Diagnostic imaging: update and important information for avian practitioners (Proceedings)

Veterinary medicine has made advances in diagnostic imaging capabilities that parallel human medicine.  Imaging techniques available to veterinarians include radiography, contrast radiography, ultrasonography, endoscopy, computed tomography and magnetic resonance imaging.  All of these techniques have and can be used on psittacine species.  For the safety of the avian patient and veterinary staff and quality of images it is recommended to place the bird under isoflurane or sevoflurane general anesthesia.

A critical assessment of the patient should be made before the animal is placed under general anesthesia for radiographic evaluation.  Even if the bird appears to be an excellent candidate for general anesthesia the owner(s) must be informed on the dangers of general anesthesia and rare but real chance the patient may die.  Precautions for emergency therapy are necessary for any avian hospital administering general anesthesia.  The first step in emergency preparations is putting together a “crash kit”.  Recommended items to include in a “crash kit” would be therapeutic agents to revive a patient that is not breathing, has a slow or nonexistent heart beat, syringes, needles, tracheal tubes, tape and an ambu – bag.  If the bird presents in a condition that is not conducive for general anesthesia it may be amenable for quick images that may be diagnostic.  These patients may not move or specific diagnostic results are desired by he veterinarian (e.g., heavy metal objects or a foreign body within the gastrointestinal system).  All birds should be intubated when placed under general anesthesia for radiographs.  Birds are induced via a facemask and then intubated while maintained on isoflurane or sevoflurane anesthesia.  If face images are desired an air sac tube may be placed in the caudal thoracic air sac after induction of the patient.  To maintain an avian patient in the proper plane of anesthesia with administration of the anesthetic agent through an air sac tube, oxygen flow and percent of anesthetic agent should be higher than routine tracheal intubation. 

For radiography, high definition – screen film combinations have been recommended for the best image results.1  Rare – earth screens have a higher intensifying capacity without the loss of detail found in calcium tungstate screens.  The higher intensifying capacity is generally needed given the short exposure times used on pet bird species.  The best combination of screen and film for avian practice at this time is a rare-earth screen and film emulsified on one side.1  rare earth screens with greater intensifying capabilities (sensitivity of 200) are needed for marginal quality machines.  The small size of avian patients require short exposure times (0.015-0.05s) using at least a 200-300 mA machine.1 to obtain the highest degree of contrast 10-20 KV is optimal.1  Inflating the air sacs during radiographic exposure has shown the ability to increase contrast of internal organs.  For smaller avian patients, <100 grams, and specific focal areas of interest in larger birds (e.g., toes) dental radiographic units using high detail dental film will provide greater detail, in most cases, than larger radiographic views. 

As mentioned previously, general anesthesia is recommended for most avian patients being radiographed or undergoing a diagnostic imaging examination.  Prior to induction, the radiographic team needs to designate an anesthetist, have the endotracheal tube and tape prepped for placement, the positioning board ready and the emergency “crash kit” available.  A designated anesthetist is a very important person on the radiographic team.  The anesthetist must understand their position, be able to focus on the job and relay the vital information requested to the team manipulating the patient for radiographic images.  There are many concerns for the person positioning and measuring the patient and determining the correct settings on the vaporizer and oxygen flow in relation to the patient's condition is not one of them. 

Diagnostic imaging and a patient under general anesthesia does not lend itself to multitasking. 

For positioning a Plexiglas® restraint board is recommended.  The restraint board allows for consistent positioning between patients and when taking serial radiographs of the same patient.  The head/neck restraint allows for one to stretch the patient for better positioning.  The standard avian radiographic views are ventrodorsal and lateral.  The legs are stretched down and the wings stretched out with all extremities taped to the board on the ventral dorsal view.  When positioned in lateral recumbancy the wings are placed over the dorsum then taped and the bottom leg stretched cranially and the top leg stretched caudally then taped to the board.  Other positions can be achieved depending on the case.  For radiographic gastrointestinal constrast studies, iohexol is the contrast material of choice although barium sulfate has been used successfully in the past.  The main benefit of iohexol over barium sulfate is that iohexol has been proven to cause less tissue damage to the lungs if aspirated.2 Iodine based agents have been used when intravenous contrast material has been needed for urography or computed tomography studies.

Ultrasongraphy is a relatively recent advance in avian diagnostic imaging techniques.  The recommended transducer for avian patients is a sector scanner with a frequency of 7.5 MHz.1 General anesthesia is not as important for patients undergoing a songraphic examination but for sensitive heart examinations it may be beneficial when viewing those images.  Prior to  the examination, avian patients should be fasted for at least 3 hours and birds of prey for 1 – 2 days.1  Because of the avian air sac system there are only a few windows into the body that are effective in obtaining diagnostic ultrasound image results.  Caudal to the keel, with the bird in dorsal recumbancy, the transducer can be directed in a cranial dorsal direction to obtain images of the liver and heart.  Directing the transducer dorsally in the mid to caudal coelomic cavity will highlight the intestinal and urogentital tracts.

