Chapter 4 (Image quality) Flashcards
Pixel Value
Given in hounsefield units. is a numerical representation of the attenuation value of a voxel of tissue as compared with that of water for an individual CT system.
Partial Volume Averaging
Occurs when multiple types of tissue are represented by a single voxel. The attenuation coefficients for each tissue type are averaged, yielding a single pixel value whose HU attempts to represent the entire contents of the voxel.
Reducing voxel dimension =
Less need for partial volume averaging. The smaller a voxel the less there is a chance that attenuation coefficients will differ. Simplest method is to reduce section width (Thins)
Formula for Pixel Dimension (in relation to DFOV)
d= DFOV(mm) / matrix size
standard matrix size in CT imaging
512 pixels X 512 pixels
Scan field of view (SFOV)
determined by the size ( x and y axes) of the detector array. controls the diameter of the circular data acquisition field within the CT gantry. Mostly a range of 48-52 cm . Most systems have multiple choices. Varying SFOV may also vary filtration and other adaptations to improve image quality.
Display field of view (DFOV)
Chosen by the CT operator on the basis of the part size in question. Selected DFOV determines what portion of the acquired data will be displayed accross the matrix. (What you select to recon)
Choosing a smaller DFOV =
limits anatomic area displayed, increases the size of the displayed anatomy and reduces pixel dimension and the volume of each voxel
What three technical parameters are pixel dimension and voxel volume controlled by
matrix size, DFOV, section width (z)
actual size of pixel never changes, just the amount of tissue spacially localized to each voxel
Less tissue in each voxel =
reduces partial volume effect. which improves image quality.
Cartesian Coordinate System
uses the x-y-z axis, used to adjust image centering. Can be reffered to as RAS (for righ, anterior, and superior)
Magnification
electronic feature of the system software, has no effect on pixel/voxel dimension.
Windowing
used to describe the process of gray-scale mapping of the CT image. (Controls Image contrast)
how many bits of data per pixel can the typical CT system display
12
typical range of hounsefield scale CT numbers
-1024HU to + 3071HU
How many shades of gray are possible and how many can we see?
4096 are possible
dyanmic range of monitors can display 256
human eye can only differentiate 60-80 individual shades
What happens to pixels with values ouside of the image window
they are displayed as either black or white
pixels with a CT number lower than the set range?…higher?
lower= appears black higher= appears white
how many pixel values for each shade of gray will a WW(window width) setting of 256 demonstrate?….WW2560?
There are 256 shades of gray available to display. 4096 possible pixel number. WW determines how many consecutive CT hounsefield units will be displayed by a single shade of gray. Therefore
WW256= 1 pixel value per shade of gray
WW2560= 10 pixel values per shade of gray
Window Level
The pixel value, given in HU, at the center of the window width (controls brightness)
. Usually set at or near the hounsefield value of the primary tissue of interest. Example Window width of 150 (-25 to +125) is centered at a CT number of +50HU
Image display filters
do not affect the process of raw data reconstruction; rather they alter the display characteristics of an image after the reconstruction process.
name 2 analytic functions of the CT system
region of interest (ROI)
Linear distance measurement
Region of Interest (ROI)
Provides a quantitative analysis of the hounsefield values of a specific anatomic area.
Linear Distance Measurement
graphic used for precise size determination, distance for interventional procedures and so on.
Isotropic data det
describes voxels with equal dimensions along the x, y and z axes. yields high quality images with equal resolution in any reconstructed plane, eliminating the need for data acquisition in two planes.
Multiplanar reformation (MPR)
describes the process of displaying CT images in a different orientation from the one used in the original recon process.
orthogonal planes
at right angles to each other ex: coronal and sagittal
Maximum Intensity Projection (MIP)
display only the maximum pixel value along a ray traced through the object to the viewers assumed perspective in front of the viewing monitor. Tissues with lower attenuation values are not displayed leaving high-attenuation structures such as bone and contrast-enhanced soft tissue structures free of superimposition.
Minimum Intensity Projections (min-IP)
display minimum pixel value along each ray to the viewer. used primarily in the evaluation of the biliary tree, colon, lungs and trachea.
surface rendering
also termed shaded surface display (SSD) is a 3D technique. uses an adjustable threshold to only display certain voxels based on the adjusted threshold. example: set a threshold of >300 and you can recreat a skull….
volume rendering
adjusts the opacity of the voxels included in the 3D model according to their tissue characteristics. unlike threshold, it does not exclude voxels, it adjusts them.
perspective volume rendering
provides viewpoint of being within the lumen
orthographic volume rendering
the method of externally viewing a 3D reconstruction from the perspective of being outside the body.
about how much storage does one CT image take
0.5 MB , but there can be hundreds of images in one CT study.
Step artifact
when the separation of each axial section is apparent. Happens with thicker anisotropic section.
What level of overlap is suggested for optimal reformatted image quality?
50% example: 1.0 mm sections reconstructed every 0.5 mm
Spatial Resolution
The ability of a CT imaging system to display fine details seperately . Describes ability to resolve small, closely spaced objects when they are surrounded by material that is very different in density. Measure in lp/cm (line pairs per centimeter)
2 components of spatial resolution for helical CT systems
in-plane spatial resolution
longitudinal spatial resolution
Modulation Transfer Function (MTF)
graphical representation of a CT systems response to a spatial frequency. The ability of a CT scanner to record a spatial frequency and thereby resolve an object. MTF of 1.0 the image is a perfect reproduction of the object
point spread function
quality control. uses phantom with small wire to quantify the amount of burring that occurs within an image of an object.
focal spot size
small focal spot size improves in-plane spatial resolution
detector size
smaller, more closely spaced detectors improve spatial resolution.
reconstruction algorithm
also reffered to as a convolution kernel., the reconstruction algorithm shapes the spatial frequencies used during the image reconstruction process. High spatial frequency algorithms= superior spatial resolution
pixel dimension
combo of large matrix and small DFOV (large zoom factor) results in smaller pixel dimension and an increase in the in-plane spatial resolution.
sampling frequency
the number of views obtained by the CT system during the data acquisition of an image sampling rate may also be referred to as views per rotation (VPR)
nyquist theorem
dictates that the data sampling frequency must be at least twice the objects spatial frequency in order for the object to be resolved by the CT system
Slice sensitivity profile (SSP)
represents the amount of broadening that occurs along the z-axis during volumetric data acquisition.
spiral interpolation algorithm
an inherent component of the recon process that is not influenced by the operator. algorithm is applied to the image recon of helically acquired images to reduce SSP-broadening-effects. The 180LI algorithm is most commonly used.
pitch
increase in pitch=widening of SSP= loss of longitudinal spatial resolution
contrast resolution
the ability of the CT system to detect and object with a small difference in linear attenuation coefficient as compared to the surrounding tissue.
beam collimation
scatter radiation significantly reduces contrast resolution. Increasing beam collimation reduces scatter radiation and improves contrast resolution.
what effect does a narrow window width have on contrast resolution
improves it by exaggerating the difference in gray shade assignment to pixels with only small differences in attenuation values.
detector collimation
increased detector collimation results in the recon of thinner sections. as section width decreases, the photon flux for each pixel also decreases, unless the mA is raised to compensate, an increase in noise occurs. Any increase in noise results in a decrease in contrast resolution.
noise
increase in noise= decrease in contrast resolution
decrease in noise= better contrast resolution
temporal resolution
describes the ability of the CT system to freeze motion and provide an image free of blurring.
Signal to noise ratio (SNR)
descriptive term used to quantify the amount of noise in a displayed CT image. goal is to create images with a high SNR while maintaining dose and spatial resolution at appropriate levels.
