The opportunity to refer cases for advanced imaging on an inpatient and outpatient basis is increasing rapidly. Many referral hospitals now offer outpatient ultrasonography, computed tomography (CT), and magnetic resonance imaging (MRI).
Key to a successful patient referral for diagnostic imaging is managing owner expectations. Cost, risks, benefits, diagnostic expectations, and the limited ability to perform interventional procedures on outpatient cases should be discussed before referral. The following tells how to determine the best modality for your patient as well as which cases are appropriate for inpatient and outpatient imaging.
INPATIENT VS. OUTPATIENT
From an imaging perpective, an outpatient is one being referred exclusively for an imaging diagnosis. This situation is ideal for clients looking to follow up with their regular veterinarian for further diagnostics or treatment. However, the lack of familiarity and lack of a doctor-client-patient relationship may preclude the use of sedatives and interventional procedures.
Alternatively, from an imaging perspective, an inpatient is simply one that has met with, and been examined by, a doctor in the specialty practice. Although this may yield a more thorough and complete consultation, and may allow use of medications, interventions, and treatments, this type of referral tends to be more costly.
Some specialty practices offer a mixed model in which a patient is referred exclusively for diagnostic imaging but receives a brief physical examination and assessment by a specialty practice doctor so that sedation or anesthesia can be performed. This model is particularly common at outpatient CT and MRI centers. A discussion with the specialty center before referral will help determine which imaging modality and referral type is best for an individual patient.
It is best practice to instruct owners to fast the patient before an ultrasonographic examination. This prevents a food-filled stomach from obscuring visualization of the abdomen and also allows for safe use of sedation if deemed necessary. Ideally, fractious, uncooperative, or aggressive patients are best suited for inpatient referral to allow for adequate observation after the use of sedation, anesthesia, or both. However, sedation will likely not be given for outpatient cases; thus, fractious, uncooperative, aggressive patients are best suited for inpatient referral.
Ultrasonographic examinations are becoming more commonplace in veterinary medicine and can be a useful tool in certain situations. In managing an owner's expectations, it is crucial to communicate that the majority of ultrasonographic findings are nonspecific, and additional diagnostic steps are often required to obtain a definitive diagnosis.
These cases are likely the least appropriate outpatient referral cases since 80% leave outpatient imaging for hepatic disease without a diagnosis. A biopsy or aspirate is often required to establish a definitive diagnosis. Exceptions would be gallbladder and biliary disease (e.g. gallbladder mucocele, cholelithiasis, obstruction) in which abdominal ultrasonography may be a definitive test. Patients with suspected portosystemic shunts should be referred for inpatient ultrasonography since they may require sedation or further diagnostic tests.
Ultrasonography of the spleen is most often performed to detect a mass. However, it can be difficult to assess whether the mass is benign or malignant with ultrasonography, and rarely can a definitive diagnosis be reached without further diagnostics (e.g. fine-needle aspiration, biopsy).
Suspected ureteroliths, cystoliths, pyelonephritis, ectopic ureters, prostatic abscesses, and cysts make ideal referral ultrasonography cases. A definitive diagnosis can often be reached based on imaging findings.
The utility of ultrasonographic examination of the gastrointestinal tract varies. Foreign bodies can be the most straightforward diagnosis, whereas inflammatory and neoplastic diseases are the least straightforward because of the overlap in ultrasonographic appearance. Generally, further testing in the way of ultrasound-guided diagnostic techniques, endoscopy, or laparoscopic or abdominal exploratory surgery may be necessary to distinguish between these two disease entities and to differentiate the types of neoplasia.
Since it is impractical to examine the entire thorax ultrasonographically, a targeted approach based on radiographic findings is usually employed. Although pleural effusion is easily detected and characterized, rarely is a definitive cause found on ultrasonography.
Pulmonary nodules may be detected if they are located peripherally. In these cases, ultrasonography is used to help guide fine-needle aspiration or biopsy to help make a definitive diagnosis.
CT is performed on an inpatient or mixed inpatient-outpatient basis. As for all referral cases, ensure owners are aware of the costs, necessity and risks of anesthesia, and duration of hospitalization. Like ultrasonography, additional diagnostics may be required to reach a definitive diagnosis. Almost all cases referred for CT benefit from preemptive communication between the referring doctor and the referral center to discuss the imaging procedures, risks, and likelihood of reaching a definitive diagnosis.
