Image Quality and Quality Assurance in Computed Tomography Flashcards
relates to how well the image represents the object scanned.
Image quality
In CT, image quality is directly related to
its usefulness in providing an accurate diagnosis
critical to optimize radiation dose to the patient and image quality
appropriate selection of mAs and kVp
allow shorter scan times to be used
Higher mA settings
avoiding image degradation as a result of patient motion
short scan time
Dose is also reduced if
kVp is reduced while the mAs is held constant.
using digital technology, the image quality is not directly linked to the dose, so even when an mA or kVp setting that is too high is used, a good image results.
Uncoupling Effect
two main features used to measure image quality are:
Spatial Resolution
Contrast Resolution
the ability to resolve (as separate objects) small, high-contrast objects.
Spatial Resolution
the ability to differentiate between objects with very similar densities as their background.
Contrast Resolution
Spatial resolution is also known as
detaiil resolution
This is the system’s ability to resolve, as separate forms, small objects that are very close together.
Spatial resolution
Spatial resolution can be measured using two methods:
- Direct measurement using a phantom.
2. Data analysis is known as the modulation transfer function (MTF).
made of acrylic and has closely spaced metal strips imbedded.
line pairs phantom
The phantom is scanned, and the number of strips that are visible are counted.
Direct measurement using a phantom
number of line pairs visible per unit length.
spatial frequency
If objects are large, not many will fit in a given length
They are said to have low spatial frequency.
If the objects are smaller, many more will fit into the same length.
These are said to have high spatial frequency.
most commonly used method of describing spatial resolution ability, not only in CT, but also in conventional radiography
Modulation Transfer Function (MTF)
the ratio of the accuracy of the image compared with the actual object scanned
Modulation Transfer Function (MTF).
indicates image fidelity
MTF
The MTF scale is from
0 to 1
If the image reproduced the object exactly,
MTF of the system would have a value of 1.
If the image were blank and contained no information about the object
MTF would be 0.
actual MTF calculated from most objects is between these two extremes
it will have a value between 0 and 1.
Resolution in the xy direction is called
in-plane resolution
resolution in the z direction is called
longitudinal resolution.
The greater the total pixels present in the image
the smaller each individual pixel
determines how much raw data will be used to reconstruct the image.
display field of view (DFOV)
Increasing the DFOV
increases the size of each pixel in the image.
reflects how much patient data is contained within each square
pixel size
will include more patient data
large pixel
The relationship between pixel size, matrix size, and DFOV is apparent in the equation:
pixel size = DFOV/matrix size
thinner slices produce
sharper images because to create an image the system must flatten the scan thickness (a volume) into two dimensions (a flat image)
The thicker the slice
the more flattening is necessary
improves the images’ longitudinal resolution.
Narrowing the slice
When the imaging voxel is equal in size in all dimensions
there is no loss of information when data are reformatted in a different plane
depends on which parts of the data should be enhanced or suppressed to optimize the image for diagnosis
appropriate reconstruction algorithm
Bone algorithms produce
lower contrast resolution (but better spatial resolution)
soft tissue algorithms improves
improve contrast resolution at the expense of spatial resolution.
larger focal spots cause
more geometric unsharpness in the image and reduce spatial resolution
increasing the pitch
reduces resolution
depends on the detector configuration and the CT projection data interpolation scheme used.
Optimal choice of pitch
creates blurring in the image and degrades spatial resolution.
Motion
may help improve spatial resolution to the extent that they may reduce the effects of both involuntary motion (e.g., heart) and overt patient motion.
Shortened scan times
Contrast resolution also known as
low-contrast sensitivity
This ability to distinguish an object that is nearly the same density as its background is referred to as
low contrast detectability
superior to all other clinical modalities in its contrast resolution
CT
the undesirable fluctuation of pixel values in an image of a homogeneous material
Image noise
as the grainy appearance or “salt-and-pepper” look on an underexposed image.
Noise
occurs when there are an insufficient number of photons detected.
Quantum mottle
In CT, the number of x-ray photons detected per pixel is also often referred to as
signal-to-noise ratio (SNR)
influences the number of x-ray photons used to produce the CT image, thereby affecting the SNR and the contrast resolution.
mAs selected
will improve contrast resolution, but at the cost of a higher radiation dose to the patient.
increasing mAs
Keeping all other scan parameters the same, as pixel size decreases,
the number of detected x-ray photons per pixel will decrease.
Because thicker slices allow more photons to reach the detectors
they have a better SNR and appear less noisy
larger patients attenuate more x-rays photons, leaving fewer to reach the detectors
This reduces SNR, increases noise, and results in lower contrast resolution.
refers to how rapidly data are acquired
Temporal resolution
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.
