Magnetic Resonance Imaging

MRI is based on the magnetization properties of atomic nuclei. A powerful, uniform, external magnetic field is employed to align the protons that are normally randomly oriented within the water nuclei of the tissue being examined. This alignment (or magnetization) is next perturbed or disrupted by introduction of external radio frequency (RF) energy. The nuclei return to their resting alignment through various relaxation processes and in so doing emit RF energy. After a certain period following the initial RF, the emitted signals are measured. Fourier transformation is used to convert the frequency information contained in the signal from each location in the imaged plane to corresponding intensity levels, which are then displayed as shades of gray in a matrix arrangement of pixels. By varying the sequence of RF pulses applied and collected, different types of images are created. Repetition Time (TR) is the amount of time between successive pulse sequences applied to the same slice. Time to Echo (TE) is the time between the delivery of the RF pulse and the receipt of the echo signal.

Tissue can be characterized by two different relaxation times – T1 and T2. T1 (longitudinal relaxation time) is the time constant which determines the rate at which excited protons return to equilibrium. It is a measure of the time taken for spinning protons to realign with the external magnetic field. T2 (transverse relaxation time) is the time constant which determines the rate at which excited protons reach equilibrium or go out of phase with each other. It is a measure of the time taken for spinning protons to lose phase coherence among the nuclei spinning perpendicular to the main field.

MRI can be used to address questions of the structural integrity of the brain. Additionally, MRI can be a measure of functionality of the brain. Advanced sequences can be used to visualise fluctuations in the haemodynamic response, flow dynamics, and white matter connectivity and composition. From these attributes, functionality can be inferred.

Through our research partners, Macquarie MindScan has an extensive list of equipment available for stimulus presentation and collection of participant responses to facilitate activation studies. Follow the links for more details on the MRI scanners installed and the current functional and structural sequences available via Macquarie Medical Imaging.

 

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