Hsiao-Wen Chung

PROPELLER EPI: An MRI technique suitable for diffusion tensor imaging at high field strength with reduced geometric distortions†

A technique suitable for diffusion tensor imaging (DTI) at high field strengths is presented in this work. The method is based on a periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) k‐space trajectory using EPI as the signal readout module, and hence is dubbed PROPELLER EPI. The implementation of PROPELLER EPI included a series of correction schemes to reduce possible errors associated with the intrinsically higher sensitivity of EPI to off‐resonance effects. Experimental results on a 3.0 Tesla MR system showed that the PROPELLER EPI images exhibit substantially reduced geometric distortions compared with single‐shot EPI, at a much lower RF specific absorption rate (SAR) than the original version of the PROPELLER fast spin‐echo (FSE) technique. For DTI, the self‐navigated phase‐correction capability of the PROPELLER EPI sequence was shown to be effective for in vivo imaging. A higher signal‐to‐noise ratio (SNR) compared to single‐shot EPI at an identical total scan time was achieved, which is advantageous for routine DTI applications in clinical practice. Magn Reson Med, 2005.

Principles and Limitations of Computational Algorithms in Clinical Diffusion Tensor MR Tractography

SUMMARY: There have been numerous reports documenting the graphic reconstruction of 3D white matter architecture in the human brain by means of diffusion tensor MR tractography. Different from other reviews addressing the physics and clinical applications of DTI, this article reviews the computational principles of tractography algorithms appearing in the literature. The simplest voxel-based method and 2 widely used subvoxel approaches are illustrated first, together with brief notes on parameter selection and the restrictions arising from the distinct attributes of tract estimations. Subsequently, some advanced techniques attempting to offer improvement in various aspects are briefly introduced, including the increasingly popular research tracking tool using HARDI. The article explains the inherent technical limitations in most of the algorithms reported to date and concludes by providing a reference guideline for formulating routine applications of this important tool to clinical neuroradiology in an objective and reproducible manner.

Automatic spike detection via an artificial neural network using raw EEG data: effects of data preparation and implications in the limitations of online recognition

OBJECTIVE Automatic detection of epileptic EEG spikes via an artificial neural network has been reported to be feasible using raw EEG data as input. This study re-investigated its suitability by further exploring the effects of data preparation on classification performance testing. METHODS Six hundred EEG files (300 spikes and 300 non-spikes) taken from 20 patients were included in this study. Raw EEG data were sent to the neural network using the architecture reported to give best performance (30 input-layer and 6 hidden-layer neurons). RESULTS Significantly larger weighting of the 10th input-layer neuron was found after training with prepared raw EEG data. The classification process was thus dominated by the peak location. Subsequent analysis showed that online spike detection with an erroneously trained network yielded an area less than 0.5 under the receiver-operating-characteristic curve, and hence performed inferiorly to random assignments. Networks trained and tested using the same unprepared EEG data achieved no better than about 87% true classification rate at equal sensitivity and specificity. CONCLUSIONS The high true classification rate reported previously is believed to be an artifact arising from erroneous data preparation and off-line validation. Spike detection using raw EEG data as input is unlikely to be feasible under current computer technology.

Fat and Water Separation in Balanced Steady-State Free Precession Using the Dixon Method

In this work the feasibility of separating fat and water signals using the balanced steady‐state free precession (SSFP) technique is demonstrated. The technique is based on the observation (Scheffler and Hennig, Magnetic Resonance in Medicine 2003;49:395–397) that at the nominal values of TE = TR/2 in SSFP imaging, phase coherence can be achieved at essentially only two orientations (0° and 180°) relative to the RF pulses in the rotating frame, under the assumption of TR << T2, and independently of the SSFP angle. This property allows in‐phase and out‐of‐phase SSFP images to be obtained by proper choices of the center frequency offset, and thus allows the Dixon subtraction method to be utilized for effective fat–water separation. The TR and frequency offset for optimal fat–water separation are derived from theories. Experimental results from healthy subjects, using a 3.0 Tesla system, show that nearly complete fat suppression can be accomplished. Magn Reson Med 51:243–247, 2004.

Heroin-induced spongiform leukoencephalopathy : Value of diffusion MR imaging

The diffusion-weighted (DW) magnetic resonance (MR) imaging findings of a patient with subacute stage of heroin-induced vacuolating myelinopathy are reported. The diffuse decrease of apparent diffusion coefficient (ADC) of the white matter on DW imaging is attributed to restricted water diffusion, which is known to be caused by fluid entrapment within the myelin lamellae without demyelination.

