7/25/2009 Practical Procedures for Achieving and Maintaining ACR CT Accreditation • Requirements for physics testing • Practical aspects of ACR testing CT systems • Performance P f off pre-application li ti testing t ti and annual surveillance testing • Establishment of routine quality assurance protocols and record keeping David R. Pickens, PhD Vanderbilt University Medical C t Center Nashville, Tennessee ©D Pickens AAPM 2009 Outline 1 ©D Pickens AAPM 2009 2 Image Requirements ACR Web Page for CT Accreditation http://www.acr.org/accreditation/computed.aspx Source for instructions, forms, dose spread p sheets, and FAQ ACR also has people available by phone to answer questions ©D Pickens AAPM 2009 3 Physics Testing Clinical Testing • SMPTE • Phantom Alignment • CT Number Calibration and slice thickness • Low L contrast t t resolution l ti • Uniformity • High contrast resolution • CTDI Images of head, body, pediatric phantom • Specific images from representative studies done using protocols at the testing location • Images from studies performed with the system being tested • Specific instruction for head/neck, chest, abdomen for adult and pediatric examinations ©D Pickens AAPM 2009 4 Video Test Pattern for First Box of Film Sheet Film Output for Submission Of Phantom Images SMPTE test pattern or similar required in first box of each of two submitted sheets If the facility has a film printer the submitted printer, physics images must be on film in 3 x 4 format, up to 21 images. No aliasing of bar patterns or other artifacts 95% square must be visible (white) 5% square must be visible (black) Facilities with no film printers can submit physics images on CD (call ACR) ©D Pickens AAPM 2009 5 ©D Pickens AAPM 2009 6 1 7/25/2009 Equipment Requirements Equipment Requirements • Standard phantom • Calibrated meter and CT probes • CTDI phantom sets • Miscellaneous equipment Inclinometer ACR CT Accreditation Phantom Level and Lead Ruler 100 mm ionization chambers ©D Pickens AAPM 2009 ©D Pickens AAPM 2009 7 Phantom Scanning Form Equipment Requirements • • • • Nested phantom set 15.5 cm long Body: 32 cm Head: 16 cm (peds body) Peds Head: 10 cm Plus plastic pins to fill holes Accepts 100 mm ionization chamber • • D Pickens AAPM 2009 9 ACR Table 1 For a 16 Slice System Used for Adults kVp mA Brain W/out Routine Abd Renal Mass (Adult head) (HR Chest) (Adult Abdomen) 120 120 120 380 ma 133.3 ma 433.3 ma (570 mAs) (100mAs) (325 mAs) Rotation time (s) 1.5 0.75 0.75 Scan FOV 50 50 50 Display FOV (cm) 22 35 35 Recon Algorithm UC L B Axial or Helical Z-axis collimation A A H 4.5 0.6 1.5 (T, in mm) # data channels used 4 2 16 18 1.2 22.5 (N) Axial table increment mm or Hel table speed mm/rot (I) N/A N/A 0.938 Recon scan width (mm) 4.5 1.2 3 Recon scan interval (mm) 4.5 10 3 Pitch 8 Read the instructions Be sure you understand what the site uses for standard acquisitions Be sure you know what patient types are done Be sure you understand the characteristics of the system and how used for each study – Detector configuration – Number of data channels ((N)) used – Maximum number of data channels (Nmax) – Z-axis collimation (T) – Table increment (I) -- axial or for each tube rotation in helical Fill out Table 1 of the ACR Phantom Scanning Forms FOV appropriate for the size of the ACR phantom is permitted. Not so with the dose phantoms: must use FOV for the protocol being tested ©D Pickens AAPM 2009 10 ACR Phantom • Four sections or modules 24 mm asymmetrical detector (4 x 1.5, 16 x 0.75, 4 x 1.5) Allows various combinations 32 detectors, 16 data channels – Module 1: Alignment, CT #, and slice width – Module 2: Low contrast resolution – Module 3: Uniformity and noise, distance accuracy, and slice sensitivity profile (SSP) – Module 4: High contrast resolution Pitch = Table speed/N*T In mm/rotation 1. Must be filled out correctly 2. Will use this to do scans 3. Will do either axial or helical scans depending on your protocols 4. Important to use axial scans for module 1 5. Modules 2-4 done either axial or helical according to your protocol 6. Dose phantoms must be done axial • 20 cm diameter,, 4 cm spacing p g between modules,, total length 16 cm • Alignment beads located in center of module 1 and 4 (1 mm steel) • Markings at center of each section plus marking for head, foot, top • Phantom holder to aid in positioning No dose reduction used for phantom imaging ©D Pickens AAPM 2009 11 ©D Pickens AAPM 2009 12 2 7/25/2009 Module 1: Alignment, Slice Thickness, HU Values Phantom Support and Positioning • Phantom alignment – This must be correct – Beads (1 mm) must be seen uniformly or phantom will fail – Longer bar must be located in center of slice measurement section on 2 mm slice. This is more sensitive to alignment than beads • Water value must be less than or equal to +// 7 HU (+/( / 5) using abdominal protocol and ROI of ~200 mm2 • Images required for water values at all kVps system uses • Polyethylene (-107 to-87 HU), acrylic (110 to 130 HU), bone (850 to 970 HU), air (-1005 to 970 HU) • Slice thickness determined to nearest 0.5 mm by counting visible bars over approximate 3,5,and 7 mm slice thicknesses. Use next larger value system will do Nylon standoffs can cause artifacts; Position carefully ©D Pickens AAPM 2009 13 ©D Pickens AAPM 2009 14 Positioning of ACR Phantom Notes on Phantom Alignment • Centering of slices depends on how system specifies start – Our 16 slice system specifies at center of array – Our 64 slice system specifies at beginning of array • External lasers can be out of alignment a significant amount (one of ours ~1 cm) • Inner laser usually properly aligned • For axials from a helical protocol, be sure that the image of interest will fall in the center of the array to avoid cone beam artifacts (40 mm arrays and wider) ©D Pickens AAPM 2009 15 ©D Pickens AAPM 2009 16 Proper Alignment of the ACR Phantom Phantom Alignment Careful alignment with a level assumes that the external lasers are properly aligned and the gantry is properly leveled. Alignment without cradle is often very difficult Window width = 1000, Window level = 0 Hi resolution chest protocol ©D Pickens AAPM 2009 17 ©D Pickens AAPM 2009 18 3 7/25/2009 Module 1 Alignment Problem Module 2: Low Contrast Detestability • Alignment incorrect: 1. BBs not shown at all four locations 2. Long bar on slice thickness ramp not properly visible on t off iimage top • Adult abdomen protocol • ©D Pickens AAPM 2009 Low contrast performance – Large rod 25 mm – 100 mm2 ROI over 25 mm rod and next to it – Series of rods of 6 mm, 5 mm, 4 mm, 3 mm and 2 mm – For adult head and adult abdomen protocols, must see clearly all 4 rods 6 mm diameter at window width of 100 and widow level of 100 This section interacts with dose measurements – Must be able to see rods – Must keep dose below ACR recommendations – Must keep dose as low as possible – Measurements made without system dose reduction turned on For systems with established protocols, may be more efficient during testing to use specified protocol that is known to be below the ACR dose requirements to make sure the rods are visible. ©D Pickens AAPM 2009 19 Low Contrast Section of ACR Phantom Pediatric Abdomen Protocol Low Contrast Section of ACR Phantom 345 mAs ~23 mGy Adult Head Protocol: 120 kVp, 540 mAs, 4.5 mm WW = 100, WL =100 300 mAs ~20 mGy 200 mAs ~13.3 mGy At the required window settings of 100, 100: ACR requirements not to exceed 25 mGy with all four 6 mm rods visible How you view the image can make a difference: Viewing distance, size on monitor, room lighting ©D Pickens AAPM 2009 20 21 ©D Pickens AAPM 2009 22 Module 3 Uniformity and Noise Measurements Module 3: Uniformity • Uniform section of water-equivalent material • Two small bbs spaced at 100 mm Each ROI about 400 mm2 WW = 100, WL = 0 – Distance measurements – SSP Edge to edge variation no more than 5 HU • Use for uniformity testing – Edge and section HU differences of > 5 HU – ROIs must be placed properly and be of proper size for this – Center number using adult abdomen protocol must be -7 to 7 HU or better (+/- 5 HU better) – Image must not show artifacts with ww = 100 and wl = 0. Center should be 0 +/- 5 HU • Adult abdomen protocol ©D Pickens AAPM 2009 23 ©D Pickens AAPM 2009 24 4 7/25/2009 An Example of a Problem Phantom Scan Four Slice Scanner Abdomen protocol Problems HU high for water HU not uniform in field Low contrast artifact Module 4: High Contrast Resolution • Use with two protocols – Adult abdomen – High resolution chest • For abdomen, must see 5 lp/cm bars • For HRC, must see 6 lp/cm bars • Window width = 100, window level ~ 1100; there is some flexibility with setting the level • Note that the four beads must be visible for proper alignment Cause – temperature variations in room ©D Pickens AAPM 2009 25 Highest resolution for system 7-8 lp/cm visible here 2 mm slice thickness Alignment correct No significant artifacts 7 lp/cm 26 ACR Phantom Notes High resolution Chest Protocol, Module 4 4 lp/cm ©D Pickens AAPM 2009 ACR requires 5 lp/cm for adult abdomen, 6 lp/cm for adult HRC • Machine should be calibrated before you get there; doesn’t always happen • If you get low contrast, wide, ring artifacts, y help an air cal may • If there are calibration or artifact issues, check