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CT Physics Terms I
Basic Principles of CT and Data Acquisition
| Question | Answer |
|---|---|
| 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
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