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Low‐field strength scanners present an opportunity for more inclusive imaging exams and bring several challenges including lower signal‐to‐noise ratio (SNR) and longer scan times. Magnetic resonance fingerprinting (MRF) is a rapid quantitative multiparametric method that can enable multiple quantitative maps simultaneously. To demonstrate the feasibility of an MRF sequence for knee cartilage evaluation in a 0.55T system we performed repeatability and accuracy experiments with agar‐gel phantoms. Additionally, five healthy volunteers (age 32 ± 4 years old, 2 females) were scanned at 3T and 0.55T. The MRI acquisition protocols include a stack‐of‐stars T1ρ‐enabled MRF sequence, a VIBE sequence with variable flip angles (VFA) for T1 mapping, and fat‐suppressed turbo flash (TFL) sequences for T2 and T1ρ mappings. Double‐Echo steady‐state (DESS) sequence was also used for cartilage segmentation. Acquisitions were performed at two different field strengths, 0.55T and 3T, with the same sequences but protocols were slightly different to accommodate differences in signal‐to‐noise ratio and relaxation times. Cartilage segmentation was done using five compartments. T1, T2, and T1ρ values were measured in the knee cartilage using both MRF and conventional relaxometry sequences. The MRF sequence demonstrated excellent repeatability in a test–retest experiment with model agar‐gel phantoms, as demonstrated with correlation and Bland–Altman plots. Underestimation of T1 values was observed on both field strengths, with the average global difference between reference values and the MRF being 151 ms at 0.55T and 337 ms at 3T. At 0.55T, MRF measurements presented significant biases but strong correlations with the reference measurements. Although a larger error was present in T1 measurements, MRF measurements trended similarly to the conventional measurements for human subjects and model agar‐gel phantoms. [ABSTRACT FROM AUTHOR]

Purpose: To compare the performance of a learned magnetization‐prepared gradient echo (L‐MPGRE) sequence against a commonly used sequence for 3D T2 and T1ρ mapping of the knee joint, the magnetization‐prepared angle‐modulated partitioned k‐space spoiled gradient echo snapshots (MAPSS), on bi‐exponential (BE), stretched‐exponential (SE), and mono‐exponential (ME) relaxation models. Methods: We used a combined differentiable and non‐differentiable optimization to learn pulse sequence structure and its parameters for 3D T2 and T1ρ mapping of the knee joint using ME, SE, and BE models. The learned pulse sequence framework was used to improve quantitative accuracy and SNR and to reduce filtering effects. We compare the measured multi‐compartment values between the two sequences (n = 8), and their repeatability (n = 4) in healthy volunteers (n = 12 total). Results: The voxel‐wise median absolute percentage difference (MAPD) between the T2 and T1ρ maps obtained with each sequence was 18.6% and 19.9%, respectively. The T2 and T1ρ repeatability tests showed a MAPD of 18.5% and 19.1% for MAPSS, and 16.8% and 15.5% for L‐MPGRE. Bland–Altman region of interest (ROI)‐wise analysis shows that bias is small, close to −1.5%, and the coefficient of variation is around 5.5% when comparing ROIs from both sequences. Conclusion: The L‐MPGRE sequences can be used as a replacement for MAPSS for T2 and T1ρ mapping in the knee cartilage with advantages, achieving similar accuracy and 15% better repeatability in only half of its scan time. [ABSTRACT FROM AUTHOR]

