Advanced imaging in exotics (Proceedings)

While standard radiographic and ultrasound imaging techniques are common diagnostic tools in exotic animal medicine, the use of more advanced imaging techniques such as computed tomography (CT) and magnetic resonance imaging (MRI) currently appear to be underutilized for exotic patients. Although radiographs are often considered the first choice for the initial imaging modality in the exotic patient, very often a second imaging technique is needed, especially in cases where the clinical signs suggest a condition for which radiographs may not produce adequate imaging to rule out the suspected problem, e.g. a gastrointestinal blockage due to a plastic foreign body. It is therefore often useful to combine different imaging modalities in order to improve the accuracy of the diagnosis and ensure that the limitations of one imaging modality are overcome by a second mode of imaging.

Ultrasound

Ultrasound is considered a standard diagnostic approach in traditional pet species, and should be more commonly used in exotic species. New publications are appearing every year highlighting the significant diagnostic benefits of this tool for soft tissue imaging. Ultrasound should be considered for every pathological process that might have a soft tissue component. In addition in dogs, cats and large animals US can be used to assess for osteomyelitis, bone involvement of soft tissue tumors, assess fracture healing, or to guide biopsies as well. The non-invasive character as well as the ability to obtain real-time images with magnification and to assess blood flow using color and spectral Doppler make this tool truly indispensable. The most common uses of ultrasound as an imaging technique include: documentation of pregnancy, monitoring of the reproductive cycle, evaluation of the internal organs for shape, size, architecture and homogeneity, echocardiography, and as a guide for invasive techniques such as Tru-cut biopsies or fine needle aspirates. Ultrasound images allow visualization of small details that might not be seen in survey radiographs. For example, small (< 3 mm), bladder stones, which can cause significant urinary tract problems in the Guinea pig may not be visible on plain radiographs even though they are radiodense. For this reason it is advisable to routinely perform ultrasound examinations on small mammals showing signs of urinary tract disease that have negative findings on radiographic images.

The two major disadvantages of ultrasound are the inability to completely image bony structures, and the fact that the ultrasound waves will not travel through air, making it difficult to examine birds, because of their airsacs, or mammals with large amounts of gas in the GI tract, e.g. herbivores. However imaging of the surface of normal bony structures (or deeper bony structures in cases of disruption due to neoplasia or infection) are sometimes possible.

Computed tomography

While traditional radiographic images are easily obtained in almost every clinic, accessibility to CT scanners is less common, although they are becoming more available. While the actual CT image is produced by traditional x-rays, the process of acquiring it is dynamic. The x-rays are passed through the patient in a full 360° circle by rotating the x-ray producing tube, with the x-ray detectors positioned opposite, around the patient. As the x-rays pass through the patient, they experience differential attenuation based on the density of tissues they encounter along their path. Contrast in a CT image is based upon this differential attenuation. Radiolucent tissues, such as air-filled structures, attenuate fewer x-rays and appear darker, while more radiodense tissues, e.g. bone or metal, attenuate more x-rays and appear whiter. The dynamically acquired images can then be visualized as variably sized slices or summed to provide reconstructions in various planes. One of the downfalls of CT is the significantly higher amount of radiation experienced by the patient compared with that of traditional radiography. In order to obtain information regarding dynamic processes in the body, intravenous iodinated contrast media, e.g. iodine, can be used to improve the contrast between different body tissues. With the addition of specialized software, a 3D model of the animal or lesion can be created in order to demonstrate the spatial correlation of pathological processes. This tool has significant benefit for accessing a mass or structure when planning radiation treatment or surgery.

The average time to obtain a scan in a modern CT unit is about 10 minutes, and the patient needs to be anesthetized to prevent blurring due to movement. When contrast is used the scan is repeated, effectively doubling the time of anesthesia. From start to finish a complete session usually requires the patient to be under general anesthesia for approximately 45 minutes.

The most significant advantage of CT images over traditional radiographs is the fact that each slice, which can be as thin as 1 mm, provides superior information about the tissue in question over traditional radiographs due to the fact that no superimposition of tissues hinders the interpretation. In addition, images obtained from the CT scan can always be modified later, and also maintain the original 3D character of the structures imaged. The most significant disadvantages of the CT scan over traditional radiographs are the increased costs associated with the imaging, less accessibly to the equipment, and the need for prolonged anesthesia during the scan.

One of the most useful applications of CT scans is in the diagnosis of dental problems in small mammals. The benefit here lies in the ability to completely assess the involvement of tooth, tooth root and surrounding structures such as the mandible or the maxilla in the problem. Very often important information about the relationship of the teeth and their environment gets lost in traditional radiographs due to superimposition of structures encountered in a 2-dimensional image. In general, a CT scan provides excellent images of all bony structures. With 1mm thick slices of the skull, small fractures are easily appreciated. Subtle bony lysis of turbinates due to chronic upper respiratory tract infections, abnormalities of the tympanic bulla cavities due to chronic ear infections, and even skull tumors can easily be detected and assessed, all of which might not be easily appreciated with traditional radiographs.

