A high quality spiral CT exam of the musculoskeletal system requires tailoring to the body part being imaged and to the clinical question being addressed. Factors such as collimation, pitch, and reconstruction algorithm have an important influence on image quality and, thus, diagnostic ability. This paper describes some of the technical aspects and current musculoskeletal applications of spiral CT and points out potential future applications.
Since its inception in 1990,1 spiral (or helical)
computed tomography (CT) has been increasingly applied to the
musculoskeletal system. Although systematic comparisons of spiral
technology with conventional axial CT are generally lacking, spiral
CT is an excellent choice for many routine musculoskeletal CT
applications. This paper describes some of the technical aspects
and current musculoskeletal applications of spiral CT and points
out potential future applications.
Advantages of spiral/helical CT
Spiral CT allows rapid acquisition of
volumetric data of any body part of interest. In contrast to
conventional CT, spiral CT acquires images at an angled plane of
section during simultaneous patient translation and x-ray exposure.
This continuous acquisition of images is enabled because of the
slip-ring technology of the gantry construction which replaced the
former electrical cabling, allowing for continuous rotation of the
Transaxial images are then reconstructed based on interpolation
from the volumetric data set. This allows for reconstruction of
images at arbitrary intervals, providing overlapping images of the
area of interest and the potential for high quality multiplanar and
The advantages of spiral CT include the
elimination of respiratory misregistration and diminished patient
motion artifact due to the continuous scanning ability and rapid
acquisition of images during a single breath-hold. Because imaging
of bones and joints does not routinely require breath-holding, this
relative advantage over conventional CT is less for musculoskeletal
applications than for applications in the chest and abdomen. For
musculoskeletal imaging the major advantage is in reconstruction of
overlapping image sections for 2D and 3D reformation. This
attribute of spiral CT maximizes longitudinal resolution without
increasing radiation exposure.
Multiplanar reconstructions can be useful in the evaluation of
traumatic injuries in anatomically complex areas, characterization
of complex fractures and dislocations, and assessment of
They can be applied toward preoperative planning as well as to
postoperative or postreduction analysis and have relevance in both
the adult and pediatric populations.
The early disadvantages of spiral CT included
limited x-ray tube heat capacity, resulting in relatively low mA
settings for continuous scanning, as well as limited computing
power. However, with recent advances in both hardware and software
these limits have vanished. Current generation scanners produce
high tube current for exposure durations of up to 60 seconds, and
gantry rotation periods between 0.5 and 0.8 seconds per image are
becoming routine. Many manufacturers are also now providing
machines with multiple detectors, providing for faster throughput,
increased longitudinal coverage, or increased longitudinal spatial
resolution (thinner collimation) during a given acquisition. High
quality spiral CT images are thus routinely feasible.
Paramount to the success of spiral imaging is
the correct application of scanning parameters, such as
collimation, pitch, and reconstruction interval. The collimation,
or slice thickness, varies depending upon the body part being
examined. Compared with body applications, the longitudinal
coverage required for musculoskeletal imaging usually is limited.
In general, we favor the use of relatively narrow collimator
settings (3 to 5 mm in the larger body parts such as the hips and
pelvis, and 1 to 2 mm in the wrists and hands) and extend the scan
duration or pitch as necessary to provide for longitu-dinal
Pitch is defined as the speed of table
translation divided by the collimation. For musculoskeletal
applications pitch rarely needs to be increased above 1.5. As pitch
increases, there is the potential to decrease longitudinal
resolution due to slice profile broadening,
and to increase image artifacts.
Reconstruction intervals, or slice indices,
vary depending upon the body part being imaged and the goal of the
examination. Finer reconstruction intervals result in superior
multiplanar reformation and 3D rendering. For 2D reformations,
reconstruction should occur at approximately 50% overlap to
maximize longitudinal resolution. We typically utilize the high
frequency or bone algorithm for reconstruction in this setting. For
3D reformations, reconstruction should occur at 50 to 75% overlap
and should utilize the standard algorithm (for example, a 3 mm
collimation reconstructed every 1 mm).
