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CT Physics Terms I

Basic Principles of CT and Data Acquisition

Ability of a system to resolve, as separate forms, small objects that are very close together. Also called high-contrast resolution or detail resolution spatial resolution
Ability of the system to differentiate between objects with similar densities. Also called contrast resolution or contrast detectability. low-contrast resolution
How rapidly data are acquired. It is controlled by gantry rotation speed, the number of detector channels in the system, and the speed with which the system can record changing signals. temporal resolution
Plane that correlates to the slice thickness, or depth, of the CT slice z-axis
Mechanical hardware that resembles small shutters and adjusts the opening based on the operator's selection collimators
Picture element. Two-dimensional square of data. When arranged in rows and columns, they make up the image matrix. pixel
Volume element. Three-dimensional cube of data acquired in CT voxel
Grid formed from the rows and columns of pixels matrix
Phenomenon by which an x-ray beam passing through a structure is decreased in intensity or amount because of absorption and interaction with matter. The alteration in the beam varies with the density of the structure it passes through beam attenuation
An x-ray beam that is nearly unimpeded by an object; typically shown as dark gray or black on an image low attenuation
An x-ray beam is greatly impeded by an object; typically shown as light gray or white on an image high attenuation
Amount of x-ray beam that is scattered or absorbed per unit thickness of the absorber linear attenuation coefficient
Contrast agents that are of a higher density than the structure being imaged. Most contain barium or iodine positive contrast agent
A contrast agent that is of a lower density than the surrounding structure, such as air or carbon dioxide negative contrast agent
Measure of the beam attenuation capability of a specific structure. Also called pixel values, density numbers, or CT numbers Hounsfield units
An x-ray beam that is composed of photons with varying energies polychromatic x-ray energy
Artifacts that result from lower-energy photons being preferentially absorbed, leaving higher-intensity photons to strike the detector array beam-hardening artifacts
Artifact that results from beam hardening. It appears on the image as a vague area of increased density in a somewhat concentric shape around the periphery of an image, similar to the shape of a cup cupping artifacts
Process by which different tissue attenuation values are averaged to produce one less accurate pixel reading. Also referred to as _____ volume averaging; partial volume effect
All measurements obtained from the detector array and sitting in the computer waiting to be made into an image. Also called _____ raw data and scan data
Use of raw data to create an image image reconstruction
Image reconstruction that is automatically produced during scanning prospective reconstruction
Process of using the same raw data to later generate a new image retrospective reconstruction
Scan method in which the CT table moves to the desired location and remains stationary while the x-ray tube rotates within the gantry, collecting data; the images will appear perpendicular to z axis and parallel to every other slice; AKA axial scanning step-and-shoot scanning
Scanning method that includes a continually rotating x-ray tube, constant x-ray output, and uninterrupted table movement. Also called ____, ____, or ____ spiral scanning; helical, volumetric, or continuous acquisition scanning
Scanner design in which there are many parallel rows of detectors. A single rotation can produce multiple slices multidetector row CT scanning
Energy of motion kinetic energy
Ring-shaped part of the CT scanner that houses many of the components necessary to produce and detect x-rays gantry
X-ray tube design includes a cathode, which emits electrons, and an anode, which collects electron anode
Area of the anode where the electrons strike and the x-ray beam is produced focal spot
Measured in thousandths of an ampere, or mA, it controls the quantity of electrons propelled from cathode to anode tube current
Ability of the tube to withstand the heat heat capacity
Ability of the tube to rid itself of heat heat dissipation
Measures the number of photons that strikes the detector, converts the information to a digital signal, and sends the signal to the computer data acquisition system
A complete set of ray sums view
Component that interprets computer program instructions and sequences tasks. It contains the microprocessor, the control unit, and the primary memory central processing unit
CT component that assigns a group of Hounsfield units to each shade of gray display processor
Number of photons absorbed by the detector; dependent on the physical properties of the detector face (e.g., thickness, material) absorption efficiency
Converts the analog signal to a digital format analog-to-digital converter
A brief, persistent flash of scintillation that must be taken into account and subtracted before image reconstruction afterglow
Used as a reference point when planning the scout image anatomic landmark
Mechanical filter that removes soft, or low-energy, x-ray beams, minimizing patient exposure and providing a more uniform beam intensity bow tie filters
Ability with which the detector obtains photons that have passed through the patient capture efficiency
Filters the x-ray beam to reduce the radiation dose to the patient; help to minimize image artifact and improve image quality compensating filters
Cooling mechanisms included in the gantry, such as blowers, filters, or devices that perform oil-to-air heat exchange cooling systems
Element in a CT system that collects attenuation information. It measures the intensity of the transmitted x-ray radiation along a beam projected from the x-ray source to that particular detector element detector
Size of the detector opening detector aperture
