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Computed tomography scanning, also called CT scan, CAT scan, or computerized axial
tomography, is a diagnostic tool that provides views of internal body structures using x
rays. In the field of mental health, a CT scan may be used when a patient seeks medical
help for symptoms that could possibly be caused by abrain tumor. These symptoms
may include headaches, emotional abnormalities, or intellectual or memory problems.
In these cases, a CT scan may be performed to "rule out" a tumor, so that other tests can
be performed in order to establish an accuratediagnosis .
Purpose
CT scans are used to image bone, soft tissues, and air. Since the 1990s, CT equipment
has become more affordable and available. CT scans have become the imaging exam of
choice for the diagnoses of most solid tumors. Because the computerized image is sharp,
focused, and three-dimensional, many structures can be better differentiated
(visualized) when compared with standard x rays.
Common indications for CT scans include:
y Sinus studies. The CT scan can show details of sinusitis, bone fractures, and the
presence of bony tumor involvement. Physicians may order a CT scan of the
sinuses to provide an accurate map for surgery.y Brain studies. Brain CT scans can detect hematomas (blood clotted mass),
tumors, strokes, aneurysms (a blood vessel that ruptures), and degenerative or
infected brain tissue. The introduction of CT scanning, especially spiral CT, has
helped reduce the need for more invasive procedures such as cerebral
angiography (inserting a wire through an artery to where it will reach brain
vessels for visualization in real time).
y Body scans. CT scans of the chest, abdomen, spine, and extremities can detect
the presence of tumors, enlarged lymph nodes, abnormal collection of fluid, and
vertebral disc disease. These scans can also be helpful in evaluating the extent of
bone breakdown in osteoporosis.
y Heart and aorta scans. CT scans can focus on the thoracic (chest) or abdominal
aorta to locate aneurysms and other possible aortic diseases. A newer type of CT
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scan, called electron beam CT, can be used to detect calcium buildup in arteries.
Because it is a new technology, it is not yet widely used and its indications are not
yet well-defined.
y Chest scans. CT scans of the chest are useful in distinguishing tumors and in
detailing accumulation of fluid in chest infections.
Precautions
Pregnant women or those who could possibly be pregnant should not have a CT scan,
particularly a full body or abdominal scan, unless the diagnostic benefits outweigh the
risks. If the exam is necessary for obstetric purposes, technologists are instructed not to
repeat films if there are errors. Pregnant patients receiving a CT scan or any x ray exam
away from the abdominal area may be protected by a lead apron; most radiation, known
as scatter, travels through the body, however, and is not totally blocked by the apron.
Contrast agents are often used in CT exams, though some types of tumors are better
seen without it. Patients should discuss the use of contrast agents with their doctor, and
should be asked to sign a consent form prior to the administration of contrast. One of
the common contrast agents, iodine, can cause allergic reactions. Patients who are
known to be allergic to iodine or shellfish should inform the physician prior to the CT
scan; a combination of medications can be given to such patients before the scan to
prevent or minimize the reaction. Contrast agents may also put patients with diabetes at
risk of kidney failure, particularly those taking the medication glucophage.
Description
Computed tomography, is a combination of focused x-ray beams and the computerized
production of an image. Introduced in the early 1970s, this radiologic procedure has
advanced rapidly and is now widely used, sometimes in the place of standard x rays.
CT equipment
A CT scan may be performed in a hospital or outpatient imaging center. Although the
equipment looks large and intimidating, it is very sophisticated and fairly comfortable.
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The patient is asked to lie on a gantry, or narrow table, that slides into the center of the
scanner. The scanner looks like a doughnut and is round in the middle, which allows the
x-ray beam to rotate around the patient. The scanner section may also be tilted slightly
to allow for certain cross-sectional angles.
CT procedure
The gantry moves very slightly as the precise adjustments for each sectional image are
made. A technologist watches the procedure from a window and views the images on a
computer screen. Generally, patients are alone during the procedure, though exceptions
are sometimes made for pediatric patients. Communication is possible via an intercom
system.
It is essential that the patient lie very still during the procedure to prevent motion
blurring. In some studies, such as chest CTs, the patient will be asked to hold his or her
breath during image capture.