Accelerated proton echo planar spectroscopic imaging (PEPSI) using GRAPPA with a 32‐channel phased‐array coil

Parallel imaging has been demonstrated to reduce the encoding time of MR spectroscopic imaging (MRSI). Here we investigate up to 5‐fold acceleration of 2D proton echo planar spectroscopic imaging (PEPSI) at 3T using generalized autocalibrating partial parallel acquisition (GRAPPA) with a 32‐channel coil array, 1.5 cm3 voxel size, TR/TE of 15/2000 ms, and 2.1 Hz spectral resolution. Compared to an 8‐channel array, the smaller RF coil elements in this 32‐channel array provided a 3.1‐fold and 2.8‐fold increase in signal‐to‐noise ratio (SNR) in the peripheral region and the central region, respectively, and more spatial modulated information. Comparison of sensitivity‐encoding (SENSE) and GRAPPA reconstruction using an 8‐channel array showed that both methods yielded similar quantitative metabolite measures (P > 0.1). Concentration values of N‐acetyl‐aspartate (NAA), total creatine (tCr), choline (Cho), myo‐inositol (mI), and the sum of glutamate and glutamine (Glx) for both methods were consistent with previous studies. Using the 32‐channel array coil the mean Cramer–Rao lower bounds (CRLB) were less than 8% for NAA, tCr, and Cho and less than 15% for mI and Glx at 2‐fold acceleration. At 4‐fold acceleration the mean CRLB for NAA, tCr, and Cho was less than 11%. In conclusion, the use of a 32‐channel coil array and GRAPPA reconstruction can significantly reduce the measurement time for mapping brain metabolites. Magn Reson Med 59:989–998, 2008.

Are trueFISP images T2/T1-weighted?

Images acquired using the TrueFISP technique (true fast imaging with steady‐state precession) are generally believed to exhibit T2/T1‐weighting. In this study, it is demonstrated that with the widely used half‐flip‐angle preparation scheme, approaching the steady state requires a time length comparable to the scan time such that the transient‐state response may dominate the TrueFISP image contrast. Two‐dimensional images of the human brain were obtained using various phase‐encoding matrices to investigate the transient‐state signal behavior. Contrast between gray and white matter was found to change significantly from proton‐density‐ to T2/T1‐weighted as the phase‐encoding matrix size increased, which was in good agreement with theoretical predictions. It is concluded that TrueFISP images in general exhibit T2/T1‐contrast, but should be more appropriately regarded as exhibiting a transient‐state combination of proton‐density and T2/T1 contrast under particular imaging conditions. Interpretation of tissue characteristics from TrueFISP images in clinical practice thus needs to be exercised with caution. Magn Reson Med 48:684–688, 2002.

Correlation of bone marrow lipid water content with bone mineral density on the lumbar spine.

Study Design. Prospective study. Objectives. To assess the proton MR spectroscopy (1H MRS) of vertebral bone marrow and correlate the lipid water ratio (LWR) and spectral line width (LW) with bone mineral density (BMD) in female subjects. Summary of Background Data. The mechanism of bone marrow fat accumulation and bone mineral content is poorly understood. Proton MR spectroscopy was used to demonstrate the lipid and water spectra in the bone marrow. We try to assess the possible interaction between the bone marrow lipid content, aging, and BMD. Methods. Proton MRS and BMD of the lumbar spine were performed in 52 female subjects (mean age, 58 years; SD, 10 years). They were 13 premenopausal and 39 postmenopausal women. The BMD (g/cm2) was measured using dual energy radiograph absorptiometry at the lumbar spine. Single voxel 1H MRS was measured at L3 vertebral body by stimulated echo-acquisition mode (STEAM) sequence and demonstrated two major peaks (lipid and water). Comparisons of the differences between the two subgroups were made. Pearson’s correlation was also calculated to explore the association of 1H MRS measurements with age and BMD. Partial correlation was further conducted when controlling the variable such as age or BMD. Results. BMD and LWR had statistically significant difference between the pre- and postmenopausal subgroups (P < 0.001), while lipid LW had a borderline difference and water LW had no difference. LWR was positively correlated with age (r = 0.52 and P < 0.0001) and negatively correlated with BMD (r = −0.40 and P = 0.003) for all the subjects. Lipid LW was negatively correlated with age (r = −0.32 and P = 0.0197) and positively correlated with BMD (r = 0.67 and P < 0.0001). When controlling for BMD effect, only LWR is statistically correlated with age (partial r = 0.39, P = 0.0045), while only the lipid LW is statistically correlated with BMD when controlling for age (partial r = 0.63, P < 0.0001). None of the correlations between water LW and age or BMD was significant. In the subgroups, only the lipid LW is significantly correlated with BMD (r = 0.78, P = 0.0016 in premenopausal women; r = 0.62, P < 0.0001 in postmenopausal women). Conclusion. The LWR had a positive correlation with the age, while the lipid LW had a positive correlation with BMD, even after controlling the age factor. The bone marrow lipid water content and metabolism acted as important roles in the internal environment of bone and influenced bone mineralization.

