detector and a second scan is required to achieve thenecessary axial sampling for volumetric data recovery(3). Movementof eitherpatientor detectorscreatesseveral problems, especially with short-lived isotopes.Data are lost during the time required for the movementand interleaved image planes taken at a later timecontain different physiologic data. For short half-liferadionucides, such as rubidium-82 (82Rb)and oxygen15 (‘SO),a second scan cannot be obtained withoutinjecting the radionuclides a second time. During thetime required to obtain adequate counts on the firstscan, activity decays sufficiently to prevent acquisitionofa second scan. These limitations hinder three-dimensional quantitation ofthe physiologic process with veryshort-lived radionuclides, unless a PET camera withhigh axial sampling is used.

The POSICAM camera had the following designgoals to overcome problems of data loss and activitydecay:

1. Three-dimensional volumetric imaging with a singlescan.

2. Fast imaging with high count rate capability forshort-lived isotopes such as 82Rband ‘SO.

3. High resolution for the heart (whole body) and brain.

To reach these goals, a new detector design wasdeveloped and patented (4). Every other detector ineach ring is staggered by one-half position to achievethe same spatial sampling frequency as moving thedetectors in the axial direction for an interleaved slice.When compared to conventional detector geometrywithout staggering, the staggered design halves the sampling distance in the axial direction, and almost doublesthe number of slices collected simultaneously by thecamera. The detector design also allows shorter septa,which in turn allows a smaller, more compact mechanical gantry. The detector design uses conventional ¾―photomultiplier tubes (PMT) and a conservative 2:1crystal to PMT positioning scheme in order to minimizespecialized electronics and simplify the encodingscheme for more stable operation. The system has beendesigned to accept a variety of different scintillators,including barium fluoride (BaF2) (5) and bismuth ger

A high-resolution,whole-bodypositroncamera,POSICAM6.5 BGO, has been designed,built, and tested; resultsfrom it are presented. The camera utilizes 1,320 BOOcrystals and 720 PMTS in a staggered geometry to produce high resolution of 5.8 mm FWHM and 21 imageplanes simultaneously. The axial resolution of the camerais measured at 11.9 mm at the center. Highaxial samplingis achieved with 5.1 25 mm separation of the image planessuch that three-dimensional imaging of an object can becarried out in a single scan. Recovery of volumetric distribution of radioactivity and object dimensions in axial andsagittal views is demonstrated by imaging spherical objects 13 mm to 39 mm in diameter.

J NucIMed 1990;31:610—616

positron emission tomography (PET) is usedmore extensively for in vivo quantitation of regionalperfusion and metabolism, better PET camera designis needed for more accurate volumetric data recovery.Consequently, fine axial sampling is essential in addition to in plane sampling in order to produce threedimensional imaging for systems based on multipletwo-dimensional image-planes.

The importance of finer axial sampling for threedimensional imaging ofthe heart, and greater diagnosticaccuracy, has been demonstrated by Senda et al. (1) inboth phantoms and patient studies. They showed increased sensitivity of detecting myocardial perfusiondefects by incorporating a second scan of the heartobtained by moving the patient one-half the imageplane separation. Specificity also improved due to thefiner axial sampling provided by the second interleavedscan, thereby avoiding artifactual false-positive results.In our own clinical experience with three-dimensionalimaging of the heart with TOFPET I (2), even withclose spacing of image planes (10.8 mm separation for11 mm axial resolution), movement of the patient or

ReCeiVed May 17, 1989; revisIon accepted Dec. 21, 1989.For reprintscontact: NizarMullani,Divisionof Cardiology.IkiiverSityof

TexasMedicalSchool,6431Fannin,Houston,TX77030.

610 The Journalof NuclearMedicine•Vol. 31 •No. 5 •May1990

Design and Performance of Posicam 6•5BGOPositron CameraN.A. Mullani, K. Lance Gould, R.K. Hartz, R.E. Hitchens, W.H. Wong, D. Bristow, S. Adler,E.A. Philippe, B. Bendriem, M. Sanders, and B. Gibbs

Positron Diagnostic and Research Center, The University of Texas Health Science Center at Houston, Texas; PositronCorporation, Houston, Texas; and Brookhaven National Labs, Upton, New York

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minate (BGO) (6). A time-of-flight (TOF) option canalso be added to the BaF2 system with little change inthe detectors and gantry designs. This paper describespreliminary performance of the POSICAM 6.5 havingsix rings of BOO detectors (five staggered rings) simultaneously producing 21 overlapping image planes.

