The presence of active gastrointestinal bleeding canusually be diagnosed by a combination of history,physical examination, stool guaiac tests, and serial bloodcounts. It may be more difficult, however, actually tofind the site of active gastrointestinal bleeding; this mayrequire endoscopy and/or contrast angiogmaphy. Eventhen there are patients in whom the bleeding site cannotbe found because the hemorrhage is intermittent. In anexperimental dog model, abdominal scanning followingan i.v. injection of Tc-99m sulfur colloid located areasof gastrointestinal hemorrhage with bleeding matesasslowas 0.1 ml per mm (I). While the sulfurcolloidprocedure is simple to perform, the colloid is rapidlycleared from the blood pool (tip 2 mm) and it onlydetects bleeding that occurs at the time of tracer injection. Moreover, although bleeding sites in the lowerabdomen may be identified, those in the upper abdomen

Received March 2, 1979; revision accepted May 7, 1979.For reprintscontact:GaryG. Winzelberg,Dept.of Radiology,

MassachusettsGeneralHospital, Boston,MA 02114.

may be masked by the activity in the liver and/or spleen,especially when these are enlarged.

Clearly it would be better to have a tracer that wouldnot preferentially accumulate in the reticuloendothelialsystem and would remain in the circulation long enoughto permitrepeatimaging.Theopportunitytodetectintermittent bleedingovera 24-hr periodwouldbe increased, and the detection of upper gastrointestinalbleeding should be simplified. The present study evaluated red blood cells labeled in vivo with Tc-99m as ablood-pool agent to detect sites of active or intermittentlyactive gastrointestinal hemorrhage.

MATERIALS AND METHODS

Two groups of patients were studied. The first consisted of 28 control patients, ( 12F + I6M), ages 28through 65 yr, undergoing gated blood-pool scintigramsfor possible cardiac abnormalities. They were studied todefine the normal abdominal distribution of RBCs Iabeled in vivo with Tc-99m (Fig. I). There was no historyof GI bleeding in any of these patients, and their stoolguaiacs on admission were negative.

I080 THE JOURNAL OF NUCLEAR MEDICINE

Evaluationof GastrointestinalBleedingby RedBloodCellsLabeledin Vivo with

Technetium-99m

Gary G. Winzelberg, Kenneth A. McKusick, H. William Strauss, Arthur C. Waltman, and Alan J. Greenfield

Massachusetts General Hospital, Boston, Massachusetts

To determinethe effectivenessof abdominalimagingwith RBCslabeled in vivowith Tc-99m, for the detection of gastrointestinal (GI) bleeding, 28 control subjectsand ten patients wfth suspected bleeding underwent scintigraphy at 0-24 hr aftertracer injection. Colonic activfty was noted in one of the controls within 3 hr of injection, and in five of ten controls at 24 hr, all of whom had inftial gastric activity.Of the ten patients wfth suspectedGI bleeding,eight had documentedactivebleeding; seven of these had positive scintigrams. Nasogastric (NG) suction markedly decreased the presence of initial gastric activfty in the patients wfth activebleeding.With thisblood-poolradiopharmaceutical,frequentimagingof the abdomen over 24 hr can be doneto test for active bleeding.ContinuousNG suctionisrecommended to reduce accumulation of gastric activfty. These resufts suggestthat red bloodcells labeled in vivo with Tc-99m providea sensitivemethodof detecting active GI bleeding.

J NuclMed 20:1080—1086,1979

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$@ i

SPECIFICABDOMINALORGANSIN28CONTROLPATIENTSAFTER IN-VIVO LABELING OFREDBLOOD

CELLSImaged

at Imagedat2—3hr241wN8(%)

N10(%)

TABLE2. INCIDENCEOFTc-99mACTIVITY,AFTERIN VIVO LABELING OF REDBLOODCELLS,

INSPECIFICABDOMINALORGANSINTENPATIENTSWIThSUSPECTEDLOWERGlBLEEDINGImaged

at Imagedat0—101w24hrN10(%)

N4(%)

PRELIMINARY NOTES

B.

