Mechanisms of tissue–iron relaxivity: Nuclear magnetic resonance studies of human liver biopsy specimens

Abstract

MRI is becoming an increasingly important tool to assess iron overload disorders, but the complex nature of proton-iron interactions has troubled noninvasive iron quantification. Intersite and intersequence variability as well as methodological inaccuracies have been limiting factors to its widespread clinical use. It is important to understand the underlying proton relaxation mechanisms within the (human) tissue environment to address these differences. In this respect, NMR relaxometry was performed on 10 fresh human liver biopsy specimens taken from patients with transfusion-dependent anemia. T 1 (1/R 1 ) inversion recovery, T 2 (1/R 2 ) single echo, and multiecho T 2 CPMG measurements were performed on a 60-MHz Bruker Minispectrometer. NMR parameters were compared to quantitative iron levels and tissue histology. Relaxivities R 1 and R 2 both increased linearly with hepatic iron content, with R 2 being more sensitive to iron. CPMG data were well described by a chemical-exchange model and predicted effective iron center dimensions consistent with hemosiderin-filled lysosomes. Nonexponential relaxation was evident at short refocusing intervals with R 2 and amplitude behavior suggestive of magnetic susceptibility-based compartmentalization rather than anatomic subdivisions. NMR relaxometry of human liver biopsy specimens yields unique insights into the mechanisms of tissue-iron relaxivity. Magn Reson Med 54:1185-1193, 2005.

Different forms of iron accumulation in the liver on MRI

“…This area is also seen hyperintense on R2* map (within the circle), which is consistent with hypersiderosis in a siderotic liver. a b c portal hepatocytes (39). Therefore, periportal siderosis can be observed in early phases of hepatic iron overload or after chelation treatment (Fig.…”

Magnetic resonance imaging measurement of iron overload

“…This means that the signals received from different areas in the tissue remain coherent with one another and the signals last for a long duration (bright images without much contrast). Iron deposits, however, act like little magnets when placed in a strong magnetic field; protons diffusing along different paths experience wildly different magnetic profiles, disrupting coherence among the protons and darkening the image more quickly [37,38].…”

Magnetic Resonance Imaging Assessment of Excess Iron in Thalassemia, Sickle Cell Disease and Other Iron Overload Diseases

“…More importantly, it can also estimate iron concentrations in a wide range of tissues such as the heart (7,8,16,17). Magnetic resonance imaging does not image the iron directly but instead images water protons as they diffuse near iron deposits in the tissue of interest (18,19). The iron acts as little magnets, destroying the homogeneity of the magnetic field in iron laden tissues.…”