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Anatometabolic Image Fusion: PET/MRI/CT for Locating Increased Activity, Slides of Anatomy

A study that demonstrates the feasibility of digital image fusion between body PET and MRI or CT to produce hybrid 'anatometabolic' images. These images contain both the exquisite anatomic detail of CT or MRI and the unique physiological information from PET, allowing for more accurate localization of increased activity in the body. The study was conducted by researchers at the University of Michigan Medical Center.

What you will learn

  • What are the benefits of using anatometabolic images in body PET studies?
  • How accurate is the fusion process in defining the location of increased activity in the body?
  • How is digital image fusion used to produce anatometabolic images of the body?

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bg1
uptake of FDG is so high that relatively little soft-tissue
background activity is identified. In such cases, determin
ing where foci of increased FDG uptake are located can be
difficult. This problem is even more apparent with high
contrast parametricimages (12). Fusion of metabolic emis
sion images from positron emission tomography (PET) to
the much higher resolution anatomic informationavailable
from magnetic resonance imaging (Mifi) or computed to
mography (CT) scans seems desirable (13). This approach
has been applied by several groups to fuse PET on single
photon emission computed tomography (SPEC!') emission
images of the brain with MR or C1@anatomic images
(14,19). Indeed, tracer uptake has been mapped onto pa
tient-specffic or atlas-based brain anatomy (14—23).Since
the normalbrain has reasonablyhigh levels of FDG uptake
throughout it and moderate anatomic information is avail
able, anatomic informationfromemission brain images can
be used in the realignments (1824).
Whereas anatomically defining the location of PET
tracer uptake in the brain is feasible, the situation in the
body, where many normal tissues have much lower levels
of FDG accumulation than the brain and thus provide less
anatomy, is more challenging. In addition, while the brain
and its contents are fixed, the body can twist and bend. In
the present study, we demonstrate the feasibility of digital
image fusion between body PET and MRI or CT to pro
duce hybrid “anatometabolic―images of the body, which
contain both the exquisite anatomic detail of CF or Mill
and the uniquephysiological informationfromPET. This is
achieved using a rigid rotate-translate scale model and a
realignment algorithm based on operator-identification of
external markers present on both emission PET and Cr1
Mill as well as, in some instances, unique anatomic points
defined onboth transmission PET images and the anatomic
study.
METHODS
Ten patientswere studied.Each patientwas > 18yr old and
providedwritteninformedconsentforparticipationin thestudy.
The PET Studieswere performedas parts of ongoingclinical
Positron emission tomographic (PET) Images of v@ceralcan
cars are commonlyvisualizedas “hOtSpots―ofincreased activity
with relativelylittlenormal anatomy discemable, when 2-[18F]-
fluoro-2-deoxy-D-glucose(FDG)Is used as the tracer. We de
scribe a method by which computed tomography or magnetic
resonance anatomic images can be digitallyfused in three di
mensions, using a dgid rotate-translate scale model with PET
“metabolic―images, to simuftaneouslyd@ay registered ana
tomic and metabolic information. Such “anatometabolic―fusion
images were produced in 10 patients witha vwlety of visceral
cancers. E@ctemaIfiducialmarkers placed during both the ana
tomic and the metabolic study, as well as internal anatomic
fiducials defined from landmarks observed on reconstructed
transmission Images, were used to achieve image fusion. The
mean error magnitude ±sam. offidudal reg@trationInthe nine
patients with successful realignmentswas 5.0 ±0.8 mm. The
mean accuracy In realignment between known anatomic struc
tures seen on both the anatomicstudy and onthe emissionPET
scan (but notused In realignment)was 6.3 ±0.8 mm. Localiza
tion offoci of Increased FDG uptake to specificanatomic struc
tures was achieved by this method, which represented an en
hancement over the rudimentaiyanatomy available from the
emission images alone. Anatometabolic fusion images made
using this reasonably simple method should prove useful inthe
management of patients with cancer and other diseases.
J NucIMed1993;34:1190-1197
positron-emitting analog of glucose, 2-['8F]-fluoro-2-
deoxy-D-glucose (FDG), rapidly accumulates in many hu
man neoplasms following intravenous injection (1—li).In
deed, when the FDG accumulation of neoplasms is
compared to that of most normal tissues, a relatively high
target-to-background ratio is often observed (7,8).
Although high target-to-background ratios are useful for
tumor detection, they may also be problematic if tumor
ReceivedOct 19,1992;revIsionaccepted Feb. 16, 1993.
For correspondence or reprints contact Richard L Wahi, MD, DMsbn of
NudearMedIc@ne,UniversityofMichiganMedicalCenter,1500E.Med@Center
DrIve,B1G412,AnnAibor,Ml48109-0028.
1190 The Journalof NudearMedicine•Vol.34 •No. 7 •July1993
‘‘Anatometabolic' ‘Tumor Imaging : Fusion of
FDG PET with CT or MRI to Localize Foci of
Increased Activity
Richard L. Wahi, Leslie E. Quint, Richard D. Cieslak, Alex M. Aisen, Robert A. Koeppe and
Charles R. Meyer
Departments ofRadiology and InternalMedicine, Division ofNuclear Medicine, University ofMichwan Medical Center,
Ann Arbor, Michigan
pf3
pf4
pf5
pf8

