MRI

B9402037陳名揚

原理

Signal intensity depends on - proton density and the relaxation times (T1 time and T2 time)

T1 weighted image

• Spin-lattice or longitudinal relaxation time• Z 分量的回復 (63%)• Slow molecular motion of fat nuclei, longitudinal

relaxation occurs rather rapidly and longitudinal magnetization is regained quickly. short T1 time for fat. strong signal

• High mobility of the water molecules. Water nuclei do not give up their energy to the lattice (surrounding tissue) as quickly as fat. long T1 time. weak signal

T2 weighted image• Spin-spin or transverse relaxation time• X-Y 分量的歸零 (37%)• temperature dependent - At a lower temperature

molecular motion is reduced and the decay times are reduced.

• Fat has a very efficient energy exchange and therefore it has a relatively short T2. weak signal

• Water is less efficient than fat in the exchange of energy, and therefore it has a long T2 time. strong signal

FLAIR

• Fluid Attenuation Inversion Recovery• The TI time of the FLAIR pulse sequence is

adjusted to the relaxation time of the component that should be suppressed. erase the signal of fluid

• Differentiation of brain and spine lesions.(Lesions that are normally covered by bright fluid - T2WI)

Contrast agents - gadolinium

• Positive contrast agents • cause a reduction in the T1 relaxation time

(increased signal intensity on T1 weighted images).

• Gadolinium, Manganese, or Iron. • have unpaired electron spins in their outer

shells and long relaxivities.

T1WI (T1 weighted image) T2WI (T2 weighted image)

Bone, air No signal No signal

Fat Stronger signal Weaker signal

Water Weaker signal Stronger signal

White matter(contain fat tissue)

Stronger signal Weaker signal

Gray matter Weaker signal Stronger signal

口訣•Ｔ１是『肥肥白白』！•fat, dermoid tumor, lipoma, atheroma, some metastasis

•Ｔ２是『水水亮亮』！•CSF, cyst, hygroma, cerebromalacia, ischemia, edema, demyelination, most malignant tumors

Reference

• http://www.mr-tip.com/serv1.php?type=welcome

• 住院醫師丹尼爾的部落格 - http://tw.myblog.yahoo.com/daniel-doctor/article?mid=740&prev=915&next=482&l=f&fid=29

• http://www.ym.edu.tw/cnl/summerschool/miao.pdf(這個算是延伸閱讀吧 )