Stephan Kannengiesser

General formulation for quantitative G-factor calculation in GRAPPA reconstructions.

In this work a theoretical description for practical quantitative estimation of the noise enhancement in generalized autocalibrating partially parallel acquisitions (GRAPPA) reconstructions, equivalent to the geometry (g)‐factor in sensitivity encoding for fast MRI (SENSE) reconstructions, is described. The GRAPPA g‐factor is derived directly from the GRAPPA reconstruction weights. The procedure presented here also allows the calculation of quantitative g‐factor maps for both the uncombined and combined accelerated GRAPPA images. This enables, for example, a fast comparison between the performances of various GRAPPA reconstruction kernels or SENSE reconstructions. The applicability of this approach is validated on phantom studies and demonstrated using in vivo images for 1D and 2D parallel imaging. Magn Reson Med, 2009.

Field‐of‐view limitations in parallel imaging

Parallel imaging is one of the most promising developments in recent years for the acceleration of MR acquisitions. One area of practical importance where different parallel imaging methods perform differently is the manner in which they deal with aliasing in the full‐FOV reconstructed image. It has been reported that sensitivity encoding (SENSE) reconstruction fails whenever the reconstructed FOV is smaller than the object being imaged. On the other hand, generalized autocalibrating partially parallel acquisition (GRAPPA) has been used successfully to reconstruct images with aliasing in the reconstructed FOV, as in conventional imaging. The disparate behavior of these methods can be easily demonstrated by a few simple illustrative examples. Additional in vivo examples using GRAPPA and modified SENSE (mSENSE) make this distinction clear. These experiments demonstrate that SENSE fails to reconstruct correct images when coil sensitivity maps are used that do not automatically account for the object size and therefore the aliasing in the reconstructed images. However, with the use of aliased high‐resolution coil sensitivity maps, accurate SENSE reconstructions can be generated. On the other hand, GRAPPA produces images with an aliasing appearance that is exactly as would be expected from normal nonaccelerated acquisitions. An understanding of these effects could potentially lead to fewer operator‐dependent errors, as well as a better understanding of the differences between the underlying reconstruction processes. Magn Reson Med 52:1118–1126, 2004.

Liver fat quantification using a multi‐step adaptive fitting approach with multi‐echo GRE imaging

The purpose of this study was to develop a multi‐step adaptive fitting approach for liver proton density fat fraction (PDFF) and R2* quantification, and to perform an initial validation on a broadly available hardware platform.

Direct parallel image reconstructions for spiral trajectories using GRAPPA

The use of spiral trajectories is an efficient way to cover a desired k‐space partition in magnetic resonance imaging (MRI). Compared to conventional Cartesian k‐space sampling, it allows faster acquisitions and results in a slight reduction of the high gradient demand in fast dynamic scans, such as in functional MRI (fMRI). However, spiral images are more susceptible to off‐resonance effects that cause blurring artifacts and distortions of the point‐spread function (PSF), and thereby degrade the image quality. Since off‐resonance effects scale with the readout duration, the respective artifacts can be reduced by shortening the readout trajectory. Multishot experiments represent one approach to reduce these artifacts in spiral imaging, but result in longer scan times and potentially increased flow and motion artifacts. Parallel imaging methods are another promising approach to improve image quality through an increase in the acquisition speed. However, non‐Cartesian parallel image reconstructions are known to be computationally time‐consuming, which is prohibitive for clinical applications. In this study a new and fast approach for parallel image reconstructions for spiral imaging based on the generalized autocalibrating partially parallel acquisitions (GRAPPA) methodology is presented. With this approach the computational burden is reduced such that it becomes comparable to that needed in accelerated Cartesian procedures. The respective spiral images with two‐ to eightfold acceleration clearly benefit from the advantages of parallel imaging, such as enabling parallel MRI single‐shot spiral imaging with the off‐resonance behavior of multishot acquisitions. Magn Reson Med, 2006.

