Effect of Positioning on Coelomic Radiography with and without Contrast in the Green Iguana (Iguana iguana)
Abstract
A prospective study was performed in 12 green iguanas (Iguana iguana) to compare the radiographic appearance of coelomic viscera. Five radiographic projections (views) of the coelom were obtained: dorsoventral, right lateral vertical beam, left lateral vertical beam, right lateral horizontal beam, and left lateral horizontal beam. One week later, the same radiographic projections were obtained after oral administration of barium. In 4 of the 12 green iguanas, an additional vertical beam ventrodorsal view was obtained. In comparing lateral projections, lung, liver, and gonads were best imaged in horizontal beam lateral projections, before distention of the stomach with barium, whereas the gastrointestinal tract including the colon was best visualized in vertical beam lateral projections. The shape and size of the cardiac silhouette was considered equivalent between analogous lateral projections in most green iguanas (10 [83%] of 12). In comparing ventrodorsal and dorsoventral projections, the ventrodorsal projections improved visibility of the cardiac silhouette and lung in 4 (100%) of 4 green iguanas, while reducing cardiac length. Dorsoventral projections were preferred for the evaluation of the gastrointestinal tract. This study demonstrates that radiographic beam orientation and patient position result in obvious radiographic differences of the iguanid coelom and that projections are not interchangeable.
Introduction
Radiography is commonly used in the diagnostic workup of reptiles. For the evaluation of the coelomic cavity, at least two orthogonal projections are required: lateral and dorsoventral (or ventrodorsal) views (Holmes and Divers, 2019). Historically, horizontal beam lateral projections have been recommended, however; in many settings, rotation of the anode to a horizontal plane is not possible.
In a previous study of echocardiographic and radiographic findings in healthy green iguanas (Iguana iguana), horizontal beam right lateral projections improved visualization of the cardiac silhouette and great vessels compared with vertical beam right lateral projections (Gustavsen et al., 2014). In dogs and cats, position and conspicuity of thoracic and abdominal structures have been well documented to vary between projection angles (Ruehl and Thrall, 1981; Armbrust et al., 2000; Avner and Kirberger, 2005; Marolf et al., 2008; Kirberger et al., 2009; Vander Hart and Berry, 2015; Thrall, 2017; Mavromatis et al., 2018). The normal radiographic gastrointestinal anatomy and barium sulfate contrast transit time have been described in the green iguana (Smith et al., 2001) and bearded dragon (Pogona vitticeps) (Grosset et al., 2014) by means of dorsoventral and horizontal beam right lateral views. Our group is unaware of published literature investigating the effect of horizontal vs. vertical radiographic beam orientation and the potential differences between right and left lateralities in lizards.
The present study was conducted to assess the effects of radiographic beam orientation and patient laterality on the appearance of the iguanid coelom. Green iguanas were selected for this study because they are relatively common, frequently presented for veterinary attention, and serve as an accepted saurian model.
Our hypothesis was that shifting of viscera would result in clinically relevant differences in the appearance of organs between horizontal and vertical beam projections and ventrodorsal and dorsoventral positioning, but that due to the patient's small size, no difference would be observed between right lateral horizontal beam and left lateral horizontal beam.
Materials and Methods
The study included 12 sub-adult green iguanas, 8 males and 4 females, weighing 0.46–1.05 kg (mean: 0.59 kg). The green iguanas were housed in Association for Assessment and Accreditation of Laboratory Animal Care–accredited laboratory facilities according to species-specific requirements, and all methods were evaluated and approved by the University of Georgia institutional animal care and use committee (A2019 08-025). All green iguanas were acclimatized for 7 days in a controlled environment at 29.4–35°C (85–95°F) and 65% humidity and were deemed healthy on physical examination.
