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W illiam P . S h u m an , M D # { 1 4 9 }D avid R . H ay n or , M D , P h D # { 1 4 9 }A rth u r W . G u y , P hD {149}eorge E . W esb ey , M D 2 # { 1 4 9 }D an i e l J. S chae fe r , P hD # { 1 4 9 }A lb er t A M o ss, M D

S u p erfic ia l- a n d D eep -T issu e T em p era tu reIn crea ses in A n esth e tized D o g s d u r in g

E x p o su re to H ig h S p ec ific A b so rp tio nR a tes in a 1 .5 -T M R Im a g er

1 . 2 .Figures 1 , 2 . (1 ) C T -sc ann ed p ro ject ion ra d io gra ph of a d og ly ing on its side . N eed les in thelive r, m id pem ito ne um , gro in , a nd th ig h m usc le are id en tifie d . C T sca ns w ere th en ob tainedat the leve l o f eac h nee d le tip (arro w s ) to co nfirm its lo cation . (2 ) C T sc an thro ug h liv er atth e lev el of the n eed le tip d em on stra tes its po sition (a rro w ). T h e d og w as ly ing o n its le ft

55 1

S u p er fic ia l- an d d eep -t issu e h eatin gw as m easured in five d og s d u rin gh igh -sp ecif ic -a b sorp tio n -ra te rad io -freq u en cy (R F ) irrad ia tion to seew h eth er sig n ifican t tem p eratu rech an ges cou ld b e p ro d u ced b y a 1 .5 -T c lin ica l m ag netic reso n an ceim ag er . T h e R F p ow er ou tp u t em -p lo yed w a s 6 .3 tim es th a t req u iredfor rou tin e im ag in g . T em p eratu rep rob es w ere p la ced in b oth d eepa n d su p er fic ia l t issu es, an d tern -p eratu res w ere reco rded b efore , d u r-in g , an d a fter exp osu re . In each d og ,th ere w as a lin ea r tem p eratu re in -crea se o f sev era l d eg rees d u r in g R Fexp o su re ; th e m axim a l averagech an g e w as 4 .6 {176 }Cn th e u rin a ryb la d der . T h e tem p erature in creasew as sligh tly g rea ter in d eep tissu esth an in su p er ficia l t issu es. T h e ca l-cu la ted sp ec ific a b sorp tio n ra te ,b a sed on th e tem p era tu re ch an ge ,a v era g ed 7 .9 W /kg fo r all five dog s.T h ese fin d in gs argu e for con tin u edcau tion in th e d esign an d op erationo f im ag ers cap a b le o f h ig h sp ec if ica b sorp tio n rate s, p ar ticu lar ly w h enth ey a re u sed fo r im ag in g in fan ts orp a tien ts w ith a ltered th erm o regu la -t or y c ap ab ility .

Inde x term s: M a gne tic re son anc e (M R ), b io log-ica ef fects # { 1 4 9 }a gne tic re so nan ce (M R ), e xpe ri-me nta l # { 1 4 9 }ag net ic resona nce (M R ), tec hn olo gy

R adio lo gy 1 98 8; 167 :55 1-5 54

I F rom the D epa rtm en t o f R a dio log y , U n ive r-s ity of W a shington H ospita l, 195 9 N .E . P a cificS t., S ea ttle , W A 9 81 95 (W .P .S ., D .R .H ., G .E .W .,A .A .M .); the C en ter fo r B ioen gin eer ing , U ni-v ers ity o f W a sh ing ton , S e att le (A .W .G .); andth e M edical S ystem s G rou p , G en era E lec tricC om pa ny , M ilw a uk ee (D .J.S .). R e ceiv ed O c to-b er 2 , 1 987 ; rev isio n req ue sted N ove m b er 6 ; re -v isio n rece ive d N o vem be r 3 0 ; ac cep ted D ece m -b er 22 . S u ppo rte d in par t b y a gra n t f rom G en -e ra E lec tric M ed ical S ystem s. A d dress rep rin trequ ests to W .P .S .

