Feasibility of 4D T2* quantification in the lung with oxygen gas challenge in patients with non-small cell lung cancer.

Blood oxygen level-dependent (BOLD) MRI is a non-invasive diagnostic method for assessing tissue oxygenation level, by changes in the transverse relaxation time T2*. 3D BOLD imaging of lung tumours is challenging, because respiratory motion can lead to significant image quality degradation. The purpose of this work was to explore the feasibility of a three dimensional (3D) Cartesian multi gradient echo (MGRE) sequence for T2* measurements of non-small cell lung tumours during free-breathing. A non-uniform quasi-random reordering of the pahse encoding lines that allocates more sampling points near the k-space origin resulting in efficient undersampling pattern for parallel imaging was combined with multi echo acquisition and self-gating. In a series of three patients 3D T2* maps of lung carcinomas were generated with isotropic spatial resolution and full tumour coverage at air inhalation and after hyperoxic gas challenge in arbitrary respiratory phases using the proposed self-gated MGRE acquisition. The changes in T2* on the inhalation of hyperoxic gas relative to air were quantified. Significant changes in T2* were observed following oxygen inhalation in the tumour (p < 0.02). Thus, the self-gated MGRE sequence can be used for assessment of BOLD signal with isotropic resolution and arbitrary respiratory phases in non-small cell lung cancer.

Full utilization of conjugate symmetry: combining virtual conjugate coil reconstruction with partial Fourier imaging for g-factor reduction in accelerated MRI.

PURPOSE: In this study we propose a method to combine the parallel virtual conjugate coil (VCC) reconstruction with partial Fourier (PF) acquisition to improve reconstruction conditioning and reduce noise amplification in accelerated MRI where PF is used. METHODS: Accelerated measurements are reconstructed in k-space by GRAPPA, with a VCC reconstruction kernel trained and applied in the central, symmetrically sampled part of k-space, while standard reconstruction is performed on the asymmetrically sampled periphery. The two reconstructed regions are merged to form a full reconstructed dataset, followed by PF reconstruction. The method is tested in vivo using T1-weighted spin-echo and T2*-weighted gradient-echo echo planar imaging (EPI) sequences, using both in-plane and simultaneous multislice (SMS) acceleration, at 1.5T and 3T field strengths. Noise amplification is estimated with theoretical calculations and pseudo-multiple-replica computations, for different PF factors, using zero-filling, homodyne, and projection onto convex sets (POCS) PF reconstruction. RESULTS: Depending on the PF algorithm and the inherent benefit of VCC reconstruction without PF, approximately 35% to 80%, 15% to 60%, and 5% to 30% of that intrinsic SNR gain can be retained for PF factors 7/8, 6/8, and 5/8, respectively, by including the VCC signals in the reconstruction. Compared with VCC-reconstructed acquisitions of higher acceleration, without PF, but having the same net acceleration, the combined method can provide a higher SNR if the inherent benefit of VCC is low or moderate. CONCLUSION: The proposed technique enables the partial application of VCC reconstruction to measurements with PF using either in-plane or SMS acceleration, and therefore can reduce the noise amplification of such acquisitions.

Simultaneous T1 and T2 measurements using inversion recovery TrueFISP with principle component-based reconstruction, off-resonance correction, and multicomponent analysis.

PURPOSE: To improve the reconstruction quality for quantitative T1 and T2 measurements using the inversion recovery (IR) TrueFISP sequence and to demonstrate the potential for multicomponent analysis. METHODS: The iterative reconstruction method takes advantage of the high redundancy in the smooth exponential signals using principle component analysis (PCA). Multicomponent information is preserved and allows voxel-by-voxel computation of relaxation time spectra with an inverse Laplace transform. Off-resonance effects are analytically and numerically investigated and a correction approach is presented. RESULTS: Single-shot IR TrueFISP in vivo measurements on healthy volunteers demonstrate the improved reconstruction performance compared to a view sharing (k-space weighted image contrast [KWIC]) reconstruction. Especially, tissue components with short apparent relaxation times T1 * are not filtered out and can be identified in the relaxation time spectra. These components include myelin in the human brain (T1 * ≈ 130 ms) and extra cranial subcutaneous fat. CONCLUSION: The PCA-based reconstruction method improves the temporal accuracy and preserves multicomponent information. Spatially resolved relaxation time spectra can be obtained and allow the identification of tissue types with short, apparent relaxation times.

Stable and efficient retrospective 4D-MRI using non-uniformly distributed quasi-random numbers.

The purpose of this work is the development of a robust and reliable three-dimensional (3D) Cartesian imaging technique for fast and flexible retrospective 4D abdominal MRI during free breathing. To this end, a non-uniform quasi random (NU-QR) reordering of the phase encoding (k y -k z ) lines was incorporated into 3D Cartesian acquisition. The proposed sampling scheme allocates more phase encoding points near the k-space origin while reducing the sampling density in the outer part of the k-space. Respiratory self-gating in combination with SPIRiT-reconstruction is used for the reconstruction of abdominal data sets in different respiratory phases (4D-MRI). Six volunteers and three patients were examined at 1.5 T during free breathing. Additionally, data sets with conventional two-dimensional (2D) linear and 2D quasi random phase encoding order were acquired for the volunteers for comparison. A quantitative evaluation of image quality versus scan times (from 70 s to 626 s) for the given sampling schemes was obtained by calculating the normalized mutual information (NMI) for all volunteers. Motion estimation was accomplished by calculating the maximum derivative of a signal intensity profile of a transition (e.g. tumor or diaphragm). The 2D non-uniform quasi-random distribution of phase encoding lines in Cartesian 3D MRI yields more efficient undersampling patterns for parallel imaging compared to conventional uniform quasi-random and linear sampling. Median NMI values of NU-QR sampling are the highest for all scan times. Therefore, within the same scan time 4D imaging could be performed with improved image quality. The proposed method allows for the reconstruction of motion artifact reduced 4D data sets with isotropic spatial resolution of 2.1 × 2.1 × 2.1 mm3 in a short scan time, e.g. 10 respiratory phases in only 3 min. Cranio-caudal tumor displacements between 23 and 46 mm could be observed. NU-QR sampling enables for stable 4D-MRI with high temporal and spatial resolution within short scan time for visualization of organ or tumor motion during free breathing. Further studies, e.g. the application of the method for radiotherapy planning are needed to investigate the clinical applicability and diagnostic value of the approach.

Controlling the object phase for g-factor reduction in phase-Constrained parallel MRI using spatially selective RF pulses.

PURPOSE: Parallel imaging generally entails a reduction in the signal-to-noise ratio of the final image. Phase-constrained methods aim to improve reconstruction quality by using symmetry properties of k-space. Noise amplification in phase-constrained reconstruction depends heavily on the object background phase. The purpose of this work is to present a new approach of using tailored radiofrequency pulses to optimize the object phase distribution in order to maximize the benefit of phase-constrained reconstruction, and to minimize the noise amplification. METHODS: Intrinsic object phase and coil sensitivity profiles are measured in a prescan. Optimal phase distribution is computed to maximize signal-to-noise ratio in the given setup. Tailored radiofrequency pulses are designed to introduce the optimal phase map in the following accelerated acquisitions, subsequently reconstructed by phase-constrained methods. The potential of the method is demonstrated in vivo with in-plane accelerated (8x) and simultaneous multislice (3x) acquisitions. RESULTS: Mean g-factors are reduced by up to a factor of 2 compared with conventional techniques when an appropriate phase-constrained reconstruction is applied to phase-optimized acquisitions, enhancing the signal-to-noise ratio of the final images and the visibility of small details. CONCLUSIONS: Combining phase-constrained reconstruction and phase optimization by tailored radiofrequency pulses can provide notable improvement in the signal-to-noise ratio and reconstruction quality of accelerated MRI. Magn Reson Med 79:2113-2125, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Desynchronization of Cartesian k-space sampling and periodic motion for improved retrospectively self-gated 3D lung MRI using quasi-random numbers.

PURPOSE: To demonstrate that desynchronization between Cartesian k-space sampling and periodic motion in free-breathing lung MRI improves the robustness and efficiency of retrospective respiratory self-gating. METHODS: Desynchronization was accomplished by reordering the phase (ky ) and partition (kz ) encoding of a three-dimensional FLASH sequence according to two-dimensional, quasi-random (QR) numbers. For retrospective respiratory self-gating, the k-space center signal (DC signal) was acquired separately after each encoded k-space line. QR sampling results in a uniform distribution of k-space lines after gating. Missing lines resulting from the gating process were reconstructed using iterative GRAPPA. Volunteer measurements were performed to compare quasi-random with conventional sampling. Patient measurements were performed to demonstrate the feasibility of QR sampling in a clinical setting. RESULTS: The uniformly sampled k-space after retrospective gating allows for a more stable iterative GRAPPA reconstruction and improved ghost artifact reduction compared with conventional sampling. It is shown that this stability can either be used to reduce the total scan time or to reconstruct artifact-free data sets in different respiratory phases, both resulting in an improved efficiency of retrospective respiratory self-gating. CONCLUSION: QR sampling leads to desynchronization between repeated data acquisition and periodic respiratory motion. This results in an improved motion artifact reduction in shorter scan time. Magn Reson Med 77:787-793, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

In vivo imaging of the spectral line broadening of the human lung in a single breathhold.

PURPOSE: To present a technique, which allows for the in vivo quantification of the spectral line broadening of the human lung in a single breathhold. The line broadening is an interesting parameter of the lung because it can provide information about important lung properties, namely: inflation and oxygen uptake. The proposed technique integrates the asymmetric spin-echo (ASE) approach, which is commonly used to quantify the line broadening, with a single shot turbo spin-echo pulse sequence with half-Fourier acquisition (HASTE), to reduce the acquisition times. MATERIALS AND METHODS: Imaging experiments were performed at 1.5 Tesla on 14 healthy volunteers, using a ASE-prepared HASTE sequence. The line broadening was quantified using a two-points method. Data were acquired at different breathing states: functional residual capacity (FRC) and total lung capacity (TLC), and with different breathing gases: room-air and pure-oxygen. Image acquisition was accomplished within a single breathhold of approximately 15 s duration. The violation of the Carr-Purcell-Meiboom-Gill conditions, deriving from inhomogeneities of the static magnetic field, was overcome by means of radiofrequency-phase cycling and generalized autocalibrating partially parallel acquisitions (GRAPPA) reconstruction. RESULTS: Significant increase of the line broadening was observed with both lung inflation and oxygen concentration (P < 0.0001). Values of the line broadening obtained within the lung parenchyma at different breathing states (1.48 ± 0.29 ppm at FRC and 1.95 ± 0.43 ppm at TLC) are in agreement with previous reports and show excellent reproducibility, with a coefficient of variation <0.03. The mean relative difference observed with oxygen-enhancement was approximately 14%. CONCLUSION: The presented technique offers a robust way to quantify the spectral line broadening of the human lung in vivo. Image acquisition can be accomplished in a single breathhold, which could be suitable for clinical applications on patients with lung diseases. J. Magn. Reson. Imaging 2016;44:745-757.

Comparison of phase-constrained parallel MRI approaches: Analogies and differences.

PURPOSE: Phase-constrained parallel MRI approaches have the potential for significantly improving the image quality of accelerated MRI scans. The purpose of this study was to investigate the properties of two different phase-constrained parallel MRI formulations, namely the standard phase-constrained approach and the virtual conjugate coil (VCC) concept utilizing conjugate k-space symmetry. METHODS: Both formulations were combined with image-domain algorithms (SENSE) and a mathematical analysis was performed. Furthermore, the VCC concept was combined with k-space algorithms (GRAPPA and ESPIRiT) for image reconstruction. In vivo experiments were conducted to illustrate analogies and differences between the individual methods. Furthermore, a simple method of improving the signal-to-noise ratio by modifying the sampling scheme was implemented. RESULTS: For SENSE, the VCC concept was mathematically equivalent to the standard phase-constrained formulation and therefore yielded identical results. In conjunction with k-space algorithms, the VCC concept provided more robust results when only a limited amount of calibration data were available. Additionally, VCC-GRAPPA reconstructed images provided spatial phase information with full resolution. CONCLUSIONS: Although both phase-constrained parallel MRI formulations are very similar conceptually, there exist important differences between image-domain and k-space domain reconstructions regarding the calibration robustness and the availability of high-resolution phase information.

