Physics And Instrumentation Flashcards
Measurement of the attenuation that occurs along each path through the patient from X-ray tube to detector
Data Acquisition
Angle of the tube and detectors in relationship to the patient position during scout acquisition is
Azimuth
Smaller focal spots improve the geometric efficiency of the X-ray beam, leading to a
Greater spatial resolution
Spatial resolution
Ability of a CT imaging system to display fine details separately (lp/cm)
Amount of X-ray photons passing through a specified area
Photon fluence
Rate at which the photon fluence passes through a unit area over unit time
Photon flux
Describes the calculated mAs value per acquired slice.
Main controlling factor is table speed.
Effective mAs
Effective mAs = mAs/
Pitch
Describes the spacing between two adjacent CT images.
Measured as the distance between the center of one section and the center of the next adjacent section.
Section interval
Those acquired with equal section thickness and interval
Contiguous images
Those acquired with an interval greater than the section width
Noncontiguous images
Reconstructed with a section interval that is less than the section width
Overlapping images
Describes the relationship between collimation and table movement during scanning
Pitch
Original definition of pitch may now be referred to as
Detector pitch
Original definition of pitch
SSCT
Pitch = table feed per rotation/
Section width
New adaptation of pitch in MSCT is termed beam pitch. Formula:
Beam pitch =
table feed per rotation/
Total collimation
Total collimation = number of sections x section width
The reduction of intensity of a radiation beam as it passes through a substance
Attenuation
Desired qualities of a CT detector:
High efficiency
Rapid signal decay
High dynamic range
Convert X-ray energy directly into an electrical signal
Gas ionization detectors
Convert X-ray energy first into light energy, which just then be converted into electrical signal
Solid-state scintillation crystal detectors
All modern MSCT systems utilize this type
Determines the width of the reconstructed section
Detector collimation
Beam collimation no longer controls section width
Refers to the number, length, and organization of the individual detector elements in an MSCT system
Detector configuration
3 general formats of MSCT detector configuration:
Uniform matrix-array: utilizes multiple detectors in the longitudinal direction, each the same length. (Same size collimation)
Adaptive-array: thinnest widths at the center, surrounded by detectors of incrementally increasing widths. (5,2.5,1,1,2.5,5)
Hybrid-array: 2 detector sizes. Narrower midline and wider outside.
Electronic combination of signal from adjacent detectors to form a reconstructed slice that is thicker than the individual detector width
Binning
Electronic components responsible for measuring the transmitted x-radiation absorbed by the detectors
DAS
Data acquisition system
The CPU is capable of performing multiple tasks simultaneously, a function called
Parallel processing
An additional feature of CT computers systems that further improves the speed and efficiency with which it can perform multilingual simultaneous functions
Pipelining
Primary data processing component of the CT system:
Is a separate computer with its own CPU
Responsible for receiving scan data from the host computer, performing all of the major processing of the CT image, and returning the reconstructed image to the storage memory of the host computer
Array processor
Portion of the X-ray beam transmitted through the patient and incident upon a single detector
Ray
The measurement of transmitted radiation made by an individual detector
Ray sun
Ability of an object to attenuate the X-ray beam is assigned a value termed
Linear attenuation coefficient
Primary goal of CT image reconstruction is to
Spatially distribute the attenuation data recorded by the detector array
Mathematical process primarily responsible for CT image reconstruction is called
Back-projection
2D grid of numbers arranged in rows and columns
Matrix
Process applied to reduce image unsharpness
Convolution
Current method for image reconstruction in modern MSCT
Filtered back projection
Mathematical process whereby data from tube rotations just above and just below a given slice position are used for image reconstruction
Interpolation
CT # (HU) of dense bone
+1500 to +2000
HU of muscle
+50 to +65
HU of white matter
+40 to +46
HU of gray matter
+35 to +40
HU of blood
+30 to +35
HU of CSF
+15
HU of water
0
HU of fat
-50 to -100
HU of lungs
-550 to -950
HU of air
-1000
Occurs when multiple types of tissue are represented by a single voxel.