CAMERA DESIGN

A drawing of the POSICAM camera is shown inFigure 1. A brief description of certain elements of thecamera is included.

The donut-shaped assembly for holding the detectorsis made of half-inch steel for structural support andshielding against scattered radiation. The detectors andsepta are mounted on a moving plate inside this structure, which move eccentrically(wobble)with a diameterof 22 mm for better sampling during the scan (7). Thesepta are donut-shaped and are made of 1.5 mm thicklead, 7 cm long. They are attached to the detectormodule plate and wobble with the detectors as a unit.Lead shielding, varying from 5 to 7 cm in thickness, isattached to the stationary part ofthe gantry to minimizethe scattered radiation reaching the detectors from thepatient's body outside of the axial field of view. Thedesign of the septa is essentially dictated by the compromise between accepted randoms, scatter events, andsensitivity for true counts (8,9).

The POSICAM 6.5 BOO camera utilizes 1,320 BOO

FIGURE1Artist'sdrawingof thePOSICAMmechan@aIgantry.Severalfeatures ofthe system such as rotation, tilt, and patient couchrotationare illustratedin the drawing.A large footprint of thebase distributes the weight over a large area for easy installationwith few on-sitestructuralchanges.

scintillation crystals 8.5x20x30 mm in size. These

crystals are viewed by 720 PMTs, which detect thescintillation light and convert it to electrical pulses. Thedetectors and PMTs are grouped in 120 modules, eachwith 11 crystals and 6 PMTs that are placed in a circlewith a diameter of 78 cm. Each module of 11 crystalsand 6 PMTs is light-tight and self-contained. The modules are placed radially around the circle such that thesix PMTs are aligned with the axial direction of thegantry.

The staggered detector design doubles the axial sampling and the number of slices, while reducing the sliceseparation to 5.125 mm. Every alternate detector in thecircle of crystals is offset by one-half the detector position, axially. The offset produces axial sampling whichis equivalent to manually moving the detectors by onequarter of the detector length.

The placement of PMTs and crystals is shown inFigure 2. The crystals are numbered 1 through 11 andthe PMTs are labeled A through F. Light output fromcrystal 1 is collected mostly by PMT A, and similarly,the light output from crystal 3 is collected mostly byPMT B. The light from crystal 2 is shared equallybetween PMT A and PMT B. The system for collectinglight from 11 crystals to 6 PMTs is referred to as a 2:1system because of two-to-one splitting of light for encoding. The 2:1 detector positioning is easily encodedand decoded since there is a large difference in thesignals between the two PMTs for the three crystalpositions. The large separation of the PMT outputs forcrystal identification also permits greater variation ofPMT performance without significantly affecting detector encoding accuracy.

FIGURE2A plandrawingof thecrystalsandPMTarrangementfor thePOSICAMdetectormodule.The crystalsare numbered1through 11 and the PMT5 are labeled A through F. Lightoutput from crystal 1 is primarily detected by PMT A, whilelight output from crystal 2 is detected by PMTs and A and Bequally.

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611Posicam 6.5 BOO •Mullani et al

POSICAM GANTRY FEATURES

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Data acquisition and image processing are handledby a multiprocessor system comprised of two VMEbased systems and a VAX computer. The VME-basedsystems allow simultaneous operation of data acquisition and image reconstruction while the VAX functionsas the host and image processing system. The dataacquisition system is capable ofoperating at@ 1 millioncps in the binned profile mode or list mode of datacollection (2).

METHODSThe POSICAM 6.5 BGO system has been tested for prelim

mary results and the camera design has been simulated withthe Geant III Monte Carlo code from CERN (10). The testingand simulation covers several aspects ofthe camera (includingresolution and sensitivity) for performance comparison withtheoretically possible performance. The following tests werecarried out on the POSICAM camera to evaluate its overallperformance.

