the STEAM sequence can be an efficient tool to obtain images free of flow artifacts in anatomical situation where the spin echo failed. The simplest way to eliminate flow artifacts is to exploit the dephasing induced by motion in magnetic field gradients and to reduce to zero the signal from moving tissues. This can be achieve by increasing the time elapsed between the spin excitation and the signal observed. Because of T2 relaxation, such an increase results in a signal decrease when the spin echo sequence is used. The STEAM sequence has the unique property that the time elapsed between observation and excitation can be increased without change in T2 value and so allows a good suppression of signals from the moving spins with short TE. Our results demonstrate that, although the stimulated echo intensity is only half that of a spin echo taken at the same read out time, the advantages of STEAM imaging can compensate for this partial loss in signal to noise in some particular clinical situations. The influence of mixing time on contrast has been evaluated using thoracic spine imaging and it has been shown that contrast between spine and CSF can be significantly improved (+ 60%) when TM is increased (from 17 ms to 50 ms). In the same time, the contrast between spine and fat issue decreases (40%). This last effect facilitates the adjustment of contrast window. Suppression of motion artifacts has first been evaluated with thoracic spine imaging, using a whole body coil. Suppression of artifacts was better than that obtained with a flow compensated spin echo sequence, especially in the case of kyphotic patients when a presaturation band was inefficient. In a second step suppression of motion artifacts has been evaluated from posterior fossa examination after injection of a paramagnetic contrast agent. The images obtained with the stimulated echo sequence show a dramatic reduction of signal from blood in the lateral sinus, and therefore an increase of quality by elimination of motion artifacts.

Adipose Tissue↗
PubMed1995

Quantitative magnetic resonance and isotopic imaging: early evaluation of radiation injury to the brain.

PURPOSE: Using magnetic resonance (MR) and isotopic imaging to investigate the cerebral alterations after highdose single-fraction irradiation on a pig model. We assessed the nuclear magnetic resonance (NMR) relaxation times as early markers of radiation injury to the healthy brain. METHODS AND MATERIALS: A total of 17 animals was studied; 15 irradiated and 2 unirradiated controls. Pigs were irradiated with a 12 MeV electron beam at a rate of 2 Gy/min. Ten animals received 40 Gy at the 90% isodose, five animals received 60 Gy, and two animals were unirradiated. The follow-up intervals ranged from 2 days to 6 months. T1-weighted scans, T2-weighted scans, and scintigrams were performed on all animals to study neurological abnormalities, cerebral blood flow, and blood-brain barrier (BBB) integrity. T1 and T2 relaxation times were measured in selected regions of interest (ROIs) within the irradiated and contralateral hemispheres. A ratio T1 after irradiation/T1 before irradiation, and a ratio T2 after irradiation/T2 before irradiation, were calculated, pooled for each dose group, and followed as a function of time after irradiation. RESULTS: Scintigraphy visualized the brain perfusion defect and BBB disruption in all irradiated brains. The ratio T2 after irradiation/T2 before irradiation was proportional to the effective dose received. The T2 ratio kinetics could be analyzed in three phases:an immediate and transient phase, two long-lasting phases, which preceded compression of the irradiated lateral ventricle, and edema and necrosis at later stages of radiation injury, respectively. The magnetic resonance imaging (MRI) observations correlated well with histological analysis. CONCLUSION: The results show that quantitative imaging is a sensitive in vivo method for early detection of cerebral radiation injury. The reliability and dose dependence of T2 relaxation time may offer new opportunities to detect and understand brain pathophysiology after high-dose single-fraction irradiation.

Animals↗
PubMed1995

Chemical shift imaging from simultaneous acquisition of a primary and a stimulated echo.

An imaging method is presented for obtaining chemical shift images from only one acquisition. Images are acquired with the same spatial resolution as in regular spin-echo imaging. The sequence is based on the simultaneous acquisition of a spin echo and a stimulated echo in a single pass. The use of 90 degrees radiofrequency pulse flip angles results in the same proton density, T2 and T1 weighting for the two echoes. Application of this sequence to chemical shift imaging is discussed for three fat suppression techniques (chopper, CHESS, and hybrid). Imaging was performed on phatoms and volunteers. The image quality was the same as those obtained by the chopper and the hybrid methods and the acquisition time was reduced by a factor of two.

Adipose Tissue↗
PubMed1995

[Nuclear magnetic resonance spectroscopy: methodology and applications to the study of asphyxia neonatorum].

Cerebral metabolism has been extensively studied by magnetic resonance spectroscopy (MRS). MRS allows the study of neonates brain maturation as well as the onset and the evolution of brain injury. The use of phosphorous spectroscopy allows the quantification of phosphorylated metabolites. Thus, the measurement of the relative concentrations of creatine-phosphate and inorganic-phosphate is a prognostic factor of the outcome of a neonate after birth asphyxia. Absolute concentrations have more recently been studied and seem to be more significant. Proton MRS gives access to brain metabolites such as choline, lactate, N-acetyl aspartate and taurine. Its use is more recent than the phosphorous spectroscopy but first results already show its potential in neonatology.