Following the procedure, films of the images are usually printed for the radiologist and
referring physician to review. A radiologist can also interpret CT exams on the computer
screen. The procedure time will vary in length depending on the area being imaged.
Average study times are from 30 to 60 minutes. Some patients may be concerned aboutclaustrophobia (a feeling of being "closed in") but the width of the "doughnut" portion of
the scanner is such that many patients can be reassured of openness. Doctors may
consider giving sedatives to patients who have severe claustrophobia or difficulty lying
still (such as small children).
The CT image
While traditional x-ray machines image organs in two dimensions, often resulting in
organs in the front of the body being superimposed over those in the back, CT scans
allow for a more three-dimensional effect. CT images can be likened to slices in a loaf of
bread. Precise sections of the body can be located and imaged as cross-sectional views.
The screen before the technologist shows a computer's analysis of each section detected
by the x-ray beam. Thus, various densities of tissue can be easily distinguished.
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Contrast agents
Contrast agents are often used in CT exams and in other radiology procedures to
illuminate certain details of anatomy more clearly. Some contrasts are natural, such as
air or water. A water-based contrast agent is sometimes administered for specific
diagnostic purposes. Barium sulfate is commonly used in gastroenterology procedures.
The patient may drink this contrast or receive it in an enema. Oral or rectal contrast is
usually given when examining the abdomen or cells, but not when scanning the brain or
chest. Iodine is the most widely used intravenous contrast agent and is given through an
intravenous needle.
Patient lying on mobile table, entering a CT (computed tomography or CAT)
scanner.
(Volker Steger/Science Photo Library, Science Source/Photo Researchers, Inc.
Reproduced by permission.)
If contrast agents are used in the CT exam, these will be administered several minutes
before the study begins. Patients undergoing abdominal CT may be asked to drink a
contrast medium. Some patients may experience a salty taste, flushing of the face,
warmth or slight nausea, or hives from an intravenous contrast injection. Technologists
and radiologists have the equipment and training to help patients through these minor
reactions and to handle more severe reactions. Severe reactions to contrast are rare, but
do occur.
Newer types of CT scans
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The spiral CT scan, also called a helical CT, is a newer version of CT. This type of scan is
continuous in motion and allows for the continuous re-creation of images. For example,
traditional CT allows the technologist to take slices at very small and precise intervals
one after the other. Spiral CT allows for a continuous flow of images, without stopping
the scanner to move to the next image slice. A major advantage of spiral CT is the ability
to reconstruct images anywhere along the length of the study area. Because the
procedure is faster, patients are required to lie still for shorter periods of time. The
ability to image contrast more rapidly after it is injected, when it is at its highest level, is
another advantage of spiral CT's high speed.
Electron beam CT scans are another newer type of CT technology that can be used to
detect calcium buildup in arteries. These calcium deposits are potential risk factors for
coronary artery disease. Electron beam CT scans take pictures much more quickly than
conventional CTs, and are therefore better able to produce clear images of the
Computerized axial tomography (CAT) scan of a human brain with
Parkinson's disease showing atrophy.(GJLP/CNRI/Phototake. Reproduced by permission.)
See color insert for color version of photo.
heart as it pumps blood. Because it is a newer and expensive test, electron beam CT
scanning is not widely used.
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Some facilities will have spiral, electron, and conventional CT available. Although spiral
is more advantageous for many applications, conventional CT is still a superior and
precise method for imaging many tissues and structures. The physician will evaluate
which type of CT works best for the specific exam purpose.
Preparation
If a contrast medium is administered, the patient may be asked to fast for about four to
six hours prior to the procedure. Patients will usually be given a gown (like a typical
hospital gown) to be worn during the procedure. All metal and jewelry should be
removed to avoid artifacts on the film. Depending on the type of study, patients may
also be required to remove dentures.
Aftercare
Generally, no aftercare is required following a CT scan. Immediately following the exam,
the technologist will continue to watch the patient for possible adverse contrast
reactions. Patients are instructed to advise the technologist of any symptoms,
particularly respiratory difficulty. The site of contrast injection will be bandaged and
may feel tender following the exam.
