Brain Surgery Information:

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CHAPTER 6: INVESTIGATING BRAIN DISORDERS

There are many ways to investigate a brain disorder:

·  Computerized tomography (CT) or Computer-assisted tomography (CAT) scanning: This technique involves a patient being put onto a sliding table and, while holding still and lying flat, being slowly advanced into a relatively wide doughnut-like scanning apparatus or gantry. A fast-rotating X-ray system within the gantry rapidly takes many x-rays of the patient. CT scans may be carried out with or without a contrast agent or dye. The use of a contrast, which is typically administered through a vein before the study, is helpful in picking up a blood vessel abnormality, or in detecting “enhancement” or abnormal contrast uptake and leakiness in, say, certain tumors. What CT scans lack in detail or resolution compared with other types of scans, they make up for in their speed, cost-effective convenience and noninvasiveness, that is, unintrusiveness. A combination of CT scanning and angiography referred to as CT angiography (CTA), where a larger amount of intravenous (IV) dye is introduced into the patient at the time of CT scanning, is currently gaining popularity as a very good means of detecting and characterizing blood vessel abnormalities such as aneurysms and AVMs. CTA can also be used to look at the brain’s venous system, where it is referred to as CT venography (CTV). CTA may one day replace conventional cerebral angiography (see below), as the former is so much quicker, less expensive, and less invasive compared with the latter. Both techniques involve some degree of X-radiation, however. The ability to create high-resolution and color 3-dimensional (3D) images with CTA is very useful for surgeons planning to operate on these lesions.

·  Magnetic resonance imaging (MRI): This technique relies on electromagnetic interaction between a large magnet and molecules within the patient’s head tissues and brain. MRI scans takes longer and are more expensive to obtain than CT scans, but do not involve X-rays, are noninvasive, and can provide very high resolution images. MRI scans, like their CT counterparts, involve the patient being slid on their backs into a doughnut-like gantry. Often, this gantry is longer and narrower than the CT’s gantry, and so claustrophobic patients usually do not tolerate MRI scanning, although they may find it more comfortable with some form of temporary sedation. It should be noted that there are newer open MRI scanners which are uncovered on the top, and therefore less claustrophobic, however, such scanners at present don’t produce very high definition studies. A well-tolerated IV contrast dye such as Gadolinium® or Magnevist® may be administered for the study. MRI scans are excellent for picking up most abnormalities of the brain, even very subtle ones. There are special forms of MRI known as MR angiography or arteriography (MRA) and MR venography (MRV) that are designed to look specifically at brain arteries and veins, respectively. These are being developed to perhaps replace formal catheter cerebral angiography (see below) as MRA and MRV, like CTA and CTV, are noninvasive ways at looking at brain blood vessels. MRA and MRV techniques are used in screening for brain aneurysms and AVMs. A special MR sequence known as diffusion-weighted imaging (DWI) is used to detect strokes, certain tumors and infections, DAI, and so forth. Finally, another special form of MRI known as MR spectroscopy (MRS) is being developed as a noninvasive technique to detect the spectroscopic pattern or biochemical signatures of certain molecules in the brain, and lesions within the brain. It may be helpful in determining the more exact location of a tumor cell mass or differentiating regrowth or recurrence of certain tumors from post-radiation change or infection.

·  Cerebral angiography: Cerebral angiography involves injection of an opaque dye into the blood stream of a patient. The injection is made through a catheter or thin piece of tubing inserted into, and advanced towards the neck from, the femoral artery in the groin area (Figure 14). The dye eventually reaches the brain circulation, and X-rays are taken at this point. The dye is "radio-opaque" in that X-rays don't pass as easily through it as they do through neighboring brain tissue, so the dye stands out. This way, a roadmap of the brain circulation is obtained, telling a physician about the state of the arteries and veins in terms of their course, their pattern of communication, their diameters and lengths, and any other abnormalities. The abnormal blush or sometimes rich blood supply of a tumor may be seen, or an aneurysm or AVM detected and characterized. At present, cerebral angiography remains the best method or “gold standard” investigation for brain aneurysms and AVMs. Arteries in spasm, say following rupture of a brain aneurysm, can be detected and potentially treated using catheter angiography techniques.

