Brain Imaging
Authors: Judy Zhu and Cassie Wang
CT Scan
Computerized axial tomography (CT or CAT) is the most common imaging test. It combines
a series of X-ray images taken from different angles around your body and uses computer
processing to create cross-sectional images (slices) of the bones, blood vessels and soft
tissues inside your body. When used for detecting brain injury, it produces a series of images
showing cross-sections of the brain. It is particularly good for hemorrhage, trauma and
hydrocephalus.
Risks
The amount of radiation you get when having a CT scan is greater than you would get during a plain X-ray because the CT scan gathers more-detailed information. The low doses of radiation used in CT scans have not been shown to cause long-term harm, although at much higher doses, there may be a small increase in your potential risk of cancer.
In certain cases, your doctor may recommend that you receive a special dye called contrast material, which can be something to drink before your CT scan, or something that is injected into a vein in your arm or inserted into your rectum. Although rare, the contrast material can cause medical problems or allergic reactions. Most reactions are mild and result in a rash or itchiness. In rare instances, an allergic reaction can be serious, even life-threatening.
MRI
Magnetic resonance imaging (MRI) employs powerful magnets which produce a strong magnetic field that forces protons in the body to align with that field. When a radiofrequency current is pulsed
through the patient, the protons are stimulated and spin out of equilibrium, straining
against the pull of the magnetic field. When the radiofrequency field is turned off, the MRI
sensors are able to detect the energy released as the protons realign with the magnetic field.
The time it takes for the protons to realign with the magnetic field, as well as the amount of
energy released, changes depending on the environment and the chemical nature of the
molecules, which allows physicians to tell the difference between various types of tissues.
Because MRI does not use x-rays or other radiation, it is the imaging modality of choice when frequent imaging is required for diagnosis or therapy, especially in the brain. The brain can be seen much more clearly with MRI than with regular x-rays and CT. In the brain, MRI can differentiate between white matter and grey matter. However, MRI is more expensive than x-ray imaging or CT.
Risks
It employs a strong magnetic field that extends beyond the machine and exerts very powerful forces on objects of iron, some steels, and other magnetizable objects. Patients should notify their physicians of any form of medical or implant prior to an MRI scan.
DTI Scan
Diffusion tensor imaging (DTI) is a specialized type of MRI that measures the movement
of water in the brain, detecting areas where the normal flow of water is disrupted. This
test is particularly useful to image the white matter fiber tracts, or neurons, of the brain.
Disadvantages of X-rays, MRIs, and CTs
X-rays, MRIs, and CT scans can detect fractures, hemorrhages, swelling, and certain kinds of tissue damage, but they do not always detect traumatic brain injury. This is because TBI, especially in its milder forms, often involves subtle trauma to the brain that causes chemical and physical changes to brain tissues rather than structural damage. These changes often cannot be found with standard imaging procedures.
SPECT Scan
Single-photon emission computerized tomography (SPECT) scan is a type of nuclear
imaging test. It involves the injection of a small amount of short-lived radioactive particles
into the blood. A special camera capable of detecting these particles then rotates around
the patient and takes pictures from many angles. A computer then uses these pictures to
form a cross-sectional image. A SPECT scan is particularly useful in detecting blood flow
and showing what areas of your brain are more active or less active.
Risks
If you receive an injection or infusion of radioactive tracer, you may experience:
-
Bleeding, pain or swelling where the needle was inserted in your arm
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Rarely, an allergic reaction to the radioactive tracer
For women who are pregnant or breast-feeding, the radioactive tracer may be passed to the developing fetus or the nursing baby.
You are exposed to a small amount of radiation, but it is not associated with any long-term health risks.
PET Scan
Positron emission tomography (PET) uses short-lived radioactive substances to produce three-dimensional colored images. It studies metabolic activity or body function of substances
functioning within the body. The scan uses a special dye containing radioactive
tracers. These tracers are either swallowed, inhaled, or injected into a vein in
your arm depending on what part of the body is being examined. The tracer will
collect in areas of higher chemical activity, which is helpful because certain
tissues of the body, and certain diseases, have a higher level of chemical activity.
These areas of disease will show up as bright spots on the PET scan.
Risks
The PET scan involves radioactive tracers, but the exposure to harmful radiation is minimal.
Angiogram
An angiogram is used to examine blood vessels. When diagnosing a brain injury,
the test involves injecting dye into an artery that supplies blood to the brain,
usually through a catheter inserted in the groin. The dye highlights the blood
vessels on x-ray, and shows any leakage from those vessels.
Risks
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stroke (if the catheter loosens plaque inside a blood vessel)
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damage to the blood vessels, including puncturing an artery
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blood clots, which can form around the catheter tip
ICP Monitor
Intracranial pressure monitoring (ICP) is a device used to measure
pressure within the skull. It is used in two ways. One way is to place a
small, hollow tube (catheter) into the fluid-filled space in the brain
(ventricle). Other times, a small, hollow device (bolt) is placed through
the skull into the space just between the skull and the brain.
EEG
An electroencephalogram (EEG), is a safe and painless test
used to measure brain activity. During an EEG, numerous
electrodes in the form of patches are applied to the head.
These electrodes detect the electrical activity in the brain,
and a graph of that activity is generated.
Magnetoencephalography (MEG)
MEG also uses magnetic forces to detect ionic current changes during brain
activity. It is usually used when performing visual, auditory, sensory and motor
tasks in studies and shows the specific location of activity.
Compared to MRI, a MEG scanner is a helmet fixed on the head and it does not
make noise. Individuals can move around during MEG scans but have to lie still
on MRI scanners. Moreover, it provides more detailed information in higher
resolution than fMRI.
Transcranial Magnetic Stimulation (TMS)
TMS, like MRI and MEG, measure the function of the human
brain via magnetic field. But what makes TMS unique is that
it stimulates specific areas of interest by inducing minimal
electric currents to activate neurons. TMS is not frequently
used for clinical diagnosis, although some studies showed
that TMS has the potential to be implemented as a treatment
of depression, stroke and other neurological diseases.
References
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