MRI

MRI

Magnetic resonance imaging (MRI) is a non- invasive method of mapping structure and various aspects of function within the body by producing images by the virtue of gyromagnetic property of protons, with the greatest advantage of not using ionizing radiation for imaging

Principle of MRI        :  NMR given by Purcell and Bloch (1952)

Invention of MRI       : Lauterbeur and Mansfield (2003)

Functional MRI          : Ogawa and Rosen

Magnetic resonance is a phenomenon whereby the nuclei of certain atoms, when placed in a magnetic field ,absorb and emit energy at a specific or resonant frequency.

          Nuclei suitable for MRI contain odd no of protons and neutrons .Almost all clinical MR images are produced using the simplest of all nuclei that of hydrogen, Which is present in virtually all biological material and exhibits relatively high MR sensitivity.

PARTS OF MRI

. Maxwell Coil –Gradient coil used to create magnetic field gradients  along the direction of the main magnetic field

. Radio frequency coils : to generate stronger magnetic field along the scan field

.Faraday cage/shield/Hoffman Box  - blocks out external static electric fields. Made up of copper

.Superconductors – Electromagnets measured in Tesla

HOW IMAGE IS ACQUIRED IN MRI

.In the absence of magnetic field all protons have random movement

. Under the influence of magnetic field, alignment of proton takes place

. Realignment occurs under the influence of radiofrequency pulse

. After switching off the radiofrequency pulse, the atoms return to their original position and release energy in the form of electrical voltage signal

.When proton releases energy to surrounding lattice, it is called as spin lattice relaxation or T1 relaxation

. When proton releases energy  to  the surrounding spin ,it is called as  spin-spin relaxation or T2 relaxation

. This electrical signal (produced by receiver coils) is digitized and analyzed in a computer, to produce MR images.

          Using T1 and T2 relaxation time , two types of images are broadly acquired using this principal; T1 weighted MR and T2 weighted MR.

CONTRAINDIVATIONS OF MRI

              It is a common saying that MRI means  “  metal results in injury”  however it is not true in current scenario an there are many metallic devices compatible with MRI and students should be aware of those , particularly devices made up of TITANIUM which are very much compatible with MRI.

       Few important devices or conditions where MRI is usually contraindicated are  :

. Cardiac pacemakers

. Cochlear implants

. Intraoccular  Metallic foreign body

. Electronically, magnetically and mechanically activated implants

. Ferromagnetic or electronically operated active devices like defibrillators

. Aneurysmal clips

. Prosthetic heart valves- metallic

. Insulin pumps and nerve stimulators

. stapedial implants

. Claustrophobia

. First trimester of pregnancy

MRI can be safely performed in

. Orthopedic implants (usually made of titanium). In cases of implants made of stainless steel, MRI can still be done however, heating is a problem so it is usually recommended for a shorter time and on a lower strength magnet. In permanent tattoos also because of the heavy metal dyes, heating may be an issue.

.Pregnancy after 1^{s t}  trimester. Safety profile of MRI is not established in first trimester so, should not be done in 1^st trimester

.cholecystectomy clips, sternal sutures

.IUCD

.Non metallic foreign bodies

.Breast implants

.Pediatric patients   ( to avoid for ionizing radiation)

.Coronary stents usually drug eluting stents are used and MRI can be done after 3 months.