Human PATHOLOGY VOLUME

April

23

1992

NUMBER

4

Symposium Magnetic Resonance Imaging of Prostate Cancer FARHAD PARIVAR, VICTOR WALUCH,

MD, FRCS(ED), MD, PHD

AND

Overthe

pa.st 10 years, dramatic developments in hardware and software have made magnetic resonance imaging a very powerful diagnostic ttool for imaging body organs. In this review, the technique as it applies to prostate imaging is discussed, and the literature is reviewed IO provide an overview of the current status of prostate magnetic resonance imaging as a tool for diagnosis and staging of prostate cancer. HUM PATHOI. 23:335-343. Copyright 1~ 1992 by W.B. Saunders Company (:olll~)llt(‘~l

lotnogr~lpll\’

and

sof‘t tissue ~~otittxt. This. conlt)itwd bitts its multiplana1~ iuqing c ;1p;ihilitk and vak1hle E‘OL’. tll;~ke~ MRI ;I powwfiil tool to irkge lhe prostatr. l~ei‘ipl-ostatic, tissues. xld pet\is. Ii1 ;1 twrnl tnultic~eiilt.r ~~qxxitive ctudv in I tw Iliiitecl Statea. the relative twtdit:i of‘ hlRl ;~tid TRl!S-P in cletcc‘ting and staging c.linic ally Ioc;1li/ed o~~ost;ilt’ ~~;II~cc’I~ wet-e e~aluatd.” This studs showed I hiit AIRI is ;II Irast 11~good ;ts ‘I‘RI ‘S-P in cletet tiorl ;IIKI low1 stayiiig of this carcinoitm ’ In this reviwv, the tcc~hniques ol’lwrfi~rming x1 MRl csmiin;1tioti of the prostate. ;ilqxx;ii1w of tiornial prostate. and features of cmdnotna AS sew on MRl arc’ tkusscd. 111 ddition. recent clevelopn1t~nts in MRI tee tiriolo~~ will be presented ttidt will tuake iiitcleai~ nia~netk resorixiw. in niarly instmcm, ttrc. niodalit\- of‘ c.hoiw in proslatic~ diagnosis mti tiwlii~c~iil.

trans1wx1lLlltrasoLlIlcl

(TRL’S-I’) have 60th been usrd to detect anti tomography is apatdg~ prostaIr cancer. (kmiputed prosinMel\~ tiO’q xcuratr in detecting prirnar-y prostate it off&x more advantage in c&g(‘;ul(‘el‘. I,2 ~tlth~~Llgll rlcGlg distmt Iwtastases. Over the past 10 years, TRUS1’ has l’oui~d 111ow lavor ;inloi~g radiologists mid urologisls iii del~~ctioli mti kGd staging of‘ prostatc cmct‘r. It is noninvasi~~, f’ast. and relatively inexpensi\~e; it has I tw xtl\ant;igr of‘ producing real-time images that allow guided needle l)iopsy. However, it is at best 59% accurate in the tlet~:.c.tion of canwr.L) Furthermore. the limited fic,ld oi’\iew (FOi’) and diffraction artikts of“~RUS-I’ t~~cic~ it ;I f)oor 1001 fi)r imaging periprostatic structures and, therelim~. fibr staging. 111 rcc‘twt ywx, magnetic resonance imaging (MRI) h;1s hetw itic rasing1y used to image the pelvis and prostatt‘. Ttw gwat benefit of MRI derives from its very high

olprostatc~

MAGNETIC TECHNIQUE

RESONANCE

IMAGING

Maglietic~ rcsonmct‘ imaging is prrlc It n1ed with hotno,geneou~ prrniauent, elect t-oniqnetk. or superconducting magnets of‘ field strength ranginp~ t)etwc.en 0.0-l to 2.0 ‘I’. Many of the clinical n1agnets conmel-ciall> available todw operate at 1.5 T, which pro~icfe5 ;1 good (~4mproniise I,etween image resolution. imaging tirnr. and uejc. In lower field strength ~na~iiets tlic image rescllution for prostate imaging is too poor to he of much clinical value aticl imaging times are getierd?~ pr~~loriged, making them undesirabk. A bocl!; coil is generally used as the tmnsniitte~~ of radiofrequenc~ pulws. Image acquisitiol1 is performed with either large txdy (oils cstcrnal to the body acting as ;I rec.eivcLr 01 with 3 small surface cx)il applied directly o\.er the prostate. This type 0E surface coil is niounted over ;I catheter that is me&d t~ectally nntl is held in position agaimt thci prostate by III~‘;II~S of x3 inflatable Ix1lloon:‘~” hIor ret tmtly. the 11st 335

