In recent years, cerebral imaging has exploded with new techniques, software, and capabilities. Two types of imaging—magnetic resonance angiography (MRA) and computed tomography angiography (CTA)—have gained in use and availability. If your patient needs either test, you'll want to know how these tests are performed and what they tell the healthcare team. You're already familiar with conventional magnetic resonance imaging (MRI) and computed tomography (CT), so understanding MRA and CTA will be a snap.
MRA is a type of MRI. Magnetic resonance works by manipulating hydrogen, the most common element in human tissue. Hydrogen creates a radiofrequency signal when exposed to a magnetic field. MRI uses this principle to evaluate cerebral structures. Because radiofrequency signals differ from structure to structure based on how much hydrogen they have, a computer can assemble the radiofrequency signals into a readable picture.1
MRA refines this process even further. The radiofrequency signals created by hydrogen traveling in the arteries create an image, but the computer takes away the images of the other structures, providing a clear image of the arteries and any pathology. Some MRA is capable of providing three-dimensional images. Gadolinium, an I.V. contrast medium, is used to highlight the arteries in MRA.1 Imaging is done without contrast first, then again after the gadolinium injection.2
An MRA can evaluate for intracranial or extracranial atherosclerosis, arteriovenous malformation, intact aneurysms, or other cerebrovascular disease.2 To prepare your patient, focus on education. Explain that generally, the only uncomfortable part of the procedure is when vascular access is established for contrast injection. Tell the patient about the importance of being still during the scan; motion reduces the clarity of the images produced. Because MRI produces an extremely loud noise, like a jackhammer, patients will most likely be offered ear protection such as earplugs.
Screen your patient for any contraindications to MRI. Many institutions have a screening form to assess for metal in the body; you'll need to carefully question your patient and document the responses. Also screen your patient for claustrophobia; if necessary, obtain an order for an anxiolytic. (Open MRI, another alternative for claustrophobic patients, yields poor-quality MRA images.1)
If sedation is used, you'll need to use MRI-compatible monitoring equipment. Many patients can be comforted by knowing healthcare personnel are just outside the room, and the patient can communicate with them if necessary.1
Be alert that most magnetic resonance scanners have a weight limit. Finally, carefully assess women of childbearing age for possible pregnancy. Although MRA doesn't use radiation, and no adverse effect to the fetus from magnetic resonance or gadolinium have been documented, this doesn't mean adverse effects don't exist. The need for the MRA needs to be weighed against the safety of the fetus in the case of pregnancy. If your patient is breastfeeding, you may want to check with your lactation consultant about the patient's ability to nurse after contrast injection. Generally, it's considered safe.3
After the scan, monitor the patient's renal function. Although gadolinium is less nephrotoxic than conventional contrast, it can affect renal function.2 If you administered sedation, continue to monitor the patient's cardiopulmonary status.1 Outpatients should have made arrangements for transportation home, and should receive discharge instructions on not driving for 24 hours. Encourage patients to drink fluids for 24 to 48 hours after MRA, unless contraindicated.
A type of CT scan, CTA uses a computer to produce images that are taken via X-ray. A CT scanner must have a multidetector to be capable of performing a CTA (these scanners are typically called helical or spiral CTs). The multidetector lets the CT quickly take high-quality pictures of the brain.
I.V. contrast media is used to help produce a clear picture of the cerebral arteries. The healthcare provider can see an aneurysm, even if it's ruptured.1 The best application for CTA is in diagnosing aneurysms.
Preparing your patient for CTA is similar to preparing a patient for MRA. Establish vascular access for the contrast injection and explain the need to lie still to assure good-quality images. The scanner also produces noise, but it's not as loud as magnetic resonance. Patients undergoing CTA also can communicate with healthcare personnel if need be. Make sure patients don't have anything in their hair or on their head, such as bobby pins and barrettes, because this can affect the image.1
Carefully screen patients for a history of adverse reactions to intravascular iodinated contrast media. Document the patient's reaction when exposed to intravascular contrast media, communicate this information to the radiologist, and follow your institution's policy on using contrast in these patients. Intravascular contrast can make patients feel warm, so explain to the patient the difference between this type of expected reaction and unanticipated adverse reactions. Mild reactions commonly include nausea and vomiting and local urticaria and pruritus; moderate reactions include vasovagal reactions, bronchospasm, and mild laryngeal edema; severe reactions are rare and include seizures and cardiac arrest.3
Screen female patients for pregnancy. Although a pregnant patient can be shielded to minimize exposure to the fetus, ideally, a CTA shouldn't be performed during pregnancy unless the potential benefits outweigh the risks.1 Like gadolinium, the contrast used in CTA hasn't been documented to affect the fetus, but that doesn't mean it won't, furthering the argument for waiting until after pregnancy, if possible. Check with your facility's lactation consultant if your patient is breastfeeding, although the contrast used in CTA isn't considered harmful to infants.3
In the event your patient is critically ill, be sure that standard monitoring and resuscitation equipment accompany the patient. The prescriber may decide to premedicate the patient with a diuretic if the patient has intracranial pressure (ICP) compromise because the patient must be supine for the duration of the scan.1
After the scan, encourage oral fluid intake, unless contraindicated, to enhance urinary excretion of the contrast agent. Monitor the patient's renal function and assess for signs and symptoms of adverse reactions to intravascular contrast media. If your patient is critically ill, monitor ICP and neurologic status for signs of compromise.1
Pros and cons
CTA and MRA have definite advantages and disadvantages. The primary advantage is the less invasive nature of both imaging studies compared with conventional angiography. The lack of arterial access in CTA and MRA means less complications. Additionally, MRA has no radiation exposure, unlike CTA and conventional angiography.1,2
Another advantage is that CTA and MRA typically take less time to perform than conventional angiography. This is especially helpful when your patient is critically ill or otherwise unable to endure a long imaging study.
One drawback to CTA and MRA is their reliability in detecting cerebrovascular diseases. Because CTA and MRA haven't been used as long as conventional angiography has, using them as a sole diagnostic tool isn't always appropriate. This is especially true when surgical intervention is required. For example, studies indicate that MRA may miss aneurysms, but CTA has a high sensitivity for finding smaller aneurysms.4,5 Another study found that MRA had a specificity and sensitivity of only 80.6% and 93%, respectively, in diagnosing carotid artery stenosis.2 Another disadvantage is a lack of universal availability of both imaging studies.
However, as CTA and MRA become more common, you'll see them more frequently in your practice. Understanding these studies can help you better prepare your patient.