what is a name for the preliminary (or localizer) image taken at the start of a CT examination?
Topogram, scout or scanogram
the ability of a system to differentiate, on the image,objects with similar densities is know as ____
Low-contrast resolution
the two-dimensional square of data that make up the ct image is called a ___
pixel
how many pixels are contained in a 1024 matrix image?
1,048,567
the total number of pixels in a matrix is the product of the number of rows and number of columns
what is beam attenuation?
the phenomenon by which an x-ray beam passing through a structure is decreased in intensity or amount because of absorption and interaction with matter
which of the following is a low-attenuation structure?
B. ) trachea
an object is slightly less dense than water.what is the expected Houndsfield measurement?
-10
why does the administration of iodinated contrast media result in an enhanced image?
iodinated contrast material increases the ability of the enhanced structure to attenuate the x-ray beam
an object that appears on the image but is not present in the object scanned is called a(an)___.
artifact
what is an advantage of filtering the x-ray beam?
reduces radiation dose to the patient
scan thickness is primarily important for the part it plays in:
volume averaging
explanation: thinner slices reduce volume averaging by decreasing the amount of patient information included in each voxel
how many CT numbers are assigned to each pixel in the image matrix?
1
what is another name for raw data?
scan data
what are the 3 main advantage of CT over conventional radiography?
1.) the elimination of superimposed structures,
2.) the ability to differentiate smalle differences in density
3.) superior quality of the images
How is image quality evaluated?
spatial resolution, low-contrast resolution, and temporal resolution
What is housed within the gantry?
high-frequency generator, slip rings, detectors
the power capacity of the generator is listed in ___
kilowatts (kW)
describe a slip ring device.
slip rings permit the gantry frame to rotate continuously, making helical scan modes possible
what is a disadvantage of a small focal apot size?
reduced heat capacity
what is an advantage of a small focal spot size?
better spatial resolution
the ability of the tube to withstand by-product heat is called ____
heat capacity
regarding the detectors in the CT system, geometric efficiency is controlled primarily by:
detector spacing and aperture
what are the 3 methods in which scanners acquire data?
1. preliminary scanning
2. axial scanning
3. helical scanning
compared with x-rays, CT scout images ___
are of slightly poorer image quality with the same amounts of radiation
what are the key aspects to axial data acquisition?
the table remains stationary while the x-ray tube rotates within the gantry, collecting data
what causes the interscan delay inherent in an axial scan?
table movement between data acquisitions.
grouping more than one axial scan in a single breath hold is referred to as ____
clustering
what is the primary disadvantage of axial scanning when compared to helical scanning?
total examination time is longer and data reconstruction is limited
Reducing the heat load on the x-ray tube will:
extend the life of the tube
Hounsfield units are used to:
quantify the beam attenuation capablility of a given object
Temporal resolution is primarily affected by:
gantry rotation speed and number of slices acquired per rotation
What determines the number of Hounsfield units represented on a specific image?
window width
Software that automatically adjusts the tube current to fit specific anatomic regions:
adapts mAs from one gantry rotation to the next
he network is fundamental to the success of a PACS because it:
regulates the movement of data and directly affects all useres
What component measures the number of photons that strike the detector, converts the information to a digital signal, and then sends the signal to the computer?
data acquisition system (DAS)
CT generators produce kV in what range?
120-140 kV
Artifacts that result from preferential absorption of the low-energy photons, which leaves only higher-intensity photons to strike the detector array, are called:
Beam hardening artifacts
Which determines how much of the collected raw data are used to create an image?
display field of view (DFOV)
Assuming the mAs is kept constant, what is an advantage of using a high mA setting?
allows for a shorter scan time that may reduce motion
What are bow-tie filters used for?
they are body scanning filters used to reduce beam intensity at the periphery of the beam
What defines the z-axis?
The thickness of the cross sectional slice
Why does slice thickness vary?
Smaller parts of the body need to be seen in more detail but it also increases the patient dose
Explain the function and purpose of source collimators?
Reduces patient dose by hardening the beam and getting rid of low intensity photons that would be absorbed by the patient. And reduces scatter reaching the detectors.
what are the optimal characteristics of detectors?
1.) High efficiency
2.) Low-to-no afterglow-
3.) High scatter suppression
4.) High stability
what is the difference in raw data and image data?
Raw data has not been made into and image and must be accessed to reformate images. Image data results once the computer has processed raw data and is the visualized image
areas of a CT image containing abrupt changes in tissue density are electronically represented by:
high spatial frequencies
the multiple-scan average dose (MSAD) is used to describe what?
average dose to a single section within a scan of multiple sections
what is the central volume principle?
CBF=CBV/MTT
what is the primary interaction between xray photons and tissue during CT?