Fast isotropic banding-free bSSFP imaging using 3D dynamically phase-cycled radial bSSFP (3D DYPR-SSFP).

AIMS: Dynamically phase-cycled radial balanced steady-state free precession (DYPR-SSFP) is a method for efficient banding artifact removal in bSSFP imaging. Based on a varying radiofrequency (RF) phase-increment in combination with a radial trajectory, DYPR-SSFP allows obtaining a banding-free image out of a single acquired k-space. The purpose of this work is to present an extension of this technique, enabling fast three-dimensional isotropic banding-free bSSFP imaging. METHODS: While banding artifact removal with DYPR-SSFP relies on the applied dynamic phase-cycle, this aspect can lead to artifacts, at least when the number of acquired projections lies below a certain limit. However, by using a 3D radial trajectory with quasi-random view ordering for image acquisition, this problem is intrinsically solved, enabling 3D DYPR-SSFP imaging at or even below the Nyquist criterion. The approach is validated for brain and knee imaging at 3 Tesla. RESULTS: Volumetric, banding-free images were obtained in clinically acceptable scan times with an isotropic resolution up to 0.56mm. CONCLUSION: The combination of DYPR-SSFP with a 3D radial trajectory allows banding-free isotropic volumetric bSSFP imaging with no expense of scan time. Therefore, this is a promising candidate for clinical applications such as imaging of cranial nerves or articular cartilage.

Acoustic-noise-optimized diffusion-weighted imaging.

OBJECTIVE: This work was aimed at reducing acoustic noise in diffusion-weighted MR imaging (DWI) that might reach acoustic noise levels of over 100 dB(A) in clinical practice. MATERIALS AND METHODS: A diffusion-weighted readout-segmented echo-planar imaging (EPI) sequence was optimized for acoustic noise by utilizing small readout segment widths to obtain low gradient slew rates and amplitudes instead of faster k-space coverage. In addition, all other gradients were optimized for low slew rates. Volunteer and patient imaging experiments were conducted to demonstrate the feasibility of the method. Acoustic noise measurements were performed and analyzed for four different DWI measurement protocols at 1.5T and 3T. RESULTS: An acoustic noise reduction of up to 20 dB(A) was achieved, which corresponds to a fourfold reduction in acoustic perception. The image quality was preserved at the level of a standard single-shot (ss)-EPI sequence, with a 27-54% increase in scan time. CONCLUSIONS: The diffusion-weighted imaging technique proposed in this study allowed a substantial reduction in the level of acoustic noise compared to standard single-shot diffusion-weighted EPI. This is expected to afford considerably more patient comfort, but a larger study would be necessary to fully characterize the subjective changes in patient experience.

Echo time dependence of observed T1 in the human lung.

BACKGROUND: This work is intended to demonstrate that T1 measured in the lungs depends on the echo time (TE) used. Measuring lung T1 can be used to gain quantitative morphological and functional information. It is also shown that this dependence is particularly visible when using an ultra-short TE (UTE) sequence with TE well below 1 ms for T1 quantification in lung tissue, rather than techniques with TE on the order of 1-2 ms. METHODS: The lungs of 12 healthy volunteers (aged 22 to 33 years) were examined at 1.5 Tesla. A segmented inversion recovery Look-Locker multi-echo sequence based on two-dimensional UTE was used for independent T1 quantification at five TEs between TE1 = 70 µs and TE5 = 2.3 ms. RESULTS: The measured T1 was found to increase gradually with TE from 1060 ± 40 ms at TE1 to 1389 ± 53 ms at TE5 (P < 0.001). CONCLUSION: Measuring T1 at ultra-short echo times reveals a significant dependence of observed T1 on the echo time. Thus, any comparison of T1 values should also consider the TEs used. However, this dependence on TE could also be exploited to gain additional diagnostic information on the tissue compartments in the lung.

Dynamically phase-cycled radial balanced SSFP imaging for efficient banding removal.