RESULTS

Controls. The distribution of the Tc-tagged RBCs inthe abdomen of 28 control patients is outlined in TableI. Ontheinitial images,made2—3hr aftertracerinjection,activitywasalwaysseeninthegreatvessels,liver,and spleen; renal, gastric, and bladder activity was notedin60,50,and40%ofcases,respectively.Colonicactivitywas noted in only one patient (4%). No Tc-99m activitywas seen in the region of the duodenum, jejunum, orileum. On delayed images obtained at 24 hr, the distributionof activityin thegreatvessels,liver,spleen,and

Greatvessels 100 100Liver 100 100Spleen 100 100Kidneys 60 60Stomach 50 20Bladder 40 20Smallbowel 0 0Colon@ 4 50Thyroldt 0 NIt

. One patient without history of GI complaints or bleeding.

Hct. 42% and stool guaiac negative.t @yeepatients with gastric activity hed neck area imaged

for thyroid activfty.t NI = not Imaged.

Great vessels 100 100Liver 100 100Spleen 100 100Kidney 70 75Stomach 10 0Bladder 70 100Colon 60 50Smallbowel 30 0

Volume 20, Number 10 1081

FIG. 1. AnterIor abdominal sclntigrams In control subject, at following times: (A) 1 hr (B)3 hr (C)24 hr. Note activity fri livor, spleen,great vessels, and bladder.Bowel activfty is not seen.

The second group consisted of ten adults, (4M + 6F),ages 19 through 89 yr, with a history of maroon-coloredstools and/or passing blood clots per rectum. They wereall referred for location of the site of bleeding in anticipationof possibleangiogmaphy.Informedconsentwasobtained from all patients.

Images of the abdomen were recorded in the anteriorposition using a large-field Anger camera equipped withan all-purpose parallel-hole collimator. A 20% windowwas centered at 140 keY. A commercial kit* containing1 mg stannouschloride, 10mg pymophosphate,and 20mg sodium triphosphate was diluted in 3 cc of sterilesaline and injected directly into an antecubital vein usinga plastic syringe and 20-gauge needle. Thirty minuteslater, 20 mCi of [99mTcjpertechnetate were injecteddirectly into an antecubital vein using a 20-gauge needleto obtain in-vivo-labeled red blood cells (2). In the control group, 500,000-count anterior images of the abdomen were obtained 2 to 3 hr following tracer injection,and delayed 24-hr anterior abdominal views were obtamed in ten patients. In the patients with suspectedbleeding, sequential 500,000-count anterior images ofthe abdomen were obtained at 5-mm intervals for 30 mm,and at 1and 2 hr. Additional oblique and lateral imageswere made for better triangulation of regions of abnormal tracer concentration. If the findings indicated activebleeding, the patients were immediately referred forangiography. Delayed scans were obtained on thosepatients with normal 2-hr images, either when the patients' clinical signs indicated renewed bleeding, or at 24hr after initial tracer injection to monitor for recurrentbleeding not clinically suspected.

The scintigrams were prospectively interpreted beforeabdominal angiography. They were read as showingactive bleeding sites when focal collections of activitywere noted in the abdomen in regions normally free ofactivity.

Angiography was performed using the Seldingertechnique. The specific vessels injected and the order ofinjections depended on the clinical presentation. Theangiograms were recorded on cut film and were considered positive when extravasation of contrast materialwas seen.

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TABLE3.SUMMARYOFCLINICALDATAINTENPATiENTSWITHSUSPECTEDLOWERGIBLEEDINGContinuousPatIentnasogastricTransfusionHospitalNo.HistorysuctIonScintigramAnglographyrequirementcourse

WINZELBERG. MCKUSICK, STRAUSS, WALTMAN, AND GREENFIELD

52yr malepassingred blood clots(RBC) per rectum

for 24 hr.Hematocrit (Hot)37%, NGaspiratenegative.

2 76 yr female withcirrhosis.Incarceratedsmall bowel;resected.Developed lowerGIbleeding.Hot.21%, NGaspirate negative

3 75yrfemalepassingRBCperrectum. Hot.23%, NGaspirate negative

4 l9yrmalewlthmelena, negativeBAE,Meckel'sscan equivocal.RBCper rectum.Righthemicolectomy,but bleedingpersisted. Hct.31%, NGaspirate negative

5 43yrfemalewfth

lungcancermetastatic tobowel. Partiallyresected.Persistent lowerGIbleeding.Hot.30%, NGaspirate negative.

6 76yr female.RBCper rectum for 24hr. Hot. 30%, NGaspirate negative.UGIendoscopynegative

Yes Positive5 mlnInleft lowerquadrant.

