Ankle and Foot 47 - Department of Radiology - University of ...

2238 VII Imaging of the Musculoskeletal System
flexion and dorsiflexion. US is used to evaluate small
superficial structures that are sometimes difficult to see on
MRI, such as Morton’s neuromas or the plantar plate. US
can also be used to characterize soft tissue masses, particularly
to assess the degree of vascularity or to determine if
the mass is cystic or solid. US is also extremely sensitive
for the detection of subcutaneous foreign bodies in the
extremities, particularly wooden splinters, which can be
difficult to detect with radiographs, CT, or MRI. (A discussion
of US of the ankle and foot is beyond the scope of
this chapter.)
Nuclear medicine (NM) plays a limited role when it
comes to imaging the ankle and foot, although in certain
circumstances a bone scan can be helpful. Fractures of the
sesamoid bones of the great toe tend to be less conspicuous
on MRI than on bone scans, especially when the both-feeton-detector
view is used. In neuropathic feet in which
radiographs show Charcot changes of collapse and bone
fragmentation, a bone scan combined with a white blood
cell scan can be as sensitive and more specific for the detection
of osteomyelitis than MRI. (A discussion of nuclear
medicine is beyond the scope of this chapter.)
• Radiography
Because radiographs are a necessary first step in the workup
of the ankle or foot, let us now briefly review how these
should be obtained.
• Ankle Radiography
Ankle radiographs can be either weight bearing or non–
weight bearing, depending on the preference of the ordering
clinician. A standard radiographic ankle series consists
of three projectional views: anteroposterior (AP), mortise,
and lateral (Fig. 47-41). The mortise view is similar to the
AP view, with the leg internally rotated 15 degrees to obtain
a better profile of the ankle mortise.
When obtaining radiographs of the ankle, it is important
that the technologist include the base of the fifth
metatarsal on at least one view. Patients with fractures of
the base of the fifth metatarsal clinically present complaining
of lateral ankle pain, and this can cause the clinician
to request radiographs of the ankle rather than of the foot.
Figure 47-41 is such a case, where the Jones* fracture can
be seen at the edge of the lateral view.
*Sir Robert Jones (1857-1933) was the father of orthopedic surgery in England
and revolutionized the care of wounded soldiers during World War I. An early
proponent of x-rays, Jones imaged the transverse extra-articular fracture across
the proximal diaphysis of the fifth metatarsal just a few months after Röntgen published
“On a New Kind of Rays” (December 28, 1895). Jones first described this
fracture after having sustained such an injury himself “whilst dancing.” (This was
not ballroom dancing; rather it was “dancing in a circle round the tent pole” with
his military colleagues. There was no mention as to whether alcohol was involved.)
He subsequently identified this fracture on radiographs of two other patients and
published his series of three in the Annals of Surgery in 1902, “Fracture of the Base
of the Fifth Metatarsal Bone by Indirect Violence.”
• Foot Radiography
It is preferable to obtain radiographs of the foot with the
patient standing to visualize the bones in their weightbearing
alignment. 5 AP and oblique views (Fig. 47-42A and
B) can be obtained by placing the x-ray cassette on the floor
and having the patient stand on the cassette while the x-ray
beam is pointed downward. It is important to closely scrutinize
the alignment of the tarsometatarsal joints on both
of these views when assessing for a Lisfranc fracturedislocation.
Normally, the first metatarsal should line up
perfectly with the first (medial) cuneiform, the second
metatarsal with the middle cuneiform, the third metatarsal
with the third cuneiform, and the fourth and fifth metatarsals
with the cuboid.
The standing lateral view of the foot (Fig. 47-42C) is
somewhat more difficult to obtain because it is usually not
possible to lower the x-ray tube all the way down to the
floor. Consequently, we use a set of wooden steps (Fig. 47-
43). This elevates the feet to a level where the x-ray beam
can be oriented horizontally while the cassette is held
between the feet. Figure 47-44 is an example of differences
that can be seen between standing and non–weight-bearing
views.
• Computed Tomography
• Overview
Bone CT protocols have evolved as scanner technology has
progressed. In the broadest terms, a CT gantry consists of
a spinning ring on which is mounted an x-ray tube. The
tube emits a fan-shaped x-ray beam, aimed through the
center of the ring to an array of x-ray detectors mounted
on the other side. The patient lies on a padded table that
moves through the spinning gantry. With early generations
of single-slice CT scanners the gantry would spin clockwise
one rotation, then stop and spin counter-clockwise one
rotation to prevent tangling of the power cables supplying
the x-ray tube. The patient table would be stationary during
each of the scanning rotations while the gantry was spinning
and the tube was emitting x-rays. The table would
move between gantry rotations and stop at each slice position.
These were the days of true CAT, in which “A” stood
for “axial,” and scans consisted only of a series of axial
slides. Scans were relatively slow because time was lost
stopping the gantry’s clockwise momentum to reverse its
rotation.
With the innovation of slip-ring technology, tangled
power cables were eliminated and the gantry could spin in
one direction continuously while the table moved continuously
through it. Thus, helical CT (also known as spiral CT,
analogous to a spiral-sliced ham) was born. Now the data
stream coming out of the x-ray detectors no longer represents
individual axial slices, but rather a continuous volume
of patient imaging information. The raw data are then
reconstructed into a series of axial slices that we refer to as
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