Brain Scan – Types and differences, Potential risks

Brain Scan

Also called: Neuroimaging


Brain scans are imaging tests taken of the head to identify possible injury or disease of the brain. A physician might recommend a brain scan for many reasons, such as diagnosing disease, monitoring disease progression or healing, and planning treatment.

Various neurological conditions can be diagnosed with a brain scan. These include brain tumors, cerebral aneurysm, stroke, cranial neuralgia, infection, cerebral palsy and some movement disorders (e.g., multiple sclerosis, amyotrophic lateral sclerosis).

Different types of imaging technology are available for brain imaging purposes. The most common technologies used for brain scans include magnetic resonance imaging(MRI), computed axial tomography (CAT) and positron emission tomography (PET).

Brain scans are typically painless and minimally invasive. In many cases, any potential risks are outweighed by the information obtained that is helpful in diagnosis and treatment. In certain cases, information obtained through brain scans can lead to life-saving treatments.

Preparation for a brain scan depends on the type of imaging test being used. Patients are generally asked to remove all metallic objects, and in some cases, they might be asked to restrict what they eat or drink for a certain length of time prior to the examination. Patients should also inform their physician about any medical conditions (including allergies), medications they are currently taking, and whether they are pregnant or breastfeeding.

Most brain imaging scans are performed by a technician in a hospital or outpatient clinic. The exact steps of the procedure differ, depending on the imaging technology used. In most cases, a patient is asked to lie still on a scanning table, surrounded by imaging equipment. Machinery noise is common in most procedures, although patients may be given earphones or headphones to help them relax. The length of the test varies, although brain scans on average take about an hour to complete. Patients can generally return to their regular daily routines immediately after the brain scan is completed. After a brain scan has been performed, a radiologist or physician will analyze the images and the results will then be reported to the patient and to the patient’s physician.

About brain scans

Brain scans use imaging tests to produce images of the brain. This can reveal whether injury or disease of the brain is present.

A variety of different imaging technologies (e.g., computed axial tomography [CAT] scans, magnetic resonance imaging [MRI]) can be used for obtaining brain scans. The type of scan selected depends on a number of different factors. Primary among these is the condition or disease a physician suspects or wishes to rule out.  

Ideally, a physician will choose a test that is capable of revealing the widest range of possible causes for a patient’s symptoms. For example, symptoms that indicate increased pressure within the skull may be due to a variety of causes including tissue inflammation, brain tumor, blood vessel rupture or cerebral aneurysm. A physician will select the test that can produce the best image of the material or structure suspected of being damaged. For example, a CAT scan can detect changes in bones better than other imaging tests, whereas an MRI detects soft tissues changes more precisely. Thus, an MRI is likely to be used to obtain precise images of diseased or damaged brain and nerve tissue. Positron emission tomography (PET) scans measure cell and tissue metabolism, highlight blood flow and can assess certain general aspects of brain function.

In addition, the potential risks associated with certain imaging tests can make the test unacceptable for certain patients. For example, CAT scans that require a contrast medium should not be used in patients with allergies to iodine, and MRIs should not be used in patients with metallic foreign objects inside their bodies. Implanted objects such as vagus nerve stimulators. while not strictly prohibitive, must be turned off in order for an MRI to be performed. However, the benefits of a specific test usually outweigh the potential risks.

Brain scans can be performed for a variety of reasons, including:

  • Diagnosing neurological injuries, conditions and diseases

  • Determining the progress of a previously diagnosed condition

  • Guiding a planned brain biopsy (surgical removal of a tissue sample)

  • Planning treatment such as radiation therapy or brain surgery

  • Monitoring the effectiveness of certain treatments

Brain scans provide information that can help physicians confirm a particular diagnosis. A lack of certain findings can also be used to rule out certain conditions. Bleeding in or around the brain can indicate head injury, blood vessel malformation, hemorrhagic stroke or ruptured cerebral aneurysm. Brain tissue abnormalities may indicate seizures or birth defects such as cephalic disorders. Inflammation in and around the brain can be indicative of an infection, such as meningitis. Damage to nerve tissue in the brain can help diagnose multiple sclerosis or other demyelinating process.

