by Jack Hoch
Medical diagnostic imaging is a complex field requiring highly trained and specialized medical professionals to administer these procedures and interpret the results. As technology and understanding of disease pathology evolves, combinations of diagnostic images are being used in an integrated and layered approach. In some cases, imaging technology, which has been around for a decade or more, is being altered and used in new ways. This can make the testing process easier and less invasive or lead to new approaches in the diagnosis of a disease.
This article’s purpose is to describe the two most common imaging technologies and their use in the cavernous angioma diagnosis process.
For many years, the first line of diagnosis was the computed tomography (CT), or “CAT Scan” (the “A” in “CAT” stands for “axial”, meaning looking at one’s head from the top down). CT is a technology that has been in use for roughly 30 years, and has improved with time.
Initially slow and prone to patient movement artifacts, getting a sharp and detailed image was problematic. Regardless, it was worth the time and trouble because the process was non-invasive and gave doctors a good look at the soft tissue structures of the brain. CT is still widely used today, especially in emergency rooms where trauma doctors need to get a first look at a patient’s problem. It helps that CT is less expensive than MRI, and it’s also adept at imaging fresh blood.
Drawbacks to the use of CT technology include the use of x-rays to create the resultant image, and the image detail is less than other technologies. Like MRI, CT may include the use of a contrast agent (dye) to enhance certain aspects of the image. Patients who don’t like needles won’t appreciate this portion of the test, but at least it’s only a single injection.
MRI is the gold standard in diagnostic imaging. Invented in the late 1980s, MRI has revolutionized the diagnosis of certain diseases, including cavernous malformation (CCM). While the images generated by this technology appear similar to those produced by CT, the process by which these images are rendered is completely different.
There are two physical types of MRIs: open and closed. Close MRIs require the patient to enter a very narrow tube, and lie flat and still during the procedure which can take 30 minutes or more. For those who are claustrophobic, it can be extremely challenging.
Open MRI’s are not comprised of a closed tube, so there are really no problems for claustrophobic patients. There is, however, a trade-off; in most cases, open MRIs are less precise than their closed counterparts. If a patient can handle the claustrophobic aspects of a closed MRI, then a closed MRI is the optimal procedure to undergo.
An MRI develops a very strong magnetic field resulting from the generation of radio waves focused at a certain part of the body. As of now, there are no known health problems from the occasional exposure to high strength magnetic fields. However, because of the generated magnetic field, no metal objects are allowed in the actual MRI room. Due to the high strength magnetic field, certain patients cannot undergo an MRI if they have an implanted pacemaker, or metal plates or screws surgically inserted somewhere in the body.
Although there are no preparation requirements (such as fasting, other restrictions, etc.) that must be met before the procedure, there is a requirement that the patient remain absolutely still during the imaging portion of the exam. Movement will result in blurred, useless images. Additionally, MRI is a noisy process and requires the use of hearing protection.
Like CT, MRI generates an image “slice by slice”. These slices are normally a few millimeters thick, so that the rendered image is detailed and clear. Likewise, the slicing orientation is controlled by the technician: axial (top of head looking down; coronal (back of head looking forward), and sagittal (side of head, looking toward other side of head). MRI scans can be run with a multitude of settings, depending upon the expected results and location of the mass or entity to be studied.
The garden variety MRI is the spin-echo MRI. Spin-echo refers to the type of MRI pulse that is used during the procedure. There are different weightings and spin-echo sequences that radiologists will use depending upon the individual case. In general, there are two weightings of spin-echo images which are most widely used:
T1 – Longitudinal relaxation time – hemorrhages, especially newer ones, appear brighter than surrounding brain tissue
T2 – Transverse relaxation time – hemorrhages appear darker than surrounding brain tissue.
Both T1 and T2 times are adjustable by the radiologist, so that the best contrast relative to background brain tissue can be depicted. Again, the settings will be optimized for the expected location of study in the brain, as well as the type of finding expected. In those cases where a brain scan is ordered without pre-existing knowledge of lesion, general “template” T1 and T2 settings are used to have the best chance of picking up abnormalities.
Gradient-echo and susceptibility weighted imaging differs from spin-echo and allow detection of very small (punctuate or pin-sized) anormalities. This is especially critical for potential cavernous angioma patients, as even small lesions can have big neurological consequences. For one’s first diagnostic scan, when the root cause of clinical symptoms is yet unknown, performing a gradient-echo MRI or an MRI with susceptibility weighting (SW) is highly recommended. When in doubt, be sure to ask that “gradient-echo” and/or "susceptibility weighting" be specified by the MRI prescription issued by the referring doctor.
