Comp. Med. Imag. ond Graphics, Vol. 12, No. I, pp. 59-66, 1988 0895-611 I/88 $3.00 + .OO Printed in the USA. All rights reserved. Copyright 8 Pergamon Press Inc

THREE-DIMENSIONAL COMPUTED TOMOGRAPHY OF THE FOOT: OPTIMIZING THE IMAGE

STEVEN J. ADLER,* MICHAEL W. VANNIER,t LOUIS A. GILULA and ROBERT H. KNAPP

Mallinckrodt Institute of Radiology, Washington University School of Medicine, 510 S. Kingshighway Blvd, St Louis, MO 63110, U.S.A.

(Received 24 February 1987; received for publication 6 July 1987)

Abstract-The complex anatomy of the foot can be imaged using high resolution computed tomography. High resolution serial nonoverlapping CT scans of foot anatomy have a high degree of soft tissue contrast, and excellent geometrical accuracy (no magnification error). Three-dimensional surface reconstruction from CT scans of the foot were performed using specially developed computer software. These surface recon- structions display the osseous and soft tissue anatomy of the foot in a form similar to anatomic preparations. The removal of overlying skin, disarticulation of the ankle, tarsals and metatarsals was accomplished using computer methods. Major factors necessary to optimize three-dimensional images are presented and illustrated. The technique has been applied in living subjects with arthritis, carpal coalitions, osteochondritis dissecans, and fractures. These images have been useful in communicating the findings on high resolution CT scans to referring clinicians, correlating CT findings in areas of complex anatomy, and eliminating overlying or obscuring structures by mathematically disarticulating the foot and individual tarsal bones.

Foot, computed tomography Foot, abnormalities ities Computed tomography, image processing

Ankle, computed tomography Ankle, abnormal-

INTRODUCTION

Techniques for three-dimensional imaging of bones and soft tissue have recently been developed using reconstruction of data obtained during routine planar computed tomography. Computer programs for such surface image reconstruction have been successful especially in the skull, wrist and pelvis [1-a.

The foot is another site where complex anatomy makes routine radiography inadequate to demonstrate certain abnormalities. Computed tomography has been found to be of value in demonstrating complex foot anatomy [7-l 51. However, the integration and interpretation of multiple sectional images is often difficult for the clinician. Communication of the information contained in these images in a form readily understandable by the clinician may be difficult for the radiologist. Three-dimensional surface reconstruction has been found useful in facilitating the display of morphologic derangements in other skeletal areas [l-6].

Our initial attempts to perform three-dimensional reconstruction on 21 foot CT cases collected at the Mallinckrodt Institute over the past several years were rather unrewarding in that the detail of the three-dimensional images was suboptimal, despite the use of the same software successfully used previously in craniofacial, wrist and other applications. This was due to the nature of the data available to us as it was archived in floppy discs-namely processed standard two-dimensional CT images. We did not have the original raw data collected at the time of scanning. While scans stored on floppy disks are perfectly suitable for routine diagnosis, they are suboptimal as input for a three-dimensional reconstruction program. This is true even when the original images are optimized by judicious choice of kV, mA, scan time, slice thickness and interval as it would be in most well performed routine CT studies.

Because of our initial disappointing results, we sought to optimize the three-dimensional images produced by a CT scanner. Conceptually, there are three steps necessary to produce a three- dimensional surface reconstruction from original patient anatomy. First, raw data must be collected

* Present address: Stanford University School of Medicine, Department of Radiology, Stanford, CA 94305, U.S.A. t To whom correspondence should be addressed.

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60 STEVEN J. ADLER et al

Fig. 1. Cadaver CT scan of the feet obtained through the talus and calcaneous. Both feet are in the field of view to facilitate comparison. This scan was obtained at relatively low magnification (zoom 2, pixel size 0.5mm, 512 x 512

matrix).

Fig. 2. Surface reconstruction of the ankles from low reso- lution CT scans through the feet and ankles in a cadaver.

