Frequently Asked Questions About Contrast Material Usage
What kinds of contrast are available?
- There are three broad kinds of contrast available: IV, PO, and PR (rectal). IV contrast is either gadolinium for MRI or iodinated contrast for CT. PO contrast for all ER and inpatient CT scans is dilute iodinated contrast (same agent used for IV contrast in CT). Barium is available as an alternative for ER patients with allergic reactions to iodinated contrast agents; currently the Department of Radiology recommends premedication for PO iodinated contrast in patients with a history of moderate or severe allergy (see departmental policy). Rectal contrast like oral contrast is dilute iodinated contrast, but administered through a rectal tube.
What is the difference between a CT angiogram and a CT scan with IV contrast?
- An angiogram is a specific type of CT scan with contrast. In a CT angiogram the contrast is timed so that it will highlight either the arteries or veins (venogram) of interest. For instance, a CT angiogram of the chest to evaluate for PE will have the timing set so the contrast is present in the pulmonary arteries. A CT angiogram of the chest to evaluate for aortic dissection will have the timing set so the contrast is present in the aorta. Meanwhile, a general CT of the chest with IV contrast will have the contrast timed so that it is present within the capillary bed of the soft tissues. For the abdomen and pelvis, contrast timing is more complicated, because there are both systemic and portal venous capillary beds. Most CT scans of the abdomen and pelvis are timed so that the contrast is in the portal veins. However, trauma studies are timed for the late arterial/early portal venous phase.
CT scans of the brain: when is IV contrast used?
- IV contrast is used in brain CT when performing a CT angiogram (or venogram) or for evaluating an abscess or malignancy. In general, workups start with a non-contrast brain CT study and then may progress to MRI or contrast enhanced CT when necessary.
When is IV contrast used for abdomen and pelvis CT?
- There are several times when IV contrast is not necessary. These are when evaluating for ureteral calculi or retroperitoneal hematoma. For all other purposes IV contrast is strongly desirable (including evaluating for infection associated with kidney stones). IV contrast improves tissue contrast, the parameter that distinguishes one organ from the next) and greatly aids in identifying masses or inflammatory/infectious processes. If a patient has a contraindication to IV contrast, the study can be performed without it.
What is the Oral Contrast Policy for Abdominal CT in ED Patients?
How long does my patient have to wait after drinking PO contrast before having the study?
- Generally, there is a 45-60 minute delay after finishing PO contrast before performing the CT study.
When would I need rectal contrast? How do I order it?
- There are very few indications for rectal contrast in the ER. In the setting of penetrating trauma to the pelvis, rectal contrast may be necessary. Otherwise, it can be useful to evaluate for surgical complications or colonic fistulas, but extreme caution must be exercised in deciding whether to insert a rectal tube in a potentially friable or traumatized/post-surgical rectum. To order rectal contrast, order a CT scan as you would otherwise, specify your desire for rectal contrast in the order, and specifically request it when communicating verbally with the ER Radiologist.
What are the contraindications for CT IV contrast?
- There are two main contraindications for the administration of iodinated IV contrast: contrast induced nephropathy and allergy to iodinated contrast.
- The attending ER Radiologist and the referring clinician may allow for patients with renal failure to receive intravenous contrast when the risks are felt to be outweighed by the benefits. Read the current policy on renal function and use of Iodinated Contrast Patients who only have one kidney or are kidney transplant recipients can still receive IV contrast as long as renal function is adequate. See policy details here.
- Patients who have had a severe anaphylactoid reaction to iodinated contrast should never receive iodinated contrast unless the radiologist and ordering provider are both in agreement that the scan is needed for medical care and benefit outweighs risks. The ED team should be prepared to treat the patient with the radiology team if such a scan is needed and the patient should receive standard Steroid and Benadryl premedication if patients clinical stability allows.
Do I need to hold my patients Metformin if they are getting a CT with contrast?
- In general, no. Patients taking any drugs with Metformin do NOT need to stop taking the drug unless eGFR is under 30 as the risk of contrast related AKI in patients with eGFR > 30 is extremely low. Policy for diabetic patients on Glucophage or Glucovance.
What are the contraindications for MRI IV contrast?
- There are two main contraindications for the administration of gadolinium based IV contrast agents used in MRI: risk of nephrogenic systemic fibrosis (NSF) and allergy to gadolinium.
- The most commonly used MRI contrast agent in YNHHS is a macrocyclic agent for which the risk of NSF is thought to be extremely low, if present at all (even in patients on dialysis or with acute renal failure). For current policy on receiving gadolinium based agents click here. Gandolinium Contrast Information.
- Anaphylactoid reactions to IV gadolinium are much less common than reactions to IV iodinated contrast. Nevertheless, allergic reactions do occur, and patients at risk should be premedicated according to Department of Radiology policy if they have a documented allergy to gadolinium-based contrast agents. Pre-Medication Policy.