Entire collection of detectors included in a CT system; detector elements are situated in an arc or a ring detector array
Ability of the detector to capture transmitted photons and change them into electronic signals detector efficiency
Measured from the middle of one detector to the middle of the neighboring detector; accounts for the spacing bar detector spacing
Ratio of the maximum signal measured to the minimum signal the detectors can measure. The range of x-ray intensity values to which the scanner can accurately respond dynamic range
This system uses a large electron gun as its x-ray beam source. A massive anode target is placed in a semicircular ring around the patient. Neither the x-ray beam source nor the detectors move, and the scan can be acquired in a short time electron beam imaging
Scanner configuration that uses a detector array that is fixed in a 360 degree circle within the gantry. Sometimes referred to as rotate-only scanners fourth-generation design
Opening in the gantry; range of aperture size is typically 70 to 90 cm gantry aperture
Produces high voltage and transmit it to the x-ray tube high-frequency generators
Listed in kilowatts (kW); determines the range of exposure techniques available on a particular system power capacity
Shape the beam and are located below the patient and above the detector array predetector collimators
Limit the x-ray beam before it passes through the patient prepatient collimators
Included in the detector array and help to calibrate data and reduce artifacts reference detectors
Time required for the signal from the detector to return to zero after stimulation of the detector by x-ray radiation so that it is ready to detect another x-ray event response time
Occur with third-generation scanners and appear on the image as a ring or concentric rings centered on the rotational axis. They are caused by imperfect detector elements—either faulty or simply out of calibration ring artifacts
Number of samples taken per second from the continuous signal emitted from the detector sampling rate
Degree to which a table can move horizontally. Determines the extent a patient can be scanned without repositioning scannable range
Electromechanical devices that use a brushlike apparatus to provide continuous electrical power & electronic communication across a rotating surface, permitting gantry frame to rotate continuously, eliminating the need to straighten twisted system cables slip rings
Process of moving the table by a specified measure. Also referred to as feed, step, or index table incrementation
When the table position is manually set at zero by the technologist table referencing
Scanner configuration that consists of a detector array and an x-ray tube that produces a fan-shaped beam that covers the entire field of view and a detector array. Sometimes referred to as rotate-rotate scanners third-generation design
Digital image acquisitions that are created while the tube is stationary and the table moves through the scan field. Referred to by various names, depending n the manufacturer, such as scout, topogram, scanogram, and pilot localizer scans
Scan method where the CT table moves to desired location & remains stationary while the xray tube rotates w/in the gantry, collecting data; scans produced w/ this method result in images that are perpendicular to the z axis & parallel to every other slice axial scanning; AKA step-and-shoot scanning
The practice of grouping more than one scan in a single breath-hold clustered scans
Method of acquiring slices in which one slice abuts the next contiguous
The radiation emitted from the collimated x-ray source in single-detector row CT systems fan beam
The radiation emitted from the collimated x-ray source in multidetector row CT systems cone beam
Scanning method that includes a continually rotating x-ray tube, constant x-ray output, and uninterrupted table movement. Also called helical, spiral, or volumetric scanning continuous acquisition scanning
Image noise resulting from the scattering of x-ray photons by adjacent detectors crosstalk
Table movement per rotation divided by beam width beam pitch
Table movement per rotation time divided by the selected slice thickness of the detector detector pitch
CT design that uses two sets of x-ray tubes and two corresponding detector arrays in a single CT gantry dual source
Detector rows that have variable widths and sizes. Also called nonuniform or hybrid arrays adaptive array
Detector rows that have variable widths and sizes. Also called adaptive or nonuniform arrays hybrid array
Complex statistical methods to, in effect, take the slant and blur out of the helical image and create images that closely resemble those acquired in a traditional axial mode helical interpolation methods
Relation to table speed to slice thickness. It is most commonly defined as the travel distance of the CT scan table per 360 degree rotation of the x-ray tube, divided by the x-ray beam collimation width pitch
When the slice thickness displayed on the image is wider than that selected by the operator slice thickness blooming
Thickness of the slice that’s actually represented on the CT image, as opposed to the size selected by the collimator opening. slice-sensitive profile; AKA effective slice thickness
Detector rows that are parallel and of equal size uniform array
Early systems, which contained only a single row of detectors in the z axis, obtained data for one slice with each rotation single-detector row CT (SDCT)
Scanner design in which there are many parallel rows of detectors. A single rotation can produce multiple slices multidetector row CT (MDCT)
A technique of interpolating helical scan data for SDCT systems using 180 degree linear interpolation 180Ll
A technique of interpolating helical scan data for SDCT systems using the 360 degree linear interpolation 360Ll
Occurs when a patient breathes differently with each data acquisition. The difference in breathing places the 2nd group of scans in an incorrect anatomic position relative to the 1st set of slices. Valuable information may be missed b/c of this effect slice misregistration
Created by: sspatel