Table 1.
Studies evaluating the accuracy and stability of gray values (GVs) in CBCT
| Study | CBCT models | Phantom | Materials | Reference | Resultsa |
|---|---|---|---|---|---|
| Aranyarachkul et al34 | 1 | Human cadavers (n = 9) | Bone | Quantitative CT | R2 = 0.85–0.96 |
| Azeredo et al35 | 3 | Small cubic phantoms | Air, water, wax, acrylic, plaster, gutta-percha | CT | R2 = 0.8818–0.9947 (average, 0.9370) |
| Bamba et al36 | 3 | Cylindrical PMMA phantom | Air, LDPE, PMMA, POM, PTFE, aluminium | CT | Uniformity in axial and coronal plane: variable results, shifts of >1000 GV for one CBCT model Effect of scan parameters (kVp, mAs) and environment (amount of PMMA) demonstrated |
| Bryant et al37 | 1 | Assymetric acrylic phantom Terraced phantom Pyramid phantom Water phantom |
Acrylic Water | N/A Theoretical Hounsfield units (i.e. 0 HU) |
GVs affected by mass in slice and mass outside slice. Stable region of 11 cm in centre of FOV Correction equation proposed based on mass of 7 g per slice |
| Bujtár et al38 | 1 | Cadaver head | PE, POM, borosilicate glass, aluminium, water | CT | Effect of position: SD, 70–135 (maximum shift, 193–488 GV) CBCT-CT: R2, 0.82–0.98 |
| Cassetta et al39 | 1 | Mandibles (n = 10) | Cortical and cancellous bone | N/A | Effect of mAs (approximately 47% reduction): cortical bone: shift of 161 GV cancellous bone: shift of 65 GV |
| Cassetta et al40 | 1 | Mandibles (n = 20) | Cortical and cancellous bone | CT |
R2 = 0.87–0.96 (average, 0.93) Average difference between CBCT and CT, 235 GV |
| Chindasombatjaroen et al41 | 1 | Cylindrical water phantom | Iodine contrast medium 0.0625–8%, concentration doubled with each step | CT |
R2 = 0.9997–0.9999 Effect of mAs: GV shifts up to approximately 150 GV (Effect of kVp also demonstrated) |
| Eskandarloo et al42 | 3 | Rectangular phantom filled with water and oil | PE, polyamide, PVC, bone pieces | CT | Effect of position in FOV: Model 1: average shift, 125 GV; maximum, 291 GV Model 2: average shift, 39 GV; maximum, 141 GV Model 3: average shift, 106 GV; maximum, 395 GV |
| Hohlweg-Majert et al43 | 1 | Cylindrical phantom | HA (100, 200, 400, 1000 mg cm−3) | CT | Differences between axial slices: average shift, 24 GV; maximum, 67 GV |
| Katsumata et al44 | 2 | Cylindrical plastic, filled with water, dry mandible and cervical vertebrae | Bone, water | CT | Difference in GV of water at buccal and lingual sides GVs and lingual-buccal differences affected by objects outside the FOV Objects outside the FOV lead to GV shifts of up to 10% of the gray scale Relative densities (normalized to bone) also not stable |
| Katsumata et al45 | 1 | Cylindrical plastic, filled with water, dry mandible and cervical vertebrae | Bone, water | N/A | Highest variability for smallest FOV, decreased variability for larger FOVs GVs severely inconsistent between FOVs (difference of 374 GV for bone) Shifts due to objects outside FOV, 10–74 GV |
| Lagravère et al46 | 1 | PMMA box filled with water | POM, acrylic, nylon, cork, celfortic pink foam, spruce | Physical density | Correlation between density and Hounsfield units: R2 = 0.986 Standard error of 27 HU |
| Lagravère et al47 | 1 | PMMA box filled with water | Canadian spruce, nylon, POM | Physical density | Shifts in GV due to object location up to 85–214 GV (not statistically significant) Correlation between density and Hounsfield units: R2 = 0.893 |