All green iguanas were fasted for 12 h before sedation. Each green iguana was sedated with 0.1 mg/kg dexmedetomidine (Dexdomitor, 0.5 mg/ml, Zoetis, Orion Corporation, Parsippany, NJ, USA) IM. The head was wrapped using self-adherent bandaging tape (Vet Wrap, 3M Canada Inc., London, ON, Canada) to induce a vaso-vagal response and minimize light, sound, and movement during imaging (Smith et al., 2001). After image acquisition, animals were reversed 80 min after induction with 1 mg/kg atipamezole (Antisedan, 5 mg/ml, Zoetis, Orion Corporation) IM.
Before image acquisition, pectoral limbs were extended cranially and pelvic limbs were extended caudally and held in position, away from the coelom, by using self-adherent bandage tape. Radiographs were acquired using a digital radiography system (RadPRO® Elite XM, Canon USA Inc., Melville, NY, USA) with wireless digital detectors (Canon CXDI-70C, Canon USA Inc.).
Radiographic projections were obtained of each green iguana in the following order: 1) vertical beam dorsoventral (DV), 2) left lateral horizontal beam (LHB), 3) right lateral horizontal beam (RHB), 4) left lateral vertical beam (LVB), and 5) right lateral vertical beam (RVB). All lateral radiographs were acquired using a technique of 58 kVp and 2 mAs, whereas DV radiographs were acquired using a technique of 48 kVp and 2 mAs. All radiographs were acquired with a focal distance of 105 cm, with a field of view including the entire coelom and centering of the light beam on the mid-coelom. For the horizontal beam projections, the x-ray tube was rotated 90° from vertical and the digital detector was positioned vertically, perpendicular to the x-ray tube. Each green iguana was positioned in ventral recumbency against the digital detector on a 10-cm-thick foam pad so as to be elevated from the tabletop and positioned at the center of the digital detector (Fig. 1). For the vertical beam lateral projections, the iguanas were placed in lateral recumbency on the radiography table and restrained with tape positioned over the previously applied self-adherent bandaging tape. After radiographs, in the same anesthetic episode but a separate experiment, green iguanas were administered iohexol (Omnipaque, 350 mg iodine/ml, Novaplus®, Marlborough, MA, USA) at 600 mg iodine/kg IO.
Citation: Journal of Herpetological Medicine and Surgery 31, 1; 10.5818/JHMS-S-20-00023
One week later, a positive contrast gastrointestinal study was performed with the same radiographic projections obtained in the same order as described previously. A 60% (w/v) barium sulfate oral suspension (Liquid EZ-PAQUE®, E-Z-EM Canada Inc., Anjou, QC, Canada) was diluted with tap water to a 25% (w/v) concentration. A 25-ml/kg dose of this barium sulfate dilution was administered through a stainless steel feeding tube (10-gauge × 6 in. [curved], Veterinary Specialty Products, Shawnee, KS, USA) inserted through the mouth and into the distal esophagus, as described previously (Smith et al., 2001). In the first four green iguanas, an additional vertical beam ventrodorsal (VD) radiograph was performed following the routine set of radiographs by using the same DV technique. However, because the fourth green iguana regurgitated the barium, VD techniques were not performed on subsequent green iguanas.
Radiographic images were retrospectively evaluated by a board-certified veterinary radiologist with 8 yr of experience (KPG) and a first-year diagnostic imaging resident (MCMB) by using a web-based diagnostic imaging viewer (NilRead, Lexmark Canada Inc., Toronto, ON, Canada). Adjustments to image contrast and brightness were permitted, and a consensus opinion was obtained for each criterion. In total, 124 images were evaluated (60 survey images and 64 images from the positive contrast gastrointestinal study). Position, shape, opacity, and margination of the coelomic organs were compared between projections. For each image, the following characteristics were recorded and compared between analogous projections: 1) shape, definition, and caudal extent of the cardiac silhouette relative to the costosternal junctions; 2) degree of inflation of the lungs (VD and DV projections only): degree of inflation of the lungs was graded as poor if the outline of the cranial coelom had a concave appearance, moderate if the cranial coelom had a straight appearance, and good if the cranial coelom had a convex appearance; 3) opacity and contour of the caudal lung fields; 4) shape, opacity, and definition of the liver and gallbladder; 5) position, shape, and definition of the gastrointestinal tract (stomach, small intestine, and colon) and influence of positive contrast medium; and 6) visibility of other organs: gonads, spleen, urinary bladder, and kidneys.