Temporal resolution
Temporal resolution is typically reported in
milliseconds (ms)
is of particular importance when imaging moving structures (e.g., heart) and for studies dependent on the dynamic flow of iodinated contrast media (e.g., CT angiography, perfusion studies).
High temporal resolution
designed to ensure that the CT system is producing the best possible image quality using the minimal radiation dose to the patient
Quality control programs
Quality assurance programs should be designed around three basic concepts:
- The tests that make up the program must be performed on a regular basis
- The results from all tests must be recorded using a consistent format
- Documentation should indicate whether the tested parameter is within specified guidelines
can be calculated from the analysis of the spread of information within the system using the MTF.
Spatial Resolution
given as the maximum number of visible line pairs (lead strip and space) per millimeter
Spatial Resolution
The spatial resolution of current scanners when images are reconstructed in a high-resolution algorithm is in the range of
10 to 20 lp/cm
To evaluate contrast resolution a phantom is used that contains
objects of varying sizes
At the minimum, contrast resolution should be such that with a density difference of
0.5% a 5-mm object can be displayed
Measurements of selected slice thickness are determined using a phantom
that includes a ramp, spiral, or step-wedge.
For a slice thickness of 5 mm or greater
the slice thickness should not vary more than ±1 mm from the intended slice thickness.
For a slice thickness of less than 5 mm
the slice thickness should not vary more than ±0.5 mm.
Slice Thickness Accuracy
test is usually performed
semiannually
Contrast Resolution (Low-Contrast Resolution) test is performed
monthly in most programs
are used extensively for patient positioning and alignment
Laser lights located both inside and outside the gantry
The light field should coincide with the radiation field to within
2mm
Laser Light Accuracy test is usually performed
semiannually
Phantom used in noise and uniformity test
water phantom
is measured by obtaining the standard deviation (SD) of the CT numbers within a region of interest (ROI)
Noise
refers to the ability of the scanner to yield the same CT number regardless of the location of an ROI within a homogeneous object
Uniformity
refers to the relationship between CT numbers and the linear attenuation values of the scanned object at a designated kVp value
Linearity
Phantoms used for Radiation Dose
made using standard head and body CT dose index (CTDI) phantoms and a pencil ionization chamber.
are defined as anything appearing on the image that is not present in the object scanned.
Artifacts
Artifacts can be broadly classified as:
- physics-based (resulting from the physical processes associated with
data acquisition), - patient-based,
- or equipment-induced
X-ray beam passes through an object, lower-energy photons are preferentially absorbed, creating a “harder” beam.
Beam Hardening
The beam is hardened more by
dense objects (e.g., more by bone and less by fat).
Two types of artifact can result from this effect (beam hardening),
cupping artifacts (the periphery of the image is lighter) and the appearance of dark bands or streaks between dense objects in the image
CT systems use three features to minimize beam hardening:
filtration, calibration correction, and beam hardening correction software
The best strategy available to the operator to avoid beam hardening is to select the appropriate
SFOV to ensure the correct filtration, calibration, and beam-hardening correction software is used.
can occur when dense objects lie to the edge of the SFOV and are only present in some of the views used to create the image
Partial volume artifacts
The best method of reducing partial volume artifacts
Use thinner slices
Insufficient projection data (for instance, when the helical pitch is greatly extended) is known as
undersampling
Undersampling causes inaccuracies related to reproducing sharp edges and small objects and results in an artifact known as
aliasing
in which fine stripes appear to be radiating from a dense structure.
Aliasing
Aliasing artifacts can be combated by
slowing gantry rotation speed (i.e., increasing scan time) or by reducing the helical pitch
results in streak artifact or shading (both light and dark) arising from irregularly shaped objects that have a pronounced difference in density from surrounding structures
The edge gradient effect
typically appear as shading, ghosting (objects appear to have a shadow), streaking or blurring
Motion artifact
Manufacturers have built features into the CT systems to reduce motion artifacts such as
overscan and partial scan modes, software correction, and cardiac gating
presence of metal objects in the scan field can lead to severe streaking artifacts.
Metallic artifacts
They occur because the density of the metal is beyond the normal range that can be handled by the computer, resulting in incomplete attenuation profiles.
Metallic artifacts
caused by anatomy that extends outside of the selected SFOV
Out-of-field artifacts
occur with third-generation scanners and appear on the image as a ring or concentric rings centered on the rotational axis.
Ring artifacts
A common cause of equipment-induced artifact occurs when there is an undesired surge of electrical current (i.e., a short-circuit) within the x-ray tube.
Tube Arcing
lines appear in a windmill formation
Cone beam effect
Occur in helical scanning attributable to the helical interpolation and reconstruction process
Helical and Cone Beam Effect
