Advanced Imaging Options and Usage: When Ultrasound or Radiographs
Don't get me wrong - I love ultrasound! My fascination with this modality is one of the big reasons that I became a radiologist. However, there are times that an ultrasound or radiographs may not provide you with the most complete information about your patient. That's why we are privileged to have options like CT, MRI, fluoroscopy, and nuclear medicine. These modalities, like ultrasound and radiographs, allow us to non-invasively "look" inside an animal's body to help us decide whether proceeding with more invasive procedures makes sense. It is often difficult to gain access to all these modalities outside of academia, so we are proud to offer you CT, fluoroscopy, and nuclear medicine here at DVMS, in addition to ultrasound and digital radiography. We are equally pleased to a have a good relationship with a local practice that gives us access to their MRI, allowing us to provide your patients with a full spectrum of leading-edge imaging options.
When might we want to put aside ultrasound for another modality?
Although ultrasound can potentially identify portosystemic shunts, it can often be difficult to evaluate the vascular anatomy in small dogs in general and especially in young, wiggly dogs that do not remain still during ultrasound. Gas in the GI tract (particularly the stomach) can also obscure the anatomy. Ultrasound is useful in identifying supporting signs of a portosystemic shunt (small liver and intrahepatic portal vein, large kidneys, and bladder stones) even if a shunt vessel cannot be clearly identified. We have the option of performing nuclear scintigraphy or CT at our facility to evaluate for shunts. Nuclear scintigraphy involves injecting a radioactive pharmaceutical into the spleen and watching the path of the pharmaceutical through the portal vasculature on our gamma camera. This procedure takes less than 15 minutes and requires only light sedation. However, the animal is radioactive after the procedure and must remain in isolation for a few hours after the procedure, so this option may not be ideal for patients who have seizures. CT is the best way to completely evaluate the portal vasculature since it is a 3D modality that eliminates the complication of superimposition and gas shadowing that can occur with ultrasound. CT requires the patient to be under heavy sedation or anesthesia.
If a mass does not extend to the thoracic body wall, ultrasound is not a viable tool since it cannot penetrate aerated lungs. It's also difficult to image very dorsal masses with ultrasound due to the proximity of the ribs. CT is the imaging modality of choice for lungs, and we can sometimes, depending on the location of the mass, obtain CT-guided aspirates or biopsies. In addition to identifying the location, appearance, and extent of thoracic masses for potential surgical planning, CT also allows us to evaluate the remaining pulmonary parenchyma for subtle metastatic disease that may not be apparent on radiographs. CT is also generally better for evaluating for causes of spontaneous pneumothorax (such as bulla or penetrating foreign bodies) since the superimposition of gas from pneumothorax can obscure these lesions on radiographs. Thoracic CT requires general anesthesia so that we can maintain respiratory control to help eliminate motion and more completely inflate the lungs during the examination.
Subcutaneous and limb swelling/masses:
Diffuse limb swelling is very challenging to evaluate with ultrasound, as inflammation and edema make everything appear generally bright and coarse and often obscure complete evaluation of the region of interest. CT again allows us to eliminate these 2D challenges and fully evaluate the extent of any mass or foreign body that may be a cause of limb/subcutaneous swelling. Small vascular changes can be challenging, and MRI is a better modality if a vascular anomaly is suspected.
Depending on their insertion point and whether they tunnel intramurally, ectopic ureters can be challenging to completely evaluate with ultrasound. If the insertion point is within the pelvis, ultrasound will not be able to evaluate the area since ultrasound cannot penetrate bony tissues. Any dilation of the more proximal ureter(s) and the kidneys resulting from chronic ectopia is usually obvious on ultrasound. A fluoroscopic contrast IVP (intravenous pyelogram) or CT-IVP is helpful in these cases. The flow of contrast through the ureters can be seen in real time with fluoroscopy, so the insertion can be identified. With CT, repeat serial images may be necessary to see the contrast just at the insertion point, so the exam may take longer, but it usually clearly identifies the insertion. Sedation is usually required for IVP and anesthesia for CT-IVP. Patients who are currently dehydrated or azotemic, however, are not good candidates for either procedure.
Shifting lameness/lameness that is difficult to isolate/subtle lameness:
Nuclear bone scintigraphy involves injecting an IV radioactive pharmaceutical and looking for any active areas of bone using the gamma camera. Bone scintigraphy does not always clearly differentiate between benign and aggressive disease, but it can help identify a region that needs to be radiographed for further evaluation. This procedure requires sedation and an overnight stay in the hospital until the patient is no longer radioactive.
Ultrasound cannot penetrate bone, much of the urethra lives in the pelvis, and structures outside of the abdomen (i.e., the perineum and prepuce) are not often imaged with ultrasound, so detecting subtle changes can be challenging. Fluoroscopic retrograde urethrography is very useful to evaluate for luminal obstructive lesions, strictures, or urethral trauma. Again, these changes can be evaluated in real time to help identify the location and severity of disease and to decide on the best treatment plan. CT is preferred if a mass is suspected since the entire mass can be evaluated for surgical planning with better tissue resolution and without the superimposition of 2D imaging modalities.
Other diseases that may be better evaluated with advanced imaging modalities other than ultrasound include tracheal trauma, hiatal hernias, neck masses, and non-healing wounds and/or abscesses.
If you are unsure about how to proceed with a challenging case, please call Desert Veterinary Medical Specialists to discuss options. We are always happy to help you choose the right imaging for your patient.