The skull of lizards belonging to the infraorder Iguania
is roughly triangular in dorsal view, with a short pre-orbital region. It retains all the characteristics of the ancestral lizard skull with no secondary closure of the skull openings [15
]. The main difference between Iguanidae
is in the nature of tooth implantation – acrodontal rather than pleurodontal – although most agamids have at least some pleurodont teeth in the front lower jaw [15
Standard positioning is a prerequisite for good film quality [29
]. As in snakes, the junctions between the bones composing the lower jaw are relatively loose [13
]. Therefore, radiographic positioning quality should be evaluated, in our opinion, primarily through the symmetry and superimposing of the bilateral structures of the snout and neurocranium both in lateral and sagittal projections.
All the radiographs and CT images shown in the figures were obtained from live animals. It is the authors’ opinion that, although a direct comparison between anatomical and diagnostic images was not possible, a significant correlation in the matched images was achieved. Moreover, the use of contrast medium enabled good visualisation of the soft tissues and overcame the lack of soft tissue detail encountered in other similar works performed only on cadavers [8
The CT images of the iguana (Figure
) provided good detail when displayed both in bone and soft tissue windows. The CT images of the tegu (Figure
) displayed in a bone window provided good detail of the bony structures whereas in the images displayed in a soft tissue window only the eyes were clearly distinguishable from the remaining soft tissues. The CT images of the bearded dragon (Figure
) were of a relatively lower quality in both the bone and soft tissue windows. This lack of detail is due to the small size of this species and to the impossibility to reduce the field of view of our CT scanner to less than 16
cm. Despite this lack of detail, most of the bony structures were identified in CT images displayed in the bone window. In the CT images displayed in the soft tissue window, only the eyes and the Harderian glands were distinguishable from the other soft tissues of the head.
The nasal cavity of all three species was clearly visible both in VD (Figures
C, C, C) and LL (Figures
C, C, C) radiographs. The septomaxilla and the nasal glands were very prominent in the LL radiographic projection of the iguana (Figure
C), giving the nasal cavities an overall higher radiopacity than in the tegu (Figure
C) and the bearded dragon (Figure
C). The nasal cavity of the iguana appeared almost entirely occupied by the nasal glands both in cross-sections and CT scans (Figures
B, C, D). The nasal glands in the bearded dragon (Figures
B, C, D) were similar in appearance but less prominent in the nasal cavity. The nasal glands in the tegu were more medially located and less evident both in cross-sections and in CT images (Figures
B, C, D).
The scleral ossicles were clearly visible both in the iguana (Figure
C) and the tegu (Figure
C) in LL radiographic projections, whereas they appeared less evident in the LL radiographic projection involving the bearded dragon (Figure
C). Furthermore, the scleral ossicles were evident in CT images of all the examined species (Figures
F,G, F,G, F,G) but were hardly visible in anatomic cross-sections (Figures
E, E, E).
The bones composing the lower jaw were not individually evident either in LL or VD radiographic projections in any of the examined species because the sutures between the bones of the lower jaw are smaller than the minimum radiologic resolution.
The oesophagus was well defined in LL radiographs of all the examined species (Figures
C, C, C). It appeared as a U-shaped radiolucency bordering the caudal aspect of the lower jaw in the iguana (Figure
C) and bearded dragon (Figure
C) whereas it appeared straighter in the tegu (Figure
C). Furthermore, the dorsal portion of the oesophagus is partially superimposed on the highly developed masticatory muscles in the iguana and bearded dragon (Figures
A and A, respectively) and is grossly much larger, as can be seen in the radiographs.
The trachea was clearly identified in the LL radiographic projection partially superimposed on the ventral aspect of the oesophagus in the iguana (Figure
C) and tegu (Figure
C), while it was difficult to identify in the bearded dragon (Figure
C). In the iguana (Figure
C) the trachea showed a peculiar outline turning two ninety-degree angles at the inlet of the coelomic cavity. All the animals were deeply sedated during the imaging procedures; the authors are unable to determine whether this latter feature is due to muscular relaxation induced by anaesthesia or is normal even in unsedated iguanas.
The eyes were very evident in all the species examined, both in cross-sections and CT images displayed in a soft tissue window (Figures
E, G, E, G, E, G). The lens and the vitreous were clearly delineated both in cross-sections and CT images in the iguana (Figures
E, G) and tegu (Figures
E, G). The lens and the vitreous were not individually identified either in cross-sections or CT images in the bearded dragon (Figures
E, G). In the iguana the Harderian glands were hardly visible in cross-sections (Figure
E) while they were very prominent and clearly distinguishable from the underlying sinus orbitalis
in CT images (Figure
G). A radiodense line was noticed on the ventral aspect of the sinus orbitalis
in post-contrast CT images of the iguana displayed in a soft tissue window (Figure
G). In the tegu the Harderian glands were clearly evident in cross-sections (Figure
H) but were difficult to identify in the CT images (Figure
L). In the bearded dragon the Harderian glands could be identified both in cross-sections (Figure
E) and CT images (Figure
G) but were less evident than in the iguana.
The head of lizards can be affected by several pathologies; abscesses, metabolic bone diseases, fractures, osteomyelitis, and neoplasia [30
Bites from preys, traumas, foreign bodies, pyogenic infections may lead to the formation of abscesses in the head of lizards; this is especially true in captive specimens, as they are often immunocompromised and more prone to develop inflammation [31
]. Successful treatment of the reptilian abscess is due to the complete removal of abscess cavity and surrounding fibrous capsule [31
]. Therefore both radiographic and CT studies could be helpful to determine the extension and the number of involved structures for a good pre-surgical evaluation. Moreover, CT studies may become mandatory to achieve a correct diagnosis in deep abscesses that have no external protrusion.
Metabolic bone disease may result from different underlying pathologies in reptiles [32
]. To this effect, conventional radiology and dual-energy x-ray absorptiometry (DXA) techniques of the head of lizards are obvious diagnostic tools, and may also successfully contribute to the follow up evaluation of the relative treatments [2
Skull fractures of traumatic or pathological origin (i.e.: metabolic bone disease, neoplasia) are not uncommon in reptiles [34
]. Plain or conventional radiology is not considered as the technique of choice to assess a skull fracture due to: 1) the relatively small size of the bones composing the skull, and, 2) the high risk of border effacement due to different tissues superimposition that is intrinsic to the technique. On the contrary it is the authors’ opinion that CT may be useful to correctly assess skull fractures in lizards and, more in general, in reptiles.
The mandibula and the maxilla are the most common sites of osteomyelitis in the head; changes in their bony structure include osteolysis and less frequently new bone formation [30
]. Although plain radiographs may be sufficient to diagnose this pathology [30
]; nevertheless, in our opinion, CT scans may provide additional information on the extension of the infectious process and aid in planning a correct therapy.
Head/skull neoplasia is considered as a quite common disease affecting reptiles [33
]. Since lizard patients often require sedation for diagnostic imaging procedures [30
], it is the authors’ opinion that, CT may be the imaging technique of choice if neoplasia of the head is suspected. The mayor advantages of CT are: the possibility to fully visualize the extension of the neoplastic process; and the possibility to perform CT guided fine needle aspirates or biopsies [35
The skull of reptiles develops from a post-hatching cranium, the chondrocranium, to its definitive form in adults, the osteocranium [15
]. Often the bones composing the chondrocranium have not reached full adult form [15
]. All the tables presented in this work refer to adult animals and care should be taken when using these tables to interpret radiographs or CT studies of immature animals.