Similar to ultrasonography as an adjunct to radiography, CT scans are an equally valuable modality for imaging small changes that cannot be seen with traditional radiographs. CT should be considered especially when lung metastases are suspected in cancer patients. Radiographs will often miss even large numbers of small nodules, which will become immediately apparent on a CT scan.

Evaluation of the brain parenchyma is also significantly better on CT scan than with radiographs, and so, a CT scan should be recommended for any pathologies involving the head, including trauma. The abdomen should also be scanned in cases where an abdominal ultrasound exam was inconclusive. The application of CT scans in neurology cases are of interest when imaging the spinal canal or in head trauma cases due to superior image contrast over radiography, and the fact that even critical patients can undergo this relatively short procedure in comparison to an MR scan. The use of iodine intravenous contrast material can enhance and differentiate between cystic or solid lesions as well as gain an idea of vascularity or aggressiveness of a lesion, e.g. tumors, and to document physiological processes such as elimination via the kidneys.

Magnetic Resonance Imaging (MRI)

MR images are obtained by placing the animal into a very strong magnetic field (up to 100,000 times stronger than the earth's magnetic field) and visualizing the movement of the hydrogen atoms in the body in reaction to the magnetic field changes. This is obviously an oversimplification of a very complicated process, but the imaging therefore does not require any radiation or other form of ionizing rays, and is non-invasive making obtaining the image relatively risk-free.

An MRI scan is the diagnostic imaging modality of choice for neurological problems since MR images of the CNS are of superior quality to CT images. The second largest practical use of MRI scans is in the imaging of certain soft tissues such as joints and muscles. As with CT scans, the use of contrast material (Gadolinium) can be used intravenously to make interpretation of the images more diagnostically valuable. Use of contrast material is considered a routine procedure for MRI scans in order to compare pre- and post-contrast images, while the use of contrast with CT scans is considered to be elective.

While the image quality of the MRI scan is superior to any other imaging modality, there are significant disadvantages associated with this procedure. While the images of the neurologic tissues are significantly better with MR, the prolonged scanning time compared to a CT scan, which produces less detailed images, prevents the MR scan from being the imaging modality of choice for critical neurological patients. Very often a CT scan is preferred for unstable patients due to the speed of the procedure, and therefore, less anesthetic time. In addition to the longer scan time, the costs associated with MR are also much higher than for a CT scan. In addition, the local availability of MR scanners is often less than with CT. However, because MR scanners are becoming physically smaller, and less expensive, they are now being installed in many referral centers.

When scanning very small exotic patients such as small rabbits or rats, care must be taken to check with the radiologist about the resolution of the image obtained with the available magnet to see if the procedure will result in a diagnostic image of the tissue of interest. A MR scanner equipped with a magnet of 1.3 Tesla field strength appears to be suitable to image the brain of smaller mammals such as small rabbits or ferrets. However, 'weaker' magnets with a field strength of only 0.2-0.4 Tesla will have significantly less resolution. At UGA we have the possibility to scan very small patients with a 7 Tesla magnet, which provides superior images for patients as small as 30 gr. Larger field strengths have a larger signal to noise ratio permitting not only higher resolution images, but faster scan times. Scan times have been significantly reduced with new sequences and higher field strengths so that length of scan time is becoming less problematic This is similar to a camera taking pictures with 1 mega-pixel or 6 mega-pixels resolution. These images may not have enough resolution to be diagnostically useful in the smaller patients.

Other 'new' modalities

While CT and MR imaging modalities are not new to human medicine or traditional pet medicine by any means, their use in exotic or non-traditional pets is vastly underutilized. In addition, other imaging modalities exist which may have value in exotic patients. CT and MR scanners combined with other specific equipment can be used to detect and trace radioactive material as it is injected into the patient, e.g. Positron Emission Tomography (PET) scans, and have been used in some cases to diagnose clinical problems1 in exotic patients. PET scans are also being used to assess pain in birds. Scintigraphy is an additional imaging modality and it has recently been used by the author in the diagnosis of hyperthyroidism in a Guinea pig. In addition, the use of thermography in the early detection of rabies in a raccoon was recently published.

Without a doubt, advanced imaging techniques in exotic pets are becoming more and more popular. It is in the best interest of the patient, the client and the veterinarian to be aware of these different modalities, and to recognize their applications and limitations in order to integrate them successfully into the routine workup of challenging cases.

References

Souza MJ, Greenacre CB, Jones MP, Hadley TL, Adams WH, Avenell JS, Wall JS, Daniel GB. Clinical use of microPET and CT imaging in birds. Proceedings of the Annual Conference of the Association of Avian Veterinarians. 2006: 13-16.

Paul-Murphy J, Sladky KK, McCutcheon RA, et al. Using positron emission tomography imaging of the parrot brain to study response to clinical pain. Proc AAZV, AAWV, AZA/NAG Joint Conf. 2005;140-141

Dunbar, M., Maccarthy K. A.. 2006. Use of infrared thermography to detect signs of rabies infection in raccoons (Procyon lotor). Journal of Zoo and Wildlife Medicine 37:518-523