Volume 23, No. 4 (April 1992)

HUMAN PATHOLOGY

FIGURE 1. Comparison of axial Tl weighted (TRITE 600/200) body coil image of the prostate (left) and TR/TE/TI 2,500/40/300 IR image of the same slice obtained with the rectal coil (right). Note the improved resolution over a smaller FOV in the right panel. The small arrows indicate a transurethral resection of prostate defect containing urine that has hyperintense signal surrounded by hypointense signal due to scarring. Boundaries of the prostate (“capsule”) and the rectum are highlighted by a black rim (arrowheads) in this sequence. R, rectum; P, prostate

of a double surface coil, one to be placed intrarectally and one suprapuhically, has been advocated.’ An examination performed with a large body coil has the benefit of a larger FOV (typically 24 cm), permitting screening of the pelvis for lymphadenopathy. A typical 5 mm-thick slice in these images over a matrix of 128 X 256 generates a pixel size of 1.8 X 0.9 mm (voxel, 8.1 mm”). On the other hand, rectal coil images have the advantage of being capable of producing much thinner slices (2 to 3 mm) over a smaller FOV of 10 to 12 cm with better image quality. The typical pixel size in these images is 0.9 X 0.4 mm (voxel, 1 .O mm”), a significant improvement in resolution. One can routinely image in three planes (axial, sagittal, and coronal) and any other additional planes as needed. The imaging plane and the pulse sequence selected depend on the information desired. For practical purposes, transaxial images are generally used to depict prostate pathology and to evaluate the remainder of the pelvis in the majority of cases. However, for an exceptionally large prostate, coronal or sagittal views are more useful. Images in the sagittal plane are also useful if spread of tumor to the seminal vesicle or base of the bladder is suspected. Tissue contrast in MRI depends strongly on the choice of pulse sequence used. Short repetition time (TR) and echo time (TE) (600/20) and long TR/TE (2,500/40,80) spin echo (SE) images are routinely obtained in all patients. Short TR/TE images (Tl-weighted) offer the best contrast between prostate, seminal vesicles, periprostatic fat, blood vessels, and surrounding muscle; however, little information is provided about the architectural details of these structures. Long TR/ 336

TE images (T2-weighted) are best suited for depicting internal architecture of the prostate and seminal vesicles. On these images zonal anatomy of the gland, benign prostatic hypertrophy (BPH) nodules, prostatic ducts, ejaculatory, ducts, and, occasionally, prostatic urethra are clearly Identified. A technique combining ultralong TE (120-l 60) and a narrow band-width, which suppresses fat. has been reported by Mitchell et al to increase the rate of detection of prostate carcinoma.# Fat suppression can also be carried out by the inversion recovery (IR) technique using inversion times (TI) of 160 to 180 ms to suppress the periprostatic and intraprostatic fat signals. This allows better visualization of the boundaries of the gland. Longer TIs of up to 300 ms create an edging effect around the prostate, rectum, seminal vesicles, and other periprostatic tissues, which may be extremely useful in local staging of the disease. The value of contrast agents in the MRI of prostate is not well established. Gadolinium-DTPA, the only MRI contrast agent that has been approved for human use, has been tried but has yielded no significant improvement in the accuracy of detection or staging of prostate cancer.’ MAGNETIC

RESONANCE

ANATOMY

The microscopic anatomy of’ the prostate has been described by McNeal.‘” There are four morphologically separate zones of the prostate: periurethral, transitional, central, and peripheral. On MRI, the periurethral, transitional, and central zones cannot be differentiated clearly and, for practical purposes, are collectively called

MRI OF PROSTATE

CANCER

(Parivar & Waluch)

FIGURE 2. Axial T2 weighted (TR/TE 2,500/40) body coil image. The PZ (small arrows) and CG (large arrow) are seen distinguished in this sequence R, rectum; B, bladder

1t1et entral glared ((Xi). OrI

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ilit;lsc’.

