Myocardial Iron Estimation-MRI

A 19 year old male, a known case of thalassemia on regular transfusions has been evaluated for left ventricular dysfunction on account of myocardial iron over load on MRI. It shows an approximate value of T2* as 8-10 m/sec (red zone) with an iron equivalent of 1.41 mgs / gm of myocardium suggesting chelation with deferoxime would helpful. Case submitted by Dr MGK Murthy, Dr Krishna Mohan, and Mr Venkat.

Teaching points :

• Iron is invisible on MRI, however MR detects iron’s influence on magnetic milleu of water protons diffusing in the tissues.
• There are three types of iron a) Labile iron, though clinically toxic is  magnetically silent. b) Ferritin (bodies defense against circulating free iron) is weakly magnetic, c) Haemosiderin – is well identified on MR as darkening.
• All organs darken with increased echotimes in general, however those that contain iron darken more rapidly (at the earlier echotimes).
• T2* represents echotimes necessary for the tissue to become twice dark, R2* represents exponentially rate of darkening.
• Some prefer R2* values rather than T2* because it is directly proportional to the iron concentration (R2* = 1000 / T2*).
• The technique of acquisition is simple by using SPGR sequence and marking out various regions of interest within the myocardium at multiple echotimes and analyzing the data by the formula       Fe = 45 (T2*) – 1.22.
• The normal myocardium usually has  T2* more than 20 ms = 1.16 mgs/gm = normal ventricular function (green zone).
  If 10-20 ms = Caution zone, less than 10 ms = RED zone
  If less 6 ms = 50% risk of heart failure in one year due to cardiomyopathy.
• False positives of darkening can occur on account of powerful diuretics because of decreased water content.
• False negatives can occur on account of fluid overload.
• Treatment involves chelation with I.V desferoxime.
• The cardiac iron is 4 times slower to get out of the body than the hepatic iron, and takes approximately 13-14 months.