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15. I had a seizure
at work and was brought to the emergency room. I had a CT scan, which was
abnormal, but then the doctor also ordered an MRI scan. Why do I need both?
Most patients have
both a CT scan and an MRI because they provide different information. Many
emergency rooms use CT (computed tomography) scans as an initial screen for
tumors, stroke, hemorrhage, and other neurological conditions. CT is more
widely available, less expensive, and can be done in a matter of minutes. An
MRI (magnetic resonance imaging) scan typically takes much longer and may
not be immediately available. Some patients cannot have an MRI because they
have metal pacemakers or other metal devices implanted in their bodies.
However, MRI does not use x-rays or iodine contrast, which makes it safer
for most patients. In addition, MRI provides more detailed,
three-dimensional pictures of the brain. These detailed pictures are
particularly important when planning surgery.
CT scans typically
show only a single plane of the brain (an axial image). Axial images
are slices through the brain that begin at the crown and end at the bottom
of the skull. They are useful because right and left hemispheres of the
brain are normally mirror images of each other. It is easy to see any
distortion of one of the hemispheres with an axial image. Two other types of
imaging planes, sagittal and coronal, are seen only with MRI.
A sagittal image divides the brain into left and right, as if divided from
the tip of the nose to the center of the back of the head. This type of
image is particularly helpful in showing tumors in the exact center of the
brain. Coronal images divide the brain into front (anterior) and back
(posterior) and show the deeper and more central areas of the brain. Each
image on an MRI includes other information, such as the thickness of the
slice in millimeters, the number in the sequence, and the right and left
orientation of coronal and axial images.
The first scans that
you have (those that are performed before surgery and before any
medications are prescribed, including steroids) are very important. These
scans may be needed to help determine your treatment, especially if surgery
and radiation therapy are being considered. Although many hospitals insist
that your scans are the property of the hospital and must be returned, you
can and should ask for copies of your MRIs and CTs. Some facilities will
charge for copying each sheet of your scans. Other facilities will not
charge for a copy as long as it is taken to one of your doctors. With each
follow-up scan, you should ask for a copy to be made at the same time as the
original. These copies can be taken to your appointments and reviewed by
your doctors, but you should keep them in your possession afterwards. Keep
all of your scans together, dry and flat (under the bed is ideal).
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16. How does MRI
work?
This is extremely
difficult to explain in laymen's terms, but the very name, magnetic
resonance imaging, is a brief description of the principle of MRI. Everyone
is familiar with magnets and the fact that magnets have a north and south
pole. There is a magnetic field of the Earth, but the magnetic force of an
MRI in a hospital is at least one thousand times stronger than the Earth's
magnetic force. If you imagine that the nuclei of the hydrogen molecules of
the brain are magnets, you can understand that they have a north and south
pole. In the magnetic field of an MRI unit, all of the north poles of the
hydrogen nuclei of the brain align in one direction. The MRI unit sends a
radio signal to the nuclei, which causes them to flip 90 degrees. When the
radio signal switches off, the nuclei go back to their original position. As
the nuclei change position, they emit an electromagnetic signal (resonance)
that is captured on a computer. The computer determines exactly where the
signal is coming from and it is this localization that produces an image.
The strength of the electromagnetic signal from abnormal tissue in the brain
is different from the signal from the normal brain, producing a different
shade of gray or different radiosignal. Most patients suspected of
having a tumor will be given an intravenous injection of a chemical agent
called gadolinium. The gadolinium makes the blood vessels distinctly white
against the gray background of the normal brain. Some tumors will also show
bright areas of enhancement when gadolinium contrast is used.
Although it is an
oversimplification to say that MRI detects the subtle differences in the
hydrogen content of the structures of the brain, that's exactly what it
does. All of the clicks, buzzes, and banging that you hear during an MRI
examination are circuits causing the magnets of the hydrogen nuclei to flip
back and forth. A typical MRI scan includes several different types of
images. Each image provides different information. Your doctor will specify
whether you need an intravenous injection of contrast and whether special
views or thinner slices are required.