Yes Positive1hr Inright lowerquandrant(RLQ)

Yes Negative11w;positiveat 101win ascendingandtransverse colon

Yes Positive1hr forduodenal activity

with sequential

activity In smallandlargebowel

No Negative 1 1w;posftive at 51wIn right colon

Extravasatlon low 1000 ml/24 hrin descendingcolon

Extravasatlon in 3000—4000mI/24midjejunal branch hr

of superiormesentericartery

Treated withIntraarterial (IA)

pitressin (P).Goodresponse.Barium enema(BAE)showedcolonicdiverticula

Respondedto IA P.Developed fatal

sepsis and liver

failure

Poor response to IAP. Underwentcolectomy forbleeding

diverticulum.Goodrecovery

Persistent bleedingwithout responseto IA P.Endoscopyshowed duodenalbleeding.Treatedwith vagotomyand pyloroplastywithgoodresponse

Continuedslowbleeding.Receivedradiation therapy.Died.

Persistent slow GIhemorrhageneedingtransfusions.R

hemicolectomy.Microscopyshoweddiverticulosis with

superficialmucosalhemorrhage Rcolon

Negative at 1 and101w.Repeatat

36 hr showedextravasation intransverse colon

Negative at 24 and 1500 mI/24 hr48 hr

Diffuse hyperemiaof colonwithout focalextravasation

2000—3000mI/24

hr

Yes Positiveat 31/2 Negativehours In RLQ

500—1000mI/24

hr

500—1000mI/24

hr

I082 THE JOURNAL OF NUCLEAR MEDICINE

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PatientContInuousnasogastricTransfusIonHospitalNo.History suction SclntlgamAnglography requirementcourse

Table3 (Continued)

TABLE3. SUMMARYOFCUNICALDArAINTENPATIENTSWiThSUSPECTEDLOWERGIBLEEDING

Maroonstools for24hr.Transfused7 unIts.Then no

further bleeding.

BAE showed

diverticulosls

Clinicallystable.Nofurther bleeding.Feftto havebledatS.B..

anastomosis

NofurtherbleedIng.Discharged.Admitted 3 wk

later Withredclots per rectum.At surgery

Meclcel'sdiverticulum

Felt to haveIntermittentbleeding fromhamartomas.Nofocal spot found.

7 65yrfemalewlthahistory of sigmoidresection forbleedingdiverticula.

Admitted for RBCper rectum. Hot.19%,NG

aspirate negative8 20 yr female. Small

bowel obstructionfrom adhesions.S.B. resectIon,with postop.

bleeding. Hot.

20%, NG

aspiratenegative.TransfusedtoHct.32%. No

further bleedIng9 30 yr male. Upper

GI bleedIng 1973.BIlIroth II forbleeding ulcer; 3

further episodesof lower GIbleedIng.Admitted withRBCper rectum12hr beforeadmission. Hot.40%,NGaspirate negative

10 89yrmalewfthIron-defIcIencyanemia.Muftlplegastric andduodenal

hamartomasonendoscopy.Stoolgualac positive.Requiredtransfusionevery3 wk

PosItiveat 2 hr inLLQ

NegatIveover241w

Yes Not done

Yes Notdone

Yes NegatIveover 3 hr Negative

No Negativeover24 Negativehr

1000—1500ml/24

hr

0

0

50 ml/24 hr

kidneys appeared unchanged, but colonic activity wasobserved in 50% of the patients, all of whom had initialgastric activity. This colonic activity appeared similarto the activity seen in patients with positive scans forcolonic bleeding, but was usually less intense. No controlsubjects were imaged between 3 and 24 hr. In three ofthe control subjects with initial gastric activity, no thyroid activity was detected in the neck at 3 hr followingtracer injection.

Patients.The patients'histories,scintigraphicresults,

blood transfusion requirements, and hospital courses areoutlined in Tables 2 and 3. Ten patients who were clinically thought to have gastrointestinal bleeding were meferred for study. Eight of these had continuous drainageof gastric contents via nasogastric tub@s.

Of the ten patients, seven had positive scintigrams. Ofthat group, all patients were subsequently shown to havecontinued transfusion requirements and instability ofvital signs, and were considered to have continuing activehemorrhage. Three patients' scans were interpreted as

1083Volume 20, Number 10

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WINZELBERG.MCKUSICK,STRAUSS,WALTMAN, AND GREENFIELD

showing no evidence of active gastrointestinal hemorrhage. Two of those three had no further transfusionrequirements and were totally stable. The third patient(Patient 10) continued to have a transfusion requirementof one unit of blood every 10 days over a 3-mo period.