This corresponds to the slices seen in Fig. 1.

by the CT scanner. The process is dependent on the choice of kV and mA, scan time, slice thickness and interval, as well as the ability to immobilize the patient during scanning. Second, this raw data must be processed by the scanner’s computer to form the usual planar or two dimensional image (Fig. 1). This step can be influenced by user-selected factors including magnification or zoom, convolution kernel (a mathematical feature of the reconstruction algorithm determines contour smoothness and edge sharpness, and spatial vs contrast resolution), and centering. Third, the two-dimensional scans serve as input to the three-dimensional reconstruction programs for production of the final surface images (Fig. 2). In this step, the radiologist can modify the final result by selective erasure of unwanted or interfering structures from the planar scans. Production of surface images of bones or of soft tissue elements such as ligaments and tendons can be achieved by choice of the proper CT attenuation threshold.

MATERIALS AND METHODS

To demonstrate these steps in producing three-dimensional images of optimal anatomic and diagnostic quality we scanned the feet of a fresh cadaver and of a patient with known osteochondritis dissecans of the talus. We used a standard CT scanner (Siemens Somatom DR-H) and selected a technique of 120 kV, 280 mA with 5 s scan time and 2 mm contiguous slices. These factors were chosen as being typical for a routine CT examination of the foot at Mallinckrodt Institute of Radiology. 960 as opposed to 480 or 1440 projections per section were used. The higher spatial resolution but smaller field of view head imaging mode, as opposed to the body mode on the Siemens unit, was used due to the small size of the parts being imaged. Images as shown in Fig. 1 were obtained.

The zoom or magnification function supplied with the standard CT scanner software was employed in conjunction with recentering of the area of interest to magnify the image up to 10 times. Because this function operated on raw data, the result was increased spatial resolution rather than mere enlargement of the individual pixels. We performed these operations immediately after scanning while the raw data was still available. However, if necessary the raw data can be archived on magnetic tape or floppy disc for future processing. We empirically chose an edge sharpening convolution kernel to yield high resolution of bony detail.

The two-dimensional CT slice images were input into the three-dimensional surface reconstruction program, which is run on the control computer incorporated in the CT scanner. This program runs without supervision for approximately 20 min to produce six or more standard three-dimensional images from a set of up to 84 original scans. Three-dimensional reconstructions were made from scans of the cadaver foot with various magnifications (Figs 3 and 4). Lighter structures are closer to the viewer, and darker ones are more distant.

3-D CT of the feet 61

Fig. 3. (a) Medium resolution CT scan reconstruction with edge enhancement obtained by reprocessing raw scan data. In thns section, the zoom factor was 6, pixel size 0.17 mm, 512 x 512 matrix). (b) Same as (a) except malleoli have been removed (black areas). (c) Anterior 3-D surface view of the ankle obtained from CT scans at medium resolution at a zoom of 6 (a). Darker structures, the fibula, tibia and talus are farther away, and the lighter structures, the tarsals and metatarsals seen end-on, are closer to the viewer. (d) Anterior 3-D surface view of the ankle obtained after removal of the distal tibia and fibula from (c). (e) Anterior view of the ankle from (a) at high resolution or zoom 10 (pixel size is 0.1 mm). (f) Anterior view of the talus in (a)

after mathematical removal of the surrounding tarsals, tibia, and fibula.

If desired, before three-dimensional reconstruction, portions of the original CT scans can be selectively erased in order to eliminate structures obscuring or overlying the part of interest [Figs 3(b,d,f), 4(b)]. This is accomplished with software also run on the unmodified CT scanner using the built in electronic pad and stylus. Depending on the complexity and size of the area to be erased, this takes us between 30 and 90 s per CT section, or a total of about 30 min per case. When the malleoli are erased and three-dimensional reformatting is performed, the dome of the talus can be uncovered

62 STEVEN J. ADLER et al.

Fig. 4. (a) Posterior 3-D surface view of the ankle obtained from CT scans at medium resolution or zoom 6. The darker portion of the talus and sustentaculum tali (arrow) are farther away, and the posterior aspect of the tibia (T) and calcaneous (C) are lighter and closer to the viewer. (b) Posterior 3-D surface view of the ankle obtained after removal of the distal tibia and fibula. The posterior subtalar articulation is evident

(arrows).

