Analysis of intramedullary cell density by MRI using the multiple spin-echo technique

Changes in Bone Marrow Fat upon Dietary-Induced Weight Loss

Background: Bone marrow fat is implicated in metabolism, bone health and haematological diseases. Thus, this study aims to analyse the impact of moderate weight loss on bone marrow fat content (BMFC) in obese, healthy individuals. Methods: Data of the HELENA-Trial (Healthy nutrition and energy restriction as cancer prevention strategies: a randomized controlled intervention trial), a randomized controlled trial (RCT) among 137 non-smoking, overweight or obese participants, were analysed to quantify the Magnetic Resonance Imaging (MRI)-derived BMFC at baseline, after a 12-week dietary intervention phase, and after a 50-week follow-up. The study cohort was classified into quartiles based on changes in body weight between baseline and week 12. Changes in BMFC in respect of weight loss were analysed by linear mixed models. Spearman’s coefficients were used to assess correlations between anthropometric parameters, blood biochemical markers, blood cells and BMFC. Results: Relative changes in BMFC from baseline to week 12 were 0.0 ± 0.2%, −3.2 ± 0.1%, −6.1 ± 0.2% and −11.5 ± 0.6% for Q1 to Q4. Across all four quartiles and for the two-group comparison, Q1 versus Q4, there was a significant difference (p < 0.05) for changes in BMFC. BMFC was not associated with blood cell counts and showed only weaker correlations (<0.3) with metabolic biomarkers. Conclusion: Weight loss is associated with a decrease of BMFC. However, BMFC showed no stronger associations with inflammatory and metabolic biomarkers.

Metabolic regulation of hematopoietic and leukemic stem/progenitor cells under homeostatic and stress conditions

Hematopoietic stem cells (HSCs) exhibit multilineage differentiation and self-renewal activities that maintain the entire hematopoietic system during an organism's lifetime. These abilities are sustained by intrinsic transcriptional programs and extrinsic cues from the microenvironment or niche. Recent studies using metabolomics technologies reveal that metabolic regulation plays an essential role in HSC maintenance. Metabolic pathways provide energy and building blocks for other factors functioning at steady state and in stress. Here we review recent advances in our understanding of metabolic regulation in HSCs relevant to cell cycle quiescence, symmetric/asymmetric division, and proliferation following stress and lineage commitment, and discuss the therapeutic potential of targeting metabolic factors or pathways to treat hematological malignancies.

Quantitative MRI analysis of craniofacial bone marrow in patients with sickle cell disease

BACKGROUND AND PURPOSE

Assessment of bone marrow is most commonly performed qualitatively in the spine or other large long bones. The craniofacial bones are less ideal for bone marrow analysis because of the relatively small bone marrow volume. Because patients with SCD often undergo repeated brain imaging to evaluate for cerebral vaso-occlusive disease, quantitative assessment of craniofacial bone marrow is a reasonable possibility in these patients. The purpose of this study was to investigate specific sickle cell disease changes in craniofacial bone marrow quantitatively by analyzing T1, T2, and secular-T2 relaxation times and volume with the use of quantitative MRI.

MATERIALS AND METHODS

Fourteen patients with SCD and 17 control subjects were imaged with the mixed TSE pulse sequence at 1.5T. The craniofacial bones were manually segmented by using 3D Slicer to generate bone marrow volumes and to provide T1, T2, and secular-T2 relaxation times.

RESULTS

All subjects exhibited a bimodal T1 histogram. In the SCD group, there was a decrease in amplitude in the first T1 peak and an increase in amplitude in the second T1 peak. The first T1 peak showed a significant increase in relaxation time compared with control subjects (P < .0001), whereas there was no significant difference in the second T1 peak. T2 and secular-T2 relaxation times were significantly shorter in the SCD group (T2, P < .0001; secular-T2, P < .0001). Increasing numbers of blood transfusions resulted in a decrease in T2 and secular-T2 times. Patients with SCD exhibited a larger bone marrow volume compared with control subjects, even after standardization.

CONCLUSIONS

Patients with SCD exhibited significant quantifiable changes in the craniofacial bone marrow because of failure of red-to-yellow marrow conversion and iron deposition that can be identified by qMRI relaxometry and volumetry. Both qMRI relaxometry and volumetry may be used as noninvasive tools for assessment of disease severity.

Optimal echo spacing for multi-echo imaging measurements of Bi-exponential T2 relaxation

Calculations, analytical solutions, and simulations were used to investigate the trade-off of echo spacing and receiver bandwidth for the characterization of bi-exponential transverse relaxation using a multi-echo imaging pulse sequence. The Cramer-Rao lower bound of the standard deviation of the four parameters of a two-pool model was computed for a wide range of component T(2) values and echo spacing. The results demonstrate that optimal echo spacing (TE(opt)) is not generally the minimal available given other pulse sequence constraints. The TE(opt) increases with increasing value of the short T(2) time constant and decreases as the ratio of the long and short time constant decreases. A simple model of TE(opt) as a function of the two T(2) time constants and four empirically derived scalars is presented.

