Bernard
A. Birnbaum,
J. Megibow,
Alec Evan
M. Kaminer,
Hepatic ofMR, Blood
#{149} Marilyn
MD
MD MD
E. Noz, PhD Maguire, Jr, PhD L. Kramer, MD
Elissa
#{149}
of red
blood
with
cells
technetium-99m.
Image analysis was performed in 20 patients with 35 known hepatic hemangiomas. After section thickness and pixel sizes of the different studies were matched, intrinsic
landmarks
were
chosen
to
identify anatomically corresponding locations. Regions of interest (ROIs) drawn on the CT and/or MR images were translated, rotated, and reprojected to match the areas of interest on
the
corresponding
SPECT
images
by means of a two-dimensional polynomial-based warping algorithm. Analysis of ROIs on 30 SPECT-MR and
20 SPECT-CT
pairs
of registered
images provided absolute confirmation that 34 suspected hemangiomas identified on SPECT images correlated exacily with lesions seen on CT and/or
MR
images.
Accuracy
of fu-
sion was within an average of 1.5 pixels ± 0.8 (±1 standard deviation). The technique enabled diagnostic confirmation of hemangiomas as small as 1.0 cm and proved useful for evaluating lesions located adjacent to intrahepatic vessels. Index
terms:
Liver
neoplasms,
neoplasms,
plasms,
gastrointestinal tract, CT #{149} Images, analysis #{149} CT, 761.1211, 761.30 #{149} Liver
Angioma,
#{149} Emission
76i.3194
diagnosis,
761.3194
MR studies,
761.1214,
neoptasms,
radionucide
Radiology
1991;
ported
#{149} Liver
761.30
studies,
neo-
#{149} Liver
761.1299,
761.30
181:469-474
I From the Department York, NY 10016 (B.A.B., puter Science, Columbia
Received
#{149} Jeffrey
Chapnick, C. Weinreb,
Hemangiomas: Diagnosis CT, and Tc-99m-labeled Cell SPECT Images’
A method of image analysis was developed for correlation of hemangiomas detected at computed tomography (CT) and/or magnetic resonance (MR) imaging with increased blood pool activity evident at single photon emission CT (SPECT) performed after labeling
#{149}Jeffrey
Q.
#{149} Gerald
P
increasingly
HYSICIANS
required
and
April 25, 1991; revision
requested
pare data obtained with different cross-sectional imaging modalities. Quantitative nuclear medicine tomographic techniques such as single photon emission computed tomography (SPECT) and positron emission tomography are capable of providing unique functional information, but interpretation
may
be limited
by
high-
contrast, poor-resolution images that offer little anatomic detail. For instance, tissue-specific information regarding abnormal concentrations of tumor-associated antigen can be demonstrated with radiolabeled monoclonal antibody (MoAb) imaging with SPEd, but the images frequently lack sufficient anatomic landmarks for precise identification of the structure in which the abnormal concentration is localized (i). On the other hand, computed tomography (CT) and magnetic resonance (MR) imaging provide high-resolution structural anatomic detail, but currently lack functional tissue specificity. Image registration or “fusion”methods that enable precise matching and alignment of data from different imaging modalities-was developed in part to facilitate comparison of functional and structural cross-sectional images to complement and enhance the information provided by each modality. Improved interpretation of the results of brain imaging, with regard to quantitative analysis (2,3)
and
radiation
treatment
plan-
ning (4), has been achieved with use of this technique. Nonneurologic applications within the thorax have recently been explored; fusion of CT
May 17; revision Body
corn-
Tomography
received and
June General
1; accepted Electric
J. Sanger,
#{149}Joseph
with Red
are being
to interpret
of Radiology, New York University Medical Center, 560 First M.E.N.,J.C., J.J.S., A.J.M., J.C.W., E.M.K., ELK.) and Department University, New York (G.Q.M.). From the 1990 RSNA scientific
in part by the Society of Computed Address reprint requests to B.A.B. ( RSNA, 1991
MD MD
MD
Fusion
and SPECT images has been used in studies of lung physiology (5) and for mediastinal staging of patients with lung cancer (6). Although Kaplan reported use of CT-SPECT fusion in the chest and abdomen for analysis of imaging with gallium-67, indium-illlabeled leukocytes and technetium99m-labeled red blood cells (RBCs) (7), abdominal applications neventheless have been limited. We recently described the use of CT-SPEd fusion to correlate uptake of radiolabeted MoAb with abdominal CT findings in eight patients with colorectal adenocarcinoma (1). In the present study, we fused tabeled-RBC SPECT images with CT and MR images obtained in patients with known hepatic hemangiomas to explore the potential role of image registration in characterizing liver tissue. Because hemangiomas often demonstrate a well-defined region of increased activity at SPECT imaging, this patient population was thought to be an ideal clinical model for further investigation and refinement of this technique and would simultaneously allow us to improve and optimize our software design. MATERIALS
AND
METHODS
Subjects Twenty patients (ii men and nine women) with 35 known hepatic hemangiomas
were
studied.
