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Biomedical subjects

A Valavanis

Publications and source records attributed to A Valavanis.

At least 19 recordsLinked to original sources
PubMed2006

Influence of SENSE on image properties in high-resolution single-shot echo-planar DTI.

Limited spatial resolution is a key obstacle to the study of brain white matter structure with diffusion tensor imaging (DTI). In its frequent implementation with single-excitation spin-echo echo-planar sequences, DTI's ability to resolve small structures is strongly restricted by T2 and T2* decay, B0 inhomogeneity, and limited signal-to-noise ratio (SNR). In this work the influence of sensitivity encoding (SENSE) on diffusion-weighted (DW) image properties is investigated. Computer simulations showed that the PSF becomes narrower with increasing SENSE reduction factors, R, enhancing the intrinsic resolution. After a brief theoretical discussion, we describe the estimation of SNR on a pixel-by-pixel basis as a function of R. The mean image SNR behavior is manifold: SENSE is capable of increasing SNR efficiency by reducing the echo time (TE). Each SNR(R) curve reveals a maximum that depends on the amount of partial Fourier encoding used. The overall best SNR efficiency for an eight-element head coil array and a b-factor of 1000 s/mm2 is achieved at R = 2.1 and partial Fourier encoding of 60%. In vivo tensor maps of volunteers and a patient, with an in-plane resolution of 0.78 x 0.78 mm2, are also presented to demonstrate the practical implementation of the parallel approach.

Arteriovenous Malformations↗
PubMed2005

Resolving fiber crossing using advanced fast marching tractography based on diffusion tensor imaging.

Magnetic resonance diffusion tensor tractography is a powerful tool for the non-invasive depiction of the white matter architecture in the human brain. However, due to limitations in the underlying tensor model, the technique is often unable to reconstruct correct trajectories in heterogeneous fiber arrangements, such as axonal crossings. A novel tractography method based on fast marching (FM) is proposed which is capable of resolving fiber crossings and also permits trajectories to branch. It detects heterogeneous fiber arrangements by incorporating information from the entire diffusion tensor. The FM speed function is adapted to the local tensor characteristics, allowing in particular to maintain the front evolution direction in crossing situations. In addition, the FM's discretization error is reduced by increasing the number of considered possible front evolution directions. The performance of the technique is demonstrated in artificial data and in the healthy human brain. Comparisons with standard FM tractography and conventional line propagation algorithms show that, in the presence of interfering structures, the proposed method is more accurate in reconstructing trajectories. The in vivo results illustrate that the elucidated major white matter pathways are consistent with known anatomy and that multiple crossings and tract branching are handled correctly.

Algorithms↗
PubMed2004

SENSE-DTI at 3 T.

While holding vast potential, diffusion tensor imaging (DTI) with single-excitation protocols still faces serious challenges. Limited spatial resolution, susceptibility to magnetic field inhomogeneity, and low signal-to-noise ratio (SNR) may be considered the most prominent limitations. It is demonstrated that all of these shortcomings can be effectively mitigated by the transition to parallel imaging technology and high magnetic field strength. Using the sensitivity encoding (SENSE) technique at 3 T, brain DTI was performed in nine healthy volunteers. Despite enhanced field inhomogeneity, parallel acquisition permitted both controlling geometric distortions and enhancing spatial resolution up to 0.8 mm in-plane. Heightened SNR requirements were met in part by high base sensitivity at 3 T. A further significant increase in SNR efficiency was accomplished by SENSE acquisition, exploiting enhanced encoding speed for echo time reduction. Based on the resulting image data, high-resolution tensor mapping is demonstrated.

Anisotropy↗
PubMed2004

Normalisation of ocular pulse amplitude after embolisation of dural cavernous sinus arteriovenous fistula.

