Without reference, identify principles relating to Computed Tomography Clinical Applications with at least 70 percent accuracy

COMPUTED TOMOGRAPHY CLINICAL APPLICATIONS

Without reference, identify principles relating to Computed

Tomography Clinical Applications with at least 70 percent accuracy


The History of Computed Tomography Computed Tomography (CT) imaging is also known as

"CAT scanning" (Computed Axial Tomography) Tomography is from the Greek word "tomos" meaning

"slice" or "section" and "graphia" meaning "describing"

CT was invented in 1972 by British engineer Godfrey Hounsfield of EMI Laboratories, England, and independently by South African born physicist Allan Cormack of Tufts University, Massachusetts

Hounsfield was later awarded the Nobel Peace Prize and honored with Knighthood in England for his contributions to medicine and science


Applications of CT Unlike other medical imaging techniques, such as

conventional x-ray imaging, CT enables direct imaging and differentiation of soft tissue structures, such as • Liver • Lung tissue • Fat

Therefore CT is a valuable tool, for instance, in searching for large space occupying lesions, tumors and metastasis

CT scans can not only reveal the presence but also the size, spatial location and extent of a tumor


CT imaging of the head and brain can detect tumors, show blood clots and blood vessel defects, show enlarged ventricles (caused by a build up of cerebrospinal fluid) and image other abnormalities such as those of the nerves or muscles of the eye

Due to the short scan times of 500 milliseconds to a few seconds, CT can be used for all anatomic regions, including those susceptible to patient motion and breathing

For example, in the thorax, CT can be used for visualization of nodular structures, infiltration of fluids, and fibrosis

CT exams are fast and simple and enable a quick overview of possibly life-threatening pathology and rapidly enable a dedicated surgical treatment. Therefore, CT is becoming the method of choice for imaging trauma patients


The first clinical CT scanners were installed between 1974 and 1976

The original systems were dedicated to head imaging only, but "whole body" systems with larger patient openings became available in 1976

CT became widely available by about 1980 There are now about 6,000 CT scanners installed

in the U.S. and about 30,000 installed worldwide The latest multi-slice CT systems can collect up to

4 slices of data in about 350 ms and reconstruct a 512 x 512 matrix image from millions of data points in less than a second


The X-ray System Tube and gantry • The x-ray tube is mounted on a circular gantry

assembly, which rotates around the patient's body • There are two ways to supply power to the tube

while it rotates Cables » Designed to only make a couple of rotations » The gantry must be stopped and rotated in the other

direction to uncoil the cables Sliding electrical contacts (or slip rings) - they permit

continuous high-speed rotation


Collimation - two sets of collimators • One set of collimators determines the angular span of

the beam • The other set of collimators determines the thickness

of the beam Filtration - CT x-ray beams are filter for two

purposes • Beam hardening

In CT imaging the x-ray beam creates an image artifact because of the peripheral tissue is exposed to a lower average photon energy than the inner portion of the slice

This filtration reduces patient exposure by selectivity removing the low-energy low-penetration part of the x-ray beam


• Compensation Compensates for the non-uniform thickness of the human

body It is thicker near the edges and is sometimes referred to as

the bow-tie filter Power supply - typically a constant potential type that

can produce relatively high KV and MA values for a sustained period of time

Detectors - the radiation receptor is an array of many small detectors that are mounted within the gantry assembly • Function

Absorbs the radiation it intercepts Produces an electrical signal proportional to the radiation

intensity.


• Configurations The way in which the detectors are arranged and moved

during the scanning process has changed during the evolution of the CT scanner and is different among scanners used today

There are four generations of detector configuration » First Type A. Used a single detector element that was moved,

along with the tube, in a straight line across the patient's body to form one view

B. Then the tube and detector assembly was rotated 1°and scanned across the body to form the second view

» Second type - used multiple detectors and reduced the number of rotations required to achieve a full scan


» Third type (Rotate-rotate scanner) A. An array of individual detector elements that is just large enough to form one view is mounted on the gantry B. It rotates along with the x-ray tube

» Fourth Type (Stationary-rotate scanner) A. A ring of detectors that completely encircles the patient B. The detectors remain stationary as the tube rotates around

the patient Computer - performs several functions • Control

After the operator selects the appropriate scanning factors and initiates the scan, the procedure progresses under the control of the computer

It coordinates and times the sequence of events during the scan » Turning the beam and detectors on and off at the appropriate times » Transferring data » Monitoring the system operation


• Processing - directly involved in the formation of the CT image through processing data into the image

• Storage and retrieval - it transfers, stores, and retrieves images and data

Display unit and camera • Display unit displays an image on a CRT or video monitor • Camera converts image to film

CT image formation The formation of a CT image is a distinct three phase process The CT image is, for all practical purposes, an image of three

densities of the tissue The scanning phase • The x-ray beam is scanned around the body • The amount of radiation that penetrates the body is measured by

the detectors and converted into data


The reconstruction phase • Back projection

The data produced is not a complete image, but a profile of the objects that have been x-rayed

Only enough data in the profile allows the computer to draw in streaks As the x-ray beam rotates around the body, obtaining different views,

we see the beginnings of an image • CT number

A digital image of CT is in the form of a matrix of pixels A part of the reconstruction phase is to calculate a CT number for each

image pixel Water is the reference material for CT numbers and has an assigned

value of zero Materials with density greater than water will have a positive CT

number Materials that is less dense than water will have a negative Ct number CT numbers are measured in Hounsfield Units


The digital to analog conversion phase • The digital image, consisting of a matrix of pixels with

each pixel having a CT number, is converted into a visible image represented by different shades of gray or brightness levels by windowing

• Windowing controls contrast in CT imaging • The window is the range of CT numbers that will be

displayed with the different shades of gray, ranging from black to white

• Tissues within the window will have different shades of gray (brightness) and will have visible contrast

• All tissues and materials that have CT numbers above the window will be all white and no contrast within this range


• All that have CT numbers below the window will be all black and without contrast

• The level control adjusts the center of the window • The width control adjusts the range of CT numbers

that will be displayed with contrast • The width controls the contrast in the displayed

image • Reducing window width increases the displayed

image contrast among the tissues • The ability to window is what gives CT a very high

contrast sensitivity • This is because a window can be set to display and

make visible very small differences in tissue densities


• All tissues and materials that have CT numbers above the window will be all white and no contrast within this range

• All that have CT numbers below the window will be all black and without contrast

• The level control adjusts the center of the window • The width control adjusts the range of CT numbers that

will be displayed with contrast • The width controls the contrast in the displayed image • Reducing window width increases the displayed image

contrast among the tissues • The ability to window is what gives CT a very high

contrast sensitivity • This is because a window can be set to display and make

visible very small differences in tissue densities


Documents

Without reference, identify principles relating to Computed Tomography Clinical Applications with at least 70 percent accuracy