Dental disease is common in rabbits. Molar and premolar overgrowth is often diagnosed on the basis of findings of proper oral examination, but imaging techniques are also beneficial in the diagnosis and evaluation of dental disease. Although radiography can be helpful, numerous views are often required to make an accurate diagnosis. Moreover, interpretation of the images is difficult because rabbits have small skulls and superimposition of tooth roots and crowns occurs.1
Computed tomography overcomes some limitations of standard radiology by permitting cross-sectional images of the rabbit head in multiple planes without superimposition and with high-contrast resolution. The technique is suitable for early detection of even small changes in bone and adjacent soft tissues of the head and provides clear images of dental anatomy and pathological changes.2–4 The use of CT in the diagnosis of dental abnormalities in exotic animals is emerging5–8 and an increasing number of published clinical reports1,7,9,10 involve CT of the dentition. In addition to the diagnosis of dental diseases in rabbits, determination of a more accurate prognosis and treatment plan is possible with CT.5
In veterinary medicine, CT scanners intended for use in humans are frequently used. The spatial resolution of such equipment is of the order of 1 mm or just below this value. When scanning small objects such as rabbit skulls, this results in images that contain a small number of pixels, resulting in low spatial resolution.4,5 The disadvantage of low spatial resolution of CT images versus radiographs can be compensated for by the ability to view slices in various planes. The spatial resolution of CT images is superior when a multi-slice spiral scanner is used instead of a single-slice spiral CT unit and when axial scanning is performed instead of helical scanning.11 Increased scanning time can also improve image quality. Because of the small size of the structures involved in dental disease in rabbits, interpretation of helical CT data can still be challenging.
Micro-CT scanners or CT scanners designed for use in human patients provide superior spatial resolution, compared with conventional CT scanners. The spatial resolution of current laboratory-used micro-CT systems has improved from tens of micrometers to only a few micrometers. High-resolution micro-CT has been described as an important tool for biological research in veterinary medicine.12 It has the potential to replace serial histologic examinations as the reference standard in many in vitro studies, requiring little sample preparation and relatively simple data postprocessing,13 and it provides a practical approach for collection of quantitative information during some types of longitudinal investigations in vivo.14 The promising advantages of micro-CT, compared with helical CT, include high spatial resolution, high sensitivity to bone and lung tissue, and cost-effectiveness. Disadvantages for clinical use of micro-CT in exotic animals, compared with helical CT, include a long scanning time and a higher cost for the owner because the equipment is more expensive. The maximum size of objects that can be scanned with this equipment is approximately 30 cm3. Similar to helical CT, there is relatively poor contrast of soft tissue.15 The purpose of the study reported here was to characterize the anatomic features of dentition and surrounding structures of the head in rabbits by use of a newly developed micro-CT device.16
Materials and Methods
Sample—Cadavers of 7 commercially raised clinically normal Dendermonde White adult rabbits weighing 2.5 to 3 kg were used in this study. None of the rabbits had a history of dental disease, and abnormalities were not detected during physical and radiographic examinations. Two radiographic images were obtained for each rabbit (left lateral and dorsoventral). The rabbits were humanely slaughtered in an officially recognized Belgian slaughterhouse conforming to all relevant regulations and were subsequently flayed and decapitated at the level of the occipital condyle. The rabbit heads were then frozen at −14°C to prepare the sample for micro-CT scanning.
Micro-CT—Each frozen head was placed in a cardboard tube filled with paper tissue to isolate the sample and prevent it from thawing. The diameter of the cardboard tube was kept as small as possible to obtain maximal resolution. The cardboard tube was tightly secured onto the sample manipulator of the micro-CT scanner to ensure the sample would not move during scanning. The micro-CT scanner used in the study was custom designed and constructed at the Ghent University Centre for X-ray Tomography16 and had a dual-head open-type radiation source. The sample manipulator had a rotation motor with an ultraprecision stage and a load capacity of 5 kg.
Micro-CT was performed to obtain a series of transverse (perpendicular to the hard palate) images for each rabbit head from the nares to the occipital condyles, with a voxel size of 55 × 55 × 55 μm3. The distance between the centers of 2 adjacent voxels, the so-called voxel pitch, was 55 μm. The high voltage on the x-ray tube was 90 kV, and a directional W target was used on which 30 W of beam power was delivered in a focal spot of approximately 30 μm, filtered with 1 mm of copper. A flat-panel digital x-ray image detectora with a resolution of 1,820 × 1,460 pixels (pixel size, 127 μm) with a CsI scintillator was used. The scan settings were chosen to highlight the bony structures on the basis of the equipment designers' experience. Scanning time for each head was approximately 45 minutes.
