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Science to Practice |
1 Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins Hospital, 600 N Wolfe St, MRI Room 143, Baltimore, MD 21287. dbluemke@jhmi.edu
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Iron overload is most commonly due to multiple transfusions or abnormally elevated iron absorption. The body iron stores are normally between 0.5 and 1.5 g of iron (see diagram); each unit of blood contains 200–250 mg of iron. Since the body has no mechanism for excreting iron, repeated transfusions result in iron accumulation in the reticuloendothelial system of the liver, spleen, and bone marrow. With very high levels of excess iron, there is iron deposition in organs such as the heart and pancreas (1). In this issue of Radiology, Wang et al (2) report the results of their evaluation of magnetic resonance (MR) imaging for measurement of iron levels in the heart.
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When excess iron stored as either ferritin or hemosiderin can no longer be bound by transferrin, the unbound iron deposited in the heart or other organs is highly toxic and may result in oxidative damage to membrane lipids and proteins, with subsequent heart failure. The onset of heart failure often signifies irreversible cardiac damage and a poor prognosis. Current tests (measurement of serum ferritin, liver biopsy, non-transferrin-bound iron in plasma) correlate poorly with cardiac iron levels and with death from iron-induced cardiomyopathy (1). Echocardiograms and multiple gated acquisition (or MUGA) scans can depict heart failure at a late stage. Thus, it is highly desirable to have a direct noninvasive measurement of iron in the heart.
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The Practice
Clinical use.—Clinical studies are needed to determine the amount of iron that causes cardiomyopathy.
Anderson et al (4) noted that a T2* value of less than 20 msec is associated with heart failure in patients with thalassemia major. Because myocardial biopsy specimens are difficult to obtain, previous investigators have not related the T2* values to myocardial iron levels. However, because iron deposition is nonuniform and associated fibrosis may affect T2* measurements, a calibration curve relating T2 or magnetic susceptibility to iron content is very desirable. By measuring the MR signal, it is possible to triage patients for iron chelation therapy and to noninvasively monitor the progress of this therapy (5).
For the liver, it has been possible to correlate iron content measurements in biopsy specimens with signal intensity changes on MR images, but this has not been done for the heart. The T2 of the heart can be derived by using a fast spin-echo pulse sequence and by measuring signal changes that occur when the echo time is changed but the repetition time is held constant. Ideally, the T2 value is then directly related to iron concentration. However, besides iron content, T2 is also affected by magnetic field strength, tissue oxygenation level, and the structure of iron as ferritin or hemosiderin. Alternatively, the magnetic susceptibility of the heart can be derived by using a gradient-echo pulse sequence and by measuring the change in the phase angle of the MR signal at the border of two tissues. The magnetic susceptibility should increase linearly with iron concentration. Wang et al found the best correlation by measuring both magnetic susceptibility and T2 of the heart (2).
Future opportunities and challenges.—The magnetic susceptibility of the human heart may be very heterogeneous because of fibrosis. In the gerbil experiments by Wang et al (2), the magnetic susceptibility could not be measured accurately in the intact heart, in which the tissue was very heterogeneous, and measurements apparently varied at different positions in the heart. Thus, the cardiac tissue was instead lysed into a solution to make the MR signal more homogeneous. To determine whether the results of these experiments in gerbils apply to humans, a correlation between findings at MR imaging and at heart biopsy in humans will ultimately be necessary. Since oxygenation affects the T2 of the heart, in vivo MR measurements must be performed. For magnetic susceptibility measurement with the method used by Wang et al, the susceptibility of a reference tissue adjacent to the heart must also be known. Finally, all of these MR measurements must be amenable to acquisition with a technique that freezes the motion of the beating human heart.
Summary
A noninvasive MR imaging "biopsy" to determine iron levels within the heart is within reach, as demonstrated by Wang et al. Human studies are now needed to validate proposed methods.
FOOTNOTES
See also the article by Wang et al in this issue.
REFERENCES
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