Fig. 5. (a) Plain A-P radiograph of the ankle with a defect of osteochondritis dissecans at the medial aspect of the talar dome (arrow). (b) A-P pluridirectional tomographic section of the ankle shows defect in the talar

dome (arrow).

Fig. 6. In the transverse plane, a low resolution CT scan slice through the talus shows the defect (arrows) in the posteromedial talar dome with separate bony fragments (arrowheads).

3-D CT of the feet 63

from beneath the tibia [Figs 3(d) and 4(b)]. Similarly by mathematical disarticulation, a single bone can be isolated from the remainder of the surrounding osseous structures [Fig. 3(f)].

After testing these techniques on cadaver feet, a patient with known osteochondritis dissecans of the talus (Fig. 5) was examined to optimize the production of three-dimensional surface images (Figs 6 and 7).

RESULTS

The isolated talu,s of the patient with osteochondritis dissecans demonstrates two bony fragments within the large defect at the medial aspect of the talar dome [Fig. iT(d,e,f)].

Structures in three-dimensional images oriented in directions perpendicular to the plane of scanning are less well defined because spatial resolution in those direction is limited by slice thickness-2 mm in this case-while resolution in the plane of section can approach 100 pm at zoom 10, the upper limit on magnification for our scanner [Fig. 7(e)]. Still, the abnormality can be seen in this superior view of the talus [Fig. 7(f)].

Viewing the talus from below reveals the sinus tarsi and tarsal canal [Fig. 7(g)]. We know of no other imaging modality able to demonstrate these structures in their entirety. This emphasizes the value of 3-dimensional surface reconstruction in showing surface anatomy which cannot otherwise be imaged.

Areas for further work include investigation of 3-D imaging of soft tissue surfaces such as ligaments, tendons and muscles (Fig. 8). Imaging showing tendons about the ankle [Fig. 8(a)] including the Achilles tendon posteriorly [Fig. 8(c)], can easily be produced by setting an appropriate soft tissue threshold to the attenuation coefficient detected by the 3-D reconstruction program. Surfaces of structures with CT attenuation values equal to or greater than the selected threshold will then be displayed.

DISCUSSION

Three-dimensional techniques are able to display complex sectional anatomy in a form similar to the gross anatomy understood by all physicians. The impact this can be significant in teaching, and to facilitate communication between radiologists and referring physicians.

Currently, several manufacturers offer three-dimensional software packages for CT scanners as well as specialized dedicated workstations. For optimal use of this technology, we advise that when three-dimensional reformatting is contemplated, raw scan data be saved. Subsequent to the collection of the scans, these data may be reconstructed into slices more nearly optimal for three-dimensional surface reconstruction. The modification of slice reconstruction processing via zoom, centering and convolution kernel selection is critical to the quality of the final three-dimensional image, even though high quality diagnostic two-dimensional CT images can be made routinely without much attention to these factors. These factors operate only on the raw data, and cannot be altered when only the processed planar images are stored, as is often the case in clinical practice.

CONCLUSION

Three-dimensional surface reconstruction images of the feet can display osseous anatomy in a form more readily understood by nonradiologist physicians. Alterations in the CT scanning procedure, including target or high resolution slice reconstruction from stored raw projections data and edge enhancement, can result in superior three-dimensional surface image quality. Three-dimensional surface reconstruction for the feet can be a useful tool in communicating the results of high resolution CT scan examinations to nonradiologist physicians in selected cases of osseous surface abnormalities.

