CT for technologists CHP 6.

5. Define spatial resolution.

Spatial resolution (also called detail resolution) is the systems ability to resolve, as separate forms, small objects that are very close together.

6. How is the CT image affected by too low a mAs setting? How is it affected if the mAs setting is too high?

Too low a mAs setting will cause quantum mottle. Increasing mAs will improve contrast resolution, but this will increase dose to the patient.

3. What factors defines the intensity of the x-ray beam?

kVp setting defines the quality, or average energy, of the beam.

9. A: List the factors that affect spacial resolution. B: List the factories that affect contrast resolution.

A: Matrix size, DFOV, pixel size, slice thickness, reconstruction algorithms, pitch, patient motion, focal spot size. B: mAs, pixel size, slice thickness, reconstruction algorithm, patient size.

Higher ma settings

Allow shorter scan times to be used.

8. How does a change in slice thickness affect the spacial resolution? How does it affect contrast resolution? What affect will it have on patient dose?

Because thicker slices allow more photons to reach the detectors they have a better SNR and appear less noisy. However, this improvement comes at the cost of spacial resolution in the z-axis. Because the image appears to have less noise contrast resolution will improve. Slice thickness does not affect patient dose, but thinner slices increases noise which may temp the user to increase mAs which would in tern increase the patient dose.

7. Give examples of examinations in which images my need to be reconstructed twice, using different algorithms.

Deciding what algorithm to use depends on what data needs to be enhanced or suppressed. Some algorithms will "smooth" the data more heavily, by reducing the difference between adjacent pixels, this helps to reduce the appearance of artifacts (fillings), but do so at the cost of special resolution. Conversely, some filters accentuate the difference between neighboring pixels to optimize spacial resolution, at the cost of low contrast resolution. Certain studies require the data to be reconstructed with two separate algorithms. One optimizes low contrast detectability in the soft tissue. The second provides optimal high spacial resolution and is preferable for bone. ( example = inner ear = tissue densities are limited to bone and air.)

1. What factors are referred to as scanning parameters?

Factors that the operator can control and are referred to as scanning parameters are: mA level, scan time, slice thickness, FOV, reconstruction algorithm, kVp, pitch.

2. What factors define the quantity of x-ray energy produced?

Milliampere level and scan time together define the quantity of the X-ray energy.

4. Define partial scan and overscan.

Partial scan: It is possible to obtain a partial scan, which is acquired from 180° plus the degree of arc of the fan angle. These are referred to as half scans. They are inferior to the standard 360° scans, but they are good for studies that require sharp scan times. Overscan: a 400° scan is called an overscan. These scans acquire the same amount of data as the regular 360° scans and an additional 40°. Overscans were used to help fix the problem of motion by allowing some overlap of data from the first and last tube positions.