Background: Three‐dimensional MR fingerprinting (3D‐MRF) techniques have been recently described for simultaneous multiparametric mapping of knee cartilage. However, investigation of repeatability remains limited. Purpose: To assess the intra‐day and inter‐day repeatabilities of knee cartilage T1, T2, and T1ρ maps using a 3D‐MRF sequence for simultaneous measurement. Study Type: Prospective. Subjects: Fourteen healthy subjects (35.4 ± 9.3 years, eight males), scanned on Day 1 and Day 7. Field Strength/Sequence: 3 T/3D‐MRF, T1, T2, and T1ρ maps. Assessment: The acquisition of 3D‐MRF cartilage (simultaneous acquisition of T1, T2, and T1ρ maps) were acquired using a dictionary pattern‐matching approach. Conventional cartilage T1, T2, and T1ρ maps were acquired using variable flip angles and a modified 3D‐Turbo‐Flash sequence with different echo and spin‐lock times, respectively, and were fitted using mono‐exponential models. Each sequence was acquired on Day 1 and Day 7 with two scans on each day. Statistical Tests: The mean and SD for cartilage T1, T2, and T1ρ were calculated in five manually segmented regions of interest (ROIs), including lateral femur, lateral tibia, medial femur, medial tibia, and patella cartilages. Intra‐subject and inter‐subject repeatabilities were assessed using coefficient of variation (CV) and intra‐class correlation coefficient (ICC), respectively, on the same day and among different days. Regression and Bland–Altman analysis were performed to compare maps between the conventional and 3D‐MRF sequences. Results: The CV in all ROIs was lower than 7.4%, 8.4%, and 7.5% and the ICC was higher than 0.56, 0.51, and 0.52 for cartilage T1, T2, and T1ρ, respectively. The MRF results had a good agreement with the conventional methods with a linear regression slope >0.61 and R2 > 0.59. Conclusion: The 3D‐MRF sequence had high intra‐subject and inter‐subject repeatabilities for simultaneously measuring knee cartilage T1, T2, and T1ρ with good agreement with conventional sequences. Evidence Level: 1 Technical Efficacy: Stage 1 [ABSTRACT FROM AUTHOR]

Background: Multiparameter characterization using MR fingerprinting (MRF) can quantify multiple relaxation parameters of intervertebral disc (IVD) simultaneously. These parameters may vary by age and sex. Purpose: To investigate age‐ and sex‐related differences in the relaxation parameters of the IVD of the lumbar spine using a multiparameter MRF technique. Study Type: Prospective. Subjects: 17 healthy subjects (8 male; mean age = 34 ± 10 years, range 20–60 years). Field Strength/Sequence: 3D‐MRF sequence for simultaneous acquisition of proton density, T1, T2, and T1ρ maps at 3.0T. Assessment: Global mean T1, T2, and T1ρ of all lumbar IVDs and mean T1, T2, and T1ρ of each individual IVD (L1–L5) were measured. Gray level co‐occurrence matrix was used to quantify textural features (median, contrast, correlation, energy, and homogeneity) from T1, T2, and T1ρ maps. Statistical Tests: Spearman rank correlations (R) evaluated the association between age and T1, T2, and T1ρ of IVD. Mann–Whitney U‐tests evaluated differences between males and females in T1, T2, and T1ρ of IVD. Statistical significance was defined as P‐value <0.05. Results: There was a significant negative correlation between age and global mean values of all IVDs for T1 (R = −0.637), T2 (R = −0.509), and T1ρ (R = −0.726). For individual IVDs, there was a significant negative correlation between age and mean T1 at all IVD segments (R range = −0.530 to −0.708), between age and mean T2 at L2‐L3, L3‐L4, and L4‐L5 (R range = −0.493 to 0.640), and between age and mean T1ρ at all segments except L1‐L2 (R range = −0.632 to −0.763). There were no significant differences between sexes in global mean T1, T2, and T1ρ (P‐value = 0.23–0.76) The texture features with the highest significant correlations with age for all IVDs were global T1ρ mean (R = −0.726), T1 energy (R = −0.681), and T1 contrast (R = 0.709). Conclusion: This study showed that the 3D‐MRF technique has potential to characterize age‐related differences in T1, T2, or T1ρ of IVD in healthy subjects. Level of Evidence: 2 Technical Efficacy: Stage 1 [ABSTRACT FROM AUTHOR]