2 C urren t add res s: D ep artm e nt of R ad io lo gy ,S cripps M e m o rial H o sp ita l, L a Jo lla , C alif.

C R S N A , 1988

side

A D I O - F R E Q U E N C Y (R F) rad ia tioncan cau se hea ting in liv ing tis -

sue . T he curren t F ood and D rug A d-m in is tra tio n (F D A ) recom m end ationsta tes tha t R F rad ia tion exp osu re dun-in g m ag ne tic resonance (M R ) im ag -in g o f hum ans represen ts an ins ig -n ifican t risk if it does no t exceed aw ho le -b ody average spec if ic ab som p -tion m ate o f 0 .4 W /kg (1 ) . T h is ra tew as derived fro m rep orts ind ica tingth at th e on se t o f the rm al e ffec ts(w ork stop page in laborato ry an i-m als ) took p lace a t 4 W /k g ; to d erivea broad safe ty m arg in , th is va lu e w asd iv ided b y a fac to r o f 1 0 (1 ). M o re m e-cen t reports have su ggested tha t, u n -der ce rta in co nd itions, a h ig her spe-cific abso rp tion ra te (0 .4 -4 .0 W /kg)m ay be used fo r M R im aging in hu-m an s w ithou t resu lting in dang eroustis sue hea tin g (2 -6 ) . In these s tud iestem pera tu re ch an ges in an im als an dhum ans w ere m easu red im m edia te lybefo re and afte r (b u t no t dur in g) im -ag ing . T em pera tu res w ere m easu redin the m outh (sub lingua l pocke t) andesoph ag us and on the surface of th esk in ; o ral and esop hagea l tem pera -tu res in c reased b y u p to 0 .5 #{17 6}C ,andsurface sk in tem p eratu res by up to3 .5 #{17 6}C .T hese find in gs a re cons isten t

w ith theory , w hich pred ic ts g rea te rR F -indu ced h ea ting superf ic ia llythan in m ore in te rna l loca tio ns (2 ,6 -9 ) .

W e undertoo k th is s tu dy to m ea-su re bo th sup erficia l- an d deep-tissueh ea ting du rin g im ag in g a t a spec ificab sorp tio n ra te th at w as m uch h igherthan is n orm ally used clin ica lly bu ttha t w as s till ach ievab le w ith a clin i-ca l M R im ager. T o accom plish th is, itw as necessa ry to use an an im al tha t issm aller than ad u lt hum ans and to by-pass bo th th e R F po w er m on ito r o fthe c lin ica l M R im ager and the ex ist-ing so ftw are , w hich h as bu ilt-in sa fe -guards d es igned to preven t the use o fh ig h spec ific absorp tion ra tes.

M A T E R IA L S A N D M E T H O D SW e stud ied five hea lthy m ongrel dogs

w e igh ing 1 8-2 8 k g . E a ch d og w a s g iven3 0 m g /kg o f so d iu m p entoba rb i ta l in tra-ven ou sly to in duc e a surg ic al leve l of an-esth es ia ; a n ad dit ion al 1 5 m g/k g w a s ad-m in iste red to ea ch d og du rin g th e ex per i-m e nt to m ain tain ane sth esia . E a ch d ogw a s in tu ba ted a nd allo w e d to breatheroom a ir. T he ven tila to ry m ate w as m on i-to re d a t app rox im ate 2-m in ute in te rva ls.

W ith ea ch do g , lon g (15-cm ) 16 -ga ugen eed les , o rig inal ly d esigne d for p lacing

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3 . 4 .Figu res 3 , 4 . (3 ) C T sca n thro ug h th e d ogs abd om en a t th e leve l of the ne ed le t ip in them idpe rito ne um (a rro w ). (4 ) C T scan d em ons tra tes the po sition of the n eed le in the sub cu ta-neo us t issu e o f the g ro in (a rrow ).

55 2 . Rad io logy M a y 198 8

th erap eu tic hy per the rm ia se nso r th ern tis-to rs (D eseret M ed ical ; S an dy , U tah ), w erepe rcu tan eou sly p osi tion ed in th e pa ra-sp ina l n eck m u scle an d /o r th igh m u scle ,th e m id liver an d m idp eriton eum (gu tm ese n tery reg ion ), a nd th e su bcu tan eou stis sue s of th e ax illa an d gro in (m id lin e).A f ter th e n eed les w ere p la ced , th e po si-tio n of e ach n eed le tip w as co nfirm e dw ith com pu ted tom og raph y (C T ) . A C T -sc ann ed p ro ject ion ra d io gra ph (S cou t-V ie w ; G ene ral E le ctr ic , M ilw auk ee ) w a sf irst ob tain ed to loca te eac h n eed le tip(F ig . 1 ), an d the n ax ial C T sc an s w e re ob-tam ed at th e leve l o f e ach tip (F igs . 2 -4).O c cas ion ally , the po sit ion of o ne o r m oreneed les w as ad ju sted an d the new po si-tion c on firm ed by m e an s o f C T .

A ne sthe sia w as indu ce d just b efo rep lace m e nt o f the n ee d les so th at it con tin -ued fo r the dura tion o f the C T stud y (1 8-25 m inutes) , the m o ve to the ad jac en t M Rim a gin g su ite (3 m in ute s), and the setupo f th e M R im aging exp er im en t (12 -15m inu tes ). D urin g th is tim e, n o attem ptw as m ade to ex terna lly re gu late o r m a in-ta m the dog s bo dy te m p era tu re .

E a ch 16 -ga uge nee d le fit snug ly ov er ab l ind p lastic ca the ter tw ice a s lon g as th ene ed le; the m e tal ne ed le w as sc ore d lo n-g i tud ina lly so that it c ou ld be ba cke d ou tof tissu e (leav ing the cathete r in the tis -su e), sp l it lo ng itud ina lly , and rem ove dfro m the p las tic catheter w itho u t d is od g-ing the c athe ter t ip . A hig h-re sis tan cethe rm isto r se nso r on the e nd of a w ireco u ld b e slid do w n to th e e nd o f the b lin dca theter a fter the nee d le w as rem ove d;dep th m a rks on the catheter an d thethe rm isto r w ire w ere u sed to en sure po si-t ion ing o f the therm is to r at the end o f thecatheter .

In th ree of th e do gs, a therm is to r w asinse rted in to th e ur inary b lad der th ro ug ha F ole y ca the ter, a fter the b la dde r w asd is ten ded w ith 50 m L of roo m -tem pera-tu re salin e so lu tio n . T h e saline w as left inthe b lad der for ap pro x im ate ly 8 m inu tesbefo re th e sta rt o f scann in g in order toach iev e p artia l eq u il ib ratio n w ith bo dytemperatu re .

A ltho ug h th e h igh- resista nce th erm is-to rs w ere de sig ned to m e asu re tem pe ra-tu re in a h igh -R F env iro nm ent (3 0-1 , 00 0M H z), p lac ing them in snu g-fittin g p las -tic ca the ters prov ided so m e deg ree ofe lec tric al in su lation fro m ad jac en t tissu ea nd m in im ized any in duc ed curren t inthe th erm isto r w ires . In a dd itio n , th e te r-m inal 3 0 .5 cm of th e co nn ection w a sh ig h-resistan ce carb on -im pre gna ted T e f-lon rather tha n w ire . T he therm is to rw ires w ere con ne cted by a 5 .5 -rn cab le toa B S D -30 0 m ob ile co m p ute rize d tem pe ra-tu re rec ord ing sys tem d esig ne d to co llec t,d isp lay , a nd re cord tis sue tem peratu reda ta dur ing the rap eu tic h yp erth erm ia(B S D M ed ica l C o rpo rat ion , S alt L a keCity ) .

T h e h igh -res istanc e the rm isto r p rob esw e re com pu ter ca lib rate d before ea ch ex -pe rim ent w ith a p ro be cal ibration bathan d a sta nd ard probe calib ra ted by th eN ationa l B urea u o f S tand ard s. T h e p rob eacc ura cy w as 0 .1# { 176 }C . D u ring eac h ex pe ri-

m e nt, th e te m p era tu re rec ord ing sys temw as loca ted jus t ou ts ide th e m agne ticsh ield of th e M R im a ger an d w a s the re-fo re in a m a gne tic f ie ld o f on ly 0 .0 03 Teven th ough it w a s ju st 5 .5 m fro m thecen ter of the bo re o f the 1 .5 -T m agn et.

W e used a sta nd ard c lin ica l 1 .5 -Tw ho le-bod y M R im a ger (G e neral E le ctric)o pe rating a t 64 M H z w ith a qu adratu rec o il . T h e softw a re and pow er m o nito r o fth is system ha d be en m odified to al lowu se of a c alc u la ted spec ific a bsorp tion m ateg rea ter th an 0 .4 W /kg . A re sea rch v ers iono f ope rating so ftw are w a s em plo ye d , av ersion ava ila b le o n ly to in stitu tio ns thath ave app rov al from their loca l hum ansu bje cts rev ie w bo ard to ex po se hu m a nsto greate r th an 0 .4 W /k g . T his sof tw area llow s th e sp ecific-absorp tio n-ra te m o ni-to rs to be ov erridde n w ith a spec ific key -s troke com mand .