The
mean
age
of the
patients was 53 years (range, 33-79 years). In i9 patients (34 lesions) previously reported, the diagnosis of hemangioma was confirmed by correlating findings at charactenistic Tc-99m-labeled RBC SPECT and MR imaging with results obtained at clinical and serial imaging follow-up (8). The remaining patient had a single lesion that
Aye, New of Cornassembly.
June Medical
17. SupSystems.
MoAb = monoctonal antibody, RBC = red blood cell, ROI = region of interest, SD = standard deviation, SPECT = single photon emission CT.
Abbreviations:
469
was characterized as hemangioma on the basis of results of confirmatory SPECT (perfusion-blood pool mismatch with delayed blood pool uptake), CT (a mass that appeared hypoattenuating at nonenhanced imaging but which demonstrated peripheral enhancement and progressive, complete isoattenuating fill-in after administration of contrast medium), and MR imaging
(hyperintense,
welt-marginated,
homogeneous mass on 12-weighted images with signal intensity equal to that of cerebrospinal fluid). Eleven patients had solitary lesions and nine had multiple tesions (range, two to six lesions). Alt patients underwent labeted-RBC SPECT and MR imaging studies, and 16 patients underwent CT studies, but digitized CT data could be retrieved in only 12 cases.
Imaging
Techniques
SPECT.-RBC labeling was performed by using the modified in vitro technique with 30 mCi (i,iiO MBq) of Tc-99m pertechnetate (9). After an interval of 90-120 minutes after the patient’s blood was reinjected, SPECT scans of the liver (360#{176} rotation, 64 view angles, 25-30 seconds per projection) were acquired with a largefield-of-view tomographic gamma camera fitted with a high-resolution, low-energy collimator (Starcam 400-AC/T cameracomputer system; GE Medical Systems, Milwaukee). After correction for center-ofrotation deviation and camera nonuniformity, the projections were reconstructed into 6-mm-thick transaxiat sections by using commercially available filtered-back projection and Chang’s method of attenuation correction (10). Sagittal and coronal sections were re-sorted from these transaxial slices and were merged pairwise to yield 1.2-cm-thick contiguous axial, sagittat, and coronal sections (64 x 64 pixel matrix). CT-Twelve CT studies performed at our institution were used for image registration; these examinations were initially performed for detection of lesions. For dynamic incremental CT scanning, a bolus
infusion of contrast medium was administered. A total of 200 mL of 43% contrast medium (Conray 43; Mallinckrodt, St Louis) was administered [50-mL botus (2.5 mL/s), followed by a 150-mL infusion (1 mL/s)] performed by means of a power injector (Angiomat CT injector; LiebetFlarsheim, Cincinnati). Scanning commenced approximately 10-15 seconds after initiation of the infusion phase. Preliminary
nonenhanced
and
delayed
images were not routinely obtained in these cases. Studies were performed with either of two scanners (CT/I 9800 Quick or CT/I 9800 HiLight, GE Medical Systems). Axial 10-mm-thick CI sections of the liver were obtained at 12-mm intervals in a 512 x 512-pixel matrix. A variety of equipment was used in CT studies performed at other institutions; because technique in these studies was not standardized, the results were not used for image registration. In the patient 470
#{149} Radiology
not included in a previous study, a dynamic characterization study with delayed images was used to help confirm the diagnosis of hemangioma. MR imaging-MR imaging was performed with a 0.5-1 superconducting magnet (Gyroscan; Philips Medical Systems, Shelton, Conn). Muttiecho 12weighted axial images were obtained through the entire liver with a repetition time of 2,000 msec, echo times of 50 and 100 msec, and two excitations. Section thickness was 10 mm with a 2-mm intersection gap. Sections were acquired with a 128 x 128 matrix and were ultimately reconstructed to a 256 x 256-pixel matrix. Flow compensation and motion reduction techniques were not employed. An echo time of 100 msec was chosen because it allowed an entire study of the liver in a reasonable amount of time, provided an optimal signal-to-noise ratio on our system, and enabled sufficient 12 weighting for characterization of tissue (8). In our experience,
use
of spin-echo
pulse
se-
quences with echo times longer than 100 msec stilt may not ensure that all hemangiomas are differentiated from hypervascular or necrotic neoptasms.
