Abstracts-part VI (Continue in Part VII)

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Haakil Lee, Douglas H. Moms, Sudeep Chandra, and Paul C. Lauterbur. Biomedical Magnetic ... 4) D. N. Guilfoyle, P. Gibbs, R. J. Ordidge and P . M a n s f i e l d ...
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Encoding Velocity Information in NMR Images by Phase Tagging Haakil Lee,Douglas H. Moms, Sudeep Chandra, and Paul C. Lauterbur Biomedical Magnetic Resonance Laboratory, University of Illinois, Urbana IL 61801 convenient resolution. NMR i m a g e s o f moving s p i n s , a s u s e d i n a n g i o g r a p h y or c a r d i a c i m a g i n g , c a n d i s p l a y p h y s i o l o g i c a l and p a t h o l o g i c a l c o n d i t i o n s of organs. Therefore, p u l s e sequences designed t o e n c o d e v e l o c i t y i n f o r m a t i o n i n NMR i m a g e s a r e a l w a y s of i n t e r e s t . W e p r o p o s e a s e q u e n c e t h a t can g e n e r a t e phase s h i f t s proportional t o t h e v e l o c i t y . This modulation p a t t e r n is s i m i l a r t o C r a n s i e n t m a g n e t i z a t i o n g r a t i n g s (1) a n d SPAMM ( Z ) , w h i c h a r e g a i n i n g a t t e n t i o n i n i m a g i n g o f moving s p i n s . The s e q u e n c e , when combined w i t h s t a n d a r d phase-encoding imaging techniques, produces intensity-modulation p a t t e r n s f o r s i g n a l s from moving s p i n s , A leaving stationary spins unaffected. c o n s e q u e n c e of t h i s i s t h a t i m a g e s o f f l o w i n g s p i n s c a n b e made w i t h or w i t h o u t s t a t i o n a r y s p i n s i n t h e i m a g e s . The s e q u e n c e c a n e a s i l y be extended t o t h e encoding of a l l t h r e e v e l o c i t y components w i t h a m i n i m a l i n c r e a s e in acquisition t i m e .

E x p e r i m e n t s were d o n e on a S p e c t r o s c o p y Imaging S y s t e m s C o r p o r a t i o n (SISCO) 4 . 7 T / 3 3 c m NMR i m a g i n g s p e c t r o m e t e r s y s t e m w i t h a 2 5 . 4 cm c l e a r b o r e w i t h i n t h e g r a d i e n t and shim c o i l s . NMR i m a g e s were a c q u i r e d w i t h a n 8 c m d i a m e t e r saddle c o i l . The v e l o c i t y e n c o d i n g by p h a s e m o d u l a t i o n was p e r f o r m e d p r i o r t o t h e n o r m a l A 90° s p i n echo imaging sequence ( F i g . 1 ) . f l i p a n g l e (a) was u s e d . Images were a c q u i r e d with 128 phase-encoding s t e p s and a r e l a x a t i o n The m o d u l a t i o n i s g i v e n by A@ = d e l a y of 1 s . y G v a ( a + b ) , where y i s t h e gyromagnetic r a t i o of t h e n u c l i d e , G t h e g r a d i e n t f i e l d s t r e n g t h , v i s t h e v e l o c i t y o f t h e moving s p i n s , a n d a and b a r e t h e g r a d i e n t d u r a t i o n t i m e and i n t e r - g r a d i e n t p u l s e d e l a y , r e s p e c t i v e l y . The f o r m a l 1:l b i n o m i a l p u l s e i s d i v i d e d b y a rc pulse t o avoid dephasing via field inhomogeneity ( 3 ) . The r e s u l t i n g i m a g e e x h i b i t s p h a s e m o d u l a t i o n b a s e d o n l y on t r u e velocity.

