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    Liver and Iron - MR protocol
    Yves Gandon - Medical Imaging, Rennes, France.

     

    MR technique is based on two or three gradient echo (GRE) sequences. This type of sequence is more sensitive to the presence of iron, and decrease the detection threshold (7, 24, 27) by comparison to spin echo sequences (18). A precise technique is mandatory to limitate the quantification errors..
     

       MR sequences

    Using GRE sequences the decrease of the liver signal is specific to iron excess, if we avoid "out-of-phase" TE. If not, a decrease of liver signal can be due to steatosis (29-30).

    The use of a body coil is mandatory to get a homogeneous signal in the whole MR slice, and to be able to compare the liver signal intensity to that of the paraspinous muscles.

    The MR protocol is based on GRE sequences with a TR of 120 ms and with differents TE « in phase » (4-30 ms) and two pulse angles (20°-90°) in order to get T1, proton density (PD) and T2 images. The selected TE must be "in-phase" (ie a multiple of 6.8 ms/Bo, Bo is corresponding to the magnetic field in Tesla). For this reason the MR protocol was adapted to the magnetic field of the MR unit.

    Selected sequences :
     
    .5 T 1.0 T 1.5 T
    GRE "T1" sequence 120/14/90° 120/7/90° 120/4/90°
    GRE "PD" sequence 120/14/20° 120/7/20° 120/4/20°
    GRE "T2" sequence 120/28/20° 120/14/20° 120/9/20°
    GRE "T2+" sequence - 120/21/20° 120/14/20°
    GRE "T2++" sequence - - 120/21/20°
     

    The field of view must be adapted to the patient abdominal diameter (30-45 cm). Using a single excitation and a 128x256 matrix, the acquisition time is 15 seconds. Slice thickness is usually 10 mm (15 mm could be usefull at .5 T to increase signal to noise ratio). Presaturations are not needed.


     
       Practically

    It is usefull to begin by threshold coronal slices.

    The "T1" GRE (breathhold) sequence gives a morphological analysis of the upper abdomen but this sequence is not mandatory to quantify liver iron.

    The proton density "PD" GRE (breathhold) sequence is used to quantify moderate or major overload (between 100 and 250 µmol/g).

    Then:

    • in case of major overload the liver signal disappears (L/M signal ratio<.2) and MR cannot yield a precise estimation. However, in that case, it is not so important to distinguish between a 300µmol/g or a 700µmol/g iron overload. There is no relation between the level of the overload and the risks. On the other hand, the age of the patient is more important. If he has more than 45 yo, it could be interesting to cover the whole liver with a high resolution sequence (if possible in breathhold) to be sure that there is not a iron free nodule.
    • if the overload is not obvious (L/M signal ratio>.5), more sensitives sequences are needed ("T2", "T2+", or even "T2++" GRE sequences).
    The whole study takes about 15 minutes.

     
       MR signal intensity measurements

    Signal intensity measurements are done by means of ROIs (« Region Of Interest ») of usually about 1cm2. It is a good habit to place 3 ROIs on the right part of the liver avoiding artifacts, and 2 ROIs on the paraspinous muscles (right and left). These ROIs, plotted for each sequence, must be placed on the same image in order to avoid global signal intensity variations between images.

    Be carefull to avoid liver vessels, heterogeneous areas, artifacts and particularly the decrease of signal intensity of the paraspinous muscle adjacent to the posterior lung bases (diamagnetic effect of the air).
     

      Results ...  
     

    Update: June 10th, 2001 Write to the webmaster