T h e room am bien t te m p era tu re w asm ain tain ed at 1 8# {1 76} -2 0#{ 1 76} Cith a re la tivehu m id ity o f 35% -4 0% ; the m a gn et b orefa n w a s on du rin g e ach exp erim e nt toprov ide a m e asu red airflo w of 0 .1 0 m 3 /m m . A fter th e h igh- res ista nce therm is to rse nso rs w e re p osi tion ed , eac h a nes the -tize d d og w as p la ced in the m a gn et b orew ith its bod y w ith in the co nfin es o f theR F co il. (T he co il w as 150 cm long , andthe d og s va rie d in leng th f rom 8 0 to 100c m . )

B ase lin e tem pe ratu res w ere reco rde deve ry m inute fo r 15 m in ute s from e ach o fthe s ix th erm isto r se nso rs (F ig . 5 ) . A 25 .6-m in u te m a xim a (1 6-k W ) R F p ow er ou t-p u t pu lse se que nc e w a s then p erform ed(w ith o n ly 0 .2 dB of R F tra nsm it at ten ua-tio n). T e m pe rature s w ere re corded e veryminute dur ing th e sequ en ce. In d og 5 , thep u lse seq uen ce laste d 35 .6 m in ute s be-c ause o f a n im ag er m a lfun ction . S inc e op -tim a l im ag ing fo r th ese do gs w ou ld h av eu sed a n R F tran sm it atte nu atio n o f ab out8 .0 dB (fo r a 90 #{ 176 }u lse) , the po w e r ou tpu tfo r th is m ax im al seq ue nce (no tra nsm ita tten ua tion ) w a s ab out 6 .3 tim e s g rea terth an th at requ ired for rou t ine im ag ing .T he sp in-ech o pu lse se que nce em ploye d arep etitio n tim e (T R ) o f 1 ,000 m se c; e chotim es of 20 , 40 , 6 0 , an d 80 m sec; six repe ti-

tio ns ; e ig h t 5-m m -th ick se ctio ns w ith 2 .5 -m m inte rva ls; an d a 256 X 2 56 m atrix .T em pera tu re record ing w as co n tin ued fo r1 5 m inu tes a fter the en d of the pulse se -quence .

In dog 3 w e a lso es tim ated th e spec ificab sorp tion m ate by com pu tin g th e netpeak absorbed pow er . D uring th e pu lseseq uence , th e fo rw ard , re flec ted , an dloa d-d ive rted R F p ow er w a s m ea sured .T he ne t peak co il lo ss p ow er (m easured a tthe sam e R F m a gne tic f ie ld s tren g th an ddeterm ine d w ith a sm a ll, los sles s p ha n-tom ) w as sub trac ted after th e pe ak p ow erleve ls w ere con ve rted in to av era ge le velsw ith use of the pu lse w ave form s. S in cethe T R w as know n, d iv id in g the ne t peaka bsorb ed po w e r b y the m a ss of the do g(2 2 kg ) y ield ed the sp ecif ic ab sorp tio nra te in w atts per k ilog ram .

R E S U L T ST he resp irato ry ra te w as som ew hat

dep ressed (to four to e igh t b rea thsper m inu te ) in a ll o f the d ogs d uringthe early p ortion of the exp er im en t,p resum ably because of th e anes th e-s ia. In tw o do gs, the resp ira to ry ra teacce le ra ted to 60 b reath s p er m inu tetow ard the end of the pu lse se -quence . D ur in g the in itial 15 -m in u tetem p eratu re m onito ring period (noR F ), a ll p robes recorded stab le tem -pera tum es in a ll dog s (chang e w as< 0 .3#{ 176} C ) (F ig . 5 ). D uring the pu lse se -quence (R F o n), the re w as a linea rtem p eratu re in c rease fo r a ll p ro bes ina ll dog s (F ig . 5 ). T he range of te rn -pera tu re inc rease w as 2 .4#{ 176} -4 .7#{ 176} Cav -erag e , 3 .7 #{17 6}C )fo r n eck m u sc le , 1 .3 #{ 176} -4 .3 #{1 76}C average , 2 .5# {176 }C )fo r th igh m us-d c, 3 .2 #{ 17 6} -5 .6 #{ 17 6} Ca ve ra ge , 4 .1 #{ 17 6} C)f orthe liv er , 3 .2#{1 76}-5 .3 #{17 6}Caverage , 4 .4#{ 176} C)fo r the m idp er iton eu m , 2 .i#{ 176} -5 .3#{1 76}C(av erag e , 3 .8 #{17 6}C )fo r sub cu taneo us ax-i lla ry ti ss ue , 2 .1 #{ 17 6} -5 .3 #{ 17 6} Ca ve ra ge ,3 .4 {176}C)o r su bc u ta neo us gro in tissue,a nd 3 .4 #{ 17 6} -5 .6 #{ 17 6} Caverag e, 4 .6#{1 76}C )fo r

