Biomedical Engineering Reference
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of R2*. A number of methods have been proposed to compensate for susceptibility
artefacts, including increasing spatial resolution, alternating slice selection, tailoring
rf pulses or utilizing 3D z-shimming. A method that combined 3D high-resolution
DB 0 maps and image post-processing was reported to provide improved R2*
measurements that would have been a twofold overestimate if left uncorrected
(Dahnke and Schaeffter 2005 ).
Besides improving cellular iron loading, improvements in hardware and image
processing can increase detection sensitivity. Nevertheless, it remains a considerable
challenge to detect small numbers of cells against native low signals from, for example,
haemosiderin or haemorrhagic artefacts, or the presence of metals such as calcium.
Furthermore, the detection of labelled cells is limited by partial volume effects, which
can be minimized by decreasing voxel sizes at the sacrifice of signal-to-noise ratios
(Heyn et al. 2005 ). A post-processing algorithm based on phase map cross correla-
tion was proposed for robust identification of labelled cells even in images with low
SNR (Mills et al. 2008 ). Deoxyhemoglobin in small, slow flowing vessels is another
source of native hypo-intensity that can have similar appearances to labelled cells,
especially in 2D images. The problem can be circumvented by the administration
of paramagnetic contrast agent prior to cellular imaging (Anderson et al. 2005 ) or
the use of carbogen inhalation (95% O 2 and 5% CO 2 ) to allow vasodilation by CO 2
(Himmelreich et al. 2005 ).
Acknowledgements EL was supported by project grants from Australian NHMRC, and JC
received salary support from Singapore NMRC (CSA/012/2009). Appreciation is expressed to
Debbie Ng for proofreading of the manuscript.
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