Seven of the eight actively bleeding patients, and oneof the two nonbleeding patients, were studied angiographically. Of those with positive scans, only three weresubsequently found to have extravasation of contrast onselective visceral angiography: two had bleeding colonicdiverticula, and one had a bleeding small-bowel anastomosis. Jn one patient (Patient 3), the initial scintigramwasnegative,as wastheangiogram.At 10hr. thescanbecame positive in the ascending and transverse colon,but a repeat angiogram remained negative although thepatient was still requiring transfusions to stabilize herhematocrit. Another repeat angiogram at 36 hr waspositivefor a bleedingcolonicdiverticulumnear thesplenic flexure. At surgery, the bleeding point in thesplenic flexure was found, along with fresh blood clotsin the ascending and transverse colon. A subtotal colectomy was performed.

In the five remaining patients with active bleeding,four had positive scintigrams. These included two patientswithbleedingdiverticula,and oneeachwithanactiveduodenalulcerand metastatictumorimplantstothe small bowel (proven by surgery, endoscopy, or bariumcontraststudies).Onepatient(Patient10)withveryslow bleeding from gastrointestinal hamartomas hadboth a negative angiogram and a negative scintigram.Two patients who had stopped bleeding clinically, onefrom a small-bowel anastomosis and the second from aMeckel's diverticulum, had negative scintigrams.

The radionuclide scans became positive at varioustimeintervalsfollowinginjectionof tracer.Threewerepositive within the first hour (Patients 1, 2, and 4). Thesepatients' transfusion requirements were approximatelyI800 ml per 24 hr. The remaining four scans becamepositive at 2, 3@/2,5, and 10 hr (Patients 7, 5, 6, and 3)respectively. Their transfusion requirements varied from500 ml to3000 ml per24-hrperiod.

The positive scans correctly located the site of hemorrhage to the colon in the four patients with bleedingdiverticula (Figs. 2 and 3). The images suggestingsmall-bowel hemorrhage were found in patients with ableeding small-bowel anastomosis, bleeding metastaticimplants to the small bowel, and a bleeding duodenalulcer (Fig. 4). Ofthe seven scintigrams that were positiveby 10 hr after tracer injection, only one had initial gastricactivity.

DISCUSSION

The advent of endoscopy and selective visceral angiography (3) has greatly improved the location of sitesof gastrointestinal hemorrhage. Endoscopy is especially

..

I@@

B.

FIG. 2. Anterior abdominalscintigrams of Patient 1. (A) At5 mm abnormal tracer appearsin region of left colon (singlearrow);(B) at 15 mmfocal activity Is seen in lower descending colon (double arrow). (C)Angiogram performed a shorttime later confirms activebleeding divertlculum in descending colon (single whitearrow).C

A.

.@@.

;..1t@@‘5i@@. Ii@

FIG.3. AnterIorabdominalscintigramsofPatient3:(A)at 1hr.showing normal distribution; (B)at 10hr markedtracer activity appears in ascending (single arrows) and transverse colon (curvedarrow). (C)Angiogramat 36 hr showsactive extravasationin distaltransversecolon (singlewhitearrow).At surgery,bleedingpointwasidentified along with recent clots throughoutentire colon.

I084 THE JOURNAL OF NUCLEAR MEDICINE

.. .@ ..;@. .44@

.‘.@

-@. ;.,

-0'

B.

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PRELIMINARY NOTES

usefulin determiningthesiteof bleedingin uppergastrointestinal hemorrhage, but the procedure is difficultto perform in the poorly prepared colon. Angiographyis useful in both location and control of hemorrhage fromboth the upper and lower gastrointestinal tract. A largenumber of negative angiograms are performed in lowergastrointestinal hemorrhage because it is difficult todetermine whether the patient has stopped hemorrhagingat the time of angiography. Therefore, a noninvasivetechnique that could detect the presence of continuinghemorrhage as well as locate the site of bleeding wouldbe helpful in the management of acute gastrointestinalhemorrhage.

