Susceptibility mapping of air, bone, and calcium in the head
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PURPOSE: To demonstrate the mapping of structures with high susceptibility values, such as the sinuses, bones and teeth, using short echo times. METHODS: Four in vivo datasets were collected with a gradient-echo sequence (TE1 = 2.5 ms, TE2 = 5 ms and TE3 = 7.5 ms). Complex division was performed to remove the phase offset term and generate the phase at TE = 2.5 ms. Susceptibility maps were generated from unwrapped phase images, using a geometry-constrained iterative algorithm, by preserving phase information in the extracerebral tissues. The susceptibility results were improved by updating the missing phase information inside structures with no signal using the predicted phase at each iteration step. Simulated phase images of a three-dimensional brain model and tooth phantom were used to validate the proposed method. RESULTS: Improved susceptibility maps were obtained once the phase information in the extracerebral tissue region was incorporated, for both the model and in vivo data. For in vivo data, the average susceptibilities of air (sphenoid sinus), bone and calcium (teeth) were found to be (in ppm): Δχ(sinus-tissue) = +9.2 ± 1.3, Δχ(bone-tissue) = -2.1 ± 0.6 and Δχ(teeth-tissue) = -3.3 ± 1.2, respectively. CONCLUSION: High susceptibility structures with little or no signal can be imaged using quantitative susceptibility mapping and can be used to improve background field removal.