Table 1.
Unique synCT generation RPN and SOD scores before and after suggested modifications
| Failure mode | Failure pathway | Before mitigation |
Suggested Change | After mitigation |
||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| S | O | D | RPN | SOD | S | O | D | RPN | SOD | |||
| 1. Bone segmentation | ||||||||||||
| Incorrect bone classification | Bone/air indistinguishable in MR images | 4 | 9 | 1 | 36 | 491 | Consult with radiologists; apply consensus guidelines; attend education programs through national meetings (eg, RSNA) | 4 | 9 | 1 | 36 | 491 |
| Nonbone material classified as bone | 5 | 8 | 1 | 40 | 581 | 5 | 4 | 1 | 20 | 541 | ||
| Bone volume underestimated | 4 | 7 | 4 | 112 | 474 | 4 | 4 | 3 | 48 | 443 | ||
| Bone volume overestimated | 4 | 8 | 4 | 128 | 484 | 4 | 5 | 3 | 60 | 453 | ||
| Uncertainty from interobserver differences in manual bone segmentation | 4 | 5 | 3 | 60 | 453 | 4 | 4 | 3 | 48 | 443 | ||
| 2. Tissue classification/density assignments | ||||||||||||
| SynCT not representative of average anatomy | Long scan time leads to changes in internal anatomy (bladder/rectal filling) | 4 | 7 | 1 | 28 | 471 | Minimize number of acquired sequences; minimize acquisition time for each sequence | 3 | 7 | 1 | 21 | 371 |
| Varied physiologic states for different data sets needed for synCT | 4 | 7 | 1 | 28 | 471 | 3 | 7 | 1 | 21 | 371 | ||
| Changed target location because of state | 6 | 5 | 2 | 60 | 652 | 4 | 5 | 2 | 40 | 452 | ||
| Patient-specific distortion corrections for air/tissue may be inaccurate | 4 | 4 | 5 | 80 | 445 | 3 | 4 | 5 | 60 | 345 | ||
| Tissue misclassification/inaccurate HU assignment | Inaccurate autosegmentation | 4 | 6 | 3 | 72 | 463 | Standardize sequences; increase the number of patients to ensure a representative group of patients in the training set | 3 | 4 | 3 | 36 | 343 |
| Patient not well represented by population-based values | 4 | 3 | 9 | 108 | 439 | 4 | 1 | 9 | 36 | 419 | ||
| Population-based values derived from a nonrepresentative set of patients | 4 | 1 | 9 | 36 | 419 | 2 | 1 | 9 | 18 | 219 | ||
| Not enough patients used to determine population-based values | 4 | 1 | 9 | 36 | 419 | 2 | 1 | 9 | 18 | 219 | ||
| Inaccurate segmentation | Image nonuniformity affecting automated intensity-based segmentation approaches | 4 | 5 | 5 | 100 | 455 | Check constancy of vendor-implemented correction software; Implement independent postprocessing assessment and correction tools and QA procedures | 4 | 3 | 4 | 48 | 434 |
| Inadequate distortion correction | 6 | 6 | 3 | 108 | 663 | 2 | 4 | 3 | 24 | 243 | ||
| 3. Overall synCT process | ||||||||||||
| External contour incorrect | System-level geometric distortion not taken into account | 5 | 9 | 3 | 135 | 593 | Implement robust QA/QC including verification tests performed on phantoms; training of radiation oncology staff with respect to proper coil use | 5 | 5 | 3 | 75 | 553 |
| Image artifacts preventing accurate external delineation | 3 | 8 | 1 | 24 | 381 | 3 | 6 | 1 | 18 | 361 | ||
| External anatomy incomplete | 3 | 8 | 1 | 24 | 381 | 3 | 6 | 1 | 18 | 361 | ||
| Anatomy deformed by coils | 3 | 7 | 2 | 42 | 372 | 3 | 4 | 2 | 24 | 342 | ||
| Inaccurate synCT | Missing images required for generating synCT | 4 | 2 | 1 | 8 | 421 | Standardize sequences | 4 | 1 | 1 | 4 | 411 |
| Organ location inaccurate | System-level geometric distortion not taken into account | 6 | 7 | 4 | 168 | 674 | Standardize sequences; optimize sequence parameters to minimize acquisition time; implement vendor-independent postprocessing software | 2 | 2 | 5 | 20 | 225 |
| Patient-induced distortions near interfaces present | 6 | 7 | 5 | 210 | 675 | 5 | 6 | 5 | 150 | 565 | ||
| Patient anatomy is not standard for patient—unable to reproduce anatomy | 7 | 3 | 3 | 63 | 733 | 7 | 1 | 3 | 21 | 713 | ||
| Long scan time leads to changes in internal anatomy (bladder/rectal filling) | 6 | 5 | 2 | 60 | 652 | 4 | 5 | 2 | 40 | 452 | ||
| Varied physiologic states for different data sets needed for synCT | 6 | 3 | 2 | 36 | 632 | 5 | 2 | 2 | 20 | 522 | ||
| Changed target location because of state | 8 | 2 | 2 | 32 | 822 | 6 | 2 | 2 | 24 | 622 | ||
Abbreviations: HU = Hounsfield unit; MR = magnetic resonance; QA = quality assurance; QC = quality control; RPN = risk priority numbers; RSNA = Radiological Society of North America; SOD = severity-occurrence-detectability; synCT = synthetic computed tomography.