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uptake of FDG is so high that relatively little soft-tissue

background activity is identified. In such cases, determin

ing where foci of increased FDG uptake are located can be

difficult. This problem is even more apparent with high

contrast parametricimages (12). Fusion of metabolic emis

sion images from positron emission tomography (PET) to

the much higher resolution anatomic informationavailable

from magnetic resonance imaging (Mifi) or computed to

mography (CT) scans seems desirable (13). This approach

has been applied by several groups to fuse PET on single

photon emission computed tomography(SPEC!') emission

images of the brain with MR or C1@anatomic images

(14,19). Indeed, tracer uptake has been mapped onto pa

tient-specffic or atlas-based brain anatomy (14—23).Since

the normalbrainhas reasonably high levels of FDG uptake

throughout it and moderate anatomic information is avail

able, anatomicinformationfrom emission brainimages can

be used in the realignments (1824).

Whereas anatomically defining the location of PET

tracer uptake in the brain is feasible, the situation in the

body, where many normal tissues have much lower levels

of FDG accumulationthan the brain and thus provide less

anatomy, is more challenging. In addition, while the brain

and its contents are fixed, the body can twist and bend. In

the present study, we demonstrate the feasibility of digital

image fusion between body PET and MRI or CT to pro

duce hybrid “anatometabolic―images of the body, which

contain both the exquisite anatomic detail of CF or Mill

andthe uniquephysiological informationfrom PET. This is

achieved using a rigid rotate-translate scale model and a

realignment algorithm based on operator-identification of

external markers present on both emission PET and Cr

Mill as well as, in some instances, unique anatomic points

defined on both transmission PET images and the anatomic

study.

METHODS

Ten patientswere studied.Eachpatientwas > 18yr old and

providedwritteninformedconsentforparticipationin the study.

The PET Studieswere performedas partsof ongoingclinical

Positron emission tomographic (PET) Images of v@ceralcan

cars are commonlyvisualizedas “hOtSpots―ofincreased activity

with relativelylittlenormal anatomy discemable, when 2-[18F]-

fluoro-2-deoxy-D-glucose(FDG)Is used as the tracer. We de

scribe a method by which computed tomography or magnetic

resonance anatomic images can be digitallyfused in three di

mensions, using a dgid rotate-translate scale model with PET

“metabolic―images, to simuftaneouslyd@ay registered ana

tomic and metabolic information. Such “anatometabolic―fusion

images were produced in 10 patients witha vwlety of visceral

cancers. E@ctemaIfiducialmarkers placed during both the ana

tomic and the metabolic study, as well as internal anatomic

fiducials defined from landmarks observed on reconstructed

transmission Images, were used to achieve image fusion. The

mean error magnitude ±sam. offidudal reg@trationInthe nine

patients with successful realignmentswas 5.0 ±0.8 mm. The

mean accuracy In realignment between known anatomic struc

tures seen on boththe anatomicstudy and on the emission PET

scan (butnot used Inrealignment)was 6.3 ±0.8 mm. Localiza

tion of fociof Increased FDG uptake to specificanatomic struc

tures was achieved by this method, which represented an en

hancement over the rudimentaiy anatomy available from the

emission images alone. Anatometabolic fusion images made

using this reasonably simple method should prove useful in the

management of patients withcancer and other diseases.