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How
to Read Your Own MRI Scans
Patients who have been
diagnosed with a brain or spinal cord tumor are usually carrying around a
huge x-ray file folder of their films to their neurosurgeon, radiation
oncologist, or neuro-oncologist for months before they figure out that these
films are, literally, the key to their future. A neuroradiologist can look
at someone's brain MRI and predict what will happen next. For example, a
neuroradiologist can tell if the patient's right leg will become weaker, if
part of the patient's visual field will be lost, and if the patient will
develop speech problems. Of course, most neuroradiologists do not actually
see the patient. It is up to the doctor taking care of the patient to use
the information provided by neuroradiologist to make a treatment plan that
will prevent further disability.
Neuroradiologists
are doctors who have been trained in general radiology (interpreting a
variety of images) and receive further training in the interpretation of
images of the brain and spine. Neuroradiologists typically see thousands of
brain tumor cases during their training. This experience gives them a unique
advantage over most other radiologists and even other neurological
specialists. However, learning some of the basic principles of MRI
interpretation can be helpful to the patient (and almost every brain tumor
patient has sneaked a peek at the new scans when given the opportunity).
If an MRI of your
brain has already been done at another facility, bring it with you when you
have a new scan, even if it is several months old. A neuroradiologist looks
for changes over time. If your doctor is ordering an MRI scan now,
even though you had one three months ago, it is because he is expecting that
there are or could be changes in the appearance of the brain in the
interval.
Secondly, a
neuroradiologist looks for changes in symmetry. The tumor or swelling
around the tumor may have created a distortion of the center line of the
brain called midline shift. If the distortion involves compression
against another section of the brain, this is called mass effect.
Third, you have
noticed that MRI scans are black and white images, and their interpretation
depends on what can be subtle changes in gray scale, reflecting
changes in radiosignal. Black and white photographs have negatives, but
MRI scans are not positive and negative images in the same sense. Although
there are often several different kinds of images included in a complete MRI
study of the brain, the T2-weighted (or T2 scans) and T1 weighted
gadolinium-enhanced images (often marked with adhesive labels stating the
brand of contrast agent used) are among the most important.
Figure 3 is a T2-weighted axial image. This type of image
shows the spinal fluid and any excess water in the brain as white. Edema
(swelling) around a tumor may be very striking, as in the image here.
Remember that your symptoms can be caused by this edema, which is why your
doctor usually studies these images carefully. However, even when the tumor
has been completely removed, there may still be a lighter margin around the
area of the surgery for several months because T2 scans are so sensitive to
residual water. There are some types of tumors, including some low-grade gliomas, that are detected only on T2 scans. Effects of radiation therapy
can also show changes on T2 scans. Many other changes within the brain,
including strokes, multiple sclerosis, and infection, can also cause changes
on T2 scans.
Figure 4 is an axial image of a primary central nervous
system lymphoma after an intravenous injection of gadolinium contrast. The
tumor is much more obvious with contrast because it has caused what neuroradiologists refer to as "breakdown of the blood-brain barrier."
In other words, the presence of the tumor has created tiny leaks in the very
fine blood vessel networks in the tumor. However, other abnormalities of the
brain can also show this dense whiteness, including the changes that occur
at the margin of the tumor after surgery. For this reason, many
neurosurgeons feel that MRI can be misleading in judging how much tumor
remains following surgery. Also, a dense area of white may appear in the
brain even before contrast is given, which corresponds to recent bleeding
within the brain (Fig. 5).
Finally, one of the
most difficult changes that a neuroradiologist must detect is changes
that occur as a result of therapy. Most of the time, a neuroradiologist
does not know what therapy has been given. For example, a dense, white area
on the scan of a patient who has received radiosurgery may be a recurrent,
growing tumor or may be dead tumor (radiation necrosis). It can be
impossible to tell the difference without a biopsy, although under certain
circumstances positron emission tomography (PET) can be helpful in
discriminating between them (see Question 20).
Figure 6 is the T2-weighted axial image of a patient who
has an oligodendroglioma that was first diagnosed by biopsy 10 years ago.