ResolutionA single 1 mm in diameter germanium-68 line source

positron source encased in a steel tube with an approximateoutside diameter of 2 mm was scanned at three differentlocations from the center, i.e. at radial position of r = 0, r =10, and r = 20 cm. The point source was scanned in air at ahigh count rate of —250,000cps. Radial and tangential measurements of resolutions were obtained from the reconstructedimages by taking profiles through the images and fitting Gaussian profiles to the data.

Fluorine- 18 sources -@-2mm in diameter were used tomeasure axial resolution. The sources were attached to thepatient pallet and, under computer control, these sources werescanned at many different axial locations stepped in increments of 2 mm. A region of interest (ROl) was drawn aroundthe point source in the image plane. A profile ofaxial responsefunction was obtained from several scan positions. The ROldata was then plotted as a function of patient pallet positionand the full width at half maximum (FWHM) value wasmeasured for different slices (assuming a Gaussian distributionin the axial direction). The axial resolution was measured forthree different radial positions of the source at r = 0, r = 10,and r = 20 cm from the center.

SensitivityThe sensitivity of the camera was measured by placing a

uniformly distributed source of radioactivity in a cylinder of20cmdiameterand19cmlength.Thelengthofthecylinderwas arbitrarily chosen in order to cover the 12.3-cm axial fieldof view. The source was scanned at a low radioactivity concentration of -.@0.1 MCi/cc where the deadtime losses andrandoms are negligible. The energy threshold of the systemwas typically set at 350 keV and sensitivity was measured asthe total number of counts collected by the system, includingscatter,after subtractionfor randoms.

ScatterScatter was measured with a line source in air and in a 20-

cm diameterscatteringmedium. Scatterdatawerealsosimulated with the Geant Monte Carlo simulation program andthe scatter fraction (defined as the ratio of scatter to true

counts) was estimated from the simulated data by Bendriemet al. (10). A scatter deconvolution filter was derived andapplied to the binned profiles before image reconstruction.

A uniform phantom of 20 cm diameter with a 5-cm fillingdefect also was scanned with the hole positioned at the centeras well as off-center for evaluation of the scatter correctionalgorithm. A ROl was drawn in the 5-cm hole, as well as inthe surrounding activity, to assess the amount of scatteredradiation and the residual error after subsequent scatter correction.

RandomsA uniform phantom source, 20 cm in diameter by 19 cm

long (5,440 cc), was scanned with high levels of activity inexcess of 20 mCi and allowed to decay. Sequential scans wereconducted to correlate the activity level to the detected counts.The number of randoms was estimated from the tails of thebinned profiles that extend from radial positions of greaterthan r = 20 cm, in order to minimize the amount of scatterin the randoms estimation. Randoms were also calculatedfrom the singles rate measured by the system. The true andrandom events were recovered from the ROI drawn aroundthe uniform phantom. The true counts and random countswere plotted against known activity levels in the phantom forthis large uniform distribution phantom exceeding the axialfield of view.

Deadtime LossesDeadtime losses are an important characteristic ofthe PET

camera since they limit the collection of counts from thepatient. These losses are complex enough that they cannot betreated with a single correction factor unless the system iswell-characterized (1 1). There are several areas of deadtimelosses: (a) the singles at the module; (b) the encoder andcoincidencecircuits;and (c)the data transfer.Deadtimelossesare measured from the true counts obtained after randomsubtraction.A 20-cmdiametersourcefilledwith a short-livedisotope is scanned several times at different concentrations.True counts are obtained from the data and the expectedtruecounts at high count rates are linearly extrapolated from thelow count-rate values to obtain an estimation of deadtimelosses.

AxialSamplingThe effect of inadequate axial sampling in both axial direc

tions and the partial volume errors, caused by angulation ofan organ with respect to the imaged plane, can be simultaneously demonstrated by a special phantom (12). Several 1-cmthick fingers of radioactivity were placed in the field of viewat angles ofO, 30, 45, 60, and 90 degrees to the axial direction.These fingersof radioactivitysimulatingthe myocardiumatdifferent positions were scanned with a single scan. The reconstructed images were displayed by creating long axis slicesthrough them such that all the fingers of radioactivity areviewed. The drawing of the phantom is shown in Figure 3.This phantom has been designed to quantitate the recoverycoefficients for volumetric imaging as a function of the objectsize and the angle ofthe involved plane. Detailed informationis presented in a separate publication by Mullani (12).

