Positron Emission Tomography

PET gives a three-dimensional image with brain areas colored according to radiotracer uptake (e.g., indicating metabolic activity). The spatial resolution is about 4-8 mm3.

How It Works

A radioactive tracer (radiotracer) is injected or inhaled, goes to the brain, and accumulates in regions with, for example, high metabolism, emitting gamma rays. Images revealing these regions are computed from the data from tens of thousands of gamma-ray detector elements positioned around the head.

Functional Magnetic Resonance Imaging

This technology measures blood flow activity and produces three-dimensional images with regions colored according to calculated neural activity levels. An fMRI scan can show a live brain in action down to the cubic millimeter level—a relatively high spatial resolution. However, there can be a million neurons in that space. The resolution is low with respect to neural architecture; thus, fMRI cannot reveal finer cellular architecture or activity. The time resolution is low—on the order of hundreds of milliseconds. In contrast, EEG and MEG have higher time resolution than fMRI (on the order of milliseconds) and lower spatial resolution (on the order of centimeters rather than millimeters).

How It Works

fMRI measures magnetic properties. Active neurons consume more oxygen and gain specific magnetic properties. These magnetic properties are detected by exposing the brain to fluctuating magnetic fields, resulting in magnetic responses, which can be measured, analyzed, and used to build three-dimensional images.


EEG measures gross neural activity near the scalp, is suitable for detecting abnormal firing patterns, and can be used for diagnosing epilepsy. Sleep researchers use EEG to monitor sleep stages. EEG responds fast to electrical activity but has low spatial resolution.

How It Works

EEG measures electric potentials created by many neurons, using electrodes placed on the scalp.


MEG measures gross neural cortical activity. MEG has a high temporal resolution of up to one millisecond, almost as fast as measurement of neural activity with electrodes. MEG has low spatial resolution, but the technology has potential for improvement.

How It Works

MEG detects magnetic fields generated by electricity flowing through neurons, using sensors on the scalp. The fields are so weak that MEG equipment is housed in magnetically shielded rooms.

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