Yi-Ru Lin

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.

Fast mapping of the T2 relaxation time of cerebral metabolites using proton echo-planar spectroscopic imaging (PEPSI).

Metabolite T2 is necessary for accurate quantification of the absolute concentration of metabolites using long‐echo‐time (TE) acquisition schemes. However, lengthy data acquisition times pose a major challenge to mapping metabolite T2. In this study we used proton echo‐planar spectroscopic imaging (PEPSI) at 3T to obtain fast T2 maps of three major cerebral metabolites: N‐acetyl‐aspartate (NAA), creatine (Cre), and choline (Cho). We showed that PEPSI spectra matched T2 values obtained using single‐voxel spectroscopy (SVS). Data acquisition for 2D metabolite maps with a voxel volume of 0.95 ml (32 × 32 image matrix) can be completed in 25 min using five TEs and eight averages. A sufficient spectral signal‐to‐noise ratio (SNR) for T2 estimation was validated by high Pearsons correlation coefficients between logarithmic MR signals and TEs (R2 = 0.98, 0.97, and 0.95 for NAA, Cre, and Cho, respectively). In agreement with previous studies, we found that the T2 values of NAA, but not Cre and Cho, were significantly different between gray matter (GM) and white matter (WM; P < 0.001). The difference between the T2 estimates of the PEPSI and SVS scans was less than 9%. Consistent spatial distributions of T2 were found in six healthy subjects, and disagreement among subjects was less than 10%. In summary, the PEPSI technique is a robust method to obtain fast mapping of metabolite T2. Magn Reson Med 57:859–865, 2007.

Comparison of arterial spin labeling and first-pass dynamic contrast-enhanced MR imaging in the assessment of pulmonary perfusion in humans: The inflow spin-tracer saturation effect

The flow‐sensitive alternating inversion recovery (FAIR) and the first‐pass dynamic contrast‐enhanced MR imaging (CE‐MRI) techniques have both been shown to be effective in the assessment of human pulmonary perfusion. However, no comprehensive comparison of the measurements by these two methods has been reported. In this study, healthy adults were recruited, with FAIR and CE‐MRI performed for an estimation of the relative pulmonary blood flow (rPBF). Regions of interest were encircled from the right and left lungs, with right‐to‐left rPBF ratios calculated. Results indicated that, on posterior coronal slices, the rPBF ratios obtained with the FAIR technique agreed well with CE‐MRI measurements (mean difference = −0.02, intraclass correlation coefficient RI = 0.78, 95% confidence interval = [0.67, 0.86]). On middle coronal slices, however, FAIR showed a substantially lower rPBF by up to 43% in the right lung compared with CE‐MRI (mean difference = −0.38, RI = 0.34, 95% confidence interval = [−0.09, 0.68]). The location‐dependent discrepancy between measurements by FAIR and CE‐MRI methods is attributed to tracer saturation effects of arterial inflow when the middle coronal slice contains the in‐plane‐oriented right pulmonary artery, whereas the left lung rPBF is less affected due to oblique orientation of the left pulmonary artery. Intrasequence comparison on additional subjects using FAIR at different slice orientations supported the above hypothesis. It is concluded that FAIR imaging for pulmonary perfusion in the coronal plane provides equivalent rPBF information with CE‐MRI only in the absence of tracer saturation effects; hence, FAIR should be carefully exercised to avoid misleading interpretations. Magn Reson Med 52:1291–1301, 2004.

Short- and long-term quantitation reproducibility of brain metabolites in the medial wall using proton echo planar spectroscopic imaging.

