Physics Testing for Performance Based Protocol version 3. 0 (Now based on ACRIN 6678) QIBA Group 1 C 1
Goal From a specification of image quality performance, determine scanner settings which achieve that performance
Statement of Image Quality Performance n QIBA 1 C Image Quality Performance Specification: n n n 120 k. Vp 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 6 lp/cm on ACR phantom spatial resolution section in ACR Phantom Module 4 Standard Deviation (proxy for noise) of 17± 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.
General Outline of Testing Protocol n Record required data on accompanying spreadsheet, n n Note that spreadsheet has two tabs to allow two Scan Field of View (or Patient Size) settings to be used if necessary. If Scanner has multiple Patient size (Toshiba) or SFOV (GE) settings, then perform measurements twice, once each for: (a) (b) n n Set Scan Field of View or Patient Size (or equivalent). Set 120 k. Vp Set Slice thickness between 0. 75 to 1. 25 mm (depending on scanner’s available reconstructed slice thickness) Set nominal beam collimation (Nx. T such as 16 x 0. 5 mm, or 128 x 0. 6 mm, 320 x 0. 5 mm) rotation time and pitch such that scan cover a 35 cm thorax in 15 seconds or less n n n SFOV or Patient Size that is appropriate for size of phantom (20 cm diameter) AND SFOV or Patient Size that would be appropriate for adult thorax (approx. 35 cm diameter) 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 m. As or effective m. As setting, given beam collimation, pitch and rotation time.
Specific Instructions for Testing Protocol 1. On accompanying spreadsheet, record: a) b) c) d) e) 2. Set k. Vp to 120 or just lower if there is no 120 k. Vp setting a) 3. Record selected setting on accompanying spreadsheet If Scanner has multiple Patient size (Toshiba) or SFOV (GE) settings, then perform measurements twice, once each for: a) b) c) d) 4. Scanner Manufacturer (e. g GE) Model (e. g. VCT) Site id (e. g Scanner 95) Software version Testing date (12 -26 -09) SFOV or Patient Size appropriate for phantom size (20 cm diameter) AND SFOV or Patient Size appropriate for adult thorax (approx. 35 cm diam) Record selected setting on accompanying spreadsheet Use different tabs for each set of measurements Set reconstructed slice thickness between 0. 75 to 1. 25 mm (depending on scanner’s available reconstructed slice thickness) 1. Record selected setting on accompanying spreadsheet
Specific Instructions for Testing Protocol Set the following parameters such that the scan could cover a 35 cm thorax in ≤ 15 s 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: 5. a) b) c) Nominal beam collimation (Nx. T such as 16 x 0. 5 mm, or 64 x 0. 625 mm, 320 x 0. 5 mm, etc. ) Rotation time (in seconds) Pitch (Values near pitch 1 are preferred)
Specific Instructions for Testing Protocol ITERATE on recon kernels to get 6 lp/cm in ACR phantom 6. 1 Scan phantom once through section 4 of ACR CT accreditation phantom, using: 6. a) b) c) d) e) k. Vp 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 m. A, m. As/rotation or effective m. As – for this step, use high enough m. A to get 200 effective m. As; spreadsheet has calculator to confirm 200 effective m. As (even if you specify m. A). a) f) Record m. A or m. As or eff. m. As setting on spreadsheet. Instructions continued on next page
Specific Instructions for Testing Protocol 6. 2 Reconstruct images using several recon kernels, display with Level =1100, Window=100 a) For each recon filter used: a) b) b) Select smoothest kernel that allows user to see 6 lp/cm: a) b) B 30 Record the name of recon filter Record the size of the line pair object (e. g. 6 lp/cm etc. ) observed for that filter Record the name of recon filter selected Provide image (DICOM AND jpeg) reconstructed with this filter Zoomed in B 45 6 lp/cm 7 lp/cm ACR Spatial Resolution Section (Module 4) 7 lp/cm
Specific Instructions for Testing Protocol 7 ITERATE on m. A or eff. m. As settings to meet Standard Deviation Specification (17± 1 HU) 7. 1 Scan and reconstruct images of section 3 (homogeneity) of ACR CT accreditation phantom, using: a) b) c) d) e) f) k. Vp 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 m. A (or m. As or effective m. As) setting a) g) Record initial m. A or eff. m. As setting on spreadsheet From Reconstructed image, measure standard deviation by placing an ROI of approximately 400 mm 2 at the center of the image a) Record standard deviation for this setting on spreadsheet
Specific Instructions for Testing Protocol 7. 2 If standard deviation is within specification (17± 1 HU), then: a) Record m. A (or eff. m. As setting) b) Record scanner reported CTDIvol c) Provide image (DICOM AND jpeg) using this setting. 7. 3 If standard deviation is NOT within specification (17± 1 HU), then: a) b) Adjust m. A (or eff. m. As) 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
Example of Testing Protocol to Determine Parameters That Meet Specification n Step 7 – Example on Siemens Sensation 64. n 100 eff. m. As Tech. Params: • 120 k. Vp • 1 mm • Pitch 1 • 0. 5 sec • B 30 Spec: std dev= 17± 1 HU MEETS SPEC! 100 eff. m. As; std dev = 17
Testing Protocol to Determine Parameters That Meet Specification n Conclusion: For Siemens Sensation 64, these settings meet QIBA 1 C specifications: n n n n 120 k. Vp Reconstructed Slice thickness 1. 0 mm Nominal beam collimation 64 x 0. 6 (actually 32 x 0. 6 with flying focal spot) Rotation time 0. 5 seconds Pitch 1. 0 Recon kernel B 30 Effective m. As setting 100 CTDIvol = 7. 7 m. Gy
For Scanners with Multiple Patient Size (Toshiba) or SFOV (GE) Settings n n Repeat Steps 1 -7 for 2 nd patient size setting Record results on second tab of spreadsheet
Next Scanner?
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