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

2242 VII Imaging of the Musculoskeletal System
Figure 47-46. Anteroposterior (A) and lateral
(B) scout CT views. The scanning field should cover
both ankles and should extend from above the
syndesmosis to below the calcaneus (white
rectangles). In cases of pilon fractures, more of the
distal tibia is covered (dashed rectangle).
A
B
We can achieve the highest resolution on the reformatted
images by reconstructing the source images at a width
equal to the width of the narrowest detector. The reconstruction
interval between source images is equal to onehalf
the detector width, and this allows for a 50% overlap
between slices. We reconstruct two sets of source images;
one uses an edge-enhanced bone algorithm for the twodimensional
multiplanar reformatted images, the other a
smoothing standard algorithm for reformatting in threedimensions.
Both sets of source images are sent to the
PACS for archival storage and can be accessed any time in
the future if additional two- or three-dimensional reformatted
images are desired. Because these thin overlapping
slices yield hundreds of source images, we store these
source images on the PACS in their own folder—a folder
separate from where we store the reformatted images,
which are images most of us primarily view.
Reformatting Technique
At the UW we have identified at least 15 different ways that
we are commonly asked to create two-dimensional reformatted
images of the ankle and foot. These are delineated
on our downloadable protocol sheets, portions of which
are shown in Figure 47-47. The reformatting protocols are
centered on the anatomic divisions illustrated in Figures
47-5 and 47-48.
Ankle/Distal Tibia Protocol. Our ankle/distal tibia protocol
(see Fig. 47-47A) is centered on the ankle joint. This
protocol is used for scanning fractures of the distal tibia
(e.g., pilon, malleoli, triplane, juvenile Tillaux) or of the
talar dome (e.g., osteochondral lesions). Using a midsagittal
reference image, straight axial images are created in a
plane parallel to the bottom of the foot. Then, using an
axial reference image through the top of the ankle mortise,
mortise coronal and mortise sagittal images are created
parallel and perpendicular to an imaginary line through
the anterior cortex of the medial and lateral malleoli. For
distal tibial fractures, we find that creating reformatted
images that are 3 mm thick at 3-mm intervals (no gap or
overlap between reformatted slices) yields crisp images
that do not appear noisy. However, for osteochondral
lesions of the talar dome, our surgeons prefer that the
mortise coronal and mortise sagittal images be reformatted
at 1 mm, yielding images of higher resolution but also with
more noise.
Hindfoot/Midfoot Protocol. Our hindfoot/midfoot protocol
(see Fig. 47-47B) is centered on the Chopart joint and
is used to evaluate hindfoot fractures (e.g., calcaneus, talar
body) and the subtalar joint (e.g., tarsal coalitions). Using
an axial reference image, straight sagittal images are reformatted
along a plane parallel to the long axis of the foot.
The other three planes are reformatted off a midsagittal
reference image. Straight axial images are reformatted in a
plane parallel to the bottom of the foot. Oblique coronal
and oblique axial images are reformatted in planes both
perpendicular and parallel to the posterior facet of the
subtalar joint.
Forefoot/Midfoot Protocol. Our forefoot/midfoot protocol
(see Fig. 47-47C) is primarily used to assess the alignment
of the Lisfranc joint and the integrity of the adjacent
bones. We find that it works best to create reformatted
images in three planes relative to the first metatarsal shaft.
A sagittal reference image that best delineates the entire
length of the first metatarsal is selected. Long-axis and
short-axis planes are reformatted both parallel and perpendicular
to the sagittal length of the first metatarsal. The
third plane, sagittal to the first metatarsal, is best obtained
off an axial reference image that has been obliqued to
include the entire length of the first metatarsal.
Navicular Protocol. Our dedicated navicular protocol
(see Fig. 47-47D) is used to assess the healing of a known
navicular fatigue fracture that has perhaps been previously
diagnosed by MRI. Because these navicular fatigue fractures
tend to be incomplete hairline cracks, we increase the resolution
by creating thin (1 mm) reformatted images in a
small (6 cm) FOV. Oblique coronal and oblique axial
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9/9/2008 5:34:23 PM