Renxin Chu

Signal-to-noise ratio and parallel imaging performance of a 16-channel receive-only brain coil array at 3.0 Tesla†

The performance of a 16‐channel receive‐only RF coil for brain imaging at 3.0 Tesla was investigated using a custom‐built 16‐channel receiver. Both the image signal‐to‐noise ratio (SNR) and the noise amplification (g‐factor) in sensitivity‐encoding (SENSE) parallel imaging applications were quantitatively evaluated. Furthermore, the performance was compared with that of hypothetical coils with one, two, four, and eight elements (n) by combining channels in software during image reconstruction. As expected, both the g‐factor and SNR improved substantially with n. Compared to an equivalent (simulated) single‐element coil, the 16‐channel coil showed a 1.87‐fold average increase in brain SNR. This was mainly due to an increase in SNR in the peripheral brain (an up to threefold SNR increase), whereas the SNR increase in the center of the brain was 4%. The incremental SNR gains became relatively small at large n, with a 9% gain observed when n was increased from 8 to 16. Compared to the (larger) product birdcage head coil, SNR increased by close to a factor of 2 in the center, and by up to a factor of 6 in the periphery of the brain. For low SENSE acceleration (rate‐2), g‐factors leveled off for n > 4, and improved only slightly (1.4% averaged over brain) going from n = 8 to n = 16. Improvements in g for n > 8 were larger for higher acceleration rates, with the improvement for rate‐3 averaging 12.0%. Magn Reson Med 51:22–26, 2004. Published 2003 Wiley‐Liss, Inc.

Temporal dynamics of the BOLD fMRI impulse response

Using computer simulations and high-resolution fMRI experiments in humans (n=6) and rats (n=8), we investigated to what extent BOLD fMRI temporal resolution is limited by dispersion in the venous vasculature. For this purpose, time-to-peak (TTP) and full-width at half-maximum (FWHM) of the BOLD impulse response (IR) function were determined. In fMRI experiments, a binary m-sequence probe method was used to obtain high-sensitivity model-free single-pixel estimates of IR. Simulations of postcapillary flow suggested that flow-related dispersion leads to a TTP and FWHM increase, which can amount to several seconds in larger pial veins. fMRI experiments showed substantial spatial variation in IR timing within human visual cortex, together with a correlation between TTP and FWHM. Averaged across the activated regions and across subjects, TTP and FWHM were 4.51+/-0.52 and 4.04+/-0.42 s, respectively. In regions of interest (ROI) weighted toward the larger venous structures, TTP and FWHM increased to 5.07+/-0.64 and 4.32+/-0.48 s, respectively. In rat somatosensory cortex, TTP and FWHM were substantially shorter than in humans (2.73+/-0.60 and 2.28+/-0.63 s, respectively). These results are consistent with a substantial macrovascular dispersive contribution to BOLD IR in humans, and furthermore suggest that neurovascular coupling is a relatively rapid process, with a resolution below 2.3 s FWHM.

Hunting for neuronal currents: absence of rapid MRI signal changes during visual-evoked response

While recent reports have advocated the use of magnetic resonance imaging (MRI) to detect the effects of neuronal currents associated with human brain activity, only preliminary experimental data have been presented so far to demonstrate the feasibility of the method. Furthermore, it has not been adequately demonstrated that (1) MRI can separate neuronal current (NC) effects from other effects such as blood oxygen level-dependent (BOLD) contrast; (2) MRI has adequate sensitivity to detect NCs in vivo. In this work, we introduce a method that can separate slow (e.g., BOLD) processes from potential rapid (e.g., NC) processes and apply this method to investigate whether MRI allows detection of an NC response to a visual stimulus. MRI studies (n = 8) at 3.0 T using a sensitive multichannel detector showed insignificant effects related to NCs (averaged t < 0.05), in the presence of a highly significant BOLD signal (t = 6.15 +/- 0.90). In contrast, magnetoencephalography (MEG) experiments performed under similar conditions on the same subjects showed highly significant electrical activity (t = 7.90 +/- 2.28). It is concluded that, under the conditions used in this study, the sensitivity of MRI to detect evoked responses through NCs is at least an order of magnitude below that of BOLD-based functional MRI (fMRI) or MEG and too low to be practically useful.

Scalable multichannel MRI data acquisition system.