Asphyxia Neonatorum↗
PubMed1995

Measurement of epidermal moisture content by magnetic resonance imaging: assessment of a hydration cream.

The moisture content of the epidermis was measured by magnetic resonance imaging (MRI), using transverse relaxation time. The spatial resolution was 86 microns, allowing a quantitative, accurate and localized determination of variations in epidermal hydration. The wrists of 15 volunteers were studied before and after application of a hydration cream. Results showed an increase of 15% of epidermal T2 after application of the cream. Moisture content curves varied according to different degrees of skin dryness. This study demonstrates that MRI is a useful tool in evaluation of epidermal hydration.

Administration, Cutaneous↗
PubMed1995

Magnetic resonance imaging in vivo monitoring of T2 relaxation time: quantitative assessment of primate brain maturation.

This work describes quantitative MRI assessment of primate brain maturation. Nine young baboons were followed from the age of one to 30 months. Assessment of myelination was based on the gray/white matter contrast on MR images and the evolution of T2 relaxation time respectively. The brain maturation began in the posterior fossa and progressed to the olfactory bulbs corresponding to decreasing white matter T2 values. Relaxation parameters provide new opportunities to trace the myelination process in vivo.

Aging↗
PubMed1995

T1 mapping from spin echo and stimulated echoes.

We present an imaging method to obtain a map of the spin-lattice relaxation time. Images were acquired with the same spatial resolution and in the same time as for a regular spin-echo acquisition. The sequence was based on the simultaneous acquisition of a spin echo and several stimulated echoes with the same intensity except for T1 weighting which increases with the interval between the excitation pulse and the readout pulse. T1 values obtained on phantoms were compared to those from the inversion-recovery method and show the accuracy (2%) and the precision (5%) of the method. T1 images of the brain of a healthy volunteer are presented and demonstrate the ability of the method to obtain T1 mapping in vivo in 12 min and without susceptibility artifacts. In vivo and in vitro results were compared to those obtained by a TOMROP sequence in the same acquisition time.

Biophysical Phenomena↗
PubMed1995

Experimental MR study of cerebral radiation injury: quantitative T2 changes over time and histopathologic correlation.

PURPOSE: To use the pig brain as a large-animal model to examine the effects of high-dose single-fraction irradiation on MR images, T2 relaxation time, and histologic integrity. METHODS: A total of 24 Meishan pigs were studied: 20 irradiated animals and 4 unirradiated controls. A high dose was delivered to the right hemisphere of the animals, using a 12-MeV electron beam. Ten animals received 40 Gy at the 90% isodose, and 10 animals received 60 Gy. Quantitative measurement of T2 relaxation time was compared with qualitative analysis of T2-weighted images and histologic studies. RESULTS: Quantitative analysis revealed a reproducible increase of the T2 parameter within the irradiated areas. The T2 kinetic could be analyzed in two phases, which appeared before the visualization of ventricle compression, necrosis, and edema. The first is characterized by vascular inflammation and the latter by radiation necrosis and edema. Both are dose dependent. CONCLUSION: These results underline the ability of quantitative MR for early diagnosis of brain radiation lesions in vivo.

Animals↗
PubMed1994

Concurrent changes in intracranial pressure, cerebral blood flow velocity, and brain energy metabolism in rabbits with acute intracranial hypertension.

The relationship between intracranial pressure or cerebral perfusion pressure (CPP), cerebral blood flow, and brain energy failure is unpredictable throughout the development of acute intracranial hypertension. The purpose of the present study was to correlate intracranial pressure with cerebral blood flow velocities and brain energy metabolism in adult rabbits. The acute intracranial hypertension was achieved by pressure transmission. Transcranial Doppler wave-forms were obtained from the basilar artery for monitoring cerebral blood flow velocities. 31P-Magnetic resonance spectroscopy was used to assess brain energy metabolism. The diastolic blood flow velocity began to decrease significantly (34.5%) when the intracranial pressure was equal to half the diastolic arterial pressure for a CPP of 36 +/- 18 mmHg. Circulatory cerebral resistances increased significantly (55%) for the same value of CPP. Diastolic frequency was near zero when intracranial pressure approached diastolic arterial pressure (51 +/- 12 mmHg), corresponding to a CPP of 30 +/- 15 mmHg. At the same time, only a tendency for brain energy metabolism to decrease was observed. Consequently, transcranial Doppler sonography could be proposed for the follow-up of intracranial hypertension. Magnetic resonance spectroscopy could help to monitor these patients and could be especially proposed in case of high intracranial pressure (near diastolic arterial pressure). The joint use of these two methods would help in making appropriate therapeutic decision in humans.

Animals↗