Improving susceptibility mapping using a threshold‐based K‐space/image domain iterative reconstruction approach

To improve susceptibility quantification, a threshold‐based k‐space/image domain iterative approach that uses geometric information from the susceptibility map itself as a constraint to overcome the ill‐posed nature of the inverse filter is introduced. Simulations were used to study the accuracy of the method and its robustness in the presence of noise. In vivo data were processed and analyzed using this method. Both simulations and in vivo results show that most streaking artifacts inside the susceptibility map caused by the ill‐defined inverse filter were suppressed by the iterative approach. In simulated data, the bias toward lower mean susceptibility values inside vessels has been shown to decrease from around 10% to 2% when choosing an appropriate threshold value for the proposed iterative method. Typically, three iterations are sufficient for this approach to converge and this process takes less than 30 s to process a 512 × 512 × 256 dataset. This iterative method improves quantification of susceptibility inside vessels and reduces streaking artifacts throughout the brain for data collected from a single‐orientation acquisition. This approach has been applied to vessels alone as well as to vessels and other structures with lower susceptibility to generate whole brain susceptibility maps with significantly reduced streaking artifacts. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Bolus‐tracking MRI with a simultaneous T1‐ and T‐measurement

The aim of this study was to propose and evaluate a methodology to analyze simultaneously acquired T‐weighted dynamic susceptibility contrast (DSC) MRI and T₁‐weighted dynamic contrast enhanced (DCE) MRI data. Two generalized models of T‐relaxation are proposed to account for tracer leakage, and a two‐compartment exchange model is used to separate tracer in intra‐ and extravascular spaces. The methods are evaluated using data extracted from ROIs in three mice with subcutaneously implanted human colorectal tumors. Comparing plasma flow values obtained from DCE‐MRI and DSC‐MRI data defines a practical experimental paradigm to measure T‐relaxivities, and reveals a factor of 15 between values in tissue and blood. Comparing mean transit time values obtained from DCE‐MRI and DSC‐MRI without leakage correction, indicates a significant reduction of susceptibility weighting in DSC‐MRI during tracer leakage. A one‐parameter gradient correction model provides a good approximation for this susceptibility loss, but redundancy of the parameter limits the practical potential of this model for DSC‐MRI. Susceptibility loss is modeled more accurately with a variable T‐relaxivity, which allows to extract new parameters that cannot be derived from DSC‐MRI or DCE‐MRI alone. They reflect the cellular and vessel geometry, and thus may lead to a more complete characterization of tissue structure. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Long‐term clinical and radiological assessment of untreated severe cartilage damage in the knee: a natural history study

We aimed to evaluate whether and to what extent an isolated deep cartilage lesion localized within the tibiofemoral and the patellofemoral joint has an impact on the clinical outcomes and osteoarthritis (OA) progression when it is left untreated. From 1991 to 1994, 4121 consecutive knee arthroscopies were performed, and 37 of them in patients with a single isolated chondral lesion of Outerbridge grade 4 located within weight‐bearing areas of the femoral and tibial condyles (FT group) and patella (P group). The lesion size ranged from 2 to 4 cm². Outcomes were reported at a mean 15.3‐year follow‐up using the Lysholm score, the Tegner activity scale and the Womac score. The mean Lysholm, Tegner and Womac score in the FT group was 87.7, 5.6 and 88.7, respectively. In the P group, it was 83.8, 4.8 and 84.6, respectively (P<0.0.5). Osteoarthritic changes were found in 39% of the patients. There was no difference in OA severity between an injured and an uninjured knee. In patients of the FT group, there was a relationship between the incidence of tibiofemoral OA and patellofemoral OA (P=0.00075). Severe isolated single chondral damage left with no treatment has a limited influence on clinical outcomes and the development of OA.     

Impact of motion on T1 mapping acquired with inversion recovery fast spin echo and rapid spoiled gradient recalled‐echo pulse sequences for delayed gadolinium‐enhanced MRI of cartilage (dGEMRIC) in volunteers

Purpose:

To evaluate the impact of motion on T1 values acquired by using either inversion‐recovery fast spin echo (IR‐FSE) or three‐dimensional (3D) spoiled gradient recalled‐echo (SPGR) sequences for delayed gadolinium‐enhanced magnetic resonance imaging of cartilage (dGEMRIC) in volunteers.

Materials and Methods:

Single‐slice IR‐FSE and 3D SPGR sequences were applied to perform dGEMRIC in five healthy volunteers. A mutual information‐based approach was used to correct for image misregistration. Displacements were expressed as averaged Euclidean distances and angles. Averages of differences in goodness of fit (Δχ²) tests and averages of relative differences in T1 values (ΔT1) before and after motion correction were computed.

Results:

Maximum Euclidean distance was 3.5 mm and 1.2 mm for IR‐FSE and SPGR respectively. Mean ± SD of Δχ² were 10.18 ± 8.4 for IR‐FSE and ?1.37 ± 5.5 for SPGR. Mean ± SD of ΔT1 were 0.008 ± 0.0048 for IR‐FSE and ?0.002 ± 0.019 for FSPGR. Pairwise comparison of Δχ² values showed a significant difference for IR‐FSE, but not for 3D‐SPGR. Significantly greater variability in T1 values was also noted for IR‐FSE than for 3D‐SPGR.

Conclusion:

Involuntary motion has a significant influence on T1 values acquired with IR‐FSE, but not with 3D‐SPGR in healthy volunteers. J. Magn. Reson. Imaging 2010;32:394–398. © 2010 Wiley‐Liss, Inc.