64 STEVEN J. ADLER et al.

- --. Fig. 7

3-D CT of the feet 65

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4.

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Fig. 8. (a) Low resolution anterior 3-D surface reconstruction of both ankles with ligaments (white band structures) visible. (b) low resolution anterior 3-D surface reconstruction of both ankles after removal of ligaments. (c) Low resolution right lateral 3-D surface reconstruction of the ankle with ligaments visible. The achilles tendon is seen to the left (arrow). The dorsum of the foot is on the right. (d) Low resolution right lateral 3-D surface reconstruction of the ankle after removal of ligaments. The brightest component of this

scene is the lateral malleolus (arrow-talus; arrowhead-tibia).

REFERENCES P. M. Weeks, M. W. Vannier, W. G. Stevens, D. Gayou and L. A. Gilula, Three-dimensional imaging of the wrist, J. Hand Surg. lOA, 32-39 (1985). A. S. Verbout, T. H. M. Falke and J. Tinkelenberg, A three-dimensional graphic reconstruction method of the vertebral column from CT scans, Eur. J. Rud. 3, 167-170 (1983). M. W. Vannier, J. L. Marsh and J. 0. Warren, Three-dimensional CT reconstruction images for craniofacial surgical planning and evaluation, Radiology 150, 179-184 (1984). W. G. Totty and M. W. Vanner, Complex musculoskeletal anatomy: analysis using thee-dimensional surface recon- struction, Radiology 150, 173-177 (1984). D. L. Burk, D. C. Mears, W. H. Kennedy, L. A. Cooperstein and D. L. Herbert, Three-dimensional computed tomography of acetabular fractures, Radiology 155, 1833186 (1985).

Fig. 7. Facing page. (a) In a paracoronal plane, a low resolution CT scan slice through the talus shows the talar dome defect (arrow). (b) With zoom 6 or medium resolution, this CT scan is reconstructed from the same raw data as used in (a). Pixel size is 0.167 mm. (c) Zoom 10 or high resolution CT scan reconstruction from the same raw data used in (a). Pixel size is 0.1 mm. (d) Same as (c), except surrounding osseous structures have been removed (erased). (e) Posterior 3-D talar surface reconstruction from high resolution CT scans (c) after removal of surrounding osseous structures. The osseous fragments (arrows) are present in the talar dome defect. (f) Superior 3-D talar surface reconstruction from high resolution CT scans (c) after removal of surrounding osseous structures. The osseous defect (black arrows) containing the bone fragments is again evident in the posteromedial talar dome. The distal end of the talus (white arrow) is darker and farther from the viewer. (g) Inferior 3-D talar surface reconstruction from high resolution CT scans of (c) after removal of surrounding osseous structures. The tarsal canal (between arrowheads) is clearly evident with the sinus

tarsi to the viewer’s right (between arrows).

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6. J. L. Marsh and M. W. Vannier, Surface imaging from computerized tomographic scans, Surgery 94, 159-165 (1983). 7. M. A. Solomon, L. A. Gilula, L. M. Oloff, J. Oloff and T. Compton, Computed tomography scanning of the foot and

ankle: Part I: Normal anatomy, Am. J. Roentg. 146, 1192-1203 (1986). 8. M. A. Solomon, L. A. Gilula, L. M. Oloff and J. Oloff, Computed tomography scanning of the foot and ankle: Part II:

Clinical applications and review of the literature, Am. J. Roentg. 146, 12041214 (1986). 9. C. Zinman and N. D. Reis, Osteochondritis dissecans of the talus: Use of the high resolution computed tomography

scanner, Acta orthop. scan 53, 697-700 (1982). 10. N. D. Reis, C. Zinman et al., High-resolution computerized tomography in clinical orthopaedics, J. Bone Joint Surg. 64B,

2&24 (1982). 11. R. W. Smith and T. W. Staple, Computerized tomography (CT) scanning technique for the hindfoot, C/in. Orthop. Rel.

