Apart from being non-invasive, sampling occurs in a large cross-section of the liver, as opposed to a biopsy, which typically represents 1/50,000 of the whole liver.
The acquisition for both methods described below is usually short and usually takes up 10 minutes of magnet time only.
There are two principle methods of liver iron quantification by MRI: T2 relaxometry and signal intensity ratio.
Iron within liver has paramagnetic properties and results in a decrease in T2 relaxation times. The accelerated relaxation is proportional to iron content. Decay models are attributed to the average signal intensity at different echo times. To obtain reliable measurements of T2 times at different levels of iron overload, acquisition sequences with several different echo times are needed. Signal intensity as a function of echo time can be used to obtain a T2 parametric map 1.
An online tool is available to perform the relevant calculations 4.
Signal intensity ratio (SIR)
The ratio between the signal intensity of the liver and the signal intensity of paraspinal muscle that does not accumulate iron can be used to determine liver iron concentration. Using the same slice, signal intensity measurements are performed using ROIs, avoiding large vessels in the liver. The sequences used are gradient recalled echoes, as these are more sensitive to the paramagnetic effect of iron.
One of the most recognized SIR methods was developed at the University of Rennes, France 2.
T2 relaxometry vs. SIR methods
Estimating liver iron content is easier to perform with SIR compared with T2 relaxometry. SIR methods are less accurate at an iron concentration of >350 micromol Fe/g. However, SIR methods are highly specific at all levels of iron concentration and are reproducible. Overall, T2 relaxation methods are more accurate, but not yet standardized, and normally require off-site processing, which attracts additional cost. An advantage of T2 relaxation methods is the utility of myocardial iron concentration measurement 3.
- 1. St Pierre TG, Clark PR, Chua-anusorn W et-al. Noninvasive measurement and imaging of liver iron concentrations using proton magnetic resonance. Blood. 2005;105 (2): 855-61. doi:10.1182/blood-2004-01-0177 - Pubmed citation
- 2. Gandon Y, Olivié D, Guyader D et-al. Non-invasive assessment of hepatic iron stores by MRI. Lancet. 2004;363 (9406): 357-62. doi:10.1016/S0140-6736(04)15436-6 - Pubmed citation
- 3. Alústiza Echeverría JM, Castiella A, Emparanza JI. Quantification of iron concentration in the liver by MRI. Insights Imaging. 2012;3 (2): 173-80. doi:10.1007/s13244-011-0132-1 - Free text at pubmed - Pubmed citation
- 4. Git KA, Fioravante LA, Fernandes JL. An online open-source tool for automated quantification of liver and myocardial iron concentrations by T2* magnetic resonance imaging. (2015) The British journal of radiology. 88 (1053): 20150269. doi:10.1259/bjr.20150269 - Pubmed - Online calculator