Gastrointestinal (GIT) disorders belong to the most commonly observed problems in small animal medicine. In the course of working up a patient with a GIT disorder, diagnostic imaging plays an essential role. A number of modalities and techniques are used to assess the esophagus, stomach, small and large intestine, pancreas, mesentery, regional lymph nodes as well as the liver. Although many of the alternative imaging modalities such as CT, MRI, or echoendoscopy may provide important contributions to the diagnosis, they are often unavailable and expensive. Therefore, survey radiography and ultrasonography maintain a key role for the evaluation of gastrointestinal disorders. Plain radiographs provide an excellent overview, are rapidly and easily performed and are best for detecting disorders requiring immediate surgery such as with obstructions. Although radiography is often the modality of choice in patients with vomiting and diarrhea, 2-D ultrasound has become just as important. It provides insight into the stomach and bowel wall architecture, GIT content, and motility. Furthermore, it is a method which is readily available, non-invasive thus repeatable at any time. However, it must be recognized that the accuracy of GI ultrasonography is highly dependent on the experience of the operator. Today, with the combination of radiography and ultrasound, most of the information required in the decision making process can be gained.
Ultrasound guided fine needle aspiration or core biopsy tissue sampling is a frequently used technique, has a low complication rate and aids therapeutic decision making. Sampling of the liver, pancreas, bowel wall and lymph nodes can be performed. Many new innovations in sonography are also increasingly available. These include harmonic imaging, contrast enhanced harmonic imaging, ultra-high resolution transducers (>20MHz) and echoendoscopy. However, the expertise and equipment for these newer techniques is often not (yet) widely available. This may change, though, as more information about the diagnostic advantages of these techniques become available in the near future. The limitations of survey radiographs and ultrasound are well known: intrathoracic lesions of the esophagus are not readily appreciated, and both modalities are limited in the assessment of bolus propagation, gastric emptying and GI transit. In assessing GI motility, contrast studies are still valuable (with dynamic scintigraphy being the gold standard) imaging modalities. Positive contrast gastrograms either with barium sulphate suspensions alone, combined with food or in the form of BIPS (Barium-impregnated polyethylene spheres) are the most widely used. In selected patients, oral administration of non-ionic iodinated contrast media have their merits as well.
Two-dimensional grey scale imaging is a highly sensitive tool for detecting alterations in the gastrointestinal tract, but lacks specificity. Inflammatory and neoplastic infiltrates may appear similar. Differentiating between the causes of inflammatory disorders such as food hypersensitivity and inflammatory bowel disease (IBD) in chronic enteropathies is difficult. A loss of bowel wall layering, however, is an important finding that indicates a higher likelihood of neoplastic disease being present. The quantification of gastrointestinal blood flow with Doppler Ultrasound may provide new insights into the pathophysiology of both inflammatory and neoplastic disease. With this approach the hemodynamics of the splanchnic system reacting to different allergens, infections, tumors and inflammation can be assessed. Another new field of interest is the examination of tissue alterations in the liver and lymph nodes, where the lack of specificity for many changes also exists. The use of ultrasound contrast media may provide much more information on the hemodynamic nature and architecture of parenchymal lesions and will hopefully lead to increased diagnostic accuracy in gastrointestinal disorders in the future.
Over the last several years, new transducer technologies and new processing philosophies have been introduced. Transducers with matrix-architecture, transrectal transducers and endovascular systems are some to name a few. One of the most important steps forward in ultrasonography was the development of high frequency transducers (7,5-13MHz) for improved spatial resolution. Ultra-high frequency transducers, for example 50MHz have the ability to depict structures up to 15µm. Some reports have even described the use of 100MHz systems. As depth penetration is severely limited at these frequencies, only superficial structures can be imaged with such transducers. Endoscopic ultrasound (EUS) also provides new possibilities in conjunction with high frequency transducers. Endoluminal, ultrasonographic scanning has been developed to overcome the limitations in conventional transabdominal and transthoracic scanning. The "air-barrier" in cases of thoracic diseases and deep lying structures as in abdominal disorders are less of a problem with endoluminal scanning. With EUS the transducer can be brought into close proximity to the organ of interest using the natural passage ways of the esophageal and bowel lumens. Depth penetration is no longer a problem and high frequency transducers can be used. Transluminal, interventional techniques in conjunction with conventional gastroduodenoscopy can also be applied. Miniature ultrasonic probes (MUPs) have been developed with frequencies of 20-30 MHz. These may be advanced through the working channel of the endoscope for evaluation of the bowel mucosa.
