Term
|
Definition
| a mathematical process used to process scan information into a display image |
|
|
Term
|
Definition
| the angle between which different data samples are obtained. Differs fundamentally between third and fourth generation scanners. |
|
|
Term
|
Definition
An opening. In CT the circular opening in the gantry through which the pt passes, or the face of the detector through which x-rays pass.
usually 50-85 cm
|
|
|
Term
|
Definition
| A device placed over a 4th generation scanner's detector array to partly shield the aperture of each individual detector. Reduces the effective detector aperture, improving spatial resolution but decrasing the capture efficiency. |
|
|
Term
|
Definition
| A fast reconstruction processor, housed in a cabinet adjacent to the computer. |
|
|
Term
|
Definition
| In the reconstructed image, an error that has no counterpart in reality. The most common causes are pt motion, very dense bone, ventricular shunt, and small dense objects. |
|
|
Term
|
Definition
| A decrease in the intensity of a beam or in the # of photons w/i the beam, caused by absorption and interaction w/ matter. |
|
|
Term
|
Definition
| A component of the reconstruction algorithm, it is the mathematical method of using raw data to create an image. |
|
|
Term
|
Definition
| The process of filtration of a polychromatic beam by the preferential absorption of lower energy photons in tissue. This shifts the average photon KeV upward. |
|
|
Term
| Beam Hardening Correction |
|
Definition
| Weighting factors applied to CT data to correct for the beam hardening effect. These corrections may be first-order or second-order corrections. 1st order are applied to the data initially, based on tissue thickness. 2nd utilize the data from the initaial recon. to further refine the attenuation data. |
|
|
Term
Bowtie filter
(Beam Shaping Filter) |
|
Definition
| An x-ray beam filter composed of a material such as graphite, teflon or aluminum, which when placed at the x-ray tube more heavily attenuates peripheral portions of the x-ray beam which pass through thinner parts of the pt. |
|
|
Term
|
Definition
| Process used to achieve homogeneity w/i the fov and maximize CT # accuracy. |
|
|
Term
|
Definition
| A measure of the ability to display structures of slightly differing density. This factor is measured w/ the test objects containing homogeneous structures of slightly differing densit. Depends on noise: it is poorer when noise is greater. It is improved by large pixel size, high mAs, thick slice, and low pass filter. |
|
|
Term
|
Definition
| The process of applying a predesigned function curve(spatial filter) to acquire data to modify the depiction of detail. The specific function chosen that is applied to the data of each view. |
|
|
Term
|
Definition
| Y axis, The plane dividing your front to back. |
|
|
Term
|
Definition
| The numbers used to define x-ray attenuation in a picture element of a CT image. In the commonly used HU scale. air is -1000, water is 0, and dense bone is about +3000. |
|
|
Term
| Data Acquisition System (DAS) |
|
Definition
| The assembly of components that cathers the raw data signals from all of the detectors. |
|
|
Term
|
Definition
| A single element of a detector unit or array, which receives x-rays and responds by producing an eletrical or light signal. |
|
|
Term
|
Definition
| The physical width of the face of a detector, which is sensitive to x-rays. |
|
|
Term
|
Definition
| The entire assembley of detectors including their interspace material. |
|
|
Term
|
Definition
| The interconnected series of components from a single detector through its photodiode amplifier, analog-to-digital convertor, offset and gain correction circuitry, and other components, to its point of entry into the computer. |
|
|
Term
|
Definition
| The use of two polychromatic x-ray beams of different kVp. Because of the predictable absorption characteristics of various elements at different x-ray energies, the elemental content of tissue can be more accurately calculated, tissue attenuation numbers are more accureate, and artifacs cased by beam hardening can be minimized. Can increase noise. |
|
|
Term
|
Definition
| A method of obtaining CT scans in rapid sequence. Done to follow passage of contrast material through vessels or tissue or to decrease exam time. |
|
|
Term
|
Definition
| The portion of noise in an image that is introduced by electronic components and electrical interference w/i DAS. |
|
|
Term
Field of View(FOV)
SFOV, DFOV |
|
Definition
Refers to the # of detectors collecting data during Scan.
