The recently developed multi-acquisition with variable resonance image combination (MAVRIC) and slice-encoding metal artifact correction (SEMAC) techniques can significantly reduce image artifacts commonly encountered near embedded metal hardware. These artifact reductions are enabled by applying alternative spectral and spatial-encoding schemes to conventional spin-echo imaging techniques. Here, the MAVRIC and SEMAC concepts are connected and discussed. The development of a hybrid technique that utilizes strengths of both methods is then introduced. The presented technique is shown capable of producing minimal artifact, high-resolution images near total joint replacements in a clinical setting. Magn Reson Med 65:71-82, 2011.

No abstract

The desire to apply magnetic resonance imaging (MRI) techniques in the vicinity of embedded metallic hardware is increasing. The soft-tissue contrast available with MR techniques is advantageous in diagnosing complications near an increasing variety of MR-safe metallic hardware. Near such hardware, the spatial encoding mechanisms utilized in conventional MRI methods are often severely compromised. Mitigating these encoding difficulties has been the focus of numerous research investigations over the past two decades. Such approaches include view-angle tilting, short echo-time projection reconstruction acquisitions, single-point imaging, prepolarized MRI, and postprocessing image correction. Various technical advances have also enabled the recent development of two alternative approaches that have shown promising clinical potential. Here, the physical principals and proposed solutions to the problem of MRI near embedded metal are discussed.

Purpose To develop a robust sequence that combines T1ρ and T2 quantifications and to examine the in-vivo repeatability and diurnal variation of T1ρ and T2 quantifications in knee cartilage. Materials and Methods Six healthy volunteers were scanned in the morning and afternoon on two days using a combined T1ρ and T2 quantification sequence developed in this study. Repeatability of T1ρ and T2 quantification was estimated using root-mean-square coefficients-of-variation (RMS-CV). T1ρ and T2 values from morning scans were compared to those from afternoon scans using paired t-tests. Results The overall RMS-CV of in-vivo T1ρ and T2 quantification was 5.3% and 5.2% respectively. The RMS-CV of AM scans was 4.2% and 5.0% while the RMS-CV of PM scans was 6.0% and 6.3% for T1ρ and T2 respectively. No significant difference was found between T1ρ or T2 values in the morning and in the afternoon. Conclusions A sequence that combines T1ρ and T2 quantification with scan time less than 10 minutes and is robust to B0 and B1 inhomogeneity was developed with excellent repeatability. For a cohort with low-level daily activity, although no significant diurnal variation of cartilage MR relaxation times was observed, the afternoon scans had inferior repeatability compared to morning scans.

Purpose: To demonstrate accelerated imaging with both artifact reduction and different contrast mechanisms near metallic implants.Materials and Methods: Slice-encoding for metal artifact correction (SEMAC) is a modified spin echo sequence that uses view-angle tilting and slice-direction phase encoding to correct both in-plane and through-plane artifacts. Standard spin echo trains and short-TI inversion recovery (STIR) allow efficient PD-weighted imaging with optional fat suppression. A completely linear reconstruction allows incorporation of parallel imaging and partial Fourier imaging. The signal-to-noise ratio (SNR) effects of all reconstructions were quantified in one subject. Ten subjects with different metallic implants were scanned using SEMAC protocols, all with scan times below 11 minutes, as well as with standard spin echo methods. Results:The SNR using standard acceleration techniques is unaffected by the linear SEMAC reconstruction. In all cases with implants, accelerated SEMAC significantly reduced artifacts compared with standard imaging techniques, with no additional artifacts from acceleration techniques. The use of different contrast mechanisms allowed differentiation of fluid from other structures in several subjects.Conclusion: SEMAC imaging can be combined with standard echo-train imaging, parallel imaging, partialFourier imaging, and inversion recovery techniques to offer flexible image contrast with a dramatic reduction of metal-induced artifacts in scan times under 11 minutes.