The micro-CT scans produced 1,000 transverse projections/head, which were reconstructed into a stack of CT slices by use of a reconstruction package developed at the Ghent University Centre for X-ray Tomography.17,b Unlike medical CT, in micro-CT with flat-panel detectors, the voxels are almost always isotropic; thus, the slices had the same thickness as the reconstructed pixel size. Raw acquisition data were transformed into a stack of 2-D cross sections through the sample, resulting in a 3-D data set. The 3-D rendering was performed with dedicated software.c
The micro-CT images were reviewed and evaluated by 2 investigators (LMDR and IMG), and identifiable anatomic structures in images obtained from the first rabbit head were labeled with the aid of texts on rabbit anatomy4,18 and in accordance with accepted anatomic terminology.19 Afterward, the identified structures were evaluated in all micro-CT images of the other 6 rabbit heads.
Results
Ten representative transverse micro-CT images extending from the nares to the occipital condyles of 1 rabbit were selected for initial evaluation. The 10 levels at which micro-CT images were obtained were indicated on the left lateral radiographic view (Figure 1).
Left lateral radiographic image of the head of the cadaver of a clinically normal adult rabbit. Parallel lines (A through K) indicate locations at which transverse micro-CT images were obtained in a study to identify anatomic structures of the head and teeth in rabbits by use of this technology.
Citation: American Journal of Veterinary Research 73, 2; 10.2460/ajvr.73.2.227
Left lateral radiographic image of the head of the cadaver of a clinically normal adult rabbit. Parallel lines (A through K) indicate locations at which transverse micro-CT images were obtained in a study to identify anatomic structures of the head and teeth in rabbits by use of this technology.
Citation: American Journal of Veterinary Research 73, 2; 10.2460/ajvr.73.2.227
Left lateral radiographic image of the head of the cadaver of a clinically normal adult rabbit. Parallel lines (A through K) indicate locations at which transverse micro-CT images were obtained in a study to identify anatomic structures of the head and teeth in rabbits by use of this technology.
Citation: American Journal of Veterinary Research 73, 2; 10.2460/ajvr.73.2.227
All dental structures and numerous bony structures were identified on transverse micro-CT images (Figure 2). The first and second incisors as well as all maxillary and mandibular cheek teeth were distinct in the images. All teeth, outlined within dental alveoli, could easily be discerned, and pulp cavities were clearly visible. The apical aspects of teeth could be evaluated against alveolar bone. The occlusal surfaces of opposing cheek teeth were very distinct, and microstructure of the teeth was clearly outlined. A thin layer of dentin was seen surrounding the roots of all incisors and cheek teeth, and on the labial aspect, enamel was detected as a thin radiopaque line.
Transverse micro-CT images of the head of the same rabbit in Figure 1. Images were obtained at slices A through K as defined in Figure 1. M1 = First molar. M2 = Second molar. M3 = Third molar. P2 = Second premolar. P3 =Third premolar. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 73, 2; 10.2460/ajvr.73.2.227
Transverse micro-CT images of the head of the same rabbit in Figure 1. Images were obtained at slices A through K as defined in Figure 1. M1 = First molar. M2 = Second molar. M3 = Third molar. P2 = Second premolar. P3 =Third premolar. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 73, 2; 10.2460/ajvr.73.2.227
Transverse micro-CT images of the head of the same rabbit in Figure 1. Images were obtained at slices A through K as defined in Figure 1. M1 = First molar. M2 = Second molar. M3 = Third molar. P2 = Second premolar. P3 =Third premolar. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 73, 2; 10.2460/ajvr.73.2.227
The various skull bones (eg, incisive bone, nasal bone, maxilla, palatine bone, sphenoid bone, frontal bone, parietal bone, zygomatic bone, temporal bone with tympanic bulla, occipital bone, and mandible) were also identifiable in micro-CT images. In some of these, the 2 layers (tabula externa and interna) of cortical bone were seen surrounding the spongious or trabecular bone (Figure 2). The intricately formed web of numerous trabeculae was most easily detected in the incisive bones and mandibles. The frontal sinus of the frontal bone was also clearly filled with trabecular bone. At the level of the nasal cavities, the ethmoturbinates, nasal meati, dorsal and ventral conchae, and nasal septum were identifiable. Inside the rostral aspect of the nasal cavity, the spiral-shaped nasal conchae that form sinuses were clearly defined. In addition to the larger spirals of the ventral nasal conchae, further bifurcations of these structures were evident in micro-CT images. Caudally, the nasal conchae passing into the endoturbinates were seen as short and very fine bony lamellae forming a spiral web, the path of which could be followed in detail inside the nasal cavity. The dorsal and ventral parts of the maxillary sinus and the vomeronasal organ were also visible. Soft tissue accompanying bony parts of the nasal cavity was identified as the nasal mucosa. Because the edges of cortical bone were discernible, the gap between the nasal and incisive bones (or so-called nasoincisive incisure), the palatine fissure that flanks the lateral aspects of the palatine process of the incisive bone, and the mandibular symphysis were observed on micro-CT images. The edges of the basilar part of the occipital bone and the petrous portion of the temporal bone were very distinct and revealed the lack of a bony connection between the base of the cranium and the petrous portion of the temporal bone.