This image demonstrates abnormal increased brightness corresponding to
increased radiosignal in the left occipital lobe. Notice that there is
little distortion or mass effect on the adjacent brain structure. These
changes of increased radiosignal, without significant distortion of the
adjacent brain structures, are consistent with a low-grade tumor.
Figure 7 is the contrast-enhanced image of a patient who
has a malignant glioma that was diagnosed two years before this scan was
performed. The tumor causes a distortion of the normal structures of the
brain, with both prominent mass effect on the lateral ventricle and midline
shift from right to left. After the administration of gadolinium contrast,
there is marked increased radiosignal within the tumor. Although the tumor
originated in the right frontal lobe, it has now crossed the corpus callosum
and extends into the left frontal lobe.
Figure 8 is the T1 weighted, contrast-enhanced view that
demonstrates multiple abnormalities distributed throughout several lobes of
the brain. In this patient with a known history of lung cancer, the
abnormalities depicted are most consistent with metastatic lung cancer.
Virtually all metastatic tumors enhance with contrast.
Finally, it is
unfortunate, although not uncommon, that tumors that have been changing
slowly, if at all, on MRI scans may rapidly recur, even over a few weeks. Figure 9 is a T1-weighted,
contrast-enhanced coronal view of a patient who has undergone previous
surgery for an oligodendroglioma of the right frontal lobe. Eight weeks
later, the same area shows that the tumor has recurred as a more aggressive,
high-grade glioma (Fig. 10). Remember that even relatively subtle changes
on an MRI may indicate that a change in therapy is indicated, and these
changes may be detected only by performing MRI scans at regular intervals.
M.L's comment:
I have had a lot of
MRI scans (over a dozen now), and although I'm not claustrophobic, I can see
why a lot of people would have a difficult time with an MRI. Lying on a
table and having this machine slide you in to what feels like a very cramped
tunnel can make anyone feel like they need to get out! One technique that
seems to work for me is to close my eyes and take deep breaths. I focus on
the breath that I'm taking in and the one that I'm breathing out. I try not
to think about the fact that I'm in a "tunnel." By using this technique, I
usually come pretty close to falling asleep, especially because my
medication makes me kind of sleepy anyway. Also, make sure that the MRI
technician gives you some earplugs. The MRI machine can be kind of loud. The
earplugs really help cut down on the noise, and sometimes they make it
easier for me to fall asleep. When the technician gives me the contrast dye,
I try not to let it get to me, but I just don't like being stuck with
needles. So far, I've been pretty lucky and haven't had anyone hurt me.
Typically, those that have to take blood or give contrast dye on a regular
basis have the procedure down so well that they know how to do it so that it
doesn't really hurt. I NEVER look at the location where the needle is being
stuck; I just always make sure and tell the technician, "Please don't hurt
me!"
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17. I had surgery and
radiation therapy for a brain tumor. How often should I have a follow-up MRI
scan?
Your doctor considers
many factors when determining the frequency of follow-up MRI scans. Although
there are no specific guidelines for the follow-up of brain tumors, a
general rule is that if the result of the scan would impact on the patient's
decision for further therapy, a scan is recommended.
MRI scans are often
obtained after surgery and several weeks following the completion of
radiation therapy. If there is residual tumor present on your MRI scan
following radiation therapy, your doctor may order an MRI scan 2 or 3 months
later to determine whether the tumor appears to be stable. If there is clear
evidence of tumor recurrence over that period of time, your doctor will
discuss with you possible treatment options. On the other hand, if the
residual tumor appears to be stable or improving, your doctor may continue
to monitor your progress with both regular neurological examinations and MRI
scans.
Patients on clinical
trials always have regular evaluations to determine the success of their
treatment, often with measurements of tumor growth or shrinkage on MRI.
Patients receiving chemotherapy will also have reassessment on a regular
basis, but this may be determined by the cycle length of the drug (see
Question 43).
Patients with
slow-growing tumors may have follow-up MRI scans every 6 to 12 months.
Patients with new symptoms may require follow-up MRI scans more frequently,
sometimes as often as once a month, until it is determined whether tumor
growth, edema, radiation necrosis, or another factor is responsible for the
change.
