CT is often superior to MRI and used for assessment of margins of osseous or mineralized structures compared to MRI. CT can assess soft tissue changes and differences fairly well by narrowing windows and levels under standard algorithms to see differences of attenuation of the x-rays, but cannot manipulate the soft tissues due to their molecular structure as MRI can in order to enhance or null their differences. Therefore, MRI is often far superior to CT at assessing for subtle changes within soft tissues due to the dramatic contrast enhancement. MRI is most often utilized in veterinary medicine and is the modality of choice when you are trying to assess soft tissue structures not easily accessed by an ultrasound probe or are looking for diseases that may not be appreciated via any other modality. MRI is used primarily for neurologic (brain and spine) imaging and joint imaging concerning cartilage, ligaments, and/or menisci. Keeping those general statements in mind, depending on the type of disease you are assessing for you may be able to appreciate the abnormalities on both modalities, so either study may be adequate for diagnosis. References are provided with specific imaging protocols for MWDs.1-5

 

Sedation/Anesthesia

The patient must be either heavily sedated or anesthetized while the study is taking place. CT studies of the thorax and abdomen require general anesthesia and intubation of the patient, with closure of the pop-off valve on the anesthetic machine during image acquisition. Depending on how advanced the CT machine is and slice thicknesses needed, this may or may not be a problem for the patient, as the breath hold may have to last for several seconds. Always ensure anesthesia pop-off valves are not left closed, to avoid pneumothorax.

 

Contrast Administration

Intravenous iodinated contrast may be used during a CT study in order to further enhance margins of soft tissue structures. If a CT is being conducted to assess an abnormal soft tissue mass or structure, intravenous iodinated contrast should be administered after acquisition of routine images prior to contrast administration for comparison purposes. This contrast administration allows for further characterization of the abnormal soft tissue as only the vascular portions of the structure will enhance.

  • The current standard for use of contrast during CT is non-ionic iodinated contrast media, with the two most common types being iohexol and iopamidol. Iohexol is most commonly used in MWDs. For a vial of iohexol at a concentration of 240mg/mL, the intravenous contrast dose is 400 mg/kg (rule of thumb is 1 mL of contrast agent per pound of body weight, not to exceed 60 mL).
  • IV catheterization of the patient is required for contrast administration, and the contrast is a thick, sticky solution which needs to be bolused to the patient, so use 18 gauge catheters and syringe needles.
  • After bolusing the contrast to the patient, only the study in the standard algorithm needs to be repeated.
  • If the patient is dehydrated, the patient should be rehydrated prior to the CT study if possible or at least on IV fluids to correct the problem if unavoidable.
  • Adverse side effects are rare with non-ionic contrast media in correctly hydrated patients.