I dusted this off from the

I dusted this off from the depths of my hard drive:) may be helpful:

 

Reptile Ultrasound Examination (V525)

Western Veterinary Conference 2007

Scott J. Stahl, DVM, DABVP (Avian)
Stahl Exotic Animal Veterinary Services, Vienna, VA, USA

Objectives of the Presentation

To introduce clinicians to reptile ultrasound. The lecture will focus on the clinical application of ultrasonography in reptiles emphasizing imaging techniques, normal anatomy and some common pathological findings seen in snakes, lizards and chelonians.

General^2,7,8,9,10

Why Ultrasound?

A useful and yet under-rated technique, ultrasound has only recently gained popularity in reptile diagnostics, particularly with regard to examining tissue parenchyma, guiding biopsy needles, and, with color flow Doppler, investigation of cardiac disease.

Ultrasound is non-invasive, and anesthesia is generally not required. Ultrasonography allows visualization of internal anatomy, especially the texture of internal organs and the ability to identify soft tissue abnormalities such as neoplasia, abscesses and cyst formation.

The ultrasound is also useful in evaluating the celom for free fluid and the presence of cystic calculi.

The interpretation of a two-dimensional, gray-scale image takes time and experience to master, but with practice, ultrasound can be a useful adjunct to radiography.

The authors find the use of ultrasound most rewarding for the assessment of:

1.  Reproductive function and disease, especially ovarian activity and distinguishing between pre-ovulatory ova stasis and post-ovulatory egg stasis

2.  Liver and gall bladder

3.  Kidneys and where present, bladder

4.  Any soft tissue mass

5.  Ocular and retrobulbar disease

6.  Cardiac disease (using color flow Doppler)

Limitations of Use with Reptiles

Success depends on the ability of the ultrasonographer and knowledge of normal anatomy of the reptile being scanned.

 Ultrasound waves do not penetrate well through mineralized tissue (boney shell of chelonians and heavily keratinized scales of some reptiles).

 Ultrasound waves are disrupted by air so sonography has limitations when investigating respiratory and gastro-intestinal diseases.

 Scanning during ecdysis is difficult as the air between the layers of skin can cause interference.

 Ultrasound can be used to guide biopsy collection, but when dealing with small reptile patients the use of endoscopy offers superior, direct visualization and has proven to be more valuable for accurate and safe biopsy collection.

Probe Size and Techniques for Improving Imaging through Keratinized Reptile Scales

The giant species will require a 5MHz probe while a 7.5MHz probe will suffice for most reptiles. When dealing with very small specimens (or for the ultrasound examination of eyes) a 10MHz probe with a small footprint is more appropriate.

Good contact and imaging generally requires copious quantities of gel or a water bath.

Improve imaging of thick keratinized reptile scales by:

 Bathing or soaking the animal in a warm water bath for 10-15 minutes prior to scanning.

 Apply the coupling gel 10-15 minutes prior to scanning.

 Use copious amounts of coupling gel to improve contact.

 For small reptiles a water bath may be utilized and the patient scanned ventrally using coupling gel between the plastic container and the probe.

 If possible and by protecting the ultrasound probe (i.e., wrap in surgical glove to waterproof etc. if necessary) imaging in a warm water bath will improve contact and image quality.

 The use of a suitable standoff in conjunction with copious gel may be helpful to improve image.

 Scan lizards and snakes from the ventrum to minimize interference from ribs.

 Scan chelonians in available soft tissue “windows” to avoid interference from the shell.

Lizards^7,8,10

Techniques

The celomic structures of lizards are typically scanned in dorsal recumbency. However, if stressed on their backs they can be scanned in a normal sitting position with the use of a scanning table or by slightly elevating their bodies with towels. Scanning obese lizards in sternal recumbency laterally to avoid scanning through fat pads may be helpful in some cases. Other techniques as described above can be utilized to improve the scanning image (soaking in a bath prescan, using copious amounts of coupling gel, or the use of a water bath during scanning). The ultrasound technique for lizards is to scan on the midline in a cranial to caudal direction in both longitudinal and transverse sections making sure to scan the entire celomic cavity.

Heart

 The heart in lizards varies in its location from a very cranial location between the shoulders as in the green iguana (Iguana iguana) to midbody between the liver lobes as in monitor lizards (Varanus spp.). The heart is oval in shape with three chambers, a right and left atrium and one ventricle.

