Radiographic
Examination of the Equine Foot
W Rich Redding DVM, MS, DACVS
Clinical Associate Professor
Equine Sports Medicine Program
College of Veterinary Medicine
North Carolina State University
Raleigh, North Carolina
Radiographic examination of the digit is the
most common radiographic study performed by the equine practitioner.
The practitioner should strive to obtain good quality radiographic
equipment and develop good radiographic technique. In addition,
an accurate diagnosis requires an understanding of good working
knowledge of anatomy and pathophysiology of disease. The practitioner
needs to understand the complex anatomy of the foot and recognize
that there is a range of normal variations seen in many of
the anatomical structures in the foot. Good radiographic technique
necessitates an understanding of the relationship of limb
position, cassette position and x-ray tube angle. It is also
important to recognize that inadequate foot preparation, inappropriate
projections and motion (of the animal and examiner) can create
artifacts. Errors in interpretation often occur due to correctable
technical problems.
A detailed clinical exam should be performed before a specific
radiographic study can be recommended. The clinical examination
should include a complete lameness evaluation with specific
manipulative test, flexion tests, and hoof tester application.
Ancillary diagnostics such as palmar digital nerve block,
distal interphalangeal joint block, navicular bursa block
and digital flexor tendon sheath block may further direct
the clinical examination. The response to each one of these
blocks may direct the clinician to include more definitive
radiographic projections. Advanced imaging techniques such
as thermography, diagnostic ultrasound, and nuclear scintigraphy
can direct the clinician to perform a specific study or to
include specific projection to further elaborate an area of
interest. In the future, magnetic resonance imaging will become
more common but will not preclude the radiographic exam. It
is hoped that MR will validate certain radiographic changes
seen on the standard foot examination. Radiographic examination
of the digit will remain the common imaging technique between
the field practitioner and the referral hospital in lameness
and prepurchase examinations of the horse.
Equipment
The equine practitioner should develop a working knowledge
of the equipment that is currently available. There are a
limited number of systems available that meet the requirements
of an ambulatory practitioner (MinXray, Medison, Bowie, &
Sternes). Most of these manufacturers produce lightweight
high frequency units that have greater exposure capabilities.
Cost and physical characteristics (weight) of the equipment
may influence the suitability of a system for a specific practice.
It is preferably to purchase a system with light collimator
or laser pointer and a line voltage measuring unit included.
Rare earth systems are of great benefit in radiographic examinations
of equine limbs. There are many manufacturers of screens and
film (Kodak, AGFA, 3M, MCI OPTONIX). These systems can produce
high quality radiographs with portable radiographic systems
because they are much faster than traditional calcium tungstate
systems. These imaging systems can produce diagnostic images
with lower exposure times, which are necessary to minimize
motion artifacts and decreases radiation exposure to personnel.
The specific imaging system purchased should be based on several
factors beyond the scope of this paper but should be influenced
by the practitioner’s preference to have a good contrast or
good latitude film.
Computed radiography (CR) and direct digital radiography (DDR)
have recently been introduced to the veterinary market. There
are several systems available and many of these systems have
been in use in human medicine for the last decade. These systems
can be utilized with the high frequency radiographic systems
already in use with current radiographic imaging systems.
The major advantage of these systems is that the images have
more exposure latitude (more shades of grey) which makes it
possible to see both bone and soft tissue detail in the same
image. CR & DR systems have the added benefit of software
that allows the image to be manipulated (magnification, contrast,
brightness, cropping, edge enhancement) as well as allowing
the image to be measured and enhanced with drawings. The main
disadvantages include cost and the need for specialized training.
Safety equipment that should be incorporated in the production
of radiographs include film badges, lead aprons and gloves
(.5 mm lead equivalent), and aluminum cassette holders. These
safety items are necessary to minimize exposure of personnel
to scatter radiation. The holder and horse handler should
not be in the primary beam.
Labeling the radiograph
The radiograph is a legal document and part of the permanent
medical record for each horse. The legal requirements for
maintaining these radiographic studies vary by state and should
be determined before discarding any radiograph. In most states
the veterinarian is required to maintain the radiographs for
5-7 years. Because many horse owners are keeping horses for
longer periods of time it may be advantageous to maintain
the radiographs for the life of the horse. The owner’s name,
patient name or number, name of the veterinarian or clinic,
location of the clinic, date of the examination, and the limb
examined (RF, LF, RH, LH) are required. It is helpful to include
the specific projection used to generate each specific radiograph.
As a rule, the labeling marker should be placed on the lateral
and/or dorsal aspect of the plate.
There are several options to label the film but the most common
are radio-opaque marking tape with personalized holder blocker
and/or light flasher system.
Preparation of the foot
It is essential to clean the foot of dirt and debris before
performing a radiographic examination. The bottom of the foot
is not the only area that needs to be cleaned but should include
the heels and wall. In most instances shoe(s) removal will
be necessary to adequately examine the navicular bone and
P3 as well as to minimize scatter radiation (which can diminish
detail of the radiograph). Skill and the appropriate equipment
are necessary to remove the shoes without damaging the foot.