One week after the last radiographic evaluation, the green iguanas were humanely euthanized and necropsies were performed. Samples of liver, kidney, and heart were routinely collected, along with any other abnormal tissues. Tissues were collected into 10% neutral buffered formalin, sliced to 5 μm in thickness, dehydrated, stained with H&E, and examined by two resident pathologists (AW, RR) and a boarded pathologist (NLG).
Results
During the course of the study, one animal was found dead 96 h after the preliminary sedation and imaging series; therefore, this animal was excluded from the study. A second animal was found dead 72 h after the second sedation, but this animal was not excluded as the two imaging events had been completed. Necropsy in both green iguanas revealed subacute, Gram-negative bacterial septicemia, with granulomatous pneumonia, necrotizing granulomatous myocarditis, hepatitis, splenitis, nephritis, and nematodiasis. Histologic findings in all other green iguanas (11 of 12) were judged by the pathologists as minor and unlikely to translate into detectable radiographic changes: multifocal hepatic lipid deposition (8 of 12), lipid deposition in the liver capsule (1 of 12), multifocal myocardial degeneration (4 of 12), and renal tubular lipidosis (3 of 12).
Vertical beam (VB) and horizontal beam (HB) projections were not considered equivalent for evaluation of the coelomic organs. In the four subjects in which both DV and VD radiographs were obtained, these views were not considered equivalent for evaluation of the coelomic organs.
The cardiac silhouette was found at the cranial aspect of the coelom within the pectoral girdle, extending on average in VD to the level of the caudal endplate of T1 (range cranial aspect of T1 to T1/2 intervertebral disc space) and in DV to the caudal endplate of T2 (range caudal endplate of T1 to the caudal endplate of T3). In 4 (100%) of 4 green iguanas, the cardiac silhouette was better defined in the VD projection compared to the DV projection (Table 1). In lateral projections, the cardiac apex was located at the level of the first or second sternocostal junction. In 10 (83%) of 12 green iguanas, the shape and size of the cardiac silhouette were considered equivalent between LVB/LHB and RVB/RHB.
In all green iguanas (12 [100%] of 12), HB projections subjectively allowed a better visualization of the caudal lung fields compared with VB projections (Table 2). In all LVB/RVB images, the stomach superimposed over the caudal lung fields. Lung inflation was improved in 4 of 4 green iguanas in VD views, with resultant decreased organ superimposition (Fig. 2). In the corresponding DV projections of these same four lizards, lung inflation was graded poor (1 of 4) or moderate (3 of 4) (Table 1). No radiographic abnormalities were noted in the green iguana that later died from necrotizing pneumonia.
Citation: Journal of Herpetological Medicine and Surgery 31, 1; 10.5818/JHMS-S-20-00023
In 6 (50%) of 12 green iguanas, the dorsal aspect of the liver was ill defined and lucent in HB projections compared with the analogous VB projections (Fig. 3). Distension of the stomach with contrast resulted in effacement of the caudal hepatic margin. The liver and gallbladder were poorly defined and similarly poorly opaque in VD and DV projections (Table 1). Hepatic shape as seen in lateral projections was variable between animals, ranging from elongated/rounded (4 of 12) to a well-defined trapezoid shape (8 of 12), but in the same animal remained similar between VB and HB lateral projections.
Citation: Journal of Herpetological Medicine and Surgery 31, 1; 10.5818/JHMS-S-20-00023
The esophagus could be easily identified in survey lateral images with the presence of variable fluid and gas in both VB and HB projections (12 of 12). After administration of barium, a small-to-moderate amount remained in the mid- to caudal aspect of the esophagus, highlighting the longitudinal folds in all projections.