 On ultrasound the heart is oval in shape. The round atriums are hypoechoic; the ventricle is also hypoechoic and is surrounded by a hyperechoic pericardium.

 In larger species (~ ½ kg or larger) the hyperechoic valves can be seen.Lungs in lizards can make it difficult to sometimes visualize the heart.

 Due to the thick ventricle (highly trabeculated) it is difficult to diagnose hypertrophy or other cardiac disease; M-mode and Doppler studies are difficult in lizards and chelonians due to the cranial location of heart and heart size.

 Continued research in heart contractility and blood flow (with the use of color Doppler) in larger species of lizards will hopefully provide more information on cardiac function and physiology. Unique reptile physiology may also make these studies more difficult due to external factors that can affect cardiopulmonary function in reptiles such as body temperature.

 Currently, cardiac diseases or abnormalities that may be detected with ultrasound include pericardial effusions, presence of urate crystals and tophi, severe endocarditis, cardiac neoplasia, thrombi and cardiomyopathy.

Lungs

 Lungs can be extensive in lizards and can overlay many of the celomic organs making detection of some organs more difficult.

Liver/Gall Bladder

 The liver in lizards is bi-lobed with the gall bladder attached to the caudal portion of the right liver lobe. The liver begins just caudal to the heart and may be overlaid ventrally and laterally by fat bodies. The liver borders can typically be noted associated with other celomic organs, including the heart cranially, gastrointestinal tract and ovary caudally.

 On ultrasonography the liver gives a hypoechoic homogenous image with no obvious capsule noted. Anechoic blood vessels are typically noted within the liver parenchyma. Specifically the Vena cava caudalis and the vena cava portae can be seen as anechoic lines associated with the liver. The liver differs from fat bodies, which are brighter and more granular in appearance. Fat bodies are also divided into lobes that are outlined by hyperechoic lines. The gall bladder can be noted as a round or oblong anechoic structure surrounded by the hypoechoic liver in both the transverse and longitudinal views.

 Lungs and ribs in lizards can interfere with sonography of the liver.

 The liver should be thoroughly scanned to evaluate size and for any parenchymal changes, which could include cysts, abscesses or neoplasia. The gall bladder is evaluated for size and possible abnormal content such as gall bladder stones or concretions.

 Liver biopsies and aspirates guided by the ultrasound are useful in larger species of lizards in an attempt to determine etiologies for abnormal architecture.

Ovaries/Testes

 Ultrasound can be useful to determine sex in monomorphic species of lizards such as monitors, Gila monsters and beaded lizards.

 Ultrasonography is also useful to determine the reproductive status of female reptiles by determining ovarian activity and possible pregnancy.

 Caudal and dorsal to the liver the ovaries are located on both the left and right side in a paramedian position.

 The immature ovary and the testes are often not seen with the ultrasound.

 The scanning image of the ovary varies depending on the developmental stage of the follicles from anechoic to hypoechoic.

 Previtellogenic follicles are round in shape with anechoic image, arranged in a grape like or string fashion one after another in the longitudinal scan and can be seen when they are at least 1.5-2.0 mm in diameter.

 Vitellogenic (preovulatory) follicles are readily scanned, round and arranged grape like or in a cluster and are more hypoechoic and homogenous.

 Vitellogenic follicles that are in regression show an inhomogeneous pattern where their surface is not as smooth and there may be an inner anechoic area as the follicles regress. Corpora lutea typically are not discernable with the ultrasound.

 Postovulatory follicles/eggs may appear more oblong and in a line rather than a group and tend to have more variable echogenicity with hypoechoic center and a hyperechoic line associated with a calcified egg.

 Late in gestation in live-bearing lizards the spines of neonates (live = moving) and beating hearts may be detected by ultrasound.

 Testes are located cranial to the kidney, are typically bean shaped and variable in size due to reproductive activity. They can be difficult to consistently scan and may only be detectable in larger lizards with active testes. If noted the testes will scan with a medium echogenicity.

Kidneys

 The kidneys of lizards are located dorsally and paramedian, caudal to the gonads and cranial to or within the pelvis. The colon separates the kidneys medially; this is seen readily on the transverse section. The reptile kidney is not divided into medulla, cortex and pelvis as in mammals. For lizards with the kidneys located within the pelvis such as the green iguana (Iguana iguana) and rock iguanas (Cyclura spp.) the kidney can be scanned by placing the ultrasound probe at the base of the tail laterally in a craniodorsal orientation.

