Chapter 6 image quality

• SCANNING PARAMETERS1) milliampere (mA) level2) scan time

3) slice thickness4) field of view5) reconstruction algorithm6) 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.

• 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.

- 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.

- 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).

• 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.

• two reasons to change mAs rather kVp

- First, the choice of mA is more flexible (from 20 to800 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 mAor kVp setting is too low,because quantum noise willresult and provide evidence of the inadequate exposuresettings.

• 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 controltechniques report a 15% to 40% reduction in dose,

• 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 whatsection, 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.

• 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.

• 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 separateobjects) small, high-contrast objects.

Contrast Resolution—the ability to differentiate between objects with very similar densities as their back ground.

• 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 andhas closely spaced metal strips).

- 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 FrequencyIs The number of line pairs visible per unit length.

• Evaluating Spatial Resolution Using the MTF

- Used in Ct also in Conventional radiography.

- It is often used to graphically represent a system’scapability 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

-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)

- The limiting resolution is the spatial frequencypossible 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.

• 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 toreconstruct the image.

- 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 isless likely that they will contain different densities, therefore decreasing the likelihood of volume averaging

• 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.

- 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 dataloss with either multiplanar reformation (MPR) or volume rendering (VR).

- Sampling Teorem(Nyquist Sampling Theorem)

- because an object may not lie entirely withina pixel, the pixel dimension should be half the size ofthe object to increase the likelihood of that object beingresolved.

- This theorem accounts for the element of random chance in the creation of a CT image.

- Random chance plays a role in whether a smallobject 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.

• 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

• 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).

• 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 thatcontain objects, typically

cylindrical, of varying sizes and

with a small difference in density

(typically from 4 to 10 HU).

• 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 perpixel is also often referred to as signal-to-noise ratio (SNR)

• 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 reachthe detectors they have a better SNR and appear lessnoisy.

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.

• 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 canrecord changing signals.

- High temporal resolution is of particular importancewhen imaging moving structures (e.g., heart) and for studies dependent on the dynamic flow of iodinated contrast media (e.g., CT angiography, perfusion studies).