In addition brain scans are sometimes repeated, or an additional type of study performed for some people. For example, a patient taken to the hospital for stroke will immediately undergo a CAT brain scan to confirm that a stroke and/or bleeding has occurred. Several days later, the same patient might undergo an MRI of the brain to identify the extent of any brain damage and help determine possible treatment options.

Brain scans are typically painless procedures that are minimally invasive (e.g., when contrast material is injected into a patient’s vein) or noninvasive (in which nothing enters or penetrates the body).

The dyes used during certain brain scans help to show structures within the head (e.g., blood vessels, bones, brain and nerve tissue) more clearly. Contrast media used during a CAT scan usually contain iodine that highlights certain normal and abnormal findings in brain structure. Contrast material used during MRI usually contains gadolinium, a chemical element that responds to nearby molecules (especially blood vessels and blood vessel leakage) by making them appear brighter.

Contrast material used during radionuclide imaging such as Positron emission tomography (PET) contains radioactive materials (isotopes) that accumulate in areas of high cellular activity, such as diseased cells, within tissues and organs of the body. Once inside these areas, the isotopes release atomic particles called positrons. The positrons combine with electrons in the body to produce gamma rays (a type of high-energy ray), which can be detected by a special camera. Radionuclide imaging involves detecting rays that are being emitted from within the body by the dye isotopes.

Types and differences of brain scans

All brain scans produce images of the brain. These images can involve the entire brain or focus on specific areas. The images produced can also include other structures within the head besides brain tissue, such as bones, blood vessels and nerve tissue.

The main differences among brain scan procedures are related to the specific imaging technology being used. Which technology is used depends on various factors, including the medical condition a physician suspects or wishes to rule out, as well as imaging test availability, potential risks and cost. Imaging technology used for brain scans includes:

  • Magnetic resonance imaging (MRI). Instead of using x-rays, MRIs use powerful magnets and computer-generated radio waves to create detailed, cross-sectional and three-dimensional images of internal body organs. MRIs may be performed with or without use of a contrast medium. MRIs typically produce precise images of soft tissue – even tissue that would otherwise be obstructed by bones or foreign bodies. MRIs also detail surface and deep brain structures, making them the preferred imaging test for diagnosing most brain disorders. They are often selected over other imaging tests to avoid radiation exposure.

    MRIs of the head are often used to distinguish healthy brain or nerve tissue from diseased or damaged tissue. They can be used to confirm a brain tumor, or to identify damage to the myelin sheath that protects central nervous system nerves  and becomes scarred in multiple sclerosis. MRIs may also be useful in documenting brain abnormalities in patients with dementia.

    A type of MRI known as a functional MRI (fMRI) is particularly useful in some cases (e.g., head injury, stroke) because of its ability to assess certain aspects of brain function. An fMRI uses the blood’s magnetic properties to produce real-time images of blood flow to particular areas of the brain. During an fMRI, the patient will be asked to perform a task while the machine scans the brain and records brain activity in relation to the task being performed. An fMRI can identify active areas of the brain and the duration and sequence of brain activity. Another type of MRI called magnetic resonance spectroscopy (MRS) uses signals generated by atoms of phosphorus instead of the hydrogen atoms of conventional MRI. MRS can help to depict brain metabolism that is specific to a variety of disease states. While not accurately diagnostic for many situations, MRS can be helpful in differentiating between specific diseases without the need for open brain biopsy.

    MRIs may not be appropriate for patients with certain types of implanted metal or electrical devices (e.g., pacemaker, vagus nerve stimulator, deep brain stimulation). In the case of vagus nerve stimulators, the device needs to be turned off in order to allow for MRI scanning.  Metal can interfere with the magnetic field created during the test and affect the accuracy of test results. In addition, the unexpected motion of certain metallic foreign bodies can also result in injury to internal organs brought on by inadvertent motion of such objects under the influence of a strong magnetic field as produced by MRI scans. 

