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 http://radiology.yale.edu/patientcare/policies/oralcontrast.aspx). 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; additionally, excretory phase images are done through the kidneys to detect collecting system injuries.
CT scans of the brain: when is IV contrast used?
- In general 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 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. Current policy suggests a creatinine of less than or equal to 1.5 mg/dl and no evidence of acute kidney injury for IV contrast administration. The attending ER Radiologist and the referring clinician may allow for patients with mild renal failure to receive intravenous contrast when the risks are felt to be outweighed by the benefits The Department of Radiology’s policies allow patients who have had a severe anaphylactoid reaction to receive IV contrast if the ER attending believes the study is needed emergently and the patient receives premedication (http://radiology.yale.edu/patientcare/policies/premedication.aspx). Patients who only have one kidney or are kidney transplant recipients can still receive IV contrast; per departmental policy, the volume of contrast will be decreased (http://radiology.yale.edu/patientcare/policies/singlekidney.aspx). Departmental policy states that patients taking Glucophage (Metformin) should have the medication withheld for 48 hours following contrast administration and should have their renal function re-tested prior to re-starting (http://radiology.yale.edu/patientcare/policies/glucophage.aspx).
What are the contraindications for MRI IV contrast?
- There are two main contraindications for the administration of gadolinium IV contrast: risk of nephrogenic systemic fibrosis (NSF) and allergy to gadolinium. To prevent NSF current Department of Radiology policy sets a cut off of eGFR > 30 ml/min/1.73m2 or > 40 ml/min/1.73m2 for severe liver disease (http://radiology.yale.edu/patientcare/policies/gadolinium.aspx). The attending ER Radiologist and the referring clinician may allow for patients with eGFR greater than those thresholds to receive intravenous gadolinium when the risks are felt to be outweighed by the benefits and there is a subsequent completed attending nephrology/hepatology consult prior to ordering the study. Anaphylactoid reactions to IV gadolinium are thought to be 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 (http://radiology.yale.edu/patientcare/policies/premedication.aspx).
What are the contraindications for PO contrast for abdomen and pelvis CT?
- PO contrast is not necessary for the evaluation of kidney stones or a retroperitoneal hematoma. Otherwise, for ER purposes, oral contrast is almost always strongly desired, even in trauma patients. PO contrast highlights the lumen of the bowel; it is essential for identifying when the wall of the bowel is inflamed and for separating bowel loops from non-bowel loops in the abdomen (i.e. lymph nodes, abscesses, masses). Currently all full trauma patients are scanned without the administration of any PO contrast, and all modified trauma patients should have PO contrast attempted (but no delay following ingestion of PO contrast is mandated) prior to performing the study. However, even in these trauma patients, the administration of PO contrast, and, allowing for a stable patient, waiting 45 minutes following the administration of contrast, can be very helpful.
How long does my patient have to wait after drinking PO contrast before having the study?
- Ultimately it is up to the discretion of the attending ER Radiologist in consultation with the attending ER Physician (different radiologists have different preferences). Generally, there is a 45 minute delay after finishing PO contrast before performing the study. The exception is for CT scans where appendicitis is a significant diagnostic possibility, when a 2 hour delay is strongly preferred. Modified trauma patients who receive PO contrast can be scanned immediately after ingestion of contrast; however, forty-five minute delays improve sensitivity and specificity and are desirable, when possible.
Is it possible to do an appendicitis CT scan without waiting for PO contrast?
- Most attending ER Radiologists at Yale believe using PO contrast and waiting for two hours is in the patient’s best interest. In obese patients some attending ER Radiologists may consider forgoing PO contrast as long as IV contrast can be administered. The intra-abdominal fat helps to outline the bowel loops and makes PO contrast less essential. There are other approaches to appendicitis imaging, but currently these are the only two approaches in favor at Yale.
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.