| Mah et al48 | 11 | Small cylindrical phantom (in air and in small/large water containers) | (Air), adipose, water, PMMA, muscle, cancellous bone, cortical bone, aluminium | Linear attenuation coefficients | Highest correlation at effective beam energy Hounsfield units derived through attenuation coefficients Variability in CBCT-derived Hounsfield units when scanning in air vs small or large water container: average shift 127 HU; maximum 1258 HU Large differences in derived Hounsfield units between CBCTs |
| Molteni28 | 2 | Small cylindrical phantom (in air and in small/large water containers); | (Air), adipose, water, PMMA, muscle, cancellous bone, cortical bone, aluminium | Linear attenuation coefficients | Less artefacts for small FOVs Increase in GV towards the top of FOV for one CBCT model No influence of resolution mode on GV for one CBCT model Effect of exo-mass, presence of water and central/peripheral phantom position Discrepancy between central and peripheral gray values for water: −135 to +125 GV Correlation between attenuation coefficients and CBCT GV: R2, 0.935–0.973 |
| Nackaerts et al49 | 5 | Rectangular bone mineral phantom | Water, HA (75, 150 mg cm−3) | CT | Poor stability of CBCT GV throughout FOV (CV, 10%; CT, 0.1%) Large effect of rotation and off-centre positioning of phantom Correlation of CBCT-CT: R2 = 0.095–1.000 (average, 0.764; median, 0.969) |
| Naitoh et al50 | 1 | Patients (n = 16) | Mandibular cancellous bone | CT (BMD) | Correlation CBCT GV and CT-derived BMD: R2 = 0.931 BMD from CBCT deviates 1–182 mg cm−3 (average, 46 mg cm−3) from CT Deviations of up to approximately 200 GV |
| Naitoh et al51 | 1 | Patients (n = 15) Reference bone block | Mandibular cancellous bone HA (200 mg cm−3) | CT (BMD) | BMD from CBCT deviates 1–123 mg cm−3 (average, 38 mg cm−3) from CT 84% of bone sites correctly classified as > or <200 mg cm−3 using reference block 38% in 150–250 mg cm−3 range misclassified |
| Nishino et al52 | 1 | Cylindrical phantom | Air, LDPE, acrylic, PTFE | N/A | Effect of relative z-position of phantom: maximum shift, 19–69 GV (depending on material) |
| Nomura et al53 | 1 | Water phantom | Aluminium, iodine solutions (0–100 mgI ml−1, 10 mgI ml−1 step size) | CT | Differences due to kVp or mA variation: up to approximately 12 GV CV: CBCT, 3.7–7.8%; CT: 0.7% Linear regression, CBCT-CT: R2 = 0.982, non-linear regression: R2 = 0.989 |
| Nomura et al54 | 1 | Water phantom or in-air | HA (0–239 mg cm−3, 40 mg cm−3 step size) | CT | Correlation: R2 = 0.9983–0.9999 Large influence of air/water background Presence of hard tissue: average shift 6 GV; maximum, 9 GV Presence of metal: average shift, 15 GV; maximum, 30 GV (metal inside FOV), average shift, 13 GV; maximum, 28 GV (metal outside FOV) Variation along z-axis: SD, 8.3–10.8 GV |
| Oliveira et al55 | 1 | Water phantom | K2HPO4 (200–1200 ng cm−3, 200 mg cm−3 step size) | N/A | Effect of kVp: average shift, 122 GV; maximum, 290 GV Effect of mA: average shift 40 GV; maximum, 108 GV Effect of exo-mass: average shift; 183 GV; maximum, 307 GV Effect of implant in FOV: average shift, 5 GV; maximum, 28 GV |
| Parsa et al56 | 1 | Mandibles (n = 10) | Bone | CT | Correlation CBCT-CT: R2 = 0.937 |
