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Physics Testing for Performance Based Protocol version 2.1 QIBA Group 1C

Physics Testing for Performance Based Protocol version 2.1 QIBA Group 1C. Goal. From a specification of image quality performance, determine scanner settings which achieve that performance. Statement of Image Quality Performance. QIBA 1C Image Quality Performance Specification: 120 kVp

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Physics Testing for Performance Based Protocol version 2.1 QIBA Group 1C

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  1. Physics Testing for Performance Based Protocolversion 2.1 QIBA Group 1C

  2. Goal From a specification of image quality performance, determine scanner settings which achieve that performance

  3. Statement of Image Quality Performance • QIBA 1C Image Quality Performance Specification: • 120 kVp • Reconstructed slice thickness between 0.75 to 1.25 mm • Beam Collimation (Detector configuration), Pitch and rotation time set to that scan through an entire thorax could be completed in under 15 seconds* • Spatial Resolution of 7 lp/cm on ACR phantom spatial resolution section in ACR Phantom Module 4 • Standard Deviation (proxy for noise) of 12±1 HU in ACR Phantom water equivalent Uniformity Module- Module 3 * Example of a breathold constraint; technically not an image quality performance specification, but related.

  4. General Outline of Testing Protocol • Record required data on accompanying spreadsheet, • Set 120 kVp • Set Scan Field of View or Patient Size (or equivalent) setting to values that are appropriate for phantom being scanned. • Set Slice thickness between 0.75 to 1.25 mm (depending on scanner’s available reconstructed slice thickness) • Set nominal beam collimation (NxT such as 16 x 0.5mm, or 128x0.6mm, 320 x 0.5 mm) rotation time and pitch such that scan can cover a 35 cm thorax in 15 seconds or less • Values near pitch 1 are preferred. • ITERATE (hopefully only a few times) on reconstruction kernels to meet spatial resolution spec. • ITERATE (again, hopefully just a few times) on mAs or effective mAs setting, given beam collimation, pitch and rotation time.

  5. Specific Instructions for Testing Protocol • On accompanying spreadsheet, record: • Scanner Manufacturer (e.g GE) • Model (e.g. VCT) • Site id (e.g Scanner 95) • Software version • Testing date (12-26-09) • Set kVp to 120 or just lower if there is no 120 kVp setting • Record selected setting on accompanying spreadsheet • Select Patient Size (Toshiba), Scan Field of View (SFOV – GE) or equivalent for each scanner. The value selected should be appropriate for size of phantom being scanned (ACR Phantom is 20 cm in diameter); do not use setting for adult thorax. • Record selected setting on accompanying spreadsheet • Set reconstructed slice thickness between 0.75 to 1.25 mm (depending on scanner’s available reconstructed slice thickness) • Record selected setting on accompanying spreadsheet

  6. Specific Instructions for Testing Protocol • Set the following parameters such that the scan could cover a 35 cm thorax in ≤ 15s AND that reconstructed slice thickness selected in step 4 is possible. The accompanying spreadsheet calculates scan time for 35 cm long thorax given selected parameters Select: • Nominal beam collimation (NxT such as 16 x 0.5mm, or 64x0.625mm, 320 x 0.5 mm, etc.) • Rotation time (in seconds) • Pitch (Values near pitch 1 are preferred)

  7. Specific Instructions for Testing Protocol • ITERATE on recon kernels to get 7 lp/cm in ACR phantom 6.1 Scan phantom once through section 4 of ACR CT accreditation phantom, using: • kVp selected in step 2 • Patient size or SFOV selected in step 3 • Reconstructed slice thickness selected in step 4 (use contiguous reconstructions) • Other acquisition parameters (nominal beam collimation, rotation time, pitch) as selected in step 5. • Select mA, mAs/rotation or effective mAs – for this step, use high enough mA to get 200 effective mAs; spreadsheet has calculator to confirm 200 effective mAs (even if you specify mA). • Record mA or mAs or eff. mAs setting on spreadsheet. • Instructions continued on next page

  8. Specific Instructions for Testing Protocol 6.2Reconstruct images using several recon kernels, display with Level =1100, Window=100 • For each recon filter used: • Record the name of recon filter • Record the size of the line pair object (e.g. 6 lp/cm etc.) observed for that filter • Select smoothest kernel that allows user to see 7 lp/cm: • Record the name of recon filter selected • Provideimage (DICOM or jpeg) reconstructed with this filter Zoomed in Zoomed in B30 B45 6 lp/cm 6 lp/cm 7 lp/cm 7 lp/cm ACR Spatial Resolution Section (Module 4)

  9. Specific Instructions for Testing Protocol 7 ITERATE on mA or eff. mAs settings to meet Standard Deviation Specification (12±1 HU) 7.1 Scan and reconstruct images of section 3 (homogeneity) of ACR CT accreditation phantom, using: • kVp selected in step 2 • Patient size or SFOV selected in step 3 • Reconstructed slice thickness selected in step 4 (use contiguous reconstructions) • Other acquisition parameters (nominal beam collimation, rotation time, pitch) as selected in step 5. • Reconstruction filter as selected in Step 6 • Select an initial mA (or mAs or effective mAs) setting • Record initial mA or eff. mAs setting on spreadsheet • From Reconstructed image, measure standard deviation • Record standard deviation for this setting on spreadsheet

  10. Specific Instructions for Testing Protocol 7.2 If standard deviation is within specification (12±1 HU), then: • Record mA (or eff. mAs setting) • Record scanner reported CTDIvol • Provideimage (DICOM or jpeg) using this setting. 7.3 If standard deviation is NOT within specification (12±1 HU), then: • Adjust mA (or eff. mAs) setting and repeat steps 7.1.f and 7.1.g until standard deviation is within specification. • When specification is met, then perform step 7.2

  11. Example of Testing Protocol to Determine Parameters That Meet Specification • Step 7 – Example on Siemens Sensation 64. • Low Dose (25 eff. mAs) • Conventional Dose (250 eff. mAs) Spec: std dev= 12±1 HU • Tech. Params: • 120 kVp • 1mm • Pitch 1 • 0.5 sec • B45 MEETS SPEC! 25 eff. mAs; std dev = 60 250 eff. mAs; std dev = 12.2

  12. Testing Protocol to Determine Parameters That Meet Specification • Conclusion: For Siemens Sensation 64, these settings meet QIBA 1C specifications: • 120 kVp • Reconstructed Slice thickness 1.0 mm • Nominal beam collimation 64x0.6 (actually 32x0.6 with flying focal spot) • Rotation time 0.5 seconds • Pitch 1.0 • Recon kernel B45 • Effective mAs setting 250 • CTDIvol = 18.0 mGys

  13. Next Scanner?

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