 The scanning image of the kidney is generally homogenous and more hyperechoic than the fat bodies. Kidneys have a distinct granular and speckled image and like the liver the capsule is not seen.

 Kidneys should be evaluated for any morphological changes altering the shape and size.

 Pathological findings seen with ultrasound may include: Neoplasia, abscesses, gout formation, cysts. Ultrasound guided biopsy is often possible in lizards.

Bladder

 The lizard bladder if present is located cranial to the pelvis and ventral to the kidneys. The bladder is variable in size and its fluid contents result in an anechoic image with an echoic border.

 Sometimes sand like hypo to hyperechoic urate material can be seen within the bladder and by gently agitating the area this material can be seen floating around like the material in a snow globe. Cystic calculi may also be detected as a hyperechoic solid material.

Gastrointestinal Tract

 The stomach and intestinal tract of lizards are located between the liver cranially, fat bodies laterally and bladder caudally.

 The stomach wall can be seen as a hypoechoic line. Intestines scan cloud like and often contain air resulting in a poor image. The small intestine has moderate echogenicity, however the large intestine has an increased echogenicity, as there is less fluid. Intestines may have an anechoic ring with a hypoechoic center indicative of free fluid surrounding intestinal contents. Evaluation of the intestinal tract is often hindered by the presence of gas. The spleen and pancreas are difficult to scan consistently unless there is severe pathology.

Fat Bodies

Lizards have paired fat bodies (pads) that are situated laterally originating ventrally from the inguinal body wall region and extending cranially. In obese lizards these fat bodies can extend to the heart and cover the liver laterally and ventrally. Fat bodies are medium in echogenicity and homogenous, but have granular or speckle appearance with distinct hyperechoic lines between the small individual lobes that make up the larger fat body. Fat bodies differ from the liver in that they are brighter and more granular in appearance. Fat bodies are also divided into lobes that are outlined by hyperechoic lines. Fat bodies are an excellent reference for comparison to other organs based on echogenicity and they tend to be very consistent in their echogenicity.

Snakes^2,8,9

Technique

 Celomic ultrasound in snakes is severely hampered dorsally by the spine and laterally by the ribs. Technique for snakes is to scan the entire celomic cavity on the ventral midline in a cranial to caudal direction in both the longitudinal and transverse sections.

 Another technique is to examine snakes with the use of a water bath where each segment can be moved through the bath while doing a ventral scan. Unfortunately, this can be difficult to accomplish without anesthesia but is an excellent technique to improve the scanning image.

 Start by scanning cranially and moving caudally.

Heart

 The heart is the first major organ to be evaluated, positioned cranioventral to the lungs, at the end of the first 25-30% of the snake.

 On longitudinal section the heart is ovoid and lying almost parallel to the to the body’s axis.

 On ultrasound the heart is oval in shape. The round atriums are hypoechoic; the ventricle is also hypoechoic and is surrounded by a hyperechoic pericardium.

 In larger species (~ ½ kg or larger) the hyperechoic valves can be seen.

 Currently cardiac diseases or abnormalities that may be detected with ultrasound include pericardial effusions, presence of urate crystals and tophi, severe endocarditis, cardiac neoplasia, thrombi and cardiomyopathy.

 Study by Snyder et al, 1999 on the echocardiographic anatomy of the heart in Burmese pythons (Python morulus bivittatus):

 Reptile heart still poorly understood with much work needed to understand anatomy, physiology and embryology.

Current Understanding of Blood Flow in the Snake Heart

Venous return of blood (deoxygenated from circulation) empties into the sinus venosus then enters the right atrium from the sinus venosus through the sinus venosus valve. The blood then passes through the right atrioventricular valve into the cavum venosum (one of the 3 separate chambers of the ventricle). During diastole the blood from the cavum venosum is directed into the cavum pulmonale. Then, during systole, the blood is pumped from the cavum pulmonale into the pulmonary artery (the horizontal septum directs the blood out toward the pulmonary artery).

Then oxygenated blood from the pulmonary veins enters the left atrium where it is directed into the cavum arteriosum. Then during systole the blood is pumped from the cavum arteriosum over the vertical septum, through the interventricular canal, into the cavum venosum and then exits the heart through the left and right aortic arches.

Thus the snake ventricle is divided into three separate regions by two incomplete septa, the horizontal septum and the vertical septum.

