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

2240 VII Imaging of the Musculoskeletal System
A
B
Figure 47-43. Method of obtaining weight-bearing lateral view. In
most radiology departments the x-ray tube cannot be lowered to the
floor. Weight-bearing lateral views can be obtained by having the
patient stand on a wooden box. The central x-ray beam (dashed
arrow) passes through the foot, from lateral to medial, striking the
x-ray cassette held upright between the feet.
C
Figure 47-42. Weight-bearing radiographic foot series in an
asymptomatic 41-year-old male volunteer. A, Anteroposterior view;
B, oblique view; C, lateral view. The upward-pointing white arrow was
placed by the technologist to indicate the patient was standing. The
heel spur (black arrow in C) at the origin of the plantar fascia is of
doubtful clinical significance in this normal volunteer who has never
had heel pain. (This person is not standing on screws, but on the
wooden box in Figure 47-43, held together by screws.)
reconstructed into relatively thin slices, often the width of
a detector element. To minimize the stair-step quantization
artifact that can occur between axial slices, the source
images should be reconstructed at intervals such that they
overlap each other. We have found that a 50% overlap
(interslice interval equals one-half slice width) works well.
We use an edge-enhanced reconstruction algorithm (called
a “bone” algorithm by some CT manufacturers) to yield
two-dimensional reformatted images with sharp cortical
detail.
Thin and overlapping source images also yield good
three-dimensional reformatted images. However, threedimensional
images by their nature represent a smooth
rendering of the volumetric data, and edge-enhanced
source images can yield excessively noisy threedimensional
images. We create a second set of source
images, using a smoothing reconstruction algorithm (called
a “standard” algorithm by some manufacturers) for threedimensional
reformatting.
Depending on the length of the body part being
scanned, the thin overlapping source images may consist
of hundreds, or sometimes thousands, of images—twice
that if both edge-enhanced and smoothed data sets are
created. At our institution we choose to store these source
images permanently on our PACS, although we save them
in a separate imaging folder from the multiplanar reformatted
images, which typically consist of merely dozens of
images per plane.
• Protocol for Foot, Ankle, and Tibia (Distal)
Scanning Technique
At the UW we have developed our “F/A/T” protocol—a single
scanning protocol that allows us to create multiplanar
reformatted images optimized to visualize the foot,
ankle, and distal tibia. (The latest versions of all the
UW musculoskeletal protocol sheets can be viewed
and downloaded for free at www.Radiology.Wisc.Edu/
MSKprotocols.)
The patient is positioned supine on the CT table, with
legs straight, feet together in the center of the gantry, and
toes pointing up at the ceiling (Fig. 47-45A). We typically
scan through both ankles and feet simultaneously because
this position is most comfortable for the patient and allows
us to compare the injured side with the contralateral
normal side when questions arise regarding subtle alignment
abnormalities. Unless the contralateral foot contains
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9/9/2008 5:34:17 PM