| Parsa et al57 | 1 | Mandibles (n = 20) | Bone | CT | Difference CBCT GV and Hounsfield units: average, 155 Correlation CBCT-CT: R2 = 0.794 |
| Parsa et al58 | 2 | Mandible (n = 1) | Bone | N/A | Model 1: effect of FOV size: maximum shift, 402 GV effect of time 1 (resolution mode): average shift, 3 GV effect of time 2 (rotation mode): average shift, 29 GV Model 2: effect of FOV size: maximum shift, 116 GV effect of mA: average shift, 36 GV effect of exposure time: average shift, 4 GV |
| Pauwels et al59 | 7 | Cylindrical PMMA phantom | Air, PMMA, HA (50, 100, 200 mg cm−3), aluminium | CT | Correlation CBCT-CT: All materials: R2 = 0.6864–0.9996 (average, 0.9689) Medium density: R2 = 0.7303–0.9909 (average, 0.9156) Uniformity of GV in axial plane, 4–21% |
| Pauwels et al60 | 13 | Cylindrical PMMA phantom | Air, PMMA, HA (50, 100, 200 mg cm−3), aluminium | CT | Correlation CBCT-CT: All materials: R2 = 0.7014–0.9996 Medium density: R2 = 0.5620–0.9991 Error after calibration: 35–1562 GV |
| Plachtovics et al61 | 1 | PMMA cylinder | PTFE, LDPE, acrylic, air, water | CT | Effect of phantom rotation: average shift, 15 GV (excluding air), maximum, 51 GV Effect of projection mode: average shift, 128 GV (excluding air), maximum, 263 GV Centre vs periphery (64 mm from centre): average shift, 209 GV; maximum, 240 GV (except double-exposure overlap mode: shift, 10 GV) |
| Reeves et al62 | 2 | Reference object in patient's mouth | Cortical bone, trabecular bone, PMMA, water | Linear attenuation coefficients | Clinical application of Mah et al48 <3% difference between average derived and actual Hounsfield units |
| Sennerby et al63 | 1 | Cylindrical phantom with water bath | PE, PS, nylon, PC, PMMA, HA (200, 800, 1000, 1500 mg cm−3) | Linear attenuation coefficients, hydroxyapatite concentration |
R2 = 0.9574–0.9979 Effect of scan setting (binning): maximum shift, 10 GV Deviations up to 25 GV seen from correlation plots Variability up to approximately 20 GV between slices |
| Silva et al64 | 1 | Mandibles (n = 20) | Trabecular bone | CT | Difference in average GV between CBCT and CT: 105 GV |
| Spin-Neto et al65 | 6 | Human skull in acrylic | All voxels in FOV | N/A | GV shifts between consecutive exposures: <5 GV for four CBCTs, ≤17 GV and ≤109 GV for two other CBCTs |
| Valiyaparambil et al66 | 1 | Cylindrical phantoms (n = 2) | K2HPO4 (50–1000 mg ml−1), HA (100, 200, 400, 800 mg cm−3) | CT | Correlation CBCT-CT: R2 = 0.98–0.99 |
| Yamashina et al67 | 1 | Soft tissue-equivalent phantom in water container | Air, epoxy resin, water | CT | Reproducibility: average shift, 3 GV; maximum, 5 GV Effect of position in FOV: Air: maximum shift, 144 GV Epoxy resin: maximum shift, 107 GV Water: maximum shift, 129 GV |
BMD, bone mineral density; CV, coefficient of variance; FOV, field of view; GV, gray value; HA, hydroxyapatite; K2HPO4, dipotassium phosphate; LDPE, low-density polyethylene; mgI, milligrams of iodine, N/A, not applicable; PC, polycarbonate; PE, polyethylene; PMMA, polymethyl methacrylate; POM, polyoxymethylene; PS, polystyrene; PTFE, polytetrafluoroethylene; PVC, polyvinyl chloride; SD, standard deviation.
a
Recalculated to R2 when explicitly stated that R was used. Certain results provided in this table were retrospectively derived from graphs and tables provided in the respective article.