Skeletal Radiol DOI 10.1007/s00256-011-1198-z
CASE REPORT
Cone-beam computed tomography: a new low dose, high resolution imaging technique of the wrist, presentation of three cases with technique Jens De Cock & Koen Mermuys & Jean Goubau & Simon Van Petegem & Brecht Houthoofd & Jan W. Casselman
Received: 13 April 2011 / Revised: 28 April 2011 / Accepted: 2 May 2011 # ISS 2011
Abstract Cone-beam computed tomography (CBCT) is a relatively new technique. It generates a 3D image by emitting a pulsed cone-shaped X-ray beam. CBCT has become a very useful and widely used technique for dentomaxillofacial imaging over the last decade. It provides clear, high resolution multiplanar reconstruction images. Previously, the images could only be generated while the patient was sitting with his/her head fixed in position. With the presented new generation CBCT (NewTom 5G, QR, Verona, Italy), a more free positioning of the patient, either lying or sitting, is possible. In this way, skeletal imaging of various body parts becomes possible. In this article we present three clinical cases of CBCT imaging of the wrist, J. De Cock : K. Mermuys : S. Van Petegem : B. Houthoofd : J. W. Casselman Department of Radiology, A.Z. St.-Jan Hospital, Bruges, Belgium K. Mermuys e-mail:
[email protected] S. Van Petegem e-mail:
[email protected] B. Houthoofd e-mail:
[email protected] J. W. Casselman e-mail:
[email protected] J. Goubau Department of Orthopedic Surgery, A.Z. St.-Jan Hospital, Bruges, Belgium e-mail:
[email protected] J. De Cock (*) Molenbeekstraat 32, 2160 Wommelgem, Belgium e-mail:
[email protected]
describe the background of the technique, and discuss the advantages and disadvantages of CBCT imaging. Keywords Cone-beam computed tomography . Wrist . Methods . Fractures . Bone
Introduction The first CT-like application of cone-beam computed tomography (CBCT) was reported by Mozzo et al. in 1998 for use in dental imaging [1]. Six years later, in 2004, Gupta et al. used a prototypical cone-beam volume tomographic scanner with a digital flat-panel detector system at an isometric resolution of 0.15 mm [2]. It proved to provide a better definition of the fine osseous structure than the multislice high resolution CT (MSCT) [2, 3]. In the last decade, CBCT has become widely used as a valuable technique for dental imaging with low radiation dose [3–5]. Since the introduction of larger imaging fields, CBCT is no longer limited to the dentomaxillofacial region. There has been a growing interest in using this technique for primary paranasal sinus imaging and as a substitute for conventional cephalometric images in craniocephalometry [6]. Recently, CBCT has even been successfully applied for imaging of the inner ear, e. g., for postoperative assessment of cochlear implantation [7, 8]. All these results were very encouraging, but due to the restriction that the patient needed to be in a seated position, the possible indications were strictly limited to the head. With the latest generation CBCT (Newtom 5G, QR, Verona, Italy), however, a more free positioning of the patient, either lying or sitting, is possible, permitting high resolution CBCT imaging of the skeletal system for the first time. We present three cases in which we successfully used the CBCT imaging technique
Skeletal Radiol
of the wrist to make the diagnosis. To our knowledge no similar report has yet been published.
Case reports In the following three cases, CBCT imaging of the wrist led to the patient’s diagnosis. Case 1 A 46-year-old man presented with decreased range of flexion movement and persisting local tenderness of the left wrist after conservative treatment for a distal fracture of the radius. On conventional radiographs (not shown) the distal radius appeared normal, but on CBCT a small 1 mm residual intra-articular step-off (Fig. 1) was visible. The patient was treated with intra-articular corrective osteotomy after which he recovered fully. Case 2 A 63-year-old woman presented with increasing pain for eight months of the ulnar side of the left wrist without initial trauma. On clinical examination she had a
painful hyperflexion and an impaired range of motion. CBCT arthrography revealed a degenerative tear of the triangular fibrocartilage complex (small arrow). There was an ulnar plus variance with ulnar abutment of the lunate and secondary cartilage loss (large arrow) (Fig. 2). The patient was treated arthroscopically with debridement of the triangular fibrocartilage complex tear, synovectomy, and resection of the distal ulna. Case 3 A 37-year-old man presented to the orthopedic clinic with persisting pressure pain in the anatomical snuffbox after he had sustained a scaphoid fracture 6 months earlier. He had been treated with a Herbert screw. CBCT showed good incorporation of the screw in the proximal pole of the scaphoid, but no incorporation in the fragmented distal pole of the scaphoid (Fig. 3). The patient was treated operatively for the pseudarthrosis of the scaphoid with the Matti-Russe technique. After 3 months, the patient’s pain had decreased significantly, and he had almost returned to his former level of activity.
