Powerpoint Flashcards
Ct detectors are called?
Does what?
Solid state detectors
- turns photons into lights
Voxel profile
Contents in the profile
Any Ct acquisition can be reconstructed with one or more reconstruction filters( bone, soft tissue)?
True
Data can be classified as(4)
Measurement data
Raw data
Convoluted data
Image data
Measurement data
Arise from detectors and are subject to preprocessing to correct the measurement data before the image algorithmn is applied
Raw data
The result of pre- processed scan data subject to the reconstruction algorithmn
Convolved data
Image reconstruction used by current Ct scanners is filtered back projection
Raw data is first filtered with mathematical filter or kernel. Process is called convolution technique which improves image quality by removing blur
Most popular reconstruction algorithmn
Filtered back projection
Kernel
Mathematical filter
Image data
Reconstructed data are convolved data that have been back projected into the image matrix to create Ct images displayed on a monitor
Image quality Ct
General image quality can be described by several,key performance parameters. High contrast, low contrast, spatial resolution, temporal resolution
Determining factors image quality
Artifacts
Contrast resolution
Linearity
Noise
Spatial resolution
Artifacts
- are?
- arise (4)?
Distortion or error in an image that is unrelated to the subject being scanned. It can degrade image quality , affect the perceptibility of detail.
Arise from number sources :
- inappropriate selection of protocol
- reconstruction process
- problems relating to equipment( malfunctions)
- fundamental limitations to physics
Ct # 40
Blood
Ct # 1000
Air
Streak artifacts
Caused by errors of isolated Channels or views and enhanced by the convolution process and manifested into lines during the back projection
Why streaks?
Improper sampling data Partial volume averaging Patient motion Metal Beam hardening Noise Spiral/ helical scanning Mechanical failure
Shading artifact
Often appear near objects of high densities and can be caused by beam hardening , partial volume, averaging, spiral/ helical scanning , scatter, off focal radiation and incomplete projections
Why shading artifact
Partial volume averaging Beam hardening Spiral/ helical scanning Scatter radiation Off focal radiation Incomplete projections
Ring and bands artifacts
Produced when the projection readings of single channel or group of channels consistently deviate from the truth . It can be result of a defective detector cells or das channels, deficiencies in system calibration or suboptimal image generation
Why ring and band artifact?
Bad detector channels in 3rd gen Ct scanners
Patient based artifacts
Motion artifact
Transient interruption of contrast
Motion artifact
Decrease scan time , increase interscan delay
Transient interruption of contrast
Use of filters ( algorithmns)
Physical based artifacts
Beam hardening
Metal artifact/ high density foreign material artifact
Partial volume averaging
Beam hardening
Artifact that occurs as streaks at bone edges adjacent to soft tissue. Seen most commonly in posterior fossa of brain
Metal artifact/ high density foreign material artifact
Produce streak artifacts metal artifact correction can be used
Partial volume averaging
Occurs when tissues of widely different absorption are encompassed on the same Ct voxel producing a beam attenuation proportional to the average value of these tissues
Physics based artifacts
Quantum mottle
Aliasing
Quantum mottle
Influenced partially by the number of photons that strike the detectors. Photon starvation can occur as a result of poor patient positioning in the scan fov and poor selection of exposure technique, scan speed and limitation of scanner. Photon starvation often leads to sever streak artifacts
More photons means less
Noise and stronger detector ; combined with electronic noise and logarithmic operation
Aliasing
Arises from insufficient projection sampling or insufficient view sampling( number of photons)
Hardware based artifacts
Ring/ band artifact
Helical and multichannel artifacts
Cone beam effect
Scatter
Ring or band artifact
Produced when the projection readings of a single channel or a group of channels consistently deviate from the truth. It can be result of defective detector cells or das channels, deficiencies in system calibration or suboptimal generation
Cone beam effect
Caused by incomplete or insufficient projection samples as a result of cone beam geometry of msct
Scatter radiation produced by
Patient
Sample
Data that has been taken from all over
Contrast resolution
Signal to noise ratio
Contrast to noise ratio
Scatter radiation reduction
Signal to noise ratio
How much true signal of anatomy versus how much Noise a particular image has- higher signal lower noise better
Contrast to noise ratio
Measure to determine image quality
Scatter radiation reduction
Corrected with algorithmn by carefully measuring or estimating the scatter distribution in the projection. The estimated scatter can then be removed from the measured intensities to arrive to the true signals representing scanned object
Linearity
Relationship of Ct numbers to linear attenuation coefficients of the object to be imagined. Can be checked by daily calibration
Water and other materials tested
Two points are defined precisely on Ct number scale, water with Ct number ______and air Ct number is _____
0
-1000
Because ____ is similar to soft tissue in terms of attenuation characteristics, it’s important to establish accuracy
Water
All Ct manufactures provide a phantom filled with water for such testing. When tested the Ct number should be fairly close to______?