  • Computed axial tomography (CAT) scan. Uses multiple x-ray images, taken at different angles, and a computer to produce a three-dimensional view of the area scanned. CAT scans of the brain (sometimes called cranial CAT scans) can be performed with or without a contrast medium. In many hospital settings, CAT technology is more common than MRI technology. CAT scans are completed faster than MRI scans, which facilitates obtaining quick results in emergency situations.

    Of all the imaging tests, CAT scans are best able to detect changes in bone. Because of this, it is the preferred technology for producing clear images of abnormalities that affect the skull and spine. In addition, CAT brain scans are commonly used to confirm a diagnosis of stroke. A technique called CAT angiography (CT angiography) can detect the presence and location of blood vessel obstruction in the brain, which typically are the cause of ischemic stroke. CAT imaging technology is most effective at locating the source of cranial bleeding when the test is performed within the first 24 hours after a stroke, although later images can also suggest bleeding, depending on the initial volume of hemorrhage.

    However, CAT scans produce less precise images of soft tissue (particularly tissue and structures deep in the brain) than MRIs and may not be able to detect inflammation of the meninges (membranes covering the brain), which can indicate meningitis. In addition, CAT scans involve the use of potentially harmful x-rays, the effects of which may not be evident until much later.

  • Positron emission tomography (PET) scan. Produces images of brain function rather than structure in a similar manner to fMRI. PET is a type of radionuclide imaging in which radioactive material (isotopes) are injected into the bloodstream and then drawn to areas of high metabolic activity within the body. A scanner is used to detect the gamma rays (a type of high-energy ray) that are then emitted from the area. The result is an image with different colors of varying intensity, depicting areas where chemical changes are occurring in body tissue. A fluorodopa PET uses a radioactive form of levodopa (fluorodopa) to detail the function of dopamine cells in the brain of people with Parkinson’s disease.

    PET scans are often used to detect the presence, stage and spread of brain cancer. They are also used to evaluate patients with seizures, assess brain abnormalities caused by traumatic head injury, assess brain function after stroke and highlight the presence or lack of blood flow.

    PET scans can identify metabolic changes in brain tissue before other changes are apparent. PET scans can be used to get more information after an MRI or CAT scan has already been performed.

  • Single-photon emission computed tomography (SPECT) scan. A type of radionuclide imaging that evaluates brain function by highlighting blood flow to brain tissue. Like a PET scan, radioactive isotopes are introduced into the patient’s bloodstream by injection. These isotopes are drawn to areas of increased blood flow within the brain. SPECT brain scans are sometimes used as a follow-up test to an MRI to diagnose infections, fractures and tumors.

  • Ultrasound. A noninvasive procedure that uses sound waves to produce images of internal tissues and organs. A special device (transducer) that emits sound waves is placed against a patient’s skin. The sound waves that bounce back from structures within the body are recorded and displayed on a computer screen or television-type monitor.

    Ultrasound brain scans are used to evaluate blood circulation in the brain of infants and young children and may help diagnose various brain conditions or diseases. Ultrasounds can also detect the direction or presence of blood flow in the brain. The procedure is painless and free of risks or complications. When ultrasounds are performed to analyze blood flow within the body, they are sometimes called vascular ultrasounds. The immature skull structure of young children allows ultrasound techniques to provide images that cannot be produced in adult patients.

    A neurosonography is an ultrasound of the brain and/or spine. It is commonly used with premature or sick newborns because of its safety and ease of use.

    Transcranial doppler (TCD) ultrasounds evaluate the blood vessels and arteries leading to the brain. TCDs may be used as a diagnostic tool or to monitor blood flow to the brain during surgical procedures.

In some cases, x-rays of the skull may be performed after a head injury to identify fractures or structural abnormalities, although the usefulness of plain skull x-rays is typically very limited. However, in the case of evaluating brain shunt systems, x-ray technology is helpful. This imaging technology uses electromagnetic radiation to create images of the bones of the body. However, because CAT scans use x-ray technology but can depict more than conventional x-rays, CAT or other imaging technology is often preferred for brain scans.

Before the brain scan

Patients should wear comfortable clothes to the health facility for the brain scan. Immediately before the scan, patients may be asked to change into a hospital gown.

