Abdominal Imaging--Correlation of Radiography, Computed Tomography and Ultrasound Findings
Erik R. Wisner, DVM
With the recent advances in diagnostic imaging technology, abdominal imaging in veterinary medicine has become more sensitive and specific for disease diagnosis. In addition to survey and contrast radiographic procedures, ultrasound and computed tomography now provide highly detailed cross-sectional images of parenchymal and hollow viscera, masses, infiltrative lesions, effusions and vascular flow. Ultrasound is also uniquely suited for aspiration or tissue core biopsy using real-time image guidance. Many of the traditional contrast radiographic procedures used for diagnosis of abdominal disease are now being replaced by these newer imaging techniques. Despite this trend, a variety of diagnostic imaging approaches may successfully be employed for diagnosis of a particular abdominal abnormality and personal preference may dictate the selection of an imaging modality or procedure.
Historically the mainstay of abdominal imaging, conventional planar radiography, is gradually being supplanted by newer imaging modalities and, with few exceptions, is evolving to a screening test rather than a definitive diagnostic test. The diagnostic vagaries of a mid-abdominal mass detected on conventional abdominal radiographs are clarified by ultrasound or CT imaging which more accurately and specifically define the origin and character of the mass and provide an avenue for definitive diagnosis through image guided fine-needle aspiration or tissue core biopsy. Despite this trend away from radiography, abdominal radiographs may still play an important role in diagnosis of such varied abnormalities as pneumoperitoneum, gastric foreign body, intestinal obstruction, ureteral calculi, pelvic disease (not accessible to ultrasound examination) and abdominal disease with involvement of adjacent skeletal structures (e.g., regional metastasis of a caudal abdominal neoplasm to the caudal lumbar vertebra) or the body wall (e.g., traumatic or inguinal hernia).
As the availability of diagnostic ultrasound continues to increase and expertise using the modality becomes more widespread, the frequency with which radiographic contrast procedures are performed will continue to decrease. The number of upper GI examinations, intravenous urograms, cystograms, urethrograms and contrast peritoneograms performed at our institution has dropped precipitously over the last 10 years as they have been replaced by routine non-invasive ultrasound examinations that are easier to perform, more quickly completed and often provide more diagnostic information. Despite this trend, there are still specific instances in which these contrast radiographic procedures play a role.
Gastrogram: Although mural lesions of the stomach and gastric motility are usually well visualized using ultrasound, intraluminal abnormalities such as foreign bodies and pedunculated masses are often better seen on survey abdominal radiographs and contrast gastrography. These studies are also better for identifying gastroesophageal reflux.
Upper GI exam: Although ultrasound is now routinely used for detecting intestinal masses, infiltrative disease, foreign bodies, complete and partial obstruction and motility abnormalities, these lesions are sometimes more apparent on upper GI examination. We now generally reserve this study for patients that are believed to have a primary small intestinal disorder that has not been identified or adequately characterized using ultrasound. Contrast studies of the lower intestinal tract are also occasionally performed particularly for diagnosis of descending colon or rectal strictures.
Intravenous urogram: These studies have been almost entirely replaced by ultrasound examination with the exception of those patients in which a qualitative estimation of kidney renal function is required for prognosis or treatment planning. Even in these patients, renal scintigraphy is preferred because it can provide a quantitative estimate of single kidney and global glomerular filtration rate. Intravenous urogram my still play a role in evaluation of the ureters and is often used for diagnosis of ureteral ectopia though both ultrasound and computed tomography are now often used for this purpose. We still use positive contrast nephropyelography to unequivocally determine the presence and location of a ureteral obstruction. Iodinated contrast material is injected into the renal pelvis using ultrasound guidance followed by serial abdominal radiographs to follow the normograde flow of contrast from the renal pelvis into the proximal ureter.
Cystography/Urethrography/Vaginography: Cystography has been almost entirely replaced by ultrasound with the exception of evaluating the bladder neck/urethra particularly when the urinary bladder is positioned in the pelvis and ultrasound access is limited. Retrograde urethrography and vaginography still play an important role in diagnosis of lower urinary/reproductive tract disease due to the limitations of ultrasound in the pelvis.
Contrast Peritoneography: These studies play an occasional role in diagnosis disruption of the diaphragm or body wall when ultrasound findings are equivocal.
The introduction of ultrasound into veterinary clinical practice has been the single most important development in diagnostic imaging since radiography came in to use. As ultrasound technology has progressed, the diagnostic quality of clinical ultrasound scanners has markedly increased while cost has dropped precipitously. In addition, technological advances such as high resolution transducers and Doppler imaging have resulted in real-time images of exquisite anatomical detail and the potential to evaluate vascular flow and tissue perfusion.