There is a caveat to susceptibility weighted imaging (SWI). If you have had an MRI with gradient-echo sequencing in the past and you have the hereditary form of the illness, it is likely that twice as many lesions will appear using SWI. They will also appear larger than life. This can be disturbing, but please keep in mind that these are not additional lesions - they are simply lesions that were too small to see with gradient-echo sequencing. Typically, the newly visualized lesions are 2 mm or less in size. Also remember that SWI does exaggerate the size of lesions.
There are additional MRI sequences, such as turbo (fast) spin-echo and functional MRIs, among others. Turbo spin-echo is simply a quicker way of accomplishing a regular spin echo scan, yielding certain advantages (and disadvantages). Functional MRI is very useful in certain pre-surgical situations. These and other diagnostic tests, such as angiography, will be highlighted in a future article.
It has been established that people who suffer from claustrophobia, possess metallic implants, or who cannot remail still for an extended period of time may have difficulty undergoing a successful MRI examination. What about children? Young ones can also experience serious medical difficulties requiring diagnostic examination. Trying to keep kids from squirming during a 30 or 45 minute MRI procedure is practically impossible.
General anesthesia has been the fallback, but this is tough on the kids, not to mention their parents. If you’ve ever seen a preschooler regaining consciousness from an anesthesia-based procedure, it’s an eye opener. There are side-effects, such as headache, and other dangers. For a non-invasive procedure, anesthesia seems like overkill, but until recently it was the only realistic alternative.
Fortunately, new technology is being used in many places that hopefully will relegate general anesthesia to the trash bin for follow-up pediatric MRIs. It appears that this new software reduces the negative effects of motion, resulting in high definition scans even with a squirmy kid. This means that many young children will no longer require sedation to undergo follow-up MRIs. Also, the overall MRI process is quicker, possibly cutting the exam duration by 40 or 50%. Since these faster MRIs use turbo spin-echo sequences, it may not be the best choice for an initial scan, but it could be a great choice for follow-up and “expectant management” of cavernous angiomas.
An angiogram (also known as an arteriogram) is a diagnostic test used to gauge the integrity of blood vessels within the body. It is an indispensable element in determining the root cause of a problem, either by positive identification or by ruling out certain possibilities. An angiogram will only “see” areas where there is blood flow above a specific threshold rate. As such, it cannot image cavernous angiomas directly, but it may help to do so by process of elimination. When used in conjunction with an MRI, an angiogram provides an invaluable look at blood vessel irregularities previously viewable only through surgery or at autopsy. The combination of the two generally results in a diagnosis of high confidence.
Of course, nothing in life is ever easy, and that is the case with angiography. Technological advances have yielded additional angiography choices that can complicate the decision-making process, potentially adding stress or anxiety to an already confusing situation. Even so, it’s nice to have alternatives, especially non-invasive ones, which weren’t available 20 years ago.
The rest of this article will provide some details on the three different types of angiography commonly used in today’s medical facilities: CTA, MRA, and conventional angiography.
CTA is the least accurate and the least expensive angiography alternative. In essence, its strengths and weaknesses are similar to the CT vs. MRI comparison discussed in the first part of this diagnostic imaging article. Basically, its availability is more widespread than MRA; it costs less, and there are fewer restrictions in that CTA can be used on patients with metal in their bodies (pacemakers, screws, rods, plates, etc.). Unfortunately, like its CT cousin, the precision is not as high as with MRI, and it requires an iodine based contract injection, which can be detrimental to some at-risk patients.
MRA is rapidly becoming the diagnostic test of choice for blood vessel imaging. It can detect blood vessels that are both bulging (aneurism) or narrowing (stenosis). Likewise, it can pick up high blood flow lesions such as arteriovenous malformations (AVMs). CTA can as well, but not with the degree of precision found in MRA. The degree of precision is what gives MRA a big advantage over CTA in terms of early detection.
MRA also offers important advantages over conventional angiography. Unlike the latter, MRA is non-invasive and is much quicker. While the risks with an invasive procedure are relatively small, those risks are still present. MRA completely removes this concern.
Regardless, some of the more intransigent medical facilities consider MRA (and CTA) somewhat experimental and prefer to use conventional angiography.