One of the more common imaging techniques used on avian patients is endoscopy.  Rigid endoscopes having a diameter of 1.7 – 2.7 mm are commonly used for minimally invasive coelomic, respiratory and cloacal imaging.  As with any imaging technique there is a learning curve and endoscopy is no exception.  Knowledge of the avian anatomy is imperative when viewing images of the coelomic cavity through the endoscope or on a video monitor.  Required instrumentation for endoscopic imaging includes the scope, biopsy port cannula, blunt tipped trocar, biopsy forceps and scissors.  It is extremely useful to have aspiration capabilities and an endoscopic camera with a video monitor to significantly enhance ones ability to perform this procedure. Common sites of entry into the coelomic cavity with an endoscope are the caudal air sac, choanal slit, upper infraorbital sinus, trachea and cloaca.  Other areas of the bird may be accessed but the aforementioned sites are the most common viewed.  By accessing the caudal thoracic air sac the entire coelomic cavity may be viewed.  Through this portal the lungs, heart, liver, kidney, proventriculus, spleen, kidneys, gonads, urogentital tract and intestinal tract can be assessed.  If any suspicious pathology is encountered a biopsy sample may be taken of that organ or tissue.  Through the choanal slit viewing the nasal bifurcation, granulomas may be observed.  When viewing the choana with the endoscope aspiration capabilities are warranted.  Often there is a buildup of mucoid nasal discharge in and around the infraorbital sinus extending down through the choanal slit.  This nasal discharge should be removed to obtain an unobstructed view of the anatomy in question.  An air sac breathing tube is required for the endoscopic examination of the trachea.  The most common disease condition identified through examination of the trachea is granuloma formation due to aspergillosis.  Aspiration and culture of the granuloma will often provide a definitive diagnosis of the disease for the veterinarian.  To view the cloacal mucosa sterile saline is needed to fill this anatomical structure during the examination.  Non-specific cloacal prolapses are often the most common disease presentation for the need to examine the area of an avian patient.  Biopsies of the cloaca mucosa may provide an answer for the cloacal prolapse presentation.  To view the upper infraorbital sinus, an incision is made in the depression found between the medial canthus and nare.  The endoscope is then placed through this incision and the cranial aspect of the infraorbital sinus is viewed.  Granulomas and tissue masses may be seen from this location and biopsy samples taken of lesions observed.

Computed tomography (CT) is a relatively new imaging technique that was first introduced in 1972.  Computed tomography images are radiography-based thin cross-sectional scans that remove the complications of superimposed structures.  Transverse and longitudinal scans can be performed on a patient in 1 to 5 mm slice thickness or a distance recommended by the clinician.  With the updated computer programs 3 dimensional models can be generated from most CT studies.  The advantages of computed tomography are based on the ability to separate the anatomic structures within a serial study.  A complete transverse scan of an avian patient can take place within a 15 minute time frame.  Birds should be under general anesthesia for computed tomography scans and preparation of the anesthesia unit for table movement during the scan must be taken into consideration before the scan is initiated.  To highlight tissue masses intravenous iodine-based contrast material is often used as a comparison to a survey scan.  One of the major disadvantages when using computed tomography technology is cost.  A complete avian scan may cost in the range from $250 - $350.

Another high cost but very useful image technology is magnetic resonance imaging (MRI).  This technique uses a superconducting magnet as part of its imaging technology.  Magnetic resonance imaging is more complicated to use for avian patients because of the superconducting magnate.  No metal that would be affected by magnetic forces can be in the room during the scan.  Unless nonconductive material is used in the construction of the anesthesia machine, injectable general anesthetic agents are required.  Any monitoring equipment must also be compatible with the super conducting magnet, thereby making them extremely expensive to purchase.  As with computed tomography scans magnetic resonance imaging scans are relatively expensive when compared to the other available diagnostic techniques currently available.

References

Krautwald-Junghanns ME, Trinkaus K, Imaging techniques. In: Avian Medicine, Tully TN, Lawton MPC, Dorrestein GM (Eds). Butterworth-Heinemann, Oxford 2000;52-73

Ernst S, Goggin JM, Biller DS, Carpenter JW, Silverman S, Comparison of iohexol and barium sulfate as gastrointestinal contrast media in mid-sized psittacine birds, J Avian Med Surg 1998;12(1):16-20.