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T a b l e 1S pec ific A bsorp tion R a te B ased on A v erage T em peratu re C han ge

D ura tio n of R FP u lse S e que nce A verage T em peratu re S p ecif ic A bso rp t ion

D o g W e igh t (k g) (m m ) C han ge (# {1 76} C ) R ate (W /k g)*1 28 2 5 .6 2 .6 5 .92 26 2 5.6 2 .9 6 .53 22 2 5.6 3 .6 8 .14 18 2 5 .6 4 .7 10 .55 22 3 5.6 5 .1 8.3

T he ave rag e rate w as 7 .9 W /k g .

V o lu m e 167 N u m b er 2 Rad io logy # { 1 4 9 }53

. N*p: i;o a

T E M P : \ 0> 7,. .0.

.55:55 55.P

T IM E M IN UT ES )

F igu re 5 . T em p era ture T EMP changeso ver tim e in do g 2 . F or clarity , tem pe ratu re sa re p lo tt ed fo r on ly four of the six tis sue s.T he tem pora l rela tionsh ips and the linea rityo f the tem pera tu re inc rease dur in g R F ex po-sure se en in th is gra ph w ere typ ica l. N oteth e re la t ively sm all ch an ge in the per iod sbefo re an d afte r R F exp osu re .

the u rina ry b ladd er. T h e estim atedm ean spec ific abso rp tion ra te (assum -ing no hea t loss ), w h ich w as derivedby averag ing th e tem pera tu rechanges fo r a ll p robes in each dog(us ing an av erag e tis sue-spec ific hea to f 0 .83 k cal [3 .47 kJ]/k g / #{ 176} C ),angedfrom 5.9 to 10 .5 W /kg (T ab le 1 ). F ordog 3 , th e ca lcu la ted sp ec ific ab sorp -tion m ate based on the average tem -pem atum e change fo r a ll tissues w as8 .1 W /kg , and the m ate de term in ed b ym easur in g the ab sorb ed p ow er in thedog -co il com b in ation w as 7 .1 W /kg .

D I S C U S S I O NA reduced ab ility to pe rfo rm tasks

has been o bserv ed in ra ts and m on -keys w hose bo dy tem p eratu res w ereinc reased b y 1 # {176 }Cfte r w ho le -bo dyR F exp osu re of 4 W /kg (10). In hu-m ans, a p ruden t spec ific -abso rp tio n -ra te th resh o ld o f 1 .5 W /kg fo r long -dura tion s tu d ies (2 0 m in u tes o rm ore) and 4 W /kg for s tu d ies o f 1 0m inu tes o r less has been sugges ted(2 ). T h is su ggestion is based on a b as-a l m etabo lic m ate in hum ans of 1 .5 W Ik g du ring s leep an d 15 W /kg du ringh eav y exerc ise. O ther s tu d ies havesug gested th at 4 W /k g m ay b e them -m ally accep tab le fo r longer periods(3 , ii). W ith h igh m etabo lic ra tes,such as those tha t occu r during exem -c isc, hea lthy hum ans are ab le to d is -s ipa te hea t th rou gh th e evapora tiono f sw ea t o ver a large sk in su rface areaand , to a lessen ex ten t, th rou gh an in -c reased resp ira to ry ra te , so tha t thecom e bo dy tem pera tu re does n o t rises ign ifican tly . T he coo ling of sk in d ueto evaporation of sw eat is dependen to n am b ien t tem pera tu re , h um id ity ,and a irf lo w . C o n tro llin g co re te rn -p em atu re w ith these m echan ism s isim por tan t, s ince a rise o f 4#{ 176} Crg reate r can p roduce hea t s tress, ce llu -lam dam age , an d even dea th .

