CT physics and instruments
Different names for Filter Function
Algorithm, convulsion filter, or kernel
ADC
Analog to Digital Converter. Part of DAS converts
Filter function
Applied to the scan data before back projection occurs to minimize artifacts, can only be applied to raw data.
Cupping artifacts
Artifact that results from beam hardening. It appears on the image as a vague area (dark streaks) of increased density in a somewhat concentric shape around the periphery of an image, similar to the shape of a cup
ATCM
Automatic Tube Current Modulation: adjust the mA throughout an acquisition to reduce patient radiation dose to minimum
Desired qualities of a CT detector
a) High efficiency b)Rapid signal decay: fast measure, limited afterglow c) high dynamic range
Noncontiguous images
acquired with an interval greater than section width. 1mm sections recon. every 10mm (9mm missing)
Slice sensitivity profile
may be used to describe the reconstructed CT section
Photon fluence
quantity of x-ray photons passing through a specifies area, controlled by mAs
dual-energy CT
rapidly switch between or alternate kVp settings during data acquisition, allowing for improved contrast between substances of differing densities
Overlapping images
reconstructed with section intervals less than section width (used with MPR and 3D). 1mm sections recon every 0.5mm (50%overlap)
Reconstruction algorithm (kernel)
reconstructive algorithms are digital filters which determine both the appearance and resolution of the image. can smooth and enhance edges of required anatomical structures.
DFOV
the area from the SFOV (scan field of view) that will be displayed on the monitor.
Effective mAs
the calculated mAs per acquired slice with a multislice CT (MSCT) system.
Detector pitch
the ratio of table feed per gantry rotation to the acquired section width. This definition of pitch is used for helical-slice CT (SSCT)
Beam pitch
the ratio of table feed per gantry rotation to the total collimation used during acquisition with a multi-slice (MSCT) system. Beam pitch= table feed per sec/total collimation
Dose profile
the section of the patient exposed to radiation as measured at isocenter of gantry
Total collimation
# of sections x section width
Factors producing high Standard Deviation
-Mixed attenuation tissue within ROI -ROI that includes a streak artifact -ROI that is not inside the margins of the object being mesured
Prospective gating
-simultaneous ECG monitoring, scanning occurs only during diastole (least motion)
Monitor can display ____ shades of gray
256
Beam hardening
Beam hardening is caused by filtration - hardening - of the beam by structures within the body such as bone. The direction of the artifact is parallel to the direction of the source beam.
Helical interpolation
Blurring out helical image (tilt), causes loss of image resolution
Contiguous images
CT images acquired with equal section thickness and reconstruction intervals. Completely cover, no unmeasured tissue
dual-source CT
CT system that consists of two seperate x-ray tubes and detector arrays mounted 90 degrees from each other within the gantry, allowing for dual-energy CT aquisition.
Slip Rings
Consist of brushes fit into grooves to permit the current and voltage to the X-ray tube to be supplied while the tube is in continuous rotation around the gantry.
Raw Data (Scanned Data)
Data that have not yet been sectioned to create pixels; hence, HU values haven't been assigned.
Image Data
Data the computer has processed and assigned one HU value to each pixel.
Window Width
Determines the quantity of Housfield units. Controls digital image contrast
DFT
Discreet Fourier Transform:
Smaller focal spots improve:
Geometric efficiency of the x-ray beam, leading to greater spatial resolution.
Prospective reconstruction
Image reconstruction that is automatically produced during scanning
Out of field artifact
Look like streaks or shading on CT images. Occur because the anatomy outside of the scan FOV contributes to the attenuation & beam hardening, but is ignored during image recon
Windowing
Manipulation of window width and window level to optimize image contrast
Data-Acquisition System DAS
Measures the number of photons that strikes the detector, converts the information to a digital signal, and sends the signal to the computer
Gantry
Part of the CT scanner that consists of the x-ray tube, detector array, and collimator
Volume averaging
Process by which different tissue attenuation values are averaged to produce one less accurate pixel reading. Also referred to as partial volume effect.
Photon flux
Rate at which photon fluence (quantity of radiation) or (mAs) passes through a unit area over unit time.
Bow filter
Reduces the beam intensity at the periphery of the beam, corresponding to the thinner areas of the pt anatomy
Pitch setting less than 1 :
Reduces the table speed for each gantry revolution. Increases acquired data and improves image quality Higher pt dose
SFOV
Scan field of view. Determines the area (circular shape), within the gantry, from witch the raw data are acquired.
Step and Shoot Scanning
Scan method in which the CT table moves to the desired location and remains stationary while the xray tube rotates within the gantry, collecting data. Scans produced this way result in images that are perpendicular to the z axis and parallel to every other slice, like slices of a sausage, also called axial scanning.
Window Level
Selects wchich Hounsfield values are displayed as shades go gray Produces changes in density/brightness
Adaptive Statistical Iterative Reconstruction
Starts with an assumed image, computes projection from the image, campers it with the original projection data and updates the image on the basis of the difference between the calculated and actual projection. Reduce image noise, improve image quality by improving low-contrast detectability. Compered to standard back-projection method this technique reduce the radiation dose to the PT by 50%
Wider window width: (noise)
Suppress the display of noise. Used on obese patients and with metallic artifacts
Convolution
The process of applying a filter function to an attenuation profile
Back projection
The process of converting data from the attenuation profile to a matrix to create image; produces streak artifacts is a star pattern; require filtering before applied to matrix
Retrospective reconstruction
The process of reconstructing images from raw data using modified parameters, after initial image reconstruction is complete
Filtered back projection reconstruction
Use Fourier theory to reduce statistical noise and create an image pleasant to eye.
Image reconstruction
Use of raw data to create an image
Standard Deviation
Variations in HU within ROI Higher SD greater variation
Overbeaming
When xray penumbra falls outside the active detectors; this occurs when collimators are opened so that the same xray intensity reaches all of the detectors in an MDCT system.
MSCT beam is shaped like a
cone
Flying focal spot technology:
electromagnetic steering of the electron beam emitted from the cathode. magnetic steering of electron beam from cathode. Beam is directed toward 2 sep locations on the anode, resulting in 2 sources of x-radiation.
SSCT beam is shaped like a
fan
Wider window width:
greater anatomic diversity, but density lost (contrast)
Narrow window width provide:
greater density, and contrast
Changing the DFOV affect:
image quality by changing the pixel size.
kVp vs contrast
kVp up - contrast down kVp down- contrast up
Dual filaments are controlled by:
mA setting, SFOV
Window level should be set at..
roughly the same value as the average attenuation number of the tissue interest
Small focal spot produce better
spatial resolution, sharper images (reduced penumbra)
Section interval
the distance between the center of one CT section and the center of the next adjacent section.
Overranging
the process of applying radiation before and after the acquisition volume to ensure sufficient data collection for interpolation algorithims of helical ct reconstruction. Add to the PT dose
Interpolation
the process of estimating a value between two known quantities