SPECT-CT/MR
Registration
The SPECT, were transferred workstation
CT, and MR imaging data off-line to a Sun 3/180 (Mountain
View,
Calif).
Im-
age fusion required accurate data matching. To minimize differences in image geometry, the CT and MR imaging was performed with a section interval of 12 mm, which matched the SPECT section thickness. Pixel dimensions were matched within the plane of reconstruction by interpolating
the
images
were
were
chosen
by
two
of the
authors
(B.A.B., J.C.) (15). Approximately eight to ten such internal landmarks (eg, liver edge, splenic tip, inferior vena cava, and aorta) were selected on matching sections; these
were
occasionally
point pairs seen on CT scatter seen polynomial from these
supplemented
by
that correlated skin surface and MR sections with edge on SPECT sections (Fig 2). A transformation generated landmarks allowed rotation,
translation,
scale,
and
skew
to be
applied
to either the image to be registered or the polynomial vertices of a previously drawn region of interest (ROI) (12,13). With the use of a standard Spectrum, Chapel
resulted ing
Jaszczak phantom (Data Hill, NC), this technique
in correspondence
fused
images
of the result-
to within
±1.3
pixels
(±3.9 mm) (16). Accurate landmark placement was first verified by using trial ROIs traced around normal anatomic structures (Fig 3). This allowed precise delineation and correlation with
of boundaries the various
on images modalities.
obtained To identify
the hemangiorna, ROIs were then drawn on either the CT or MR image. A simultaneous display of this ROI, generated by the program, was “warped” onto the corresponding registered SPECT image. The warped ROI on the SPECT image was then evaluated for an obvious focus of blood pool activity thought to represent hemangioma uptake (Fig 4). The fusion algorithm also functioned in a reversible manner; ROIs generated over focal “hot spots”
on
the
SPECT
images
could
also
be
warped onto the corresponding registered CT and/or MR images. For each pair of images, registration typically required approximately 20-30 minutes.
to a corresponding
matrix size. Standard bilinear interpolalion was used to enlarge the 64 x 64 SPECT images to a 128 x 128-pixel format, and the 512 x 512 CT images and 256 x 256 MR images were reduced in matrix size to 128 x 128. Data conversion, handling, and standardization was performed by using “qsh,” a hardware-independent image display and handling toolkit cornposed of multiple software modules previously described in detail (1 1). The SPECT, MR. and CT images were simultaneously displayed side by side with a raster display-type frame buffer. Individualized color scales were assigned to each image, and background and saturation values were optimized to facilitate hemangioma visualization. The images were reviewed, and sections (located at matching anatomic levels) that best demonstrated hemangioma(s)
istered
selected
(Fig
1).
