Summary of imaging modalities^a.

Technique	Depth	Spatial resolution	Temporal resolution	Strength	Limitation	Clinical use	Physical principle
SPECT	Whole brain	1–5 mm	Minutes	Non-invasive, lower cost compared to PET, longer half-life (∼6 h) for radio tracers, allow longer imaging time, sensitivity: 10−10 to 10−11 mol L−1	Poor spatial resolution, radiation dose exposure, limited number of radionuclides (99mTc, 123I)	Yes	Low-energy γ-rays
PET	Whole brain	1–5 mm	Seconds to minutes	High sensitivity compared to SPECT, radionuclides used in PET (11C, 13N, 8F) are abundant in the body and can be tailored for endogenous biomolecules such as carbohydrates, fats, nucleic acids and proteins, sensitivity: 10−11 to 10−12 mol L−1	Short half-life (∼75 s) for the radio tracers and hence must be produced onsite before imaging, high-cost compared to SPECT, limited imaging time window	Yes	High-energy γ-rays
CT	Whole brain	50 μm	Minutes	High spatial resolution, fast and cross-sectional images of the brain, sensitivity: 10−6 mol L−1	Structure of the brain, not its function, low contrast	Yes	X-rays
fMRI	Whole brain	25–100 μm	Minutes to hours	No radiation, structural and functional data, high spatial resolution, greater contrast for soft tissues, imaging agents with lower toxicity, sensitivity: 10−3 to 10−5 mol L−1	High cost, long scanning time, sensitive to motion artifacts, relatively low sensitivity and low contrast	Yes	Radio waves
Ultrasound	1-5 cm	50–500 μm	Seconds to minutes	Low cost, no radiation, high speed, portable, sensitivity with microbubbles: 10−12 mol L−1	Low contrast	Yes	High frequency sound waves
EEG	Scalp	5–10 cm	Millisecond	Inexpensive, portable, high temporal resolution, electrical activity of brain	Low spatial resolution, prone to error due to environmental noise, localization of signal is difficult	Yes	Electrical
MEG	Scalp	1-5 mm	Millisecond	High temporal resolution	Expensive	Yes	Electromagnetic
fNIR	< 1 cm	2–3 mm	Seconds to minutes	No radiation, inexpensive, sensitivity: 10−9 to 10−12 mol L−1	Scattering due to tissues may be a problem, low penetration depth	Preclinical	Near-infrared light
Photoacoustic	0.6–5 cm	10 μm to 1 mm	Seconds to minutes	No radiation, label-free, high spatial resolution and low cost compared to CT/PET, sensitivity: 10−6 to 10−12 mol L−1	Distortion of acoustic signal due to skull, temperature dependent signal, weak absorption at shorter wavelengths	Preclinical	Pulsed laser and sound wave
VSDI	1 mm	50 μm	Millisecond	High temporal resolution	Invasive, prone to photobleaching of dye, toxic to cells	Preclinical	Voltage sensitive dye
Bioluminescence	1-2 cm	3–5 mm	Seconds-Minutes	No radiation, high sensitivity, inexpensive, sensitivity: 10−15 to 10−17 mol L−1	Scattering due to tissues may be a problem, spatial resolution is low	Preclinical	Visible light
LSCI	0.5–1 mm	10 μm	Microseconds	Label-free, high temporal resolution	Invasive	Preclinical	Visible and near infrared laser
Two-photon	1 mm	1 μm	Microseconds	High spatial resolution	Invasive, photobleaching issues with dyes, scattering due to tissues	Preclinical	Infrared laser
FTIR	<1 cm	5–12 μm	Seconds to minutes	Label-free method, short imaging time	High attenuation in liquid environment, difficult to distinguish closely related molecular structures	Preclinical	Infrared light
Raman	5 mm	<1 μm	Minutes to days	Label-free analysis, high spatial resolution, can work with liquid environment	Complex statistical analysis may be required to separate analytes, long imaging time required for imaging large area at high resolution	Preclinical	Visible and near infrared laser

^aSPECT: Single Photon Emission Computed Tomography; PET: Positron Emission Tomography; CT: Computed Tomography; fMRI: functional Magnetic Resonance Imaging; EEG: electroencephalogram; MEG: magnetoencephalography; fNIR: functional Near-Infrared Imaging; VSDI: Voltage-Sensitive Dye Imaging; LSCI: Laser Speckle Contrast Imaging; FTIR: Fourier Transform Infrared Microscopy.