sliding

scale

velocity

The a d v a n t a g e o f t h e s e q u e n c e i s t h e a b i l i t y t o i d e n t i f y a n d q u a n t i f y t h e moving s p i n s s i m u l t a n e o u s l y . The r e s u l t i n g image i s a n a l o g o u s t o a v e l o c i t y c o n t o u r map. Encoding of a l l three orthogonal v e l o c i t y components c a n b e a c c o m p l i s h e d e i t h e r by r e p e a t e d t a g g i n g a l o n g t h r e e d i r e c t i o n s or by applying three orthogonal gradients s e q u e n t i a l l y i n s t e a d of one a s i n t h i s example. This t h r e e dimensional v e l o c i t y t a g g i n g c a n be f o l l o w e d e i t h e r by 2 D i m a g i n g o r 3D volume i m a g i n g . A l s o , s t a t i o n a r y s p i n s c a n be s u p p r e s s e d if t h e s e c o n d a p u l s e i s s h i f t e d 90 d e g r e e s i n p h a s e r e l a t i v e t o t h e f i r s t a p u l s e . A r e c e n t a p p l i c a t i o n of t h i s s e q u e n c e t o EPI g a v e r a t h e r d i s t o r t e d f l o w i m a g e s ( 4 ) . NO s u c h a r t i f a c t s h a v e b e e n f o u n d i n o u r s t a n d a r d phase encoding experiments, which show t h a t t h i s s i m p l e s e q u e n c e c a n g i v e clear images and accurate velocity information.

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1) T . R. S a a r i n e n a n d C S . J o h n s o n , Res. , 18, 257 ( 1 9 8 8 ) .

J . , Mag.

2 ) L . Axel and L . Dougherty, R a d i o l o g y , U, 8 4 1 (1989). 3 ) H. D . M o r r i s , H . L e e , a n d P . C . L a u t e r b u r , E x p e r i m e n t a l N.M.R. C o n f e r e n c e , S t . L o u i s 1991 4 ) D . N . G u i l f o y l e , P. Gibbs, R. J . Ordidge a n d P . M a n s f i e l d , Magn. R e s o n . Med. 1 8 , 1 (1991).

Results F i g u r e s 2 a n d 3 show t h e e f f e c t s of t h e gradient pulses on t h e image. I n both f i g u r e s , t h e g r a d i e n t p u l s e l e n g t h ( a ) was 10 m s . a n d i n t e r g r a d i e n t p u l s e d e l a y ( b ) was 4 ms.. The o u t e r t u b e ( d i a m e t e r 2 . 0 Cm) was f i l l e d w i t h a n a g a r gel t o p r o v i d e t h e stationary spins. The i n n e r c o m p a r t m e n t ( d i a m e t e r 0 . 4 c m ) was a t u b e of f l o w i n g w a t e r w i t h a mean v e l o c i t y o f 0 . 2 6 c m / s . C o n s t a n t f l o w r a t e was m a i n t a i n e d by a s y r i n g e pump. I n F i g . 2 w e n o t e t h a t i n t h e absence of a g r a d i e n t no p h a s e m o d u l a t i o n i s a p p a r e n t . ~n F i g . 3 t h e t o t a l p h a s e m o d u l a t i o n o f 90 d e g r e e s i n a g r a d i e n t of 1 . 5 G/cm g i v e s a v e l o c i t y of 0 . 2 6 c m / s . The p e a k v e l o c i t y o f 0 . 4 c m / s g i v e s a p h a s e s h i f t o f 1 3 5 degrees. In f i g . 3 t h i s gives a bright plateau a t the c e n t e r of t h e t u b e i n s p e c t r a l power mode. A t any g i v e n s t r e n g t h and d u r a t i o n of t h e g r a d i e n t p u l s e t h e modulation i s a f u n c t i o n of velocity alone. Altering t h e gradient strength o r t h e g r a d i e n t p u l s e d u r a t i o n w i l l change t h e phase modulation frequency. This allows f o r a

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Fig. 2