Proton echo planar spectroscopic imaging (PEPSI) is a fast magnetic resonance spectroscopic imaging (MRSI) technique that allows mapping spatial metabolite distributions in the brain. Although the medial wall of the cortex is involved in a wide range of pathological conditions, previous MRSI studies have not focused on this region. To decide the magnitude of metabolic changes to be considered significant in this region, the reproducibility of the method needs to be established. The study aims were to establish the short- and long-term reproducibility of metabolites in the right medial wall and to compare regional differences using a constant short-echo time (TE30) and TE averaging (TEavg) optimized to yield glutamatergic information. 2D sagittal PEPSI was implemented at 3T using a 32 channel head coil. Acquisitions were repeated immediately and after approximately 2 weeks to assess the coefficients of variation (COV). COVs were obtained from eight regions-of-interest (ROIs) of varying size and location. TE30 resulted in better spectral quality and similar or lower quantitation uncertainty for all metabolites except glutamate (Glu). When Glu and glutamine (Gln) were quantified together (Glx) reduced quantitation uncertainty and increased reproducibility was observed for TE30. TEavg resulted in lowered quantitation uncertainty for Glu but in less reliable quantification of several other metabolites. TEavg did not result in a systematically improved short- or long-term reproducibility for Glu. The ROI volume was a major factor influencing reproducibility. For both short- and long-term repetitions, the Glu COVs obtained with TEavg were 5-8% for the large ROIs, 12-17% for the medium sized ROIs and 16-26% for the smaller cingulate ROIs. COVs obtained with TE30 for the less specific Glx were 3-5%, 8-10% and 10-15%. COVs for N-acetyl aspartate, creatine and choline using TE30 with long-term repetition were between 2-10%. Our results show that the cost of more specific glutamatergic information (Glu versus Glx) is the requirement of an increased effect size especially with increasing anatomical specificity. This comes in addition to the loss of sensitivity for other metabolites. Encouraging results were obtained with TE30 compared to other previously reported MRSI studies. The protocols implemented here are reliable and may be used to study disease progression and intervention mechanisms.

Resting‐State Functional Magnetic Resonance Imaging: The Impact of Regression Analysis

To investigate the impact of regression methods on resting‐state functional magnetic resonance imaging (rsfMRI). During rsfMRI preprocessing, regression analysis is considered effective for reducing the interference of physiological noise on the signal time course. However, it is unclear whether the regression method benefits rsfMRI analysis.

Application of model-free analysis in the MR assessment of pulmonary perfusion dynamics

Dynamic contrast‐enhanced (DCE) MRI has been used to quantitatively evaluate pulmonary perfusion based on the assumption of a gamma‐variate function and an arterial input function (AIF) for deconvolution. However, these assumptions may be too simplistic and may not be valid in pathological conditions, especially in patients with complex inflow patterns (such as in congenital heart disease). Exploratory data analysis methods make minimal assumptions on the data and could overcome these pitfalls. In this work, two temporal clustering methods—Kohonen clustering network (KCN) and Fuzzy C‐Means (FCM)—were concatenated to identify pixel time‐course patterns. The results from seven normal volunteers show that this technique is superior for discriminating vessels and compartments in the pulmonary circulation. Patient studies with five cases of acquired or congenital pulmonary perfusion disorders demonstrate that pathologies can be highlighted in a concise map that combines information of the mean transit time (MTT) and pulmonary blood volume (PBV). The method was found to provide greater insight into the perfusion dynamics that might be overlooked by current model‐based analyses, and may serve as a basis for optimal hemodynamic quantitative modeling of the interrogated perfusion compartments. Magn Reson Med 54:299–308, 2005.

Optimization of PROPELLER reconstruction for free-breathing T1-weighted cardiac imaging.

PURPOSE Clinical cardiac MR imaging techniques generally require patients to hold their breath during the scanning process to minimize respiratory motion-related artifacts. However, some patients cannot hold their breath because of illness or limited breath-hold capacity. This study aims to optimize the PROPELLER reconstruction for free-breathing myocardial T1-weighted imaging. METHODS Eight healthy volunteers (8 men; mean age 26.4 years) participated in this study after providing institutionally approved consent. The PROPELLER encoding method can reconstruct a low-resolution image from every blade because of k-space center oversampling. This study investigated the feasibility of extracting a respiratory trace from the PROPELLER blades by implementing a fully automatic region of interest selection and introducing a best template index to account for the property of the human respiration cycle. RESULTS Results demonstrated that the proposed algorithm significantly improves the contrast-to-noise ratio and the image sharpness (p < 0.05). CONCLUSIONS The PROPELLER method is expected to provide a robust tool for clinical application in free-breathing myocardial T1-weighted imaging. It could greatly facilitate the acquisition procedures during such a routine examination.

