Mathias Nittka

Generalized autocalibrating partially parallel acquisitions (GRAPPA)

In this study, a novel partially parallel acquisition (PPA) method is presented which can be used to accelerate image acquisition using an RF coil array for spatial encoding. This technique, GeneRalized Autocalibrating Partially Parallel Acquisitions (GRAPPA) is an extension of both the PILS and VD‐AUTO‐SMASH reconstruction techniques. As in those previous methods, a detailed, highly accurate RF field map is not needed prior to reconstruction in GRAPPA. This information is obtained from several k‐space lines which are acquired in addition to the normal image acquisition. As in PILS, the GRAPPA reconstruction algorithm provides unaliased images from each component coil prior to image combination. This results in even higher SNR and better image quality since the steps of image reconstruction and image combination are performed in separate steps. After introducing the GRAPPA technique, primary focus is given to issues related to the practical implementation of GRAPPA, including the reconstruction algorithm as well as analysis of SNR in the resulting images. Finally, in vivo GRAPPA images are shown which demonstrate the utility of the technique. Magn Reson Med 47:1202–1210, 2002.

Partially Parallel Imaging With Localized Sensitivities (PILS)

In this study a novel partially parallel acquisition method is presented, which can be used to accelerate image acquisition using an RF coil array for spatial encoding. In this technique, Parallel Imaging with Localized Sensitivities (PILS), it is assumed that the individual coils in the array have localized sensitivity patterns, in that their sensitivity is restricted to a finite region of space. Within the PILS model, a detailed, highly accurate RF field map is not needed prior to reconstruction. In PILS, each coil in the array is fully characterized by only two parameters: the center of coils sensitive region in the FOV and the width of the sensitive region around this center. In this study, it is demonstrated that the incorporation of these coil parameters into a localized Fourier transform allows reconstruction of full FOV images in each of the component coils from data sets acquired with a reduced number of phase encoding steps compared to conventional imaging techniques. After the introduction of the PILS technique, primary focus is given to issues related to the practical implementation of PILS, including coil parameter determination and the SNR and artifact power in the resulting images. Finally, in vivo PILS images are shown which demonstrate the utility of the technique. Magn Reson Med 44:602–609, 2000.

32-channel 3 Tesla receive-only phased-array head coil with soccer-ball element geometry

A 32‐channel 3T receive‐only phased‐array head coil was developed for human brain imaging. The helmet‐shaped array was designed to closely fit the head with individual overlapping circular elements arranged in patterns of hexagonal and pentagonal symmetry similar to that of a soccer ball. The signal‐to‐noise ratio (SNR) and noise amplification (g‐factor) in accelerated imaging applications were quantitatively evaluated in phantom and human images and compared with commercially available head coils. The 32‐channel coil showed SNR gains of up to 3.5‐fold in the cortex and 1.4‐fold in the corpus callosum compared to a (larger) commercial eight‐channel head coil. The experimentally measured g‐factor performance of the helmet array showed significant improvement compared to the eight‐channel array (peak g‐factor 59% and 26% of the eight‐channel values for four‐ and fivefold acceleration). The performance of the arrays is demonstrated in high‐resolution and highly accelerated brain images. Magn Reson Med, 2006.

Resolution enhancement in lung 1H imaging using parallel imaging methods

Resolution in 1H lung imaging is limited mainly by the acquisition time. Today, half‐Fourier acquisition single‐shot turbo spin‐echo (HASTE) sequences, with short echo time (TE) and short interecho spacing (Tinter) have found increased use in lung imaging. In this study, a HASTE sequence was used in combination with a partially parallel acquisition (PPA) strategy to increase the spatial resolution in single‐shot 1H lung imaging. To investigate the benefits of using a combination of single‐shot sequences and PPA, five healthy volunteers were examined. Compared to conventional imaging methods, substantially increased resolution is obtained using the PPA approach. Representative in vivo 1H lung images acquired with a HASTE sequence in combination with the generalized autocalibrating partially parallel acquisition (GRAPPA) method, up to an acceleration factor of three, are presented. Magn Reson Med 49:391–394, 2003.

SEMAC-VAT and MSVAT-SPACE sequence strategies for metal artifact reduction in 1.5T magnetic resonance imaging.

