Table 1.

Conventional imaging modalities that are presently used in the clinic.

Modality	Advantage	Limitation	Input Signal type	Resolution [mm]	Penetration depth
CT	Rapid, accurate, moderate cost, reproducible, widely available	Limited resolution, imaging interpretation difficult, ionizing radiation	X-ray	25–200 μm (preclinical), 0.5–1 mm (clinical)	Unlimited
MRI	Soft tissue contrast, high resolution, customizable molecular targeting, cell tracking	High cost, large equipment required, limited sensitivity, requires contrast agent	Radio frequency	25–100 μm (preclinical), ≈1 mm (clinical)	Unlimited
USI	Rapid, accurate, low cost, reproducibility, widely available	Limited resolution, image interpretation difficult, artifacts common	Sound waves	10–00 μm (at ≈mm depth); 1–2 cm (at ≈cm depth)	10 ms
PET	Quantification of metabolism and blood flow, high sensitivity, many radionuclide tracers available	High cost, limited availability, large equipment required, short tracer half-life, single process evaluation	Radionuclide (positron emitter)	<1 mm (preclinical), ≈5 mm (clinical)	Unlimited
PAT	Reduced tissue scattering, high resolution, non-ionizing/non-radioactive, no acoustic noise, high penetration depth, high resolution	Limited path length, dependence to temperature, weak absorption at short wavelengths.	Light	5 μm-1 mm (depth-dependent)	<6 cm
SPECT	3D imaging, widely available, highly sensitive, simultaneous imaging of multiple processes	Limited temporal resolution, few radionuclide tracers	Radionuclide (γ-ray emitter)	0.5–2 mm (preclinical), 8–10 mm (clinical)	Unlimited
NIFI	Low cost, widely available	Photobleaching, low quantum yield, shallow tissue penetration	Ultraviolet to near infrared light	2–3 mm	<2 cm