Projection angle had a visible impact on the position and appearance of the stomach. In VB lateral projections, the cardia and fundus shifted dorsally, resulting in improved visualization of the pylorus (12 of 12). Dorsal bowing of the antrum was observed in the RVB only in 7 (58%) of 12 green iguanas (Fig. 4). In all green iguanas, the stomach was better identified with positive contrast compared with the survey study where animals were fasted and the stomach was difficult to distinguish from the remaining coelomic structures.
Citation: Journal of Herpetological Medicine and Surgery 31, 1; 10.5818/JHMS-S-20-00023
The first radiograph of the positive contrast gastrointestinal study was obtained 20.9 min (mean) after barium administration (range: 15–36 min), and the last radiograph (LR or VD) was performed 29.7 min (mean) after barium administration (range: 21–49 min). In 8 (67%) of 12 green iguanas on LVB projections, the barium-filled cardia had a distinctive angular/square shape that was not appreciated in any other plane orientation or in survey radiographs. The barium-distended stomach impinged on the caudal lung fields (DV view, 4 of 4), caudal aspect of the liver (lateral projections) (12 of 12), transverse colon (11 of 12), and gonads, and for these organs' evaluation survey views were preferred.
Small intestines could only be positively identified in 10 (83%) green iguanas with the presence of barium. The small intestine was inconsistently visualized, but when it was visualized, VB lateral projections provided the least amount of superimposition with the stomach (Table 2). The colon could only be identified by the presence of fecal matter or gas. In VB lateral views, the transverse colon pivoted dorsally and was confirmed in all green iguanas in which the colon could be identified (9 of 12), a phenomenon similar in magnitude to the displacement of the stomach noted above.
Gonads were identified in lateral projections as plump round-to-ovoid structures in the dorsal caudal coelom at the level of and either immediately caudal to or partially superimposed with the caudal saccular lung. In select individuals with large gonads, these indented the caudal margins of the fundus in DV when gas was present. Gonads were identified in males only, but not all males (6 of 8), and they were much better depicted in HB laterals (Fig. 5).
Citation: Journal of Herpetological Medicine and Surgery 31, 1; 10.5818/JHMS-S-20-00023
The spleen, urinary bladder, and kidneys were not definitively identified in any of the 12 green iguanas in any projections.
Discussion
In any species, proper radiographic assessment must include two projections made at right angles (Ruehl and Thrall, 1981; Armbrust et al., 2000; Avner and Kirberger, 2005; Kirberger et al., 2009), and current recommendations for saurian imaging suggest dorsoventral VB and lateral HB radiographs (Holmes and Divers, 2019). Differences between VB and HB had until now not been objectively assessed in reptiles. Practical reasons dictate this preference for lateral HB images: patients positioned in ventral recumbency require less restraint, although commonly the radiographic equipment used in general practices does not allow rotation of the anode for HB projections.
The aim of this study was to evaluate the influence of patient position on the radiographic anatomy of the green iguana coelom. Results showed clinically relevant specificities between HB and VB laterals and between DV and VD.
In this study, the shape and length of the cardiac silhouette appeared similar on lateral projections independent of beam orientation or laterality in 83% of green iguanas, whereas mild elongation and flattening was observed in RVB in the remaining 17% of green iguanas. Whether this is an effect of heart cycle or position is unclear. In green iguanas, unlike mammals, the heart lies on ventral midline within the pectoral girdle and occupies much of its width, thereby preventing movement or displacement. Therefore, patient laterality should have no substantial effect on heart position and its radiographic appearance. Previous studies have found the cardiac silhouette and great vessels of iguanas to be best visualized on HB lateral images (Gustavsen et al., 2014). In contrast, in this study, HB projections yielded no definitive advantage for the visibility or consistency in appearance of the cardiac silhouette. Beam centering, patient positioning, and patient size are factors that we suspect may explain these diverging findings. Poor differentiation of the cardiac apex from the coelom was said to be common by the above-mentioned authors and prevented cardiac measurements. Our results indicate that in VB lateral views these changes are explained by subtle cranial displacement of the liver and increased retrosternal soft tissues, effects that are potentiated by distension of the stomach (in this case with barium). In dogs, cardiac length is increased in VD radiographs compared with DV (Ruehl and Thrall, 1981). Conversely, in this study the length of the cardiac silhouette was mildly shortened in VD in most green iguanas where this view was obtained. We surmise that in this species the cardiac silhouette is foreshortened in VD due to dorsal rotation of the apex. Although the cardiac silhouette appeared better defined in VD projection, it remained poorly marginated in both VD and DV and was attributed to anatomic factors such as summation of the scapulae, clavicles, shoulder joints, and sternum (Gustavsen et al., 2014).
