Radiology of the Equine Digit
Falling Springs Theater, Versailles KY
In-Depth Short Course
R.F. Redden, DVM
June 1-3, 2007
Focus on the mission Before we take radiographs or review those of others, we must understand what we need to learn from film and how to use information that we were not expecting to find. To seek information we must have a very specific, repeatable method that delivers consistent imaging, horse after horse and foot after foot. There are many variables that can interfere with consistent imaging that we need to identify in order to achieve optimum information from film.
There is not a reliable standard set of views that adequately cover the broad spectrum of information we seek concerning basic pre-purchase exams, repositioning film and lameness exams, as each exam must be tailored to meet the demand of the specific information we seek. Taking repeatable views in an effective fashion is the first step to understanding the information we seek. Inconsistent imaging greatly diminishes our ability to learn the vast range of norm that exists between feet of various age, breed, diet, use and environment. That said, it is not surprising to experience frustration when trying to interpret the vast range of pathological lesions.
Farriers play a very intricate role with the overall health of the horse. As we all know, no foot, no horse. The meaning of this saying is quite obvious to veterinarians and farriers who team up to sort out career and life-threatening problems. Why do farriers need to know how repeatable, informative views are actually made? Having that knowledge greatly enhances their ability to learn to read the signature of the shadows as it relates to their task and responsible role as health providers. Farriers who can read and interpret vital soft tissue parameters that are influenced by the rasp, shoeing knife and shoes become a tremendous asset to the vet-farrier team, as unlimited options are made available through this information. On the other hand, veterinarians who have a good working knowledge of those same parameters and how they change due to growth, routine farrier maintenance and pathological syndromes greatly enhance their ability to provide the farrier with timely informative film and medical knowledge that is beneficial to the team effort. The benefits of knowledge are powerful and the basis of progress as vets and farriers use film as a common source of information relative to their roles as team members.
How to Make Consistent, Informative Film Standard views have been suggested as a relative means to accurately assess the health of the structures of the feet, the thought being that with a few standard views we can adequately reveal common pathological conditions that often affect feet. This is true in part if we simply look at film for what they can reveal. However, stepping out of the box and using our own x-ray equipment to answer specific questions we may have concerning a specific foot puts new life into our pursuit of discovery.
Beam selection is of utmost importance. There are several factors that we must consider when selecting projection angles. Radiation travels in straight lines much like light waves, but it leaves the machine in a way that is similar to shot gun pellets leaving a gun. Using this simple analogy let’s follow the path of each pellet, which traveling along a straight line penetrates some part of the subject (in this case, the foot) and becomes imprinted on the film, leaving information relative to how hard it hit the film, if at all. Each pellet creates a slightly different image relative to the angle it entered and exited the foot, how fast it was traveling and how much stored energy (mass and velocity) it had. Soft tissue exposures require less energy to penetrate than bone exposures. Therefore we will speak of soft, medium and hard penetration throughout this course.
The softest penetration reveals an image that clearly demonstrates the shape and densities of the hoof capsule. A severely underexposed film will reveal only the outside margin of the hoof. While even the hair will be visible at the coronary band, the bone image will not be identifiable simply because the energy behind the pellets was not adequate to penetrate bone. Thus, they never reached the film. The hardest penetration will reveal only the densest bone. Other softer or less dense areas will not be visible with normal fluorescent back lighting as the film was bombarded by the kinetic energy delivered by the overkill, so to speak.
Both extremes contain information specific to that degree of penetration. Soft tissue views offer valuable information for the podiatry team where harder penetrating views offer the veterinary surgeon and diagnostician valuable information concerning the state of health of the bone. Therefore, to gain optimum information relative to the soft tissue and bone, different machine settings and beam positioning must be considered to cover the large range. A standard technique that provides the information necessary to diagnose and treat a large variety of foot ailments does not exist.