Image registration and fusion were accomptished by means of intrinsic anatomic landmark identification and a two-dimensional polynomial-based warping algorithm (12-14). The first three terms of an nth-order polynomial was used for this purpose, as previously described in detail (14). To achieve cross correlation, anatomically corresponding point pairs on the reference image and on the image to be reg-
Methodology The SPECT viewed
Validation
registered CT-SPECT and MRimage pairs were concurrently by three of the authors (B.A.B.,
M.E.N.,
J.C.).
initially
successful
Image
analysis
of ROIs
fusion
was
if visual
re-
considered
qualitative
demonstrated
direct
cor-
relation of hemangiomas on the reference and registered images. To ascertain the precision with which the warping algonthm functioned, two quantitative methods
for
determining
the
accuracy
of ROI
placement were developed. ROl center-to-center distance-After an ROI was drawn to identify (outline) a hemangioma on a CT or MR image, the algonithm was used to warp the ROI onto the corresponding registered SPECT section. The pixels within this warped structural ROl were analyzed, and the center of the “warped ROI” was calculated by averaging
the
x and
y coordinates
of the
individ-
uat pixels that comprised the ROI. A second ROI was then drawn directly on the SPECT image to identify the hot spot thought to represent hemangioma blood pool activity. This was performed in a “blind” fashion at a separate session, during
which
played.
the
The
warped
center
ROI
of this
was
not
“direct”
November
dis-
SPECT
1991
identified
with
CT and MR was located
SPECT
and
fused
with
was 1.0 cm. This lesion at the hepatic periphery
in the midlaterat aspect of the right lobe. Seven hemangiomas were located adjacent to major intrahepatic blood vessels. cm)
Five were
tered
of these
easily
SPECT
of SPECT
readers
lesions
(1.8-8.0
identified on regisimages. In one case, images
had
diffi-
culty distinguishing a possible hemangioma from branching intrahepatic vessel. In this case, image fusion conclusively matched a suspected negion of increased blood poot activity depicted by means of SPECT with a i.3-cm hemangioma posterior to the
1.
right
portal
vein
identified
at CT
and
MR imaging (Fig 5). One 1.5-cm hemangioma located immediately adjacent to the left portal vein was not seen at SPECT, despite fusion with CT and MR sections that demonstrated the lesion. In this case, image fusion
confirmed
the
juxtavascutar
location of the suspected hemangioma; its blood pool activity, however, could not be discerned from that of the neighboring vessel. Nine patients had multiple heman-
2. Figures veal
1, 2.
(1) Matching
a 1.8-cm
sections
at corresponding
sections obtained at the same anatomic level rethe right and left branches of the portal vein. regions of internal anatomy on the SPECT and MR
cross
and
hemangioma
(2) Landmark
point
(arrows)
based
SPECT (arrows)
pairs enabled
warping
and
MR
between
correlation
alignment
of image
data
with
the
giomas. lesions liver,
fled
polynomial-
algorithm.
case, tions
In one
then
was
manner.
The
the centers ROIs was means
determined
in a similar
distance
of the calculated
in pixels warped by
and using
analysis. warped simultaneously
between
els/no.
the SPECT standard
were
differences ideal result
in their would
sizes include
directly superimposed ROl size differences “blooming” artifacts (may increase erat enhancement (may decrease
size size
and/or differences tion through the
and SPECT displayed, and were noted. equal-sized
ROIs. Whether were secondary on SPECT images of SPECT fill-in on of warped
The and to
ROl), peniphCT images CT ROl),
in section hemangiomas
level
acquisi(lesion
size variable depending on location of section) was noted. Although direct SPECT ROIs were drawn on SPECT images with similar background and saturation levels, no attempt was made to use standardized quantitative threshold techniques to define hemangioma activity on the basis of blood poot pixel intensity (17). After determining the number of pixels within the warped and SPECT ROIs, the number of pixels common to both regions was calculated (pixel overlap). The number of overlapping pixels was pressed as a percentage relative number of pixels of whichever
Volume
percentage
as follows: of pixels
no.
of overlapping
in smaller
of ROl
ROI
x 100
pix=
overlap.
