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

PURPOSE: 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. MATERIALS AND METHODS: Dedicated MR spinal cord imaging of twenty-nine sequential patients with clinically confirmed multiple sclerosis was retrospectively analyzed by two independent neuroradiologists in a novel study design. Sagittal T2-weighted and STIR sequence images from the same study for each patient were examined for MS plaques using a double-blinded review of individual images 'separated in time and space', such that STIR and T2 image pairs were never analyzed simultaneously. Number of lesions and lesion conspicuity for each lesion, using a subjective scale (1-5), were tallied for each sequence. Averages for each observer were compared using a paired t-test analysis for statistical significance, and assessment of inter-rater agreement was assessed using Cohen's kappa index. RESULTS: Significantly, more MS lesions were detected on STIR than on T2-weighted sequences for both observers (P = 0.001 and P = 0.005). In seven patients, the conventional T2 sequence detected no lesions at all, whereas STIR sequences showed significant cord involvement. Lesion conspicuity was also significantly better on STIR for both observers (P < 0.0005). This improved conspicuity leads to more uniform lesion detection. On the conventional T2-weighted sequence, there was a statistically significant difference in the number of lesions detected between the two observers (P = 0.003), but there was no statistically significant difference on STIR (P = 0.43). The kappa index showed greater interobserver agreement in both lesion count and lesion conspicuity on the STIR sequence as compared with T2. CONCLUSIONS: Short T1 inversion recovery sequence imaging not only significantly improves detection of MS lesions within the spinal cord, but also provides better contrast and conspicuity of visible lesions, creating a more confident diagnostic measure of MS extent and progression. Short T1 inversion recovery sequences of the spinal cord should be routinely obtained during initial and routine follow-up of MS.

Cerebral toxoplasmosis: case review and description of a new imaging sign.

Toxoplasmosis can have catastrophic consequences in immunocompromised patients if left untreated. Accurate diagnosis is difficult, as there is substantial overlap between the imaging findings and presenting clinical syndromes of cerebral toxoplasmosis and primary central nervous system lymphoma. This paper reviews the previously described and fairly well-known post-contrast computed tomography (CT) and T1-weighted (W) magnetic resonance imaging (MRI) target signs seen in toxoplasmosis. In addition, it offers a new imaging sign, the T2W/FLAIR (fluid attenuated inversion recovery) target sign, which is often seen in clinical practice but not well-published, as an aid to the diagnosis of cerebral toxoplasmosis.

Imaging of the internal carotid artery: the dilemma of total versus near total occlusion.

PURPOSE: To evaluate ultrasonography (US) and magnetic resonance (MR) angiography in the differentiation between occlusion and near occlusion of internal carotid artery (ICA). MATERIALS AND METHODS: Consecutive patients with occlusion or near occlusion of ICA at catheter angiography and who underwent MR angiography and US were included. MR angiography and US were compared with catheter angiography, the standard, for the ability to help distinguish occlusion from near occlusion. Noninvasive examinations were evaluated for the ability to classify near occlusions as having severe focal stenosis with distal luminal collapse versus diffuse nonfocal disease. The 95% CIs were calculated. RESULTS: In 55 of 274 patients with 548 ICAs, catheter angiography depicted 37 total occlusions and 21 near occlusions. US depicted all total occlusions; MR angiography depicted 34 (92%) (95% CI: 0.78, 0.98). US depicted 18 (86%) of 21 (95% CI: 0.64, 0.97) near occlusions; MR angiography depicted all (100%). Of 18 vessels that were determined to be patent at US, 17 (94%) (95% CI: 0.73, 0.99) were classified as having focal stenosis or diffuse disease. Because flow gaps were identified in vessels with focal and diffuse disease, MR angiography was not effective in helping to differentiate these lesions. CONCLUSION: Assuming US is the initial imaging examination, when occlusion is diagnosed, MR angiography can depict it. If occlusion is confirmed, no further imaging is necessary. US performed well in helping to differentiate vessels with focal severe stenosis from those with diffuse disease. MR angiography added little in this group. Catheter angiography remains beneficial for vessels with diffuse nonfocal narrowing.

Systems analysis of functional magnetic resonance imaging data using a physiologic model of venous oxygenation.