·  Positron emission tomography (PET) scanning: This technique involves IV injection or inhalation of a small and safe dose of radioactive tracer. As the tracer decays, its presence and location are detected by a special PET camera. Although PET scanning does not provide a highly detailed or high-resolution map of the body part scanned, it does provide potentially useful information. Further, PET images can be overlapped or coregistered with brain MRI or CT images from the same patient to provide a more detailed “functional” map of the patient’s brain. PET imaging data collected by the computer can allow a physician to tell, say, whether there is impaired blood flow to the regions of a patient’s brain, or if there is metastatic tumor present in different parts of the patients body, or whether the abnormality detected on the MRI of a brain tumor patient who has received radiation therapy is regrowth of tumor versus radiation-related change. Some limitations of PET are that it is a costly technique which relies on highly advanced technology and considerable expertise to interpret the data accurately. As a result, not all medical facilities have a PET scanner.

·  Single photon emission computed tomography (SPECT) scanning: SPECT scanning is somewhat like PET scanning in the way it works, except that it provides information more exclusively on cerebral blood flow (CBF) and cerebral blood volume (CBV). Again, a small and safe amount of IV or inhaled radioactive tracer is administered to the patient to acquire this data. The technique is excellent for looking at CBF impairment, and for assessing for location of seizure activity “hot spots” in epilepsy patients. Just as for PET, the SPECT images can be “fused” with brain MR images from the same patient, that is, MR-SPECT. SPECT imaging shares the same limitations as PET, and as a result is not available at all medical facilities.

·  Ultrasound techniques: Ultrasound techniques such as duplex or Doppler presently play no major role in the detection of brain conditions except in the setting of TIA or stroke from carotid or neck artery blockage or stenosis. Here, the ultrasound probe placed along the neck skin can be used to detect the degree of blockage and alteration in blood flow velocity across the narrowed segment. Sometimes, a small ultrasound probe is used during open brain surgery to confirm that there is good blood flow in an artery, say, after a brain aneurysm coming off the side of that vessel has been clipped or after brain bypass (www.brain-aneurysm.com). In patients who have experienced rupture of a brain aneurysm, a potentially serious complication known as cerebral vasospasm can develop (www.brain-aneurysm.com). Here, an ultrasound technique called transcranial Doppler (TCD) can be used to screen patients at the bedside for the development of vasospasm. In TCD, a small ultrasound probe is gently pressed against regions of the scalp, and a signal related to brain artery blood flow is picked up and analyzed.

·  Lumbar puncture (LP): The goal of carrying out an LP is to obtain fluid from a CSF-filled pouch called the lumbar cistern located deep to the soft tissue and spinolaminar bone of the lower back in the midline. A spinal needle is used for this, and it is inserted using local anesthetic to numb the skin and underlying tissues. There may be some tugging, but it should not be painful. Usually about 5-10 milliliters of CSF are removed. An LP can yield a lot of useful information regarding brain conditions. For example, CNS infection, MS, and certain tumors such as germ cell tumors and lymphomas frequently leave “footprints” that can be detected in the CSF. The pressure at which the CSF is under can be detected via an LP, and this information can be used to help diagnose conditions such as benign intracranial hypertension (BIHT) or pseudotumor cerebri, and other conditions involving hydrocephalus. Sometimes, a significant volume of CSF can be removed as part of a “large-volume spinal tap” to help physicians determine if a patient with BIHT or with NPH improves in his or her symptoms. Additionally, subtle amounts of blood that may not be seen on a CT scan following, say, rupture or leak of a brain aneurysm, may in fact be detected using an LP. Here, examination of the CSF for blood pigments or "xanthochromia” is carried out.

·  Blood tests: Taking a small sample of blood from a vein may be useful in the setting of following a patient for infection or inflammation that may be affecting a surgical wound, blood stream, brain tissue, brain blood vessels, and so forth. For patients with fluid and electrolyte imbalance, blood sodium and blood concentration or osmolality can also be assessed this way. Some genetic mutations and variations known as polymorphisms that are associated with certain CNS disorders can be picked up by screening the deoxyribonucleic acid (DNA) present in the patient’s white blood cells (www.brain-aneurysm.com).

·  Brain biopsy: At times, a sample of brain and meningeal tissue may be needed to help diagnose a condition whose diagnosis cannot otherwise be reliably made by the tests described above. An operation is required. The operation may involve open surgery or insertion of a needle-like brain probe to take a sample of brain tissue. The former is referred to as craniotomy for open biopsy, while the latter is referred to as stereotactic brain needle or core biopsy (Chapters 12 and 13).