chapter 6 from computed temography for technologist

1. Chapter 6 image quality

2. • SCANNING PARAMETERS
1) milliampere (mA) level
2) scan time
3) slice thickness
4) field of view
5) reconstruction algorithm
6) kilovolt-peak (kVp).
7) pitch in helical scan only.
-The total x-ray beam exposure in CT is dependent on a
combination of mA setting, scan time, and kVp setting.
mA and scan time together are referred to as mAs and
defines the quantity of the x-ray energy. kVp setting
defines the quality (average energy) of the x-ray beam.

3. • Milliampere-Second Setting(mAs):
- Thermonic emission , filament current and tube
current.
- Increasing the mA increases the number of electrons
that will produce x-ray photons.
- Use of a small fi lament size concentrates the focal
spot, reducing the penumbra, but cannot tolerate
↑mA.
- Larger filament→↓resolution.

4. - scan time is the time the x-ray beam is on for the
collection of data for each slice(the time it takes for
the gantry to make a complete 360° rotation).
- Typical choices of scan time for a full rotation range
from 0.5 to 2 seconds, in cardiac CT (0.35 to 0.45
seconds).
- Higher mA settings allow shorter scan times to be
used. A short scan time is critical in avoiding image
degradation as a result of patient motion.
- Used short scan time to avoided involuntary
movement such as peristalsis and cardiac motion.

5. - The degree to which involuntary motion affects an image
is largely dependent on the area scanned.
- ↑mAs→↑ heat produced→need↑ cooling.
- The factors aff ecting the mAs selected for a CT study are
basically the same as in conventional radiography: the
thicker and denser the part being examined, the more
mAs that is required to produce and adequate image.
- Differences in mAs of less than 20% may not result in a
visible change on the image.
- For example 280mAs have Some choices: 0.4 seconds
and 700 mA (280 mAs), 0.6 seconds and 460 mA (276
mAs), 0.8 seconds and 340 mA (272 mAs), 1.0 second
and 280 mA (280 mAs), and 2.0 seconds and 140 mA
(280 mAs).

6. • Tube Voltage or Kilovolt Peak
- In CT, kVp does not change contrast as directly as it does
in film-screen radiography.
- ↑kVp→↑beam intensity→↑penetrability.
- In adult routine exams choose 120 to 140 kVp, in
pediatric 80kVp.
• Impact of mAs and kVp Settings on Radiation Dose
- To ↓radiation dose to the patient:
1) ↓mAs+ kVp constant
2) constant mAs+ ↓kVp
So The appropriate selection of mAs and kVp is critical to
optimize radiation dose to the patient and image
quality.

7. • two reasons to change mAs rather kVp
- First, the choice of mA is more flexible (from 20 to
800 mA)
- effect on image quality is more straightforward and
predictable.
• The Uncoupling Effect
- The relationship between radiation dose and CT image
quality is complex not like FSC (↑kVp+↑mAs→↑Pt
dose” over-exposed”).
- The uncoupling effect does not play a role when the mA
or kVp setting is too low,because quantum noise will
result and provide evidence of the inadequate exposure
settings.

8. • Uncoupling Effect—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.
• Automatic Tube Current Modulation
- Software that automatically adjusts the tube current
(mAs) to fit specific anatomic regions is increasingly
used in clinical practice.
- These automatic exposure control
techniques report a 15% to 40% reduction in dose,

9. • Slice Thickness
- ↑S.T→↑detail→↑spatial Resolution.
• Field of View
- field of view (SFOV) determines the area, within
the gantry, (DFOV) determines how much, and what
section, of the collected raw data are used to create an
image.
• Reconstruction Algorithms
- By choosing a specific algorithm, the operator selects
how the data are filtered in the reconstruction process.
Filter functions can only be applied to raw data (not
image data). Therefore, to reconstruct an image using a
different filter function, the raw data must be available
for that image. It is important to differentiate
reconstruction algorithms from merely setting a window
width and level.

10. • Pitch
- Pitch is the relationship between slice thickness and
table travel per rotation during a helical scan
acquisition.
• SCAN GEOMETRY
- Another factor is tube arc (180o,360oand 400o ”
overscan “. full scan [360° (full scan) + 40 (typical
field of view) = 400° scan used in 4th generation].
- Overscan→ overlap of data from the first and last
tube positions, reduced motion artifacts.

11. • IMAGE QUALITY DEFINED
- In CT, image quality is directly related to its usefulness in
providing an accurate diagnosis.
- Image quality relates to how well the image represents
the object scanned. However, the true test of the quality
of a specific image is whether it serves the purpose for
which it was acquired.
- The two main features used to measure image quality
are:
Spatial Resolution—the ability to resolve (as separate
objects) small, high-contrast objects.
Contrast Resolution—the ability to differentiate between
objects with very similar densities as their back ground.

12. • SPATIAL RESOLUTION
- Spatial resolution can be measured using two
methods.
1) It can be measured directly
2) it can be calculated from analyzing the spread of
information within the system. This latter data
analysis is known as the modulation transfer
function (MTF).
• Direct Measurement of Spatial Resolution
- Using a line pairs phantom(made of acrylic and
has closely spaced metal strips).

13. - The phantom is scanned, and
the number of strips that are
visible are counted.
- Line pair (line +space).
- if 20 lines can be seen in a 1-
cm section in an image of the
phantom, the spatial
resolution is reported as 20
line pairs per centimeter
(lp/cm).
- Spatial Frequency
Is The number of line pairs
visible per unit length.

