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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