Louisa Bokacheva

Optimal k-Space Sampling for Dynamic Contrast-Enhanced MRI with an Application to MR Renography

For time‐resolved acquisitions with k‐space undersampling, a simulation method was developed for selecting imaging parameters based on minimization of errors in signal intensity versus time and physiologic parameters derived from tracer kinetic analysis. Optimization was performed for time‐resolved angiography with stochastic trajectories (TWIST) algorithm applied to contrast‐enhanced MR renography. A realistic 4D phantom comprised of aorta and two kidneys, one healthy and one diseased, was created with ideal tissue time‐enhancement pattern generated using a three‐compartment model with fixed parameters, including glomerular filtration rate (GFR) and renal plasma flow (RPF). TWIST acquisitions with different combinations of sampled central and peripheral k‐space portions were applied to this phantom. Acquisition performance was assessed by the difference between simulated signal intensity (SI) and calculated GFR and RPF and their ideal values. Sampling of the 20% of the center and 1/5 of the periphery of k‐space in phase‐encoding plane and data‐sharing of the remaining 4/5 minimized the errors in SI (<5%), RPF, and GFR (both <10% for both healthy and diseased kidneys). High‐quality dynamic human images were acquired with optimal TWIST parameters and 2.4 sec temporal resolution. The proposed method can be generalized to other dynamic contrast‐enhanced MRI applications, e.g., MR angiography or cancer imaging. Magn Reson Med, 2009.

Performance of an automated segmentation algorithm for 3D MR renography

The accuracy and precision of an automated graph‐cuts (GC) segmentation technique for dynamic contrast‐enhanced (DCE) 3D MR renography (MRR) was analyzed using 18 simulated and 22 clinical datasets. For clinical data, the error was 7.2 ± 6.1 cm3 for the cortex and 6.5 ± 4.6 cm3 for the medulla. The precision of segmentation was 7.1 ± 4.2 cm3 for the cortex and 7.2 ± 2.4 cm3 for the medulla. Compartmental modeling of kidney function in 22 kidneys yielded a renal plasma flow (RPF) error of 7.5% ± 4.5% and single‐kidney GFR error of 13.5% ± 8.8%. The precision was 9.7% ± 6.4% for RPF and 14.8% ± 11.9% for GFR. It took 21 min to segment one kidney using GC, compared to 2.5 hr for manual segmentation. The accuracy and precision in RPF and GFR appear acceptable for clinical use. With expedited image processing, DCE 3D MRR has the potential to expand our knowledge of renal function in individual kidneys and to help diagnose renal insufficiency in a safe and noninvasive manner. Magn Reson Med 57:1159–1167, 2007.

Estimates of glomerular filtration rate from MR renography and tracer kinetic models.

To compare six methods for calculating the single‐kidney glomerular filtration rate (GFR) from T1‐weighted magnetic resonance (MR) renography (MRR) against reference radionuclide measurements.

Functional assessment of the kidney from magnetic resonance and computed tomography renography: Impulse retention approach to a multicompartment model

A three‐compartment model is proposed for analyzing magnetic resonance renography (MRR) and computed tomography renography (CTR) data to derive clinically useful parameters such as glomerular filtration rate (GFR) and renal plasma flow (RPF). The model fits the convolution of the measured input and the predefined impulse retention functions to the measured tissue curves. A MRR study of 10 patients showed that relative root mean square errors by the model were significantly lower than errors for a previously reported three‐compartmental model (11.6% ± 4.9 vs 15.5% ± 4.1; P < 0.001). GFR estimates correlated well with reference values by 99mTc‐DTPA scintigraphy (correlation coefficient r = 0.82), and for RPF, r = 0.80. Parameter‐sensitivity analysis and Monte Carlo simulation indicated that model parameters could be reliably identified. When the model was applied to CTR in five pigs, expected increases in RPF and GFR due to acetylcholine were detected with greater consistency than with the previous model. These results support the reliability and validity of the new model in computing GFR, RPF, and renal mean transit times from MR and CT data. Magn Reson Med 59:278–288, 2008.