Quantification of water compartmentation in cell suspensions by diffusion-weighted and T(2)-weighted MRI

When studying water diffusion in biological systems, any specific signal attenuation curve may be reproduced by a broad range of mathematical functions. Our goals were to quantify the diffusion and T(2) relaxation properties of water in a simple biological system and to study the changes that occur in osmotically stressed cells. Human breast cancer cells were incubated in isotonic or hypotonic osmotic buffers. Diffusion-weighted and T(2)-weighted magnetic resonance images were acquired during sedimentation over 12 h. Diffusion-weighted imaging (DWI) data were analyzed with a biexponential fit, the Kärger model for exchange between two freely diffusing populations and the Price-modified Kärger model accounting for restricted diffusion in spherical geometry. We found that only the Price model provided an accurate quantitative description for water diffusion in both cell systems, independent of acquisition parameters, over the entire density range. Model-derived cell radii, intracellular volume fractions and transmembrane water exchange times were in good agreement with results calculated from light microscopy and with model-free exchange times. T(2) data indicated two populations in fast exchange, with volume fractions clearly different from DWI populations. Hypotonic stress led to higher slow apparent diffusion coefficient, longer T(2) and lower membrane permeability. The tortuosity in a hypotonic cell suspension complied with the Wang model for spherical geometry. Quantitative characterization of biological systems is obtainable by DWI, using appropriate modeling, accounting for water restriction and exchange between compartments.

Biexponential parameterization of diffusion and T2 relaxation decay curves in a rat muscle edema model: decay curve components and water compartments

Quantitative T2 relaxation and diffusion imaging studies of a rat muscle edema model were performed in order to determine the effects of intra- and extracellular water compartmentation on the respective decay curves. The right hind paw of rats was injected with a carrageenan solution to generate edematous muscle. A Carr-Purcell-Meiboom-Gill (CPMG) imaging sequence was used to acquire T2 relaxation decay curves from both paws. A line scan diffusion imaging (LSDI) sequence was then used to acquire diffusion decay curves from the same paws over a wide b-factor range. Measurements were made from both edematous muscle (EM) and control muscle (CM). The EM and CM T2 relaxation decay curves were best fit with biexponential functions. The fraction of the fast T2 component dropped dramatically from approximately 0.95 in CM to 0.45 in EM, consistent with a water compartmentation model in which the fast and slow T2 components reflect intra- and extracellular water, respectively. Both CM and EM diffusion decay curves required biexponential fitting functions, and the diffusion coefficients of the fast and slow components were substantially larger in EM than CM. The fraction of the fast diffusion component, however, was not radically altered between CM and EM conditions (0.84 versus 0.89 for CM versus EM). Assuming a model in which intra- and extracellular water compartments are responsible for the fast and slow T2-decay components and for the slow and fast diffusion decay components, respectively, leads to fractional sizes of the diffusion components that are not supported by experiment. We conclude that intra- and extracellular water compartmentation is a reasonable interpretation for the two T2-decay components in both CM and EM but that other factors, such as restricted diffusion and/or alternate forms of water compartmentation like surface versus volume water, most probably have profound influences on the precise shapes of the diffusion decay curves, a complete understanding of which will require significant theoretical work.

Fat fractions and spectral T2 values in vertebral bone marrow in HIV- and non-HIV-infected men: a 1H spectroscopic imaging study

Fat fractions and spectral T2 values of fat and water within the vertebral marrow of non-HIV- and HIV-infected men were measured with the use of a Carr-Purcell-Meiboom-Gill (CPMG) line scan spectroscopic imaging sequence. The fat fraction for the HIV-infected men (0.29 +/- 0.08) was significantly lower (P < 0.05, Student's unpaired t-test) than the fat fraction found in non-HIV-infected men (0.40 +/- 0.12). The mean water and fat T2 values did not differ between the two groups, and did not show any systematic dependence on fat fraction over the wide range of fat fractions encountered in this study. The marrow water and fat T2 values measured with the CPMG approach were markedly longer than the spectral T2 values reported by other groups using the more common point-resolved spectroscopy (PRESS) and stimulated-echo acquisition mode (STEAM) acquisitions. Proton spectroscopic studies of vertebral marrow revealed differences between non-HIV- and HIV-infected men that may prove useful for studying the effects of this disease and/or antiretroviral agents on body composition.

Metallic electrodes and leads in simultaneous EEG-MRI: Specific absorption rate (SAR) simulation studies

The purpose of this study was to investigate the changes in specific absorption rate (SAR) in human-head tissues while using nonmagnetic metallic electroencephalography (EEG) electrodes and leads during magnetic resonance imaging (MRI). A realistic, high resolution (1 mm(3)) head model from individual MRI data was adopted to describe accurately thin tissues, such as bone marrow and skin. The RF power dissipated in the human head was evaluated using the FDTD algorithm. Both surface and bird cage coils were used. The following numbers of EEG electrodes/leads were considered: 16, 31, 62, and 124. Simulations were performed at 128 and 300 MHz. The difference in SAR between the electrodes/leads and no-electrodes conditions was greater with the bird cage coil than with the surface coil. The peak 1 g averaged SAR values were highest at 124 electrodes, increasing to as much as two orders of magnitude (x172.3) at 300 MHz compared to the original value. At 300 MHz, there was a fourfold (x3.6) increase of SAR averaged over the bone marrow, and a sevenfold (x7.4) increase in the skin. At 128 MHz, there was a fivefold (x5.6) increase of whole head SAR. Head models were obtained from two different subjects, with an inter-subject whole head SAR variability of 3%. .