PURPOSE: Balanced steady-state free precession (bSSFP) imaging suffers from banding artifacts due to its inherent sensitivity to inhomogeneities in the main magnetic field. These artifacts can be removed by the acquisition of multiple images at different frequency offsets. However, conventional phase-cycling is hindered by a long scan time. The purpose of this work is to present a novel approach for efficient banding removal in bSSFP imaging. THEORY AND METHODS: To this end, the phase-cycle during a single-shot radial acquisition of an image was dynamically changed. Thus, each projection is acquired with a different frequency offset. Using conventional radial gridding, an artifact-free image can be reconstructed out of this dataset. RESULTS: The approach is validated at clinical field strength [3.0 Tesla (T)] as well as at ultrahigh field (9.4T). Robust elimination of banding artifacts was obtained for different imaging regions, including brain imaging at ultrahigh field with an in-plane resolution of 0.25 × 0.25 mm(2). Besides banding artifact-free imaging, the applicability of the proposed technique for fat-water separation is demonstrated. CONCLUSION: Dynamically phase-cycled radial bSSFP has the potential for banding-free bSSFP imaging in a short scan time, in the presence of severe field inhomogeneities and at high resolution.

Oxygen enhanced lung MRI by simultaneous measurement of T1 and T2 * during free breathing using ultrashort TE.

PURPOSE: To provide a robust method for the simultaneous quantification of T1 and T2 * in the human lung during free breathing. Breathing pure oxygen accelerates T1 and T2 * relaxation in the lung. While T1 shortening reflects an increased amount of dissolved molecular oxygen in lung tissue, T2 * shortening shows an increased concentration of oxygen in the alveolar gas. Therefore, both parameters reflect different aspects of the oxygen uptake and provide complementary lung functional information. MATERIALS AND METHODS: A segmented inversion recovery Look-Locker multiecho sequence based on a multiecho 2D ultrashort TE (UTE) was employed for simultaneous T1 and T2 * quantification. The radial projections follow a modified golden angle ordering, allowing for respiratory self-gating and thus the reconstruction of a series of differently T1 and T2 *-weighted images in arbitrary breathing states. The method was evaluated in nine healthy volunteers while breathing room air and pure oxygen, with two volunteers examined at five oxygen concentrations. RESULTS: Relative differences of ΔT1 between 7.9% and 12.7% and of ΔT2 * between 13.2% and 6.0% were found. CONCLUSION: The proposed method provides inherently coregistered, quantitative T1 and T2 * maps in both expiration and inspiration from a single measurement acquired during free breathing and is thus well suited for clinical application.

Generating multiple contrasts using single-shot radial T1 sensitive and insensitive steady-state imaging.

PURPOSE: Recently, the (Resolution Enhanced-) T1 insensitive steady-state imaging (TOSSI) approach has been proposed for the fast acquisition of T2 -weighted images. This has been achieved by balanced steady-state free precession (bSSFP) imaging between unequally spaced inversion pulses. The purpose of this work is to present an extension of this technique, considerably increasing both the efficiency and possibilities of TOSSI. THEORY AND METHODS: A radial trajectory in combination with an appropriate view-sharing reconstruction is used. Because each projection traverses the contrast defining k-space center, several different contrasts can be extracted from a single-shot measurement. These contrasts include various T2 -weightings and T2 /T1 -weighting if an even number of inversion pulses is used, while an odd number allow the generation of several images with predefined tissue types cancelled. RESULTS: The approach is validated for brain and abdominal imaging at 3.0 Tesla. Results are compared with RE-TOSSI, bSSFP, and turbo spin-echo images and are shown to provide similar contrasts in a fraction of scan time. Furthermore, the potential utility of the approach is illustrated by images obtained from a brain tumor patient. CONCLUSION: Radial T1 sensitive and insensitive steady-state imaging is able to generate multiple contrasts out of one single-shot measurement in a short scan time.

Blood volume fraction imaging of the human lung using intravoxel incoherent motion.