Crossover between thermally assisted and pure quantum tunneling in molecular magnet Mn12-acetate

The crossover between thermally assisted and pure quantum tunneling has been studied in single crystals of high spin (S = 10) uniaxial molecular magnet Mn12 using micro-Hall-effect magnetometry. Magnetic hysteresis and relaxation experiments have been used to investigate the energy levels that determine the magnetization reversal as a function of magnetic field and temperature. These experiments demonstrate that the crossover occurs in a narrow ( approximately 0. 1 K) or broad ( approximately 1 K) temperature interval depending on the magnitude of the field transverse to the anisotropy axis.

Quantitative determination of Gd-DTPA concentration in T1-weighted MR renography studies.

A method for calculating contrast agent concentration from MR signal intensity (SI) was developed and validated for T1‐weighted MR renography (MRR) studies. This method is based on reference measurements of SI and relaxation time T1 in a Gd‐DTPA‐doped water phantom. The same form of SI vs. T1 dependence was observed in human tissues. Contrast concentrations calculated by the proposed method showed no bias between 0 and 1 mM, and agreed better with the reference values derived from direct T1 measurements than the concentrations calculated using the relative signal method. Phantom‐based conversion was used to determine the contrast concentrations in kidney tissues of nine patients who underwent dynamic Gd‐DTPA‐enhanced 3D MRR at 1.5T and 99mTc‐DTPA radionuclide renography (RR). The concentrations of both contrast agents were found to be close in magnitude and showed similar uptake and washout behavior. As shown by Monte Carlo simulations, errors in concentration due to SI noise were below 10% for SNR = 20, while a 10% error in precontrast T1 values resulted in a 12–17% error for concentrations between 0.1 and 1 mM. The proposed method is expected to be particularly useful for assessing regions with highly concentrated contrast. Magn Reson Med 57:1012–1018, 2007.

Assessment of Renal Function with Dynamic Contrast Enhanced MR Imaging

MR imaging is a promising noninvasive modality that can provide a comprehensive picture of renal anatomy and function in a single examination. The advantages of MR imaging are its high contrast and temporal resolution and lack of exposure to ionizing radiation. In the past few years, considerable progress has been made in development of methods of renal functional MR imaging and their applications in various diseases. This article reviews the key factors for acquisition and analysis of dynamic contrast-enhanced renal MR imaging (MR renography) and the most significant developments in this field over the past few years.

What causes diminished corticomedullary differentiation in renal insufficiency

To investigate whether the loss of corticomedullary differentiation (CMD) on T1‐weighted MR images due to renal insufficiency can be attributed to changes in T1 values of the cortex, medulla, or both.

Low-temperature magnetic hysteresis in Mn12 acetate single crystals

Precise magnetic-hysteresis measurements of small single crystals of Mn12 acetate of spin 10 have been conducted down to 0.4 K using a high-sensitivity Hall magnetometer. At higher temperature ( > 1.6 K) step-like changes in magnetization are observed at regularly spaced magnetic-field intervals, as previously reported. However, on lowering the temperature the steps in magnetization shift to higher magnetic fields, initially gradually. These results are consistent with the presence of a higher-order uniaxial magnetic anisotropy (fourth order in Sz), first observed by EPR spectroscopy, and thermally assisted tunnelling with tunnelling relaxation occurring from levels of progressively lower energy as the temperature is reduced. At lower temperature an abrupt shift in step positions is found. We suggest that this shift may be the first evidence of an abrupt, or first-order, transition between thermally assisted and pure quantum tunnelling, suggested by recent theory.

Single breath-hold T1 measurement using low flip angle TrueFISP.

A method for estimating T1 using a single breath‐hold, segmented, inversion recovery prepared, true fast imaging with steady‐state precession (sIR‐TrueFISP) acquisition at low flip angle (FA) was implemented in this study. T1 values measured by sIR‐TrueFISP technique in a Gd‐DTPA‐doped water phantom and the human brain and abdomen of healthy volunteers were compared with the results of the standard IR fast spin echo (FSE) technique. A good correlation between the two methods was observed (R2 = 0.999 in the phantom, and R2 = 0.943 in the brain and abdominal tissues). The T1 values of the tissues agreed well with published results. sIR‐TrueFISP enables fast measurements of T1 to be obtained within a single breath‐hold with good accuracy, which is particularly important for chest and abdominal imaging. Magn Reson Med, 2006.