Can be reduced by minimizing the voxel dimension
Partial volume averaging
A parameter that controls the diameter of the circular data acquisition field within the CT gantry as determined by the number of activated detectors along the x-y axes
Scan field of view (SFOV)
Calibration field
The diameter of the acquired attenuation data displayed across the image matrix (zoom factor or target view)
Display field of view (DFOV)
Window width controls
Contrast
Window level controls
Brightness
Post-processing for contrast-enhanced blood vessels. Low attenuation values not displayed
MIP (maximum intensity projection) max pixel value
Post-processing for biliary tree, colon, lungs, and trachea
Min-IP (minimum intensity projection)
Technique that uses the concept of thresholding to limit the displayed volumetric data.
Bone threshold >300 to build skeletal model
Surface rendering / shaded-surface rendering (SSD)
Adjusts the opacity of voxels included in the 3D model according to their tissue characteristics. Does not exclude voxels, but instead alters their appearance
Volume rendering (VR)
Viewpoint of being within the lumen of an object
Perspective volume rendering (immersive rendering)
The unwanted appearance of individual sections on a mulitplanar or 3D reformation image resulting in a loss of sharpness and detail
Step artifact
Ability of a CT imaging system to display fine details separately. (lp/cm)
Ability to resolve small, closely spaced objects when they are surrounded by material that is very different in density.
Spatial resolution
Larger objects of uniform density are represented by
Low spatial frequency signal
Smaller dense objects and areas of sharp borders between varying densities are represented by
High spatial frequency signal
A graphical representation of a CT systems response to a spatial frequency that serves as a measurement of the systems in-plane spatial resolution
Modulation transfer function (MTF)
Smaller, more closely spaced detectors ________ spatial resolution
Improve
Represents the amount of broadening that occurs along the z-axis during volumetric data acquisition
Slice sensitivity profile (SSP)
Ability of the CT system to detect an object with a small difference in linear attenuation coefficient as compared to the surrounding tissue.
Describes the CT systems ability to differentiate between two adjacent tissues of similar attenuation values.
Contrast resolution (low-contrast detectability or sensitivity)
MSCT systems are typically capable of differentiating adjacent objects with attenuation differences as small as
3 HU
The stop-motion capability of a CT system.
Ability to freeze motion and provide an image free of blurring.
Temporal resolution
Controlling factors of temporal resolution
Gantry rotation speed
Reconstruction method
Any portion of the signal that contains no useful information.
Manifests as an overall graininess on the reconstructed image.
Noise
Measured as standard deviation on water phantom
Result of an insufficient X-ray photon flux per volume
Quantum noise
Amount of quantum noise is inversely related to the amount of radiation exposed to each voxel
Descriptive term used to quantify the amount of noise in a displayed CT image.
Calculated as the standard deviation in the ROI measurement of a water phantom image.
Signal-to-noise ratio (SNR)
Major quality factor affecting low-contrast resolution
Image noise
CT systems ability to maintain relatively consistent CT values across the entire image of an object of equal density
Spatial uniformity
The relative accuracy between calculated CT numbers and their respective linear attenuation coefficients
Linearity
Daily calibration
Artifact appears when the degree of beam hardening exceeds the CT systems ability to correct during the reconstruction process.
Typically manifest as areas of _______
Light and dark streaking bands across portions of the image
Dense bony areas of the posterior fossa of skull are especially prone.
Higher kVp may slightly reduce it (120-140)
Occurs when a structure is only partly positioned within a voxel and the attenuation for the object is not accurately represented by a pixel value
Partial volume artifact
Appears as a generalized unsharpness or haziness of the borders of objects.
PF and interface between the diaphragm and abdominal contents are common locations.
Use of thin sections help eliminate this effect
Artifact caused by the systems inability to process the high spatial frequency signal that represents the interface between two substances with widely different attenuation values
Edge gradient
The unwanted appearance of individual sections on a MPR or 3D reformation image resulting in a loss of sharpness and detail
Step artifact
