Dr. Horton is a Professor of Radiology, Dr. Johnson is an Assistant Professor of Radiology and Dr. Fishman is
a Professor of Radiology, at The Russell H. Morgan Department of
Radiology and Radiologic Sciences, Johns Hopkins School of Medicine,
Baltimore, MD. Dr. Heath is President and Mr. Ney is CEO of HipGraphics Inc., Towson, MD.
the past, 3-dimensional rendering was reserved for computed tomography
(CT) angiography and orthopedic applications. However, coupled with
advancements in multidetector (MDCT) hardware, 3-dimensional volume
rendering has become an essential tool for assessment of a range of
organ systems, including pulmonary, hepatobiliary, genitourinary and
gastrointestinal applications, due to the inherent versatility of this
robust display technique. Three-dimensional volume rendering can also
provide valuable diagnostic information about the skin, subcutaneous
soft tissues and muscle.1,2 Over the past decade, limited applications of skin imaging with CT have been reported in the literature.3–5
Nonetheless, in the authors’ experience it is extremely useful for
comprehensive evaluation of skin ulcers, infection/ inﬂammation, trauma,
soft tissue tumors, and pre- and postoperative imaging and vascular
collateralization. This may have signiﬁcant impact on patients seen in
the emergency room setting.
This article explains how to optimize
the volume-rendering technique for display of skin, soft tissues and
muscle, and it illustrates the clinical utility of this application.
MDCT data acquisition
studies were done on a 64-slice MDCT scanner (SOMATOM Sensation,
Siemens Healthcare, Malvern, PA). All patients were referredby their
treating physicians for a range of clinical indications. Unless
contraindicated, studies were performed with intravenous (IV) contrast,
either Omnipaque-350 or Visipaque-320 (GE Healthcare, Chalfont, St.
Giles, U.K.), depending on the patient’s renal status or clinical
history. Injection rate for 100 cc to 120 cc of IV contrast was 4
cc/sec. Studies are routinely done using a single arterial or venous
phase acquisition although in select cases dual-phase imaging was used.
parameters included 0.6 mm collimators, 0.75 mm slice thickness with a
reconstruction interval of 0.5 mm. The typical kVp was 120 and the mAs
between 120 mAs and 200 mAs. All images were reconstructed with a
soft-tissue kernel; the high-resolution bone kernel is not used because
the images have increased noise which degrades quality. Once the
datasets were reconstructed, all images were sent to a workstation
(Leonardo running InSpace, Siemens Healthcare) for 3-dimensional
rendering by the radiologist.
Data analysis: A practical approach
evaluation of a volumetric dataset is usually done interactively using a
combination of axial CT, multiplanar reconstruction(MPR), and 3-
dimensional postprocessing with volume rendering (VR) and maximum
intensity projection (MIP) techniques. However, for cases requiring soft
tissue imaging, volume rendering is the only technique necessary for
data postprocessing. Images can be optimized to deﬁne the tissue surface
and interfaces. Understanding how to select and adjust the
volume-rendering parameters for this type of display and analysis is
Technical aspects of soft tissue imaging
surface of the skin is best imaged using the “shaded” variant of the
volume-rendering technique. Not to be confused with shaded-surface
rendering, this shaded-rendering variant of VR is designed to showand
enhance the boundaries between materials. In the case of the skin, the
boundary is between air and the skin. The shaded-rendering technique
calculates 3-dimensional gradients in the Hounsﬁeld units of each voxel
in the data being rendered. The gradient is used for 2 purposes during
- The size (magnitude) of the gradient is used to modulate what parts
of the volume are visualized. Parts of the volume data that have a small
gradient are suppressed from the ﬁnal image; this enhances the
boundaries between structures.
- A simulated light source is placed in space, usually relative to the
viewing direction. The direction of the gradient vector is combined
with the light-direction vector to produce a shading effect in which
boundaries facing the light source are enhanced and those facing away
from the light sourceare diminished. An effect called “specular
reﬂection” can also be used. This lighting effect adds “shininess” to
the resulting image. For the surface of the skin, this often enhances
the 3-dimensional nature of the image.
The shading model is very important for visualizing skin with VR.
Unshaded techniques result in images in which the surface is difﬁcult to
visualize. Maximum intensity projection is virtually useless for
rendering skin. Surface- rendering techniques (such as marching cubes)
can make good images of skin. However, volume-rendering produces similar
images and has the advantage of allowing the userto melt away the skin
and look at structures within the body simply by changing window
settings and retaining the high image quality typical of volume
rendering (Figure 1).
The use of a simulated light source and
lighting effects such as “specular reﬂection” enhance the visualization
of places where the skin surface is curving. The “curviness” is seen
because the simulated reﬂection of the light modulates the color or
brightness of the surface over the curving section (Figure 1). Without
the simulated reﬂection it would be difﬁcult to see the curves. This is
especially true for small features such as wrinkles, nodules or bumps.