0
Das
Data acquisition scan
Flux
Change
Kernel
Average within the matrix
Noise
Measured on uniform phantoms suing roi’s
Quantum noise
Determined by X-ray flux or number of photons detected. Influenced by scanning techniques, scanner efficiency ( detector quantum efficiency and geometrical efficiency) and patient
Mottle
Grainy appearance, also can be caused the reconstruction parameters.
Ex: high resolution construction kernel produces an increased noise level. Because kernels preserve and enhance high frequency contents and noise presents itself a high frequency signals
Electronic noise caused from
Detector photodiode
Data acquisition scan
Scatter radiation
Spatial resolution
Aka high contrast resolution
Scanners ability to resolve closely placed objects that are significantly different from their background. Defined and measured within the scanning plane and cross plane
In plane resolution
Specified in terms of line pairs per centimeter( lp/cm) or lp/ mm
Line pairs is a pair of equal sized black-white bars
Smallest , most closely spaces set of lead lines is said be the spatial resolution of Ct scanner.
Mtf ( modulation transfer functions) can be used to compare performance of different Ct systems
- straight line= better mtf
Spatial resolution test is done by?
Tech
Cross plane spatial resolution
New technology has enable radiologist to view images in 3D
Mpr, mip, vr
Where it is hard to tell if images were acquired, for example, in a coronal plane or reconstructed coronal. Shows the special resolution is isotropic
Improved cross plane spatial resolution reduces_____
Artifacts
Cross plane spatial resolution is usually described by _____. Then the _____ can be calculated.
Slice sensitivity profile( ssp )
Modulation transfer function
Testing for spatial resolution should be done ______
Monthly
Temporal resolution is important to which type of Ct?
Cardiac Ct
Gaiting
Waiting for heart rate resting between beats to image using an ekg machine
Water phantom test is done by the tech______
Aka?
Daily- looking for Ct number
Linearity test
Temporal resolution
Indication Ct systems ability to freeze motions of scanned object
Reduces blurring effect caused by moving object( i.e. Heart, peristalsis , contrast through vessels)
Reduced by different techniques such as physiological gating device
In cardiac cycle , time period of no motion?
Diastolic and systolic phases
During periods of cardiac temporal resolution , what can take place?
Data acquisition
Advantage of prospective gating?
Reduces patient dose
During times when perspective gating is not optimal, what happens to the scan?
Can be turned off
Disadvantage of perspective gating?
Reliance on regularity of heart motion
Low contrast resolution or low contrast detectability
Determined using objects having a very small difference in density from the background
Referred to sensitivity of the system
Images of tissues that vary slightly in density and atomic number
Measured by phantom that contains low- contrast objects of different sizes
Lcd also implies an object visibility is highly influenced by presence of noise
Influencing factors of image quality
Focal spot size Beam geometry Detector type Motion Subject contrast Ambient lighting conditions( ww or wl) Selectable factors ( ma,kv,scan time , sfov, dfov, slice thickness, interscan spacing, filtering, ww,wl)
Calibration of scanner
Ensures optimum image quality
Should be carried out according to manufactures instructions
Includes frequent air calibrations
Less frequent calibrations with homogenous water phantoms
Air calibrations allow for acquiring information about the small differences in the response of individual detect elements
Ct calibration has to be accurate within
0.5% and air calibration allows for appropriate calibration and correction of signal recorded by each individual detector element
Calibration with phantom allows some correction of beam hardening
Portion of primary beam interacting with single detector ?
Ray
Ray
Portion of X-ray beam that falls upon single detector
Ability of Ct scanner to image high density objects controlled by the____ of the scanner
Spatial resolution
Spatial resolution
Controls ability to image small structures
Which following are commonly used to describe a Ct scanner ability to differentiate objects with similar linear attenuation coeffcients?
