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Superconducting Quantum Interference Device (SQUID)
SQUID stands for Superconducting Quantum Interference Device. This imaging modality uses a very low-power magnetic field with sensitive detectors that measure the interference of iron within the field. The sensor requires a cryogenic environment, since it must be superconducting to operate. Although SQUID is still considered investigational, linear correlations have been demonstrated between SQUID measurements and liver biopsy LIC levels (1-4).
How the test is performed
A measurement is performed by lowering the patient into a known, constant magnetic field, and then detecting the change in magnetic flux versus the change in a water reference medium.
MRI detection of hepatic iron overload
Used with permission from Fischer (5). © 1998 Wiley-VCH Verlag GmbH & Co KGaA.
Accuracy of SQUID
Although SQUID directly measures the magnetic susceptibility of ferritin and hemosiderin, at present it does not have sufficient spatial or temporal resolution to evaluate myocardial iron. In a large clinical trial, LIC data obtained by SQUID were shown to underestimate LIC values obtained from biopsy by a factor of 0.46 (6).
Correlation between SQUID and liver biopsyCorrelation between R2 MRI and liver biopsy
Used with permission from Fischer, et al (7). © 1992 Elsevier Inc.
Advantages of SQUID
SQUID is a noninvasive test for iron stores that may be frequently repeated, and has been shown to have a linear correlation with LIC assessed by biopsy.
Disadvantages of SQUID
The limited availability and high cost of this test currently restrict its use to research. In addition at present, it appears to underestimate LIC values versus liver biopsy.
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Iron and Hepatic Damage
The development of liver fibrosis
directly correlates
to liver iron concentrations and serum ferritin levels.
References
- (1) Nielsen P, Fischer R, Engelhardt R, et al, Liver iron stores in patients with secondary haemosiderosis under iron chelation therapy with deferoxamine or deferiprone. Br J Haematol. 1995;91(4):827-33.
- (2) Pootrakul P, Kitcharoen K, Yansukon P, et al, The effect of erythroid hyperplasia on iron balance. Blood. 1988;71(4):1124-9.
- (3) Brittenham GM, Noninvasive methods for the early detection of hereditary hemochromatosis. Ann N Y Acad Sci. 1988;526:199-208.
- (4) Brittenham GM, Farrell DE, Harris JW, et al, Magnetic-susceptibility measurement of human iron stores. N Engl J Med. 1982;307(27):1671-5.
- (5) Fischer R. In: Andra W, Nowak H, editors. Magnetism in medicine: a handbook. Berlin:Wiley-VCH;1998:286-301.
- (6) Piga A, Fischer R, St Pierre T, et al. Comparison of LIC obtained from biopsy, BLS and R2-MRI in iron overloaded patients with β-thalassemia, treated with deferasirox (Exjade®, ICL670). Blood. 2005;106(11):abst 2689.
- (7) Fischer R, et al. In: Hoke M et al. editors. Biomagnetism: Clinical Aspects: proceedings of the 8th International Conference on Biomagnetism, Munster, 19-24 August, 1991. Elsevier Science Publishers BV;1992:585-8.