• Topic ID: id_11038793
  • Version: 2.0
  • Date: Jan 30, 2019 9:42:50 PM

Image Quality Test Procedures

1 What to Check for IQ

This CT scanner uses 16 rows of detectors with 4 rows of output, which produces up to four images at one rotation. It also utilizes data from all selected rows in helical reconstruction by using different weighting schemes to produce better quality helical images. The scanner is also designed to perform longer helical scans at lower mAs. Because of the differences, when checking the image quality (IQ), we need to pay attention to those artifacts/appearance that are unique to the this generation of CT scanner. The following are some of the examples:

  1. Ring/band artifacts may appear as partial or non-uniform ring/band in helical images. When this happens, axial scans may need to be taken to localize the bad DAS channel(s) or detectors.

  2. If artifacts happen in images of one of the outer rows, it may indicate a misalignment of beam in Z-direction (BOW). Again, axial scans may need to be taken to further isolate the problems if the problems happened in the helical images.

  3. This CT scanner uses 16 detectors that cover more space in Z-direction than CT/i. When using thicker slice configurations (5mm/4i), axial images made by the two outer rows are more susceptible to cone beam artifact (an example of a cone beam artifact is shading off a rib into the liver). Also, the use of peristaltic correction can increase the cone beam effect. In general, the use of thinner slice configurations reduces cone beam artifact. Also, helical mode will help reduce cone beam artifact, with HQ mode having less cone beam artifact than HS mode.

Specifically, the IQ checks may include the following:

  1. Alignment

    1. MTF with GE Performance phantom, both axial and helical images, comparing with spec.

    2. Aliasing test, both cold and hot.

    3. Z-Beam motion.

  2. Noise/Artifacts

    1. Noise measurement with 20 cm water phantom, comparing with spec.

    2. Cone beam artifact.

  3. Microphonics - Check image quality (lack of rings, streak and/or center artifacts).

  4. Clever DAS Gain - Confirm that the DAS gains used in the patient scans match those in the cals.

  5. CT Number Uniformity - Check both water and poly phantoms’ CT number uniformity, comparing with spec.

2 How to Check Image Quality

For artifact definitions and numerical measurements, see Installation Manual. For Image Troubleshooting, see Installation Manual. However, if artifacts happened in helical images, take axial scans to further isolate the problems.

2.1 Alignment

2.1.1 MTF

Values should be similar to those of CT/i.

  • Phantom: GE performance phantom (if available). The wire section of the GE performance phantom is not thick enough to have 2 10mm thick images. Two scans need to be done to verify both sides (2A1A and 1B2B) of the detector.

  • Data Collection:

    1. Center the phantom in x-y direction.

    2. Landmark (in z-direction) at the center of the wire.

    3. Take scans per Table 1.

  • Data Analysis: Use the “Image Resolution” button in the “Image Analysis” tool kit to measure system MTF, compare with spec. Use images at S0.

2.1.2 Aliasing

Check ISO alignment, and check if hot ISO was executed correctly.

  • Phantom: GE QA phantom, section No. 1.

  • Data Collection:

    1. Fastcal 120 kV, head bowtie.

    2. Let tube cool 30 minutes after last fastcal or scanning.

    3. Center the GE QA phantom and take scans per Table 2.

    4. Take heating scans using the protocol for QOEC tube heating.

    5. Take scans in (4) again.

  • Data Analysis: Inspect images visually. Both cold and hot tube images should be lack of aliasing artifacts.

2.1.3 Z-Beam Motion

If artifacts happened in one of the outer rows, it may suggest that the BOW is not aligned correctly.

  • Phantom: GE QA Phantom

  • Data Collection:

    1. Fastcal 120kV, head Bowtie

    2. Let tube cool 1 hour after last fastcal or scanning

    3. Center the QA phantom water section, take scans per Table 3.

    4. Scan a series of 30 scans at 4sec with 4sec ISD per Table 4.

    5. Repeat scans in (4) again.

      Data Analysis: Check images visually, pay attention to the two outer rows. No failing ring and/or band artifacts should occur

2.2 Noise

Image noise should meet the spec.

  • Phantom: GE QA phantom.

  • Data Collection:

    1. Fastcal, 120kV, Head Bowtie.

    2. Center the QA phantom water section, and take scans per Table 5.

Data Analysis: Place ROI in the center and measure image noise (standard deviation). The spec is 2.7 < Stdv < 3.3 for both axial and helical scans at the above techniques with peristaltic off.

2.3 Cone Beam Artifact

An example of a cone beam artifact would be shading off a rib into the liver. The greater the cone angle (slice thickness in Z-direction), the more severe the cone beam artifact. Cone beam artifact is typically more pronounced in the images from the two outer rows, where the POR is not as accurate as in the inner rows, especially when using thicker slices. Images obtained using thinner slice configurations should improve image quality in terms of cone beam artifact. Helical reconstruction uses all selected row outputs to minimize cone beam artifacts.

2.4 Clever DAS Gain

A multiple DAS gain scheme is used to reduce the low signal image noise. For body bowtie, scan techniques (kV/Collimation/Spot Size) are further divided into different mA ranges based on the detector collimation. For example, for the technique of 120kV/bbt/4x500/LS we have three different mA ranges:

  • 210 - 240 mA (gain 17)

  • 250 - 300 mA (gain 21)

  • 310 - 440 mA (gain 31).

Artifacts (ring, band and center spot) may be caused by a failure of a DAS channel at one of the gains but not all of them (i.e., an image artifact at a certain mA range but not others). Run DAS check to identify or exclude DAS problems.

2.5 Microphonics

Scan phantom at low signal level to test insensitivity of the DAS, Detector, and interconnect system to noise generated by vibration sources. Sources of vibration could be gantry rotation, tube rotor, fans pumps, dust in DAS etc.

  • Phantom: 48cm Poly phantom

  • Data collection:

    1. Fastcal, 120 kV, Body Bowtie

    2. Center 48cm poly phantom, and take scans per Table 14.

  • Data Analysis: Images should have no visible rings/bands, streak and center artifacts

2.6 CT Number Uniformity

  • Phantom: GE QA, 35cm Poly and 48 Poly phantoms.

  • Data Collection: Perform 1X series scans.

  • Data Analysis: Use the “Series Means” button in the “Image Analysis” tool kit to analyze the CT number uniformity.