While prolonging the scan reconstruction time and data storage
requirements, all generated images do not need to be filmed. We
film every other to every third image and review all images on a
soft-copy workstation. The quality of 2D and 3D reformations
improves with reconstruction of overlapping sections.
We now discuss acquisition and reconstruction
parameters for specific body regions. The parameters currently in
use are shown in table 1. For all applications, attempts should be
made to carefully position the patient and to limit the field of
view to the anatomy of interest. Intravenous contrast is typically
not utilized in the evaluation of trauma or articular disorders but
can be useful in the evaluation of muscle or soft tissue for
infection or neoplasm.
Imaging of the upper extremity can be
challenging due to the anatomic complexity of the joints involved
as well as patient positioning. In the shoulder, spiral CT can be
applied to evaluate pathology of the glenohumeral or
acromioclavicular joints or the scapula. Because of the
considerable x-ray attenuation due to the normal shoulders and
upper thorax, it is important to use relatively high tube current
settings to optimize image quality. We use 250 to 300 mAs and 120
kVp. Axial images are acquired with a 3 mm collimation and a pitch
of 1 to 1.5. Reformations are made in the oblique coronal and
oblique sagittal planes with respect to the glenohumeral joint
(figure 1). The reconstructions allow ready identification and
characterization of glenoid fractures (figure 2), and humeral head
fractures and dislocations (figure 3). These reconstructions can be
critical in the evaluation of injuries in which a patient's
positioning and limited range of motion make interpretation of
axial images alone difficult (figure 4), and often they can aid the
orthopedic surgeon in spatial conceptualization. CT arthrography of
the shoulder has been widely used in the evaluation of Hill-Sachs
deformities, labral tears, and capsular injuries; however, in
general, CT has been surpassed by MR arthrography.
Spiral CT of the elbow can provide pertinent
information not readily available by radiography alone, and may
alter clinical decisions.
For evaluation of chronic trauma and impingement syndromes, we
image the elbow in both flexed and extended positions (figure 5)
with 1 to 3 mm collimation, a pitch of 1 to 1.5, and 1 to 1.5 mm
reconstruction intervals. Sagittal and coronal recon-structions
help evaluate or visualize fractures and loose bodies identified on
radiographs, and can diagnose radio graphically "occult" fractures
in patients with elbow effusions. Our orthopedic surgeons find that
2D and 3D reconstructions are helpful in planning surgery and in
communicating with patients. In order to evaluate which fragments
move with which parent bone, reformations are typically acquired in
more than one elbow position (figures 5 and 6).
The wrist and hand are optimally evaluated in
axial, coronal, and sagittal planes with a narrow collimation of 1
mm, a pitch of 1, and reconstruction intervals of 1 mm.
With such thin primary sections, creation of overlapping
reconstructions usually is not necessary. Two planes generally are
imaged directly, with one being perpendicular to the area of
interest (figure 7). Images in the third anatomic plane usually are
reconstructed from the axial images. Distal radial and ulnar
fractures are well characterized on these reconstructed images. For
the evaluation of scaphoid fractures, images are to be obtained in
an oblique sagittal plane along the long axis of the scaphoid
(figure 8), or with direct coronal imaging.
Because many scaphoid fractures run in the plane of section for
transaxial scanning to the wrist, the findings can be subtle and
in the sagittal or coronal plane can be misleading if negative. In
addition to the evaluation of fractures, CT also can aid in the
assessment of degenerative arthritis, such as that arising from
chronic carpal instability (figure 9).
Pelvis and lower extremity
The pelvis and hips are readily amenable to
characterization by spiral CT. Typically, 3 mm collimation images
with a pitch of 1 to 1.5 are reconstructed at 1.5 to 2 mm
intervals. If the study is abnormal, coronal and sagittal
reformations are routinely generated. In trauma, spiral CT with 2D
and 3D reformats can reproduce plain radiographic views, thereby
limiting radiation exposure while simultaneously providing optimal
visualization. Some investigators have used these reformatted
images as replacements for the standard Judet and Letournel views.