How is the Image Created? To simplify the basic 101 geometry of radiation, we can visualize each pellet traveling along a unique pathway that was determined as it exited the focal spot or energy release center. Each pellet will enter and exit the foot at a slightly different angle than the one next to it. When it strikes the film, a series of small dots will be imprinted on the film that describe not only the angle the pellet took through the foot, but also the angle at which it hit the film. Much like the spray of pellets from a shotgun shell, radiation traveling from the center of the explosion has the shortest, most direct rout. When headed in a perpendicular plane with the cassette and film, the image produced at this point is quite pure relative to the distortion that is produced as pellets land at an angle relative to the cassette. With low MAS output portable machines, this area appears to be approximately 25mm in diameter on the film. Pellets that travel away from their center core create slightly different shaped dots when they contact the film at an angle. The further away from the center shot, the greater this angle and the greater the distortion of the image. Therefore the image transferred to the film is relative to the route the light beam or pellet traveled through the foot, meaning beam selection is the first priority.
Optimum images can only be created by knowing what you are looking for and placing the center beam on the specific area while remaining parallel to the ground surface. The foot must be on the same plane as the machine. Perpendicular film/beam relationship must exist in order to prevent unwarranted distortion of the image. Select the penetration energy (MAS and KV) that will offer the desired image (soft to medium to dark).
Film/Subject Distance The distance between the foot and film should be zero to prevent unwarranted magnification. Why is this so important? Using my soft tissue parameters protocol requires an accurate image to justify measuring precise soft tissue space zones. When the image is magnified, the horn-lamellar (HL) zone and sole depth (SD) measurements are also magnified. The farther away the plate is from the foot the longer the magnification. Having the cassette touch the foot sets a benchmark for the standard foot film. When shoes, pads, bandages, etc. hold the foot away from the cassette, you should make note on the film jacket as such. Otherwise future references to this film may be less meaningful.
The image created on the lateral view represents the sagittal section of the foot. A three inch foot (width measured at the widest point of the foot) will measure 1 ½ inches from center line to cassette while a 6 inch foot will measure 3 inches. If we saw a cadaver foot along the sagittal plane and turned the cut surface to the cassette we would find a smaller HL zone and a thinner sole than if we radiographed it intact, simply due to magnification created by the distance from the center line to the cassette. Of course we cannot overcome this degree of magnification, but we can certainly control the film/subject distance with every exposure. Make zero the standard 100% of the time.
Let’s back up and realize the importance of how to position the horse. Sedation when possible or feasible is a tremendous aid for taking properly positioned exposures. Use two blocks made specifically for the x-ray unit of choice. Preferable beam height is ½-¾ inches over the block. Why should we use two blocks? Did you ever wonder what a lame horse was thinking as you tried to make him stand on one block for the sake of getting radiographs of his lame foot? One block is not compatible with pain tolerance and often requires heavy sedation, twitching with lip chains or even blocking, all of which are totally uncalled for as they hide enough pain response to take the film, but in turn potentially create a larger problem than you had to start with. Compassion for the animal must always be foremost in our minds. Using two blocks makes the process much easier.
A very small dose of sedation, .25 to .3cc IV of Dormosedan normally works quite well for most cases. Set one block next to the sound foot and the other beside the lame foot. Let the horse stand very quietly until he reaches optimum sedation. He will normally shake his head just before sliding into a relaxed position. Quickly push him over onto the lame foot, pick up the sound foot and aim it for the center of the block. This has to be a quick, calculated move, which you will get quite good at with experience. Quickly go to the lame side and place the foot on the block, as it is now basically unloaded. The horse will be quite willing to bring it up to the same level as the other foot. Observe the stance of the animal before picking up the feet. If he stands toed out in one leg or the other, place the blocks in that position and note the chest width. If, for example, there are 2 inches between the legs, set the blocks 2 inches apart.