Potential role of nuclear magnetic resonance for the evaluation of trabecular bone quality.

SummaryThis paper discusses two novel applications of nuclear magnetic resonance (NMR) as an investigational tool for the assessment of cancellous bone microarchitecture. It further outlines extensions of the method forin vivo clinical evaluation of bone strength in patients with skeletal disorders such as osteoporosis. The first method relies on the hypothesis that the presence of two phases of different magnetic permeability, i.e., bone and bone marrow, causes a spatial nonuniformity of the magnetic field across the measurement volume. The resulting spread in resonance frequency shortens the decay time constant (T2*) of the time domain proton signal in bone marrow or its substitute (water). Increased trabecular spacing, such as it occurs in osteoporosis, reduces the spatial field inhomogeneity and thus prolongs T2*, which has been shown bothin vitro andin vivo. Subjects with osteoporosis, characterized by either low bone mineral density and/or spine compression fractures, have T2* values that are significantly prolonged. The second method focuses on a direct measurement of micromorphometric parameters of cancellous bone, using the principles of proton NMR microscopy in conjunction with computer processing of the resulting digital images. Image contrast between the trabeculae and the intertrabecular space is based on the marrow protons providing a signal, as opposed to bone, which appears with background intensity. Once tissues have been classified (into bone and marrow), for example, by means of a histogram-based segmentation algorithm, bone area fraction, mean trabecular plate density (MTPD), and mean trabecular plate thickness (MTPT) can be computed without the need for further operator intervention. The most critical parameter for successful implementation is image slice thickness which determines the extent of partial volume blurring. At 400 MHz spectrometer frequency (9.4 T field strength), images of appropriate resolution can be obtained from a 1 cm3 specimen of vertebral cancellous bone in 1 hour or less. It is shown that for relatively isotropic cancellous bone such as the one found in the vertebrae, a slice thickness on the order of 200 μm is adequate, with an inplane resolution on the order of 50 × 50 μm2 As an illustration of the technique, the relationship among the different stereologic parameters in cadaver specimens of human lumbar vertebrae is reported, showing a strong association between the area fraction and both MTPD and MTPT. The chief benefit of the new technique is its nondestructive nature and its ability to provide histomorphometric images from multiple physical locations and in multiple planes, which is desirable because of the large spatial variations in the morphologic parameters within the bone. Finally, the technique is demonstrated to be potentially also noninvasive, as illustrated with images from the human finger, acquired on a modified 1.5 Tesla clinical magnetic resonance imaging system at a pixel size of 95 × 95 μm2

PROPELLER-EPI with parallel imaging using a circularly symmetric phased-array RF coil at 3.0 T : Application to high-resolution diffusion tensor imaging

A technique integrating multishot periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) and parallel imaging is presented for diffusion echo‐planar imaging (EPI) at high spatial resolution. The method combines the advantages of parallel imaging to achieve accelerated sampling along the phase‐encoding direction, and PROPELLER acquisition to further decrease the echo train length (ETL) in EPI. With an eight‐element circularly symmetric RF coil, a parallel acceleration factor of 4 was applied such that, when combined with PROPELLER acquisition, a reduction of geometric distortions by a factor substantially greater than 4 was achieved. The resulting phantom and human brain images acquired with a 256 × 256 matrix and an ETL of only 16 were visually identical in shape to those acquired using the fast spin‐echo (FSE) technique, even without field‐map corrections. It is concluded that parallel PROPELLER‐EPI is an effective technique that can substantially reduce susceptibility‐induced geometric distortions at high field strength. Magn Reson Med, 2006.