All metallic objects, such as watches, jewelry, eyeglasses, hairpins and hearing aids, should be removed so the patient is not harmed during testing, and the test results are not impaired. Patients will also be asked to take out any removable dental work. Tattoos and permanent makeup may also create a problem during some types of brain scans.

Patients are given sedatives in some circumstances to ensure they are relaxed and remain still when required to do so during the test.

Patients are instructed to follow all preparatory steps recommended by a physician prior to a brain scan. They are also instructed to inform their physician and the technician performing the brain scan about any implanted metal or electronic devices (e.g., pacemakers, vagus nerve stimulators, deep brain stimulation) they have inside their bodies.

If the brain scan is being performed without the use of a contrast medium, no special preparation is needed.

If a contrast medium is to be used, patients should report any history of blood-clotting disorders or allergic reactions to iodine, shellfish or strawberries to their physician and the technician administering the test. Patients should also advise the physician of all current medical conditions and medications (including vitamins and herbal supplements) as well as any recent testing that included use of a contrast medium.

Usually, patients will be allowed to eat without restrictions and to take their usual medications prior to a brain scan. However, for some types of brain scans, patients will be instructed not to eat or drink for four to six hours before testing. In some cases, patients will need to drink specific amounts of water prior to testing.

If there is any possibility that a woman is pregnant, the physician and imaging technician need to be advised. If a woman is pregnant, a physician will recommend against certain types of brain scans and might use alternate diagnostic testing methods.   Patients will be asked to sign a consent form prior to the brain scan.

During the brain scan

Brain scans are generally performed by a radiology technician in a hospital or outpatient clinic. In most cases, a patient is asked to lie down on a scanning table, which slowly slides into a short, open-ended tunnel. In some scans, the table will slide into a device that looks like a large doughnut with a hole in the center. The scanning table can usually be raised, lowered or tilted. Straps and pillows will be used to keep a patient in a fixed position. During a brain scan, the table will be fitted with a special cradle that holds the patient’s head. 

Parents or caregivers with infants or children receiving a brain scan are often permitted to remain with the child during the test. For some types of brain scans, parents accompanying children may be asked to wear a lead apron to prevent exposure to radiation, and/or remove all metallic objects from their bodies and the area.

For many brain scans, a sound system built into the imaging equipment allows a patient to communicate with the technician conducting the brain scan. A patient is usually asked to lie still, and may be asked to hold their breath for short periods of time. A hand-held buzzer may be given to the patient to stop the test in case of an emergency.

The imaging equipment can create many types of noises during the brain scan, which are a normal part of the test. Patients might be given special earphones to block out the sounds, or headphones through which music may be piped in to help a patient relax while the test is being performed. At some facilities, patients are allowed to bring their own music to listen to during the brain scan. A technician or nurse will periodically make sure that the patient is comfortable and able to continue.

If necessary, injection of a contrast medium may produce a warm, flushed feeling for several minutes or a metallic or bitter taste. Both sensations are normal and disappear quickly. In some cases, a patient can experience a temporary headache or nausea.

The precise length of a brain scan differs, depending on the type of imaging technology used, although brain scans on average take about an hour.

During magnetic resonance imaging (MRI) brain scans, a patient lies on a scanning table in a chamber surrounded by a magnetic field. Patients are usually asked to lie still during an MRI, although during a functional MRI, patients will be exposed to various stimuli or be asked to perform certain tasks while the machine scans the brain and records the activity. These tasks may include talking, remembering, calculating, listening to music, or wiggling the fingers.

During computed axial tomography (CAT) scans, an x-ray sensing unit and detectors rotate around the patient’s body while the scanning table moves by a fraction of an inch between the scans.

During positron emission tomography (PET) and single-photon emission computed tomography (SPECT) brain scans, a patient lies down on a table while a radionuclide solution is injected into the patient’s bloodstream. It can take 30 minutes or more before the solution reaches the brain. During this time, the patient will be advised to rest quietly on the table, and to avoid movement or talking. Then, sensors in an overhead scanner are activated in order to detect the gamma rays being emitted from within the patient’s brain. Patients can be required to shift positions during the test so that images can be taken from different angles. In addition, patients will also be exposed to certain stimuli or asked to perform various tasks during the scan.

