Magnetic resonance imaging assessment of excess iron in thalassemia, sickle cell disease and other iron overload diseases

Abstract

Patients with transfusion-dependent anemia develop cardiac and endocrine toxicity from iron overload. Classically, serum ferritin and liver biopsy have been used to monitor patient response to chelation therapy. Recently, magnetic resonance imaging (MRI) has proven effective in detecting and quantifying iron in the heart and liver. Tissue iron is paramagnetic and increases the MRI relaxation rates R2 and R2* in a quantifiable manner. This review outlines the principles and validation of non invasive iron estimation by MRI, as well as discussing some of the technical considerations necessary for accurate measurements. Specifically, the use of R2 or R2* methods, choice of echo times, appropriate model for data fitting, the use of a pixel-wise or region-based measurement, and the choice of field strength are discussed.

Figures

FIGURE 1

Left: gradient echo images of liver at echo times of 1.1, 4.6, 9.9, and 13.4 ms. Liver parenchyma darkens progressively with increasing echo time. Right: signal intensity as a function of time is well described by a monoexponential decay.

FIGURE 2

Plot of liver R2 vs. biopsied liver iron concentration in 104 iron overloaded patients [reprinted from (20) with permission].

FIGURE 3

Left: plot of liver R2* vs. biopsied liver iron concentration. Open circle represents value estimated in normal control subjects. Right: plot of liver R2 vs. hepatic iron concentration estimated by liver biopsy (open circles) and by R2* estimate (filled dots). Curve represents the R2 calibration curve derived by St. Pierre et al. (20) [graphs reprinted from (21)].

FIGURE 4

Plot of hepatic R2* vs. chemically assayed iron concentration in gerbil liver. There is a linear relationship until R2* surpasses 700 Hz (approximately 1.33/TEmin.) whereupon there is catastrophic breakdown of the relationship [graphs reprinted from (16)].

FIGURE 5

Left: signal decay curve from the inter ventricular septum of a patient with heavy cardiac iron burden. Signal loss is prominent in early echo times but plateaus during the later echoes. Mono-exponential fit is poor but addition of an offset correction (right) corrects the problem [reprinted from (28)].