Other investigators have used tracer techniques todetect gastrointestinal bleeding. In an animal model,Cr-51-labeled red blood cells and 1-131-labeled albuminhave been used to demonstrate sites of gastrointestinalhemorrhage (4). Chmomium-51-labeled red blood cells(5) and InCl-113m-labeledtransferrin(6) havealsobeen used to monitor upper gastrointestinal hemorrhagein humansby measurementof radioactivityin gastricaspirations of patients undergoing intensive medicaltherapy. These techniques have not gained widespreadacceptance because of their cumbersome procedures and

FIG. 4. AnteriorabdominalscintigramsinPatient4: (A)at 15mm,showIng normal distributIonof tracer; (B)at I hr abnormalarea isseen In right upper quadrant(RUQ)(singlearrow); (C)at 11/4hr activfty Is more prominentInRUO,outliningduodenum(doublearrows)and proxImaljejunum(triplearrows);and(D)at 24 hr activity IsseenIn transverse and descendingcolon.

because they are primarily used to measure, rather thanto locate, bleeding. The diagnosis of Meckel's diverticulum may be made following administration of pertechnetate, which is actively secreted by ectopic gastricmucosa, which may be present in 50%of such diverticula(7). Technetium-99msulfur colloid in an animal model(1 ) and in humans (8) has also been used to detect gastrointestinal bleeding, but has been reserved to assessacute bleeding because of its rapid clearance from theblood pool. Tc-99m-labeled albumin, a blood-pool agent,has also been used to locate acute gastrointestinalhemorrhage with a bleeding mateof 2—3ml/min in onepatient with a bleeding colonic diverticulum (9). However, Tc-99m-labeled albumin is of limited use in evaluating abdominal gastrointestinal bleeding because oftracer accumulation in the liver (10), high backgroundactivity (10), and accumulation of from 0.3 to 20% oftracer activity in the gastrointestinal tract (10).

In this study the radionuclide technique was successfulin detecting the presence of continuing hemorrhage withtransfusion requirements as small as 500 ml per 24 hr.There was a single false-negative image in a patient whowas bleeding at an exceedingly slow rate. In the controlgroup there was a single patient who showed colonicactivity on the early views. Since this occurred in onlyone of 28 controls, its significance is unclear but thiscould cause false-positive interpretations.

The normal abdominal distribution ofTc-99m RBCslabeled in vivo in a control population revealed traceractivity to be present in vascular structures on all initial

scans, and gastric activity was noted in 50% of thesepatients. Although no small-bowel activity was seen, thismay have been due to the absence of imaging between3 and 24 hr. Delayed views demonstrated that colonicactivity was present in 50% of the controls with initialgastric activity. While a labeling efficiency of 96% hasbeen reported with in vivo techniques using stannouschloride as the reducing agent (2), the accumulation ofgastric activity in 50% of our controls indicates Tc-99mactivity within the stomach or stomach wall. Such activity may not be free pertechnetate, since no thyroidactivity was seen in the necks of three control patientswith initial gastric activity. However, the thyroid maynot have been at peak activity, since the neck image wasmade 3 hr after tracer injection. The delayed colonicactivity may represent passage of complexed pertechnetate from the stomach to the large bowel, or secretionof pertechnetate into the bowel itself, as has been meported in an animal model (11). In Patient 4, duodenalactivitywas notedon the 1-hr images,indicatinghisactive duodenal bleeding. Delayed 24-hr images showedpassage of the extravasated labeled medcells into thecolon. The discrepancy noted between the control groupand bleeding patients with regard to initial gastric Tc99m activity may be related to the fact that 80% of thebleeding patients were undergoing continuous naso

C.

Volume 20,Number 10 1085

,

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WINZELBERG, MCKUSICK, STRAUSS. WALTMAN, AND GREENFIELD

gastric suction, which may have collected the Tc-99mactivity.