The radiographic appearance of the lung is highly influenced by its degree of inflation (Thrall, 2012). In this study, visceral displacement by gravity in HB projections and superior expansion of the ribcage in VD yielded the highest degree of lung inflation. The influence of respiratory phase remains to be determined in these animals. Because of slow respiration rates, shallow breathing, and long periods of apnea, no effort was made to time exposures with full inspiration. It should be remembered that dorsal recumbency is not recommended in dyspneic patients, and that in this context, DV should be considered despite its limitations.
In 50% of the green iguanas, the liver was less well defined and more lucent in HB projections compared with the analogous VB projections. In iguanas, the liver is caudal to the heart and is the largest organ of the coelomic cavity. It consists of a ventral and dorsal lobe (Oldham and Smith, 1975). Administration of oral contrast, by distending the stomach, significantly reduced the visibility of the liver in lateral projections. A limitation of this study is that although livers, hearts, and kidneys were macroscopically normal, all were histologically abnormal. Hepatic lipidosis, myocardial degeneration, and renal tubular lipidosis were interpreted by the pathologists as incidental, related to short-term anorexia or fasting, and likely agonal and not due to disease, respectively. The influence of these pathologies on the radiographic findings remains to be determined. In the creation of an experimental model, healthy animals are preferably used to produce consistent responses to clinical scenarios. One limitation of this study is that one of the animals had significant underlying morbidity; however, as no radiographic changes were detected 3 days antemortem and positional organ behavior was similar to the rest of the group, the choice was made to keep this green iguana in the study.
Barium administration is typically well tolerated in reptiles, but in a previous study involving iguanas, 3 of 5 animals showed signs of regurgitation. Barium was only radiographically detected in the esophagus of individuals that had regurgitated during administration of the suspension (Smith et al., 2001). In this study, in both survey and contrast series, the esophagus contained fluid and gas or barium, respectively, in nearly every subject. The sequence of radiograph acquisition, which began with DV and HB, rules out the presence of barium in the esophagus to be secondary to lateral recumbency. The gastroesophageal sphincter of lizards is better developed than that of snakes, which may explain why vomiting and regurgitation are rare in lizards (Summers, 2014). The presence of fluid and barium in the esophagus is suspected to be due to weakness of a gastroesophageal sphincter, possibly induced by sedation as previously described in humans (Turan et al., 2010).
In iguanas, the relationship of the lungs and the stomach is dynamic due to the lack of a diaphragm (Oldham and Smith, 1975; Holmes and Divers, 2019), and visceral displacement by gravity may be played to the clinician's advantage. The same factors that result in poor lung aeration in lateral VB improve visibility of the gastrointestinal tract, owing to decreased superimposition related to organ spread and reduced coelomic compression by the weight of the body. The stomach in LVB projections presented a distinctive angular/square shape that was not appreciated in any other plane orientation. This characteristic could be a reasonably reliable sign to recognize this projection in case labeling was missing. The stomach was the only organ in which notable differences were present between left and right lateral projections. In the four cases where both VD and DV were obtained, resolution of the gastrointestinal tract was judged best in DV due to reduced organ superimposition. Given that VD is assessed to be less useful for the investigation of the stomach and intestines, and that it carries a risk for regurgitation, DV views should be preferred when the clinical question relates to the gastrointestinal tract, particularly after administration of contrast.
