MR imaging of the fetal brain at 1.5T and 3.0T field strengths: comparing specific absorption rate (SAR) and image quality.

OBJECTIVES: Our two objectives were to evaluate the feasibility of fetal brain magnetic resonance imaging (MRI) using a fast spin echo sequence at 3.0T field strength with low radio frequency (rf) energy deposition (as measured by specific absorption rate: SAR) and to compare image quality, tissue contrast and conspicuity between 1.5T and 3.0T MRI. METHODS: T2 weighted images of the fetal brain at 1.5T were compared to similar data obtained in the same fetus using a modified sequence at 3.0T. Quantitative whole-body SAR and normalized image signal to noise ratio (SNR), a nominal scoring scheme based evaluation of diagnostic image quality, and tissue contrast and conspicuity for specific anatomical structures in the brain were compared between 1.5T and 3.0T. RESULTS: Twelve pregnant women underwent both 1.5T and 3.0T MRI examinations. The image SNR was significantly higher (P=0.03) and whole-body SAR was significantly lower (P<0.0001) for images obtained at 3.0T compared to 1.5T. All cases at both field strengths were scored as having diagnostic image quality. Images from 3.0T MRI (compared to 1.5T) were equal (57%; 21/37) or superior (35%; 13/37) for tissue contrast and equal (61%; 20/33) or superior (33%, 11/33) for conspicuity. CONCLUSIONS: It is possible to obtain fetal brain images with higher resolution and better SNR at 3.0T with simultaneous reduction in SAR compared to 1.5T. Images of the fetal brain obtained at 3.0T demonstrated superior tissue contrast and conspicuity compared to 1.5T.

MR venography of the fetal brain using susceptibility weighted imaging.

PURPOSE: To evaluate the feasibility of performing fetal brain magnetic resonance venography using susceptibility weighted imaging (SWI). MATERIALS AND METHODS: After obtaining informed consent, pregnant women in the second and third trimester were imaged using a modified SWI sequence. Fetal SWI acquisition was repeated when fetal or maternal motion was encountered. The median and maximum number of times an SWI sequence was repeated was four and six respectively. All SWI image data were systematically evaluated by a pediatric neuroradiologist for image quality using an ordinal scoring scheme: 1. diagnostic; 2. diagnostic with artifacts; and 3. nondiagnostic. The best score in an individual fetus was used for further statistical analysis. Visibility of venous vasculature was also scored using a dichotomous variable. A subset of SWI data was re-evaluated by the first and independently by a second pediatric neuroradiologist. Kappa coefficients were computed to assess intra-rater and inter-rater reliability. RESULTS: SWI image data from a total of 22 fetuses were analyzed. Median gestational age and interquartile range of the fetuses imaged were 32 (29.9-34.9) weeks. In 68.2% of the cases (n = 15), there was no artifact; 22.7% (n = 5) had minor artifacts and 9.1% (n = 2) of the data was of nondiagnostic quality. Cerebral venous vasculature was visible in 86.4% (n = 19) of the cases. Substantial agreement (Kappa = 0.73; 95% confidence interval 0.44-1.00)) was observed for intra-rater reliability and moderate agreement (Kappa = 0.48; 95% confidence interval 0.19-0.77) was observed for inter-rater reliability. CONCLUSION: It is feasible to perform fetal brain venography in humans using SWI.

Measuring venous blood oxygenation in fetal brain using susceptibility-weighted imaging.

PURPOSE: To evaluate fetal cerebral venous blood oxygenation, Yv, using principles of MR susceptometry. MATERIALS AND METHODS: A cohort of 19 pregnant subjects, with a mean gestational age of 31.6 ± 4.7 weeks were imaged using a modified susceptibility-weighted imaging (SWI) sequence. Data quality was first assessed for feasibility of oxygen saturation measurement, and data from five subjects (mean ± std gestational age of 33.7 ± 3.6 weeks) were then chosen for further quantitative analysis. SWI phase in the superior sagittal sinus was used to evaluate oxygen saturation using the principles of MR susceptometry. Systematic error in the measured Y(v) values was studied through simulations. RESULTS: Simulations showed that the systematic error in Yv depended upon the assumed angle of the vessel, θ, relative to the main magnetic field and the error in that vessel angle δθ. For the typical vessel angle of θ = 30° encountered in the fetal data analyzed, a δθ as large as ±20° led to an absolute error, δYv, of less than 11%. The measured mean oxygen saturation across the five fetuses was 66% ± 9.4%. This average cerebral venous blood oxygenation value is in close agreement with values in the published literature. CONCLUSION: We have reported the first in vivo measurement of human fetal cerebral venous oxygen saturation using MRI.

Patients with multiple sclerosis with structural venous abnormalities on MR imaging exhibit an abnormal flow distribution of the internal jugular veins.