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18. After my surgery
two years ago, I have had MRI scans regularly that have been stable. My most
recent scan shows a new abnormality near the location of the original tumor.
What are the chances that this is a new tumor? How can I find out?
Not every "new" area
on an MRI scan of a brain tumor patient is a recurrence of the tumor —
although the possibility of recurrence must be taken seriously. Some other
abnormalities that may appear on MRI scan include radiation treatment
effects, vascular abnormalities including stroke, and "artifacts." Artifacts
are false images that may be produced by the imaging process or by the
movement of the patient. The neuroradiologist interpreting the scan compares
all of the images in the series, including the coronal, axial, and sagittal
sequences, to determine whether the "new" area is an artifact. However, it
is not always possible to tell whether the new area appearing on the MRI is
clearly related to the original tumor, particularly with a small
abnormality.
Review of the scans
with your neurosurgeon may be helpful. He may recommend a follow-up MRI
within 4 to 6 weeks to determine whether the abnormality is stable. Although
a biopsy could be performed to determine if the area seen on MRI is a tumor,
this would obviously involve more risk. Some abnormalities remain stable or
even resolve completely over a period of a few weeks.
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19. My
neurosurgeon said that I have a "butterfly glioma" based on my MRI. How is
this different from any other type of glioma?
A “ butterfly” glioma
is most frequently a malignant glioma, but the "butterfly" description means
that the tumor has crossed over the midline of the brain to involve both the
right and left hemispheres. The tumor often appears symmetrical, like the
wings of a butterfly. Other types of tumors, however, may also cross the
midline to create a “butterfly” appearance. A biopsy is necessary to
confirm whether the tumor seen is a glioma or a different type of tumor.
Because the tumor involves both hemispheres, it cannot be completely
removed; however, some neurosurgeons will try to remove the larger portion
of the tumor if it is creating pressure on the surrounding brain.
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20. What is a PET
scan? Should I have one? Why does my doctor use MRI scans and not PET scans
to evaluate my tumor?
Positron emission
tomography (PET) is an important imaging tool for many types of cancer and
many types of central nervous system disease. Whereas CT and MRI scans
reveal the structure (anatomy) of the body, PET scans reveal the differences
in living tissues (physiology). PET scans require the administration of a
radioactive substance, often a radioactive sugar, such as
fluoro-deoxy-glucose (FDG), produced in a cyclotron. There are several
radioactive elements that can be used in PET scanning, and they all have an
atomic nucleus that undergoes transformation from a proton (a positively
charged subatomic particle) into a neutron, (a neutral subatomic particle).
As a result of this transformation, a positron is released. The positron
then combines with an electron, which produces energy in the form of gamma
rays. The PET scanner detects the energy formed from the gamma rays, which
is then reconstructed to form an image. PET images (see Color Plate 5) do not have the fine detail of MRI scans, but they do
show differences in the metabolism or use of energy by the brain's cells.
More than 50 years
ago, scientists discovered that glucose is taken up by living cells, and
that rapidly growing cancer cells take up more glucose than normal cells.
Although early studies using PET suggested that the radioactive tracer FDG
correlates with the rapid reproduction of cancer cells, more recent studies
suggest that there is a correlation between the number of living cancer
cells present. However, other conditions, such as infection, may also take
up radioactive glucose at higher rates than normal tissue; thus, high FDG
uptake does not necessarily indicate cancer.
The difference in
uptake of FDG in normal brain tissue and in slower-growing tumors may be
slight. Therefore, PET has been used to differentiate between malignant or
aggressive tumors (which show more intensely in the scan, indicating more
radioactive tracer in this area) and more slow growing tumors (which can
show about the same amount of radioactive tracer than the normal brain).
Prior to treatment, higher rates of FDG uptake in brain tumors have been
shown in some studies to be associated with a poorer prognosis. Following
radiation treatment, FDG-PET can be used to distinguish between residual
living tumor and tumor that is dead but still shows contrast enhancement on
MRI or CT.
There are some
limitations, however, in using PET to monitor the patient's response to
brain tumor therapy. Following therapy, many patients have both tumor
necrosis and residual, living tumor present in the brain. A PET scan that
shows high FDG uptake suggests there is living tumor present, but a low FDG
uptake does not mean that the brain is tumor-free. Small amounts of living
tumor could be present.