High‐resolution continuously acquired peripheral MR angiography featuring partial parallel imaging GRAPPA

Continuously‐moving‐table MRI, in contrast to traditional multistation techniques, potentially can improve the scan time efficiency of whole‐body applications and provide seamless images of an extended field of view (FOV). Contrast‐enhanced MR angiography (CE‐MRA) in particular requires high spatial resolution and at the same time has rigid scan time constraints due to the limited arterial contrast window. In this study a reconstruction method for continuously acquired 3D data sets during table movement was combined with a self‐calibrated partial parallel imaging algorithm (generalized autocalibrating partially parallel acquisitions (GRAPPA)). The method was applied to peripheral CE‐MRA and compared with a standard continuously‐moving‐table MRA protocol. The gain in scan time was used to increase the data acquisition matrix and decrease the slice thickness. The method was evaluated in five healthy volunteers and applied to one patient with peripheral arterial occlusive disease (PAOD). The protocols were intraindividually compared with respect to the signal‐to‐noise ratio (SNR) and contrast‐to‐noise ratio (CNR) in selected vessel segments, as well as overall vessel depiction. The combination of the continuously‐moving‐table technique with parallel imaging enabled the acquisition of seamless peripheral 3D MRA with increased resolution and an overall crisper appearance. Magn Reson Med, 2006.

Sliding multislice (SMS): A new technique for minimum FOV usage in axial continuously moving-table acquisitions

A novel technique for axial continuously moving‐table scans is described that minimizes the required extension of the scanners field of view (FOV) along the direction of table motion (z) by applying a segmented multislice acquisition technique. Any anatomical slice is acquired by applying the same phase‐encoding steps at the same spatial positions along the scanner FOV. The full k‐space data set of any anatomical slice is collected while the slice moves through the scanner from one scan position to the next. Simultaneous acquisition of multiple slices is realized by shifting the acquisition trajectories of different slices in time. It is demonstrated how the image artifact behavior that relates to varying imaging properties along the distance the table traverses during the acquisition of any given anatomical slice can be optimized simultaneously for all images. Discontinuities between the images along the slice axis are avoided because all z‐dependent scan properties are encoded identically for all slices. Flexible spatial acquisition patterns are proposed to enable data oversampling and overlapping slice acquisitions at reduced table speeds. A framework of equations is presented by which matched parameter combinations for sliding multislice acquisitions can be applied to both single‐ and multiecho sequences. The new technique is validated on phantom and in vivo measurements using a T1‐weighted fast low‐angle shot (FLASH) sequence as well as a T2‐weighted multi‐spin‐echo sequence of variable echo train lengths. Magn Reson Med, 2006.

Quantification of Hepatic Steatosis With a Multistep Adaptive Fitting MRI Approach: Prospective Validation Against MR Spectroscopy

OBJECTIVE. The purpose of this study is to prospectively compare hybrid and complex chemical shift-based MRI fat quantification methods against MR spectroscopy (MRS) for the measurement of hepatic steatosis. SUBJECTS AND METHODS. Forty-two subjects (18 men and 24 women; mean ± SD age, 52.8 ± 14 years) were prospectively enrolled and imaged at 3 T with a chemical shift-based MRI sequence and a single-voxel MRS sequence, each in one breath-hold. Proton density fat fraction and rate constant (R2*) using both single- and dual-R2* hybrid fitting methods, as well as proton density fat fraction and R2* maps using a complex fitting method, were generated. A single radiologist colocalized volumes of interest on the proton density fat fraction and R2* maps according to the spectroscopy measurement voxel. Agreement among the three MRI methods and the MRS proton density fat fraction values was assessed using linear regression, intraclass correlation coefficient (ICC), and Bland-Altman analysis. RESULTS. Correlation between the MRI and MRS measures of proton density fat fraction was excellent. Linear regression coefficients ranged from 0.98 to 1.01, and intercepts ranged from -1.12% to 0.49%. Agreement measured by ICC was also excellent (0.99 for all three methods). Bland-Altman analysis showed excellent agreement, with mean differences of -1.0% to 0.6% (SD, 1.3-1.6%). CONCLUSION. The described MRI-based liver proton density fat fraction measures are clinically feasible and accurate. The validation of proton density fat fraction quantification methods is an important step toward wide availability and acceptance of the MRI-based measurement of proton density fat fraction as an accurate and generalizable biomarker.

T2-weighted 3D fMRI using S2-SSFP at 7 tesla.