The goal is to obtain as a true a representation of static full leg stance alignment as possible. A visual plumb bob alignment of the limb in both lateral and DP views should be part of your standard protocol. Always strive to avoid misrepresentation of the digital alignment, joint space and medial-lateral balance, which can occur as a result of less than perpendicular load stance. Exception to this standard is when the horse is quite lame. In these cases, you should have already thoroughly examined the foot and developed an idea of what you are looking for, and may prefer not to cause further pain or unwarranted problems. Setting the lame foot way out front on one block can offer adequate information to help rule out fractures and extremely thin soles (found with laminitis, white line disease and other problems) before you block the horse for more informative views. Keep the horse’s head straight ahead when taking lateral and DP views for balance or soft tissue parameters, as the toes follow the nose of the horse. When the horse looks left, the limb torques left, and when the foot is stationary joint space and lamellar attachment changes even in the healthy foot relative to the increased load. Make this part of your standard and you will never have to second guess why the joint space is somewhat larger on one side than on the other, or if the medial listing is real or created by the turned head.
Shoes on vs. Shoes Off Contrary to what we have all been taught, lateral and DP soft tissue views with the beam just below the palmar surface and the shoes on offers more information than views taken without the shoes. The image taken with the shoes on provides valuable information concerning the relationship of SD, palmar angle (PA) and digital breakover (DB) to the existing shoe. Knowing when the shoe was applied and having a working knowledge of farrier science and natural foot growth makes this information more valuable. A horse with a SD of only 6mm six weeks after being shod presents a completely different history from one with 6mm of SD that has been shod within the last six days or hours. Having this information might lead you to a better solution. Simply knowing that a horse has a very thin or bulging sole prior to pulling the shoe can save a ton of grief for the animal and greatly enhance your effort to help him as you know not to subject the horse to the trauma of standing on an x-ray block or walking around without the protective benefit of the shoe.
Farriers who are familiar with the silhouette of the hoof and can interpret SD, PA and DB on the lateral and DP view with shoe on can use this information to better enhance the healing environment. These three parameters are what all farriers should be aware of before trimming the problem foot, fabricating a shoe and applying it. Using this information in an effective fashion requires time and experience working with film. Farriers must first learn to read the silhouette of the hoof, be dependant upon veterinarians to take the views most informative to them and understand the importance of knowing how the rasp, knife and shoe influence the overall health of the foot. This is a tremendous job for all of us, to say the least.
There are views that offer more information when taken with the shoes off. All 65 degree DP views and the tangental or skyline views of the navicular bone should be made with the shoe off in order to avoid unnecessary scatter radiation and superimposition of the shoe over the subject.
Cleaning the Foot When farriers are present it is best for them to clean the foot up. How well it needs to be cleaned up depends on the answers you seek. All mud and debris should be routinely removed from the hoof, sole and hair along the coronary band and pastern. This will suffice for most all lateral and DP scout views when assessing balance and soft tissue parameters that can relate to several soft tissue lesions. When navicular problems, fractures, keratomas or subtle bone remodeling are suspected, the foot should be cleaned of all irregular tags. Exfoliated sole and deep crevices often found in the base of the frog should be trimmed in a fashion to help clean the air pockets that are often interpreted as fractures of the navicular bone. Visualize all areas that will be superimposed over the subject area and you will know how well the foot must be cleaned up.
Prepare the foot for the answers you seek. Using your x-ray unit to confirm your tentative diagnosis can help you tremendously as you search for a solution. Taking film for the sake of seeing what you can find is not always productive, as often times it sets us up for misinterpretation of pre-existing lesions that may not have any significance at the current time. Radiographs without a history and a physical exam have limited information relative to the state of health of the foot. Of course there are radiographic lesions that are pathognomonic for several syndromes, but there is also a great deal of information that lies in the shadows, that when coupled with the exam and history can help us assure a more accurate assessment and open the door to options that would otherwise not be available.
How to Perform a Venogram
The equipment used at IEPC is a 100/30 MinXray portable unit, asymmetric 6/12 screens, Fuji Super HR-U 30 and 3M Ultra Detail film and a 6:1, 106 line/inch grid. Several contrast mediums have been used in the past, however we have found that higher concentration produces slightly more information. Reno-60® has been the contrast of choice for the past several years. A variety of catheters have been used. The 21-gauge butterfly is easy to use, easily procured and provides a consistent means of delivering contrast.