For all rabbits evaluated, the same structures could be seen on corresponding micro-CT images. Because settings for the micro-CT scan were adjusted for assessment of bony structures, soft tissue structures such as cartilage, subcutaneous fat, muscles, tongue, buccal mucosa, and parts of the brain could not be identified on the resulting images.
To investigate another application of micro-CT in rabbits, transverse slices were used to reconstruct 3-D images of the bone tissue. A model of the entire skull, which could be manipulated on the computer screen and observed from various angles, was made by use of the dedicated software (Figure 3). Once a sample had been scanned and its virtual 3-D image was created, it was possible to make an unlimited number of virtual sections at any location and orientation through the sample. On the resulting image, the webs of the trabecular bone and the nasoturbinates could be traced out in detail. The internal structures of the teeth were visible as well as the exact outlines of cranial bones were visible. Without opening or slicing the specimen, parts of the internal structure of the rabbit skull were revealed.
Photographs of a computer-generated 3-D reconstruction of the skull of a clinically normal adult rabbit (A and B). The image was obtained by use of a custom-developed reconstruction package and dedicated software with input from 1,000 transverse micro-CT slices collected over approximately 45 minutes. A—Rostral view of the intact reconstruction. B—Rostral view of the same reconstruction after virtual sectioning (transverse slice at the level of the cheek teeth). c = Cortical bone. ct = Cheek teeth. fc = Cribrose surface of the maxilla. I1 = Maxillary first incisor. ib = Incisive bone. m = Mandible. ma = Maxilla. n = Nasopharyx. nb = Nasal bone. no = Nasal opening. nt = Nasoturbinates. p = Parietal bone. t = Trabecular bone. zb = Zygomatic bone.
Citation: American Journal of Veterinary Research 73, 2; 10.2460/ajvr.73.2.227
Photographs of a computer-generated 3-D reconstruction of the skull of a clinically normal adult rabbit (A and B). The image was obtained by use of a custom-developed reconstruction package and dedicated software with input from 1,000 transverse micro-CT slices collected over approximately 45 minutes. A—Rostral view of the intact reconstruction. B—Rostral view of the same reconstruction after virtual sectioning (transverse slice at the level of the cheek teeth). c = Cortical bone. ct = Cheek teeth. fc = Cribrose surface of the maxilla. I1 = Maxillary first incisor. ib = Incisive bone. m = Mandible. ma = Maxilla. n = Nasopharyx. nb = Nasal bone. no = Nasal opening. nt = Nasoturbinates. p = Parietal bone. t = Trabecular bone. zb = Zygomatic bone.
Citation: American Journal of Veterinary Research 73, 2; 10.2460/ajvr.73.2.227
Photographs of a computer-generated 3-D reconstruction of the skull of a clinically normal adult rabbit (A and B). The image was obtained by use of a custom-developed reconstruction package and dedicated software with input from 1,000 transverse micro-CT slices collected over approximately 45 minutes. A—Rostral view of the intact reconstruction. B—Rostral view of the same reconstruction after virtual sectioning (transverse slice at the level of the cheek teeth). c = Cortical bone. ct = Cheek teeth. fc = Cribrose surface of the maxilla. I1 = Maxillary first incisor. ib = Incisive bone. m = Mandible. ma = Maxilla. n = Nasopharyx. nb = Nasal bone. no = Nasal opening. nt = Nasoturbinates. p = Parietal bone. t = Trabecular bone. zb = Zygomatic bone.
Citation: American Journal of Veterinary Research 73, 2; 10.2460/ajvr.73.2.227
Discussion
Noninvasive imaging technologies are recognized as useful tools for the inspection of small animal anatomy, pathological changes, and development. Many imaging technologies have been developed in addition to radiography and CT, including micro-CT, magnetic resonance microscopy, high-frequency ultrasonography, micropositron emission tomography, high-resolution single photon emission CT, and optical imaging. Micro-CT technology is generally used to provide high-resolution anatomic images of small animals, whereas micropositron emission tomography, high-resolution single-photon emission CT, and optical imaging are used to acquire functional images at relatively lower resolutions.15 Because of the relatively low cost of x-ray imaging hardware, compared with most of the previously mentioned imaging techniques, micro-CT has the potential to be among the least expensive small animal imaging technologies; however, it is still more expensive than helical CT.