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21. What is magnetic
resonance spectroscopy? What does it tell my doctor about my tumor that the
MRI does not?
Magnetic resonance
spectroscopy (MRS)
is a technique similar to conventional MRI that measures chemical compounds
within the brain. Although conventional MRI detects differences in brain
water, MRS techniques suppress brain water so that other compounds such as
choline (Cho), creatine (Cr), and N-acetyl aspartate (NAA) can be detected.
Detecting these compounds produces a chemical waveform or spectra in a tumor
that can be compared to the spectra of an area of normal brain in the same
patient. The amount of each of these compounds present in an area of the
brain that appears abnormal on conventional MRI can suggest not only the
presence of a tumor, but also the grade of tumor and whether necrosis is
present. For example, because NAA is associated with living, normal neurons,
a reduction in the NAA peak reflects an absence of neurons. On the
other hard, the choline peak on an MRS is typically higher in brain
tumors than in normal brain because choline is associated with cell membrane
metabolism and dense, rapidly proliferating tumors. Creatine, the third
compound, may be either lower or higher than the choline peak in a brain
tumor. Areas of necrosis may reveal a fourth peak on an MRS that corresponds
to lactate.
Color Plate 6
is an MRS image and its corresponding waveform from an area of a tumor and
the area of the normal brain in the opposite hemisphere. The waveform or
spectra from each location reveals the proportion of NAA, Cr. and Cho
present in the tissue. These proportions are clearly different for the two
areas, and radiologists who interpret MRS use this information to detect
whether the abnormal areas are more likely to be tumor or necrosis (dead or
dying cells). The changes in the spectra can also be evaluated after therapy
to determine if viable tumor remains.
MRS has some
advantages over PET in that it uses available MRI technology and does not
require the use of contrast or radioisotopes. It can be repeated a number
of times without risk to the patient. However, MRS has some limitations. MRS
may be difficult to interpret in areas of the brain adjacent to the skull.
The resolution of an MRS scan is relatively poor, making it unsuitable for
the detection of small abnormalities. Like other imaging modalities, MRS
cannot reliably differentiate between different tumor types and grades,
although future developments in MRS may increase its accuracy.
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22. I have
read that functional MRI can show the parts of the brain that control
movement and speech. Do I need a functional MRI before my surgery?
Functional MRI,
like conventional MRI imaging, detects differences in the magnetic
properties of brain tissue, blood vessels, and spinal fluid. In addition,
functional MRI detects changes in red blood cells and capillaries as they
deliver oxygen to "functioning" parts of the brain. For example, although it
would be easy to assume that the entire brain is involved in complex
activities such as speech, there are in fact very discrete areas of the
brain that produce spoken language. These areas actually have higher blood
flow and higher oxygen consumption when a person speaks. Functional MRI can
detect these subtle but definite differences in oxygen consumption. A map of
some of the functions of the brain can be developed by asking the patient to
perform specific tasks, such as finger tapping, reading silently, or looking
at pictures, during an MRI (Color Plate 7). Obviously, some of
the more interesting, unique talents that people have, such as artistic or
musical ability, are impossible to map on functional MRI.
Functional MRI for
localizing certain brain areas before surgery may be useful. For example, a
left-handed patient may have a tumor in the left frontal lobe that seems to
extend into the area involved in the production of speech. Language
dominance in most right-handed individuals is centered in the left
hemisphere, but in some left-handed patients language dominance is in the
right hemisphere. A functional MRI can quickly and accurately
determine the area of the brain involved in speech when the patient silently
performs a series of word recognition tasks. This may guide the neurosurgeon
around the area (if the speech area localizes on the left near the tumor) or
may give him the reassurance that he will avoid it completely (if it
localizes to the right hemisphere, opposite the tumor).
Functional MRI is not
yet widely available, and there are relatively few situations that require
preoperative assessment with it. If your neurosurgeon believes that
functional MRI may be helpful in planning your surgery, he may discuss this
with you.
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