Ultrasound images are acquired unlike those of any other imaging modality. Ultrasound pulses are transmitted from a transducer into the patient at a specific frequency such that higher frequencies result in better image resolution but less depth penetration. The opposite is true for lower frequency transducers. Following transmission of the pulse, the transducer 'listens' for returned echoes of the same frequency and uses the amplitude and timing of the returned echoes to register the tissue brightness and depth, respectively. Typically the acoustical properties of lesions will differ from the surrounding normal tissue resulting in a brighter, darker, or mixed echogenic appearance. The imaging plane and examination quality are heavily influenced by the operator. This is advantageous in that any imaging plane can be achieved but is detrimental because, unlike other imaging modalities, user experience plays a large role in the diagnostic utility of the examination.
With few exceptions, ultrasound is now our first line diagnostic imaging study for evaluation of abdominal disease because of the exquisite anatomical detail that can be achieved for all the abdominal viscera. The different imaging characteristics of certain diseases can help more clearly define a differential diagnosis list. For example, a hepatic abscess will have a different ultrasonographic appearance than a hepatic neoplasm. Certain abnormalities have a signature appearance that allow a more precise diagnosis based on the imaging characteristics. Biliary cystadenomas, for example, will appear as multi-cameral cystic lesions within the liver. However, with standard B-mode ultrasound it is rarely possible to determine the exact cellular diagnosis based on the imaging appearance of the lesion.
For definitive diagnosis, ultrasound provides an avenue for guided fine-needle aspiration and tissue core biopsy of masses, organs and lymph nodes through real-time direct visualization of the lesion. Other than the limited use of fluoroscopy for similar guided biopsy procedures, no other imaging modality provides the access, versatility and accuracy of needle placement for sample acquisition. Guided biopsy has become a standard procedure at our institution when tissues of interest can be visualized and a path of access is present for needle placement.
Although ubiquitous in human medicine, computed tomography has only recently emerged as an important diagnostic imaging approach to abdominal disease in veterinary medicine. As with diagnostic ultrasound, this is in large part due to recent technological advances that have improved image quality while simultaneously reducing study cost.
As with conventional radiography, computed tomography relies on tissue density differences as the basis for image formation. Each CT image can be conceptualized as a radiograph of a 1 to 10 mm thick "slice" of the patient. Because each image represents such a thin slice of anatomy, superimposition effects that are inevitable and often the source of confusion with conventional radiography, are minimized. Relatively new technology now results in total scan times of under a minute allowing abdominal studies to be completed in a single forced breath-hold. In our practice, with rare exception, all patients are anesthetized for computed tomography examinations such that the anesthetist can control breathing.
CT has become a diagnostic workhorse in our practice supplanting conventional radiology for many applications and supplementing diagnostic ultrasound examinations in others. In many patients an abnormality that is initially identified on ultrasound examination is more specifically characterized or clearly delineated on CT examination. Therefore, in addition to diagnostic applications, CT is often used as a tool for surgical and radiation treatment planning. Clinical applications of abdominal CT are many and varied. Just a few representative uses include:
Liver/Pancreas: Contrast-enhanced CT can be used to more accurately determine location size and number of hepatic masses initially diagnosed on US examination. When these studies are performed, image acquisition is timed to maximize the contrast difference between hepatic masses and surrounding normal hepatic parenchyma. CT can also be useful for determining surgical respectability of hepatic and pancreatic masses particularly when they involve multiple hepatic lobes or the biliary tree. Contrast CT is also used at our institution to preoperatively characterize and anatomically map portosystemic shunt vessels. In these patients, image acquisition is timed to maximize contrast concentration in the portal and central venous system.
Gastrointestinal Tract: Contrast CT can be used to delineate gastric or intestinal tumor margins. CT has been particularly useful for characterizing and determining respectability of mass lesions involving the descending colon and rectum that may not be well visualized by other means.
Urinary Tract: Typically the kidneys are well visualized using ultrasound so CT is of most value for evaluating ureters and localizing ureteral obstruction and characterizing lesions within the pelvis that encroach on the lower urinary tract. We also routinely employ contrast-enhanced CT for documenting ureteral ectopia either as an initial study or when contrast radiography or ultrasound has failed to confirm the diagnosis.
Adrenal Masses: CT is often helpful for defining the margins of adrenal masses and determining the presence of vascular invasion when not convincingly determined using ultrasound.