BACKGROUND: The difference in the ocular pulse amplitude between an individual's eyes may reflect abnormalities of the cerebrovascular circulation and has been shown to be helpful in assessing patients with cavernous sinus arteriovenous fistulas. HISTORY AND SIGNS: A 71-year-old woman presented with episcleral injection, proptosis, bruit and retro-orbital pain on the left side. Ocular pulse amplitude as measured by dynamic contour tonometry was 4.38 +/- 1.23 mm Hg in the right and 9.57 +/- 2.71 mm Hg in the left eye. Angiography revealed the haemodynamics of a dural left cavernous sinus arteriovenous fistula. THERAPY AND OUTCOME: Successful occlusion of the fistula was performed by transvenous embolisation. After embolisation, ocular pulse amplitude was 2.84 +/- 0.60 mm Hg in the right and 1.88 +/- 0.29 mm Hg in the left eye. CONCLUSION: In this case of a dural arteriovenous fistula the ocular pulse amplitude as measured by dynamic contour tonometry parallels closely both clinical findings and neuroimaging.

Aged↗
PubMed2002

Identification of the ascending middle cerebral artery branches on digital subtraction angiography.

We aimed to establish a method for identifying the ascending convexity branches of the middle cerebral artery on digital subtraction arteriography. We reviewed 100 randomly chosen lateral internal carotid arteriograms of patients with aneurysms. After identifying the Sylvian triangle, the posterior internal frontal, paracentral, superior internal parietal and pericallosal arteries, we formulated templates in order to assess the position of the angular, posterior parietal, central, precentral and prefrontal arteries. Using the Sylvian triangle as a template, we identified the angular and posterior parietal arteries in 91% and 96% of cases, respectively. Using a combination of the posterior internal frontal, paracentral, superior internal frontal and pericallosal arteries a template could be formulated in 87% of cases. This was successfully used to identify the central arteries. A template derived from the pericallosal artery and Sylvian triangle was used to identify the prefrontal arteries in 83% of cases. Templates can be formulated on the basis of the Sylvian triangle and anterior cerebral artery branches for consistent identification of branches of the ascending complex of the middle cerebral artery on digital subtraction angiography.

Angiography, Digital Subtraction↗
PubMed2002

A case of aseptic vertebral necrosis in the context of metastatic lumbar disease.

Aseptic bone necrosis has many well recognized etiologies: caisson disease, corticosteroids, alcohol, Gaucher's disease and sickle cell anemia being a few of a long list. Little attention has been paid to metastatic disease as a possible cause and we were unable to find reference in the literature to this association. We present imaging features of metastatic Ewing's sarcoma involving the vertebral spine, which, in certain aspects, mimics other well-known entities of this region, such as Kümmell's and Scheuermann's diseases.

Adult↗
PubMed2002

Evaluation of topography and vascularization of cervical paragangliomas by magnetic resonance imaging and color duplex sonography.

The management of cervical paragangliomas (PGs) depends on their specific type and their relation to adjacent vessels. The purpose of this study was to evaluate the potential of magnetic resonance imaging (MRI) and color duplex sonography (CDS) to classify PGs according to topography and vascularization. Sixteen patients harboring 22 PGs were studied retrospectively. With digital subtraction angiography as reference, the topographic relation of the tumors to the carotid arteries and the internal jugular vein and the patterns of vascularization were assessed. On MRI and CDS the typical feature of 15 carotid PGs was splaying of the carotid bifurcation, with the external carotid displaced anteriorly and the internal carotid artery and internal jugular vein located posteriorly. In five vagal PGs both modalities showed unidirectional anterior displacement of the external and internal carotid arteries. Two jugular PGs were found to extend within the lumen of the internal jugular vein. CDS completely depicted carotid body tumors but failed to delineate the high cervical portion of vagal and jugular PGs. MRI allowed us to assess the entire extent of all PGs. Nineteen lesions showed flow voids corresponding to abundant flow signal on CDS: three carotid body tumors appeared hypovascular on CDS and MRI. On CDS, intratumoral flow was directed cranially in carotid and inferiorly in vagal and jugular PGs. CDS and MRI are suitable for classification of cervical PGs as carotid, vagal or jugular PGs based on the topographic relation to the carotid arteries and internal jugular vein. Visualization of the intrinsic tumor vasculature proved an additional distinguishing criterion on CDS.

Adult↗
PubMed2000

Dynamic characteristics of oxygenation-sensitive MRI signal in different temporal protocols for imaging human brain activity.