The AV (atrioventricular valves) are bicuspid (instead of tricuspid as in mammals) and they have a fibrous medial cusp and only rudimentary lateral cusps. During diastole these AV valves cover the interventricular canal, which then helps to direct blood from the atria into the appropriate ventricular region to minimize the mixing of oxygenated and deoxygenated blood.

Scanning

Most important landmarks when scanning the heart that you should be able to find:

 Sagittal view: caudal vena cava with blood entering the right atrium (via the sinus venosus valve, or SVV)

 Transverse view: pulmonary arteries and aortic arches:

 Also look for characteristic “smoke” or “spontaneous contrast” which is an intra-atrial and pulmonary arterial swirling of blood that is normal and characteristic for reptiles and horses. If seen in humans may indicate heart disease.

 Sagittal imaging:

 May also see in the right sagittal view the caudal vena cava emptying through the sinus venosus valve into the right atrium. The caudal vena cava lays dorsal along the ventricular muscle. The pulmonary artery overlies the right atrium.

 On the medial sagittal view may see the cavum pulmonale with the pulmonary artery exiting.

 Transverse imaging:

 Best to visualize the extensive trabecular nature of the ventricle. Can see the position of the horizontal septum. If ultrasound probe is directed from the apex toward the base of the heart may see the horizontal septum (comma shaped) that separates the cavum arteriosum from the cavum pulmonale. At the base of the heart on transverse view may be able to see the pulmonary artery and left and right aortic arch.

 Structures that are not routinely seen include: Cranial vena cava, left atrium, vertical septum, small pulmonary veins

Liver/Gall Bladder/Splenopancreas

 The snake liver is caudal to the heart and extends from the caudal pole of the heart to the middle of the body.

 The snake liver is mono-lobed, longitudinal and oval in cross section. The liver is directly attached to the ventral body wall. Dorsally it is bordered by the lung and on the left side by the esophagus and stomach. Dorsal and ventral large veins run along the midline, these veins then join each other at the at the end of the liver to form one large vessel (vena cava). On ultrasonography the liver gives a hypoechoic homogenous image with no obvious capsule noted. Anechoic blood vessels are typically noted within the liver parenchyma. The liver should be thoroughly scanned to evaluate size and for any parenchymal changes, which could include cysts, abscesses or neoplasia. Liver biopsies and aspirates guided by the ultrasound are useful in snakes in an attempt to determine etiologies for abnormal architecture.

 The gall bladder is caudal to and not directly associated with the liver in snakes; it is located ventral to the duodenum in the abdominal fat.The gall bladder can be noted on ultrasound as a round or oblong anechoic structure. The gall bladder is evaluated for size and possible abnormal content, such as gall bladder stones or concretions.

 The spleen and pancreas (splenopancreas) are located at the craniodorsal pole of the gall bladder. The spleen and pancreas can be difficult to scan consistently unless there is severe pathology.

Ovaries/Testes

 A short distance caudal to the gall bladder are the gonads, they are positioned laterally on both sides of the intestine within the abdominal fat bodies. The right gonad is located cranial to the left gonad. Ultrasonography is useful to determine the reproductive status of female snakes by determining ovarian activity and possible pregnancy.

 Inactive ovaries and testicles are typically not differentiated as they are surrounded by abdominal fat. The scanning image of the mature ovary varies depending on the developmental stage of the follicles from anechoic to hypoechoic.

 Previtellogenic follicles are round in shape with anechoic image, arranged in a grape like or string fashion one after another in the longitudinal scan and can be seen when they are at least 1.5-2.0 mm in diameter.

 Vitellogenic (preovulatory) follicles are readily scanned, round and arranged grape like or in a cluster and are more hypoechoic and homogenous.

 Vitellogenic follicles that are in regression show an inhomogeneous pattern where their surface is not as smooth and there may be an inner anechoic area as the follicles regress. Corpora lutea are usually not discernable with ultrasound.

 Postovulatory follicles/eggs may appear more oblong and in a line rather than a group and tend to have more variable echogenicity with hypoechoic center and a hyperechoic line associated with a calcified egg.

 Late in gestation in live-bearing snakes the spines of neonates (live = moving) and beating hearts may be detected by ultrasound.

 Testes can be difficult to consistently scan and may only be detectable in larger snakes with active testes. If noted the testes will scan with a medium echogenicity.