C oo ling m ech an ism s in dogs arequ ite d iffe ren t from those in hu-m ans . D o gs lo se hea t m os tly b y pan t-ing , no t by sw ea tin g (12). In add i-tion , dog s have a heavy coa t o f h airas ex te rna l insu la tion an d , u n lik e hu-m ans , canno t rem o ve insu la tion toinc rease heat loss. O ur dog s in itia llyexp er ienced ven tilato ry sup press ion ,p robab ly because of th e anes the ticag en t used (p en tobarb ita l) , so th eyh ad little capac ity to ge t rid o f h eatfrom R F irrad ia tion . (P en tobam bita lby itse lf can increase the com e tem -p era tu re in dog s w hen g iven as a bo -lus , b u t a t a m uch slow er m ate , byten ths o f a d eg ree per hou r [13].)T hu s, our dogs constitu ted a nearw ors t-case situa tion from th e stand-po in t o f heat lo ss . In add ition , theseanes the tized dogs had little capac ityto genera te h ea t o r m ain ta in bod ytem pera tu re . T hey w ere also sm allenou gh to fit w ith in th e confines o fthe R F co il. T hus , th e ir en tire b od iesw ere irrad ia ted an d h ea ted . T h is situ -a tio n m ig h t s im ula te tha t o f a seda tedsm all h um an pa tien t (in fan t) w h o isw rapped in b lanke ts (unab le to ev ap-o ra te sw ea t and w ell insu la ted ).

T h e h i gh - sp e ci fi c -a b so r pt io n -r a teexpo sures fo r these ex perim en ts w erea lso d es igned to be a near w ors t-cases itua tion fro m th e s tan dpo in t o f hea tdepo sitio n . T h e h igh leve l o f R Fpow er used w as m uch grea te r thantha t requ ired fo r ty p ical im ag ing inthese dog s or in m any sm all- to aver-age-s ized hum ans . Im ag ing in heav i-em hu m an s m ay requ ire m ax im al R Fam plifie r pow er, sim ila r to w ha t w eused in th is experim en t, b u t in su chind iv idua ls the spec ific absorp tionra te w o uld typ ically b e low er be-cau se of the grea ter bo dy m ass . Inour d ogs , the low es t ra te w as in thela rges t dog (T ab le 1 ).

C om m erc ia l c lin ica l M R im agersh ave sa feguard s to p reven t th e m ad-verten t use o f excess ive ly h igh R Fp o w e r . O ne such safeg uard is theso ftw are -m anda ted calcu la tion of thespec ific absorp tion ra te fo r each pa-tien t based on the sequence selec ted

an d th e pa tien ts w eigh t. S equencesex ceed ing the F D A gu id elines fo rspec ific absorp tion ra te a re re jected .T he p eak R F p ow er needed to im agea pa tien t is a lso calcu la ted (based onm ass an d an atom ic landm arks) and isd ow n lo aded to an R F pow er m onito r;th is m onito r shu ts d ow n the sys temif the p eak R F p ow er, the p u lsew id ths , o r the d u ty cyc le are exces-s ive . A no th er sa feg uard is p resen t inthe techn iqu e used to inc rease R Fp ow er ou tpu t; the ou tpu t can be in -c reased o n ly in sm all steps (as lim it-ed by the so ftw are ), so tha t a sing lekeys troke erro r cann o t resu lt in un-in tended h ig h ex posure .

S k in is a particu la rly good o rganfo r d issipa tion of hea t, since sk inb lo od flow can b e increased by a fac-tom of 20 in response to a hy perthe r-m ic env iron m en t (1 4). Su ch in -c reased sk in b lood flow is dependen to n an in tac t and phy sio log ica lly nor-m al ca rd io vascu la r sys tem . S k in coo l-ing tha t resu lts from increased b lo odflow is augm ented by the evap ora -tion of sw ea t. G ood sk in an d subcu ta -neous tissue coo ling is im portan t,s in ce th eo ry pred ic ts th at a p ro pom -tio na lly g rea te r R F hea t depos itionfrom M R im aging w ill occur near thesu rface in cy lind rica l s truc tu res (2 , 4 -9 ). O n the b as is o f th is theory , inves-tig ato rs h ave recen tly su ggested tha tit is m ore im portan t to assess su rfacetem p eratu re than d eep -tis su e tem -pem atu re w h en eva lua ting the hea t-ing in duced by R F irrad ia tion duringM R im aging (6 ).

D eep-tissue hea t is red istribu tedand carried aw ay by an in tac t ca rd io -vascu la r sys tem as w ell. H ow ever,heat (such as tha t genera ted subcu ta -neous ly ) can a lso be carried to d eep ertis sues by the card io vascu la r sys tem ;th e degree to w hich heat gen eratedsup erfic ia lly is car ried to dee p tis sue s(th e rm odyn am ic s tir ring) is un-know n. Su bcu taneous fa t is a goodelec trica l in su la to r tha t m ay so m e-w ha t iso late d eep er in te rn al cu rren tlo ops genera ted by R F ir rad iation(1 5). T his e lec trica l in su la ting effec t

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55 4 # { 1 4 9 }adio logy M ay 198 8

of fa t m ay a lter th e p rop o rtion a l d e -po sition of R F -indu ced h ea ting be -tw een superfic ia l and deeper tis suesfrom th at p red ic ted b y th eo ry orm easured w ith a s in g le -tis sue p han-torn . S u b cu ta n eo u s fa t is a lso a go odth erm a l in su la tor , so th at h ea t d ep os -ited in or carr ied to d eep er b od y tis-sues is lo st m o re slow ly th an hea tfrom supe rfic ia l tissues.

