OPTIMIZATION Patient Dose in Digital Radiography: Using Exposure Indices to Optimize Techniques. What is meant by “optimum” optimum”? Possibities: Possibities: Least patient ESE for specified image quality (IQ) Least patient effective dose for specified IQ Best IQ for specified patient dose Etc “Optimum” Optimum” may vary according to preferences Acceptance limits develop around ideal level CR/DR Radiographic Techniques Before CR/DR techniques can be effectively optimized, they must first to stabilized. Issues: Very wide latitude (dynamic range): 103 to 104 :1 obscures concept of “correct” correct” exposure: – Exposure level and displayed “density” density” unrelated : All images processed for optimal “image density” density” Lack of visual feedback about exposure factors: – Technique factors do not “stabilize” stabilize”, often inappropriate – AEC systems may be calibrated in appropriately Patient exposure and image quality vary widely Exposure Indices (EI) Examples and data used here are mostly from Fuji and Konica computed radiography (CR). EIs are expressed per manuf. manuf. standards. In both cases: – EI inversely related to plate exposure – Calibrated at 80 kVp with no added filtration – Yields SS-number of 200 with 1 mR at plate Fuji: SS-Number S = 200/Emedian (mR) mR) – Based an examexam-specific histogram analysis Konica: SS-Value S = 200/Emean (mR) mR) – Based an examexam-specific anatomic region selection Illustrating the Issue AP LL-Spine Exam Fuji CR Phototimed (AEC) Excellent positioning Excellent Collimation S-Number: 68 Fuji range (from user’ user’s manual): 100 - 400 How well are Existing EIs working ? Clinical Ranges of Exposure Indices Manual Techniques: Average Exp Index Coeff of Variation Target Range 185 144 152 213 139 37% 48% 78% 73% 51% Not provided Not provided Not provided Not provided Not provided Site 1: AP Foot (Hosp ED) AP Femur (Konica CR) Lat Forearm Port Grid Abd Port Chest (high frustration level) How well are Existing EIs working ? Clinical Ranges of Exposure Indices Phototimed (AEC): * Average Coeff of Exp Index Variation CR versus Film: Portable AP chest Target Range Site 1: PA Chest Hosp ED AP Abdomen (Konica CR) AP L-Spine 340 211 268 16% 40% 45% Not provided Not provided Not provided Site 2: Office (Fuji CR) 515 179 168 41% 60% 45% 200-600 100-400 100-400 *-Mostly PA Chest AP Abdomen AP L-Spine Perspective: Variability of CR/Film CR: Film/Screen 1: Film/Screen 2: Std Dev of film OD * Exposure C.V.** — 0.53 OD 0.41 OD 51% 43% 32% *-Of Selected lung field region **-Based on typical sensitometry for general purpose film/scre (3.4 gamma between 1.5 and 2.5 OD, 1.8-2.0 Target OD) Contributing Factors: Technical Factors: – Calibration levels of readers (all images) – Calibration methods vs clinical use – Calibration of AEC (AEC images) Exposure Index Factors: – Lack of understanding of existing EIs (TG(TG-116) – Lack of appropriate visual feedback – VendorVendor-specific selection of relevant image region (volume of interest, VOI) for EI calculation – Exam specific target ranges (if any provided) Reader Calibration (matching) Indicated plate exposure 101020% lower than indicated by exposure index Reader Calibration and Variability: Reader 1 2 3 4 5 6 7 8 9 10 Measured S-Number* No filter TG116 Filter 160 105 175 115 183 121 175 118 200 126 200 124 162 108 167 113 163 114 175 120 Average: CV: 176 8% 116.4 6% * 80 kVp, 72" SID Action Step 1: Calibrate CR/DR Units For now, service engineers will need to adhere to manufacture’ manufacture’s calibration protocol (TG(TG-116 and IEC will eventually standardize) Assure EI matches manufactures’ manufactures’ indicated value for reference exposure level (eg (eg,, 1 mR) mR) For CR, should try to use same generator for all readers (especially reader of same model) Verify proper dosimeter calibration if using filtered beam Unit 1 2 3 4 5 6 7 8 9 10 11 12 Type S-Num HF 470 3P 365 HF 167 HF 139 HF 318 HF 270 1P 499 1P 439 1P 504 1P 521 1P 352 1P 552 Average 383 C.V.: 36% Est mR at plate 0.45 0.58 1.16 1.53 0.61 0.75 0.50 0.57 0.50 0.48 0.72 0.46 0.69 47% AEC CALIBRATION Table Bucky 8” acrylic (25x25 cm) 80 kV Fuji Carbon readers (Similar variations for upright receptor) Action Step 2: Calibrate AEC Units AEC Tests: Geometry and Phantoms Use clinically relevant phantoms (eg (eg,, acrylic) Should, as closely as possible, achieve target EI for standardized conditions (eg (eg,, 8” 8” acrylic, center AEC detector, 80 kVp, kVp, etc) L-Spine/Abdomen Phantom AEC CALIBRATIONCALIBRATION-CAVEATS Achieving a particular EI for an acrylic AEC phantom DOES NOT guarantee the corresponding exposure based on the EI calibration process was received by the detector (for example, 1 mR for a 200 SS-number) will need to determine receptor exposure directly RECEPTOR EXPOSURE CAVEATS May need “cassette frame” frame” to circumvent PBL Dosimeter usually will not fit on top of cassette Also need to verify dosimeter calibration Measuring Receptor Exposures AEC CAVEATS – Part 2 Specifying an AEC receptor dose based on a speed class (e.g., 1 mR for a 200 speed class) does NOT necessarily imply that EI target ranges should be centered on that speed class value (e.g., may not be centered on 200) Cu/Al calibration filters do not equal a patient Depends on anatomic segmentation algorithm (one selecting lung fields on a PA chest will yield different EIs than one using full anatomy) VOI Selection Fuji: Full anatomic area (exam(exam-based histogram anal to unexposed areas and raw radiation Konica: recognition of relevant anatomy (exam specific) VOI Selection Fuji/Agfa: Based on histogram analysis of full anatomic area Konica: recognition of relevant anatomy (exam specific) VOI Selection Fuji: Full anatomic area (exam(exam-based histogram anal to unexposed areas and raw radiation Konica: recognition of relevant anatomy (exam specific) Anatomic Selection: AEC Sensor Areas Suggested VOI Selection Option Allow user to select VOI corresponding to clinically used AEC fields Example: Example: PA Chest Use of VOI Corresponding to AEC Fields Based on analysis of 54 (phototimed (phototimed)) PA chest exams. EI (Konica SS-value) based manually set VOI area (approximately) corresponding to left/right AEC fields Konica Default ROI Average EI: Std Dev: C.V.: 340 53 16% VOI corresponding to L/R AEC cells 222 22 10% In addition to Final Image... show exposure based “view” view” In addition to Final Image... show exposure based “view” view” SUMMARY EIs seem to vary more widely than did film O.D “Good” Good” exams often fall outside Manuf criteria Variations not just technique selectionselection-related EIs for CR miscalibrated -Physics action item AECs miscalibrated for CR -Physics action item VOI selection process perhaps largest source of clinical EI ranges AECAEC-field based VOIs or direct visual feedback may allow for better technique stabilization Overexposed (by (by 50%) 50%) UnderOver--exposed Under