Focal benign lesions of the spine and pelvis may sometimes mimic metastasis or lesions secondary to hematologic diseases.
Although MRI using conventional T1-weighted images (T1WI) ,
T2-weighted images (T2WI) and fat saturation technique (STIR or T2 FSE FAT SAT) is a sensitive method for the assessment of bone marrow,
it sometimes lacks specificity.
In order to solve this problem,
IP/OP sequence or chemical shift imaging (CSI) due to its capacity to detect the presence of microscopic fat in an osseus lesion,
has been proposed as a helpful tool to differentiate between cellular infiltration and normal hematopoietic bone marrow (1).
CSI is based on the principle that precession frequencies of hydrogen protons in water and fat molecules are slightly different,
which means that after excitation,
they will repeatedly be in-phase and out-phase.
Therefore,
in voxels containing both water and fat molecules,
there will be drop of the signal in out-phase images.
In the normal fatty marrow, a drop in signal on OP compared with IP should not be expected,
as the voxels contain only lipid without significant amounts of water.
Also,
lesions composed of virtually 100% fat will not show a drop of signal on out-of-phase sequence; fat islands,
Modic type 2 vertebral endplate marrow changes,
as well as fatty changes secondary to radiotherapy,
will not show a drop of signal.
A typical finding of a malignant bone lesion on MRI,
is the iso or hypointensity compared to that of the muscle or intervertebral disk on T1W1 sequences with corresponding hyperintensity on T2 FSE FAT SAT or STIR images,
and lack normal signal dropout on out-of-phase compared with in-phase images.
Non-neoplastic conditions that replace normal fatty marrow MR signal,
which include include red marrow reconversion,
edema,
traumatic injuries or and inflammation,
do not completely replace fat,
but generally preserve sufficient fat to allow a 20 % signal loss on out-of-phase imaging (2).
Loss of signal observe con OP compared with IP images can be appreciated visually,
but more precisely expressed as a quantitative measure using the relative SIR,
calculated with the formula: SI (IP) / SI (OP),
and can also be expresed as a percentage signal drop using: (SI (IP) – SI (OP)) / (SI (IP)) X 100. Many authors have stablished that those processes that replace or displace marrow will contain no intravoxel fat,
and will consequently have an SIR > 0.8 (signal drop < 20%) (3).
From the technical point of view,
rapid dual-echo gradient echo imaging allows fast acquisition at precise echo times to produce IP/OP images in a single sequence.