Kidneys

 The kidneys of snakes are located just caudal to each gonad, lateral to the intestines and within the abdominal fat. The kidneys are elongated, spindle shaped and triangular on cross section and lobulated. The ureter and renal vein are located dorsally running parallel to the longitudinal axis of the kidney. The scanning image of the kidney is generally homogenous and more hyperechoic than the fat bodies. Kidneys have a distinct granular and speckled image and like the liver the capsule is not seen.

 Kidneys should be evaluated for any morphological changes altering the shape and size.

 Pathological findings seen with ultrasound may include neoplasia, abscesses, gout formation and cysts. Ultrasound guided biopsy and aspirates of kidneys are often possible in snakes.

Gastrointestinal Tract

 The stomach may be able to be seen as a hypoechoic line. Intestines typically appear cloud like and since they often contain air they create a poor image. The small intestine has moderate echogenicity; large intestine echogenicity increases, as there is less fluid. Intestines may have an anechoic ring with a hypoechoic center indicative of free fluid surrounding intestinal contents. Evaluation of the intestinal tract is often hindered by the presence of gas.

 From the caudal pole of the kidney to the cloaca only the colon and fat bodies are evident on the ultrasound.

Fat Bodies

 The fat bodies in snakes consist of two elongated lobes, which cover all the celomic organs on the right and left laterally.

 Fat bodies are medium in echogenicity; they are homogenous but have granular or speckled appearance with distinct hyperechoic lines between the small individual lobes that make up the larger fat body. Fat bodies differ from the liver in that they are brighter and more granular in appearance. Fat bodies are also divided into lobes that are outlined by hyperechoic lines. Fat bodies are an excellent reference for comparison to other organs based on echogenicity, and they tend to be very consistent in their echogenicity.

Chelonia^4,5,7

Technique

 The chelonian shell represents an obvious barrier to ultrasound, and there are few imaging windows available.

 Cervicobrachial (mediastinal) window–the heart, major vessels, liver and gall bladder can be well visualized from a cranial point at the junction of the caudal neck and forelimb. Occasionally, the stomach wall can be appreciated but the presence of gastric gas limits further interpretation.

 Prefemoral (inguinal) window–the probe is placed into the prefemoral fossa just cranial to the hind limb and is especially useful for examining the ovaries, oviducts, and bladder. Depending upon the size and conformation of the prefemoral fossa it is often possible to evaluate the kidneys and liver (caudo-lateral portion), but is often poor for gastro-intestinal evaluation due to the presence of intestinal gas.

 Plastron–soft-shelled turtles and pancake tortoises (Malacochersus spp.) allow scanning through the plastron itself on the ventral midline as per lizards and snakes due to the soft nature of the shell.

Heart

 In chelonians the heart is located in the midline between the two hepatic lobes.

 The chelonian heart is three chambered with a left and right atrium (right is larger) and a single ventricle. The chelonian ventricle transversely expands, and can be identified by the well-developed trabeculae.

 On ultrasound the heart is oval in shape. The round atriums are hypoechoic; the ventricle is also hypoechoic and is surrounded by a hyperechoic pericardium.

 In larger species (~ ½ kg or larger) the hyperechoic valves can be seen.

 Currently, cardiac diseases or abnormalities that may be detected with ultrasound include pericardial effusions, presence of urate crystals and tophi, severe endocarditis, cardiac neoplasia, thrombi, and cardiomyopathy.

Liver/Gall Bladder

 The chelonian liver is bi-lobed. The left lobe covers most of the stomach and the larger right lobe covers part of the small and large intestines.

 The gall bladder lies on the dorsal side of the right liver lobe near the right border.

 On ultrasonography the liver gives a hypoechoic homogenous image with no obvious capsule noted. Anechoic blood vessels are typically noted within the liver parenchyma. The liver should be thoroughly scanned to evaluate size and for any parenchymal changes, which could include cysts, abscesses or neoplasia. Liver biopsies and aspirates guided by the ultrasound are difficult in chelonians due to the boney shell.

 The gall bladder can be noted on ultrasound as a round or oblong anechoic structure. The gall bladder is evaluated for size and possible abnormal content such as gall bladder stones or concretions.

Ovaries/Testes

 Ovaries of adult turtles are paired elongated organs attached to the peritoneum on either side of the dorsal midline, just cranial to the pelvic girdle. Ovarian follicles often may be found in different positions in between other organs such as intestinal loops, oviduct, caudal to the liver and against the body wall.

