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Comparison of
10mm slice (a.) and 1mm high resolution slice (b.)
in patient with coal worker's
pneumoconiosis CT angiography is
only possible with a spiral CT scanner due to its
speed of image acquisition. A large volume
(100-150ml) of contrast is injected into a
peripheral vein at a steady rate (c.3ml per sec)
and the scan timed to image the main pulmonary
vessels while the contrast bolus is in the
pulmonary arteries. Fairly thin slices are
obtained. Detailed analysis of the images allows
accurate diagnosis of many cases of pulmonary
embolism. CT pulmonary angiogram.
Thrombus in pulmonary arteries, outlined by
contrast, is indicated by arrows Thicker slices
allow greater coverage of the great vessels and
provides a quick and accurate method of assessing
possible aortic injury or dissection. CT angiogram. Axial image,
(a.) and sagittal reformat, (b.), show dissection
flaps in both ascending and descending aorta.
(2)
Contrast Intravenous
contrast in thoracic CT is used mainly to
demonstrate vessels. This is not always necessary
as the anatomy of many vessels in the chest is so
constant that they are obvious on the unenhanced
scan. However, problems may be encountered in the
mediastinum and hilar regions in distinguishing
lymph nodes and other soft tissue masses from
vessels and in these cases intravenous contrast can
be invaluable. Contrast medium should be
administered with care in asthmatic patients for
fear of provoking an acute asthma attack. Use of
contrast is contra-indicated in patients allergic
to iodine. Contrast can precipitate acute renal
failure in patients with impaired renal function
and such cases should be discussed with the
radiologist in advance. (3)
Windowing Windowing is
something that is done after the scan has been
performed and so can be changed at a later date if
necessary, provided one has access to the CT
scanner. Windowing is like very sophisticated
contrast and brightness controls. The amount of
information in the CT image is too great to be
presented completely on the monitor or film.
Because of this, a range of contrast and brightness
is selected by the radiographer for optimal
demonstration of the structures one is interested
in. In order to see lung parenchyma optimally
completely different windowing will be needed
compared to that used for the mediastinum. It
follows that one must have the correct windowing to
assess specific tissues. Contrast
enhanced CT scan of chest displayed on
soft tissue windows. Fat, muscle and bone
are easily distinguished by their
different levels of grey. Lung detail is
not displayed with these window
settings. CT scan
at approximately the same level displayed
on lung windows. Lung vessels, bronchi and
fissures are well seen but muscle and fat
cannot be differentiated. Bone is hard to
distinguish from soft
tissue. (4)
Reconstructions Because spiral CT
acquires a volume of data rather than individual
slices multiplanar reconstructions and 3D volume
techniques can be used to produce images that look
very impressive. Generally the most accurate data
is obtained by viewing the original axial slices,
but occasionally certain reconstructions can be of
use. There is some evidence that 3D reconstructions
of the pulmonary vessels provides a good method for
screening for pulmonary AVM's. Special computer
programmes also allow one to perform virtual
bronchoscopy, but it is worth remembering that the
images presented to the virtual endoscopist are
produced from the original axial images so that
anything present in the endoscopic tour is
generally seen more accurately on the original
axial slices. You don't get something for
nothing! 3D
surface shaded display of pulmonary AVM,
(a.) before treatment, (b.), after coil
embolisation. Small
arrow: feeding artery, Large arrow:
draining vein, Open arrow: embolisation
coil |
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