Influence of the chemical shift artifact on measurements of compact bone thickness in equine distal limb MR images.

The study investigates the impact of the chemical shift artifact, a distortion in magnetic resonance (MR) images, on thickness measurement of compact bones in horse limbs. The researchers found that the chemical shift artifact can induce errors in the evaluation of bone thickness during MR imaging, particularly when measurements are taken in the frequency encoding direction or from out-of-phase images.

Study Methodology

• In the experiment, researchers used six isolated equine (horse) limbs to understand the effect of the chemical shift artifact. This artifact typically arises due to misregistration or phase cancellation at the boundary between compact and trabecular bone types.
• Two types of MR images were taken: Sagittal T1-weighted spin echo (SE) and in-phase three-dimensional spoiled gradient echo (SPGR).
• These images were taken twice on a 1.5 T magnetic resonance unit, with change of frequency encoding direction between acquisitions. Additionally, out-of-phase SPGR images were garnered.
• The MR images gathered with different frequencies were compared against each other, as well as with radiographs of corresponding 3-mm-bone sections.

Findings

• The study established that the compact bone thickness was significantly different when comparing images obtained with different frequency encoding directions for both SE and SPGR sequences.
• When compact bone measurements in MR images were contrasted with measurements from thin section radiographs, significant differences were found in the frequency encoding direction but not the phase encoding one for the majority of the surfaced studied.
• However, when the MR measurements were corrected with the calculated chemical shift, these differences were nullified.
• Measurements of compact bone from out-of-phase SPGR sequences were significantly different than from in-phase sequences, with out-of-phase measurements typically sneaking up by an average of 0.38mm.

Implications

• The result suggests that the chemical shift artifact could lead to errors in MR assessment of compact bone thickness. This is especially the case when measurements are done in the frequency encoding direction or in out-of-phase images.
• For increased accuracy, the researchers recommend taking measurements parallel to the phase encoding direction and avoiding out-of-phase gradient echo sequences.