CT * Chapter 6 - Image Quality
Who determines the frequency of QA tests?
-Department's overall QC program -Accrediting agency specification
MTF scale is from ? To ?
0-1( 1 is perfect)
In CT, an object can be differentiated if it has a _____% contrast variation
0.5
On CT images, objects with a ____ % contrast variation can be distinguished .
0.5 %
1% contrast difference corresponds to a difference of ___ HU
10
compared with mA selection, Choices of kvp are more limited. Kvp is usually fixed at ___ to ___
120 to 140
What is the kVp range for routine body CT for adult patients?
120-140kvp
Automatic tube current modulation results in a ___ __ reduction in dose without degrading image quality
15%-40%
When checking uniformity on a phantom CT values should not differ by more than how many HU?
2
Fill in the blank : In CT, when considering IQ, its primarily __A__ rather then ____B_____. ( image thickness aka how data are reconstructed ) ( slice thickness aka how the data were accquired )
A - slice thickness aka how the data were accquired B - image thickness aka how data are reconstructed
Using a standard 512 matrix for all studies, which contains the smallest pixels?
A scan of the internal auditory canals, in which DFOV is 16cm
The operator selects how the data are filtered in the reconstruction process by choosing what ?
A specific algorithm ( can only applied to raw data )
In some Ct units , ATCD is done based on _____ , other units adjust in ______
AP and LAT scout views Real time
QA Tests and How frequently they should be checked
Accuracy and linearity of CT numbers [semi-annually] Spatial resolution (line pairs) [monthly] Contrast resolution, low-contrast resolution/detectability [monthly Noise [weekly] Uniformity [weekly] SSP (slice width) [semi-annually] Laser light alignment [semi-annually] Table indexing and section localization [semi-annually]
Software that automatically adjusts the mAs to fit the specific anatomic region. This is called ?
Automatic tube current modulation ( ATCD)
Why its more common to manipulate the mAs , rather than the kvp when modifying the radiation dose ?
Because : Choice of mA is more flexible The effect of mA on image quality is more straight forward and predictable ( high kvp is needed for penetration )
The more objects visible, the _____ the sytem's low contrast resolution capability.
Better
High noise images appear ?
Blotchy, grainy or spotty
How to measure image noise ?
By scanning a water phantom and computing a mean and standard deviation by use of an ROI
When checking uniformity on a phantom where are the ROIs placed?
Center and Periphery of the phantom
there is a ______ between spatial and contrast resolution
Compromise
What is the ability to differentiate between objects with very similar densities to their background ?
Contrast resolution
What is the formula to calculate voxel size ?
DFOV in mm / ( Matrix * slice thickness )
There are 2 methods to measure spatial/ detail resolution. Name it
Directly - a line paire phantom Data analysis - MTF
___ determines how much, and what section , of the collected raw data are used to create an image
Display view of field ( DVOF )
What is window setting used to display the image ?
Displayed contrast
T/F In CT, KVP change contrast as directly as it does in film screen radiography .
F
With digital technology ,Image quality is directly linked to radiation dose ( T/F)
F
pixels are circle ( y and z dimensions ) T/F
F Pixels are square ( x and Y dimensions)
MTF curve farther to the RT has lower resolution T/F
F - higher resolution
Lower the ratio , the better image T/F
F - higher the ratio
ATCD currently only adjust kvp , Future designs may adjust the Ma alsoT/F
False.Most scanners adjust mAs but future scanners may also adjust KVP
Name few scans geometry you know
Full scan ( 360 *)- most common 1/2 scan (180*) Over scan ( 400*) - use mainly in 4th generation scanners to reduce motion artifact
Temporal resolution is controlled by
Gantry rotation speed Number of detector channels in the sytem Speed with ehich the system can record changing signals ** Temporal resolution is reported in ms
Usually better quality = lower / higher does ?
HIGHER
What is the limiting factor in all scanners ?
Heat
Contrast resolution is improved by using
Higher mAs Thicker slices Smaller pts ( absorb less xray ) Larger pixels ( larger FOV and small matrix ) Smoothing filters *** some of this factors make spatial resolution worse
Define temporal resolution ?
How rapidly data are acquired
When discussing the quality of a CT image, what is meant by temporal resolution?
How rapidly data are acquired; it is controlled by gantry rotation speed, the number of detector channels in the system, and the speed with which the system can record changing signals
In CT, ___directly related to its usefulness in providing an accurate diagnosis .
IQ
Anything that will reduce noise will ___contrast resolution
Improve
( Increasing / Decreasing) Pitch reduces resolution or detail
Increasing
Which is a disadvantage of a bone algorithm?
It reduces the visibility of soft tissue structures.
An image of a water phantom displays a range of pixel values. What can be said about this image?
It's noisy. (because the numbers aren't uniform, if they were there would be no noise)
If mA'S >350 , ___ filament is used
Larger
Smaller the object size, the less/ more contrast resolution
Less
Increasing slice thickness, pixel size and pt dose ( mAs) results in more/ less noise ?
Less noise
Contrast resolution is also called what ?
Low- contrast detectability or system sensitivity
How does beam hardening work?
Lower energy photons are absorbed along the path, leaving higher energy photons to continue on Thus the beam that interacts with the center of a patient has a higher average photon energy than at the periphery of the patient
Name the factors that affect image quality i.e. scan parameters (operator control only )
Ma, kVp, Pitch Scan time FOV Reconstruction algorithm
____ refers how many pixels are present ,
Matrix size
What are factors that effect spatial resolution ?
