Assessment of cardiac iron by MRI susceptometry and R2* in patients with thalassemia

Abstract

A magnetic resonance imaging cardiac magnetic susceptometry (MRI-CS) technique for assessing cardiac tissue iron concentration based on phase mapping was developed. Normal control subjects (n=9) and thalassemia patients (n=13) receiving long-term blood transfusion therapy underwent MRI-CS and MRI measurements of the cardiac relaxation rate R2*. Using MRI-CS, subepicardium and subendocardium iron concentrations were quantified exploiting the hemosiderin/ferritin iron specific magnetic susceptibility. The average of subepicardium and subendocardium iron concentrations and R2* of the septum were found to be strongly correlated (r=0.96, P<.0001), and linear regression analysis yielded CIC (microg Fe/g(wet tissue))=(6.4+/-0.4).R2* (septum) (s(-1)) - (120+/-40). The results demonstrated that septal R2* indeed measures cardiac iron level.

Keywords: MRI; R2*; heart; iron overload; magnetic susceptibility; thalassemia.

Figure 1

Block diagram outlining the procedure for the quantification of magnetic susceptibility difference between the cardiac tissue and the reference.

Figure 2

Results of the phantom validation studies at the two participating sites. Error bars are smaller than the symbol. The magnetic susceptibility χ of CuSO₄ solution with concentrations of 100, 200, 300 and 400 mM was measured relative to distilled water with a regression coefficient of 0.98 (p < 0.0017) and a coefficient of determination R² = 0.9967.

Figure 3

Transverse amplitude image (A), phase image (B) and phase profile (C) in a patient with iron overload. (A): the LV cardiac wall has decreased signal intensity compared with the intercostal muscle on the amplitude image, owing to an increase in R2*. (B): the signal phase of the LV cardiac wall is increased relative to the intercostal muscle, due to an increase of the magnetic susceptibility in the cardiac tissue. The x′ and y′ axes are the same as that referred to in Figure 1. (C): the phase profile along the white bar in y′ direction of the amplitude/phase image with the magnetic field B_o perpendicular to the image plane.

Figure 3

Transverse amplitude image (A), phase image (B) and phase profile (C) in a patient with iron overload. (A): the LV cardiac wall has decreased signal intensity compared with the intercostal muscle on the amplitude image, owing to an increase in R2*. (B): the signal phase of the LV cardiac wall is increased relative to the intercostal muscle, due to an increase of the magnetic susceptibility in the cardiac tissue. The x′ and y′ axes are the same as that referred to in Figure 1. (C): the phase profile along the white bar in y′ direction of the amplitude/phase image with the magnetic field B_o perpendicular to the image plane.

Figure 3

Transverse amplitude image (A), phase image (B) and phase profile (C) in a patient with iron overload. (A): the LV cardiac wall has decreased signal intensity compared with the intercostal muscle on the amplitude image, owing to an increase in R2*. (B): the signal phase of the LV cardiac wall is increased relative to the intercostal muscle, due to an increase of the magnetic susceptibility in the cardiac tissue. The x′ and y′ axes are the same as that referred to in Figure 1. (C): the phase profile along the white bar in y′ direction of the amplitude/phase image with the magnetic field B_o perpendicular to the image plane.

Figure 4

Correlation between septal and apical R2* of left ventricular cardiac wall. The solid line is the result of linear regression when all patients and control subjects are combined (r = 0.96, p < 0.001).

Figure 5

Correlation between R2* and the averaged subendocardial and subepicardial iron concentration (CIC_avg) from the anterior apical left ventricular cardiac wall. (A) Correlation between apical R2*_apex and CIC_avg for patients and controls (solid line: r = 0.93, p < 0.001) and, separately, for control subjects (dotted line: r = -0.80, p < 0.01). (B) Septal R2*_septum correlated with CIC_avg in patients and controls (solid line: r = 0.96, p < 0.001), but not significantly for control subjects only (dotted line: r = -0.45, p = 0.22)

Figure 5

Correlation between R2* and the averaged subendocardial and subepicardial iron concentration (CIC_avg) from the anterior apical left ventricular cardiac wall. (A) Correlation between apical R2*_apex and CIC_avg for patients and controls (solid line: r = 0.93, p < 0.001) and, separately, for control subjects (dotted line: r = -0.80, p < 0.01). (B) Septal R2*_septum correlated with CIC_avg in patients and controls (solid line: r = 0.96, p < 0.001), but not significantly for control subjects only (dotted line: r = -0.45, p = 0.22)