ResolutionUniformityIn order to evaluate the uniformity of resolution across the

20-cm field of view, the Derenzo (13) phantom was scanned

612 The Journal of Nuclear Medicine •Vol. 31 •No. 5 •May1990

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TABLE2RelativeSensitivityof each Slice as Measured with a 20-

cm Diameter Uniform Phantom with Starting ActivityLevelsof Approximately1MCi/ceRelative

RelativeSlicesensitivity Slicesensitivity1

0.32 121.0520.50 131.2530.79 141.0540.95 151.1951.17 161.0260.96 171.1771.14 180.9581.02 190.7591.21 200.48101.01 210.27111.19 — —

Reso

r=0lution

in mmFWHM

r=10r=20Radial5.86.37.9Tangential5.86.36.3Axial

(average)1 1.91 2.814.3

simulation of the POSICAM 6.5 BOO is 5.5 mmFWHM at the center while the measured resolution is

SensitivityThe measured sensitivity was -@@-180,000cps/@sCi/cc,

for an energy setting of @-350keV. Table 2 lists therelative sensitivity ofeach image plane and the variationin sensitivity from slice to slice. The variation in thesensitivity from slice to slice was found to be < ±13%of the average sensitivity for the 17 inner slices.

ScatterThe percent of scatter in an image was estimated by

scanning a line source in air and in a scattering mediumof 20 cm in diameter. The scatter data was then deconvolved from out of the binned profile data using thedeconvolution method described previously (10). Anexperimental estimate from this technique which accurately corrects for scatter is 18% ofthe total data forthe 20-cm uniform phantom. Monte Carlo simulationof scatter ranged from 18% to 24%, depending on theenergy threshold settings. Scatter correction for thePOSICAM 6.5 also was demonstrated using the 20-cmdiameter source with a 5-cm defect in the center. Aresidual error of <2% was seen in the hole followingthe scatter correction.

RandomsThe relationship between randoms and trues as a

function of the total radioactivity in the field of view isshown in Figure 4. The concentration at which randomsequaled trues was 2.4 MCi/cc and the total activity in

IMAGE PLANES .____ 1 5.8 mm FWHM. The average axial resolution for all____ A slices was measured at 11.9 mm FWHM at the center.

T The averageaxial resolution at r = 10 cm was 12.8 mmI FWHM; r = 20 was 14.1 mm FWHM. Within a 20-cm

____ I diameter region, the radial, tengential, and axial reso____ I lutions changed <10% from those values at the center.____ y Fora40-cmdiameterobject,therewas<20%changes

2 1 in the tangential and axial resolutions but a 36% change

in radial resolution.

PARTIAL VOLUME PHANTOM

FIGURE3Special partial volume and axial sampling phantom whichsimulatesthe myocardiumat differentangleswith respect tothe imaged plane. The walls of the myocardium are 1 cm thickand indinedat 0, 30, 45, 60 and 90 degrees. Axialsamplingand quantitationerrors due to partialvolumeerrors are demonstrated by takingcoronalsections throughthe data.

SpheresThree-dimensional volumetric quantitation of POSICAM

was tested by scanning, with a single scan, several spheres (14)which are 13, 16, 21, 24, 31, and 39 mm in diameter. Thespheres were positioned in a circle of —5cm radius and placedin a 20-cm diameter, uniform distribution of attenuatingmedium such as water. All the sphereswere filled with thesameconcentrationof radioactivity.

Profiles were drawn through the spheres in the reconstructed images and the 50% points of the maximum in theprofiles were used as edges ofthe spheres. Sagittal and coronalviews also were obtained from all the slices. Regions of interestwere drawn over each sphere in each slice and three-dimensional count density was plotted against the known volume ofthe spheres. The plotted density was used to assess the system'scapabilityofrecoveringthree-dimensionalvolumetricconcentrations with a single scan.

RESULTS

TABLE IRadial,Tangential,and AxialResolutionsMeasuredwitha PointSourcein Air at ThreeDifferentPositionsin the

Field of View

with a low concentration of radioactivity and reconstructedusing the calculated attenuation correction.