Refers to the portions of a slice that's reconstructe and displayed |
|
|
Term
|
Definition
| Removes undesirable soft radiation from beam and shape beam intensity |
|
|
Term
|
Definition
| The point on the x-ray tube anode from which x-rays are emitted. The size helps determine spatial resolution. |
|
|
Term
|
Definition
| A camera capable of exposing multiple images on a single film. |
|
|
Term
|
Definition
| A scan obtained from a 360 degree tube rotaion around the pt. |
|
|
Term
|
Definition
| A scan obtained from a 360 degree tube rotaion around the pt. |
|
|
Term
|
Definition
| the physical opening in the CT scanner through which the pt is moved for the exam. |
|
|
Term
|
Definition
| A scan acquired when the tube rotates slightly more than 180 degrees around the gantry aperture. |
|
|
Term
|
Definition
| A convolution filter or kernel that accentuates high spatial frequency areas such as bone edges, while suppressing differences between structures of low contrast. |
|
|
Term
| Housnfield Unit(CT Number) |
|
Definition
| The generally accepted unit for attenuation values w/i pixels. 0 is the value of water and -1000 is the value of air. All other attenuation values are based on these two reference points. |
|
|
Term
| Incremented Dynamic Scanning |
|
Definition
| Method of scanning in which the table is moved (incremented) between scans. The purpose is to permit maximum opacification of all structures during the injections of congtrast |
|
|
Term
|
Definition
A mathematical method of averaging or smoothing images that are bieng displayed on a larger number of pixels than that for which they were originally reconstructed. If the pixels are magnified, their edges become more apparent and distracting.
Reduces this effect and also permits displaying images at varios FOV's on a fixed pixel matrix. |
|
|
Term
|
Definition
| The array of rows and columns of pixels. Usually between 256 and 512 rows and columns of pixels. |
|
|
Term
|
Definition
| There is a minimum frequency at which an object must be sample to accurately represent its spatial frequency on an image. The theorem states that a structure must be sampled at least twice its spatial frequency. |
|
|
Term
|
Definition
Sometimes the edge gradient is attributed to this. Synonymous w/ the volume averaging artifact.
Linear-inaccuracy in CT # caused by the presence of a structer w/i only a part of the slice thickness.
Non Lineary-A streak artifact caused by inconsistency in ray projection data from objects of complex shape and high spatial frequency w/i a slice. |
|
|
Term
|
Definition
| Indicates that the beam is composed of x-rays of varying frequencies or energies. |
|
|
Term
|
Definition
| a geometrical feature of a 3rd generation scanner in which the tube and detector are shifed by one quarter of the effective detector aperture from the center of rotation of the FOV. This causes rays obtained at 180 degrees to represent slightly differen projections, increasing the # of samples. |
|
|
Term
|
Definition
| The mathematical method chosen to construct a CT image from the raw data consisting of all the views w/i a scan. |
|
|
Term
|
Definition
| a plane dividing your rt and lt side |
|
|
Term
|
Definition
| A digitized signal representing the intensity of x-rays entering the detector |
|
|
Term
|
Definition
| Scan images created from a portion of the data from a full or overscan. Equivalent to partial scans in quality. Done to help follow changes over time. Such as passage of contrast material through a blood vessel |
|
|
Term
|
Definition
| The ratio of the strength of the signal for information content in the image to the noise level. |
|
|
Term
|
Definition
| A measurement of the ability to reproduce detail on an image. Measured by using a test object containing small structures w high contrast such as small holes in a dense block. |
|
|
Term
| Volume Averaging Artifact |
|
Definition
| Artifacts caused by the partial projection of a structure into a scanned slice. May lead to the inappropriated reading of attentuation values or to steaks emanating from the structure in question. |
|
|
Term
|
Definition
| The Ct attenuation number representing the center of a display window. |
|
|
Term
|
Definition
| The range of CT numbers w/i which the entire gray scale is displayed. |
|
|
