Calibration of myocardial T2 and T1 against iron concentration.

BACKGROUND: The assessment of myocardial iron using T2* cardiovascular magnetic resonance (CMR) has been validated and calibrated, and is in clinical use. However, there is very limited data assessing the relaxation parameters T1 and T2 for measurement of human myocardial iron. METHODS: Twelve hearts were examined from transfusion-dependent patients: 11 with end-stage heart failure, either following death (n=7) or cardiac transplantation (n=4), and 1 heart from a patient who died from a stroke with no cardiac iron loading. Ex-vivo R1 and R2 measurements (R1=1/T1 and R2=1/T2) at 1.5 Tesla were compared with myocardial iron concentration measured using inductively coupled plasma atomic emission spectroscopy. RESULTS: From a single myocardial slice in formalin which was repeatedly examined, a modest decrease in T2 was observed with time, from mean (± SD) 23.7 ± 0.93 ms at baseline (13 days after death and formalin fixation) to 18.5 ± 1.41 ms at day 566 (p<0.001). Raw T2 values were therefore adjusted to correct for this fall over time. Myocardial R2 was correlated with iron concentration [Fe] (R2 0.566, p<0.001), but the correlation was stronger between LnR2 and Ln[Fe] (R2 0.790, p<0.001). The relation was [Fe] = 5081•(T2)-2.22 between T2 (ms) and myocardial iron (mg/g dry weight). Analysis of T1 proved challenging with a dichotomous distribution of T1, with very short T1 (mean 72.3 ± 25.8 ms) that was independent of iron concentration in all hearts stored in formalin for greater than 12 months. In the remaining hearts stored for <10 weeks prior to scanning, LnR1 and iron concentration were correlated but with marked scatter (R2 0.517, p<0.001). A linear relationship was present between T1 and T2 in the hearts stored for a short period (R2 0.657, p<0.001). CONCLUSION: Myocardial T2 correlates well with myocardial iron concentration, which raises the possibility that T2 may provide additive information to T2* for patients with myocardial siderosis. However, ex-vivo T1 measurements are less reliable due to the severe chemical effects of formalin on T1 shortening, and therefore T1 calibration may only be practical from in-vivo human studies

Distortion in formalin-fixed brains: Using geometric morphometrics to quantify the worst-case scenario in mice

Although morphometric studies of fixed mammalian brains are an integral part of neuroscience, the nature of fixation-related morphometric artifacts is not well understood beyond assessments of size changes over fixation time. This study is the first to quantitatively co-evaluate the effects of the most common brain tissue fixative-formalin-on brain shape, size, and weight, using two-dimensional landmark analysis of mouse brains fixed in unbuffered, non-saline formalin from fresh specimens up to 213 days of preservation. The brains show a typical swelling reaction with subsequent decline in size and weight. Weight initially under- and later over-estimates size, so that the practice of using weight to estimate volume can be problematic. Time to recovery of original size resembled that of much larger brained mammals, suggesting that the slow reaction of formalin with tissue components mainly determines recovery times. Non-size related (anisotropic) distortion of different brain areas accounted for around a quarter of overall change suggesting that the use of "all-brain" fixation correction factors can introduce considerable error. Distortion occurs mostly after the first day of fixation, and extended fixation times impact mostly on size, not shape. Fixation effects relatively wider and stouter brain dimensions, except the cerebellum whose shape changes less. Evidence from the literature suggests that this pattern may be common to mammals due to structural commonalities