A comprehensive guide covering what one can expect prior to and during the procedure may be found on WebMD.
Long the gold standard for blood vessel diagnostics, conventional angiography has been around a very long time. One can think of it as an X-ray of one’s blood vessels. The procedure is more involved than that for MRA or CTA in that it requires an incision, normally in the femoral artery near the groin area (local anesthesia). Once this incision is made, a catheter is inserted into the artery and “snaked” into the blood vessel of concern. To image blood vessels in the brain, this requires the catheter to be guided through the torso and neck and into the head. Once the catheter is in place, contrast material is injected and images are taken of the affected area. A complete description of the procedure can be found here.
The advantage to conventional angiography is that the images are taken with close proximity perspective. Of all of the imaging methods, it is the most precise. This precision, however, comes at a higher relative risk of complications such as infection, hemorrhage from the catheter damaging a blood vessel or even stroke. Also, there is a recovery time associated with the operation of at least four hours, which requires keeping one’s leg immobilized for that period of time.
No doubt, the cop-out answer of “it depends”. In reality, most general cases can probably be handled by MRA as long as the hospital staff is well trained on the latest technology and diagnostic procedures. If one doctor recommends a conventional angiogram, ask why not an MRA? Good engineering practice always stipulates that one chooses the simplest and safest of two procedures if the expected results are the same. Make the physician explain to you why conventional angiography should be used in lieu of MRA.
Be aware that when discussing the darker side of medicine and inherent conflicts of interest, conventional angiography procedures receive a higher insurance reimbursement rate than do MRAs. All other factors being equal, some unscrupulous doctors may choose conventional angiography over MRA to grab that higher reimbursement rate.
Also, don’t lambaste emergency medical room staff if they order a CTA. In many cases, a CTA is a great first look at an emerging problem where time is of the essence.
The real kicker is that after having read all of this, if one’s angiography is “negative” (normal), that only rules out high flow lesions such as AVMs. If a lesion imaged by MRI is suspected as the underlying cause of symptoms, then the absence of anything unusual on an angiogram heightens the possibility that the lesion may be an angiographically occult vascular malformation (AOVM), such as a cavernous angioma, which by nature is low flow.
Functional MRI is a recently developed, and still advancing, procedure allowing the non-invasive measurement of blood flow in the brain. Images can be taken rapid-fire, allowing an almost movie-like synthesis of image frames so that doctors can identify currently active brain regions. This imaging and identification is usually done in conjunction with a patient task-oriented test. By assigning a task (playing a game, moving an arm or leg, etc.) to a patient and then immediately taking pictures of the brain, changes in blood flow rate and distribution can be measured. In this way, exact regions of the brain can be mapped as to function.
The key assumption here is that the flow of oxygenated blood, which fMRI indirectly measures, is directly related to the task demands and area of the brain requiring this enhanced flow. Except for the “tasks”, the fMRI procedure itself is very similar to a regular MRI, especially considering that one’s head must remain immobilized during the imaging process.
fMRI is useful as a mapping tool prior to surgery. The surgeon can use the following fMRI procedures to differentiate between important tissue (speech center, motor area, etc.) and tissue which is not as eloquent. This can make all of the difference in surgical success rate for those operations requiring a very low margin of error.
fMRI comes in different flavors, and the “flavor of the day” is dependent upon the particular aspect of the patient’s case the attending physician wishes to study. Ideally, the following fMRI techniques involve measuring cerebral blood flow while performing a before and after mental state test of a patient. Hopefully all other variables during this test are kept constant. The four main fMRI types in use today are: bold, perfusion, diffusion-weighted, and MRI spectroscopy.
Bold-fMRI depicts oxygenated blood in the brain as bright areas on the film. The assumption is that blood content high in oxygen is being delivered to those areas of the brain in use or needing it most at that given time. By giving a patient a singular and simple task (holding an object), the doctor can see the areas of the brain that are activated during the task. Images are rapidly taken before and during the task so that they may be contrasted with each other. Bold-fMRI is optimal for studying functions that can be quickly turned on and off like language, vision, movement, hearing, and memory.
Like Bold-fMRI, Perfusion-fMRI attempts to measure blood flow in the brain. It differs in that it does not measure blood oxygenation.
