Table 1.
Sensitivity and Resolution of Cancer Imaging Modalities as a Function of Cell Number and Contrast Agent/Radiotracer Concentration
| Modality | Typical Voxel/Pixel Dimensions (Resolution) | Maximum No. of Cells per Voxel/Pixel | Clinically Available Exogenous Contrast Agent(s) | Radiotracer/ Contrast Agent Hydrodynamic Diameter* | Contrast Agent Concentration per Voxel/Pixel Required for Detection† | No. of Molecules of Contrast per Voxel/Pixel Required for Detection† | Notes |
|---|---|---|---|---|---|---|---|
| US | 1 μL (1×1×1 mm) | 106 | Microbubbles | 1 μm | NA | NA | Microbubbles remain intravascular in most tissues |
| CT | 1 μL (1×1×1 mm) | 106 | Iodine | ≈1 nm | 0.5 M | 3 × 1017 | Requirement for molar concentrations precludes targeted imaging |
| MRI | 1 μL (1×1×1 mm) | 106 | Chelated Gd3+ | ≈1 nm | 50 μmol/L | 3 × 1013 | Would require > 107 Gd3+ atoms per cell for detectability |
| SPECT | 1.7 cm3 (12×12×12 mm) | 1.7 × 109 | 99mTc | ≈1 nm | 0.3 pM†‡ | 3 × 108 | On average approximately 0.2 radioatoms per cell†‡ |
| 111In | (≈8 × 103 Bq/voxel) | ||||||
| 67Ga | |||||||
| PET | 0.5 cm3 (8×8×8 mm) | 5 × 108 | 18F | ≈1 nm | 0.02 pM†‡ (≈7 × 102 Bq/voxel) | 6 × 106 | On average approximately 0.01 radioatoms per cell†‡ |
| Optical (2-D) | 0.01 mm2 (0.1×0.1 mm) | 103 | ICG | ≈1 nm | ≈10-100 nmol/L | 6 × 107§ | Surface only NIR fluorescence, requires approximately 104−105 fluorophores per cell for detectability |
| Optical (3-D) | 1 cm3‖ (1×1×1 cm) | 109 | ICG | ≈1 nm | ≈10-100 nmol/L | 6 × 1013 | NIR tomography-based, requires approximately 104−105 fluorophores per cell for detectability |
Abbreviations: US, ultrasound; NA, not applicable; CT, computed tomography; MRI, magnetic resonance imaging; Gd3+, gadolinium; SPECT, single photon emission computed tomography; 99mTc, technetium 99m; 111In, indium 111; 67Ga, gallium 67; PET, positron emission tomography; ICG, indocyanine green; NIR, near-infrared.
After conjugation of a targeting molecule to a contrast agent or radiotracer, final hydrodynamic diameter increases proportionally.
Does not take into account the significant effect of background and tissue attenuation on detectability. Values shown are theoretical only.
Assumes 100 counts, 5 minutes scan, SPECT/PET attenuation of 0.2/0.1, sensitivity of 0.02%/0.5%, half-life of 6/1.8 hours, and maximum specific activity.
Assuming a tumor thickness of 0.1 mm (10 cells thick).
Approximate resolution at a depth of 10 cm in soft tissue such as breast.