PURPOSE: To evaluate extracranial venous structural and flow characteristics in patients with multiple sclerosis (MS). MATERIALS AND METHODS: Two hundred subjects with MS from two sites (n = 100 each) were evaluated with magnetic resonance (MR) imaging at 3 T. Contrast-enhanced time-resolved MR angiography and time-of-flight MR venography were used to assess vascular anatomy. Two-dimensional phase-contrast MR imaging was used to quantify blood flow. The MS population was divided into two groups: those with evident internal jugular vein (IJV) stenoses (stenotic group) and those without (nonstenotic group). RESULTS: Of the 200 patients, 136 (68%) showed IJV structural abnormalities, including unilateral or bilateral stenoses at different levels in the neck (n = 101; 50.5%) and atresia (n = 35; 17.5%). The total IJV flow normalized to the total arterial flow of the stenotic group (56% ± 22) was significantly lower than that of the nonstenotic group (77% ± 14; P < .001). The arterial/venous flow mismatch in the stenotic group (12% ± 15) was significantly greater than that in the nonstenotic group (6% ± 12; P < .001). The ratio of subdominant venous flow rate (Fsd) to dominant venous flow rate (Fd) for the stenotic group (0.38 ± 0.27) was significantly lower than for the nonstenotic group (0.59 ± 0.23; P < .001). The majority of the stenotic group (67%) also had an Fsd of less than 3 mL/s, a Fd/Fsd ratio greater than 3:1, and/or a total IJV flow rate of less than 8 mL/s. CONCLUSIONS: MR imaging provides a noninvasive means to separate stenotic from nonstenotic MS cases. The former group was more prevalent in the present MS population and carried significantly less flow in the IJVs than the latter.

Correlation of putative iron content as represented by changes in R2* and phase with age in deep gray matter of healthy adults.

PURPOSE: To establish a correlation between putative iron content using susceptibility weighted imaging (SWI) phase and T2* weighted magnitude values in the basal ganglia and the thalamus as a function of age in healthy human brains. MATERIALS AND METHODS: One hundred healthy adults (range, 20-69 years; mean, 43 years) were evaluated for this study using a gradient echo sequence. The original magnitude and high pass filtered phase data were analyzed as proxy variables for iron content in the substantia nigra, red nucleus, globus pallidus, putamen, caudate nucleus, thalamus, and pulvinar thalamus. Each structure was broken into two parts, a high iron content region and a low iron content region. RESULTS: Both magnitude and phase data showed an increase in putative iron content with age. However, the high iron content region revealed two new pieces of information: both the average iron content per pixel and the area of high iron increased with age. Furthermore, significant increase in iron uptake as a function of age was found past the age of 40. CONCLUSION: A two region of interest analysis of iron is a much more sensitive means to evaluate iron content change over time. Contrary to the current belief that iron content increases level off with age, the putative iron deposition in the high iron content region is seen to increase with age.

Cross-hemispheric functional connectivity in the human fetal brain.

Compelling evidence indicates that psychiatric and developmental disorders are generally caused by disruptions in the functional connectivity (FC) of brain networks. Events occurring during development, and in particular during fetal life, have been implicated in the genesis of such disorders. However, the developmental timetable for the emergence of neural FC during human fetal life is unknown. We present the results of resting-state functional magnetic resonance imaging performed in 25 healthy human fetuses in the second and third trimesters of pregnancy (24 to 38 weeks of gestation). We report the presence of bilateral fetal brain FC and regional and age-related variation in FC. Significant bilateral connectivity was evident in half of the 42 areas tested, and the strength of FC between homologous cortical brain regions increased with advancing gestational age. We also observed medial to lateral gradients in fetal functional brain connectivity. These findings improve understanding of human fetal central nervous system development and provide a basis for examining the role of insults during fetal life in the subsequent development of disorders in neural FC.

Dynamic contrast-enhanced magnetic resonance imaging of vascular changes induced by sunitinib in papillary renal cell carcinoma xenograft tumors.

To investigate further the antiangiogenic potential of sunitinib for renal cell carcinoma (RCC) treatment, its effects on tumor vasculature were monitored by dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) using an orthotopic KCI-18 model of human RCC xenografts in nude mice. Tumor-bearing mice were treated with various doses of sunitinib, and vascular changes were assessed by DCE-MRI and histologic studies. Sunitinib induced dose-dependent vascular changes, which were observed both in kidney tumors and in normal kidneys by DCE-MRI. A dosage of 10 mg/kg per day caused mild changes in Gd uptake and clearance kinetics in kidney tumors. A dosage of 40 mg/kg per day induced increased vascular tumor permeability with Gd retention, probably resulting from the destruction of tumor vasculature, and also caused vascular alterations of normal vessels. However, sunitinib at 20 mg/kg per day caused increased tumor perfusion and decreased vascular permeability associated with thinning and regularization of tumor vessels while mildly affecting normal vessels as confirmed by histologic diagnosis. Alterations in tumor vasculature resulted in a significant inhibition of KCI-18 RCC tumor growth at sunitinib dosages of 20 and 40 mg/kg per day. Sunitinib also exerted a direct cytotoxic effect in KCI-18 cells in vitro. KCI-18 cells and tumors expressed vascular endothelial growth factor receptor 2 and platelet-derived growth factor receptor beta molecular targets of sunitinib that were modulated by the drug treatment. These data suggest that a sunitinib dosage of 20 mg/kg per day, which inhibits RCC tumor growth and regularizes tumor vessels with milder effects on normal vessels, could be used to improve blood flow for combination with chemotherapy. These studies emphasize the clinical potential of DCE-MRI in selecting the dose and schedule of antiangiogenic compounds.