It should be stressed that if tracer activity is seen inthe large bowel on early views following tracer injection,active bleeding should be suspected. If there has been noinitial gastric accumulation o(Tc-99m activity, delayedviews at 24 hr may also indicate active bleeding. However, ifgastric activity is noted on the initial views, thisactivity may eventually accumulate in the lower smallbowel and colon and may give false-positive delayedimages. This problem may be minimized by keeping thepatients on continuous nasogastric suction during the24-hr imaging period. In the future, in vitro labeling ofmedblood cells might reduce some of the gastric activityseenwith invivolabelingmethods.The tracermightbeimproved by labeling the RBCs with In-i 11, which hasa longer half-life and is notexcmeted into the GI tracts

The data from this study suggest that red, blood cellslabeled in vivo with Tc-99m can provide an effectivediagnostic tool in the management of patients with intermittent gastrointestinal' hemorrhage, especially inpatients with lower gastrointestinal bleeding. It can beparticularly useful when the indications df continuedactive hemorrhage are equivocal. In this setting a negative scintigram is good evidence against active bleeding.Because the tracer remains in the blood ‘pool,repeatscanning can be performed over a span of several hoursto screen for active bleeding which, if present, can befurther evaluated with angiography or endoscopy.

FOOTNOTE

* Pyrolite; New England Nuclear Corp., Boston, MA.

ACKNOWLEDGMENT

We thank Ms. ConnieAlbani and Ms. EleanorPlati for their assistance in the preparation ofthis manuscript.

This work wassupportedin part by NIH CardiovascularNuclearMedicineTrainingGrant No. l-T32-HL074I6-0l.

REFERENCES

I . ALAVI A, DANN RW, BAUM S, et al: Scintigraphic detectionof acute gastrointestinal bleeding. Radiology I24: 753-756,I977

2. PAVELDG, ZIMMER AM, PATTERSONVN: In vivo labelingof red bloodcellswith @mTc:A newapproachto bloodpoolvisualization.J NucI Med I8:305-308,I977

3. ATHANASOULIS CA, WALTMAN AC, NOVELLINE RA, et al:Angiography:Its contribution to theemergencymanagementofgastrointestinal hemorrhage. Radiol Clin N Amer 14: 265-280,1976

4. D'ADDABBO A, FER5INI M: Detection of intra-abdominalbleeding by radioisotope scanning after experimental extravasation.JNuclBiolMed 18:170—174,1974

5. ARIEL IM: Thesiteof uppergastrointestinalbleeding:Detectionby radioactive-tagged red blood cells. JAMA I 80: 2 12-2 14,I962

6. SHAPIRO SH, CASTRONOVO FP, CALLAHAN Ri, et al: Utilization of indium-I I 3m for the detection of occult gastrointestinalbleeding.J Nucl Med I6: 569, 1975 (abst)

7. BERQUISTTH, NOLAN NG, STEPHENSDH, et al: Specificityof [99mTcjpertechnetate in scintigraphic diagnosis of Meckel'sdiverticulum: Review of 100 cases. J NucI Med 17: 465-469,I 976

8. BARRYJW, ENGLECV: Detectionof hemorrhagein a patientwith cecal varices using 99mTcsulfur colloid. Radiology 129:489-490, 1978

9. MISKOWIAK J, NIELSEN SL, MUNCK 0, et al: Abdominalscintiphotography with @mTclabeled albumin in acute gastrointestinal bleeding.Lancet2: 852-854, 1977

Jo. RHODESBA: Considerationsin radiolabelingof albumin.SeminNuclMed4: 281-293,1974

ii. TAYLOR AT, ALAZRAKI N, HENRY JE, Ct al: Intestinal concentrationof@mTc@pertechnetateinto isolatedloopsof rat bowel.JNuclMed 17:470-472 1976

I086 THE JOURNAL OF NUCLEAR MEDICINE

14th INTERNATIONAL SYMPOSIUMRADIOACTIVEISOTOPESIN CLINICALMEDICINEANDRESEARCH

January 9-12, 1980 Bad Gastein, Austria

The @4thInternational Symposium on “RadioactiveIsotopes in Clinical Medicine and Research―will be held January 9-12. 1980 in Bad

Gostein, Austria.

The Symposium will be opened by the “Gastein-Lecture―which will be given by on invited guest lecturer. There isno thematic limitation for

the program.

Opportunities for smaller expert discussions will be available as well as the traditional “FreePaper―sessions.

For further information contact:Prof. Dr. R.HoferHead, Dept. for Nuclear Medicine2nd Medical Univ.ClinicGarnisongasse 13,A-1090 Vienna, Austria

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1979;20:1080-1086.J Nucl Med.   Gary G. Winzelberg, Kenneth A. McKusick, H. William Strauss, Arthur C. Waltman and Alan J. Greenfield  Technetium-99mEvaluation of Gastrointestinal Bleeding by Red Blood Cells Labeled in Vivo with

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