The temporal characteristics of cerebral blood oxygenation during human brain activation were monitored with dynamic echo-planar imaging (EPI) using the blood oxygenation level dependent (BOLD) fMRI. We investigated oxygenation-sensitive signal changes: 1. during repetitive block stimuli, to determine the latency of the activation-induced signal change in the primary visual cortex; 2. on shortening the rest periods between constant stimulated phases, to investigate the limitations that this latency poses in temporal resolution of the technique; and 3. on sustained steady-state stimulation, to characterise oxygenation changes during prolonged brain activation using different stimuli. Delayed intrinsic haemodynamic response and a finite signal-to-noise ratio limit the temporal resolution achieved with BOLD fMRI. Separate activation periods were resolved when the delay between consecutive stimulations was at least 2 s. In this study oxygenation remained elevated throughout sustained activation, suggesting a constant rate of oxygen consumption by the primary cortical neurones during activation. Characterisation of fMRI signal dynamics in dynamic temporal protocols is significant both in terms of optimising stimulation protocols and the potential to gain insight into the physiological mechanisms underlying neuronal activation which could increase the clinical applicability of the technique.

Adult↗
PubMed2000

MRI and CT of a haemangioma of the mandible in Kasabach-Merritt syndrome.

Since the description in 1940 of Kasabach-Merritt syndrome (KMS) in patients with capillary haemangiomas, several other vascular tumours have been recognised as possible causes of this coagulopathy. The literature suggests a specific histological pattern of vascular tumours responsible for KMS, excluding capillary haemangioma [1]. There is an extensive literature on, haemangiomas accompanied by thrombocytopenia, and imaging of thrombosis in the lesion, especially cavernous haemangioma of the liver. However, no report has described a haemangioma of the mandible in the acute stage of the coagulopathy, or serial examinations of such a lesion. We report the features of a mandible lesion with KMS and discusses the interpretations of the changes observed.

Child, Preschool↗
PubMed2000

Transient oedema of the cervical spinal cord.

Transient but very intense oedema of the cervical spinal cord was observed in two patients with obstruction of the cerebrospinal fluid (CSF) pathways. Both presented with hydrocephalus, one due to an infratentorial obstructing mass and the other due to postmeningitic adhesive obstruction of the outlet foramina of the fourth ventricle. In animal experiments with obstruction of CSF pathways (due to outlet foramina obstruction or to downward tentorial herniation) flattening and stretching of the ependymal cells along the central canal is observed, followed by disruption and splitting of the ependymal lining and then by extracellular oedema of the subependymal tissue. Without treatment, frank cavity formation develops in a fourth stage. In our two patients, however, most probably because of appropriate decompressive therapy, the oedema disappeared completely without a residual spinal cord lesion.

Adult↗
PubMed2000

Near-real-time guidance using intraoperative magnetic resonance imaging for radical evacuation of hypertensive hematomas in the basal ganglia.

OBJECTIVE: To report our preliminary clinical experience in treating patients with hypertensive hemorrhage in the basal ganglia using a minimally invasive approach facilitated by intraoperative real-time imaging of an open magnetic resonance imaging (MRI) system and a newly designed cutting suction device. METHODS: We developed an artifact-free device for use during intraoperative MRI consisting of a guiding base that locks into a burr hole, a side-cutting composite-based cannula connected to a standard aspirator, and a handpiece that allows aspiration strength to be regulated by the surgeon. Thirteen patients with hypertensive bleeding in the basal ganglia were included in the study. Outcome was evaluated by mortality, Glasgow Outcome Scale score, activities of daily living score, and Rankin score at 2 weeks and at a median of 4.2 months after the hemorrhage. RESULTS: In this group of 13 patients, complete evacuation was achieved in 8 patients (62%) and subtotal evacuation of 75 to 90% of the initial volume in 4 patients (31%); the evacuation was partial in 1 patient (8%). Vascular malformations were preoperatively excluded angiographically. There was no rebleeding during surgery or postoperatively, as demonstrated by immediate postoperative MRI and computed tomography on the 1st postoperative day. Hematomas were evacuated on median Day 4 after the hemorrhage, varying between Day 1 and Day 8; evacuation was performed on Day 21 after the hemorrhage in one patient. Twelve of the 13 patients survived during a median follow-up time of 4.2 months. Neurological function improved in 11 of the 12 patients eligible for assessment. One patient with an additional head injury died 15 days after surgery from pulmonary embolism. CONCLUSION: This study shows an excellent outcome with regard to mortality and a positive trend regarding neurological outcome for the specific group of patients with hypertensive hematomas in the basal ganglia. This minimally invasive approach is feasible in the open intraoperative MRI in combination with the cutting suction device developed in our institution. Online imaging is extremely helpful for planning, guiding, and real-time monitoring of the procedure.

