Radiographic Alternative for Opening Joint Spaces

We all know the basics of opening joint spaces on our radiographs.  Keep the joint space opening perpendicular to the image receptor while aligning the central ray directly to the joint.  No problem... x-ray 101 right?  But what about those joint spaces like wrists and ankles?  You know, the ones where some of the radiologists say "you should have gotten more of the tibia (or forearm)!"  Sometimes it's good to include a little extra - especially if your radiologists prefer it when there is obvious deformity.

The problem lies within x-ray beam geometry.  I was going to make a video to demonstrate a basic principle in beam geometry, but Peter Gleeson did an exceptional job on his YouTube channel... watch his short video before we move on to some possible solutions.

Now that we know a tube angle does not change the distortion of an object because the x-ray source does not change position, we can evaluate our options... below, we have a traditional positioning setup for an AP ankle.

Sometimes we need a bit more of the distal tib-fib.  One proposed solution I have seen is to simply move the joint to the bottom of the image receptor, while placing the central ray a few inches above the joint space.  This accomplishes our goal of including more of the distal tibia, but can also close the ankle joint because it no longer lies directly in the central ray.  The beam is divergent at the point of intersection with the joint.

I have seen the following method used in another state that does not have very strict guidelines on radiation protection.  The tech would center to the joint space, but open the collimated field beyond the edge of the image receptor.  I do NOT recommend this for ALARA purposes, but I thought I would throw this example in there because I have seen it before.

I propose we can take the best of both worlds with the following solution.  Start out with the central ray over the joint space as in the previous example.  Without moving the x-ray tube (longitudinal, transverse, or vertical) from its current location, angle it cephalic so the crosshair is a few inches proximal to the joint space.  Collimate vertically so the field size does not go beyond the image receptor.

If you watched the video, you understand that the actual source of photons does not change with the tube angle.  There is still a perpendicular path of x-ray photons opening the joint space, while we include more of the proximal tib-fib.  The only disadvantage is there will be some elongation of the tibia near the periphery of the field of view.  But also take note that the same amount of elongation would occur in the prior method due to the same beam divergence.

Don't believe me?  Take any 3-dimensional object and place it directly under your perpendicular central ray.  Open the collimation and angle the tube in either direction.  You will notice (just as in the video) that the object's shadow does not move.  As long as you do NOT move the tube from its current location prior to tube angulation, the beam divergence will be exactly the same with the tube angle as if you simply opened the collimation.

I should mention that the BEST way to avoid this whole scenario would be to have the physician order a tib-fib (or correct corresponding body part).  But since we as technologists cannot order the exams, sometimes we have to make the best out of imperfect situations.