of vectorial
ROI overlap-The ROIs
smaller,
181
#{149} Number
2
then
exto the ROI was
RESULTS Image fusion was successfully performed in every case in which it was attempted and resulted in a total of 50 registered structural-functional image pairs. Twenty CT-SPECT and 20 MRSPECT image pains obtained in 12 patients (20 hemangiomas) and 10 MRSPECT image pairs obtained in eight patients (15 hemangiomas) were aligned. Of the patients in whom MRSPECT image pairs were obtained, one had six hemangiomas on one section, and two each had two lesions located at different levels. Hemangioma size was 1.0-11.0 cm (mean, 3.5 cm). Analysis of ROIs drawn on corresponding registered sections
provided
absolute
confirma-
tion that 34 suspected hemangiomas identified as hot spots on axial SPECT images correlated precisely with hyperintense lesions seen on MR images and/or peripherally enhanced lesions seen at CT. The smallest hemangioma
with
multiple
the identiat the same anatomic level. In this individual SPECT and MR secwere fused. Use of the warping six
throughout
hemangiomas
algorithm ROI
patient
dispersed
resulted
were
in “global”
data
alignment throughout the image, which enabled correlation of each focus of increased blood pool activity with a hypenintense lesion on the registered MR section. In the other patients with multiple hemangiomas, the lesions were located at different anatomic levels, and multiple section levels were fused. Image landmarks in these cases were “translatable”; that
is, after
anatomic
landmarks
were
selected at a given level, these same landmarks were used to register subsequent sections without having to select new sets of point pains. In two patients with multiple lesions (hemangioma levels located 4.8 cm and 8.0 cm positions
apart, respectively), were modified
and
landmark reposi-
tioned to facilitate acceptable image registration. Hemangioma blood poot activity was identifiable in 48 of the 50 registered structural-functional image pairs (29 MR-SPECT and 19 ClSPECT image pairs), which permitted quantitative ROl comparison in these cases.
The
image cated
pains of the hemangioma adjacent to the left portal
MR-SPECT
and
CI-SPECT
Radiology
tovein #{149} 471
4.
3.
Figures
3, 4. (3) To assess on the matching the ROl becomes outlines its borders.
displayed gorithm, curately
displayed ing
on the matching
“hot
cisely
spot”
(arrow).
correlates
with
were
accuracy of landmark placement, an ROl SPECT section (top left), the ROl appears registered with the SPECT image (bottom (4) An ROl is drawn closely around the
the
included
not
SI’ECT After
section
application
focus
(top left), the ROl appears of
the
of hemangioma
in this
possible
ing,
15
with
studied
with
pool
MR
ROIs
In this
imaging
anatomic
±
2.4)
(±1
standard
ation [SD]). ln 31 of these 53 instances, warped ROl contained fewer than did the SPECT ROl. This differential was thought to be the blooming effects of SPECT cases,
peripheral
±
The warped
ranged
from
percentage
and 45.5%
the pixels size due to in 21
were
tive
472
#{149} Radiology
centers
of
permitted
hepatic
internal
or
(eg, level
it was
an ade-
essential
point
to correlate
skin
pairs
and
now
pre-
scatter
from
SPECT was
sets
the
sections.
This
necessary
oven
the
entire
points image
for the polynomial
algorithm to result tion of the locations
5. CT-SPECT image pair analysis of hemangioma adjacent to the vessel. Matching SPECT and CT sections demonstrate two hernangiomas within the right lobe of the liver. An ROl drawn on a CT image
(arrow,
1.3-cm
seen
top
lesion
right
vein.
appears
ROI onto the SPECT image
focus
on
edge
body
sun-
branch
(top
which
ROI this
aligns
(arrows
and
on
confirmation
correlated
at CT
the
the
registered the ROI spot
the
warping
enabled
of activity portal
left),
After
hot
seen of the
on
corresponding (bottom left),
images),
this
of
displayed
liver.
perivascular
hemangioma
with
the
a smaller,
to a branch
section
outside
the
identifies
When
SPECT
matching
that
right)
adjacent
portal
SPECT
with
was
not
the
a
vessel.
supplemarks.