ResolutionThe radial and tangential resolutions are shown in

Table 1. The resolution predicted by the Monte Carlo

613Posicam 6.5 BOO •Mullani et al

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FIGURE5Axial view obtained from the 21 slices of the axial samplingphantom show the uniform recovery of myocardial activityregardless of the tiltangle withrespect to the imageplane.

Figure 6. Fifty million counts were collected in thephantom. The image was reconstructed using the rampfilter.

Imagingof SpheresSpheres of 13, 16, 21, 24, 33, and 39 mm diameter

were filled with uniform activity and placed in scattering medium consisting of a 20-cm diameter phantom.Axial views ofthe spheres obtained through two spheresat a time (Fig. 7) show the ability of POSICAM systemto recover three-dimensional activity distribution.

Regions of interest were drawn in all the slices forthe six spheres, and activities in these ROIs for thedifferent slices were summed together to form a threedimensional ROI over the sphere. PET counts meas

ured with these three-dimensional ROIs were then compared with the volumes of the spheres and shows the

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FIGURE 4The relationshipbetween trues and randoms counts as detected by POSICAMin a 20-cm diameteruniform source 19cm deep as a functionof the total activity in the phantom.

the 20 cm diameter by 19 cm long phantom was 13mCi. The ratio oftotal singles in the system to the truecounts measured at low count rates is ‘@@-38.In otherwords, -@.‘2.8%of the total activity detected by thedetectors results in true coincidences.

DeadtlmeLossesDeadtime loss for the POSICAM 6.5 BOO system

was demonstrated by monitoring the count rates atseveral concentration levels using a short-lived source.The data is plotted in Figure 4 for decaying radioactivityin the 20-cm diameter uniform phantom. Randomswere measured at the edges of the profiles and the totalcounts collected from the 20-cm region are brokendown into trues and randoms. The point at which 50%of the trues are lost due to deadtime occurs with 12mCi in the phantom.

Axial SamplingThe axial sampling capability ofPOSICAM was dem

onstrated by scanning the special phantom which sim

ulates myocardial activity at 0, 30, 45, and 60 degreeangles to the image plane, as shown in Figure 3. All 21slices are collected simultaneously in a single scan, withlong axis views subsequently obtained from these images (Fig. 5). The simulated myocardial activity at allangles were recovered uniformly because the POSICAMsystem has sufficient axial sampling with respect to itsaxial resolution. No sampling artifacts are observed inthe images.

ResolutionUniformityThe resolution capability of the system and uniform

response within the 20-cm field of view were demonstrated with the Derenzo phantom (13) as shown in

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the ramp filter. The

614 The Journal of Nuclear Medicine•Vol.31 •No. 5 •May 1990

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3D MEASUREMENTOF OBJECTDIMENSIONSBY POSICAM6.5 BGO.

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recovery of the volumetric information from 1.3 cc to39 cc sized objects (Fig. 8).

Profiles were also drawn through the spheres in thetransaxial planes and in the axial direction. The 50%level ofthe maximum amplitude ofeach was then used

JORDANSPHERESVOLUMEMEASUREMENTPOSICAM6.5 BOO

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SPHEREDIAMETERS(mm)FIGURE9Diameterof the spheres measured in-planeand in the axialdirectionplottedagainst the knowndiametersof the spheres.Datawas obtainedfroma singlescan ofthe spheres phantom.

to measure the diameter of the spheres, which wereplotted as a function ofthe true diameters ofthe spheres(Fig. 9).

Heart PhantomThe heart phantom from Data Spectrum (Chapel

Hill, NC) was imaged with radioactivity in the myocardium which included a defect of 1 cm by 2 cm. Therewas a low background level of ventricular activity surrounding the myocardium. The 21 slices obtained fromthe heart phantom are shown in Figure 10.

CONCLUSION

This multi-slice system is designed for fast threedimensional volumetric imaging with a single scan. Theresults demonstrate accurate recovery of activity concentrations and dimensions for objects which are 16mm or greater.

The high resolution capability of the system and itsuniformity of resolution across the field of view havebeen shown. Variation in sensitivity of adjacent imageplanes also has been demonstrated to be quite small.This uniformity of sensitivity makes the signal-to-noisecharacteristics ofthe data comparable from one slice tothe other.