Risks
Radiation exposure from a CT scan is similar to, though higher than, that of a
conventional x ray. Although this is a risk to pregnant women, the risk for other adults is
minimal and should produce no effects. Severe contrast reactions are rare, but they are a
risk of many CT procedures.
Normal results
Normal findings on a CT exam show bone, the most dense tissue, as white areas. Tissues
and fat will show as various shades of gray, and fluids will be gray or black. Air will also
look black. Intravenous, oral, and rectal contrast appear as white areas. The radiologist
can determine if tissues and organs appear normal by the sensitivity of the gray
shadows.
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Abnormal results
Abnormal results may show different characteristics of tissues within organs.
Accumulations of blood or other fluids where they do not belong may be detected.
Radiologists can differentiate among types of tumors throughout the body by viewing
details of their makeup.
Sinus studies
The increasing availability and lowered cost of CT scanning has led to its increased use
in sinus studies, either as a replacement for a sinus x ray or as a follow-up to an
abnormal sinus radiograph. The sensitivity of CT allows for the location of areas of sinus
infection, particularly chronic infection. Sinus tumors will show as shades of grayindicating the difference in their density from that of normal tissues in the area.
Brain studies
The precise differences in density allowed by CT scan can clearly show tumors, strokes,
or lesions in the brain area as altered densities. These lighter or darker areas on the
image may indicate a tumor or hematoma within the brain and skull area. Different
types of tumors can be identified by the presence of edema (fluid), by the tissue's
density, or by studying blood vessel location and activity. The speed and convenience of
CT often allows for detection of hemorrhage (bleeding) before symptoms even occur.
Body scans
The body CT scan can identify abnormal body structures and organs. A CT scan may
indicate tumors or cysts, enlarged lymph nodes, abnormal collections of fluids, blood,
fat, or cancer metastasis. Tumors resulting from metastasis (movement of the cancer
from the primary site of cancer growth to a distant site) are different in makeup than
primary (original) tumors.
Chest scans
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In addition to those findings which may indicate aortic aneurysms (rupture of the
largest artery in the body), chest CT studies can show other problems in the heart and
lungs, and distinguish between an aortic aneurysm and a tumor adjacent to the aorta.
CT will not only show differences between air, water, tissues and bone, but will also
assign numerical values to the various densities. Coin-sized lesions in the lungs may be
indicative of tuberculosis or tumors. CT will help distinguish among the two. Enlarged
lymph nodes in the chest area may indicate Hodgkin's disease (a blood disorder).
Read more: Computed tomography - children, effects, adults, used, medication, brain,
effect, women, health, Definition, Purpose, Precautions, Description, Preparation,
Aftercare, Risks, Normal results, Abnormal resultshttp://www.minddisorders.com/Br-
Del/Computed-tomography.html#ixzz13nFHghb7
Skull Fractures
Skull fractures are categorized as linear or depressed, depending on whetherthe fracture fragments are depressed below the surface of the skull. Linear
fractures are more common. The bone windows must be examined carefully.A skull fracture is most clinically significant if the paranasal sinus or skull base
is involved. Fractures must be distinguished from sutures that occur inanatomical locations (sagittal, coronal, lambdoidal) and venous channels.Sutures have undulating margins both sutures and venous channels have
sclerotic margins. Venous channels have undulating sides. Depressedfractures are characterized by inward displacement of fracture fragments.
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Subarachnoid Hemorrhage
A subarachnoid hemorrhage occurs with injury of small arteries or veins on the surface
of the brain. The ruptured vessel bleeds into the space between the pia and arachnoid
matter. The most common cause of subarachnoid hemorrhage is trauma. In the
absence of significant trauma, the most common cause of subarachnoid hemorrhageis the rupture of a cerebral aneurysm. When traumatic, subarachnoid hemorrhage
occurs most commonly over the cerebral convexities or adjacent to otherwise injured
brain (i.e. adjacent to a cerebral contusion). If there is a large amount of subarachnoid
hemorrhage, particularly in the basilar cisterns, the physician should consider whether a
ruptured aneurysm led to the subsequent trauma. Cerebral angiography may be
needed for further evaluation. On CT, subarachnoid hemorrhage appears as focal high
density in sulci and fissures or linear hyperdensity in the cerebral sulci. Again, the most
common location of posttraumatic subarachnoid hemorrhage is over the cerebral
convexity. This may be the only indication of cerebral injury.