During an ultrasound, a patient lies on a table and a clear, water-soluble gel is applied to the areas of the head to be examined. This helps the transducer (device that emits sound waves) retain contact with the skin, eliminating air pockets between the device and the patient’s skin. Areas that may be examined include the back of the neck, front of the ears, above the cheekbones and over the eyelids. The transducer is then held firmly against the skin and swept back and forth over a particular area until the desired images are captured.

After the brain scan

Patients can generally return to their regular daily routines after a brain scan. If a sedative is administered before the brain scan, patients are usually advised not to drive home after the test.

Patients who have undergone a brain scan using radionuclide imaging (e.g., positron emission tomography or single-photon emission computed tomography scans) will receive special instructions after their test. This may include drinking plenty of fluids to help flush the radioactive material from their bodies, which can take up to several days. While this is happening, patients will need to flush the toilet immediately after using it to limit exposure to potentially radioactive material. In addition, they may be advised to limit their time near children and to refrain from sexual activity until the radioactive material is completely eliminated from the body.

If a contrast medium was administered and the injection site remains sore days after the procedure, patients should notify their physician. After the brain scan has been performed, a radiologist will analyze the images and provide a formal interpretation report. The results will then be reported to the patient and the patient’s physician.

Potential risks with brain scans

Brain scans are generally safe. However, some risks are possible with certain types of imaging tests used for brain scans. In many cases, the potentially life-saving benefits provided by the diagnostic information from these imaging tests outweigh any potential risks. However, patients are advised to consult the physician about any concerns regarding brain scans before tests are performed.

The risks associated with imaging tests used for brain scans may include:

  • Magnetic resonance imaging (MRI). No significant side effects have been reported regarding the use of MRIs on the human body. Patients with implanted metal or electronic devices (e.g., pacemakers, vagus nerve stimulators, deep brain stimulation) may be advised against having an MRI, due to possible interaction with the magnetic field during the test. Possible risks to a fetus are unknown and pregnant women may be advised to avoid MRIs.

  • Computed axial tomography (CAT) scan. Very little radiation is used with CAT scans, but radiation levels are higher than those used with x-rays. Exposure to radiation can potentially damage body tissue and may be harmful to patients in certain circumstances (e.g., during pregnancy).

  • Positron emission tomography (PET) scan and single-photon emission computed tomography (SPECT) scan. For most people, the amount of radiation received during radionuclide imaging, such as a PET or SPECT scan, is considered minimal and safe. However, pregnant or nursing women will be advised against these tests and other methods will be used.

  • Ultrasound. Generally, no adverse risks or side effects have been associated with the use of ultrasound imaging.

Brain scans that involve the use of a contrast medium can, in rare cases, cause an allergic reaction. This can include symptoms such as mild itching or hives (small, raised, reddened areas of skin). In more severe reactions, shortness of breath and swelling of the throat and other body areas can occur. Patients who notice any of these symptoms developing during the procedure should immediately alert the technician performing the test. Patients with asthma or allergies may have an increased risk of allergic reaction to contrast media used during brain scans. In addition, certain medical conditions can increase the likelihood of side effects to contrast media or affect the accuracy of the brain scan. These include diabetes, heart disease, kidney problems or thyroid conditions.

Questions for your doctor regarding brain scans

Preparing questions in advance can help patients to have more meaningful discussions with healthcare professionals regarding their condition or test. Patients may wish to ask their doctor the following questions related to brain scans:

  1. Why are you recommending a brain scan?
  2. What information will a brain scan provide you?
  3. What imaging technology will be used for my brain scan?
  4. What will happen during the procedure?
  5. Will the test be uncomfortable? What if I’m uncomfortable in small places?
  6. For how long will I need to remain still during the brain scan? Why do I need to remain still?
  7. How should I prepare for my brain scan?
  8. Will a contrast medium be used?
  9. When is my brain scan scheduled? Where do I need to go?
  10. When will I get the results of my scan, and who will explain them to me?
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