J NucIMed1993;34:1190-

positron-emitting analog of glucose, 2-['8F]-fluoro-2-

deoxy-D-glucose (FDG), rapidly accumulates in many hu

man neoplasms following intravenous injection (1—li).In

deed, when the FDG accumulation of neoplasms is

compared to that of most normal tissues, a relatively high

target-to-background ratio is often observed (7,8).

Although high target-to-background ratios are useful for

tumor detection, they may also be problematic if tumor

ReceivedOct 19, 1992;revIsionaccepted Feb. 16, 1993. For correspondence or reprints contact Richard L Wahi, MD, DMsbn of

NudearMedIc@ne,UniversityofMichiganMedicalCenter, 1500 E.Med@Center

DrIve,B1G412,AnnAibor,Ml48109-0028.

1190 The Journalof NudearMedicine•Vol.34 •No. 7 •July

‘‘Anatometabolic' ‘Tumor Imaging : Fusion of

FDG PET with CT or MRI to Localize Foci of

Increased Activity

Richard L. Wahi, Leslie E. Quint, Richard D. Cieslak, Alex M. Aisen, Robert A. Koeppe and

Charles R. Meyer

Departments ofRadiology and InternalMedicine, Division ofNuclear Medicine, University ofMichwan Medical Center, Ann Arbor, Michigan

MeanMean

ofNumber erroraccuracy

ofmagnitudereallgnmenttPt.

no. Cancer MOdality fiduclals(e,l)*(mm)(mm)

@ *e numberofexternalfidudals;I- numberofInternalfidudalsusedInrealignment.

tNU@Ik@eI@In parentheses Is number of points assessed for accuracy of fusion. @TechnlcalIyunsatisfactory due to patient motion during emission PET study and was not used In calculating mean error magnitude or accuracy.

TABLE I

Accuracyof Fusion InTen Patients withAnatometabolicTumor Imaging

IBreastMR4(1,3)4.66.2(3)2TestIsMR8(8,0)4.08.4(3)3TestIsMR8(8,0)5.69.3(4)4ProstateMR5(3,2)5.57.3(3)5BladderMR5(5,0)5.14.4(2)6LungCT4(3,1)2.36.6(4)7BladderMR*

(1)8LungCT5(4,1)3.61.0(2)9LungCT5 (5,0)10.424.V*

.07.4(3)10LungCT4 (1,4)1 1 (3)Mean± (1,3)3.76.

s.e.m.5.04 ±0.826.3 ±0.

because of their improved visibility, their commercial availability and their ability to define the geometric center of the markers. Crscanswereobtainedinfourpatientsinthestandardfashion

with or withoutintravenouscontrast,dependingon the clinical

situation. A GE 9800 CT (General Electric Medical Systems,

Milwaukee,WI)scannerwas usedwith a 2-secscan time. Images

were displayed using a 512 x 512 pixel image matrix. Small

cylindricallead markers, roughly2 mm in diameter x 1.0 cm in

length,were placedover the same markedskin sites used for the

PETstudy,orientedperpendicularto the scanplane.Generally,

i-cm thick contiguous tomographic sections slices were obtained.

InsomecasestheanatomicstudyprecededthePETscan.Ineach

case, however, the PET and anatomic studies were performed on

thesameday.Duringeachimagingprocedure,effortsweremade

to positionthe patientstraightand fiat on theirbacks on the

scanning table but no external frames were used.

Imag Fusion

All cross-sectional image sets were reconstructed initially on

each acquisitionsystem usingstandardvendor-suppliedrecon

struction algorithms.Image fusionswere performedon a Sun 4

workstationrunningSunOSUnixandthenativeMITX-windows

graphicaluser interface.Reconstructeddatasetswere transferred

to the workstationusingthe Internetfile transferprotocol.To

reducethe spatialgranularityof z-axis identificationof marker

location,all imagesets were first interpolatedin the z-axisdirec

tion using a fourth-order,Hamming-weightedsinc functionto

yieldthree evenlyseparatedinterpolatedimagesforeveryoriginal

cross-sectionalimage.The sinc functionwas chosen because of

its particular suitability for reconstructing sample data sets (25).