Matrix size DFOV Pixel size Slice thickness Reconstruction algorithm Focal spot size Pitch Pt motion
Which determines pixel size ?
Matrix size and DFOV
Name 3 factors that distinguish various scanners
Maximum mA Minimum interscan delay Heat dissipation rates
Disadvantage of a higher pitch
May miss pathology, especially small lesions Higher pitch = faster scan= greater distance covered = less possibility of motion problems (but can have other issues)
Define quality control program
Monitors the overall technical performance of the CT system by measuring specific aspects of its function and comparing these measurements to a set standard
What are other influences of image quality ?
Motion Subject contrast ( pt size, contrast media , density of tissue ) IR Focal spot Viewing conditions Observer performance Pixel size Type and generaion of scanner
Which plays an important role in low contrast resolution ?
Noise
What is the undesireable fluctuation of pixel values in an image of homogeneous material ?
Noise ( salt and pepper look )
Define inherent contrast
Physical properties of the object and its background
What is the relationship between slice thickeness and table travel per rotaion during helical scan acquisition called ?
Pitch
What is the formula to calculate pixel size ?
Pixel size (mm) = DFOV / Matrix size
how to determine Pixel size
Pixel size = dFOV/matrix size
KVP defines ___ of xray beam
Quality
____is a subject motion and is dependent on the purpose for which the image was acquired .
Quality
2 components of a quality control program
Quality assurance (QA): the measurement of the system Quality control (QC): implementation of corrective actions
What is an expression of the desired signal to the background ?
SNG - signal to noise ratio
___ determines the area , within the gantry , for which raw data are acquired .
Scan field of view ( SFOV )
What is the tube rotation during the acquisition for each slice?
Scan geometry
Thinner slices produce _____ image and the thicker the slice the more ____neccesary.
Sharper Flattening
A ___ ___ ___is critical in avoiding image degradation as a result os pt motion .
Short scan time
A scanner's low contrast resolution can be measured using a phantom that contains objects of varying sizes and with ( small/ big )difference in density from the background
Small
If mA's <350 , ___ filament is used
Small
Best detail of image obtained by ___ FOV and ____ matrix
Small FOV Large matrixl
Best detail of image obtained by _____ pixel and ____slice thickness
Small pixel Thin slice thickness
Detail resolution is also called ?
Spatial resolution
What is the ability to resolve ( as separate object ) small , high contrast objects ?
Spatial resolution
What are 2 features are used to measure Image Quality ?
Spatial resolution Contrast resolution
What two main features are assessed to measure image quality?
Spatial resolution and contrast resolution
CT is ( superior/ inferior ) to all other clinical modalities in its contrast resolution
Superior
Greater picth produces loss of quality T/F
T
Quantum noise increases contrast resolution T/F
T
T/F Smalle focal spot gives better detail but in CT , the effect is minimal
T
What is spatial frequency?
The number of line pairs visible per unit length
What is modulation transfer function - MTF ?
The ratio of the accuracy of the image compared with the actual object scanned .
Define linearity
The relative accuracy between calculated CT numbers and their respective linear attenuation coefficients
This effect makes CT physics somewhat different from that of film screen radiography
The uncoupling effect
Even when mA and kvp setting is too high, a good quality image is produced. When a setting is too low, an unacceptable image will result due to Quantum Noise . This is called ?
The uncoupling efffect
To create an image , the system must flatten the scan _____ into 2 dimensions (a flat image )
Thickness
Spatial resolution units
Units = line pairs/cm (lp/cm or lp cm-1)
Describe direct measurement of spatial resolution
Using a line pair phantom , phantom scanned and visible strips are counted .
How is contrast resolution measured ?
Using phantoms that contain objects of varying sizes and with a small difference in density ( from 4- 10 HU ) from the background
Low noise image appear ?
Very smooth
Slice thickness is also called
Voxel
What is the true test of the quality of a specific image?
Whether it serves the purpose for which it was acquired
What do Detail algorithms do?
accentuate the differences between neighboring pixels to optimize spatial resolution, but sacrifice low-contrast resolution.
Examples of contrast resolution?
brain white and grey matter, liver lesions
Which term describes an isotropic voxel?
cube
By______ the size of the pixel, we can increase our chance of accurately representing a small object.
decreasing
By making the pixel size half of the object we will_____ the likelihood that the object is accurately resolved on the image.
increase
What is a partial scan?
less than < 360° rotation Typically 180 ° plus the angle of the fan beam Inferior to full scan
The ability to differentiate a structure that varies only slightly in density from its surrounding is referred to as
low contrast resolution
What do detail algorithms sacrifice?
low contrast resolution
A graphical representation of the CT systems capability or passing information through it to the observer is called
modulation transfer function
What is the most commonly used method of describing spatial resolution?
modulation transfer function
What is a full scan
one 360° rotation of tube
What do smoothing algorithms do?
reduce the differences between adjacent pixels, but sacrifices spatial resolution
Which results in imgae noise ?
scatter radittion
What do smoothing algorithms sacrifice?
spatial resolution
Who performs the QA tests?
technologist or medical physicist
Define Image quality in CT
the comparison of the image to the actual object
What is an overscan?
uses information > < 360° rotation Typically adds width of sFOV to scan, resulting in 400 ° Used mostly in the old 4th generation scanners
How do beam hardening artifacts appear?
when the degree of beam hardening exceeds the CT system's ability to correct during the reconstruction process