because
of coordinate
Figure a small
with
were
surface
sections
originating
ac-
hepatic
to supplement
landmarks;
on
splenic
of the
CT or MR imaging
required of
seen
left)
possible RBCs pro-
and vascular was sufficient to anatomic borders
and
existed
eight
or percentage
was
paucity
mentation
relationship between size and either distance No correlation
of the correspond-
bottom
markwas
Tc-99m-labeled
organs
dome),
persed
ROl
location
(arrow,
intrinsic
number
used
aver-
only This
poot activity indicate the
tivity
no significant hemangioma overlap.
image
of
of internal anatomic landmarks to be chosen for cross conrelation of the SPECT, CT, and MR images. When sections of interest were at anatomic levels where a rela-
face
ROl
to the expected SPECT
x on y
external
parenchymal
these
age ROl percentage overlap was 83.8% ± 12.2% (±1 SD). Scatterplot analysis demonstrated
between
the
substantial blood poo1 activity the liver, spleen, kidneys, and retropenitoneal vessels. The
quate
be-
overlap
the
registration
using
placed.
the
vided within major
of
image by
not
blood clearly
fill-in
SPECT ROIs to 100%. The
either
landmarks;
resulting
CT in four cases, differences in section level acquisition through a hemangioma in one case, and a combination of differences in section level acquisition and the aforementioned factors in the remaining five intween
study,
because
at
stances.
placement
along
achieved
ers
dcvi-
enhancement
on
DISCUSSION
MR inlag-
The calculated distance between the centers of the warped and SPECT ROIs ranged from 0.2 to 3.9 pixels (0.6-11.7 mm). The average ROI centen-to-center distance was 1.5 pixels mm
superolaterat
direction.
was
(4.5
the liver, is registered
of preferential
warped
alone).
0.8
ROl
the
activity.
(19 le-
CT and
outside
algorithm,
in terms
assessment,
in 53 instances
studied
sions
warping
blood
because the hemangioma in this patient was not seen with SPECT. Quantitative analysis of image pairing was
is first drawn around the liver on the MR image (arrows, top right). When larger than the liver and overlaps it. After application of the warping alleft) such that the ROl now clearly matches the liver in size and more achyperintense hernangioma on the MR section (arrow, top right). When
diswere
warping
in precise correlaof anatomic land-
nat
Future image
registration clear intrinsic
identified
applications fusion of
medicine
are images studies
anatomic
(eg,
of
likely obtained in
in
which
landmarks
images
abdomi-
to involve nu-
few are
with
radiola-
November
1991
beled
monoclonal
antibodies).
these instances, landmarks atone to
accurately
cross-correlate
combination
and
of
Analysis
images;
internal
externally
necessary
a
landmarks
placed
markers
may
be
(1).
In our
ROI
In
intrinsic anatomic may be insufficient
study,
image
analysis
fusion
with
absolute conflnmation that suspected hemangiomas identified by means of SPECT with labeled RBCs correlated exactly with lesions seen on CT and MR images. The smallest hemangioma identified
by
provided
review
SPECT
and
of registered
MR-SPECT
1.0 cm in size. (triple-headed)
capable
images
New
SPECT
and
mented
cameras
fusion
a recent
tempts
study
(18)
SPECT
docu-
sensitivity
of hepatic hemangiomas 1.5 cm. We believe image
may
clinically
prove
diagnostic
are
3.5-mm-thick
improved
for detection smaller than
was
high-resolution
of acquiring
sections,
CT-
confirmation
useful of such
for small
lesions.
Image
fusion
the
evaluation
near
regions
may
potentially
of hot-spot of vascular
SPECT scanning with has limited sensitivity small near
heart this
(8). As seen study,
unsure
foci activity.
cm) vessels
SPECT a focus
whether
case
in
readers were of increased
uptake represented hemangioma or branching vessel, image fusion with ROI analysis matched a focus of suspicious
uptake
identified Small ately
a hemangioma
with
on CT and
MR images.
hemangiomas located adjacent to vessels are
SPECT
missed
on
Image
fusion
blood
may
pool
serve
to
the suspected juxtavascular of these hemangiomas;
per
se, however,
lesions
that
wilt
are
racy
ROI
enabled of
of registered
within
1.5 pixels accuracy
to that toms
Of the two developed,
anal-
the
images 0.8 (4.5
is favorable
achieved
with
with
and
images
is close of phan-
correspondence)
quantitative we believe
2.4).