Thez-axisinterpolationmaintainedanisotropicvoxels, i.e., “cu

beriles―were not made.Note thatthe interpolateddatasetwas

used only for determininggeometrictransformation.Identifica

tion of corresponding three-dimensional markers (homologous

point pairs) in each dataset were selected by the operator using

mouse-driven,pairedslider-scrolledimagesandpointer.Thecen

ter of the fiducial markers was used in the realignments. After

point pairs were identifiedin the datasets, a rigid rotate-scale

protocols to evaluate the diagnostic accuracy of PET in a variety

of cancers.Thestudypopulationis brieflydescribedin Table1.

PETimageswereacquiredwitha SiemensCli 931i08-12PET

scanner (Siemens Medical, Iselin, NJ). The method for image

acquisition has previously been described and 15 slices of ‘—6.& mm thickness were obtained per scanninglevel(810,11). In brief,

transmissionPET datasetswere firstobtainedover specificareas

ofthe body suspected ofcontaining cancer for a periodof ‘—10min per set of 15 slices (with axial coverage of —10cm) using the

retractableasGe/@Garingtransmissionsources of the PET scan

ncr. Transmissionimageswere reconstructedby filteredbackpro

jection and generallycontained5—10milliondetectedcoincidence

events per image. Following the performance of one or more transmissionscans, radioactivefiducialmarkerswere taped onto

ink-markedskin sites on the patient that were expected to be

within the field of view of the scanner. These fiducialmarkers

contained 0.5-3 iCi of FDG adsorbed onto —-2-5-mmdiameter

polystyrene beads. Markers were placed so that the suspected

tumor region was thoroughly surroundedby markers (generally covering 15—20cm in the z-axis). Up to eight markerswere placed per patient. Emission images reconstructed using filtered back

projectionwere obtainedfollowingthe intravenousinjectionof

approximately 10 mCi of FDG. The two 10-mis duration images

from50—70miii postinjectionwere used for fusions.Generally

—20cm of the patient was imaged in the z-axis, which represented

two contiguous 10-cmacquisitions.A 128 x 128image matrix

displaywas used to displayemissionimagesof approximately

mm resolution.

Six patients had MR scans obtained with a Picker Vista 0.

Tesla or GE Signa 1.5 Tesla Mill device. StandardTi-weighted,

T2-weightedand/or proton density spin-echo images were ob

tamed, which were 5—10mm in slice thickness, generallywith a

i-mm gap between images. Images were displayed using a 256 x 256 image matrix. Vitamin E capsules (iOOI.U. vitamin E, Rorer

Inc., Ft. Washington,PA)weretapedoverthesamemarkedskin

sites used forthe PETimagingstudyandwere orientedperpen

dicularto the axialscan plane. Anisotropicmarkerswere chosen

AnatometabolicTumor Imaging•Wahi at at. (^) I 191

@ I p@@GroROTATE-SCALE- Physicianassessfor I TRANSLATE I ALGORiTHM+patientmotion/

_____________H

@ I@ie@o@ma@

Ideletequestionable

@@ I FusionAnatoetbo' I

I imagesproducedover I I 20ariz-a,ds I

Flow Chart of Registration Process Between CT/MR and Emission PET to Form the “AnametabolicImage Set@'

Technologist Identifies unique anatomic points on recon structed transmission scan (ifany) and applies to registered emission PET

Adequate

FiGURE 1. Row chart detalhingmethod of anatometabolicImageformation(after

Ref.18).

a representationof stable scarringafter aggressive chemo

therapy(7,27).

The potential of the method in the care of a patientwith

newly diagnosedlung canceris illustrated in Figure 3A. In

this patient, CF shows volume loss in the rightlower lung

and a localized right pleural effusion. The corresponding

PET image shows intense central FDG uptake with a small

focus of modest FDG uptakein the anteriorleft chest, with

only minimal FDG uptake elsewhere. The fusion image

clearly demonstrates intense FDG uptake in the central

portion of the rightlower lobe collapse. This appearanceis

most consistent with central FDG-avid lung cancer,

whereas the peripheral area represents postobstructive

lung volume loss with little FDG uptake. A lower set of

images (Fig. 3B) shows that the anterior left chest FDG

activity is myocardial in origin. This fusion over multiple

image planes illustrates that the fusions are true fusions in

three dimensions,not just realignmentswithin a single

imaging plane.