ttir

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tatc’ ;lp~)e;l.r-s ;Is ;I 3c)ft tissue structure veiltr-al to the rec.ttmi. Ott 5ttorl I K/I‘E: itttages. the gland appears ho;I lower signal intensity than thr titogvtteou4 ;III~ ha stirrounding lixsurs (Fig 1, left). Prostatic urc~tira. which is not always seen on MKI. When ia normall~r c~ollapsetl, resection of tltere has I)ern ;I previous tratisurethral prostate the surgical defect is seen near the superior aqect of I he gland (Fig 1. right). These details are best sven on lhtb rectal coil images. On long ‘TK/TE: axial images. rhe ~ottal architecture (Ott be cleat$ differentiated. On these images. rhe peripheral zone (I’%) has higher signal intensity than the (~(;*I’,‘~ pi-c)babl) clue to its higher water content and less str0m.t’~ (Fig 2). The 1’7, is larger near rtie apex of the gland ;itid thins toward the base. On axial rrctal coil images the whorled appearance of BPH nodules can he readily sec~l as high intensity nodularity in the CC;. The (Xi itself looks heterogeneous due to the varyittg signal intensities of BPH nodules and prostatic ducts (Fig Z3). The two ejaculatory ducts appear as hyperintense structures on hc )th sides of the midline at the border between the PZ ancl (I(; (proximal to verumontanutn). On sagittal images, thca (Xi can be seen extending cephalad beyond the I’%. (III these images, seminal vesicles are seen postrrosuperior IO the prostate. occupying the space between the bladder and the rectum. The prostate gland is invested by a thin layer of fibrous tissue 1hat is generally regarded by pathologists Delineation of this capsule ;IS the prostatic “capsule.” iz vital in any staging examination of the prostate. On axial T2-weighted SE images, rhe capsule appears as a thin rim of very low intensity signal”’ (Fig Z3).However, the capsule is not always delineated in its entirety. even in healthy ~olrmteers.‘l~‘” On IR images (TI WO). the contour of the prostate is highlighted by a black rim that extends around the rectum and seminal vesicles. III these imagt’s, the capsule can be seen in its entirety

337

FIGURE 3. Rectal coil TR/TE 2.500/40 The small arrows indicate BPH nodules, the large arrows indicate prostatlc ducts, the long arrows indicate ejaculatory ducts, and the arrowheads indicate the capsule R, rectum: B, bladder.

rtrcotiipassitig the glatid”’ (Fig I, right) Outside tht prostatic. capsule is the periprostatic Lat. tvhich harbors 1tic periproslatic. vt‘nous ple?ius. Tttesv I vins art‘ best sevti as hypoititense structures witliitt the hyperititenst 1;tt on shot-t TR/TE images obtained with the rc*c.tal coil

FIGURE 4. TR/TE 600/20. The small arrows indicate periprostatic veins, the large arrow indicates a transurethral resection of prostate defect containing urine, and the long arrows indicate a neurovascular bundle. F, periprostatic fat; R, rectum.

HUMAN PATHOLOGY

Volume 23, No 4 (April 1992)

FIGURE 5. TR/TE 2,500/40 SE image of BPH (left) compared with TR/TE/TI 2,500/20/160 fat suppressed IR image of the same slice (right). Note the periprostatic and perirectal fat suppression in the right panel highlighting the contours of the gland. The small arrows indicate the PZ. F, fat.

nign and malignant ch;~nges.“” Howe\ er, (:arrol et al, working at I..5 T, reported that prostatic carcinoma has whereas Jaeger et al. working at low signal intensity,“’ 0.15 T, reported that prostate cancer is hyperintense.”

(Fig 4). The neurovascular bundles are only convincingly seen on the rectal coil images and are located at 5 and 7 o’clock positions, in the angle between the prostate and the rectum”’ (Fig 4). Prostatic fat signals can he nullified by IR imaging (TI 160-18(l), and this is extremely useful in demarcating the prostate (Fig 5). Sagittal images are less useful for evaluation of periprostatic structures; however, the upward course of ejaculatory ducts can be traced only on this projection. On cephalad transaxial images, seminal vesicles are seen as soft tissue structures between the rectum and the bladder. On long TR/TE rectal coil images, the tubular fluid can be seen as a high signal intensity contained by the tubular wall, which appears as low intensity. The internal architecture of seminal vesicle is best depicted on IR rectal coil images (Fig 6). CARCINOMA lntraglandular

Disease

Since 1983, MRI of the prostate gland has been extensively explored and has yielded mixed conclusions. In 1983, Hricak et al observed that malignant prostate may have a high signal intensity.” Between 1984 and 1986 various groups, working at low field (0.15 to 0.35 T), concluded that MRI cannot differentiate between benign and malignant prostatic nodules.‘X-S’ In 1987, Phillips et al retrospectively reviewed 31 patients with benign and malignant prostatic lesions who had MRI at 1.5 T and concluded that the prostate had no specific signal intensity that enabled differentiation between be-

FIGURE 6. TR/TE/TI 2,500/20/300 IR image of seminal vesicles. Tubular wall is hypointense (small arrows) and tubular fluid is hyperintense (larger arrows).