The micro-CT equipment used in the present study was also more expensive than most commercially available micro-CT scanners because of its high resolution (1 μm) and ability to perform tomographic scans of various subjects or objects ranging from biological to geologic samples. This micro-CT scanner was built inside a bunker on a large granite optical (vibration-suppressing) table to obtain the greatest possible stability. The x-ray tube's dual-head open-type source enables replacement of filaments or targets. One of the 2 tube heads allows high-power micro-CT, and the other is used for sub–micro-CT (also called nano-CT); to change from one tube to the other, the x-ray system must be rotated by 60°.16 A particular advantage of this equipment is that various detectors can be used with the scanner to optimize scanning conditions for the object or anatomic structure under investigation.16 In addition, a custom-developed reconstruction software package was used for 3-D modeling. A number of artifact and noise-reduction algorithms were integrated into the program (eg, to reduce ring artifacts, beam-hardening artifacts, center-of-rotation misalignment, detector or stage tilt, and pixel nonlinearities).17 These corrections make important contributions to image quality and, as a result, may facilitate analysis of 3-D data.
The high-resolution scans and the 3-D modeling capability of micro-CT systems can be used for imaging of various organ systems during preclinical research, including the skeleton, thoracic and abdominal organs, and brain.15 The major application of micro-CT to date has been quantification of the density and architecture of bone20–23 and other calcified tissues such as root structures in teeth.24–27 A new, interesting challenge for x-ray micro-CT is visualization of soft tissue structures.12 To date, these studies have focused on the microvasculature of the kidney,28,29 liver and biliary system,30 heart,31–34 and internal structure of the lung35–37 and on systematic phenotyping of transgenic mouse embryos.14,38 Future technical advances will likely include much faster 3-D scanning techniques and the development of new contrast techniques to provide even more detailed images of the vasculature of live animals.
Important issues to consider when designing a small animal micro-CT protocol are choice of anesthetic (because of potential sensitivity of small exotic animals to some products), trade-off between ionizing radiation dose and image quality, and choice of contrast media.12 Micro-CT will probably become a standard tool in many laboratories and universities because of its capacity to provide quantitative 3-D data.
In the study reported here, we obtained micro-CT images of rabbit heads with an emphasis on dentition. This resulted in poor soft tissue contrast on all images, and thus it was not possible to identify most soft tissue structures in rabbit heads. Changing the settings of the micro-CT scanner used in the present study would allow better differentiation between fat and soft tissues. Although soft tissue can also be enhanced by use of contrast medium in living animals, several boluses would be needed during the 45-minute scan, which is not clinically practical. Compared with helical CT, most micro-CT scanners reveal no extra soft tissue detail, except for those that use synchrotron radiation, in which electrons are accelerated to very high speeds in a storage ring.4 These high-energy, high-quality x-rays enhance the contrast of the types of soft tissues that had a rather homogenous density distribution on micro-CT images obtained in the present study.
The microstructure of cranial bones and dentition in the rabbits of the study reported here was clearly visible. It was possible to observe the cortical bone and the intricate web of trabeculae of the trabecular bone. All dental structures, such as the pulp cavity, dentin, and enamel, could be identified, and each tooth could be discerned within its alveolus.
Micro-CT has been used for the diagnosis of dental disease in a guinea pig that had lysis of the alveolar bone adjacent to the tooth root, periosteal reaction of the cortex of the maxillary and mandibular bone, and a trabecular bone lesion.1 Because guinea pigs have complete hypsodont dentition and diagnostic challenges in this species are similar to those encountered in rabbits, it may be concluded that micro-CT is a promising technique for appropriate diagnosis of dental disease in rabbits as well. Because micro-CT shows excellent detail, pathological changes may be detected more easily and at an earlier stage with this method than with helical CT.
However, there is still limited access to micro-CT equipment in veterinary practice.3 The higher cost for micro-CT equipment suggests a higher cost for the owner, compared with that of helical CT. Moreover, the micro-CT scanning of a small object requires substantially more time than that needed for helical CT scanning. In our study, approximately 1 hour was needed to scan a rabbit head. New commercially available micro-CT systems are faster, but acquisition times are still longer than those of helical CT. The limited size of objects that can be scanned with micro-CT (approx 30 cm3) also restricts practical use. Further research is needed to determine the clinical applications of micro-CT in small animals.
ABBREVIATION
| CT | Computed tomography |
VARIAN 2520V Paxscan a-Si flat panel with CsI scintillator, Varian Medical Systems, Palo Alto, Calif.
Octopus, Centre for X-ray Tomography, Ghent University, Ghent, Belgium.
VGstudio Max, Volume Graphics GMBH, Heidelberg, Germany.
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