Adult↗
PubMed2000

Spatial relationship between vestibular schwannoma and facial nerve on three-dimensional T2-weighted fast spin-echo MR images.

BACKGROUND AND PURPOSE: During surgical removal of a vestibular schwannoma, correct identification of the facial nerve is necessary for its preservation and continuing function. We prospectively analyzed the spatial relationship between vestibular schwannomas and the facial nerve using 3D T2-weighted and postcontrast T1-weighted spin-echo (SE) MR imaging. METHODS: Twenty-two patients with a unilateral vestibular schwannoma were examined with MR imaging. The position and spatial relationship of the facial nerve to adjacent tumor within the internal auditory canal (IAC) and cerebellopontine angle cistern (CPA) were assessed on multiplanar reformatted 3D T2-weighted fast spin-echo (FSE) images and on postcontrast transverse and coronal T1-weighted SE images. The entrance of the nerve into the bony canal at the meatal foramen and the nerve root exit zone along the brain stem were used as landmarks to follow the nerve course proximally and distally on all images. RESULTS: The spatial relationship between vestibular schwannoma and facial nerve could not be detected on postcontrast T1-weighted SE images. In 86% of the patients, the position of the nerve in relation to the tumor was discernible on multiplanar reformatted 3D T2-weighted FSE images. In tumors with a maximal diameter up to 10 mm, the entire nerve course was visible; in tumors with a diameter of 11 to 24 mm, only segments of the facial nerve were visible; and in tumors larger than 25 mm, the facial nerve could not be seen, owing to focal nerve thinning and obliteration of landmarks within the IAC and CPA. CONCLUSION: Identification of the facial nerve and its position relative to an adjacent vestibular schwannoma is possible on multiplanar reformatted 3D T2-weighted FSE images but not on postcontrast T1-weighted SE images. Detection of this spatial relationship depends on the tumor's size and location.

Adult↗
PubMed2000

Plasticity of the human motor cortex in patients with arteriovenous malformations: a functional MR imaging study.

BACKGROUND AND PURPOSE: The capacity of the human brain to recover from damage has been explained on the basis of plasticity, according to which remaining areas assume functions that would normally have been performed by the damaged brain. Patients with cerebral arteriovenous malformations (AVMs) involving primary motor areas may present without significant neurologic deficits. We used functional MR imaging to investigate the organization of cortical motor areas in patients with AVMs. METHODS: Cortical motor hand and foot representations were mapped in nine right-handed patients harboring AVMs occupying the hand (n = 6) or foot (n = 3) region of the primary motor cortex (M1). None of the patients exhibited motor deficits. Simple movements of the hand and foot were performed. In eight patients, both right and left extremities were tested; in one patient, only the hand contralateral to the AVM was examined. Localization of activation in the affected hemisphere was compared with that in the unaffected hemisphere and evaluated with respect to the normal M1 somatotopic organization shown in earlier functional MR imaging investigations. RESULTS: Cortical activation showed three patterns: 1) functional displacement within the affected M1 independent of the structural distortion induced by the AVM (n = 4), 2) presence of activation within the unaffected M1 ipsilateral to the moving extremity without activation in the affected M1 (n = 3), and 3) prominent activation in nonprimary motor areas without activation in either the affected or unaffected M1 (n = 2). CONCLUSION: Preliminary evidence suggests that brain AVMs lead to reorganization within the somatotopic representation in M1 and to occasional abnormal expansion into nonprimary motor areas.