Theabilityof our algorithmto realignvolumesof tissue

is also clearly shown in a patient with metastatic breast

cancer (Fig. 4). Images high in the thorax (Fig. 4A), in the

mid-thorax (Fig. 4B) and in the mediastinum (Fig. 4C)

show excellent alignment. Three foci of increased FDG

uptakewere difficultto localize on emission PET alone due

to the high target-to-background ratios. The fusion image

demonstrated that there was focal tumor in an anterior

right rib and not in a lymph node as was suspected without

the fusion images (Fig. 4B). An unexpected observation

was two foci of intense FDG uptake in the mediastinum

(Fig. 4C). The fusion image (Fig. 4C) showed that the FDG

uptake was into normal-sized lymph nodes. On follow-up

studies several months later, this patient had progressive

systemic and mediastinal involvement with breast cancer,

which implied that PET detected cancer in normal-sized

lymph nodes. Such cases where FDG activity localizes to

apparentlynormal structures on anatomic imagingstudies

would seem to represent a situation in which anatometa

bolicfusionimagingwouldbe of greatvalue. Fusion imaging was also possible in the abdomen,

although external fiducial markers were more necessary

than in the chest because relatively little internalanatomic

informationwas provided by transmission images of the

abdomen. In our small series, there were no quantitative

Matometabolic Tumor Imaging•Wahi at at. 1193

) ‘T'

___________ luniqueanatomic I

.. Ipoints from tram- I _________*—ImiuionscanontheI [MR/cs I

differences in the quality of realignmentsin comparingthe

chest and abdomen (Table 1). For example, in a patient

beingevaluatedasa partof anongoingstudyof prostatic carcinoma, excellent image realignment was achieved

(Fig. 5). This individual had an abnormal bone scan and

Ti-weighted MR image with metastatic prostatic card

noma to the acetabulum/hipregion. Emission PET demon

stratedtwo foci of FDG activity, one of moderate intensity

in the prostate itself(he had residualprostaticcarcinomain

the gland), and a more intense triangularfocus to the right

of the prostate. Fusion images showed the right-most ac

tivity to be in the right acetabulum (Fig. 5, bottom).

Although realignments with our current algorithm are sometimes imperfect (such as in Patient 7) and more so phisticated warping algorithms might be useful, it is clear

that our relatively simple approach, which can align to

within 5—6mm, has practical utility.

DISCUSSION

Our study demonstrates that it is possible to obtain rea

sonably accurate anatometabolic fusion images between

Cr orMRandFDG/PETscansbyusingarelativelysimple

rotate-translate-scalerigidmodel and manualidentification

of fiducial points from the CT, MR and transmission and

emissionPET images.Theseanatometabolicimagesap

FiGURE2. Patientwitha historyof germcellcaranomaof the

testis with muWptestable residual pulmonaiy nodules followingag

gres@vechemotherapy.TheupperlefthandimageisaTl-walghted

cm667,TE20)spin-echoMRimageshowinga retrocardiacnod

ule,a smallnoduleatthetightcosto-phrenicangleandthreevitamin

E markers positioned over the anterior chest. The FDG PET emis slon image at 60 mm postinjecdon (upper nght) clearly shows the three FDGfiducialmarkersatthe same skinlocationsas the vitamin E markers, myocard@IFDG uptake and a small photopenic area in

the retrocardiacarea.Thefusionimage(bottom)shows excellent

registrationbetweenthe FDGand vitaminE markers,demonstrates

FDG uptake in the cardiac wall and shows no FDG uptake in either pulmonarynodule. Modest FDG uptake projects in the expected

locationfortheliver.Follow-upofthsspatientshowedstabilityofthe

pulmonarynodulesmostconsistentwithscarnngand a lackof FDG

accumulation.

