Niels Oesingmann

Brain tissue sodium concentration in multiple sclerosis: a sodium imaging study at 3 tesla

Neuro-axonal degeneration occurs progressively from the onset of multiple sclerosis and is thought to be a significant cause of increasing clinical disability. Several histopathological studies of multiple sclerosis and experimental autoimmune encephalomyelitis have shown that the accumulation of sodium in axons can promote reverse action of the sodium/calcium exchanger that, in turn, leads to a lethal overload in intra-axonal calcium. We hypothesized that sodium magnetic resonance imaging would provide an indicator of cellular and metabolic integrity and ion homeostasis in patients with multiple sclerosis. Using a three-dimensional radial gradient-echo sequence with short echo time, we performed sodium magnetic resonance imaging at 3 T in 17 patients with relapsing-remitting multiple sclerosis and in 13 normal subjects. The absolute total tissue sodium concentration was measured in lesions and in several areas of normal-appearing white and grey matter in patients, and corresponding areas of white and grey matter in controls. A mixed model analysis of covariance was performed to compare regional tissue sodium concentration levels in patients and controls. Spearman correlations were used to determine the association of regional tissue sodium concentration levels in T(2)- and T(1)-weighted lesions with measures of normalized whole brain and grey and white matter volumes, and with expanded disability status scale scores. In patients, tissue sodium concentration levels were found to be elevated in acute and chronic lesions compared to areas of normal-appearing white matter (P < 0.0001). The tissue sodium concentration levels in areas of normal-appearing white matter were significantly higher than those in corresponding white matter regions in healthy controls (P < 0.0001). The tissue sodium concentration value averaged over lesions and over regions of normal-appearing white and grey matter was positively associated with T(2)-weighted (P < or = 0.001 for all) and T(1)-weighted (P < or = 0.006 for all) lesion volumes. In patients, only the tissue sodium concentration value averaged over regions of normal-appearing grey matter was negatively associated with the normalized grey matter volume (P = 0.0009). Finally, the expanded disability status scale score showed a mild, positive association with the mean tissue sodium concentration value in chronic lesions (P = 0.002), in regions of normal-appearing white matter (P = 0.004) and normal-appearing grey matter (P = 0.002). This study shows the feasibility of using in vivo sodium magnetic resonance imaging at 3 T in patients with multiple sclerosis. Our findings suggest that the abnormal values of the tissue sodium concentration in patients with relapsing-remitting multiple sclerosis might reflect changes in cellular composition of the lesions and/or changes in cellular and metabolic integrity. Sodium magnetic resonance imaging has the potential to provide insight into the pathophysiological mechanisms of tissue injury when correlation with histopathology becomes available.

3D nongadolinium‐enhanced ECG‐gated MRA of the distal lower extremities: Preliminary clinical experience

To report our initial experience implementing a noncontrast‐enhanced electrocardiograph (ECG) gated fast spin echo magnetic resonance angiography (MRA) technique for assessment of the calf arteries.

Rapid Isotropic 3D-Sodium MRI of the Knee Joint In-vivo at 7T

To demonstrate the feasibility of acquiring high‐resolution, isotropic 3D‐sodium magnetic resonance (MR) images of the whole knee joint in vivo at ultrahigh field strength (7.0T) via a 3D‐radial acquisition with ultrashort echo times and clinically acceptable acquisition times.

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.

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.

Functional renal imaging: nonvascular renal disease

Functional renal imaging—a fast-growing field of MR-imaging—applies different sequence types to gather information about the kidneys other than morphology and angiography. This update article presents the current status of different functional imaging approaches and presents current and potential clinical applications. Apart from conventional in-phase and opposed-phase imaging, which already yields information about the tiusse composition, BOLD (blood-oxygenation level dependent) sequences, DWI (diffusion-weighted imaging) sequences, perfusion measurements, and dedicated contrast agents are used.

