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Fig. 14 Sagittal images of the foot demonstrating non-uniform fat saturation. In (a) a sagittal PD
fat saturation (FS) demonstrates increased signal intensity within the metatarsal bone distally, as
well as within the phalanges and surrounding soft tissues (solid arrow), suggestive of bone marrow
and soft tissue edema, and so osteomyelitis with cellulitis. However, in the STIR image (b), there
is no increased signal in these regions, and additionally, in T1 image (c), there is no commensurate
decreased signal within the bone (
(fluid is low signal on T1) to suggest edema. Thus, the increased
signal in the toe on the PD FS image is most likely due to non-uniform fat saturation. Note that fat
saturation is maintained in the midfoot bone structures on the PD FD image (dashed arrow),
leading to the term
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non-uniform fat saturation.
with
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fluid detection characteristics to optimize the assessment of
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fluid associated
structures such as the spinal canal.
The ability of T1 sequences to visualize and assess tissue pathology may be
enhanced by gadolinium contrast administration. As previously discussed, X-rays
interact with CT iodine-based contrast, to enhance the differentiation of tissue types
and allow improved assessment of the physiologic processes and anatomic struc-
tures being visualized. MRI contrast materials, on the other hand, possess magnetic
characteristics allowing them to interact with the magnetic
field applied through the
scanner. The agents most commonly used are gadolinium chelates, which possess
paramagnetic properties (Fig. 15 ).
There are other forms of specialized MR sequencing which are also of interest in
spine imaging. Diffusion tensor imaging (DTI) and Diffusion weighted imaging
(DWI) are two examples of these specialized MR series, which measure the dif-
fusion of extracellular
fluid molecules through tissue using magnetic properties of
the tissue and organ anatomy imaged. This extracellular diffusion of
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fluid molecules
indirectly indicates the motion of water in tissue on the molecular level. DWI is
usually performed in association with apparent diffusion coef
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cient (ADC) imag-
ing. ADC is a tensor characterizing the diffusion mobility, or diffusion magnitude
[ 12 ]. Combined ADC and DWI imaging clarify the etiology of increased DWI
signal as due to restricted diffusion or
T2 shine through
(increased T2 signal due
to
fluid in the tissues). In clinical practice, DWI is used in the assessment of active
demyelinating lesions in the spine, where foci of high signal intensity indicate
restricted diffusion. Other uses of DWI in spinal imaging include assessment of
cord infarcts, as well as increasing sensitivity for detection of osseous lesions. In
combination with ADC, DWI can help differentiate acute traumatic cord injury
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