±
(16).
techniques that the ROI
distance method proprecise means of evalu-
center-to-center
a more
ating the ability of the warping algonithm to align and match data. The ROI percentage overlap method provides
complementary
that
is helpful
size
and
Volume
overlap
181
information
for evaluating of
#{149} Number
lesion
2
lesion borders.
suc-
and
MR
imag-
the
accuracy
of
the
transformation
apply
entire image, intrinsic landwere selected throughout the section and were suppleby points that defined the surface.
Slight
if the were
geometric
fiducial
not
errors
landmark
symmetrically
distnib-
uted, which is demonstrated in Figure 3, in which the definition of the tatenal aspect of the right lobe of the by
the
warped
ROl
outline
is
more accurate than is definition of the anterior aspect of the left lobe. A more exact match may have been obtained by
the
tify
was mm
CT
rule,
to make
to the marks chosen mented
tional
accu-
conrespon-
±
(1.3-pixel
vides
visible
comparison
technique;
fort
liver
demonstrate
us to validate
our
dence This
location technique
clearly
not
SPECT. Quantitative ysis
not
As a general
points
studies.
revealed
landmark placement together with the number of terms used in the polynomial determines the tightness of fit between registered images. In an ef-
occurred
confirm
the
at acquiring
body
immedioften
data
ing sections at levels designed to match SPECT section thickness and position, such factors as section misregistration due to variable nespiratory excursion may make it difficult for the center of a Cl section to coincide with that of an axial SPECT section. Nevertheless, it was encouraging that even in these cases, a portion of the warped ROl always overlapped a region of suspected hemangioma blood pool activity.
of
located on the
2.5
in a single
when
located
labeled RBC for detection
(
hemangiomas major intrahepatic
aid in
of our
cessful image fusion to be most dependent on careful placement of anatomic landmarks. Accurate landmark placement was verified by using trial ROIs drawn around normal anatomic borders or structures, and landmark point pairs were repositioned as needed to achieve a “good fit.” At times, landmark positioning was accurate, but image registration appeared to be less than perfect. We encountered this panticularly when attempts were made to fuse SPECT images with those obtained with CT and MR imaging at different levels through a hemangioma. Despite at-
the
placement
of multiple
landmarks hepatic
to specifically surface. Such
match, however, necessary. In this ROI center-to-center
only 86%;
0.5 pixel a 1.8-cm
addi-
idena perfect
is often not clinically particular case, the distance was
and ROI overlap was hemangioma was con-
firmed. The axial sections used for image registration in this study were assumed to be coplanar. A small component of geometric error may have nesulted from this assumption. The polynomial-based warping algorithm we used is capable, nevertheless, of transforming any two-dimensional section (provided landmarks are suit-
ably
chosen),
irrespective
of the
tilt
of
the section, ie, the angle formed between the section and the longitudinat axis of the body. When tilt in the coronal or sagittal planes exists, the sections can be reoriented in the transaxial plane. The details of such oblique nepnojection techniques and their application to clinical abdominal imaging have been previously reported (i,14). The time required for image registration varied with case difficulty and as changes
design.
were
made
Typically,
20-30
minutes
in our
software
approximately were
needed
to regis-
ten matching image pairs. This included time expended for patient setection; image display and review; section selection; assignment of optimized colon scales, windows and 1evels
for
individual
images;
CT,
landmark
MR.
assignment
editing (if necessary); and oma ROI analysis. Accurate positioning
was
the
most
and
SPECT
and hemangilandmark time-con-
suming aspect of image fusion. The time necessary for this should decrease in the future, however, because automated or semiautomated reproducible means of landmark selection are being devised to replace the manuat technique used in this study. The hemangiomas in our patients represented an excellent clinical model to use in refining our registration
methodology.
