computed tomography scanning

8/8/2019 Computed Tomography Scanning

1/16

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


2/16

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.


3/16

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.


4/16

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


5/16

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.


6/16

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.


7/16

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


8/16

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.


9/16

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.


10/16

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.


11/16

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


12/16

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.


13/16

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.


14/16

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


15/16

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


16/16

o Ventriculitis / Ependymitis

o Subdural effusion

o Subdural empyema

o Cerebritis / Abscess

o Vasospasm / arterial infarctso Venous thrombosis / venous infarcts

computed tomography scanning

Documents