CT is useful in the evaluation of pelvic and acetabular fractures
(figure 10), and in assessment of intraarticular bodies (figure
11). CT also is excellent for depiction of osseous sacral and
sacroiliac joint disorders (figure 12) and in the evaluation of
some developmental pediatric disorders, such as developmental
dysplasia of the hip (figure 13). For pediatric patients CT can
assess osteocartilaginous causes of frequent dislocations, such as
iliopsoas interposition and femoral anteversion, and can be useful
for imaging of patients in casts.
Although MRI is the standard modality for
imaging of the knee, CT is useful in the evaluation of fractures
for surgical planning. Axial images are acquired with a 2 to 3 mm
collimation, a pitch of 1 to 1.5, and are reconstructed at 1 to 1.5
mm intervals. Sagittal and coronal reformations can help
characterize tibial plateau fractures and the extent of depression
(figure 14). Additionally, CT is valuable in the evaluation of the
postoperative or postreduction knee, which may be casted,
subsequently limiting plain film evaluation. Other investigators
have used spiral CT in the preoperative planning of a patient with
and in the detection of insufficiency fractures in patients with
Much like the wrist and hand, the ankle and
foot usually can be oriented in more than one plane relative to the
scanner gantry, thereby allowing direct axial and coronal sections.
In most cases of fracture characterization, 2 to 3 mm collimation
is adequate, although a 1 mm collimation may be useful for fracture
detection. This is particularly true of subtle fractures of the
smaller foot bones, such as the navicular or cuneiforms. The
multiplanar reformation capabilities of spiral CT are especially
useful in the evaluation of complex fractures in which there are
multiple fracture planes. Complex tibial plafond fractures are
readily characterized by the extent of the fracture line, degree of
displacement, and disruption of the ankle mortise (figure 15).
Pretorius et al were able to classify distal tibial fractures into
those that required acute reduction and those that needed delayed
definitive arthroplasty based on interpretation of spiral CT
multiplanar calcaneal fractures are delineated with coronal and
sagittal reconstructions (figure 16).
Corbett et al have devised a classification system of calcaneal
fractures to help guide in the decisions behind conservative versus
Smaller body parts such as the digits are best imaged in the axial
or coronal planes with a 1 mm collimation, a pitch of 1, and l mm
reconstructions (figure 17).
Critical to the interpretation of spiral CT
images is an understanding of accompanying artifacts. Stair step
artifact is a common artifact visualized as a disruption of
inclined surfaces in a regular, stair step fashion.
It is most often encountered when oblique surfaces are nearly
parallel to the transverse plane rather than aligned with the
direction of table motion (figure 18).
Three-dimensional image rendering drawbacks
include those associated with volume formation, tissue
classification, and image projection.
In volume formation, artifacts can occur in preprocessing,
acquisition, or data editing. Figure 19 demonstrates a 3D rendered
image in which noise from the 1 mm acquisition is manifest as
linear disruptions in the bone contour. Moreover, this image
demonstrates a common pitfall, in which a markedly displaced
glenoid fracture fragment was inadvertently edited completely out
of the image.
The future of spiral CT is expanding rapidly
and includes advances such as quantitative prosthetic modeling and
virtual arthroscopy. Three-dimensional reconstructions of spiral CT
images are being used in bone modeling of pathology for presurgical
and preradiation therapy, as well as in surgical simulations such
as virtual osteotomies.
These 3D models are used to guide the manufacturing of prostheses,
both off-the-shelf and custom made, and the volumetric data
acquired with spiral CT also may be applied in the assessment of
post prosthetic fit by evaluating tissue displacement.
Perspective volume rendering is another new application of spiral
CT which allows visualization of volumetric data sets from an
internal viewpoint, thereby simulating endoscopy. These techniques
have been applied to the vascular system, tracheobronchial tree,
and gastrointestinal tract.
In the musculoskeletal system such techniques have the potential to
aid in surgical planning and in communications with the referring
High quality spiral CT of the musculoskeletal
system requires tailoring the examination to the body part being
imaged and to the clinical question being addressed. The
collimation, pitch, tube current, reconstruction interval, and
reconstruction algorithm have important influence on image quality.
If chosen appropriately, excellent quality and clinical value can
routinely be achieved.