Barium was present in the orad portions of the small intestines at a mean of 20.9 min postadministration, consistent with previously published normal transit times in unsedated iguanas (Smith et al., 2001). In veterinary and human patients, α2-adrenoceptor agonists such as dexmedetomidine have significant inhibitory effects on gastrointestinal motility (Cullen, 1996; Asai et al., 1997; Iirola et al., 2011). Whether these effects apply to reptiles remain to be determined.
The green iguana is a hindgut fermenter with a large, voluminous colon divided into sacculations to facilitate hindgut fermentation (Barten and Simpson, 2019). The largest compartment of the colon, the second (transverse), was found to be quite mobile, translating together with the stomach fundus dorsally in VB lateral views. Given the clinical importance of this organ, awareness of its improved identification in survey studies and VB lateral views may be relevant.
The gonadal silhouette was only visualized in male green iguanas, and much more reliably in HB radiographs, with a strong reader preference for RHB. More mature animals in active reproductive phases may well present different radiographic features.
The spleen, urinary bladder, and kidneys are not visualized in iguanas when normal (Hochleithner and Sharma, 2019; Holmes and Divers, 2019; Wilkinson and Divers, 2020), and results of this study indicate that positional radiography does not change these attributes.
Results of this study confirm our hypothesis that HB lateral projections of the iguanid coelom are essentially equivalent. As predicted, HB and VB projections are not interchangeable, because gravity-dependent organ positional shifts result in obvious radiographic differences, with the exception of the heart where all laterals were similar irrespective of beam orientation in 83% of green iguanas. Lung, liver, and gonads were best imaged in HB lateral projections, before distention of the stomach with barium. The gastrointestinal tract was best visualized in VB lateral and DV projections. VD projections were advantageous for the evaluation of the lung. Although the clinical applications of these findings remain to be confirmed, it is essential for practitioners to be aware of the effect of various projection angles in lizards so as to choose projections appropriate to the organs of interest.
Positioning a sedated green iguana (Iguana iguana) for a standard right and left lateral horizontal beam radiographs. Radiolucent positioning device (foam pad) was used to elevate the iguana above table and center the x-ray beam (A). The limbs were taped to provide an unobstructed view from coelom (B).
Dorsoventral (DV) (A) and ventrodorsal (VD) (B) radiographs of the coelom of a juvenile male green iguana (Iguana iguana). In VD, the ribcage expands, improving pulmonary inflation. In VD, cardiac length was judged mildly shorter and cardiac margins were sharper in all green iguanas. In the subject shown here, the cardiac silhouette is additionally more opaque in VD, and this observation was not repeated in other green iguanas for which this projection was available. Inset: heart (red), liver (yellow), and stomach (green).
Right lateral radiographs of the cranial coelom in a juvenile green iguana (Iguana iguana): (A) right lateral horizontal beam (RHB), (B) right lateral vertical beam (RVB), and (C) RVB positive contrast gastrointestinal study. The liver (arrows) is best outlined in horizontal beam projection before distension of the stomach with barium.
Lateral radiographs of the coelom of a juvenile female green iguana (Iguana iguana). Although left lateral horizontal beam (LHB) (A) and right lateral horizontal beam (RHB) (B) are nearly identical, vertical beam (VB) views show striking differences, particularly relating to the gastrointestinal tract. The angular shape of the fundus (black *) was only observed in LVB (C). Dorsal bowing of the pyloric antrum (arrow) was observed only in RVB (D). The urinary bladder (white *) could not be outlined in survey images, but in this series is opacified by residual iohexol administered in a separate study the week before imaging.
Right lateral horizontal beam (RHB) (A) and right lateral vertical beam (RVB) (B) radiographs of a juvenile male green iguana (Iguana iguana). The gonads (arrows) are distinctly seen in (A), but effaced in (B).
