The use of an inhomogeneous applied field improves the spatial sensitivity profile of an in vivo SQUID susceptometer

Validation of serum ferritin values by magnetic susceptometry in predicting iron overload in dialysis patients

BACKGROUND

Guidelines for treating anemia in dialysis patients accept, as high-end range of serum ferritin useful to optimize erythropoietin therapy, values high as 500 to 900 microg/L, on the hypothesis that ferritin might be not representative of iron overload.

METHODS

A superconducting quantum interference device (SQUID) was used to make direct noninvasive magnetic measurements of nonheme hepatic iron content in 40 dialysis patients treated with intravenous iron, and liver iron content was compared with biochemical markers of iron status.

RESULTS

Only 12/40 (30%) patients showed normal hepatic iron content (SQUID <400 microg/g), while 32.5% had mild (400 to 1000 microg/g) and 37.5% severe (>1000 microg/g) iron overload, although 28/40 patients (70%) had serum ferritin below 500 microg/L. Among many parameters, hepatic iron content was only correlated with ferritin (r= 0.324, P= 0.04). The receiver operating characteristic (ROC) analysis showed the best specificity/sensitivity ratio to identify iron overload for ferritin >340 microg/L (W = 0.716). Multivariate logistic regression analysis demonstrated that an increase in serum ferritin of 100 microg/L and female gender were independent variables associated with moderate to severe hepatic iron overload: OR 1.71 (95% CI 1.10 to 2.67) and OR 10.68 (95% CI 1.81 to 63.15), respectively.

CONCLUSION

Hepatic iron overload is frequent in dialysis patients with ferritin below currently proposed high-end ranges, and the diagnostic power of ferritin in indicating true iron stores is better than presumed. Safety concerns should prompt a reevaluation of acceptable iron parameters, focusing on potential gender-specific differences, to avoid potentially harmful iron overload in a majority of dialysis patients, mainly females.

Theoretical evaluation of the susceptometric measurement of iron in human liver by four different susceptometers

This paper is an evaluation of liver iron quantification using a simulated magnetic susceptibility measurement in the hepatic region. Susceptometers having homogeneous and non-homogeneous magnetizing fields coupled with axial second-order and planar first-order gradiometric magnetic detectors were considered. The intensity of magnetic flux threading the detector coils was evaluated considering samples with volume and susceptibility equivalent to liver iron, tissue and lung air individually. These volumes were represented by cylindrical and spherical geometries. The main sources of error in quantifying iron overload in susceptometric measurement of hepatic tissue were evaluated for four configurations of the susceptometer.