338

MRI OF PROSTATE

CANCER

Sii1c.e I !jSX, dramatic. improveme~~ts in IlKI tech(pxlicularl)~ endorectal coil designLi,‘.“), coupled with better appreciation of tumor biology. ha\;e markctllv increased the potential of MRI in detection and sla$ng of‘ prosla~c cxcinoma. During this period, a iiuinlxr 0I‘reporls have emerged explaining the varying apptW;lll~ t s atid rate ol‘dete~tion of prostatic carcinoma and havr tlocumcnted that MRI of the prostalr is much iJioJ~e ust~l’lil lhm prrviously lhough~.“~’ ‘25-2” Tlitm~ ;irc~ at least two factors affecting lhe ability 10 c1t.trc.t c.arcilloma notl~~le~ on MRIs: tunlor grade and cancer is ;I functiimor size. Signal intensity of‘ prostate tion 01‘ its crllularit\ and differentiation (grade). as shown I)\ Schiebb et a1 in rheir in vitro work at 1 .I) ‘I .“” Tllc&* inoistigators noted [hat well- to moderatel) grow in nodules of well-diEi t~iitiated carcinoin;~s glandular, elements with little central denwlv pdclmi sloe for Imucin storage. Therefore, the high cellularit) and reducrd lluid content of this grade of cancer genOn the other hand, rrate a loi< MRI signal intensity. poorh diftcrcaiitiated carcinomas tend to grow in an infi[tr.ativr. ll;lttern, causing little distortion of‘the normal their arc.hitc.c~u~c~ of the gland anti therch\~ averaging iirtellsit\ r+,irh xlixent norm~~I cells, making thtarn poorI) .~lthorJgh direct extrapolation of these \isil)le c;n RlKls. iti vitro rou115 10 in \,ivc, sluciies at lower fields is not 51i.aiglirli)r\v;ir-rl, cxrrrnt in \ivo studies strongly suggest ~~1’c~arcinoma have differt>nt signal tllar ditiei cn( gr-adrs iritriisilir\ ;it I ..5 T. Thirt!,-four of :S7 patients in one r(ap( )rted :,crics”” and 19 of 20 cases in another series!’ llad IOM signa intensity (xrcinomas in the I’% on long iniqt’t. In thr latter group, once patient had a TKjI‘tC high signal intc.nsity c;ircinoma in the PZ. In the Cal-r01 scariy5.“” 67% 01‘ c;ii JWJX wcJ-e hypointense. Although Iongitudi~lal studies using endorectal coil are ongoing. ;I I~YYJ~~ I)reliiiiinarj slud!. reported that 54 of 5.5 pa(ients with biopsy-proven prostate cancer had low signal intensity in the, I’% on long TR/TE imagtx”!’ Therefore. it appears that a typical cancer nodule on MRIs is a hypointense lesion in the 1’7, of the prostate; conversely, any hypoilltensc lesion in the PZ should be regarded with great suspicion (Fig 7). High-intensity lesions should be interpreted with great care, especially if they appeal ill the C(;. because thr majority of RPH nodules in this (Fig 3). There is also a rare region arc 1also hyperintense nlu~ill-secl-eting prostate carcinoma that appears as a hvperintetisc lesion regardless of its location in the prostatt*. ‘l‘hc3e are very important factors to he considrr-eci when screening for prostate cancer by MRI. is the least important of the The sire of ;I tuJnor ahove in determining the ability two farto~~s mentioned to detect the cxnc‘er. As the well-differentiated tumox grows in :siLe. it loses differentiation in the periphery. slaking margin identification and therefore volume estimation on MRls inaccurate. Thickman et a1 showed that :47% of tumors over 5 mm in greatest diameterwere were uncierestimated in volume and that “2% cover-eslimaled.~’ Kahn et al reported that there was ;1 YT.59; to 70% (mean. 10Y0) undereslimatiorl of tumor vollime by MKI in 1SYc,of their patients. No tumors in that star& werr ovcdrestimated in volume. Furthermore, there 113s no cx ~rrel;ilion brtwren fumor size and the rate of

