5 tips for improving thoracic spine
x-rays
On AP thoracic x-rays,
what do you do when the upper half of the film looks too dark, and the
bottom half looks too light? Or what if
the lower thoracic spine has barely been penetrated, and the ribs are
burned out?
On lateral thoracic
x-rays, what do you do if the area above the diaphragm is too dark, and
the area below the diaphragm is too light? Or
what if you can’t visualize the upper thoracic segments?
These are certainly common problems. Although thoracic spine x-rays are easy to
produce as far as patient positioning is concerned, the anatomy of that
region presents technical problems that make it difficult to obtain a
good quality image. Some
pointers are presented below that will go a long way toward solving
these common problems.
1. Cathode
Down
As a one-time item,
check to see that (for upright films) your anode is uppermost and your
cathode
is lowermost. (Yes, it does
make a difference!) Sometimes the
tube is clearly marked with “anode” and “cathode.”
If not, it may be marked with “+“ for anode and a
“-“ for
cathode. If your tube has neither
of these designations, just look for the small electrical cord which
goes to
only one end of your tube. This is
the electrical circuitry for the rotating anode, and it therefore
indicates
which end of the tube should be uppermost.
The strength of the
primary beam is greatest toward the cathode end of the tube.
2.
Filters
Acquire a set of
aluminum wedge compensating filters to use at your tube (in front of
the
collimator). The thick part of the
filter attenuates/decreases the beam before it reaches the thinner body
region,
while the full amount of the beam
exits below the filter so as to reach the thicker body region.
For APs, measure the
patient at his widest point (abdomen?), and then place the
filter with
the thick side up. Placing
the filter in front of the collimator will block the collimator light,
and you
can therefore see to bring the filter shadow down to about the inferior
end of
the sternum.
For laterals, measure
the patient at the outside of his shoulders, and choose
sufficient mAs to
penetrate that very thick and dense region.
Place the filter in front of the collimator with the
thick side down.
Bring the filter shadow up to just under the axilla.
Even a single filter
is much better than no filter! Usually,
however, filters come in sets of three (thin, medium, and thick) or
even five,
and these are used according to the degree of discrepancy between the
thinnest
and thickest body parts in the field of view.
The thickest filter is usually chosen for thoracic
spine x-rays.
Various types of
filters have different ways of being attached to the front of the
collimator.
Some have a plastic frame with set screws.
Others use magnets. You
can
even velcro a filter to the front of the collimator!
It really is
not possible to obtain a decent upright thoracic
spine x-ray – that shows
all the thoracic segments – without a filter.
3.
Higher kV
Convert your exposure
technique chart to a somewhat higher kV range to obtain a slightly
wider gray
scale. This will diminish the big
contrast difference between the air in the lungs and the spine.
Obviously you don’t want too high a kV,
because adequate
contrast is required to visualize the osseous structures well.
For APs, kV should be
around 75-80 for high-frequency generators, and 80-85 for single-phase
generators*.
For laterals, kV
should be around 85-90 for high-frequency generators, and 90-95 for
single-phase
generators.
(*The reason for the
difference is that high-frequency generators produce an inherently
higher kV
that is truer to the stated value, whereas single-phase generators
produce a
lower average kV than stated due to the oscillation of the wave form.)
High kV/low mAs
techniques produce a wide gray scale, and low kV/high mAs techniques
produce a
contrasty x-ray.
Exposure charts should
be based on a standardized kV/variable mAs principle.
For a certain body region the kV stays the same for
all sizes
of patients, and the mAs increases as the size of the patient
increases. There
are different recommended kV ranges for different body regions.
Variable kV exposure charts should not be used.
It is incorrect to elevate the kV as the size of the
patient increases.
If this erroneous and backwards approach is used,
very black-and-white
contrasty films will be produced on little people and very gray films
will be
produced on big people. All your
x-rays – on all sizes of patients – should look about the same as far
as
image quality is concerned.
4.
Collimation
Collimate to a 7x17”
field of view. It is not possible
to obtain an AP x-ray that shows both ribs and thoracic spine optimally.
The lower thoracic spine requires roughly five times
the amount of mAs as
is required for the ribs that are above the diaphragm.
If your clinical interest is the thoracic spine,
x-ray only the spine; if
your clinical interest is the ribs, then x-ray the ribs, but they must
be done
separately. And both are separate
from the chest.
Please note that the
above recommendations are made for the purpose of obtaining good image
quality.
Tight collimation does not help to avoid
liability by limiting the
field of view. The field of
interest must be determined by the presenting complaint and clinical
question. If the examining/treating doctor
feels that there may be a
need for a wider field of view than the spine, he/she may either choose
to
perform the required additional x-rays, or he may choose to refer the
patient
out to have the required additional x-rays produced at another site.
5.
Full Inspiration
Take all thoracic
spine films in full inspiration. This
usually places the entire thoracic spine above the diaphragm, thus
ensuring that
the spine is surrounded by similar density of tissue, thereby providing
a more
uniform density film for the entire thoracic spinal region.
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