Eamon K Doyle1,2, Samuel Thornton3, Nilesh R Ghugre4,5,6, Thomas D Coates7, Krishna S Nayak3, John C Wood1,2. 1. Department of Biomedical Engineering, University of Southern California, Los Angeles, California, USA. 2. Division of Cardiology and Radiology, Children's Hospital Los Angeles, Keck School of Medicine, University of Southern California, Los Angeles, California, USA. 3. Ming Hsieh Department of Electrical and Computer Engineering, University of Southern California, Los Angeles, California, USA. 4. Schulich Heart Research Program, Sunnybrook Health Sciences Centre, Toronto, Ontario, Canada. 5. Physical Sciences Platform, Sunnybrook Research Institute, Toronto, Ontario, Canada. 6. Department of Medical Biophysics, University of Toronto, Toronto, Ontario, Canada. 7. Division of Hematology, Children's Hospital Los Angeles, Keck School of Medicine, University of Southern California, Los Angeles, California, USA.
Abstract
BACKGROUND: Liver iron concentration (LIC) measured by MRI has become the clinical reference standard for managing iron overload in chronically transfused patients. Transverse relaxivity (R2 or R2 * ) measurements are converted to LIC units using empirically derived calibration curves. HYPOTHESIS: That flip angle (FA) error due to B1 + spatial heterogeneity causes significant LIC quantitation error. B1 + scale (b1 , [FAactual /FAspecified ]) variation is a major problem at 3 T which could reduce the accuracy of transverse relaxivity measurements. STUDY TYPE: Prospective. POPULATION: Forty-seven subjects with chronic transfusional iron overload undergoing clinically indicated LIC assessment. FIELD STRENGTH/SEQUENCE: 5 T/3 T dual-repetition time B1 + mapping sequence ASSESSMENT: We quantified the average/standard deviation b1 in the right and left lobes of the liver from B1 + maps acquired at 1.5 T and 3 T. The impact of b1 variation on spin echo LIC estimates was determined using a Monte Carlo model. STATISTICAL TESTS: Mean, median, and standard deviation in whole liver and right and left lobes; two-sided t-test between whole-liver b1 means. RESULTS: Average b1 within the liver was 99.3% ± 12.3% at 1.5 T versus 69.6% ± 14.6% at 3 T and was independent of iron burden (P < 0.05). Monte Carlo simulations demonstrated that b1 systematically increased R2 estimates at lower LIC (<~25 mg/g at 1.5 T, <~15 mg/g at 3 T) but flattened or even inverted the R2 -LIC relationship at higher LIC (≥~25 mg/g to 1.5 T, ≥~15 mg/g to 3 T); changes in the R2 -LIC relationship were symmetric with respect to over and under excitation and were similar at 1.5 T and 3 T (for the same R2 value). The R2 * -LIC relationship was independent of b1 . CONCLUSION: Spin echo R2 measurement of LIC at 3 T is error-prone without correction for b1 errors. The impact of b1 error on current 1.5 T spin echo-based techniques for LIC quantification is large enough to introduce measurable intersubject variability but the in vivo effect size needs a dedicated validation study. TECHNICAL EFFICACY STAGE: 2.
BACKGROUND: Liver iron concentration (LIC) measured by MRI has become the clinical reference standard for managing iron overload in chronically transfused patients. Transverse relaxivity (R2 or R2 * ) measurements are converted to LIC units using empirically derived calibration curves. HYPOTHESIS: That flip angle (FA) error due to B1 + spatial heterogeneity causes significant LIC quantitation error. B1 + scale (b1 , [FAactual /FAspecified ]) variation is a major problem at 3 T which could reduce the accuracy of transverse relaxivity measurements. STUDY TYPE: Prospective. POPULATION: Forty-seven subjects with chronic transfusional iron overload undergoing clinically indicated LIC assessment. FIELD STRENGTH/SEQUENCE: 5 T/3 T dual-repetition time B1 + mapping sequence ASSESSMENT: We quantified the average/standard deviation b1 in the right and left lobes of the liver from B1 + maps acquired at 1.5 T and 3 T. The impact of b1 variation on spin echo LIC estimates was determined using a Monte Carlo model. STATISTICAL TESTS: Mean, median, and standard deviation in whole liver and right and left lobes; two-sided t-test between whole-liver b1 means. RESULTS: Average b1 within the liver was 99.3% ± 12.3% at 1.5 T versus 69.6% ± 14.6% at 3 T and was independent of iron burden (P < 0.05). Monte Carlo simulations demonstrated that b1 systematically increased R2 estimates at lower LIC (<~25 mg/g at 1.5 T, <~15 mg/g at 3 T) but flattened or even inverted the R2 -LIC relationship at higher LIC (≥~25 mg/g to 1.5 T, ≥~15 mg/g to 3 T); changes in the R2 -LIC relationship were symmetric with respect to over and under excitation and were similar at 1.5 T and 3 T (for the same R2 value). The R2 * -LIC relationship was independent of b1 . CONCLUSION: Spin echo R2 measurement of LIC at 3 T is error-prone without correction for b1 errors. The impact of b1 error on current 1.5 T spin echo-based techniques for LIC quantification is large enough to introduce measurable intersubject variability but the in vivo effect size needs a dedicated validation study. TECHNICAL EFFICACY STAGE: 2.
Authors: Timothy G St Pierre; Paul R Clark; Wanida Chua-anusorn; Adam J Fleming; Gary P Jeffrey; John K Olynyk; Pensri Pootrakul; Erin Robins; Robert Lindeman Journal: Blood Date: 2004-07-15 Impact factor: 22.113
Authors: Yannick Bliesener; Xinran Zhong; Yi Guo; Michael Boss; Ryan Bosca; Hendrik Laue; Caroline Chung; Kyunghyun Sung; Krishna S Nayak Journal: Med Phys Date: 2019-04-15 Impact factor: 4.071
Authors: Nathan T Roberts; Louis A Hinshaw; Timothy J Colgan; Takanori Ii; Diego Hernando; Scott B Reeder Journal: Magn Reson Med Date: 2020-10-26 Impact factor: 4.668