Background: Three-dimensional MR fingerprinting (3D-MRF) has been increasingly used to assess cartilage degeneration, particularly in the knee joint, by looking into multiple relaxation parameters. A comparable 3D-MRF approach can be adapted to assess cartilage degeneration for the hip joint, with changes to accommodate specific challenges of hip joint imaging., Purpose: To demonstrate the feasibility and repeatability of 3D-MRF in the bilateral hip jointly we map proton density (PD), T 1 , T 2 , T 1ρ , and ∆B 1+ in clinically feasible scan times., Study Type: Prospective., Subjects: Eight healthy subjects, three patients with mild osteoarthritis (OA), and one of the OA patients had femoral acetabular impingement (FAI). A National Institute of Standards and Technology/International Society for Magnetic Resonance in Medicine (NIST/ISMRM) system phantom was also used., Field Strength/sequence: 3 T, 3D-MRF sequence for bilateral hip joint mapping. Reference sequences include Volume Interpolated Breath-hold Examination (VIBE) for T 1 mapping, and magnetization-prepared fast low-angle shot (TFL) for T 2 and T 1ρ mapping., Assessment: The signal-to-noise ratio (SNR), repeatability, scan time, and accuracy of T 1 , T 2 , and T 1ρ maps of 3D-MRF sequence were evaluated on a NIST/ISMRM phantom and human subjects. Differences in the parametric maps between OA and healthy subjects were assessed., Statistical Tests: Regression, Bland-Altman, Kruskal-Wallis, and Wilcoxon tests were used to assess for accuracy, repeatability, and subregional variation. The P-value <0.05 indicated statistically significant., Results: A 3D-MRF sequence sensitive to PD, T 1 , T 2 , T 1ρ , and ∆B 1+ within 15 minutes, achieving high SNR and low test-retest coefficient of variance (T 1 : 3.36%, T 2 : 3.99%, T 1ρ : 5.93%). Mild hip OA patients, including one with mild OA and FAI, showed elevation of 29.4 ± 9% (T 2 ) and 32.4 ± 4.4% (T 1ρ ) in femoral lateral compartment of the hip joint compared to healthy controls., Data Conclusion: 3D-MRF may be a feasible approach for simultaneous, quantitative mapping of bilateral hip joint cartilage in healthy and mild OA patients., Evidence Level: 1 TECHNICAL EFFICACY: Stage 1., (© 2024 International Society for Magnetic Resonance in Medicine.)

Low-field strength scanners present an opportunity for more inclusive imaging exams and bring several challenges including lower signal-to-noise ratio (SNR) and longer scan times. Magnetic resonance fingerprinting (MRF) is a rapid quantitative multiparametric method that can enable multiple quantitative maps simultaneously. To demonstrate the feasibility of an MRF sequence for knee cartilage evaluation in a 0.55T system we performed repeatability and accuracy experiments with agar-gel phantoms. Additionally, five healthy volunteers (age 32 ± 4 years old, 2 females) were scanned at 3T and 0.55T. The MRI acquisition protocols include a stack-of-stars T 1ρ -enabled MRF sequence, a VIBE sequence with variable flip angles (VFA) for T 1 mapping, and fat-suppressed turbo flash (TFL) sequences for T 2 and T 1ρ mappings. Double-Echo steady-state (DESS) sequence was also used for cartilage segmentation. Acquisitions were performed at two different field strengths, 0.55T and 3T, with the same sequences but protocols were slightly different to accommodate differences in signal-to-noise ratio and relaxation times. Cartilage segmentation was done using five compartments. T 1 , T 2 , and T 1ρ values were measured in the knee cartilage using both MRF and conventional relaxometry sequences. The MRF sequence demonstrated excellent repeatability in a test-retest experiment with model agar-gel phantoms, as demonstrated with correlation and Bland-Altman plots. Underestimation of T 1 values was observed on both field strengths, with the average global difference between reference values and the MRF being 151 ms at 0.55T and 337 ms at 3T. At 0.55T, MRF measurements presented significant biases but strong correlations with the reference measurements. Although a larger error was present in T 1 measurements, MRF measurements trended similarly to the conventional measurements for human subjects and model agar-gel phantoms., (© 2024 John Wiley & Sons Ltd.)

