The SNR in the ankle and foot is usually good if using a
dedicated coil, such as the extremity coil or small surface coils. For all sequences, slice thickness should not
exceed 4mm to minimize partial volume effects.
No standard protocol has been established and the sequences and imaging
planes differ according to the diagnostic question. Generally, spin echo sequences are used to
examine the foot and ankle. These
sequences allow the acquisition of high-resolution images with T1, proton
density and T2-weigh. The major drawback
to spin echo sequences is the relatively long acquisition times especially for
T2-weighted sequences. The introduction
of fast sin echo (FSE) sequence has reduced scanning time considerably whilst
providing the same contrast properties as those of conventional spin echo. However, as already mentioned there are at
least two differences between CSE and FSE.
First fat appears considerably brighter in FSE than in CSE. This affects the detection of bone marrow
abnormalities. Thus it is generally
necessary to use fat suppression with FSE sequences. Secondly, there can be loss of high
resolution detail in FSE images at short TE.
This is most noticeable when using high ETL (more than 2) longer
interecho spacing and a smaller acquisition matrix (Kneeland, 1994).
Gradient echo techniques with 2D or 3D acquisitions may
be used in the ankle for the evaluation of cartilage and ligaments. 3D provides contiguous sections of less then
3mm with a high signal to noise ratio.
The major disadvantage of gradient echo technique is the loss of signal
within the bone marrow as a result of magnetic susceptibility artefacts. Similarly to other areas of the
musculoskeletal system, our radiologists prefer spin echo (FSE or CSE)
sequences because the resultant effects of modifications of the spin echo parameters
are well known.
Source:Joseph Castillo
No comments:
Post a Comment