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