(Parivar & Waluch)

1Jo101=\

FIGURE 7. TR/TE 2,500/80 rectal coil image of carcinoma. The small arrows indicate carcinoma in the PZ and CG. Note the asymmetry of the PZ due to carcinoma in the left side.

dett.c.tioii;

tumor could he idt-lllitiett. whereas be missed.” Ill liahll et al’s study. 1ilnior gixk5 were‘ not takru into xcourlt, tbJl1 f’rom I0 the discxissiom ahove it is e\.iciclrt rli:i( ;I sinall bdl\vell-difterentiatetl ~~:ii-f~iliori~;.~ cai1 he r~aclih ~i~otiei~atelv drtectetl tx MRl whereas a Lrrge. poorly differentiatccl onr may be missed or ulldel-esrirllatc_(i in volurrlc. Therefore. disc~repar~cies in the wpoilc’dl figures of sc’nsitivitv md specificit) of prostar? MRI ;Irise from tlilferrn;~cs in tumor grade and volume in Ihosr series. For csample. fiW of carcinomas in the ser-icb5 of (Lirrol cl ;11”’ ;ind ‘i I%, of. carcinomas in thr scG5 of Kahn et aI” WCJ-Y dc.tec.ted hv MRI on long TR/‘I‘E: intages. whereas 01’ in anothrr slutib only :%6% of all C;IIIC‘C’IS and 8% the cancers greateJthan 5 inni in diamel~r wcare itlcnrified.” It shc~ulcl bc noted that aII the studies mention~ti coils for image gentxitiori. The attenabove wised ho+ md relatively pool’ image qualit\, dant Iow resolution ;I difficult problem of’ detec.tion. It is t?xesacer~)atetl a11d imagt~ petted that the dramatic gains in re\olution clualitv afforded hy the endorect;tl c-oil Lsill grally irnprostatic prove 41Rl’~ abilitv to diagnose ;iiid slage

;I Iarger

ie. a sIna

(uiilor

could

~‘;lIl(‘~r.

Capsule A signihcant contribution of MKI lvill be in Ihr cmcei-. Since ltltw ih ;I correlatio~l staging of prostate brtwern the G/e of the tumor, its tlcgr~t~ ofditferentialion, afitl the stage of the disease, it is important to tlistinguish brtwecn the operable slages of the disease mltl the norlop~r~Me stages when the c-ancer teas spread beVOIK~ the

ccu~fines

5 339

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7

of

the

capsule.

the capsule typicall\, takes place at the o’clock positions on tr-ansmial irrqvs where.

Iiivasioti

of

Volume 23, No. 4 (April 1992)

HUMAN PATHOLOGY

FIGURE 8. Capsular invasion (Left) TR/TE/TI 2,500/20/300 IR image showing deformity of the contour and discontinuity of the capsule (small arrows) due to carcinoma. (Right) TR/TE 2.500/80 image showing carcinoma in the left PZ (large arrows) with capsular invasion (small arrows). (Courtesy of M.D. Schnall, MD, Philadelphia, PA.)

the neurovasc‘ular bundle penetrates the gland. Once outside the gland, the tumor can spread to the periprostatic fat, seminal vesicles, and obturator internus, and even circumscribe the rectum. Any discontinuity in the prostatic capsule or irregularity of the margin of the gland on long TR/TE images may be a sign of capsular infiltration (Fig 8). In addition, any abnormality of periprostatic venous plexus and abnormal signal intensity in periprostatic fat may be considered as further evidence of capsular infiltration.‘” The contour of the gland is best depicted on IR rectal coil images.” Kahn et al demonstrated that MRI at 0.35 T is 37.5Yo sensitive, 100% specific, and 82% accurate in predicting extraglandular disease on long TR/TE images.” The corresponding figures by Bezzi at 1.5 T were 44% sensitive, 81% specific, and 65% accurate.“’ This must not be interpreted to mean that low field MRI is more accurate than high field MRI, but rather reflects the more stringent criteria used by Bezzi et al in defining capsular infiltration as mentioned above. To date, no data have been reported on the accuracy of high-resolution endorectal MRI in defining capsular infiltration, but studies are ongoing and preliminary results are very encouraging.

