In vivo calibration of the T2* cardiovascular magnetic resonance method at 1.5 T for estimation of cardiac iron in a minipig model of transfusional iron overload

Abstract

Background Non-invasive estimation of the cardiac iron concentration (CIC) by T2* cardiovascular magnetic resonance (CMR) has been validated repeatedly and is in widespread clinical use. However, calibration data are limited, and mostly from post-mortem studies. In the present study, we performed an in vivo calibration in a dextran-iron loaded minipig model. Methods R2* (= 1/T2*) was assessed in vivo by 1.5 T CMR in the cardiac septum. Chemical CIC was assessed by inductively coupled plasma-optical emission spectroscopy in endomyocardial catheter biopsies (EMBs) from cardiac septum taken during follow up of 11 minipigs on dextran-iron loading, and also in full-wall biopsies from cardiac septum, taken post-mortem in another 16 minipigs, after completed iron loading. Results A strong correlation could be demonstrated between chemical CIC in 55 EMBs and parallel cardiac T2* (Spearman rank correlation coefficient 0.72, P < 0.001). Regression analysis led to [CIC] = (R2* − 17.16)/41.12 for the calibration equation with CIC in mg/g dry weight and R2* in Hz. An even stronger correlation was found, when chemical CIC was measured by full-wall biopsies from cardiac septum, taken immediately after euthanasia, in connection with the last CMR session after finished iron loading (Spearman rank correlation coefficient 0.95 (P < 0.001). Regression analysis led to the calibration equation [CIC] = (R2* − 17.2)/31.8. Conclusions Calibration of cardiac T2* by EMBs is possible in the minipig model but is less accurate than by full-wall biopsies. Likely explanations are sampling error, variable content of non-iron containing tissue and smaller biopsies, when using catheter biopsies. The results further validate the CMR T2* technique for estimation of cardiac iron in conditions with iron overload and add to the limited calibration data published earlier.

Narrative review of magnetic resonance imaging in quantifying liver iron load

“…And it leads to delayed signal loss at high LIC due to the shorter TE, thus allowing the assessment of more severe liver iron overload. In recent years, Jensen et al (21) found that the upper limit of 1.5 T MRI SIR Measurement range would be extended to 115 mg/g if the relevant parameters TE = 12 ms and TR = 1,200 ms were set in the liver iron overload experiment of quantitative miniature pigs. However, this conclusion was drawn in an animal model and requires further validation in iron-loaded patients.…”

“…Then the corresponding signal decay time constants T 2 and T 2 * and the corresponding relaxation rates R 2 (1,000/T 2 ) and R 2 * (1,000/T 2 *) were obtained (1,3,22). Relaxometry is a quantitative assessment of MRI signal loss caused by the shortening of T 2 /T 2 * relaxation times (21,22). Excess iron stored in the body in the form of trivalent iron can shorten T1 and T 2 -as T1 shortens, the corresponding SI increases; The corresponding SI decreases with the shortening of T 2 (1, 3).…”

Biopsy-based optimization and calibration of a signal-intensity-ratio-based MRI method (1.5 Tesla) in a dextran-iron loaded mini-pig model, enabling estimation of very high liver iron concentrations

“…They were followed up with 1- to 3-month intervals for up to 21 months with transcutaneous liver biopsies and parallel MRI for assessment of hepatic T2*, hepatic SIR, and myocardial T2*. The myocardial T2* data have already been published recently [ 60 ]. MRI of the liver was always performed around an hour before the liver biopsy.…”