ACKNOWLEDGMENTSThe authors wish to thank Ro Edens,Claire Finn, Kathy

Norred, and Kathy Rainbird in preparing the manuscript. The

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FIGURE8Total counts measured in the 3-D ROl for the spheres areplottedagainstthe knownvolumesof the sphere.Therelationship is linear except for the smallest sphere of 1.3 ccwhere the PET measured activity is overestimated partly dueto partialvolumeeffects.

615Posicam 6.5 BOO •Mullani et al

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1. Senda M, Yonekura Y, Tamaki N, et al. Interpolating scanand oblique-angletomograms in myocardial PET using nitrogen-13ammonia. JNuclMed 1986;27:830—836.

2. Mullani NA, GaetaJ, Yerial K, etal. Dynamicimagingwithhigh resolution time-of-flightPET camera TOFPET I. IEEETransNuciSci1984;NS-31:609—613.

3. MullaniNA.Theneedforthree-dimensionalimaginginPET.Proceedings of the international symposium on current andfuture aspects of cancer diagnosis with positron emissiontomography.Tohoku, Japan, 1985:228—296.

4. Mullani NA. U.S. Patent Number 4,563,582 Positron Emission Tomography Camera.

5. WongWH, MullaniNA, Wardworth0, etal.Characteristicsofsmallbariumfluoride(BaF2)scintillatorforhighresolutiontime-of-flight positron emission tomography. IEEE TransNuciSci 1984; NS-3l:38l—386.

6. ChoZ, FarukhiMR. Bismuthgermanateasa potentialscmtillation detector in positron cameras. J Nuci Med 1977;18:840—847.

7. Mullani NA, TerPogossian MM, Higgins CS, et al. Engineering aspects ofPETT V. IEEE Trans NuciSci 1979; 26:2703—2705.

8. Derenzo SE. Method for optimizing side shielding in positronemission tomography and for comparing detector materials.JNuclMed 1980;21:971—977.

9. Mullani NA, Wong WH, Hartz RK, et al. Sensitivity improvement of TOFPET by the utilization of the inter-slice coincidence. IEEE Trans Nuci Sci 1982;29:479—483.

10. BendriemB,WongWH, Mullani NA, et al. Analysisof scatterdeconvolution technique in conventional PET using MonteCarlo simulation. JNuclMed 1987;28:681.

11. Thompson CJ, Meyer E. The effect of live time in components of a positron tomography on image quantification.IEEE Trans Nuci Sci 1987;34:337—343.

12. Mullani NA. A phantomfor quantitationof partial volumeeffectsin ECT. IEEE Trans Nuci Sci 1989;36:983—987.

13. Derenzo SE, Budinger TF, Muesman RH, et al. Imagingproperties of a positron tomography with 280 BGO crystals.IEEE TransNuciSci1981;28:81—89.

5

13

00FIGURE10Transaxialslices of the Data SpectrumHeart phantom.Twenty-one slices are collected simultaneously and a fewselected slices are enlarged in the lower part of the display. A1 cm x 2 cm defect is clearlyseen in severalslices.

authors also thank the Department of Nuclear Medicineatthe National Institute of Health for its scientific contributionsand the production of radioactivity. We also thank ProfessorJordan and Dr. Knopp at Hanover for the use of their phantoms.

This research was carried out in part as ajoint collaborativeeffort with the Clayton Foundation for Research, and wasfunded in part by DOE grant DE-F005-84ER602 10and NIHgrants ROl-Hl-26862 and ROl-HL26855.

Presented in parts at the European Society of NuclearMedicine meeting, London, August 1985, and The Society ofNuclear Medicine meeting, June 1986, Washington, DC.

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The Journal of Nuclear Medicine•Vol.31 •No. 5 •May1990

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1990;31:610-616.J Nucl Med.   Sanders and B. GibbsN.A. Mullani, K. Lance Gould, R.K. Hartz, R.E. Hitchens, W.H. Wong, D. Bristow, S. Adler, E.A. Philippe, B. Bendriem, M.  Design and Performance of Posicam 6.5 BGO Positron Camera

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