There are two types of Perfusion-fMRI: intravenous bolus tracking and arterial spin-labeling. The former uses an injection of a tracer substance such as gadolinium to depict relative blood flow. Gadolinium is the same contrast agent used during many standard MRI procedures. The tracer, or “bolus” is then mapped as it courses through the cerebral bloodstream in the area of the brain being studied. More than one bolus can be administered in a session. Unfortunately, this is somewhat invasive (gadolinium injection) and is also limited by the ability of one’s kidneys to process and eliminate the tracer substance without damage due to toxicity.
Arterial spin-labeling, on the other hand, is non-invasive and can be repeated as many times as necessary, plus, it can measure absolute blood flow. Absolute blood flow allows a series of images focusing on the same area to be taken during a specific fMRI session.
The biggest drawback is that this method is very slow, and individual images (“slices”) can only be generated every few minutes. This contrasts with Bold-fMRI, which is rapid-fire. The longer the time between slices, the greater the chance of the patient’s mental state changing in a non-controlled fashion, thereby introducing unwanted variables into the procedure. However, you may be asked to participate in studies using Perfusion-fMRI as it appears to have potential applicability to measuring the success of medications being researched to treat people with cavernous angiomas.
This procedure measures the relative mobility of water molecules in the brain. The natural, random motion of water molecules (for those of us who remember their days in physics class--Brownian motion) can be factored out such that abnormal movement of these molecules can be measured. For instance, during a de-myelinizing disease process such as multiple sclerosis, water molecules would more readily diffuse across the boundary of the myelin sheath since it is no longer intact. Most lesions and strokes cause disruption of the brain’s white matter such that diffusion-weighted imaging can hone in on these areas.
MRI Spectroscopy (MRIS)
While your basic MRI shows relative differences between areas of the brain, MRI Spectroscopy allows for detailed chemical information about specific composition of individual brain areas. For instance, MRIS can tell the difference between a tumor, and dead tissue. Use and refinement of this technique continues to evolve and will most likely play a greater role in future diagnostic procedures.
There are a host of other image-oriented tools that doctors employ to diagnose suspected problems in the brain. Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) have been around for awhile. Their greatest attribute, relative to cavernous angioma patients, is diagnosing/localizing epilepsy centers in the brain. Both of these procedures image the brain similar to fMRI, but PET and SPECT involve slower image acquisition times, are more costly, and include ionizing radiation as a byproduct of their use. Given these limitations, fMRI is almost always preferable.
MEG and EEG measure the electrical activity of the brain. EEG requires long preparation time, especially in the placement of electrodes on the patient. MEG, while quicker, requires more expensive equipment.
Ultrasound uses inaudible (to the human ear) sound waves to image a particular area. Most are familiar with its use during mid-term pregnancy to predetermine the gender of the fetus and ensure normal gestation. Currently, ultrasound doesn’t have much practical application in the diagnostic process of potential cavernous angioma patients.
MRI and fMRI will continue to be the imaging procedures of choice for the foreseeable future. While other “boutique” imaging procedures exist, most are so specialized that they are either very costly or are not applicable to cavernous angioma patients. Others require significant invasiveness, such as a craniotomy, in order to be useful.
Probably the biggest near term impact in the MRI world will be an increase in magnetic field strength. Currently, the standard field strength is 1.5 T (Tesla) or 3.0 T in newer machines. Machines will easily double that and may possibly reach 7 or 8 T once it is determined that these higher field strengths are safe for humans. Quicker MRI procedures and much higher image resolutions will be the result.
Further in the future, more molecular oriented imaging systems will be developed. If one “follows the money” in the medical research world, studies involving sub-cellular molecular structures and processes are getting their share of the funding. One day we may have readily available patient diagnostics that can show doctors changes on the molecular level. To put it another way, the imaging difference is similar to a satellite photographing the earth and viewing detail at city level versus using a different satellite to peer into a window and read the contents of a post-it note on someone’s desk.
Technology is blazing the way for new and exciting developments in diagnostic imaging. The rapid pace of development is good news for those patients harboring brain lesions. Diagnosis is usually quicker and more accurate than in years past. The days of mistaking a cavernous angioma for other diseases such as multiple sclerosis are fading fast, thank goodness. Even so, deployment of new imaging systems must pass stringent safety tests that can delay their clinical use. Just as important, medical professionals must be trained to use the new systems and properly interpret their output.
This page last updated 11/14/10.
This information is not a substitute for obtaining competent medical advice. It is for informational purposes only.