Sensitivity
Contrast resolution
Contrast resolution/ sensitivity
Ability of Ct scanner to accurately image objects with similar densities .
Ct scanner with poor contrast resolution or low sensitivity has difficulty separating tissues whose linear attention coefficients are nearly equal
Spatial resolution of a Ct scanner is usually given in unit measure?
Lp/cm
Spatial resolution quantifies is ability to visually separate small objects in the image . Unit used to describe the spatial resolution is tested by the scanning a phantom with an embedded resolution test pattern consisting of a series of small lead lines the decease in size and spacing . Smallest most closely spaces set lead lines is said be the spatial resolution of Ct scanner. A line pair _____consists of one lead line and a space immediately next to it.
Lp
Which following mathematical functions may be used to quantify the spatial resolution of Ct scanner?
Psf- point spread function
Mtf- modulation transfer function
Lsf - line spread function
Line spread function( lsf), point spread function(psf), modulation transfer function(mtf) are used for?
Quantify the spatial resolution of Ct scanner
Line spread function (lsf)
Measures ability of Ct scanner to clearly image the edge or a line
Point spread function( psf)
Clearly images extremely small , point like structures
Modulation transfer function (mtf)
Examines the fidelity of spatial frequency as it represents tissues with varying densities. Derived from the measurement of lsf and psf
Which following technical adjustments may be employed to improve the temporal resolution of mdct system?
Decreased scan time( avoid motion)
Temporal resolution quantifies Ct systems?
Ability to freeeze motion and provide an image free of blur. Controlling factors of temporal resolution: gantry rotation speed and reconstruction method
Primary advantage of an isotropic mdct data set?
High quality multiplanar refirmations( MPr)
Isotropic volumetric data set
Yields high quality images with equal resolution in any reconstructed plane
Reduces the step artifact
Reconstruction of isotropic
Overlapping
Thin section mdct
Number, length and organization of the individual detector elements in an msct system
Detector configuration
Prospectively cardiac Ct data acquisitions occurs only during the portion of the ecg?
T wave
Prospective gated cardiac Ct data acquisition occurs only during the portion ecg termed
T wave
T wave
Prospective electrocardiography monitoring, scanning only occurs during diastole, when the heart demonstrates the least motion. User defined percentage of the r- r interval is set as the trigger for data acquisition to occur during the t wave of ecg
Which of the following statements most accurately describes the relationships between slice sensitivity profile (ssp) and dose profile?
The dose profile is always wider than ssp
Slice sensitivity profile (ssp)
May used to describe the reconstructed Ct section. However, section of tissue exposed to ionizing radiation or dose profile, is greater in width than the ssp
Technique that allows the user to select the range of pixel values used in 3D ct reformation ?
Thresholding
Selected threshold allows the user to select the range of ____values rendered in 3D model .
Ex: higher bone threshold(>300 hu) can be chosen to build a skeletal model of the patients skull. Decreasing the threshold to include voxel with lower attenuation values results in 3D model with the patients soft tissue
Pixel
Which of the following corresponds to the period of the cardiac cycle known as atrial systole ?
Contraction of the left and right atrial
________ describes the period of the cardiac cycle when the left and right atria are contracting
Atrial systole
Which of the following formulas maybe used to calculate the dimensions of a pixel?
Pixel size= dfov/ matrix size
Ct scanner with a limited resolution of 15 lp/cm can resolve object as small as?
0.3mm
Minimum object size that a Ct scanner can resolve may be calculated by taking the reciprocal of value of scanner limiting resolution. Reciprocal of 15 lp/cm is 1/15 lp/cm . This is equivalent to 10/15 lp/cm which equals_____mm/lp . Because a line pair (lp) is equivalent to to a line space adjacent to it , this value may be divided in half to provide the minimum object that is scanner resolve: 0.3
0.6 mm/lp
When one is choosing a window to display Ct image, the width define?
Range Ct numbers( pixels) to displayed
______of Ct window controls the range of the pixel values that are assigned to a shade of grey. Width is centered to a level that is equal to value of tissue interest
Width
Ability of scanner to image a small high density object is controlled by the ____ of the scanner
Spatial resolution
Spatial resolution of Ct scanner controls?
Ability to image small structures