Technique In contrast to spiral MSCT with a collimated fan beam, CBCT uses a pulsed cone-shaped X-ray beam with a two-dimensional detector, generating a 3D image. The Xray beam is characterized by a very low variable mAs and a frequency of 15 pulses per second. The tubedetector system performs a complete 360° rotation around the examined body part. The raw data rotatory
Fig. 1 a, b Example of patient positioning for CBCT imaging of the wrist. The patient is sitting down at the rear of the scanner with his arm horizontally through the gantry. The patient’s wrist is fixated to reduce movement artifacts
Fig. 2 Coronal reconstructed image, 1 mm thickness, showing the high spatial resolution. Note the good visualization of a small 1 mm residual intra-articular step-off (arrow) of the distal radius after an old consolidated distal radius fracture
Skeletal Radiol
Fig. 3 Coronal reconstructed CBCT arthrogram image, 1 mm thickness. There is an ulnar plus variance with ulnar abutment of the lunate and secondary cartilage loss (large arrow). Note also the good visualization of a concomitant complex tear of the triangular fibrocartilage complex (small arrow)
projection images are then used for image reconstruction in any desired plane or for 3D views [1, 7, 9, 10]. The presented examinations were performed on the NewTom 5G. To position the wrist, the patient was seated with the arm horizontally through the gantry with the hand and wrist firmly fixated to prevent motion artifacts (Figs. 1a and b). The parameters of our protocol for the wrist are 110 kV, variable mAs (0.38, 0.54, and 1.32 mAs in the presented cases), slice thickness 0.15 mm, field of view 8×8 cm. This resulted in a set of 548 raw data axial images. The raw data were transferred to a GE Advantage Workstation and reconstructed to images with a slice thickness of 1 mm and an interslice gap of 1 mm in the axial, sagittal, and coronal plane and 3D reconstructions. All images were displayed on a bony window setting. The reconstructed images show very high detail and a clear, sharp bony delineation (Figs. 2, 3, 4). Note the relative absence of beam-hardening and metallic artifacts (Fig. 3). The given VolumeCTDI was 0.47 mGy for the patient with the distal radius fracture, 0.66 mGy for the patient with the osteosynthesis screw, and 1.97 mGy for the patient with the triangular fibrocartilage complex tear (higher dose boost protocol). For comparison, in our center the mean VolumeCTDI of an MSCT scan of the wrist (Discovery CT750 HD, GE Medical Systems, Waukesha, WI, USA) is 13.70 mGy, which is higher. The protocol parameters used for the MSCT scan are 100 kV, 150 mAs, slice thickness 0.625 mm, interslice gap 0.312 mm, and a field of view of 13 cm. However, no conclusions can be drawn from a single examination.
Fig. 4 Oblique reconstructed CBCT image, 1 mm thickness, in the axis of an osteosynthesis screw through the scaphoid. Note the absence of beam-hardening artifacts resulting in a very good evaluation of bony incorporation of the screw. There is good incorporation of the screw in the proximal pole of the scaphoid, but no incorporation in the fragmented distal pole of the scaphoid
Discussion The presented cases show that the acquisition technique of CBCT enables fast and comfortable multiplanar reconstructions with a very high spatial resolution. This makes CBCT very well suited for routine imaging of the wrist. It is more than sufficiently sensitive for assessing of subtle fractures (Fig. 2). The high resolution images make it an ideal technique for CBCT arthrography (Fig. 3) [2, 10]. An important asset of CBCT is its potential to partially overcome image degradation by metallic and beam-hardening artifacts because it is based on conventional radiographic images to generate the volumetric dataset. Only a few metallic and beam-hardening artifacts occur. This allows for a very nice assessment of the incorporation of osteosynthesis material (Fig. 4) [7, 11, 12]. A major advantage of CBCT is the low radiation dose. Multiple dentomaxillofacial studies report radiation dosages up to 20 times lower than conventional MSCT scans [13–15]. Finally, a CBCT scanner, e.g., the Newtom 5G, is a fairly compact device with a substantially lower cost, both in purchase and maintenance, than an MSCT scanner. This makes CBCT much more suited for small medical centers [15]. However, there are also disadvantages. The acquisition time, which is equal to the duration of a single gantry revolution, takes approximately 30 s. During this period the patient has to keep the wrist perfectly still. Patient movement, especially problematic in a population of children or stressed patients, causes artifacts. CBCT is very sensitive to these
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movement artifacts because of its very high spatial resolution [3]. Also, CBCT has a limited contrast resolution, which makes it less suited for imaging of the soft tissues compared to MSCT [10]. In our protocol we used a field of view of 8× 8 cm. This small field of view is sufficient to examine a welllocated zone of interest. When pathology is suspected in a larger area, a larger field of view should be applied. The small size of the gantry makes it hard to position larger joints, e.g., shoulder or hip. Moreover, because of the low radiation dose, larger joint images will not be as good as those in smaller joints. This makes CBCT much more suited for imaging of the smaller joints. Although we only discuss CBCT imaging of the wrist, we believe comparable results could be achieved when applying this technique to other peripheral skeletal indications. In skeletal imaging we believe CBCT could be used for better imaging of the small joints in trauma, imaging of subtle fractures, and for better delineation of the articular surface of joints as in CT artrography. Furthermore, CBCT could be used for better joint delineation of hand, wrist, foot, and ankle in the case of suspected inflammatory joint disease. CBCT could also be used for better post-traumatic evaluation of consolidation or evolution to delayed or nonunion, especially when osteosynthesis material has been used [16]. However, all these possible indications must still be further investigated in clinical studies.
Conflict of interest The authors declare that they have no conflict of interest.
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