The authors revisit a simple mathematical model, presented in previous work, that characterizes the response of cerebral venous oxygenation to changes in blood flow and oxygen consumption. This physiologically based model can qualitatively duplicate the results of several recent empirical studies in which other authors have tested the hypothesis of linearity in the functional magnetic resonance imaging (fMRI) response to task activation, in that the experimentally found nearly linear behavior of the system and also its subtle departures from linearity are both predicted by simulations of the model. The model is simple enough that its equations can be explicitly solved. Moreover, an amended model that incorporates a varying cerebral blood volume parameter is found to have similar if not better consistency with the empirical data; indeed, this "extended" model is shown to be solvable by the same differential equation as the authors' simple one, wherein the volume is fixed as a constant. These investigations lend further indirect support to the blood oxygen level-dependent hypothesis of venous deoxyhemoglobin as the primary mechanism for fMRI signal changes during task activation, as well as for the authors' simple system as a useful physiologic model thereof. Although the authors' mathematical model does not formally represent a linear system with respect to the flow input, its underlying linear character may help partially explain the "nearly" linear behavior of the fMRI response.

Mesiotemporal T2-weighted hyperintensity: neurosyphilis mimicking herpes encephalitis.

Bilateral mesiotemporal hyperintensity on T2-weighted and fluid-attenuated inversion recovery MR images of a patient with a clinical syndrome of encephalitis is considered to be a classic finding for herpes simplex virus infection. We report a case of neurosyphilis with identical MR imaging abnormalities and a similar clinical presentation. Because syphilis is not routinely tested for, awareness of this mimicry of herpes simplex virus encephalitis is important, considering the potential therapeutic implications.

Ability to use duplex US to quantify internal carotid arterial stenoses: fact or fiction?

PURPOSE: To determine if duplex ultrasonography (US) can help predict the degree of internal carotid arterial (ICA) stenosis. MATERIALS AND METHODS: ICA peak systolic velocity (PSV) and the ratio of the PSV in the ICA to that in the ipsilateral common carotid artery (VICA/VCCA) were compared with the degree of arteriographically measured stenosis. ICAs were arteriographically subgrouped at 10% incremental levels of stenosis and broader ranges. Mean PSV, VICA/VCCA, and SDs were calculated for each category. Histograms showing the numbers of stenotic ICAs in subgroups and for vessels with stenoses of greater than or equal to or less than 70% narrowing were constructed. The number of vessels correctly subgrouped with typical Doppler US thresholds was calculated. RESULTS: Mean PSV and VICA/VCCA increased with stenosis level (P < .01); SDs were wide. Histograms showed Doppler US values in the central groups across all disease levels. Histograms differentiating at least or less than 70% stenosis showed minimal overlap. PSV and VICA/VCCA helped classify, respectively, 185 and 181 of 204 vessels with stenoses of less than 50%, 15 and 21 of 46 vessels with stenoses of 50%-69%, and 73 and 67 of 84 vessels with stenoses of 70% or greater. When classifying stenoses as 69% or less or 70% or more, PSV and VICA/VCCA were correct in 90.6% and 90.3% of vessels. CONCLUSION: Doppler US is excellent for classifying stenoses as above or below a single degree of severity but does not function well in stenosis subclassification.

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.

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.

Functional magnetic resonance imaging reveals brain regions mediating the response to resistive expiratory loads in humans.

Obstructive lung disease is the most common form of respiratory disturbance. However, the location of brain structures underlying the ventilatory response to resistive expiratory loads is unknown in humans. To study this issue, midsagittal magnetic resonance images were acquired in eight healthy volunteers before and after application of a moderate resistive expiratory load (30 cmH2O/liter/s), using functional magnetic resonance imaging (fMRI) strategies (1.5-T magnetic resonance; repetition time: 72 ms; echo time: 45 ms; flip angle: 30 degrees; field of view: 26 cm; slice thickness: 5 mm; 128 x 256 x 1 number of excitations). Digital image subtractions and region of interest analyses revealed significant increases in fMRI signal intensity in discrete areas of the ventral medulla, ventral and dorsal pontomedullary structures, basal forebrain, and cerebellum. Upon load withdrawal, a rapid fMRI signal off-transient occurred in all activated sites. Application of an identical load immediately after recovery from the initial stimulus resulted in smaller signal increases (P < 0.02). Prolongation of load duration was associated with progressive fMRI signal decrease across activated regions. In three additional subjects, the threshold for significant MRI signal increases was established at expiratory loads > or = 15 cmH2O/liter/s and was dose dependent with increasing loads. We conclude that resistive expiratory loads > or = 15 cmH2O/liter/s elicit regional activation of discrete brain locations in humans.

A quantitative physiologic model of blood oxygenation for functional magnetic resonance imaging.

RATIONALE AND OBJECTIVES: Variations in venous deoxyhemoglobin levels in response to neuronal activation represent a complex interplay between focal changes in cerebral blood flow (CBF), cerebral blood volume (CBV), and regional metabolism. The authors present a mathematic model that characterizes the response of venous oxygenation to changes in these variables. METHODS: Using a mass balance approach, the equations for a simple input-output model are derived and solved using Matlab. Changes in blood oxygenation are related to available results from functional magnetic resonance imaging experiments. RESULTS: Increases in CBF produce declines in oxygen extraction fraction and venous deoxyhemoglobin according to Fick's law, and are quantitatively in agreement with available magnetic resonance and positron-emission tomography data. A flow-volume envelope defines the changes in CBF relative to CBV. CONCLUSIONS: It is possible to obtain a quantitative understanding of changes in blood oxygenation and to relate these changes to the observed dynamics of magnetic resonance signal change in the setting of functional stimulation.