High density blood (arrowheads) fills the sulci over the
right cerebral convexity in this subarachnoid hemorrhage.
Intraventricular Hemorrhage
Traumatic intraventricular hemorrhage is associated with diffuse axonal injury, deep
gray matter injury, and brainstem contusion. An isolated intraventricular hemorrhage
may be due to rupture of subependymal veins.
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Intraventricular hemorrhage (arrow) found in a trauma
patient. Note the subarachnoid hemorrhage in the
sulci in the subarachnoid space (arrowheads).
Stroke Subtypes
Strokes are classified into two major types - hemorrhagic and ischemic. Hemorrhagic
strokes are due to rupture of a cerebral blood vessel that causes bleeding into or
around the brain. Hemorrhagic strokes account for 16% of all strokes. An ischemic stroke
is caused by blockage of blood flow in a major cerebral blood vessel, usually due to ablood clot. Ischemic strokes account for about 84% of all strokes. Ischemic strokes are
further subdivided based on their etiology into several different categories including
thrombotic strokes, embolic strokes, lacunar strokes and hypoperfusion infarctions.
Hemorrhagic Stroke
Hemorrhagic strokes account for 16% of all strokes. There are two major oHemorrhagic
strokes account for 16% of all strokes. There are two major categories of hemorrhagic
stroke. Intracerebral hemorrhage is the most common, accounting for 10% of all strokes.
Subarachnoid hemorrhage, due to rupture of a cerebral aneurysm, accounts for 6% of
strokes overall.verall.
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Hemorrhage in the cerebellum (arrow).
Intracerebral Hemorrhage
The most common cause of non-traumatic intracerebral hematoma is hypertensive
hemorrhage. Other causes include amyloid angiopathy, a ruptured vascular
malformation, coagulopathy, hemorrhage into a tumor, venous infarction, and drug
abuse.
Thalamic hemorrhage (arrow) extending intothe left lateral ventricle (arrowheads
Hypertensive Hemorrhage
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Hypertensive hemorrhage accounts for approximately 70-90% of non-traumatic primary
intracerebral hemorrhages. It is commonly due to vasculopathy involving deep
penetrating arteries of the brain. Hypertensive hemorrhage has a predilection for deep
structures including the thalamus, pons, cerebellum, and basal ganglia, particularly the
putamen and external capsule. Thus, it often appears as a high-density hemorrhage in
the region of the basal ganglia. Blood may extend into the ventricular system.
Intraventricular extension of the hematoma is associated with a poor prognosis.
Subarachnoid Hemorrhage
In the absence of trauma, the most common cause of subarachnoid hemorrhage is a
ruptured cerebral aneurysm. Cerebral aneurysms tend to occur at branch points of
intracranial vessels and thus are frequently located around the Circle of Willis. Common
aneurysm locations include the anterior and posterior communicating arteries, the
middle cerebral artery bifurcation and the tip of the basilar artery. Subarachnoid
hemorrhage typically presents as the "worst headache of life" for the patient. Detectionof a subarachnoid hemorrhage is crucial because the rehemorrhage rate of ruptured
aneurysms is high and rehemorrhage is often fatal.
CT is currently the imaging modality of choice because of its high sensitivity for the
detection of subarachnoid hemorrhage. CT is most sensitive for acute subarachnoid
hemorrhage. After a period of days to weeks CT becomes much less sensitive as blood
is resorbed from the CSF. If there is a strong clinical indication, LP may be warranted
despite a negative CT since small bleeds can be unapparent on imaging.
On CT, a subarachnoid hemorrhage appears as high density within sulci and cisterns.
The insular regions and basilar cisterns should be carefully scrutinized for subtle signs of
subarachnoid hemorrhage. Subarachnoid hemorrhage may have associatedintraventricular hemorrhage and hydrocephalus.
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High density blood fills the cisterns (arrowheads) in
this patient with hemorrhage from the left middle cerebral
artery. Note the middle cerebral artery aneurysm (arrows).