for contrast or other material on x-ray transparent window of system Acquired as axial or helical, depending on your site protocol ©D Pickens AAPM 2009 27 ©D Pickens AAPM 2009 28 ACR CT Dose Requirements • Adult head CTDIvol – Pass/fail -- 80 mGy – Reference level – 75 mGy • Adult abdomen CTDIvol – Pass/fail – 30 mGy – Reference level – 25 mGy • Pediatric abdomen CTDIvol – Pass/fail – 25 mGy – Reference level – 20 mGy Fill in the Forms and Check as You Go ©D Pickens AAPM 2009 29 ©D Pickens AAPM 2009 30 5 7/25/2009 Phantom in Position for Head Dosimetry Dose Measurements (CTDI) • All require axial scans of dose phantoms • All require use of adult head, adult abdomen, pediatric abdomen protocols (if performed) • Head dose phantom in head holder • Body dose phantom on bare table top • Pediatric abdomen – use head phantom on table top • For helical protocols, must use axial scan with the detector configuration of the helical protocol (kVp, mA, time, N, T) • Use spread sheets provided by ACR to report CTDIvol (exposure or air kerma, depending on meter) Note positioning in head holder and tape to anchor chamber ©D Pickens AAPM 2009 ©D Pickens AAPM 2009 31 Adult Body Configuration for Dose Measurements Make Sure the Dose Phantom has all pins in place Can lead to an error in dose Likely to cause a fail of test Axial Slice at center of phantom Three measurements with 100 mm probe at both 12:00 and 3:00 ©D Pickens AAPM 2009 ©D Pickens AAPM 2009 33 34 Annual Testing System Tests ACR Dose Phantom Measurements • If your dose measurements vary when doing the three averages, check to see if the system is doing a 420 degree over scan rather than a 360 degree scan • Be B sure FOV is i sett properly l for f the th dose d scan and phantom centered properly • May be a good idea to check dose first so that you know your protocol does not exceed ACR dose limits ©D Pickens AAPM 2009 32 35 • • • • • • Alignment light accuracy Alignment of table to gantry Table/gantry tilt Table increment accuracy Slice thickness Image quality – High contrast resolution – Low contrast resolution – Image uniformity – Noise – Artifact evaluation • CT number accuracy and linearity ©D Pickens AAPM 2009 36 6 7/25/2009 Annual Testing Other Tests Laser Light – Bed Positioning Test • Display devices – Monitors – Film-printers • Dosimetry – CTDI measurements – Patient radiation doses for representative exams – Many newer systems estimate CTDIvol and DLP for exams • Visual safety inspection • State and local inspection and testing as required • ACR phantom or other suitable phantom can be used ©D Pickens AAPM 2009 Ruler with lead scale Can check bed index Can check scout position ©D Pickens AAPM 2009 37 38 Recommended Quality Control Verification of Tilt Readout Position and Repeatability • • • • Established by medical physicist Performed by technologist on regular basis Frequency based on facility and usage Image quality – – – – • • • • High and low contrast resolution Image uniformity N i Noise Artifact evaluation Alignment light accuracy Slice thickness CT number accuracy Display and output devices This device gives a check, but is not very accurate ©D Pickens AAPM 2009 ©D Pickens AAPM 2009 39 40 Technologist Quality Assurance Form Manufacturer-Provided Water Phantom CT Daily Quality Assurance Data Sheet Room # __________________________ 12:00 ---- 5.1 HU, SD=14.2 3:00 ---- 2.7 HU, SD=14.2 Center -- -1.2 HU, SD=17.8 WW=100, WL=0, ROI=400 mm2 ___/___/_____ to ___/___/_____ Date Technologist Initials Visual Inspection of Equipment Scan Type Head/Body Artifacts? (Y/N) Action Taken? (Y,N. Log below) Center H.U. Standard deviation 12 o'clock H.U. 3 o'clock H.U. 6 o'clock H.U. 16 slice scanner, lung screening protocol (120 kVp, 25 mAs, 2 mm) 9 o'clock H.U. Date Action Requires scanning water phantom daily at standard acquisition ©D Pickens AAPM 2009 41 ©D Pickens AAPM 2009 42 7 7/25/2009 Summary • Qualified medical physicist must be present for annual survey and ACR testing when dose measurements are made • Read the materials from ACR including FAQ • Use the ACR Phantom Site Scanning Data Form • Understand the operating modes and detector configuration of the scanner • Be sure Table 1 properly reflects the standard protocols for the site • Remember that clinical images with the site protocols will also be submitted • For dosimetry, be sure to use the appropriate spread sheet for the dose measurements • Be sure the system is calibrated and the periodic QC program is ongoing • Be prepared to perform annual surveillance; does not need to be submitted to ACR but needs to be available for inspection ©D Pickens AAPM 2009 43 8