Temporal correlation-based dynamic contrast-enhanced MR imaging improves assessment of complex pulmonary circulation in congenital heart disease†

A temporal correlation (TC) mapping method is proposed to help bolus chasing during dynamic contrast‐enhanced (DCE) MRI of complex pulmonary circulation (CPC) in patients with congenital heart disease. DCE‐MRI was performed on five healthy male subjects (23–24 years old) and 25 patients (nine males and 16 females, 0.25–44 years old), and TC maps were generated by performing pixel‐based computation of cross‐correlations to the pulmonary artery with a series of time shifts in all subjects. Qualitative and quantitative evaluations were performed in comparison with original DCE images. TC maps exhibited a better signal‐to‐noise ratio (SNR) by factors of 4.3 and 1.3 in the lung parenchyma, pulmonary veins, and superior artery/vein; a better intraparenchymal contrast‐to‐noise ratio (CNR) by factors of 1.5–5.4; and a significantly higher conspicuity in all regions except the pulmonary arteries when graded with a five‐point score. TC maps evaluated by two experienced clinicians significantly added relevant information (P < 0.001), and in some cases affected the final diagnosis. We conclude that TC maps facilitate bolus chasing for DCE‐MRI by reducing recirculation effects and interframe fluctuations, and hence complements morphological imaging of CPC in patients with complex congenital heart disease. Magn Reson Med, 2006.

Effects of Frequency Drift on the Quantification of Gamma-Aminobutyric Acid Using MEGA-PRESS

The MEGA-PRESS method is the most common method used to measure γ-aminobutyric acid (GABA) in the brain at 3T. It has been shown that the underestimation of the GABA signal due to B0 drift up to 1.22 Hz/min can be reduced by post-frequency alignment. In this study, we show that the underestimation of GABA can still occur even with post frequency alignment when the B0 drift is up to 3.93 Hz/min. The underestimation can be reduced by applying a frequency shift threshold. A total of 23 subjects were scanned twice to assess the short-term reproducibility, and 14 of them were scanned again after 2–8 weeks to evaluate the long-term reproducibility. A linear regression analysis of the quantified GABA versus the frequency shift showed a negative correlation (P < 0.01). Underestimation of the GABA signal was found. When a frequency shift threshold of 0.125 ppm (15.5 Hz or 1.79 Hz/min) was applied, the linear regression showed no statistically significant difference (P > 0.05). Therefore, a frequency shift threshold at 0.125 ppm (15.5 Hz) can be used to reduce underestimation during GABA quantification. For data with a B0 drift up to 3.93 Hz/min, the coefficients of variance of short-term and long-term reproducibility for the GABA quantification were less than 10% when the frequency threshold was applied.

Correction of geometric distortion in Propeller echo planar imaging using a modified reversed gradient approach.

OBJECTIVE This study investigates the application of a modified reversed gradient algorithm to the Propeller-EPI imaging method (periodically rotated overlapping parallel lines with enhanced reconstruction based on echo-planar imaging readout) for corrections of geometric distortions due to the EPI readout. MATERIALS AND METHODS Propeller-EPI acquisition was executed with 360-degree rotational coverage of the k-space, from which the image pairs with opposite phase-encoding gradient polarities were extracted for reversed gradient geometric and intensity corrections. The spatial displacements obtained on a pixel-by-pixel basis were fitted using a two-dimensional polynomial followed by low-pass filtering to assure correction reliability in low-signal regions. Single-shot EPI images were obtained on a phantom, whereas high spatial resolution T2-weighted and diffusion tensor Propeller-EPI data were acquired in vivo from healthy subjects at 3.0 Tesla, to demonstrate the effectiveness of the proposed algorithm. RESULTS Phantom images show success of the smoothed displacement map concept in providing improvements of the geometric corrections at low-signal regions. Human brain images demonstrate prominently superior reconstruction quality of Propeller-EPI images with modified reversed gradient corrections as compared with those obtained without corrections, as evidenced from verification against the distortion-free fast spin-echo images at the same level. CONCLUSIONS The modified reversed gradient method is an effective approach to obtain high-resolution Propeller-EPI images with substantially reduced artifacts.