In this study, the sensitivity of the S2‐steady‐state free precession (SSFP) signal for functional MRI at 7 T was investigated. In order to achieve the necessary temporal resolution, a three‐dimensional acquisition scheme with acceleration along two spatial axes was employed. Activation maps based on S2‐steady‐state free precession data showed similar spatial localization of activation and sensitivity as spin‐echo echo‐planar imaging (SE‐EPI), but data can be acquired with substantially lower power deposition. The functional sensitivity estimated by the average z‐values was not significantly different for SE‐EPI compared to the S2‐signal but was slightly lower for the S2‐signal (6.74 ± 0.32 for the TR = 15 ms protocol and 7.51 ± 0.78 for the TR = 27 ms protocol) compared to SE‐EPI (7.49 ± 1.44 and 8.05 ± 1.67) using the same activated voxels, respectively. The relative signal changes in these voxels upon activation were slightly lower for SE‐EPI (2.37% ± 0.18%) compared to the TR = 15 ms S2‐SSFP protocol (2.75% ± 0.53%) and significantly lower than the TR = 27 ms protocol (5.38% ± 1.28%), in line with simulations results. The large relative signal change for the long TR SSFP protocol can be explained by contributions from multiple coherence pathways and the low intrinsic intensity of the S2 signal. In conclusion, whole‐brain T2‐weighted functional MRI with negligible image distortion at 7 T is feasible using the S2‐SSFP sequence and partially parallel imaging. Magn Reson Med 63:1015–1020, 2010.

Interplatform reproducibility of liver and spleen stiffness measured with MR elastography.

To assess interplatform reproducibility of liver stiffness (LS) and spleen stiffness (SS) measured with magnetic resonance elastography (MRE) based on a 2D gradient echo (GRE) sequence.

Pancreatic Steatosis and Fibrosis: Quantitative Assessment with Preoperative Multiparametric MR Imaging

PURPOSE To evaluate the diagnostic performance of multiparametric pancreatic magnetic resonance (MR) imaging, including the T2*-corrected Dixon technique and intravoxel incoherent motion (IVIM) diffusion-weighted (DW) imaging, in the quantification of pancreatic steatosis and fibrosis, with histologic analysis as the reference standard, and to determine the relationship between MR parameters and postoperative pancreatic fistula. MATERIALS AND METHODS This retrospective study was approved by the institutional review board, and the informed consent requirement was waived. A total of 165 patients (93 men, 72 women; mean age, 62 years) underwent preoperative 3-T MR imaging and subsequent pancreatectomy (interval, 0-77 days). Fat fractions, IVIM DW imaging parameters (true diffusion coefficient [D], pseudodiffusion coefficient [D*], and perfusion fraction [f]), pancreas-to-muscle signal intensity ratios on unenhanced T1-weighted images, and pancreatic duct sizes were compared with the fat fractions and fibrosis degrees (F0-F3) of specimens. In 95 patients who underwent pancreatoenteric anastomosis, MR parameters were compared between groups with clinically relevant postoperative pancreatic fistula and those without. The relationship between postoperative pancreatic fistula and MR parameters was evaluated by using logistic regression analysis. RESULTS Fat fractions at MR imaging showed a moderate relationship with histologic findings (r = 0.71; 95% confidence interval: 0.63, 0.78). Patients with advanced fibrosis (F2-F3) had lower D*([39.72 ± 13.64] ×10(-3)mm(2)/sec vs [32.50 ± 13.09] ×10(-3)mm(2)/sec [mean ± standard deviation], P = .004), f (29.77% ± 8.51 vs 20.82% ± 8.66, P < .001), and unenhanced T1-weighted signal intensity ratio (1.43 ± 0.26 vs 1.21 ± 0.30, P < .001) than did patients with F0-F1 disease. Clinically relevant fistula developed in 14 (15%) of 95 patients, and f was significantly associated with postoperative pancreatic fistula (odds ratio, 1.17; 95% confidence interval: 1.05, 1.30). CONCLUSION Multiparametric MR imaging of the pancreas, including imaging with the T2*-corrected Dixon technique and IVIM DW imaging, may yield quantitative information regarding pancreatic steatosis and fibrosis, and f was shown to be significantly associated with postoperative pancreatic fistulas.