Ultra‐short echo time images quantify high liver iron

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Chronic anemias such as thalassemia and sickle cell disease represent the most common genetic disorders in the world. Transfusion therapy in these patients produces severe iron deposition in the liver and other organs, leading to cardiac and endocrine dysfunction as well as liver cirrhosis. Iron overload in transfusional siderosis cannot be treated with phlebotomy; instead, patients receive iron chelators that bind and remove iron. Dosing must be adjusted based on tissue iron content, necessitating reliable iron quantitation techniques. Before 2005, needle biopsies were required to measure liver iron, with their attendant risks 1 and sampling error 2. Since then, magnetic resonance imaging systems have become important tools for diagnosing and monitoring iron overload disorders including sickle cell disease, thalassemia, hemochromatosis, and neurodegenerative disorders 5. Both transverse relaxivity and magnetic susceptibility measurements have been used to estimate tissue iron concentrations in the liver, heart, endocrine glands, and brain.
MRI‐based iron quantitation at 1.5T is now standard of care 7. However, it is estimated that 50% of new magnet installations are 3T and some imaging centers use exclusively 3T magnets; this necessitates development of robust imaging techniques for high‐field systems. Previous studies have validated quantitation over lower iron burdens at 3T 8. However, liver iron quantitation with high‐field scanners (3T and above) remains limited by rapid signal decay. When increasing the field strength from 1.5T to 3T, field‐dependent enhancement causes JOURNAL/mrim/04.02/01445475-201803000-00037/math_37MM1/v/2018-01-24T161827Z/r/image-png (1/ JOURNAL/mrim/04.02/01445475-201803000-00037/math_37MM2/v/2018-01-24T161827Z/r/image-png ) decay to approximately double, leading to transverse decay times below 0.5 ms in the liver, well below the range standard gradient‐echo techniques can reliably capture 11. Inadequate sampling of rapidly decaying signal components leads to an underestimate of liver iron concentration (LIC). The development of ultra‐short echo time (UTE) sequences has dramatically decreased the minimum achievable echo time (TE), enabling acquisition of ultra‐fast decay species 12. UTE has shown promise to perform structural imaging of cartilage and bone. The reduced TE could potentially lead to significantly increased dynamic range in quantitative imaging approaches used to non‐invasively estimate LIC at 3T and above. Proof of concept in this regard was published quite recently 13.
In this work, we measured liver JOURNAL/mrim/04.02/01445475-201803000-00037/math_37MM3/v/2018-01-24T161827Z/r/image-png in human volunteers receiving treatment for transfusional iron overload using Cartesian gradient echo (GRE) and UTE sequences. We obtained 3T LIC estimates in milligrams of iron per gram of dry liver (mg/g) for comparison with clinical LIC estimates obtained at 1.5T using a previously derived relationship between liver JOURNAL/mrim/04.02/01445475-201803000-00037/math_37MM4/v/2018-01-24T161827Z/r/image-png at 1.5T and 3T 11. We demonstrate that 3D radial UTE imaging increased the achievable dynamic range of LIC estimates to match, and possibly exceed, estimates from Cartesian gradient echo images obtained at 1.5T, providing a reliable means to quantify high liver iron at 3T.

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