No abstract

Background To compare T2 relaxation time measurements between MR pulse sequences at 3 Tesla in agar phantoms and in vivo patellar, femoral, and tibial articular cartilage. Methods T2 relaxation times were quantified in phantoms and knee articular cartilage of eight healthy individuals using a single echo spin echo (SE) as a reference standard and five other pulse sequences: multi-echo SE (MESE), fast SE (2D-FSE), magnetization-prepared spoiled gradient echo (3D-MAPSS), three-dimensional (3D) 3D-FSE with variable refocusing flip angle schedules (3D vfl-FSE), and quantitative double echo steady state (qDESS). Cartilage was manually segmented and average regional T2 relaxation times were obtained for each sequence. A regression analysis was carried out between each sequence and the reference standard, and root-mean-square error (RMSE) was calculated. Results Phantom measurements from all sequences demonstrated strong fits (R2>0.8; P<0.05). For in vivo cartilage measurements, R2 values, slope, and RMSE were: MESE: 0.25/0.42/5.0 ms, 2D-FSE: 0.64/1.31/9.3 ms, 3D-MAPSS: 0.51/0.66/3.8 ms, 3D vfl-FSE: 0.30/ 0.414.2 ms, qDESS: 0.60/0.90/4.6 ms. Conclusion 2D-FSE, qDESS, and 3D-MAPSS demonstrated the best fits with SE measurements as well as the greatest dynamic ranges. The 3D-MAPSS, 3D vfl-FSE, and qDESS demonstrated the closest average measurements to SE. Discrepancies in T2 relaxation time quantitation between sequences suggest that care should be taken when comparing results between studies.

Spiral scanning is a promising MRI method, but one limitation is that off-resonance effects can cause image blurring. Most current off-resonance correction methods for spiral imaging require an accurate field map, which is difficult to obtain in many applications. Automatic methods can perform off-resonance correction without acquiring a field map. However, these methods are computationally inefficient and relatively prone to estimation error. This study describes a new semiautomatic offresonance correction method that combines an automatic method with a low resolution field map acquisition for offresonance correction in spiral scanning. Experiments demonstrate that this method is more robust than conventional automatic off-resonance correction and can provide more accurate off-resonance correction than conventional field map based methods. The proposed method is also computationally efficient and has been implemented for online reconstruction. Spiral scanning has several desirable properties in MR imaging, such as short scan time, resistance to motion artifacts, short echo time, and excellent gradient efficiency. One major limitation, however, is image blurring due to off-resonance effects. This problem is more pronounced with long readouts and at high field strength. Many off-resonance correction methods (1-14) have been developed for image deblurring in spiral scanning. Linear off-resonance correction (1,13) is a widely used method that requires little computation and is robust in low signalto-noise ratio (SNR) regions. However, this technique assumes a linear field map and, therefore, residual blurring can manifest at locations where the field value deviates from the linear variation. Image domain deconvolution (2) is a rapid pixelwise off-resonance correction method, where the off-resonance phase is approximated as a separable quadratic function and image deblurring is carried out rapidly as one-dimensional deconvolutions. SPHERE (3) is a more effective pixelwise off-resonance correction method where the k-space data is modeled from the blurred image with off-resonance phase rewinding and the final corrected image comes from a standard inverse Fourier reconstruction. Conjugate phase reconstruction (15,16) and its fast alternatives (4 -8) are widely used off-resonance correction methods in non-Cartesian acquisitions and have been shown to be more accurate than SPHERE for spiral imaging (7). Each of these pixelwise off-resonance correction methods requires an accurate field map that indicates the resonance frequency of spins in each image voxel. The image blurring remains or can be exaggerated if the acquired field map is inaccurate.Automatic off-resonance correction methods (11-14) are alternative image deblurring methods that do not require the acquisition of a field map. Noll et al. (11) developed the original automatic correction method based on the principle that a point spread function (PSF) should be real when reconstructed on resonance. Their method is often effective, but spurious minima of the objec...