Loose flakes of horn from the sole and frog should be pared
away and particular attention should be paid to trimming the
clefts of the frog sufficiently to allow removal of dirt and
debris. Packing the sole and sulci of the frog with a material
like Play-do decreases gas artifacts from the trapped air.
This packing is especially important for the navicular and
P3 examinations. Unfortunately, packing may decrease detail
because it adds soft tissue density. Once the foot is prepared
it should be placed on paper or canvas to reduce contamination
of the packing material with dirt/debris. After the radiographic
examination, the foot should be covered with an elastic tape
has to prevent the walls from being damaged when traveling
from the area of the study to the stall and later to the farrier.
Removal of the shoes is not necessary when performing a podiatry
or laminitis study, which is constituted by a 00 Dorsopalmar
(DP), & Lateromedial (LM).
Positioning the foot for the examination
Blocks are needed to elevate the foot or feet off the ground
allowing the foot to be centered in the cassette and the x-ray
beam to pass horizontally through the specific area of interest
(i.e. solar surface of the foot, DIP joint, navicular, etc.).
The size of the block is dictated by the size of the radiograph
machine. The appropriate size block can be determined by measuring
from the floor to the center of the collimator and subtract
¾ inch. Ideally the blocks should have two wires embedded
in the top of the block which course longitudinally and perpendicular
to the first. Two blocks of equal height are needed to allow
the horse to bear weight evenly on both feet while performing
the radiographic examination(Figure 1). It is important to
position the feet on the blocks in a natural position with
the cannon bone vertical to the ground, axis of the limb directly
under the horse and the foot in line with the conformation
of the horse (i.e. if the horse toes in the blocks should
be placed in line with the foot direction). The foot should
be placed as close the inside of the block when taking a lateral
image (to place the cassette as close to the foot as possible
on it’s medial surface) to reduce magnification of the subject.
(Fig.1)
Radiopaque markers can be used to assess the
exterior surface of the dorsal hoof wall in the radiographic
study. Barium paste, wire, or a metallic marker should be
placed on the dorsal hoof wall from coronary band to the distal
margin of P3 (about ¾ inches from bearing surface)
to assess the hoof wall in the lateral view of the foot. Some
have used a quarter at the proximal most aspect of the dorsal
hoof wall close to the coronary band but this has proven inadequate
to assess the distal aspect of the dorsal hoof wall. A thumbtack
can be placed in the apex of frog after it has been pared
appropriately to determine the position of the true apex of
the frog with respect to the distal aspect of P3.
Positioning the X-ray beam
To appropriately position the x-ray beam the veterinarian
or technician must have a keen knowledge of the anatomy in
the area and a specific area of interest. For instance, to
produce the lateral view of the foot, the x-ray beam should
be parallel to the bulbs of the heel and centered on the specific
area of interest. The navicular bone examination should have
the x-ray beam centered below the coronary band midway between
the dorsal hoof wall and the heels. The solar surface of P3
will be imaged best by centering the beam ¾” above
the distal aspect of the hoof wall at the quarter midway between
the heels and the toe.
X-rays obey the inverse square law, which means a small change
in focal-film distance (distance between the x-ray machine
and the cassette or FFD) can have a significant affect on
the exposure (diminishes number of x-rays with distance) (Figure
2). Because a small change in FFD can have a significant effect
on exposure, it is imperative that this distance be consistent
and should be measured on every exposure. This distance can
be measured indirectly in systems with a collimator. The examiner
should establish the correct FFD between the machine and cassette
and then setting the collimator to the appropriate cassette
size. On subsequent exposures the collimator light can then
be adjusted to fit on the cassette (provided the cassettes
are the same size). It is also possible to place to laser
pointers on the x-ray machine that are focused together at
the FFD.
