Gasser M. Hathout

Os odontoideum in identical twins: perspectives on etiology

Most authorities favor the hypothesis of an acquired etiology of os odontoideum. We present the cases of identical twin sisters with os odontoideum in association with a congenital partial fusion of the posterior elements of the second and third cervical vertebrae, and discuss the implications. We believe that this is the first report of familial os odontoideum in a context which suggests a genetic etiology.

The early response in fMRI: a modeling approach.

A mathematical model is presented to study the generation of the early response phenomenon in fMRI. Initially, we demonstrate that a simple combination of the changes taking place in cerebral blood volume and flow could create the transient early signal decrease, by analyzing their effects on total per voxel deoxyhemoglobin content. Also, we examine the traditional paradigm for the early response: that it may be caused by an early burst of oxidative metabolism and conclude that such changes also explain the early transient response. We suggest that the volume effect may play a role in the generation of the early response phenomenon along with an early upregulation of oxidative metabolism, and that this role may be important if the early response phenomenon is shown to occur at the level of the venous blood pool, and not just at the level of the capillary bed. Magn Reson Med 1999:41:550–554, 1999.

Midbrain Ataxia: Possible Role of the Pedunculopontine Nucleus in Human Locomotion

Midbrain lesions have rarely been implicated as a responsible location in patients presenting with gait instability. In animals, stimulation of the pontomesencephalic area results in rapid walking followed by running. Ablative lesions in this area cause a reduction of locomotor activity [1]. However, the center in humans is probably more dependent upon cortical and subcortical input to activate the system. We report a case of a 66-year-old male who had a subacute infarction in the left posterior tegmentum presenting primarily with gait instability, further supporting the evidence that brainstem locomotor regions also exist in humans.

The lag of cerebral hemodynamics with rapidly alternating periodic stimulation: modeling for functional MRI

A mathematical model that characterizes the response of venous oxygenation to changes in cerebral blood flow (rCBF) and oxygen consumption has been previously presented. We use this model to examine the dampening phenomenon in functional MRI (fMRI) signals with rapidly alternating periodic stimulation bursts. Using a mass balance approach, the equations for an input-output model are derived and solved using Matlab (the Math Works Inc.). Changes in venous oxygenation are related to the results of fMRI experiments using progressively shorter periods of stimulation. An impulse-response function for the model is derived in an attempt to explore the source of the lag in cerebral hemodynamics. Increasing the frequency of stimulation bursts eventually produces a dampening in the fMRI signal. The dampening phenomenon in fMRI signals occurs with stimulation of high frequency on-off alternation. The dynamics of signal dampening, as well as the impulse-response function of a blood oxygen level-dependent model, lend strong indirect support to the hypothesis that blood oxygen level-dependent contrast at the level of the venous blood pool, rather than R1 inflow effects or changes in oxygenation at the level of the capillary bed, underlies the observed signal changes in fMRI.

A Medley of Midbrain Maladies: A Brief Review of Midbrain Anatomy and Syndromology for Radiologists

The midbrain represents the uppermost portion of the brainstem, containing numerous important nuclei and white matter tracts, most of which are involved in motor control, as well as the auditory and visual pathways. Notable midbrain nuclei include the superior and inferior colliculus nuclei, red nucleus, substantia nigra, oculomotor nuclear complex, and trochlear nucleus. In addition, white matter tracts include the brachium conjunctivum, medial and lateral lemniscus, spinothalamic tracts, and the fiber tracts within the cerebral peduncles. Although neurologically vital, many of these small midbrain nuclei and white matter tracts are not easily individually identified on neuroimaging. However, given their diverse functions, midbrain pathology often leads to distinct clinical syndromes. A review and understanding of the location and relationships between the different midbrain nuclei and fiber tracts will allow more precise correlation of radiologic findings with patient pathology and symptomatology. Particular syndromes associated with midbrain pathology include the Weber, Claude, Benedikt, Nothnagel, and Parinaud syndromes. The oculomotor and trochlear cranial nerves also reside at this level. An understanding of their functions as well as their projected courses from the midbrain towards the eye allows identification of distinct locations which are particularly vulnerable to pathology.

A comparison of sagittal short T1 inversion recovery and T2-weighted FSE sequences for detection of multiple sclerosis spinal cord lesions

Multiple sclerosis (MS) is the most common disabling CNS disease of young adults. MRI is routinely used for the detection of MS plaques in the brain and spinal cord. A significant portion of patients with MS demonstrates spinal cord lesions at the time of initial workup, and these lesions are an important part of the McDonald criteria for diagnosis. However, whereas brain imaging sequences are now fairly standardized, there continues to be debate about the optimal sequences for imaging the spinal cord. The short T1 inversion recovery (STIR) sequence has been shown in the current literature to improve lesion detection with its additive T1/T2 weighting, but current spinal cord imaging protocols from the Consortium on MS Center Consensus Guidelines do not include the STIR sequence. We demonstrate that not only do STIR sequences improve lesion detection when compared directly with conventional T2‐weighted sequences, but that they also significantly improve lesion conspicuity, facilitating earlier positive diagnosis and management.