The fact that there is a high degree of curvature in such features means
that the simulated reﬂection will make them visible.
role of CT imaging of the skin and muscle is best considered part of an
in-depth CT evaluation, which would include analysis ofthe vascular map
and bone when imaging an extremity or analysis of the liver, pancreas,
kidneys, small bowel, etc. when imaging the abdomen. For the purposes of
this article, we will focus only on the imaging of the skin, soft
tissue and muscle to deﬁne this imaging technique. The list of
applications and the illustrated cases were selected for their teaching
Soft tissue inﬂammationand infection
rendering can be used to identify defects, such as skin ulcers (Figures
2 and 3) or sites of puncture wounds. The technique here can delineate
changes in subcutaneous tissues in relationship to underlying soft
tissue, muscle or bone pathologies. Further, it can deﬁne the location
and extent of soft tissue inﬂammation, including abscess (Figure 4) and
cellulitis (Figure 5).
Soft tissue and muscle injury following trauma
VR can also deﬁne skin and muscle injury to elucidate the trajectory of
the penetrating trauma or location of blunttrauma (Figure 6). It is
helpful in determining the extent of laceration and muscle involvement
in complex muscle injury and fractures (Figure 7). CT techniques allow
radiologists to correlate soft tissue injury in relation to muscle,
vascular structures and bone.
Tumors of the skin, soft tissues and bone
can use these CT shaded-rendering VR techniques to examine soft tissue
or muscle involvement in relation to underlying bone tumors. The
technique is adequate for delineating neoplasms such as melanoma
(Figures and 9), lymphoma and neuroﬁbromas.
the above clinical applications, CT imaging of the skin can be used to
deﬁne the extent of collateral vascular ﬂow in the chest wall and neck
(Figure 10). It can also be used for soft-tissue mapping in craniofacial
pathology (Figure 11). In cosmetic surgery applications, the technique
can be combined with anatomic mapping of skin in relation to underlying
The ability to create the “ CT physical
examination” provides a unique diagnostic advantage. However,
3-dimensional rendered CT display of skin and superﬁcial tissues has not
seen widespread application. Heretofore, physicians have been largely
unaware of the diagnostic potential available from this shaded-variant
volume rendering of 64-slice MDCT. For example, a recent article on
human face imaging reported that “Unfortunately, CT scanning is capable
of capturing neither high-resolution soft-tissue surface detail nor the
optical properties of soft-tissue interfaces, and thus the
photorealistic appearance of soft tissue cannot be recorded in this
However, this series of cases illustrates that
improvements in dataset resolution in conjunction with this advanced
postprocessing algorithm can generate highly realistic images of the
skin surface. The technique provides valuable information about skin and
subcutaneous pathologyto facilitate characterization (e.g. ulcer and
cellulitis). Furthermore, adjustment of volume-rendering parameters to
depict underlying muscle and bone aid in delineating the extent of
disease, as shown by the demonstrations of abscess and osteomyelitis.
Using CT to determine the relationship of the skin to underlying
anatomic and pathologic structures has proven valuable to guide
interventional procedures.2,3 Speciﬁcally, skin imaging with MPR CT has been used to guide supraclavicular subclavian vein catheter placement.3
In patients with breast cancer and closed lumpectomy cavity undergoing
interstitial brachytherapy, 3-dimensional renderings of the skin surface
have been used to plan implant positioning.2
of 3-dimensional CT applications may result in broadened utility. In
addition to the clinical applications demonstrated, the possibilities
for medical education, as well as patient education, are truly enhanced
by these new techniques. This article has addressed one of the new
opportunities provided by the coupling of 64-slice MDCT data with
advanced postprocessing techniques like volume rendering. As CT
continues to evolve and as new, advanced rendering algorithms are
developed, we can look forward to improved image resolution and ﬁdelity.
- Johnson PT, Heath DG, Bliss DF, et al. Three-dimensional: Real time interactive volume rendering. AJR 1996;167:581-583.
Calhoun PS, Kuszyk BS, Heath DG, Carley JC, Fishman EK.
Three-dimensional volume rendering of spiral CT data: Theory and method.
- Vicini FA, Jaffray DA, Horwitz E, et al. Implementation of
3D-virtual brachytherapy in the management of breast cancer: A
description of a new method of interstitial brachytherapy. I J Rad Onc Biol Phys. 1998;40:629-635.
- Jung C-W, Seao J-H, Lee W, Bahk J-H. A novel supraclavicular
approach to the right subclavian vein based on three-dimensional
computed tomography. Anesth Analg.2007;105:200-204.
- Ayoub AF, Xiao Y, Khambay B, et al. Towards building a
photo-realistic virtual human face for craniomaxillofacial diagnosis and
treatment planning. Int J Oral Maxillofac Surg. 2007;36:423-428. Epub 2007 Apr 10.