Purpose: To compare the performance of a learned magnetization-prepared gradient echo (L-MPGRE) sequence against a commonly used sequence for 3D T 2 and T 1ρ mapping of the knee joint, the magnetization-prepared angle-modulated partitioned k-space spoiled gradient echo snapshots (MAPSS), on bi-exponential (BE), stretched-exponential (SE), and mono-exponential (ME) relaxation models., Methods: We used a combined differentiable and non-differentiable optimization to learn pulse sequence structure and its parameters for 3D T 2 and T 1ρ mapping of the knee joint using ME, SE, and BE models. The learned pulse sequence framework was used to improve quantitative accuracy and SNR and to reduce filtering effects. We compare the measured multi-compartment values between the two sequences (n = 8), and their repeatability (n = 4) in healthy volunteers (n = 12 total)., Results: The voxel-wise median absolute percentage difference (MAPD) between the T 2 and T 1ρ maps obtained with each sequence was 18.6% and 19.9%, respectively. The T 2 and T 1ρ repeatability tests showed a MAPD of 18.5% and 19.1% for MAPSS, and 16.8% and 15.5% for L-MPGRE. Bland-Altman region of interest (ROI)-wise analysis shows that bias is small, close to -1.5%, and the coefficient of variation is around 5.5% when comparing ROIs from both sequences., Conclusion: The L-MPGRE sequences can be used as a replacement for MAPSS for T 2 and T 1ρ mapping in the knee cartilage with advantages, achieving similar accuracy and 15% better repeatability in only half of its scan time., (© 2024 International Society for Magnetic Resonance in Medicine.)

Background: Three-dimensional MR fingerprinting (3D-MRF) techniques have been recently described for simultaneous multiparametric mapping of knee cartilage. However, investigation of repeatability remains limited., Purpose: To assess the intra-day and inter-day repeatabilities of knee cartilage T 1 , T 2 , and T 1ρ maps using a 3D-MRF sequence for simultaneous measurement., Study Type: Prospective., Subjects: Fourteen healthy subjects (35.4 ± 9.3 years, eight males), scanned on Day 1 and Day 7., Field Strength/sequence: 3 T/3D-MRF, T 1 , T 2 , and T 1ρ maps., Assessment: The acquisition of 3D-MRF cartilage (simultaneous acquisition of T 1 , T 2 , and T 1ρ maps) were acquired using a dictionary pattern-matching approach. Conventional cartilage T 1 , T 2 , and T 1ρ maps were acquired using variable flip angles and a modified 3D-Turbo-Flash sequence with different echo and spin-lock times, respectively, and were fitted using mono-exponential models. Each sequence was acquired on Day 1 and Day 7 with two scans on each day., Statistical Tests: The mean and SD for cartilage T 1 , T 2 , and T 1ρ were calculated in five manually segmented regions of interest (ROIs), including lateral femur, lateral tibia, medial femur, medial tibia, and patella cartilages. Intra-subject and inter-subject repeatabilities were assessed using coefficient of variation (CV) and intra-class correlation coefficient (ICC), respectively, on the same day and among different days. Regression and Bland-Altman analysis were performed to compare maps between the conventional and 3D-MRF sequences., Results: The CV in all ROIs was lower than 7.4%, 8.4%, and 7.5% and the ICC was higher than 0.56, 0.51, and 0.52 for cartilage T 1 , T 2 , and T 1ρ , respectively. The MRF results had a good agreement with the conventional methods with a linear regression slope >0.61 and R 2  > 0.59., Conclusion: The 3D-MRF sequence had high intra-subject and inter-subject repeatabilities for simultaneously measuring knee cartilage T 1 , T 2 , and T 1ρ with good agreement with conventional sequences., Evidence Level: 1 TECHNICAL EFFICACY: Stage 1., (© 2023 International Society for Magnetic Resonance in Medicine.)