Objectives:To evaluate the ability of four magnetic resonance imaging (MRI) techniques to correct for metallic artifacts. These techniques consisted of 3 2D techniques and one 3D technique. In 2D imaging the techniques View Angle Tilting (VAT), Slice Encoding for Metal Artifact Correction (SEMAC) and a technique that employed a combination of the first two (SEMAC-VAT) were evaluated. In 3D imaging the technique Multiple Slab acquisition with VAT based on a SPACE sequence was evaluated (MSVAT-SPACE). Materials and Methods:Agarose phantoms and tissue phantoms with two commonly used metal implants (stainless steel and titanium) as well as two volunteers with metal implants were imaged at 1.5T. All phantoms and volunteers were imaged using VAT, SEMAC, SEMAC-VAT and MSVAT-SPACE techniques, as well as 2D and 3D conventional imaging techniques. Each technique was optimized for different image contrast mechanisms. Artifact reduction was quantitatively assessed in the agarose phantoms by volumetric measurement. Image quality was qualitatively assessed by blinded reads employing two readers. Each reader independently viewed the tissue phantom images and in vivo human images. Statistical analysis was performed using a Friedman test, Wilcoxon test and weighted Cohens kappa test. Results:T1-weighted, T2-weighted, PD-weighted and STIR image contrasts were successfully implemented with the evaluated artifact reduction sequences in both the phantom experiments and in vivo images. For all evaluated image contrasts and both metal implants, a reduction in the volume of metal artifacts was seen when compared with 2D conventional acquisitions. The 2D metal artifact volumes on average were reduced by 49% ± 16%, 56% ± 15% and 63% ± 15% for VAT, SEMAC and SEMAC-VAT acquisitions respectively. When Friedman and Wilcoxon tests were applied the difference in metal artifact volume was found to be statistically significant when VAT, SEMAC and SEMAC-VAT were compared with the 2D conventional techniques. In 3D imaging on average MSVAT-SPACE reduced metal artifact volume compared with the 3D conventional imaging technique by 72% ± 23% for all evaluated image contrasts and both metal implants. The metal artifact volume differences were statistically significant when MSVAT-SPACE was compared with the 3D conventional technique. The blinded reads demonstrated that SEMAC-VAT and MSVAT-SPACE had distinctly superior quality compared with conventional acquisitions. Quality was measured in terms of artifact size, distortions, image quality and visualization of bone marrow and soft tissues adjacent to metal implants. This was the case for both tissue phantom images and human images with good interobserver agreement. Conclusions:SEMAC-VAT (2D) and MSVAT-SPACE (3D) demonstrated a consistent, marked reduction of metal artifacts for different metal implants and offered flexible image contrasts (T1, T2, PD and STIR) with high image quality. These techniques likely will improve the evaluation of postoperative patients with metal implants.

Renal MR angiography: Current debates and developments in imaging of renal artery stenosis

Because of its safety and robustness with reproducible image quality, three-dimensional gadolinium-enhanced magnetic resonance angiography (3D-Gd-MRA) has been widely established as a diagnostic tool for screening and grading of renal artery stenosis. Accuracy and superiority over other noninvasive imaging procedures was again demonstrated in two recent meta-analyses. However, ambiguous results on the accuracy of this technique have been reported recently, again questioning the sole role of this modality for diagnostic assessment of the renal arteries. The main deficiencies of the technique are limited spatial resolution, high interobserver variability, limited anatomic coverage, as well as inability to assess the stenosis site after stent placement. In addition, a high level of competition has been introduced by techniques such as 16 detector multislice computed tomography, which generates superb image quality, with broad anatomic coverage and high spatial resolution, with minimal technical complexity. Lastly, aggressive search for renal artery stenosis by angiographic techniques in patients with hypertension is of debate, since only a limited percentage of these patients benefit from interventions. In this article, a comprehensive approach to high-resolution 3D-Gd-MRA, using parallel imaging in combination with cardiac-gated, phase-contrast flow measurements, is reviewed. This review is based on various studies and articles that address many of the problems of 3D-Gd-MRA. By making use of maximum spatial resolution and additional functional data, MRI permits accurate detection and grading of renal artery stenosis in most cases, with acceptable interobserver variability.

Metal Artifact Reduction: Standard and Advanced Magnetic Resonance and Computed Tomography Techniques

An increasing number of joint replacements are being performed in the United States. Patients undergoing these procedures can have various complications. Imaging is one of the primary means of diagnosing these complications. Cross-sectional imaging techniques, such as computed tomography (CT) and MR imaging, are more sensitive than radiographs for evaluating complications. The use of CT and MR imaging in patients with metallic implants is limited by the presence of artifacts. This review discusses the causes of metal artifacts on MR imaging and CT, contributing factors, and conventional and novel methods to reduce the effects of these artifacts on scans.