Parameter optimization for quantitative signal-concentration mapping using spoiled gradient echo MRI.

Rationale and Objectives. Accurate signal to tracer concentration maps are critical to quantitative MRI. The purpose of this study was to evaluate and optimize spoiled gradient echo (SPGR) MR sequences for the use of gadolinium (Gd-DTPA) as a kinetic tracer. Methods. Water-gadolinium phantoms were constructed for a physiologic range of gadolinium concentrations. Observed and calculated SPGR signal to concentration curves were generated. Using a percentage error determination, optimal pulse parameters for signal to concentration mapping were obtained. Results. The accuracy of the SPGR equation is a function of the chosen MR pulse parameters, particularly the time to repetition (TR) and the flip angle (FA). At all experimental values of TR, increasing FA decreases the ratio between observed and calculated signals. Conversely, for a constant FA, increasing TR increases this ratio. Using optimized pulse parameter sets, it is possible to achieve excellent accuracy (approximately 5%) over a physiologic range of concentration tracer concentrations. Conclusion. Optimal pulse parameter sets exist and their use is essential for deriving accurate signal to concentration curves in quantitative MRI.

Complex imaging features of accidental cerebral intraventricular gadolinium administration

Gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) is a contrast agent commonly used for enhancing MRI. In this paper, the authors report on 2 cases of postoperative inadvertent administration of Gd-DTPA directly into a ventriculostomy tubing side port that was mistaken for intravenous tubing. Both cases demonstrated a low signal on MRI throughout the ventricular system and dependent portions of the subarachnoid spaces, which was originally believed to be CSF with areas of T1 shortening in the nondependent portions of the subarachnoid spaces, and misinterpreted as basal leptomeningeal enhancement and meningitis. The authors propose that the appearance of profound T1 hypointensity within the ventricles and diffuse susceptibility artifact along the ependyma is pathognomonic of intraventricular Gd-DTPA and should be recognized.

An unusual case of cerebral autosomal-dominant arteriopathy with subcortical infarcts and leukoencephalopathy with occipital lobe involvement

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is an autosomal dominant angiopathy caused by a mutation in the notch 3 gene on chromosome 19. Clinically, patients may be asymptomatic or can present with recurrent ischemic episodes and strokes leading to dementia, depression, pseudobulbar palsy, and hemi- or quadraplegia. Additional manifestations that have been described include migraine (mostly with aura), psychiatric disturbances, and epileptic seizures. Neuroimaging is essential to the diagnosis of CADASIL. On imaging CADASIL is characterized by symmetric involvement by confluent lesions located subcortically in the frontal and temporal lobes as well as in the insula, periventricularly, in the centrum semiovale, in the internal and external capsule, basal ganglia, and brain stem; with relative sparing of the fronto-orbital and the occipital subcortical regions. We describe a 49 year old male with CADASIL with absence of temporal lobe findings on MRI but predominant lesions within the periventricular white matter, occipital lobes with extension into the subcortical frontal lobes, corpus callosum and cerebellar white matter. Although CADASIL characteristically presents with anterior temporal lobe involvement, these findings may be absent and our case addresses the atypical imaging findings in CADASIL.

Detection of subthreshold atrophy in crossed cerebellar degeneration via two-compartment mathematical modeling of cell density in DWI: A proof of concept study

Crossed cerebellar diaschisis (CCD) refers to transneuronal degeneration of the corticopontocerebellar pathway, resulting in atrophy of cerebellum contralateral to supratentorial pathology. CCD is traditionally diagnosed on nuclear medicine studies. Our aim is to apply a biexponential diffusion model, composed of intracellular and extracellular compartments, to the detection of subthreshold CCD on DWI, with the calculated fraction of the intracellular compartment as a proposed measure of cell density. At a voxel-by-voxel basis, we solve for intracellular and extracellular coefficients in each side of the cerebellum and compare the distribution of coefficients between each hemisphere. We demonstrate, in all six CCD cases, a significantly lower contribution of the intracellular compartment to the cerebellar hemisphere contralateral to supratentorial pathology (p < 0.01). In a separate, proof-of-concept case of pontine stroke, we also demonstrate reduced intracellular coefficients in bilateral cerebellar hemispheres, excluding middle cerebellar peduncles (p < 0.01). Our findings are consistent with a decreased intracellular fraction, presumably a surrogate for reduced cellular density in corticopontocerebellar degeneration, despite normal-appearing scans. Our approach allows detection of subthreshold structural changes and offers the additional advantage of applicability to most clinical cases, where only three DWI beta values are available.