Sedation of the horse, local analgesia of the foot and a tourniquet placed over the fetlock is required. The original venogram technique developed at IEPC was a collaboration with Dr. Chris Pollitt using his previous in vitro study model and has since been published with minor modifications. 2 The imaging sequence that has been routinely used is soft tissue lateral, lateral with grid, DP with grid, DP with soft tissue detail, followed by a soft tissue lateral. The procedure is technique sensitive in large part because the sequence of film must be taken within a time frame of 45 seconds following contrast injection. Tissue contrast injected retrograde into the palmar vein is quickly absorbed into the interstitial space, significantly reducing the value of the information obtained.
The soft tissue lateral images have proven to be especially valuable because they allow us to see coronary papillae, circumflex vessels, solar papillae and acute and chronic lamellar leakage. The grid used with the higher MAS has proven to be a reliable means of imaging the terminal arch and its tributaries. The soft tissue and hard penetration grid views were implemented to allow us to evaluate normal vascular anatomy and structural alterations within the soft tissues and bone. The soft DP view allows us to evaluate the medial and lateral coronary supply and the medial and lateral circumflex network. The grid view offers further information concerning the terminal arch and tributaries. The soft tissue lateral view taken at the end of the sequence was implemented to allow for adequate filling time required to consistently image lamellar leakage, decreasing the possibility of missing significant structural alterations. More specific beam selection is required to image vascular lesions associated with White Line Disease and Keratomas. Techniques for imaging the equine foot will continue to evolve with increasing experience performing venograms.
Tourniquet failure and perivascular injections are frequent complications encountered in the infancy of procedural skill development. Both complications result in underperfusion and can lead to grossly inaccurate interpretation and misuse of the information. Technique underperfusion should not be confused with stark loss of contrast caused by pathological vascular compromise. The distinction should be well understood before deciding the fate of a laminitic horse based on the information gathered from this valuable tool.
Technique Sedate the horse. Block the feet just above the fetlock. Use only 4-6cc of blocking agent to prevent transient edema. Place the horse’s feet on proper positioning blocks. This will insure a pure lateral projection. Set the x-ray machine in place, and have all the necessary cassettes and grids within arm’s reach. Take a scout film with soft-tissue detail, using a barium paste marker on the face of the hoof wall. Wrap 4” Elastikon around the fetlock. This will provide an anchor point for the tourniquet and prevent twisting of the skin while applying the tourniquet. Place a tourniquet over the fetlock. Avoid a mid-cannon tourniquet. Catheterize the palmar vein using a 5/8”, 21-gauge butterfly catheter. Be careful not to thread the needle to far in to the vein. You risk making a second hole in the vein. Inject 20cc of Reno-60. I recommend using two 12cc syringes instead of one 20cc syringe. A 20cc syringe builds too much back pressure and complicates injection. Injection needs to be completed in less than one minute, as the contrast will leak from the vessels quickly and skew your interpretation. Pull the knee forward slightly while injecting the second syringe. The heel should remain flat on the block. This rocking assures lamellar perfusion by unloading the deep digital flexor tendon (DDF) Clamp a hemostat on the catheter. Quickly tape the hemostat to the leg using the loose end of the tape that secures the tourniquet. Take your series of film. All exposures should be taken within 45 seconds. Move quickly. Radiographic Views Lateral, soft exposure Lateral, hard exposure with grid DP, hard exposure with grid DP, soft exposure Lateral, soft exposure Soft exposures are needed when there are significantly compromised vessels. Hard exposures offer a diagnostic image of the terminal arch and deeper vessels. The last DP view offers a look at vascular leakage that may not be apparent in the first couple films. Acute and chronic cases will exhibit similar patterns, yet there are distinct differences between the two.
Using the digital system, you may elect to take a mid-range penetration. Bare in mind though, all units have constant adjustment limits. The goal is to see the smallest vessels, tubules and solar papillae as well as the terminal arch, which requires considerably higher MAS. Remove the tourniquet and place cotton or gauze over the medial and lateral vessels. Tape in place for 5-10 minutes.