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Great questions! Let me start with the differences between the scanner
capabilities and why some protocols may be different. The 16 slice
scanner is capable of 20 mm of scan data in the Z Axis at 1.25 mm slice
thickness. The VCT scanner is capable of 40 mm in the Z Axis at 0.625 mm
slice thickness. On the 16 slice scanner in order to scan at 0.625 mm
thickness you must scan utilizing the center channels and this allows
you a maximum of 10 mm in the Z Axis scan capability. The detector
material on the 64 channel system is an improved material from the 16
slice system with better special resolution. These factors would,
obviously, make for some inherent differences in Radiologists protocol
considerations. Another factor that needs to be a part of the scan
parameters is pitch (table speed) to detector configuration.
Next is your question on exam split, explicitly in a neck, chest exam.
First part of that is it entirely up to the Radiologists to decide how
this anatomy needs to be scanned. There are very explicit conditions and
pathologic considerations to be factored in the decision process.
Another part of the equation is the artifacts that are potentially
produced by allowing the very dense make-up of the humerus bones in the
scan field. Probably the biggest consideration is: Do the carotid
arteries need to be visualized as close to CTA quality or is there a
potential cancer factor that may need for the soft tissues of the neck
to have time to absorb the iodinated contrast material? The chest part
of the exam can usually be of very good diagnostic quality in the venous
phase and it does not have many structures that would need the arterial
phase to be the first thing visualized. This requires a good history in
order for the radiologists to correctly decide how to protocol the exam.
I believe it is perfectly acceptable to acquire the scout images with
the arms up or down and to cover the entire anatomical range in the
AP/Lat images and then to use the same scout for setting up the scan
ranges for each part of the exam. Most commonly seen is scouts with the
arms down to better visualize the soft tissue structures in the neck and
for the neck portion to be scanned first followed by the chest after a
pause to reposition the arms. Again, this is totally a decision to be
made by the supervising Radiologists. In the above described scenario it
is common to inject one portion of the contrast for the neck and then
the rest is injected for the chest to "fill in" the arterial structures
as everything is basically into recirculation by that point in time. I
have seen the exam done with one injection with the prime consideration
of timing being for the neck, but this can only be a decision made by
the Radiologists (I think I have beat that point to death by now!).
Again great questions and it is always good to examine the protocols
and the processes used on a routine basis. I am with you in thinking
that the " That is the way we have always scanned things" answer is
unacceptable. I would encourage you to talk to your supervisor if this a
real concern of yours and you want a more detailed answer. I would also
encourage you and your supervisor to talk to the Radiologists about
doing a serious review of all the protocols so that everyone is
comfortable with the how's and why's to the setting of scan parameters
and to see if anything can be adjusted if needed (or wanted). Please
approach these discussions in a completely professional manner and to
address the situation as an opportunity for everyone to educate, or
re-educate, themselves on the capabilities of the scanners and their
scan parameters. You seem to want the best process to be used on your
patients so please do not forget to also use the ALARA principle in all
your radiographic endeavors. Be a champion to help reduce dose as well
as scan correctly. Good Luck!
Jack Risner
CT Product Clinical Specialist
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