Featured Applied Radiology Article: Three Dimensional Volume Rendering Of Skin Subcutaneous Tissues

Three-dimensional volume rendering and display of skin and subcutaneous tissues Karen M. Horton, MD, Pamela T. Johnson, MD, David G. Heath, PhD, Derek R. Ney, BS, and Elliot K. Fishman, MD

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n 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. Threedimensional 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. 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.

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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

All studies were done on a 64-slice MDCT scanner (SOMATOM Sensation, Siemens Healthcare, Malvern, PA). All patients were referred by 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

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arterial or venous phase acquisition although in select cases dual-phase imaging was used. Scan parameters included 0.6 mm collimators, 0.75 mm slice thickness with a reconstruction interval of 0.5 mm. The typical kVp was120 and the mAs between 120 mAs and 200 mAs. All images were reconstructed with a softtissue 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

The evaluation of a volumetric dataset is usually done interactively July–August 2009

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FIGURE 1. CT of the normal hand. All these images are at the same level, and only the rendering algorthims were changed. Each of these images can be created in a few seconds. (A) Surface rendering to accentuate the skin; the sheet under the patientʼs hand is also visible. (B) The rendering has been changed to make the skin invisible and to highlight the subcutaneous tissues, muscle and superficial vessels. (C) In this image, the soft tissues have been rendered transparent, so only the blood vessels and bones are visible. (D) In this image, all the soft tissues and blood vessels are now transparent, so only the bone is visible. (E) This is an example of how the soft tissues can be made see-through, while still being able to see the underlying bone. (F) Surface-variant volume rendering showing the effect of having the light source shining on the dorsum of the hand. (G) Surface-variant volume rendering showing the effect of having the light source positioned from right. (H) Surfacevariant volume rendering showing the effect of having the light source positioned from the left. (I) Surface-variant volume rendering showing the effect of having the light source positioned from above. (J) Surface-variant volume rendering showing the effect of having the light source positioned from below.

using a combination of axial CT, multiplanar reconstruction (MPR), and 3dimensional 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 essential. 8

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Technical aspects of soft tissue imaging

The 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 show and enhance the boundaries between materials. In the case of the skin, the boundary is between air and the skin. The shadedrendering technique calculates 3dimensional gradients in the Hounsfield units of each voxel in the data being

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rendered. The gradient is used for 2 purposes during rendering: 1) 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. 2) 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 July–August 2009

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FIGURE 2. 57-year-old man with diabetes and ulceration on the fourth digit. The CT was performed to evaluate for possible abscess or evidence of osteomyelitis. (A) Surface skin rendering of the palmar surface of the hand nicely shows the soft tissue swelling of the mid and distal fourth digit (arrow). (B) Surface skin rendering of the dorsal surface of the hand shows ulcers (arrow) on the distal fourth digit. (C) Rendering in which the skin is made translucent and the bones are visible, shows decreased mineralization of the distal tuft (arrow) of the fourth digit, compatible with osteomyelitis.

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FIGURE 3. 45-year-old woman with sickle cell disease and nonhealing ulcer over ankle. (A) Surface-variant volume rendering shows the patientʼs sock. (B) Surface skin rendering makes the sock transparent and nicely demonstrates the large skin ulcer (arrows). (C) Rendering to visualize the bone shows lucency and periosteal reaction along the distal fibula and tibia compatible with osteomyelitis.

produce a shading effect in which boundaries facing the light source are enhanced and those facing away from the light source are 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. July–August 2009

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. Surfacerendering techniques (such as marching cubes) can make good images of skin. However, volume-rendering produces similar images and has the advantage of

allowing the user to 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

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FIGURE 4. 44-year-old man with pain, swelling and redness posterior to ankle. (A) 3-dimensional rendering in which the soft tissues have been rendered transparent, so only the blood vessels and bones are visible. Significant hyperemia (arrows) is noted posterior to the ankle. (B) Sagittal reconstruction shows the abscess (arrow) posterior to the ankle, and just deep to the Achilles tendon. (C) Coronal reconstruction also shows the abscess collection.

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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.

Clinical applications

FIGURE 5. 45-year-old woman with pain and redness over lateral thigh. (A) Axial image shows a focal area of indurated skin and swelling over anterior lateral thigh. (B) 3-dimensional rendered images in which the normal skin is translucent, nicely highlights the swelling and skin thickening, compatible with cellulitis.

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The role of CT imaging of the skin and muscle is best considered part of an in-depth CT evaluation, which would include analysis of the 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 points.

Soft tissue inflammation and infection

Volume rendering can be used to identify defects, such as skin ulcers (Figures 2 July–August 2009

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FIGURE 6. 35-year-old man with trauma to the third interphalangeal joint. (A) Surface skin rendering of the dorsal surface of the hand shows a small laceration (arrow) over the third interphalangeal joint. (B) Rendering in which the skin and soft tissues are transparent, shows the hyperemia (arrow) to the third interphalangeal joint. (C) Rendering in which the skin is made translucent and the bones are visible shows no evidence of fracture.

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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).

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Soft tissue and muscle injury following trauma

Shaded-rendering VR can also define skin and muscle injury to elucidate the trajectory of the penetrating trauma or location of blunt trauma (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

FIGURE 7. 45-year-old man status post MVA. (A) Volume rendering demonstrating extensive soft tissue/muscle injury. (B) 3-dimensional rendering accentuating visualization of the bones shows fractures of tibia and fibula.

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Radiologists can use these CT shadedrendering 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 8 and 9), lymphoma and neurofibromas.

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FIGURE 8. 37-year-old man with ulcerating lesion left midaxillary line. Biopsy-proven melanoma. (A) Coronal volume-rendered image shows focal skin thickening in the left axilla (arrow). (B) Surface skin rendering also shows the skin lesion (arrow).

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FIGURE 9. 38-year-old woman with metastatic melanoma. (A) Surface skin rendering also shows numerous skin lesions (B) 3-dimensional rendering making normal skin transparent highlights the skin lesions.

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B FIGURE 11. 35-year-old man with dermatofibrosarcoma protuberans for preoperative evaluation. (A) Surface skin rendering shows the large exophytic mass arising from the left aspect of the forehead. (B) 3-dimensional rendering highlighting the bones shows only minimal bone erosion.

Miscellaneous applications

FIGURE 10. 49-year-old woman with SVC occlusion. (A) Surface rendering with transparent skin shows extensive abdominal wall collaterals. (B) 3-dimensional rendering with skin and muscles transparent, again shows extensive venous collaterals.

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Besides 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 softtissue mapping in craniofacial pathology (Figure 11). In cosmetic surgery applications, the technique can be combined with anatomic mapping of skin in relation to underlying pathology. July–August 2009

3D VOLUME RENDERING OF THE SKIN Discussion

The ability to create the “ CT physical examination” provides a unique diagnostic advantage. However, 3dimensional 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 way.”4 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 pathology to 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

Conclusion

Knowledge 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 64slice 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.

REFERENCES

1. Johnson PT, Heath DG, Bliss DF, et al. Threedimensional: Real time interactive volume rendering. AJR 1996;167:581-583. 2. Calhoun PS, Kuszyk BS, Heath DG, Carley JC, Fishman EK. Three-dimensional volume rendering of spiral CT data: Theory and method. Radiographics. 1999;19:745-764. 3. 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. 4. 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. 5. 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.

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