Currently under investigation is ultra-high frequency Doppler ultrasound that can detect the movement of blood cells down to 1mm/sec, which corresponds to capillary blood flow. This technology may give insight into the vascular architecture of lesions, such as tumors, where its use is currently under investigation.
Virtual Endoscopy--the future in animals?
The basic principle of Virtual Endoscopy (VEndo) is the 3-dimensional reconstruction of inner surfaces of hollow organs. Images generated with either computed tomography or magnetic resonance imaging can be utilized to allow the observer to "fly" either interactively or following a previously programmed path through a hollow organ and evaluate the inner surfaces. At the moment the most frequent applications in human medicine are Virtual Bronchoscopy, Colonoscopy and Virtual Angiography. Other possibilities are the virtual-endoscopic reconstruction of inner ear structures, ventricles of the brain or other hollow organs; however, most of these are still in a pre-clinical stage . What these applications have in common is that they depend on a clear density/signal intensity difference between surface and contents of the organ of interest. In the original applications (vBronchoscopy and vColonoscopy) the wall showed a higher density than the lumen. Today, however, the contrary can also occur. A high quality 3-dimensional reconstruction is strongly dependent on the quality of the primary data. With the increasing availability of multislice helical CTs, which are capable of scanning the entire human abdomen with a submillimeter resolution in a few seconds, resolution and image quality is no longer a concern.
The main advantage of virtual endoscopy for the patient is the "non-invasiveness" of the procedure. The realistic presentation facilitates the recognition of a lesion for non-radiologists, be it referring doctors or the patients themselves. This increases the acceptance and therefore the distribution of the method.
The radiologist profits from the reliable three-dimensional reconstruction by reducing the amount of images to evaluate. A 30cm abdomen examined with 1 mm thick slices in transverse orientation leads to a dataset of 300 images, all requiring analysis. Allowing only 5 seconds of analysis time per image, the time needed by the radiologist is 25 minutes. Virtual Endoscopy can reduce the evaluation time to about 5 minutes per series.
Recent software can display the original slices in 3 orthogonal reconstructed planes next to the virtual flight through the hollow organs. In this way, suspicious lesions can be examined immediately on the original images. Disadvantages lie in the dependence on the quality of the original images and in the danger of distortion of the original lesion due to the calculation process. With the use of an incorrect reconstruction algorithm, lesions can be mimicked as well as deleted. The reliability of the method is subject of ongoing research and scientific discussions.
Volume Rendering and other 3-D reconstructions of the abdomen
There are several possibilities for the 3-dimensional visualization of organs. The reconstruction of inner surfaces has been described under Virtual Endoscopy. The "high end" mode of three-dimensional reconstruction is Volume Rendering (VR). Organs are reconstructed as a complete 3-D dataset; which means, that the inner structures of the organs are included. This allows to remove parts of the reconstructed organs, allowing underlying structures to be visualized. This technology requires high-resolution original data from CT or MRI in combination with powerful 3D-reconstruction workstations, which are not widely available in veterinary medicine. This technique and most other 3-dimensional reconstructions are under investigation and may become interesting tools also in GIT imaging in the future.
The combination of radiography and ultrasound is standard in GIT disease imaging in veterinary medicine and is very sensitive for many disease processes. Ultrasound guided fine needle aspiration or core biopsy tissue sampling of the liver, pancreas, bowel wall and lymph nodes are important to overcome the lack of specificity. Doppler investigations of the splanchnic vessels are increasingly important in understanding hemodynamic in enteropathies and therefore contribute in clarifying underlying pathophysiological processes. Application and importance of harmonic imaging, contrast enhanced harmonic imaging, ultra-high resolution transducers (>20MHz) and echoendoscopy in GIT disease are currently under investigation. With increased access to helical CT-scanners und 3-D software virtual endoscopy may gain importance in veterinary medicine as well.
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