CT of the nasal cavity is the most common referral case. Clearly aggressive and clearly nonaggressive cases are easy to categorize, but a histologic diagnosis cannot be established without performing a biopsy. A common source of frustration for owners is a report without clear differential diagnoses. Most often, these cases benefit from inpatient imaging, where CT or rhinoscopic-guided biopsies can be performed to obtain tissue samples.
Some referral centers perform a CT scan to identify and categorize suspected portosystemic shunts on a referral basis. Most often there is high suspicion for a shunt (bile acids > 100 µmol/L with concomitant clinical signs). Owners should be prepared for surgical intervention if indicated. Surgical planning of large intra-abdominal or body wall masses are also common referral cases.
Conditions that would benefit from evaluation by a thoracic CT scan include primary lung tumors, metastatic disease, evaluation of thoracic lymph nodes, and interstitial pneumonias. A number of diseases (e.g. lung lobe torsion, pulmonary metastases) can be definitively diagnosed without further work-up. However, owners should be prepared for the need for additional diagnostics such as fine-needle aspiration, biopsy, transtracheal wash, or bronchoalveolar lavage based on the initial imaging results.
Elbow, hip, shoulder, and tarsal disease in young patients are commonly evaluated with CT scans. Most often a definitive diagnosis is made without further work-up.
Brain CT is also a common referral case. These patients are generally exhibiting signs of a large neoplastic lesion (e.g. adult-onset seizures, mentation changes, clinical signs consistent with pituitary disease). When possible, these cases are often more appropriately assessed with MRI.
Referral of patients for neurologic (spinal) imaging is usually on an inpatient basis given the necessity to proceed to surgery if a compressive lesion is found. Prepare owners for this outcome before imaging, as discussions regarding surgery, prognosis, and risks are best held prior to anesthesia.
The best candidates for spinal CT are middle-aged, small-breed dogs with suspected cervical or lumbar lesions with an acute onset and worsening of clinical signs. Large-breed patients, patients with static signs, or patients suspected of having parenchymal cord disease (e.g. fibrocartilagenous embolism, degenerative myelopathy, lymphoma) would likely benefit from MRI rather than CT.
MAGNETIC RESONANCE IMAGING
Although some referral centers offer outpatient MRI, it is generally performed on an inpatient basis. As with CT, owners should be aware of the costs, necessity and risk of anesthesia, and duration of hospitalization. A discussion with a referral center can help determine if CT or MRI is most appropriate and is often beneficial when counseling owners on what to expect during the referral visit.
MRI is becoming increasingly popular for evaluating nasal disease, particularly if there is concern for an intracranial component. Similar to CT, clearly aggressive and clearly nonaggressive cases are easy to categorize, but a histologic diagnosis cannot be established without performing a biopsy. Thus, these cases often benefit from inpatient imaging so that rhinoscopic biopsies may be performed.
MRI is rarely used to evaluate the abdomen. Occasionally MRI may be helpful in evaluating for portosystemic shunts or for adrenal gland and hepatic imaging.
Pulmonary parenchyma does not image well with MRI. Most magnetic resonance thoracic imaging is performed for further differentiation and surgical planning of chest wall, mediastinal, or pleural lesions, though some centers also perform cardiac MRI.
MRI is rarely used for orthopedic disease except for soft tissue stifle and shoulder imaging (e.g. cranial cruciate ligament, and biceps and supraspinatous tendons).
Evaluation of the brain and central nervous system are the most common referral cases for MRI. These patients are generally suspected to have a neoplastic lesion (experiencing adult-onset seizures, mentation changes, clinical signs consistent with pituitary disease) or inflammatory lesions. A cerebrospinal fluid tap is often performed, particularly if an inflammatory lesion is suspected.
As with CT, patients needing neurologic (spinal) imaging are usually referred on an inpatient basis given the necessity to proceed to surgery if a compressive lesion is found. It is important to discuss with the owners the possible need for surgery, the prognosis, and the risks before the patient is anesthetized and undergoes imaging.
Large-breed patients, patients with static signs, or patients suspected of having parenchymal cord disease (e.g. fibrocartilagenous embolism, degenerative myelopathy, lymphoma) may benefit from MRI's ability to evaluate spinal cord parenchyma.
MRI is the imaging modality of choice in cases of suspected brachial plexus injury and nerve sheath and soft tissue tumors.
Ryan King, DVM, DACVR Cummings School of Veterinary Medicine at Tufts University TuftsVETS North Grafton, MA