Res. 177, 3438 (1983). 12. S. E. Seltzer, B. N. Weissman, E. M. Braunstein, D. F. Adams and W. H. Thomas, Computed tomography of the hindfoot.

J. Comput. assist Tomogr. 8, 488497 (1984). 13. M. M. Riddlesberger, Computed tomography of the musculoskeletal system, Rad. Clin. N. Am. 19, 463477 (1981). 14. H. K. Genant, C. E. Cann and N. I. Chafetz, Advances in computed tomography of the musculoskeletal system, Rad.

C/in. N. Am. 19, 645-674 (1981). 15. M. W. Vannier, W. G. Totty, W. G. Stevens, P. M. Weeks, D. M. Dye, W. J. Daum, L. A. Gilula, W. A. Murphy and

R. H. Knapp, Musculoskeletal applications of three-dimensional surface reconstructions, Orthop. Clin. N. Am. 16, 543-555 (1985).

About the Author-Snvm J. ADLER, M.D. is from Brooklyn, New York and completed his undergraduate and medical studies at Washington University in St Louis, MO. Dr Adler was a resident in diagnostic radiology at the Mallinckrodt Institute of Radiology of the Washington University Medical Center, and was co-chief resident, and later a fellow in musculoskeletal radiology at the same institution. He is currently an assistant professor of radiology at the Stanford University Medical Center.

About the Author-MIcmm W. VANNIER, M.D. received his undergraduate education in engineering at the University of Kentucky, Colorado State University, Massachusetts Institute of Technology and Harvard University. After working as an engineer, he entered medical school at the University of Kentucky and eventually became a diagnostic radiology resident at the Mallinckrodt Institute of Radiology, Washington University School of Medicine, later joining the staff. He is now Associate Professor of Radiology, Assistant Professor of Surgery (Plastic and Reconstructive) and an Affiliate Professor of System Science and Mathematics.

About the Author-Lous A. GILULA, M.D. attended Southern Illinois University in Carbondale, Illinois and graduated from the University of Illinois in Chicago, Illinois to obtain an M.D. degree in 1967. He then had a one year rotating internship in San Francisco General Hospital and served two years in on-job training in radiology at Fort Belvoir, Virginia while in the Army. Doctor Gilula then started his radiology residency at St Louis City Hospital part of the St Louis University Radiology Program in St Louis, Missouri in 1970, completing his residency in 1973, and obtaining his American Board of Radiology certification in 1973. After finishing radiology training, he joined the Washington University School of Medicine, Mallinckrodt Institute of Radiology staff as Instructor of Radiology in 1973 and became a full Professor of Radiology in 1982. He became Co-Director of the Musculoskeletal Section of Radiology at the Mallinckrodt Institute of Radiology in 1975 and has remained in this position to the current date. His academic accomplishments include speaking at numerous national and international meetings and congresses he has authored or coauthored numerous articles and book chapters. Doctor Gilula is a Fellow of American College of Radiology, and belongs to the International Skeletal Society, American Medical Association, St Louis Medical Society, Missouri State Medical Association, Radiological Society of North America, Association of University Radiologists, American Roentgen Ray Society, among others.

About the Author-ROBERT H. KNAPP received a Bachelor of Music Degree in Music Education from Western Michigan University in Kalamazoo, Michigan in 1980. He received his Radiologic Technology training at the Mallinckrodt Institute of Radiology at Washington University School of Medicine in St Louis, Missouri graduating in 1982. He was Certified by the American Registry of Radiologic Technologists in the same year. He remained at Mallinckrodt Institute of Radiology as Clinical Instructor of Special Procedures Radiology and was appointed Assistant Supervisor of Special Procedures Radiology in 1983. He has been supervising 3-D imaging and image processing research since 1984 and is currently manager of the Image Processing Laboratory at the Mallinckrodt Institute of Radiology. He is a member of the ARRT, the American Federation of Musicians as well as the International Trumpet Guild.