S om e reg io ns o f the b ody a re no tperfused b y b lood , and hea t depo sit-ed in these reg io ns m ay d issipa tem o re slo w ly . T h ese reg ion s in c lu d eth e len s o f th e eye , u r in e in th e u n-nary b ladde r , and asc itic f lu id w ith inth e p eriton eu rn . T h e tem p eratu reth re sh o ld fo r gene ra tin g ca ta rac ts inth e len s o f a rab b its eye is arou n d42 { 17 6}C1 6); h ow ever , a lo ca l sp ecif icabso rp tio n ra te o f abo u t 1 00 W /k g isrequ ired to g en erate ca tarac ts (1 7 ) . Itis unk now n w h e the r lo ca l ho t spo tsm ig h t d eve lop d u rin g h igh -sp ecif ic-abso rp tio n -ra te irrad ia tion in u npe r-fused f lu id co llections .S ev era l ob servation s can b e m ad efrom ou r exp er im en ts. F ir st, th ish igh-spec if ic -absorp tion -ra te se -q uen ce p ro d u ced b y a c lin ica l M Rirnage r in c rea sed the tem pera tu re inal l of th e tissu es m easu red b y severa ld eg rees; th e g reate st a verage increasew a s 4 .6 {17 6}Cn th e u r in ary b lad d er .T h e m a xim al tem peratu res w ere4 2 .3#{ 176 }Cn subcu taneou s tissue (g ro in ,d o g 5 ), 41 .6 {1 76 }C n m u scle (n eck , d og3 ), 4 3.4 #{ 17 6} C in the live r (dog 5), 43 .2# {1 76} Cin th e m id p er ito n eu m (d og 5), an d43 .4 { 17 6}C n th e u rin a ry b la d d er (d og 5 ).In a ll d ogs, a t lea st on e tem p era tu rem easu rem en t w as abo ve 4 1 .5#{ 17 6}C .S ec -o n d , th e tem p era tu re in crease d u rin gR F irrad ia tio n w as re la tiv ely linea rover tim e (F ig . 5 ). T h ird , th e h ea tin g

o f deepe r tissu es w as sligh tly g rea terth an tha t o f sub cu tan eo us tis sues.F ourth , the heating of u rine in th eb lad de r w as sim ilar to th e hea ting ofo the r deep tissues (L e., the live r an drn idper i toneum).

T hese da ta su ggest tha t the rm a ld iffusion and b lo od flo w m ay m aketh e d og b o d y a stirred th erm o d y-n am ic sys tem , in w h ich g rad ien ts inR F pow er depo sition m ay no t resu ltin equa lly la rge g rad ien ts b etw eensuperfic ia l and in te rna l tem p era-tunes. T h e d ata a lso in d ica te th a tw itho u t ex isting sa feg ua rds , c lin ica lM R irn agers cou ld p ro d u ce s ign ifi-can t heatin g in som e tissues un de rso m e con d ition s. T hus, con tin uedcau tion in th e d es ign an d use o fth ese im age rs is p rob ab ly w arran ted ,particu la rly in u n its m odified so theycan be o pe ra ted a t sp ec if ic ab so rp tionra te s abo ve th e cu rren t F D A guid e-l ines.

E xtrapo la ting these da ta from an es-th e tized do gs to u n a n esth e tized h u -m an s w ou ld n ot b e va lid . H o w ever ,w e b elieve th at p ar ticu lar a tten tionsh ou ld be p aid to sp ec if ic ab so rp tionra te s in the im ag ing of in fan ts; pa -tien ts w ith heart cond itions, a lteredthe rm oreg u la tion due to feve r o rdrug s, o r an h id ros is; an d pa tien tsw h o can n o t com m u n ica te w e llenou gh to d escr ibe sensa tion s o fw arm th . If spec ific abso rp tion ra te sex ceed th e F D A recom men d a tio n s,care sho u ld b e taken no t to w rap p a -tien ts tigh tly w ith b lanke ts o r re -straints. In ad d ition , re lativ ely lowam bien t roo m tem pera tu re an d h u-m id ity sh ou ld b e m a in ta ined , alo ngw ith go od a irf low th ro ugh th e rn ag -n etb ore . U

R eferences1 . R ad io lo g ic al H e alth B ure au of U S .A . G u ide-

lin es for ev alu ating ele ctrom agn etic exp osu refo r tria ls o f c lin ica l N M R sy ste m s . W ash ing -to n , D .C .: F ood an d D rug A d m in istra tion ,1982 .

