BASICS OF MRI - HOW MRI WORKS

BASICS OF MRI - HOW MRI WORKS - MRI CONCEPTS

- June 14, 2011

Based on gyromagnetic property of protons(or hydrogen nucleus, H+)
It can be plain MRI or contrast MRI.M/c contrast agents used is i.v. Gadolinium
Proton Density and relaxation time are assessed by radiofrequency pulse and the computer generates a grayscale image from this data.
Magnetic field used in the range of .15 to 1.5 tesla
Relaxation time:
T1:Time taken to return to original axis(T1 images are used to find out normal anatomical details)It has got high soft tissue discrimination(CSF looks black)
T2:Time taken by protons to displace. Used to asses pathological process.(fluid looks white)
Characteristics of MR signals
CSF: Hypo Intense on T1W1 and vice versa(T2 scan shows whiter)
Fat:Hyper intense on T1W1(T1 scan shows whiter)
C/I-patients with prosthesis in the body , metallic foreign bodies, pacemaker, cochlear implants, cranial aneurysm clips should never undergo MRI.
Claustrophobia common with MRI.
There are no beam hardening artefacts in MRI as in CT. MRI is a superior imaging tool for middle cranial fossa, skull base, neural canals and intraspinal contents.
MRI has high soft tissue contrast resolution with exquisite display of gray and white matter. It is highly sensitive to presence of cerebral edema and for evaluation of chronic hemorrhages.
MRI is inferior to CT for detecting calcifications and/or hyperostosis associated with brain tumors(eg. meningioma). It is also not as sensitive to acute hemorrhages as CT.

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