PURPOSE: To present a technique for non-contrast-enhanced in vivo imaging of the blood volume fraction of the human lung. The technique is based on the intravoxel incoherent motion (IVIM) approach. However, a substantial novelty is introduced here: the need for external diffusion sensitizing gradients is eliminated by exploiting the internal magnetic field gradients typical of the lung tissue, due to magnetic susceptibility differences at air/tissue interfaces. MATERIALS AND METHODS: A single shot turbo spin-echo sequence with stimulated-echo preparation and electrocardiograph synchronization was used for acquisition. Two images were acquired in a single breath-hold of 10 seconds duration: one reference image and one blood-suppressed image. The blood volume fraction was quantified using a two-compartment signal decay model, as given by the IVIM theory. Experiments were performed at 1.5T in eight healthy volunteers. RESULTS: Values of the blood volume fraction obtained within the lung parenchyma (36 ± 16%) are in good agreement with previous reports, obtained using contrast-enhanced magnetic resonance angiography (33%), and show relatively good reproducibility. CONCLUSION: The presented technique offers a robust way to quantify the blood volume fraction of the human lung parenchyma without using contrast agents. Image acquisition can be accomplished in a single breath-hold and could be suitable for clinical applications on patients with lung diseases. J. Magn. Reson. Imaging 2015;41:1454-1464. © 2014 Wiley Periodicals, Inc.

Multiparametric oxygen-enhanced functional lung imaging in 3D.

OBJECTIVE: To develop a self-gated free-breathing 3D sequence allowing for simultaneous T1-weighted imaging and quantitative T2* mapping in different breathing phases in order to assess the feasibility of oxygen-enhanced 3D functional lung imaging. MATERIALS AND METHODS: A 3D sequence with ultrashort echo times and interleaved double readouts was implemented for oxygen-enhanced lung imaging at 1.5 T. Six healthy volunteers were examined while breathing room air as well as 100 % oxygen. Images from expiratory and inspiratory breathing phases were reconstructed and compared for the two breathing gases. RESULTS: The average T2* value measured for room air was 2.10 ms, with a 95 % confidence interval (CI) of 1.95-2.25 ms, and the average for pure oxygen was 1.89 ms, with a 95 % CI of 1.76-2.01 ms, resulting in a difference of 10.1 % (95 % CI 8.9-11.3 %). An 11.2 % increase in signal intensity (95 % CI 10.4-12.1 %) in the T 1-weighted images was detected when subjects were breathing pure oxygen compared to room air. Furthermore, a significant change in signal intensity (26.5 %, 95 % CI 18.8-34.3 %) from expiration to inspiration was observed. CONCLUSIONS: This study demonstrated the feasibility of simultaneous T2* mapping and T1-weighted 3D imaging of the lung. This method has the potential to provide information about ventilation, oxygen transfer, and lung expansion within one experiment. Future studies are needed to investigate the clinical applicability and diagnostic value of this approach in various pulmonary diseases.

Simple recipe for accurate T(2) quantification with multi spin-echo acquisitions.

OBJECTIVE: The quantification of magnetic resonance relaxation parameters T 1 and T 2 have the potential for improved disease detection and classification over standard clinical weighted imaging. Performing a mono-exponential fit on multi spin-echo (MSE) data provides quantitative T 2 values in a clinically acceptable scan-time. However, due to technical imperfections of refocusing pulses, stimulated echo contributions to the signals lead to significant deviations in the resulting T 2 values. In this work, a simple auto-calibrating correction procedure is presented, allowing the accurate estimation of T 2 from MSE acquisitions. MATERIALS AND METHODS: Correction factors for T 2 values obtained from MSE acquisitions with a mono-exponential fit are derived from simulations following the extended phase graph formulation. A closed formula is given for the calculation of the required correction factors directly from the measured data itself. RESULTS: Simulations and phantom experiments show high accuracy of corrected T 2 values for a wide range of clinically relevant T 2 values and for different nominal refocusing flip angles. In addition, corrected T 2 maps of the human brain are presented. CONCLUSION: A simple recipe is provided to correct T 2 values obtained from MSE acquisitions via a mono-exponential fit for the influence of stimulated echoes. Since all required parameters are extracted from the data themselves, no additional acquisitions are required.

Combined acquisition technique (CAT) for neuroimaging of multiple sclerosis at low specific absorption rates (SAR).

