Magnetic resonance imaging (MRI)
is non invasive method of mapping the internal structure and certain aspects of function within the body.
It uses nonionizing electromagnetic radiation and appears to be without exposure-related hazard
Name of MRI previously is nuclear magnetic resonance (NMR),first described by Bloch and Purcell in 1946 .
NMR has been used extensively as a laboratory method for studying the properties of matter at the molecular level (NMR spectroscopy).
In applications to medicine, it is now commonly referred to as magnetic resonance (MR)
Application for human study between 1973-1977
The basic principle of MRI depend on the fact that the nuclei of certine elements align with the magnetic force when placed in astrong magnetic field
At the field strengths currently used in medical imaging ,hydrogen nuclei (protons ) in water molecules and lipids are responsible for producing anatomical imaging .If radiofrequency of hydrogen is applied ,aproportion of the protons change alignment ,flipping through apreset angle ,and rotate in phase with one another,fallowing this radiofrequency pulse,the protons return (realign )to their original postion .
As the protons realign (relax ) they induce aradio signal which ,although very weak,can be detected and localize by antensana coils placed around the patient .
An image represent the distribution of the hydrogen protons can be bult up .
The strength of signal depends not only on proton density but also on tow relaxation times .T1 and T2
T1-depend on the time the proton s takes to return to the axis of magnetic fields;T2- depend on the time the protons take to diphase .
AT1-weighted images is one in which the contrast between tissue is due mainly to their T1 relaxation propereties ,while in a T2-weighted image the contrast between tissue is due mainly to their T2 relaxation propereties .
Most pathological processes show increase T1 and T2 relaxation times and these processes therefore appear lower in signal (blacker ) on a T1 –weighted scan and higher in signal (whiter ) on a T2 –weighted scan than the normal sarrounding tissue .the T1 and T2 weighting of an image can be selected by approperiatly altering the timing and sequences of radiofrequency pulses .
Atypical MRI scanner consist of large circular magnet.Inside the magnet are the radiofrequency transmitter and receiver coils as well as gradient coils to allow spatial localization of the MRI signal .Ancillary equipment converts the radiosignal into adigital form which the computer can manipulate to create image.
DVANTAGES OF MRI
· Information can directly imaged in any plane .
· No ionizaing radiation
· Bone and air do not produce artifact
· Non invasive
· No adverse biological effects from diagnostic MRI
· Soft tissue contrast is high
DISADVANTAGE
· Require longer scaning time comperd to CT scan ,so the patient keep still during scaning procedure .
· An avoidable movement from breathing mcardiac pulsation and perestalsis often degrade the image .
· Strong magnetic field used mean that it is at present contraindicated in patient with cardiac pacemakers,intraocular metallic forign bodies and certain types of aneurysm clip .
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Contrast agent for MRI
Contrast agent providing useful diagnostic information with MRI .the most widly used agents are gadolinium compounds which only cross the B.B.B. when it is damaged by disease and which concentrate in tissue and diseases processes with high blood supply .
Tissue which concentrate the agent show very high signal intensity ( they appear white ) on T1 –images
Tissue specific media ,such as iron oxide agents for reticuloendothelial cells imaging
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .