Uday Krishnamurthy

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

Abstract 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.

Measuring Venous Blood Oxygenation in Fetal Brain using Susceptibility Weighted Imaging

To evaluate fetal cerebral venous blood oxygenation, Yv, using principles of MR susceptometry.

Quantitative T2 changes and susceptibility-weighted magnetic resonance imaging in murine pregnancy.

Objective: To evaluate gestational age-dependent changes in the T2 relaxation time in normal murine placentas in vivo. The role of susceptibility-weighted imaging (SWI) in visualization of the murine fetal anatomy was also elucidated. Methods: Timed-pregnant CD-1 mice at gestational day (GD) 12 and GD17 underwent magnetic resonance imaging. Multi-echo spin echo and SWI data were acquired. The placental T2 values on GD12 and GD17 were quantified. To account for the influence of systemic maternal physiological factors on placental perfusion, maternal muscle was used as a reference for T2 normalization. A linear mixed-effects model was used to fit the normalized T2 values, and the significance of the coefficients was tested. Fetal SWI images were processed and reviewed for venous vasculature and skeletal structures. Results: The average placental T2 value decreased significantly on GD17 (40.17 ± 4.10 ms) compared to the value on GD12 (55.78 ± 8.13 ms). The difference in normalized T2 values also remained significant (p = 0.001). Using SWI, major fetal venous structures like the cardinal vein, the subcardinal vein, and the portal vein were visualized on GD12. In addition, fetal skeletal structures could also be discerned on GD17. Conclusion: The T2 value of a normal murine placenta decreases with advancing gestation. SWI provided clear visualization of the fetal venous vasculature and bony structures.

Imaging putative foetal cerebral blood oxygenation using susceptibility weighted imaging (SWI)

ObjectiveTo evaluate the magnetic susceptibility, ∆χv, as a surrogate marker of venous blood oxygen saturation, SvO2, in second- and third-trimester normal human foetuses.MethodsThirty-six pregnant women, having a mean gestational age (GA) of 31 2/7 weeks, underwent magnetic resonance imaging (MRI). Susceptibility-weighted imaging (SWI) data from the foetal brain were acquired. ∆χv of the superior sagittal sinus (SSS) was quantified using MR susceptometry from the intra-vascular phase measurements. Assuming the magnetic property of foetal blood, ∆χdo, is the same as that of adult blood, SvO2 was derived from the measured Δχv. The variation of ∆χv and SvO2, as a function of GA, was statistically evaluated.ResultsThe mean ∆χv in the SSS in the second-trimester (n = 8) and third-trimester foetuses (n = 28) was found to be 0.34± 0.06 ppm and 0.49 ±0.05 ppm, respectively. Correspondingly, the derived SvO2 values were 69.4% ±3.27% and 62.6% ±3.25%. Although not statistically significant, an increasing trend (p = 0.08) in Δχv and a decreasing trend (p = 0.22) in SvO2 with respect to advancing gestation was observed.ConclusionWe report cerebral venous blood magnetic susceptibility and putative oxygen saturation in healthy human foetuses. Cerebral oxygen saturation in healthy human foetuses, despite a slight decreasing trend, does not change significantly with advancing gestation.Key points• Cerebral venous magnetic susceptibility and oxygenation in human foetuses can be quantified.• Cerebral venous oxygenation was not different between second- and third-trimester foetuses.• Foetal cerebral venous oxygenation does not change significantly with advancing gestation.

Quantitative Flow Imaging in Human Umbilical Vessels In Utero Using Nongated 2D Phase Contrast MRI: Phase Contrast MRI of Umbilical Vessels

Volumetric assessment of afferent blood flow rate provides a measure of global organ perfusion. Phase‐contrast magnetic resonance imaging (PCMRI) is a reliable tool for volumetric flow quantification, but given the challenges with motion and lack of physiologic gating signal, such studies, in vivo on the human placenta, are scant.

Quantitative susceptibility mapping in the human fetus to measure blood oxygenation in the superior sagittal sinus

ObjectivesTo present the feasibility of performing quantitative susceptibility mapping (QSM) in the human fetus to evaluate the oxygenation (SvO2) of cerebral venous blood in vivo.MethodsSusceptibility weighted imaging (SWI) data were acquired from healthy pregnant subjects (n = 21, median = 31.3 weeks, interquartile range = 8.8 weeks). The susceptibility maps were generated from the SWI-phase images using a modified QSM processing pipeline, optimised for fetal applications. The processing pipeline is as follows: (1) mild high-pass filtering followed by quadratic fitting of the phase images to eliminate background phase variations; (2) manual creation of a fetal brain mask that includes the superior sagittal sinus (SSS); (3) inverse filtering of the resultant masked phase images using a truncated k-space approach with geometric constraint. Further, the magnetic susceptibility, ∆χv and corresponding putative SvO2 of the SSS were quantified from the generated susceptibility maps. Systematic error in the measured SvO2 due to the modified pipeline was also studied through simulations.ResultsSimulations showed that the systematic error in SvO2 when using a mask that includes a minimum of 5 voxels around the SSS and five slices remains < 3% for different orientations of the vessel relative to the main magnetic field. The average ∆χv in the SSS quantified across all gestations was 0.42 ± 0.03 ppm. Based on ∆χv, the average putative SvO2 in the SSS across all fetuses was 67% ± 7%, which is in good agreement with published studies.ConclusionsThis in vivo study demonstrates the feasibility of using QSM in the human fetal brain to estimate ∆χv and SvO2.Key Points• A modified quantitative susceptibility mapping (QSM) processing pipeline is tested and presented for the human fetus.• QSM is feasible in the human fetus for measuring magnetic susceptibility and oxygenation of venous blood in vivo.• Blood magnetic susceptibility values from MR susceptometry and QSM agree with each other in the human fetus.

Longitudinal Changes in Placental Magnetic Resonance Imaging Relaxation Parameter in Murine Pregnancy: Compartmental Analysis

Objective: To quantify gestation-dependent longitudinal changes in the magnetic resonance transverse relaxation time (T2) parameter of the major constituent regions of the mouse placenta and to evaluate their relative contributions to changes in overall placental T2. Methods: Timed-pregnant CD-1 mice underwent magnetic resonance imaging at 7.0 T field strength, on gestational day 13 (GD13), GD15 and GD17. T2 of the placenta and its constituent high and low blood perfusion regions were quantified. A linear mixed-effects model was used to fit the T2 across gestation, and the significance of coefficients was tested. Results: A decrease in the T2 values of the placenta and its constituent regions was observed across gestation. The temporal change in T2 was estimated to be -1.85 ms/GD (p < 0.0001) for the placenta, -1.00 ms/GD (p < 0.001) for the high-perfusion zones (HPZs) and -1.66 ms/GD (p < 0.0001) for the low-perfusion zones (LPZs). Conclusion: T2 of the constituent zones of the murine placenta decreases with advancing gestation. While the T2 of the LPZ is smaller than that of the HPZ, there is no difference in their decrease rate relative to that of the whole placenta (p = 0.24). The results suggest an increased role of constituent volume fractions in affecting overall gestation-dependent placental T2 decrease in mice.