Computerized axial tomography (CAT or CT scan) is an x-ray technique that produces pictures of consecutive slices of a body part.
Advantage. The advantage of a CT scan is that it can be performed quickly, often with just 5 minutes in the scanner, and it is able to detect new-onset bleeding. CT scan is used in emergency situations to rule out new hemorrhages or other serious pathology.
Limitations. While cavernous malformations can be seen using a CT scan, they are often difficult to distinguish from other types of tumors or other vascular malformations. Second, the slices of the CT scan may not be close enough to detect all of the cavernous malformations a person might have. Finally, CT scan exposes the patient to radiation, even more than used in a typical x-ray, and is, therefore, used sparingly in our patients.
Magnetic resonance imaging (MRI) is a method of imaging soft tissues of the body, such as the brain and spinal cord, by applying an external magnetic field.
Standard of care. An MRI is the standard of care for cavernous malformation, especially when performed on an MRI machine with a magnet strength of 3 Tesla (3T) or 7 Tesla (7T, currently used in research settings) and when the imaging includes susceptibility-weighted sequences (SWI). Specific recommendations for physicians who order or perform MRI are included in the Alliance to Cure Cavernous Malformation Clinical Care Consensus Guidelines.
Limitations. MRI studies require at least 20 minutes to perform, during with a patient must not move. A full spinal study can last as long as 90 minutes and so may require sedation. Second, because MRI uses a magnetic field, some metal implants may make MRI more difficult or impossible. Finally, some people may be sensitive to the contrast agent, discussed below, required for at least a baseline MRI.
Hemosiderin. A distinctive feature of a cavernous malformation when it is visualized on MRI is the hemosiderin ring around the lesion. Hemosiderin is iron, a byproduct of the breakdown of blood. Because a cavernous malformation has thin blood vessel walls comprised of cells that do not join as tightly as other blood vessel cells in the blood-brain barrier, blood is constantly oozing outside of the cavernous malformation. Blood can also be released into the brain through a hemorrhage. MRI is very good at detecting hemosiderin.
Sequences. In MRI, the brain is imaged multiple times using a variety of sequences. From a lay perspective, in each sequence, a cavernous malformation looks somewhat different: white, black, or gray. A more technical discussion of the various sequences can be found on the website Radiopaedia. For a cavernous malformation, the susceptibility-weighted sequences and T2-weighted sequences are most critical for the radiologist.
Magnet strength. An MRIs magnet strength is measured in a unit called Tesla, abbreviated T. The common magnet strengths as of this writing are 1.5T and 3T. In some research settings, a 7T magnet is in use. As you would expect, a stronger magnet allows the radiologist to see more detail, sometimes strikingly so.
Contrast. Gadolinium is the most common contrast agent used during MRI. It is injected through an IV at the halfway point of the MRI so that the radiologist has a series of images without contrast and then the same images with contrast. A contrast agent may not be appropriate for individuals with compromised kidneys, as the gadolinium is cleared out of the body through the kidneys. There is some concern about long term deposits of gadolinium in the body, but a large review did not find evidence of neurotoxicity resulting from gadolinium use in imaging. A contrast agent is not always required for a cavernous malformation MRI, especially after the first MRI. You may raise this question with the ordering physician.
Frequency. We are often asked how often an individual should repeat an MRI. This depends on the recent activity of the lesion. A lesion that has recently hemorrhaged should have more frequent imaging. Once a lesion is stable, imaging can be put on a schedule or can occur only when new symptoms arise. This is a discussion between a patient and their doctor.
Special considerations for children. Imaging for children can be complicated because of the need for some kind of sedation and by concerns about radiation and contrast agent exposure. An in-depth discussion of these issues can be found at Cavernous Angioma and Children.
Learn more. The website Stepwards, which is directed toward medical students, but accessible to patients, offers a video and corresponding webpage that walk through CT and through MRI imaging sequences for cavernous malformation.
Advanced MRI Techniques
Research is ongoing to validate and standardized two advanced techniques in MRI imaging: quantitative susceptibility mapping (QSM) and dynamic contrast-enhanced quantitative permeability imaging (DCEQP). Both of these techniques are being used in clinical drug trials to measure whether medicine is making a difference in the activity of a cavernous malformation. Both are available only in specialized research centers and are not used currently for clinical decision-making.
QSM. QSM is an MRI technique that measures iron content (a surrogate measure for the amount of blood leak) and is being developed as a biomarker of disease activity.
DCEQP. Permeability imaging attempts to measure the amount of blood product that is moving from the blood vessel into the surrounding brain as a measure of blood vessel leakiness or integrity of the blood-brain barrier. Permeability can be measured both in the cavernous malformation lesion and in other blood vessels of the brain.
A recent study documents the use of QSM and DCEQP and a current trial readiness project to validate and standardize this technology across multiple sites in preparation for clinical drug trials.
fMRI. Functional MRI (fMRI) provides information on the precise location of certain functions, for example, speech, in the brain. It may be used before surgery to allow the surgeon to plan a route to the cavernous malformation lesion that avoids a specific area. It may also guide the decision as to how much of a cavernous malformation can be safely removed if any.
Endovascular Angiography. Endovascular angiography, an invasive imaging technique, is not used to visualize cavernous malformation because cavernous malformation lesions are invisible using this technology. It is performed only to rule out other pathology, like arteriovenous malformation, if MRI results are ambiguous.
CTA and MRA. Computed Tomography Angiography (CTA) and Magnetic Resonance Angiography (MRA) are non-invasive angiographic tools that are replacing traditional angiography.