Tierärztliche Hochschule Hannover Entwicklung von Methoden zur ...

SUMMARY
measurements of hydroxylapatite, bone meal and rabbit tibiae were conducted with both the
XtremeCT and a µCT 80 Scanner (Scanco Medical AG, Brüttisellen, Switzerland).
The pixel noise measured with the XtremeCT was, at 41 µm spatial resolution and depending
on the chosen scan parameters, between 255 and 980 HU. Study of the homogeneity of CTnumbers
of water revealed variations within the FOV of up to 17 HU. A high linearity of the
CT-number-scale at the interval of 0-8000 HU was observed (R 2 = 0,996). The contrast
resolution of the XtremeCT for a 1 mm bore was, at 41 µm spatial resolution, 128 HU at
maximum. Overall, the XtremeCT tended strongly to image artefacts, of which ring artefacts
and arcuated artefacts were most pronounced. Neither a temporally nor a locally constant
occurrence of the artefacts could be observed. The investigated magnesium alloys didn't lead
to metal artefacts in the XtremeCT.
For a complete segmentation of either the magnesium implants or the tibial bone, the
measured mean CT-number ± 3 × standard deviation (σ) turned out to be optimal. The
XtremeCT showed, depending on the amount of pixel noise, a more or less pronounced
overlap between the measured CT-numbers of the magnesium implants and tibial bone,
hydroxylapatite or bone meal. The alloy LAE442 and tibial bone showed a strong overlap of
CT numbers. The CT-numbers of alloy ZEK100 had a clear intersection both with the CTnumbers
of the Substantia corticalis as well as with hydroxylapatite and bone meal. Bone
meal and hydroxylapatite formed a subset of the CT-numbers of AX30 or MgCa0,8% in
images made with the XtremeCT. A distinction between tibial bone and LAE442 proved to be
very problematic with the XtremeCT. However, both bone meal and hydroxylapatite coating
could be readily distinguished from LAE442. ZEK100, AX30 and MgCa0,8% could with the
XtremeCT, depending on the chosen scan parameters, only be reasonably well told apart from
tibial bone tissue and hydroxylapatite or bone meal coating.
Using the XtremeCT, an integration time of 300 ms and 500/180° or more projections turned
out to be required for a good representation of both the implants and bone tissue. The mean
photon energy (40 keV) could not be altered in the XtremeCT because of a fixed tube voltage.
In comparison to the XtremeCT, the lower mean photon energy (28 keV) used in the µCT 80
showed clearly better contrasts between ZEK100, AX30 and MgCa0,8% on the one hand and
the Substantia corticalis, hydroxylapatite or bone meal on the other hand. For LAE442,
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