Transcript Document
Physics Testing for Performance Based Protocol version 2.1 QIBA Group 1C 1 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 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. 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. Specific Instructions for Testing Protocol 1. 2. 3. On accompanying spreadsheet, record: a) b) c) d) e) 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 a) 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. a) Record selected setting on accompanying spreadsheet 4. 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 5. 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: a) Nominal beam collimation (NxT such as 16 x 0.5mm, or 64x0.625mm, 320 x 0.5 mm, etc.) b) Rotation time (in seconds) c) Pitch (Values near pitch 1 are preferred) Specific Instructions for Testing Protocol 6. 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: a) kVp selected in step 2 b) Patient size or SFOV selected in step 3 c) Reconstructed slice thickness selected in step 4 (use contiguous reconstructions) d) Other acquisition parameters (nominal beam collimation, rotation time, pitch) as selected in step 5. e) 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). a) Record mA or mAs or eff. mAs setting on spreadsheet. f) 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) Record the name of recon filter b) Record the size of the line pair object (e.g. 6 lp/cm etc.) observed for that filter b) Select smoothest kernel that allows user to see 7 lp/cm: a) Record the name of recon filter selected b) Provide image (DICOM or jpeg) reconstructed with this filter B30 Zoomed in Zoomed in B45 6 lp/cm 6 lp/cm 7 lp/cm ACR Spatial Resolution Section (Module 4) 7 lp/cm 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: a) kVp selected in step 2 b) Patient size or SFOV selected in step 3 c) Reconstructed slice thickness selected in step 4 (use contiguous reconstructions) d) Other acquisition parameters (nominal beam collimation, rotation time, pitch) as selected in step 5. e) Reconstruction filter as selected in Step 6 f) Select an initial mA (or mAs or effective mAs) setting a) Record initial mA or eff. mAs setting on spreadsheet g) From Reconstructed image, measure standard deviation a) Record standard deviation for this setting on spreadsheet Specific Instructions for Testing Protocol 7.2 If standard deviation is within specification (12±1 HU), then: a) Record mA (or eff. mAs setting) b) Record scanner reported CTDIvol c) Provide image (DICOM or jpeg) using this setting. 7.3 If standard deviation is NOT within specification (12±1 HU), then: a) Adjust mA (or eff. mAs) setting and repeat steps 7.1.f and 7.1.g until standard deviation is within specification. b) When specification is met, then perform step 7.2 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 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 Next Scanner?