Ischemic stroke
Ischemic strokes are caused by thrombosis, embolism of thrombosis, hypoperfusion and
lacunar infarctions. A thrombotic stroke occurs when a blood clot forms in situ within a
cerebral artery and blocks or reduces the flow of blood through the artery. This may bedue to an underlying stenosis, rupture of an atherosclerotic plaque, hemorrhage within
the wall of the blood vessel, or an underlying hypercoagulable state. This may be
preceded by a transient ischemic attack and often occurs at night or in the morning
when blood pressure is low. Thrombotic ischemic strokes account for 53% of all strokes.
An embolic stroke occurs when a detached clot flows into and blocks a cerebral
artery. The detached clot often originates from the heart or from the walls of large
vessels such as the carotid arteries. Atrial fibrillation is also a common cause. Embolic
strokes account for 30% of all strokes.
A lacunar infarction occurs when the walls of small arteries thicken and cause the
occlusion of the artery. These typically involve the small perforating vessels of the brainand result in lesions that are less than 1.5 cm in size.
Hypoperfusion infarctions occur under two circumstances. Global anoxia may occur
from cardiac or respiratory failure and presents an ischemic challenge to the brain.
Tissue downstream from a severe proximal stenosis of a cerebral artery may undergo a
localized hypoperfusion infarction. Lacunar and hypoperfusion strokes, account for the
remaining 1% of strokes of the ischemic type.
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Imaging of Stroke
"Stroke" is a clinical diagnosis; however imaging is playing an increasingly important role
in its diagnosis and management. The most important issue to determine when imaging
a stroke patient is whether one is dealing with a hemorrhagic or ischemic event. This has
crucial therapeutic and triage implications. Decisions that must be made concerningtherapy are dependent on the diagnosis and may include the following:
- Is the patient a thrombolysis candidate and should thrombolytic therapy be used?
- Intravenous or intrarterial therapy?
- Neurosurgery or neurology patient?
In addition about 2% of clinically definite "strokes" are found to be a result of some other
pathology such as a tumor, a subdural hematoma or an infection.
CT scanning
There are several advantages to performing a CT scan instead of other imaging
modalities. A CT scan:
- Is readily available
- Is rapid
- Allows easy exclusion of hemorrhage
- Allows the assessment of parenchymal damage
The disadvantages of CT include the following:
- Old versus new infarcts is not always clear
- No functional information (yet)
- Limited evaluation of vertebrobasilar system
A CT is 58% sensitive for infarction within the first 24 hours (Bryan et al, 1991). MRI is 82%
sensitive. If the patient is imaged greater than 24 hours after the event, both CT and MR
are greater than 90% sensitive.
CT Findings of Stroke
When analyzing the CT of a potential stroke victim, one of the first findings tolook for is the presence or absence of hemorrhage. Another common finding
in stroke patients is a dense middle cerebral artery or a dense basilar artery,which corresponds to thrombus in the affected vessel. There are also moresubtle changes of acute ischemia due to edema which include the following:
- Obscuration of the lentiform nuclei- Loss of insular ribbon
- Loss of gray/white distinction- Sulcal effacement
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Dense basilar artery (arrow).
CT of Subacute Infarction
The CT of a subactue infarction has the following findings in 1 -3 days:
- Increasing mass effect
-Wedge shaped low density- Hemorrhagic transformation
After 4 - 7 days the CT is characterized by:
- Gyral enhancement
- Persistent mass effect
In 1-8 weeks:
- Mass effect resolves
- Enhancement may persist
Meningitis
There are three subtypes of meningitis. Acute pyogenic meningitis is usually
bacterial. Lymphocytic meningitis is usually viral, benign and self-limited.
Chronic meningitis is often seen in immunocompromised hosts and may be
fungal or parasitic. Imaging in suspected meningitis patients is performed to
look for complications and assess safety of lumbar puncture. Imaging is not
usually performed to diagnose meningitis because imaging studies are
frequently normal despite the presence of the disease.
Complications of Meningitis
The following are common complications of meningitis that can be seen using
imaging techniques:
o Hydrocephalus
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o Ventriculitis / Ependymitis
o Subdural effusion
o Subdural empyema
o Cerebritis / Abscess
o Vasospasm / arterial infarctso Venous thrombosis / venous infarcts