Adult↗
PubMed1999

3D T2-weighted fast spin-echo MRI sialography of the parotid gland.

The diagnostic value of 3D T2-weighted MRI sialography and 2D T2-weighted fast spin-echo (FSE) images for delineation of the normal duct system and characterisation of parotid gland duct pathology was compared in a prospective study. We studied eight healthy volunteers and 18 patients with pathology of the parotid gland (tumours in 3, sialolithiasis in 6, Sjögren's disease in 4, recurrent or chronic parotitis in 4, post-traumatic stricture of the main parotid duct in 1). A heavily T2-weighted 3D FSE sequence was compared with a conventional 2D T2-weighted FSE sequence. The normal main parotid duct was always visible on 3D sialography and seen in 68% of the 2D T2-weighted FSE studies. The diagnostic reliability of both sequences for diagnosis of luminal concretions in sialolithiasis and dilatation of the duct in duct stricture or chronic parotitis was equal, although slight intraglandular dilatation was appreciated only on 3D sialography. Extraductal pathology resulting in obstruction or displacement of ducts was better characterised on 2D T2-weighted images. However, 3D MRI sialography offered the advantage of postprocessing with overview images and multiple maximum-intensity projection images in any plane.

Adolescent↗
PubMed1999

Contrast-enhanced MR angiography of the supra-aortic vessels in 24 seconds: a feasibility study.

A contrast-enhanced, gradient-echo 3D pulse sequence providing angiographic information in 24 s was tested in five healthy subjects and used prospectively in 21 patients for the investigation of the cervical arteries. Indications included suspected stenosis of the carotid (in 13), or vertebral arteries (in 1), carotid dissection (3), variants of the branches of the aortic arch (2) and extracranial carotid aneurysms (2). The results in all patients were compared with those of intra-arterial digital subtraction angiography (DSA). In patients with carotid stenosis, they were also compared with high-resolution 3D time-of-flight (TOF) MR angiography (MRA). Good quality MR angiograms of the neck vessels were obtained with the fast 3D sequence in 20 of the 21 patients. One claustrophobic patient was unable to cooperate. The degree of internal carotid artery (ICA) stenosis was graded correctly (compared to DSA) in 21 of 24 cases (87.5%). Two mild stenoses were overestimated as moderate using the fast MR sequence and one high-grade stenosis was misdiagnosed as a complete occlusion. Carotid dissection was confirmed in one case and correctly excluded in two. Four extracranial ICA aneurysms in two patients, arterial variants and stenosis of the origin of the vertebral artery were correctly diagnosed using the contrast-enhanced MR angiogram. Three-dimensional TOF MRA was unsuccessful due to motion artefacts in half of the cases of ICA stenosis.

Adult↗
PubMed1999

Endovascular treatment of cerebral vasospasm: assessment of treatment effect by cerebral angiography and transcranial colour Doppler sonography.

In a retrospective study of 30 consecutive patients with symptomatic vasospasm the mean degree of narrowing as compared to the initial angiogram was 35% (+/- 24%) in 12 intradural internal carotid arteries (ICA), 42% (+/- 17%) in 42 proximal middle cerebral (MCA) and 38% (+/- 19%) in 27 anterior cerebral arteries (ACA). The corresponding increase in mean flow velocities from baseline values obtained by transcranial colour Doppler sonography (TCD) within 12 h of the first angiogram to the time of clinical vasospasm was considerably higher, with 49% (+/- 34%) in the ICA 119% (+/- 92%) in the MCA and 147% (+/- 170%) in the ACA. Following superselective intra-arterial papaverine application in 66 arteries and balloon angioplasty of 15 arteries, 78 (96.3%) of 81 dilated. Sustained clinical improvement was achieved in 22 patients (73.3%). The mean reversal of angiographic vasospasm was 71% for the ICA (range 10-100%), 81% for the MCA (range 9-100%) and 82% (range 0-100%) for the A1 segment. The mean reduction of flow velocities after treatment was much less with 23% (+/- 21%) in the ICA, 32% (+/- 24%) in the MCA and 25% (+/- 22%) in the A1 segment.

Adult↗