2 . B ud ing er T F . N uclear m agn etic reso nan ce( N M R ) in v ivo stu d ie s: kno w n th resho lds fo rhealth ef fects. J C o m put A ssis t T om ogr 1981 ;5 :800-811.3 . S ch aefer D J, B arb er B J, G ordon C J, Z ie lon ka J,H e cke rJ. T h erm al ef fects o f m ag net ic m e w -nan ce im ag ing (M R I)(ab str .). In :B oo k o f a b-stracts: S ociety of M a gne tic R eso na nce inMedicine 1 9 8 5 . V o l. 2 . B erkeley , C alif .: S od -eof M agn etic R eso nan ce in M edic ine , 1 985 ;

4. S hello ck F G , C r u e s JV . A lte rat ion s in c u ta ne-ou s b lo od flow rela ted to tis sue hea tin g du r-ing m a gn etic re son an ce im a gin g at 1 .5 T e als(ab str.). C lin R es 1 986 ; 3 4 :3 44 A .

5 . S he lloc k F G , S ch aefe r D J , C rue s JV . T h e r m a lrespon ses to d if fere n t lev els o f ra d io fre -que nc y pow er dep osition d uring clin ica lm a gne tic re son an ce im a gin g at 1 .5 T e als(ab str.). M agn R e son Im a gin g 198 6; 4 :9 4 .

6 . S he lloc k F G , C rues JV . T em pe ratu re , h ear tra te , an d blo od p ressure ch ang es a sso cia tedw ith clin ic al M R im aging at 1 .5 T . R ad io lo gy1 9 87 ; 1 6 3: 23 9- 26 2 .

7 . B o tto m ley PA , A nd rew E R . R F m agn eticfie ld pen etra tio n , p u lse sh ift, a nd pow er d is-sip atio n in b io log ical t issue: im plic ations forN M R im a gin g . P h ys M e d B io l 197 8; 2 3 :6 30 -64

8 . B o tto m ey P A , E de lste in W A . P ow er d epo si-tio n in w h ole-bo dy N M R im a gin g . M ed P h ys1 9 81 ; 4 :5 1 0- 51 2.9. Bottomley PA , R ow lan d W R , E de ls tein W A ,S che nc k JF . E stim atin g rad iof reu enc y po w -em de po sition in bod y N M R im aging . M ag nR e son M ed 198 5; 2 :33 6-3 49 .10 . M icha elson S M . M icro w a ve bio lo g ic al ef -fac ts: a n o ve rv ie w . P roc IE E E 1 980 ; 6 8 :4 0-4 9 .

11 . A d air E R , B erg lun d LG . O n th e the rm oreg u-latory co nse qu enc es o f N M R im ag ing . M ag nR e son Im ag ing 198 6; 4:321-333 .

12 . S ug ano Y . S ea so nal cha nge s in he at b alan ceof dog s acc lim atiz ed to ou tdo or clim a te . Jpn JP hy sio l 1 981 ; 31 :465 -47 5.13 . B a um D , H alte rJB , T a borsky G J , P o rte D .P en tob arb ita l effe cts on p lasm a ca tec ho l-am ine s: tem peratu re , h ear t ra te , and b loo dp res sure . A m J P hys io l 19 85 ; 248 :E 95-E 10 0.

14 . R o w ell L B . C ard iov ascu la r aspe cts of hum antherm o reg ula tion . C irc R ca 19 83 ; 52 :36 7-3 79 .15 . G u y A W , W eb b M D , S o ren son C C . D e term i-n ation of p ow er ab so rp tion in m an ex posedto h ig h freq ue ncy ele ctrom agn etic f ie ld s bytherm o gra phic m ea surem ents o n sc ale m od -cia . IE E E T rans B iom ed E n 1976 ; 23:36 1-371 .

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17 . G u y A W , L in JC , K ram er P 0 , E m e ry A F . E f-fe ct o f 2 ,4 50-M H z rad iatio n on th e rabb it eye .IE E E T rans M icro w a ve T h eory T ech n 19 75;23:492-498 .