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SUPERCONDUCTING QUANTUM INTERFERENCE DEVICE (SQUID)

SQUID is an imaging modality that uses a very low-power magnetic field with sensitive detectors that measure the interference of iron within the field. 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. The sensor requires a cryogenic environment, since it must be superconducting to operate [47]. Although SQUID is still considered investigational, linear correlations have been demonstrated between SQUID measurements and iron levels from liver biopsy [48–50].

Technique for taking an iron measurement with SQUID

Fischer [47]. © 1998 Wiley-VCH Verlag GmbH & Co KGaA, reprinted with permission.

Linear relationship between SQUID and liver biopsy

Fischer [51]. © 1992 Elsevier Inc, reprinted with permission.

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. The use of SQUID is currently limited as there are only four facilities worldwide that have a SQUID machine available for the measurement of iron levels. In addition, during the clinical development of deferasirox, LIC data obtained by SQUID were shown to underestimate LIC values obtained from biopsy by a factor of 0.46 [52].

One of four current SQUID facilities worldwide

ECHOCARDIOGRAPHY, RADIONUCLIDE VENTRICULOGRAPHY

Accurate assessments of LVEF can be provided non-invasively by resting or stress echocardiography [53, 54] or radionuclide ventriculography [55, 56]. These techniques may be useful in the diagnosis of early iron-induced cardiac disease, since diastolic dysfunction has been shown to be of prognostic value in the development of symptomatic iron-induced cardiac disease [10, 57].

Echocardiography is the most widely used technique for the diagnosis and prognosis of left ventricular systolic function [58]. However, an important limitation of this approach is that echocardiographic abnormalities tend to develop relatively late in the course of cardiomyopathy. In addition, the technique is reliant on operator skill and requires assumptions on ventricular geometry to be made, thereby increasing the potential for inter-observer variability. As a result, measurements of resting ejection fraction by radionuclide angiography and MRI are more robust than echocardiography, and are better at recognizing preclinical systolic dysfunction [59]

Summary of primary methods available for assessment of body iron concentration.

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References

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Learn about the benefits of chelation therapy in treating transfusional iron overload.