(Fig. 2)
Subject-film distance is also an important variable
that should be consistently evaluated and minimized. In most
instances, the cassette can be placed as close to the subject
as possible. If the plate cannot be placed close to the subject
it creates magnification of the subject which reduces image
sharpness. Beam angle should be 90 degrees to the cassette
whenever possible. When the beam angle varies from perpendicular
subject distortion can be a consequence. The subject should
also be parallel to the plate to minimize distortion. Remember
that the central beam position dictates the area seen with
most clarity
Radiographic examination
Standard radiographic study of the
foot
Lateromedial
Dorso 65 degree proximal-Palmarodistal Oblique ( D45Pr-PaDiO
P3 technique)
Dorso 65 degree proximal-Palmarodistal Oblique (D65Pr-PaDiO
navicular technique)
Dorso 45 degree proximal-Palmardistal Oblique (D45Pr-PaDiO)
Palmaro 47 degree proximal-Dorsodistal Oblique (Pa45Pr-PaDiO
navicular skyline)
Dorso 0 degree palmar ( D0Pr)
Lateromedial
To obtain a lateromedial radiograph (Figure 3) the foot should
be positioned on a block of appropriate size (dictated by
the x-ray machine). The foot should be centered in the middle
of the cassette. The cassette should be placed as close to
the foot as possible. Initially, a survey film should be obtained
to include the entire foot and pastern. This is accomplished
by centering the beam on the navicular bone which is one centimeter
below the coronary band midway between the heels and the dorsal
hoof wall. The x-ray beam should be directed parallel to floor
centered. If a specific region of interest (previously determined
by the clinical exam or advanced imaging techniques) has been
determined then the x-ray beam should be directed at this
site. Typically, the x-ray beam position varies between the
solar surfaces of P3 (½ to ¾ inch above bearing
surface of the foot for podiatry and laminitis studies) to
more proximally at the coffin joint/navicular bone (beam centered
at the middle of the coronary band). The beam angle should
pass parallel to the bulbs of the heel. The radiograph demonstrates
the dorsal and palmar aspect of P3 and the end on view of
the navicular bone. With appropriate technique the dorsal
hoof wall should be apparent with most normal horses having
a thickness 15-18 mm (for most breeds). The proximal and distal
border as well as the contour of the flexor surface of the
navicular should be easy to discern when the projection is
centered at this area.
(Fig. 3)
Dorso 65 degree proximal-Palmarodistal
Oblique
The dorso 65 degree proximal-palmarodistal oblique (Figure
4) can be obtained with the foot in a weight-bearing position
or with the foot on a navicular block (upright pedal). For
the weight-bearing technique the foot is placed on a tunnel
to the front of the cassette and the beam is directed 65 degree
to the floor. With this technique the beam is directed 2 cm
proximal to the coronary band. For the upright pedal the foot
is placed on a block that holds the cassette while allowing
the foot to be placed such that the toe is 85 degrees to the
ground surface. This allows the x-ray beam to be directed
horizontal with the ground centered 2-3 cm proximal to the
coronary band on midline. The cassette is placed as close
to the solar surface of the foot. The upright pedal view creates
less distortion than the weight-bearing technique. This oblique
is utilized because of good visualization of the body, solar
margin and palmar processes of the distal phalanx.
This same projection can be modified to evaluate
the navicular bone (Figure 5). Two different views can be
used to aid interpretation and differentiate artifacts. The
navicular technique should have the beam collimated so that
navicular bone is the primary area of interest. Most radiologists
recommend the use of a grid (6:1 preferable but 8:1 acceptable).
Because this view of the navicular bone is especially susceptible
to artifacts it is necessary to thorough cleaning and packing
of the foot. The margin of P3 can be difficult to assess if
the technique is not appropriate. This projection of the navicular
bone provides the examination of the wings and body of the
navicular bone as well as the proximal and distal borders.
(Fig. 4)
(Fig. 5)
Dorso 45 degree proximal-Palmardistal
Oblique
The foot is placed on a block of appropriate size. For this
projection it is necessary to have a 45 degree slot cut into
the block to accept the cassette. The horse should stand with
the bulbs of the heel placed as close to the cassette as possible
to prevent unnecessary magnification of the digit. This technique
typically provides poor detail of P3 and the coffin joint
but does provides a good assessment of the proximal border
of the navicular bone. Good to evaluate the DIP joint, collateral
cartilages, axis of the digit and the proximal border of the
navicular bone (Figure 6)
(Fig. 6)
Palmaro 45 degree proximal-Dorsodistal
Oblique
The foot is placed on a tunnel which is placed behind the
opposite limb. The foot positioned as far caudally on the
tunnel as possible. The horse should have the majority of
it’s weight placed on the front limb. The x-ray machine should
be placed above the foot close to the thorax of the horse.
The x-ray beam should make an angle parallel with the flexor
cortex of the navicular bone (approximately 45-47 degrees).
Foot conformation can influence the beam angle by affecting
the flexor surface angle. Some clinicians take and develop
the lateral view first to determine the correct angle. It
is important to not superimpose the fetlock over the heels.
The beam should be centered on the bulbs of the heel. This
oblique projection of the navicular bone allows assessment
of the flexor cortex, medullary cavity and flexor surface
of the navicular bone as well as the palmar/plantar processes
of the distal phalanx (Figure 7).
(Fig. 7)
Dorso 0 degree palmar
The foot should be placed on the appropriate sized block to
allow the correct beam position. When performing the podiatry
examination the DP projection should have the beam horizontal
(parallel) and centered ½ to ¾ inch above the
bearing surface of the foot. The plate should be placed as
close to the bulbs of the heels as possible. The beam should
be raised if the navicular region is the primary area of interest
bone (coffin joint, navicular) in the DP plane (Figure 8).
(Fig. 8)