PURPOSE: To compare a novel combined acquisition technique (CAT) of turbo-spin-echo (TSE) and echo-planar-imaging (EPI) with conventional TSE. CAT reduces the electromagnetic energy load transmitted for spin excitation. This radiofrequency (RF) burden is limited by the specific absorption rate (SAR) for patient safety. SAR limits restrict high-field MRI applications, in particular. MATERIAL AND METHODS: The study was approved by the local Medical Ethics Committee. Written informed consent was obtained from all participants. T2- and PD-weighted brain images of n = 40 Multiple Sclerosis (MS) patients were acquired by CAT and TSE at 3 Tesla. Lesions were recorded by two blinded, board-certificated neuroradiologists. Diagnostic equivalence of CAT and TSE to detect MS lesions was evaluated along with their SAR, sound pressure level (SPL) and sensations of acoustic noise, heating, vibration and peripheral nerve stimulation. RESULTS: Every MS lesion revealed on TSE was detected by CAT according to both raters (Cohen's kappa of within-rater/across-CAT/TSE lesion detection κCAT = 1.00, at an inter-rater lesion detection agreement of κLES = 0.82). CAT reduced the SAR burden significantly compared to TSE (p<0.001). Mean SAR differences between TSE and CAT were 29.0 (± 5.7) % for the T2-contrast and 32.7 (± 21.9) % for the PD-contrast (expressed as percentages of the effective SAR limit of 3.2 W/kg for head examinations). Average SPL of CAT was no louder than during TSE. Sensations of CAT- vs. TSE-induced heating, noise and scanning vibrations did not differ. CONCLUSION: T2-/PD-CAT is diagnostically equivalent to TSE for MS lesion detection yet substantially reduces the RF exposure. Such SAR reduction facilitates high-field MRI applications at 3 Tesla or above and corresponding protocol standardizations but CAT can also be used to scan faster, at higher resolution or with more slices. According to our data, CAT is no more uncomfortable than TSE scanning.

Reducing contrast contamination in radial turbo-spin-echo acquisitions by combining a narrow-band KWIC filter with parallel imaging.

PURPOSE: Cartesian turbo spin-echo (TSE) and radial TSE images are usually reconstructed by assembling data containing different contrast information into a single k-space. This approach results in mixed contrast contributions in the images, which may reduce their diagnostic value. The goal of this work is to improve the image contrast from radial TSE acquisitions by reducing the contribution of signals with undesired contrast information. METHODS: Radial TSE acquisitions allow the reconstruction of multiple images with different T2 contrasts using the k-space weighted image contrast (KWIC) filter. In this work, the image contrast is improved by reducing the band-width of the KWIC filter. Data for the reconstruction of a single image are selected from within a small temporal range around the desired echo time. The resulting dataset is undersampled and, therefore, an iterative parallel imaging algorithm is applied to remove aliasing artifacts. RESULTS: Radial TSE images of the human brain reconstructed with the proposed method show an improved contrast when compared with Cartesian TSE images or radial TSE images with conventional KWIC reconstructions. CONCLUSION: The proposed method provides multi-contrast images from radial TSE data with contrasts similar to multi spin-echo images. Contaminations from unwanted contrast weightings are strongly reduced.

Regularization method for phase-constrained parallel MRI.

PURPOSE: To implement a regularization method for the phase-constrained generalized partially parallel acquisitions (GRAPPA) algorithm to reduce image artifacts caused by data inconsistencies. METHODS: Phase-constrained GRAPPA reconstructions are implemented through the use of virtual coils. To that end, synthetic virtual coils are generated by using complex conjugate symmetric signals from the actual coils. Regularization is achieved by applying coefficient-based penalty factors during the GRAPPA calibration procedure. Different penalizing factors are applied for the actual and virtual coils. The method is tested in vivo using T2-weighted turbo spin echo (TSE) images. RESULTS: T2 signal decay perturbs conjugate k-space symmetry and produces artifacts in phase-constrained parallel MRI reconstructions of T2-weighted TSE images. Using the proposed regularization method, artifacts are suppressed at the cost of noise amplification. However, there is still a significant SNR gain compared with conventional GRAPPA. CONCLUSION: The proposed regularization method is an efficient approach for artifact suppression and maintains the SNR benefit of phase-constrained parallel MRI over conventional parallel MRI.