invasion of seminal vesicles from without, either front the base of the prostate or from periprostatic area; and the remaining 30%) showed isolated areas of cancer within the seminal vesicle without any evidence of ex-

Seminal Vesicles There are three ways a tumor can spread to seminal vesicles, and this has been elegantly described by Wheeler in a study of 30 patients.‘” Forty percent of the patients had tumor in the semimal vesicle from direct extension up the two ejaculatory ducts; 30% had direct

FIGURE 9. Central invasion of the seminal vesicles by tumor showing as low signal intensity (arrows). B, bladder. (Courtesy of M.D. Schnall, MD, Philadelphia, PA.)

340

MRI OF PROSTATE

*o

-to

0

CANCER

(Parivar & Waluch)

-.o

PPM

FIGURE 10. 3’P MR spectrum of normal human prostate (left) and prostate cancer (right). The peaks include PME, Pi PDE, PCr. and 1, CV,and b peaks of ATP. Prostate cancer has higher PME to PCr and PME to ATP ratios and a lower PCr to ATP ratio than normal prostate (Courtesy of P. Norayan, MD, San Francisco, CA.)

FUTURE

to be hematogenous tracapsular clisease, believed spread. Magnetic resonance appearance of seminal vesicle involvement is that of a low signal intensity lesion \\ithin the hyperintense seminal vesicle on long TR/TE images (Fig 0). Replacement of normally fluid-filled tubules of seminal vesicles by highly cellular tmnor produces the reduction in signal intensit!,. Depending 011 the path of’spread, the lo\t signal lesion could either be (entrally I through the ejaculatory ducts) or peripherall!; located. If‘ the central portion of the prostate is large, centrally Ioc.a(ed seminal \,esicle tunwrs may be difficult 10 see on axial images. In this case, it is necessary to cobtain long TK/TE sagittal images. Neurovascular

PROSPECTS

Magnetic

Resonance

Spectroscopy

A vei-v important fLiture impacf of nwlear magnetit resonance ivill be in MR spectroscop! (MRS) of the prostate. In patients receikig radiorhc~rap~. chemotherap\,, or undergoing horlnone nlanil)~~latiorl. a noi1invasive asst’ssment of prostatic- ct~llular mt~tabolism would be the most direct. ,Iccuratt’. arid expeditious niethod of monitoring rhe rcslxmse 10 I hese therapies. In this regard, three nuclei (“‘phc,sl~tio~.~~s. ‘li~tlrogen, .md “‘carbo~l) MRS have been investigated. With ‘“1’ hlRS, prostatic adenosine criphosphate (ATI’), phosphoillo~loestel‘ (PME). phosphodiestrr (I’Dk:), phosphocreatinc (P(k), aiid inorganic, phosphate (15) can be quantified. Narayan et al ha\xl docume~lted ‘“I’ MRS ctiaracteristics of normal liiiiiiau prostate. prostate cancer. and BPH in viva”” (Fig IO). Thev showed Ibat prostate cancer has ;I louver 1’Cr IO AT’P ratio and higher 1’ME to ATI’ and PME to PC :r ratios than both normal prostate and the prostate tuith BPH. U’ith ‘H MRS prostatic. content of creatine;‘PCr, spermirw. citrate, and some lipids can be monitored. ~l‘h~~~~~;~!~ et al have descrihecl the ‘H MR spectrutll of norm;1.1 and malignant prostat’ ill \ivo and have demonstrated that the tactel is low in citrate”’ (Fig 1 1). Prostatic. tit rate has also been nlonitored tw natural abundance I”(: hl’KS”’ (Fig 12).