Identification of human brain regions underlying responses to resistive inspiratory loading with functional magnetic resonance imaging.

Compensatory ventilatory responses to increased inspiratory loading are essential for adequate breathing regulation in a number of pulmonary diseases; however, the human brain sites mediating such responses are unknown. Midsagittal and axial images were acquired in 11 healthy volunteers during unloaded and loaded (30 cmH2O; 1 cmH2O = 98 Pa) inspiratory breathing, by using functional magnetic resonance imaging (fMRI) strategies (1.5-tesla MR; repetition time, 72 msec; echo time, 45 msec; flip angle, 30 degrees; field of view, 26 cm; slice thickness, 5 mm; number of excitations, 1; matrix, 128 x 256). Digital image subtractions and region of interest analyses revealed significantly increased fMRI signal intensity in discrete areas of the ventral and dorsal pons, interpeduncular nucleus, basal forebrain, putamen, and cerebellar regions. Upon load withdrawal, certain regions displayed a rapid fMRI signal off-transient, while in others, a slower fMRI signal decay emerged. Sustained loading elicited slow decreases in fMRI signal across activated regions, while second application of an identical load resulted in smaller signal increases compared to initial signal responses (P < 0.001). A moderate inspiratory load is associated with consistent regional activation of discrete brain locations; certain of these regions have been implicated in mediation of loaded breathing in animal models. We speculate that temporal changes in fMRI signal may indicate respiratory after-discharge and/or habituation phenomena.

Dynamic magnetic resonance imaging of human Rolandic cortex.

Rolandic cortex was imaged with magnetic resonance (MR) in nine subjects while performing a motor activation task. Imaging was performed by a volumetric, T2-weighted pulse sequence in a conventional 1.5 Tesla scanner during both resting conditions and volitional toe flexion and extension of the dominant foot. Significant changes in MR signal intensity of 7.8 +/- 2.3% (mean +/- s.e.m.) were observed in the medial Rolandic cortex contralateral to the active foot. Changes were maximal in the vicinity of the central sulcus, but were also identified anteroposteriorly, across successive coronal planes. No significant changes were found in the ipsilateral Rolandic cortex or in other brain structures. Volumetric functional MRI strategies may provide an important non-invasive tool for assessment of cortical motor function.

MR imaging signal response to sustained stimulation in human visual cortex.

The response of signal intensity to transient (on-off) motor and sensory stimulation has been well studied; however, the dependence of signal response on the duration of stimulus requires further characterization. The objective of this study was to determine the time course of signal response in the human visual cortex to prolonged, sustained stimulation and to examine possible contributory physiologic mechanisms. Nine healthy volunteers underwent magnetic resonance (MR) imaging during sustained visual stimulation with light-proof binocular goggles. With photic stimulation, activation was observed in all subjects as an increase in signal intensity of the visual cortex. With sustained stimulation, a gradual decrease in signal intensity was subsequently observed, with progression toward an apparent steady state. Correlation with positron emission tomographic, MR spectroscopic, and visual evoked-potential data suggests that the initial uncoupling of cerebral blood flow and oxidative metabolism with a neuronal activation burst may represent a transient phenomenon. This quick-response phase may proceed to an equilibrium coupling of flow and oxidative metabolism, with a gradual normalization of venous deoxyhemoglobin levels and signal intensity.

Localization of putative neural respiratory regions in the human by functional magnetic resonance imaging.

In humans, the location of brain regions responsible for mediating the ventilatory response to CO2 remains unknown. Most of the available knowledge has been derived from animal studies or from pathophysiological correlations in patients presenting altered control of breathing. Magnetic resonance imaging at a specific pulse sequence designed to assess changes in brain tissue microcirculation was performed in 11 healthy volunteers, during steady-state conditions, while breathing 100% O2 or 5% CO2-95% O2. In one subject, 10% CO2-90% O2 was employed to examine a dose-response effect. Significant changes in image signal intensity consistently occurred in ventral and dorsal regions of medullary structures as well as in the midline pons and ventral cerebellum. These responses appeared to be dose dependent and reproducible. Magnetic resonance imaging revealed patterns of activation in brain stem and cerebellar regions during hypercapnic ventilatory challenge. These areas may underlie mechanisms for mediating the response to chemoreceptor activation.

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.