A scalable multichannel digital MRI receiver system was designed to achieve high bandwidth echo‐planar imaging (EPI) acquisitions for applications such as BOLD‐fMRI. The modular system design allows for easy extension to an arbitrary number of channels. A 16‐channel receiver was developed and integrated with a General Electric (GE) Signa 3T VH/3 clinical scanner. Receiver performance was evaluated on phantoms and human volunteers using a custom‐built 16‐element receive‐only brain surface coil array. At an output bandwidth of 1 MHz, a 100% acquisition duty cycle was achieved. Overall system noise figure and dynamic range were better than 0.85 dB and 84 dB, respectively. During repetitive EPI scanning on phantoms, the relative temporal standard deviation of the image intensity time‐course was below 0.2%. As compared to the product birdcage head coil, 16‐channel reception with the custom array yielded a nearly 6‐fold SNR gain in the cerebral cortex and a 1.8‐fold SNR gain in the center of the brain. The excellent system stability combined with the increased sensitivity and SENSE capabilities of 16‐channel coils are expected to significantly benefit and enhance fMRI applications. Magn Reson Med 51:165–171, 2004. Published 2003 Wiley‐Liss, Inc.

Workshop studies on monoclonal antibodies reactive against porcine myeloid cells

Investigators from eight laboratories analyzed the reactivity of 22 monoclonal antibodies (mAb) against porcine myeloid cells. Based on binding data, clustering analysis and inhibition studies, workshop mAb 74-22-15 (003) and 6F3 (007) were assigned a swine workshop cluster number 3 (SWC3). These mAb recognized macrophages and neutrophils; neutrophils; a monocyte/macrophage-specific mAb was not identified by this workshop.

Whole Brain Volume Measured from 1.5T versus 3T MRI in Healthy Subjects and Patients with Multiple Sclerosis

Whole brain atrophy is a putative outcome measure in monitoring relapsing‐remitting multiple sclerosis (RRMS). With the ongoing MRI transformation from 1.5T to 3T, there is an unmet need to calibrate this change. We evaluated brain parenchymal volumes (BPVs) from 1.5T versus 3T in MS and normal controls (NC).

Serum lipid antibodies are associated with cerebral tissue damage in multiple sclerosis

Objective: To determine whether peripheral immune responses as measured by serum antigen arrays are linked to cerebral MRI measures of disease severity in multiple sclerosis (MS). Methods: In this cross-sectional study, serum samples were obtained from patients with relapsing-remitting MS (n = 21) and assayed using antigen arrays that contained 420 antigens including CNS-related autoantigens, lipids, and heat shock proteins. Normalized compartment-specific global brain volumes were obtained from 3-tesla MRI as surrogates of atrophy, including gray matter fraction (GMF), white matter fraction (WMF), and total brain parenchymal fraction (BPF). Total brain T2 hyperintense lesion volume (T2LV) was quantified from fluid-attenuated inversion recovery images. Results: We found serum antibody patterns uniquely correlated with BPF, GMF, WMF, and T2LV. Furthermore, we identified immune signatures linked to MRI markers of neurodegeneration (BPF, GMF, WMF) that differentiated those linked to T2LV. Each MRI measure was correlated with a specific set of antibodies. Strikingly, immunoglobulin G (IgG) antibodies to lipids were linked to brain MRI measures. Based on the association between IgG antibody reactivity and each unique MRI measure, we developed a lipid index. This comprised the reactivity directed against all of the lipids associated with each specific MRI measure. We validated these findings in an additional independent set of patients with MS (n = 14) and detected a similar trend for the correlations between BPF, GMF, and T2LV vs their respective lipid indexes. Conclusions: We propose serum antibody repertoires that are associated with MRI measures of cerebral MS involvement. Such antibodies may serve as biomarkers for monitoring disease pathology and progression.

Multipathway sequences for MR thermometry

MR‐based thermometry is a valuable adjunct to thermal ablation therapies as it helps to determine when lethal doses are reached at the target and whether surrounding tissues are safe from damage. When the targeted lesion is mobile, MR data can further be used for motion‐tracking purposes. The present work introduces pulse sequence modifications that enable significant improvements in terms of both temperature‐to‐noise‐ratio properties and target‐tracking abilities. Instead of sampling a single magnetization pathway as in typical MR thermometry sequences, the pulse‐sequence design introduced here involves sampling at least one additional pathway. Image reconstruction changes associated with the proposed sampling scheme are also described. The method was implemented on two commonly used MR thermometry sequences: the gradient‐echo and the interleaved echo‐planar imaging sequences. Data from the extra pathway enabled temperature‐to‐noise‐ratio improvements by up to 35%, without increasing scan time. Potentially of greater significance is that the sampled pathways featured very different contrast for blood vessels, facilitating their detection and use as internal landmarks for tracking purposes. Through improved temperature‐to‐noise‐ratio and lesion‐tracking abilities, the proposed pulse‐sequence design may facilitate the use of MR‐monitored thermal ablations as an effective treatment option even in mobile organs such as the liver and kidneys. Magn Reson Med, 2011.

