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Understanding Computed Tomography- Part 2

Post contrast bone or lung algorithm CT images are not useful and their reconstruction should be avoided.

It is important to recognize the effect the subconscious mind has on the radiology reading experience. Thin-slice, bone or lung algorithm (high frequency kernel) CT images with high mAs settings and viewed with a wide window have a very pleasing appearance to us. We can see amazing bone detail, there is no visible noise, and it somehow looks like a nice MRI image or a beautiful photo shot with a Leica camera. Many readers prefer this for reviewing the spine in dogs and cats for instance.

Many viewers also claim that they can see the spinal cord better with these settings. Unfortunately they are falling into a trap. With these settings soft tissue contrast is markedly depressed, and visible soft tissue margins most likely represent rebound artefacts.

The figure above shows three transverse CT image of the T13 vertebra of a large breed dog.

The left image is a high frequency kernel image, which is aesthetically pleasing and reveals much bone detail. It also seems show a swollen spinal cord, separated by the vertebral canal by only a thin darker rim of presumed epidural fat. Note that the rim continues also in the ventral aspect of the canal, where one would expect soft tissue dense venous sinuses.

The central image is the corresponding medium frequency kernel, which to most people has a much less pleasing appearance and has resemblance to early images of CT from the 1970's and 1980's. The spinal cord is smaller on this image and the darker epidural fat margin is wider and less perfectly distributed than in the high frequency kernel image.

The right image is the corresponding myelographic CT image, demonstrating that the central image accurately depicted the dural sac and cord whereas the thin dark rim in the high frequency kernel image is a false representation of the spinal cord or dural sac. These rebound artefacts are frequently seen in the calvarium , vertebral canal, other bone edges and the pleural space.

This demonstrates that it is important to be aware of the limitations of each setting combinations and to review CT images with an informed mind and not let by aesthetic considerations.

For teleradiology interpretation we do not require any post-contrast lung or bone algorithm CT series.