Bundle

M’ith increasing popularity of the nerve-sparing radical pr-ostatectom); procedure,‘4” it is necessary to identify the neurovascular bundle on MRIs before surgery. Exttension of tumor to this structure is generall) accepted as a sign of unresectability. Only high-resolution endorectal images can identify the neurovascula1 bundle (Fig 4). Any lateral displacement of this structure or change in its signal intensity may be regarded as a sign of tumor spread. Lymphadenopathy Evalluation of pelvic lymph nodes is an integral part of the staging examination of the patient with prostate c-ancer. Metastatic and inflammatory lymph nodes canIlot be reliably differentiated on the basis of their signal intensities on MRIs. For practical purposes. clusters of Ilodes or nodes larger than 1 cm in diameter on MRI are considered pathologic.“i Because of the necessity of a large FOV, pelvic nodes can only be screened by body c.oil MRI. and short TR/TE images provide the best cx)ntrast and resolution for this purpose. An accuracy rate of WT! and specificitv of 95% have been I-eported.“’

Magnetic

Resonance

Microscopy

At the magnetic fields in clinicA USCtoda! and T\,itb currently a\ ailable hardwal-e and 5~)ft \I are. i( is now possible to attain an in-plane rrsolutiorl of- 300 X 300 (956 x 256 matrix over FOV of’ 8 cm) cx~rrcsponding to a vow1 si/e of 0.27 nd. In (he neal future, de\,elopments in nllclear magnetic resonx~cc will enable us to double the matrix size to 5 12 X .‘,12. wduw the slice thickness to 1 mm, and possibl\- t,educc: the I:OV. The resultant pixel sire of 150 X I%0 (vc~?i;c~l, 0.024 nlm”i 341

Volume 23, No. 4 (April 1992)

HUMAN PATHOLOGY

2

2

6

7

5 4

i

:

‘H MR spectrum of a healthy 26.year-old man (left) and a patient with prostate cancer (right). Peaks are FIGURE 11. as (left) 1.2, water and MDPA standard; 3.4, C2 and C5 protons of citrate molecule; and 5.6, various methylene protons of and lipids. (Right) 1.2, as the left panel; 3, residual water; 4.5, C2 and C5 protons of citrate; and 6,7, methylene protons of and lipids. Note the lower citrate content of prostate cancer in the right compared with the left panel. (Courtesy of P. MD, S&n Francisco, CA.)

will make in viva MR microscopy the power of this diagnostic tool. Computational

feasible

and add to

Staining

identified spermine spermine Narayan,

Ix~tl on the combined properties of each tissue, thereby masking diagnostic distinctions. In this new method. a series of anatomic MRls, each with a different selective sensitivity to a different tissue type, based on it> MR

Despite recent advances in MR technology, magnetic properties of normal and pathologic prostate tissue are not specific enough to be 100% diagnostic. Each tissue or cell type has MR properties (proton density, Tl, T’L etc) that differ only slightly. Conventional MRI generates only black and white images of the prostate

Llpld

(-CHZ-)

/

citrate C2.C4

r”.,“.,“,,“,,,“,,“, 100 80

60

40

20

0

-20

FIGURE 13. Computer-“stained” image of a prostate with BPH. The PZ is white in this color coding and the CG appears green. The small arrows indicate a BPH nodule. It is hoped that this color coding will be specific to carcinoma and other pathology, (Courtesy of A. Barr, PhD. Pasadena, CA.)

PPM

FIGURE 12. In vivo natural abundance 13C MR spectrum of normal human prostate obtained at 1.5 T. (Courtesy of Richard Griffey, PhD, Dallas, TX.)

342

MRI OF PROSTATE

CANCER

prt qwrtie5. is (thl;~ined. These images are the11 selectively ~‘omputer ~)lor coded to generate a preferential “stainitrg” of’ thr tissues. The aggregate data are then superilnposed. (.rt’ating a cc)lor inlagc of the prostate. This c.cmputer “staining” nla\~ be thought of in the same tvrnis as hiologic~ staining’in which certain tissues retain ;I coloring preferentiallv over others. Thr work on this methodolc)~y as it applies to the prostate is progressing md it is hoped that it will lead to a mow tissue-specific \lRI techlliqw. A prelinGnarv account of this work has hem rep(brt4” (Fig I 3).

CONCLUISION Advances in prostate MRI during the past 7 yews been remarkable. Prostate MRI. as it stands toda!-, ia ;I ven powerful diagnostic tool and its future is even nmre pknising for the clinician. Given the current rate of progress it is easy to envision routine screening of the prostate ill the very near future wherein the gland \\ ill he gr~ossl\~ and nricI-oscopically examined, cancer detected UICL st;~ped, nletabolic profiles determined, and 1herapies suggested, all from one MR esaminat ion. ha\,r

343

(Parivar & Waluch)

Magnetic resonance imaging of prostate cancer.

Over the past 10 years, dramatic developments in hardware and software have made magnetic resonance imaging a very powerful diagnostic tool for imagin...
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