Prostate cancer discrimination in the peripheral zone with a reduced field-of-view T2-mapping MRI sequence

OBJECTIVES To evaluate the performance of T2 mapping in discriminating prostate cancer from normal prostate tissue in the peripheral zone using a practical reduced field-of-view MRI sequence requiring less than 3 minutes of scan time. MATERIALS AND METHODS Thirty-six patients with biopsy-proven peripheral zone prostate cancer without prior treatment underwent routine multiparametric MRI at 3.0T with an endorectal coil. An Inner-Volume Carr-Purcell-Meiboom-Gill imaging sequence that required 2.8 minutes to obtain data for quantitative T2 mapping covering the entire prostate gland was added to the routine multiparametric protocol. Suspected cancer (SC) and suspected healthy (SH) tissue in the peripheral zone were identified in consensus by three radiologists and were correlated with available biopsy results. Differences in mean T2 values in SC and SH regions-of-interest (ROIs) were tested for significance using unpaired Students two-tailed t-test. The area under the receiver operating characteristic curve was used to assess the optimal threshold T2 value for cancer discrimination. RESULTS ROI analyses revealed significantly (p<0.0001) shorter T2 values in SC (85.4±12.3ms) compared to SH (169.6±38.7ms). An estimated T2 threshold of 99ms yielded a sensitivity of 92% and a specificity of 97% for prostate cancer discrimination. CONCLUSIONS Quantitative values derived from this clinically practical T2-mapping sequence allow high precision discrimination between healthy and cancerous peripheral zone in the prostate.

Association Between Serum MicroRNAs and Magnetic Resonance Imaging Measures of Multiple Sclerosis Severity

Importance MicroRNAs (miRNAs) are promising multiple sclerosis (MS) biomarkers. Establishing the association between miRNAs and magnetic resonance imaging (MRI) measures of disease severity will help define their significance and potential impact. Objective To correlate circulating miRNAs in the serum of patients with MS to brain and spinal MRI. Design, Setting, and Participants A cross-sectional study comparing serum miRNA samples with MRI metrics was conducted at a tertiary MS referral center. Two independent cohorts (41 and 79 patients) were retrospectively identified from the Comprehensive Longitudinal Investigation of Multiple Sclerosis at the Brigham and Womens Hospital. Expression of miRNA was determined by locked nucleic acid–based quantitative real-time polymerase chain reaction. Spearman correlation coefficients were used to test the association between miRNA and brain lesions (T2 hyperintense lesion volume [T2LV]), the ratio of T1 hypointense lesion volume [T1LV] to T2LV [T1:T2]), brain atrophy (whole brain and gray matter), and cervical spinal cord lesions (T2LV) and atrophy. The study was conducted from December 2013 to April 2016. Main Outcomes and Measures miRNA expression. Results Of the 120 patients included in the study, cohort 1 included 41 participants (7 [17.1%] men), with mean (SD) age of 47.7 (9.5) years; cohort 2 had 79 participants (26 [32.9%] men) with a mean (SD) age of 43.0 (7.5) years. Associations between miRNAs and MRIs were both protective and pathogenic. Regarding miRNA signatures, a topographic specificity differed for the brain vs the spinal cord, and the signature differed between T2LV and atrophy/destructive measures. Four miRNAs showed similar significant protective correlations with T1:T2 in both cohorts, with the highest for hsa.miR.143.3p (cohort 1: Spearman correlation coefficient rs = −0.452, P = .003; cohort 2: rs = −0.225, P = .046); the others included hsa.miR.142.5p (cohort 1: rs = −0.424, P = .006; cohort 2: rs = −0.226, P = .045), hsa.miR.181c.3p (cohort 1: rs = −0.383, P = .01; cohort 2: rs = −0.222, P = .049), and hsa.miR.181c.5p (cohort 1: rs = −0.433, P = .005; cohort 2: rs = −0.231, P = .04). In the 2 cohorts, hsa.miR.486.5p (cohort 1: rs = 0.348, P = .03; cohort 2: rs = 0.254, P = .02) and hsa.miR.92a.3p (cohort 1: rs = 0.392, P = .01; cohort 2: rs = 0.222, P = .049) showed similar significant pathogenic correlations with T1:T2; hsa.miR.375 (cohort 1: rs = −0.345, P = .03; cohort 2: rs = −0.257, P = .022) and hsa.miR.629.5p (cohort 1: rs = −0.350, P = .03; cohort 2: rs = −0.269, P = .02) showed significant pathogenic correlations with brain atrophy. Although we found several miRNAs associated with MRI outcomes, none of these associations remained significant when correcting for multiple comparisons, suggesting that further validation of our findings is needed. Conclusions and Relevance Serum miRNAs may serve as MS biomarkers for monitoring disease progression and act as surrogate markers to identify underlying disease processes.