 Previtellogenic follicles are round in shape with anechoic image, arranged in a grape like or string fashion one after another in the longitudinal scan and can be seen when they are at least 1.5-2.0 mm in diameter.

 Vitellogenic (preovulatory) follicles are readily scanned, round and arranged grape like or in a cluster and are more hypoechoic and homogenous.

 Vitellogenic follicles that are in regression show an inhomogeneous pattern where their surface is not as smooth, and there may be an inner anechoic area as the follicles regress. Corpora lutea are usually not discernable with ultrasound.

 Postovulatory follicles/eggs may appear more oblong and in a line rather than a group, and tend to have more variable echogenicity with a hypoechoic (yolk) center with a distinct anechoic albumin layer and then a hyperechoic line associated with the calcium shell.

 Testes are not typically visualized with the ultrasound.

Kidneys/Bladder

 In chelonians typically kidneys are not visualized on ultrasound unless they have associated pathology.

 The urinary bladder is located in the midline and ventral in the caudal celomic cavity. The urinary bladder is thin walled, “y” shaped or bilobed; can be variable in size. May be difficult to differentiate from accessory bladder or free fluid in celom.

 The bladder is variable in size, and its fluid contents result in an anechoic image with an echoic border. Bladder stones may be evident as hyperechoic mass.

Gastrointestinal Tract

 Lies caudal to the liver, appears typically as multiple hyperechoic structures due to intraluminal gas; may note fecal material in the intestines.

 The stomach may be able to be seen as a hypoechoic line. Intestines typically appear cloud like and, since they often contain air, cause a poor image. The small intestine has moderate echogenicity; large intestine echogenicity increases, as there is less fluid. Intestines may have an anechoic ring with a hypoechoic center indicative of free fluid surrounding intestinal contents. Evaluation of the intestinal tract is often hindered by the presence of gas.

 Typically pancreas, spleen, adrenals, and thyroid glands are not visualized with the ultrasound. The exception is with pathology to one of these organs or increased contrast due to celomic fluid accumulation.

Fat Bodies

 Fat bodies are medium in echogenicity, they are homogenous, but have granular or speckled appearance.

References

1.  Chiodini RJ, Sundber JP, Czikowsky JA. Gross anatomy of snakes. Exotic Practice 1992; 413.

2.  Isaza R, Ackerman N, Jacobson ER. Ultrasound imagery of the coelomic structures in the Boa constrictor (Boa constrictor). Vet Radiology and Ultrasound 1993; 34(6): 445-450.

3.  Morris PJ, Alberts AC. Determination of sex in White-Throated Monitors (Varanus albigularis), Gila Monsters (Heloderma suspectum), and Beaded Lizards (Heloderma horridum) using two-dimensional ultrasound imaging. Journal of Zoo and Wildlife Medicine 1996; 27(3): 371-377.

4.  Oldham, J.C., Smith, H.M. Laboratory anatomy of the Iguana. Wm. C. Brown Company, 1975.

5.  Penninck DG, Stewart JS, and Paul-Murphy J. Ultrasonography of the California desert tortoise (Xerobates agassizii): Anatomy and application. Vet Radiol 1991; 32 (3): 112-116.

6.  Robeck TR, Rostal DC, Burchfield PM, Owens DW, Kraemer DC. Ultrasound imaging of reproductive organs and eggs in Galapagos tortoises (Geochelone elephantophus spp.). Zoo Biology 1990; 9: 349-359.

7.  Sainsbury AW, Gili C. Ultrasonographic anatomy and scanning technique of the coelomic organs of the bosc monitor (Varanus exanthematicus). J. Zoo Wildl. Med 1991 22 (4): 421-433.

8.  Schildger BJ, Casares M, Kramer M, Sporle H, Gerwing M, Rubel A, Tentiu H, Gobel T. Technique of ultrasonography in lizards, snakes and chelonians. Seminars in Avian and Exotic Pet Medicine 1994; 3 (3): 147-155.

9.  Snyder PS, Shaw NG, Heard DJ. Two-dimensional echocardiographic anatomy of the snake heart (Python morulus bivittatus). Vet Radiology and Ultrasound 1999; 40 (1): 66-72.

10. Tenhu H, Schildger B, Kuckling G, Thompson G. Ultrasonographic examination and anatomy of monitors. Mertensiella. Advances in Monitor Research II 1999; 11: 81-187.