Noninvasive quantification of intracellular sodium in human brain using ultrahigh-field MRI.

In vivo sodium magnetic resonance imaging (MRI) measures tissue sodium content in living human brain but current methods do not allow noninvasive quantitative assessment of intracellular sodium concentration (ISC) – the most useful marker of tissue viability. In this study, we report the first noninvasive quantitative in vivo measurement of ISC and intracellular sodium volume fraction (ISVF) in healthy human brain, made possible by measuring tissue sodium concentration (TSC) and intracellular sodium molar fraction (ISMF) at ultra‐high field MRI. The method uses single–quantum (SQ) and triple–quantum filtered (TQF) imaging at 7 Tesla to separate intra‐ and extracellular sodium signals and provide quantification of ISMF, ISC and ISVF. This novel method allows noninvasive quantitative measurement of ISC and ISVF, opening many possibilities for structural and functional metabolic studies in healthy and diseased brains. Copyright

Comparison of the effectiveness of saturation pulses in the heart at 3T

Cardiac MRI at 3T provides a means to increase the contrast‐to‐noise ratio (CNR) for first‐pass perfusion MRI. However, both the static magnetic field (B0) and radio frequency (RF) field (B1) variations within the heart are comparatively higher at 3T than at 1.5T. The increased field variations can degrade the performance of a single rectangular saturation pulse that is conventionally used for magnetization preparation. The accuracy of T1‐weighted signal measurement depends on the uniformity of the magnetization saturation. The purpose of this study was to assess the relative effectiveness of the rectangular, pulse train, and adiabatic composite (BIR‐4) saturation pulses in the human heart at 3T. In volunteers, after nominal saturation, the mean residual magnetization within the left ventricle (LV) was different between all three pulses (0.13 ± 0.06 vs. 0.03 ± 0.02 vs. 0.03 ± 0.01, respectively; P < 0.001). Within paired groups, the mean residual magnetization was significantly higher for the rectangular pulse than for either the pulse train and BIR‐4 pulses (P < 0.001), but not different between the pulse train and BIR‐4 pulses. The performances of all three saturation pulses were comparatively poorer in the right ventricle (RV) than in the LV, respectively. Magn Reson Med, 2007.

Intraindividual comparison of MR-renal perfusion imaging at 1.5 T and 3.0 T.

Purpose:The purpose of this study was to intraindividually compare fast gradient-echo semiquantitative renal perfusion measurements at 1.5 Tesla (T) and 3.0 Tesla. Materials and Methods:Fifteen healthy male volunteers underwent renal perfusion measurements at 1.5 T and 3.0 T after the bolus injection of 7 mL of Gd-BOPTA. At both field strengths a Saturation-Recovery-fast gradient echo sequence (SR-TurboFLASH) with a temporal resolution of 4 (1.5 T) and 5 (3.0 T) simultaneously acquired slices per second was used. At 3.0 T, a parallel-imaging factor 2 was applied. For postprocessing, semiquantitative perfusion parameters including mean transit time (MTT), time to peak (TTP), and maximal signal intensity (SMax) were determined. The signal-to-noise ratios (SNR) of kidneys and aorta were determined precontrast and after enhancement. The image quality was rated by 2 radiologists. After Bonferroni correction paired t-tests were performed for statistical analysis. Results:All measurements were successfully performed. At 3.0 T, a significant 63% increase in the baseline SNR (P = 0.00005) of the kidneys was found, the peak SNR was also increased though not statistically significant. Because of the higher SNR, the SMax was also significantly (P = 0.005) increased from 406 A.U. to 522 A.U., whereas MTT and TTP were not significantly changed. The image quality was rated very good to good for the 3.0 T images but only good to moderate at 1.5 T. Conclusion:Renal perfusion measurements at 3.0 T are feasible and directly benefit from the inherently higher SNR at 3.0 T. The higher SNR also translates into an increased SMax, whereas MTT and TTP are independent of the field strength.

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.