14. • Evaluating Spatial Resolution Using the MTF
- Used in Ct also in Conventional radiography.
- It is often used to graphically represent a system’s
capability of passing information to the observer.
- The MTF is the ratio of the accuracy of the image
compared with the actual object scanned.
- If the image reproduced the object exactly,
the MTF of the system would have a value of 1.
- If the image were blank and contained no
information about the object, the MTF would be 0

15. -As expected, this graph
shows that as the size of the
object increases, the MTF also
increases.
-The relationship is not linear;
hence an object twice the size
of another object may not
necessarily possess twice the
image fidelity. (MTF indicates
image fidelity)

16. - The limiting resolution is
the spatial frequency
possible on a given CT
system, at an MTF equal
to 0.1. In this example,
the limiting resolution of
scanner A is 4.3 and
scanner B is 5.0
- Spatial resolution in
conventional radiography
more better than CT.

17. • In-Plane Versus Longitudinal Resolution
- in-plane resolution: the resolution in x-y direction
- Longitudinal resolution : resolution in z direction.
• Factors Affecting Spatial Resolution
- Depending on quality of raw data and the
reconstruction method.
• Matrix Size, Display Field of View, Pixel Size
- Pixel size plays an important role in the in-plane
spatial resolution of an image
- DFOV determines how much raw data will be used to
reconstruct the image.

18. - Pixel size = (DFOV/matrix size)
- If an object is smaller than a
pixel, its density will be
averaged with the density of
other tissues contained in the
pixel, creating a less accurate
image.
- When pixels are smaller, it is
less likely that they will contain
different densities, therefore
decreasing the likelihood of
volume averaging

20. • Slice Thickness
- 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.
- The matrix divides data into squares with an x and y
dimension. The operator’s selection of slice thickness
accounts for the z axis.
- Slice thickness plays an important role in volume
averaging, thereby affecting spatial resolution in the
image. New CT scanners allow for very thin slice
thickness; often the goal is to produce isotropic voxels.

21. - An isotropic voxel is a cube, measuring the same in the x,
y, and z directions.
- 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.
- An isotropic voxel ensures that there is no data
loss with either multiplanar reformation (MPR) or
volume rendering (VR).
- Sampling Teorem(Nyquist Sampling Theorem)
- because an object may not lie entirely within
a pixel, the pixel dimension should be half the size of
the object to increase the likelihood of that object being
resolved.

22. - This theorem accounts for the element of random chance in the creation
of a CT image.
- Random chance plays a role in whether a small
object will be seen on the reconstructed image. In (A), (B), and (C) the
object to be displayed is the same size as the pixel. The three figures show
different scenarios as to how the object could be reconstructed, each
resulting in a different level of volume averaging. In (D) and (E), a smaller
pixel size is used, and the scenarios regarding the likelihood of volume
averaging improve.

23. • Reconstruction Algorithm
- The appropriate reconstruction algorithm depends on
which parts of the data should be enhanced or
suppressed to optimize the image for diagnosis.
 Smooth: the data more heavily, by reducing the
difference between adjacent pixels→↓artifact but ↓
spatial resolution.
 the internal auditory canal in which the tiny bones of the
inner ear are displayed→ the image can be reconstructed
for spatial rather than contrast fidelity(These types of
high-contrast reconstruction algorithms are often called
bone or detail filters).
- the high-contrast filter produces a noisy effect

24. • Focal Spot Size
- larger focal spots →↑ geometric unsharpness →↓
spatial resolution.
• Pitch
- increasing the pitch reduces resolution.
- The effect in SDCT more than in MDCT systems
because data interpolation.
• Patient Motion
- Motion creates blurring in the image and degrades
spatial resolution ( using minimum time).

25. • Contrast resolution (low-contrast sensitivity or low
contrast detestability):
- Conventional radiography is 5% difference in contrast
from its background material, whereas CT is 0.5%
contrast variation.
- Contrast resolution is measured using phantoms that
contain objects, typically
cylindrical, of varying sizes and
with a small difference in density
(typically from 4 to 10 HU).

26. • Noise
- Noise is caused by the combination of many factors,
the most prevalent being quantum noise, or
quantum mottle.
- Quantum mottle occurs when there are an
insufficient number of photons detected.
- In CT, the number of x-ray photons detected per
pixel is also often referred to as signal-to-noise ratio
(SNR)

27. • Factors Aff ecting Contrast Resolution:
1) mAs/Dose
- Doubling the mAs of the study increases the SNR by
40% →↓quantum noise but The dose increases linearly
with mAs per scan.
2) Pixel Size
- as pixel size decreases, the number of detected x-ray
photons per pixel will decrease→↑noise.
3) Slice Thickness
- . Because thicker slices allow more photons to reach
the detectors they have a better SNR and appear less
noisy.

28. 4) Reconstruction Algorithm
- bone algorithms produce lower contrast resolution (but
better spatial resolution), whereas soft tissue
algorithms improve contrast resolution at the xpense of
spatial resolution.
5) Patient Size
- larger patients attenuate more x-rays photons, leaving
fewer to reach the detectors. This reduces SNR,
increases noise, and results in lower contrast resolution.
• Other Contrast Resolution Considerations
- small objects are more difficult to see than larger objects.
- The relationship between object size and visibility is called the
contrast-detail response.

29. • TEMPORAL RESOLUTION
- The temporal resolution of a system refers to how
rapidly data are acquired.
- Temporal resolution 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.
- High temporal resolution 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).