Total Knee Arthroplasty MRI Featuring Slice-Encoding for Metal Artifact Correction: Reduction of Artifacts for STIR and Proton Density–Weighted Sequences

OBJECTIVE The purpose of this article is to compare slice-encoding for metal artifact correction (SEMAC) sequences versus optimized standard MRI sequences in patients with total knee arthroplasty (TKA). SUBJECTS AND METHODS Forty-two patients with TKA underwent 1.5-T MRI. Sequences optimized for metal implant imaging (SEMAC) were compared with standard sequences optimized with high bandwidth for STIR and proton density (PD)-weighted images. In 29 patients, CT was available as reference standard. Signal void and insufficient fat saturation were quantified. Qualitative criteria (anatomy, distortion, blurring, and noise) were assessed on a 5-point scale (1, no artifacts; 5, severe artifacts) by two readers. Abnormal imaging findings were noted. A Student t test and a Wilcoxon signed rank test was used for statistics. RESULTS Signal void areas and insufficient fat saturation were smaller for the SEMAC sequences than for the optimized standard sequences (p ≤ 0.005 for all comparisons). Depiction of anatomic structures was better on STIR with SEMAC versus standard sequences optimized with high bandwidth (score range, 2.9-3.7 vs 4.2-4.9) and on PD-weighted imaging with SEMAC versus standard sequences optimized with high bandwidth (score range, 2.5-3.5 vs 3.1-3.8), which was statistically significant (p < 0.001 to p = 0.007 for different structures). Distortion and noise were lower for SEMAC than for the standard sequences (p ≤ 0.001), whereas no technique had a clear advantage for blurring. Detection of abnormal imaging findings was markedly increased for the SEMAC technique (p < 0.001) and was most pronounced for STIR images (98 and 74 findings for STIR with SEMAC for readers 1 and 2, respectively, vs 37 and 37 findings for readers 1 and 2, respectively, for STIR with standard sequences optimized with high bandwidth). Sensitivity for detection of periprosthetic osteolysis was improved for STIR with SEMAC (100% and 86% for readers 1 and 2, respectively) compared with STIR with standard sequences optimized with high bandwidth (14% and 29% for readers 1 and 2, respectively). CONCLUSION SEMAC sequences showed a statistically significant artifact reduction. The detection of clinically relevant findings such as periprosthetic osteolysis was markedly improved.

STIR sequence with increased receiver bandwidth of the inversion pulse for reduction of metallic artifacts.

OBJECTIVE The purpose of this study was to evaluate a STIR sequence with an optimized inversion pulse that entails use of increased receiver bandwidth for metal artifact reduction. CONCLUSION Image distortion, artifacts, insufficient fat suppression, and detection of relevant findings improved with the STIR optimized inversion pulse, which was associated with significant artifact reduction.

Fast method for 1D non-cartesian parallel imaging using GRAPPA

MRI with non‐Cartesian sampling schemes can offer inherent advantages. Radial acquisitions are known to be very robust, even in the case of vast undersampling. This is also true for 1D non‐Cartesian MRI, in which the center of k‐space is oversampled or at least sampled at the Nyquist rate. There are two main reasons for the more relaxed foldover artifact behavior: First, due to the oversampling of the center, high‐energy foldover artifacts originating from the center of k‐space are avoided. Second, due to the non‐equidistant sampling of k‐space, the corresponding field of view (FOV) is no longer well defined. As a result, foldover artifacts are blurred over a broad range and appear less severe. The more relaxed foldover artifact behavior and the densely sampled central k‐space make trajectories of this type an ideal complement to autocalibrated parallel MRI (pMRI) techniques, such as generalized autocalibrating partially parallel acquisitions (GRAPPA). Although pMRI can benefit from non‐Cartesian trajectories, this combination has not yet entered routine clinical use. One of the main reasons for this is the need for long reconstruction times due to the complex calculations necessary for non‐Cartesian pMRI. In this work it is shown that one can significantly reduce the complexity of the calculations by exploiting a few specific properties of k‐space‐based pMRI. Magn Reson Med 57:1037–1046, 2007.