FiGURE3. (A)CTshowsmarkedvolumeloss Inthe tightlower

lung and a lOcalIZedrightpleuraleffusion in a patientwith newly

diagnosed lung cancer. The correspondingtransverse emission

PETimageobtainedat 60 mmpostinjectionshows intensecentral

FDG uptake in the right chest modest focal FDG uptake in the

anteriorleftchest, but onlyminimalFDGuptake in the bicodpool

andthe collapsedrightlowerlobe/effusionarea.Thefusionimage

(bottom)dearly demonstratesintense FDG uptake in the central

portionofthe collapsedrightlowerlungmostconsistentwithuptake

inthe centraltumor,whereasthe remainderof the rightlunghas

actMtymostconsletentwithpostobstrucffvevolumeloss.MRI per

formedelsewheresupportedthisinterpretationandthe largecentral

turnorwastreatednonsurgically.(B)Amorecaudalseriesofimages

from the same patient as in (A) demonstrates that the left anterior

chest activityseen in (A)is myocardialInorigin,withthe leftventric

ular wall clearly delineated by PET, despite its poor delineation on

the CTstudy.Thisis bestseen onthefusionimage.

pear to be particularlyuseful in defining the anatomic lo

cation of intense foci of FDG uptake that are otherwise

difficult to localize precisely due to limited background

activity.

The method we describe, using both external fiducial

markers and available internalpoints of anatomy (derived

from transmissionimages), is a unique approachto realign

ment. The use of the transmission data in addition to the

1194 The Joumat of Nudear Medicine•Vol.34 •No. 7 •July 1993

Another approach to realigning MR and PET images of

the braindoes not requireexternal markersor transmission

images. This involves defining the interhemispheric plane

on both MR and emission PET, with subsequent operator

interactions to produce realignments in the other dimen

sions (18). Although this approach is said to require less

than an hour of processing, it can requireuser definitionof

points on the emission PET scan, including the border

between the “graymatter and CSE―This can be difficult

due to the poor resolution of PET versus CT or Mill and

because white matter mainly adjoins the CSF. This algo

rithm is also reported to be less successful if the interhemi spheric fissure is nonplanar (18).

The preceding approaches for brain image registration

reported “errormagnitudes―in the 3—4mm range. Our

error magnitude of 5—6mm compares well with these,

particularly considering that the body is much more subject

to deformation than the brain. There is, however, some

uncertainty in our assessment of accuracy of realignment

as it is sometimes challenging to accurately define the lo

cation ofvessels on emission PET, and with some vessels,

to uniquely define their z-axis location is not possible.

While it may become feasible in the future to eliminate

the need for skin surface markers in the realignmentalgo

rithm, a trained technologist can prepare and apply mark ersin a few minutes.In mostcases,onetechnologistap

plied the ‘8Fmarkersand another applied the vitamin E or

lead markers, so a “devoted―technologist was not essen

tial. Careful attention to positioning the patient “flat―on

the imagingtable was employed.

A trained technologist was able to perform the computer realignment procedure in less than 2 hr, often without

assistance from a radiologist. This seems an advantage

over methods requiringa greater familiaritywith emission

PET and Mill anatomy than the modest knowledge of

anatomy needed to define points on the transmission im

ages with our method (18). In our realignment algorithm,

anatomic input from a physician is to assess the subjective

quality and quantitative accuracy of realignment and to

determine if patient motion occurred.

A rapid reliable fusion method for body imaging that

does not require external fiducials would be preferable to

our method. By incorporating anatomic data from the

transmission images, we moved in this direction and were

able to achieve successful fusions using as few as one

external fiducial marker in three of our patients. Clearly,

themethodweappliedislesscumbersomethanhavingthe

patient rigidlyimmobilized in a mold or frame, as has been

done in brain imaging studies (22,29). It should be noted

that fusion images and/or extraction of region of interest

information also have shown promise in better localizing

abnormalities seen on SPECT imaging using @“Fc-red

blood cells for hemangioma detection and with SPECT of

radiolabeledmonoclonal antibodies (30—32).This suggests

a more general applicabilityof such fusion methods.

In summary, we describe a method which allows for the

fusion of CT or MR anatomic images in three dimensions

S

FiGURE5. Ti-weightedMRI(TA600,TE20)demonstratesab

normal signal intensity in the region of the right acetabukim In thus patient with metastatic prostatic carcinoma to bone. Emission PET demonstrated two foolof FDG activity,one inthe midlineand one to

the rightof midline.FusionimagesdemOnStratedthatthe focal

uptake in the midlinewas in the region of the prostategland (the patient had residual prostate carcinoma inthe gland), and the signal from the right of midlinefused into the area of abnormal MR signal (metastasis) bested in the right acetabulum. Fidelityof the fusion was further substantiated by the fact that the two falnt fool of FDG

uptakelocatedanteriorandto eithersideoftheprostatewereseen

at multiple levels on PET (i.e., were tUbUlar)and fused into the

locationofthe externaliliacbloodvessels.

5. Surface anatomy (and thus markerlocation) is variant

between the two studies, such as in very obese pa

tients where a change in the appearance of the pannus

of fat (and markers on it) might not reflect internal

anatomy.

These issues are partly addressed since we have been

careful to perform the anatomic and physiologic studies on

the same day with the patient laid carefully onto his/her

back, in a similarstate of hydrationor feeding. Obviously,

more sophisticated algorithmswhich include the ability to

“warp―the three-dimensional image datasets between the

two studies would be ideal for more precise realignment.

Our approach to realignment and registration of body

PET studies with CT or MIII is related to several of the

approaches described for fusing brain PET or SPECT im

ages with MR or Cl' images (14—2Z24).The realignment

algorithms for brain anatomic and emission studies have

taken several foims, but the situationin the brainis simpler

than in the body because the brain and skull do not sub

stantially deform between studies. For the brain, realign

ment algorithms have been described that eliminate the

need for surface markers. One such algorithm uses the

transmission-scan defined surface of the skull and the sur face of the scalp on Mill or CT to perform realigmnents (14,15,17).Thisso-called“headin thehat―algorithm searches for minima in the quality of surface realignments,

but is not totally operator-independent.This approach re

portedly requires —2—3hr/patient to complete (17).

1196 The Journalof NuclearMedicine•Vol.34 •No. 7 •July

@ ii

10. WahIRL, HarneyJ, HutchinsG, GrossmanMB.Imagingof renalcancer using FDG PET: pilot animal and human studies.I Urol 1991;146:1470- **1474.

  1. Harneyiv, wai@iRL, LiebertM, Ctal. Uptakeof 2-decxy,2-(18F)fluoro** d-glucoscin bladdercancer: animallocalizationand initialpatient positron emissiontomography.J Urol 1991;145:279-283. 12.WeldRL,ZasadnyKR,GreenoughR, KoeppeR. Parametricimagedis plays to enhanceVisUaliZatiOnof cancers usingFDG/PET:influxconstant **andtemporalsubtractionimaging[Abstractj.Radiology1991;181:152.
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1197

with PET metabolic images to produce a series of

anatometabolic images. The method involves the use of

external fiducial markers for all imaging modalities and is

further refined through the use of internal landmarks from

foci in the body that are uniquely defined on both the

transmission PET images and the CF or Mill study. Such

anatometabolic information should prove useful in

anatometabolic directed biopsies of areas most likely to

contain active cancer that may appear normal by other

imagingtests (29). While additional experience and simplification will be

necessary to routinely apply our method to clinical prac

tice, we believe that our relatively simple approach will

prove practical in those clinical situations in which reason

ably exact anatomic delineation of the origin of the ‘8F(or

other PET tracer) signal is necessary. Indeed, as tech

niques to produce image fusions become simpler, such

anatometabolic images may well become the standard for PET image display in patients with cancer and other ill

nesses.

ACKNOWLEDGMENTS

The authorsthankthe technologistsandchemistsof the Urn

versity of Michigan PET Center for their assistance and Ms. Manette London, Annette Betley, Barbara Burton, and Joan Fog arty for their contributions. Supported by NIH grants 5 ROl

CA53172,1 R55CA52880,CA and MOl RR00042,as well as the

“Hi-techfundinginitiative―fromtheUniversityofMichiganHos

pital.

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MatometabollcTumor Imaging•WahI atat.