In most
hemangio-
studied, “eyebatling” the nonfused images would doubtlessly have been a much faster means of anatyzing our data, given the time required for image fusion. This technique would be most useful, therefore, for evaluating hemangiomas that are eithen small on located adjacent to negions of blood pool activity (eg, intrahepatic vessels); in such cases, a more objective means of aligning image data would be preferable to visual mas
assessment. these methods
Future applications appear encouraging
of
and are likely to include fusion of SPECT-CT and SPECT-MR imaging studies to facilitate tumor-specific imaging and/or quantitation of the uptake of nadiophanmaceuticals such as monoclonal antibodies, which would be useful mates.
for U
making
dosimetry
esti-
References 1.
Kramer EL, Noz ME, SangerjJ, Megibow AJ, Maguire GQ. CT-SPECT fusion to correlate radiolabeled monoclonat antibody uptake with abdominal CT findings. Radi-
otogy 2.
Farkas brain
1989;
172:861-865.
T, Wolf glucose
AP, Jaeger metabolism
J, et at. Regional in the study of
Radiology
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3.
4.
5.
6.
7.
chronic schizophrenia. Arch Gen Psychiatry 1984; 41:293-300. Wolkin A, JaegerJ, Brodie JD, et al. Persistent cerebral metabolic abnormalities in schizophrenia as determined by PET. AmJ Psychot 1985; 142:564-571. Schad LR, Boesecke R, Schtegel W, et at. Three dimensional image correlation of CT, MR, and PET studies in radiotherapy treatrnent planning of brain tumors. J Comput Assist Tomogr 1987; 11:948-954. Moskowitz GW, VaugeoisJC, Schiff RG, Levy LM. Improvement of SPECT lung perfusion physiology with CT high resolution structural anatomy (abstr). J Nuct Med 1986; 27:1038. Kramer EL, Reddy D, Noz ME, et at. “Fusion” of CT and radiotabeted antibody (AB) SPECT to improve mediastinal staging of patients with tung cancer (abstr). J Nuct Med 1990; 31:798. Kaplan IL, Swayne LC. Composite
SPECT-CT
images:
technique
applications in chest and ing. AJR 1989; 152:865-866.
and potential
abdominal
imag-
8.
9.
10.
ii.
12.
13.
Birnbaum BA, WeinrebJC, Megibow AJ, SangerJJ, et at. Definitive diagnosis of hepatic hemangiomas: MR imaging versus Tc-99m-labeted red blood cell SPECT. Radiotogy 1990; 176:95-401. Front D, Israel 0, Groshar 0, WeiningerJ. Technetium-99m-labeted red blood cell imaging. Semin Nuct Med 1984; 14:226250. Chang LT. A method for attenuation correction in radionuclide computed tomography. IEEE Trans NucI Sci 1978; 25:638-643. Noz ME, Maguire GQ. QSH: a minimal but highly portable image display and handling toolkit. Comput Methods Programs Biomed 1988; 27:229-240. Singh
#{149} Radiology
W,
Shibita
T, Huth
in medical
imaging.
Netherlands:
Martinus
Nijhoff,
1986;
266-279.
14.
15.
Maquire GQ Jr, Noz ME, Rusinek H, et at. Graphics applied to medical image registration. IEEE Comput Graphic AppI 1991; 11(2):20-28. Hall EL. Computer image processing and recognition. New 1979; 186-189,468-554.
16.
plications
17.
18.
York:
Academic
Press,
Kramer EL, Noz ME. CT/SPECT fusion for analysis of radiolabeled antibodies: apcinoma. tnt 18:27-42.
GH,
Telfer NE. A digital technique for accurate change detection in nuclear medicine images: with apptication to myocardiat perfusion studies using thallium 201. IEEE Trans Nucl Sci 1979; 26:565-575. Maguire GQ, Noz ME, Lee E, et at. Correlation methods for tomographic images using two and three dimensional techniques. In: Bacharach SL, ed. Information processing
474
M, Frei
The
in gastrointestinal
I
Rad
Appl
and lung
Instrum
car-
[BI 1991;
Front D, Iosilevsky G, Frenkel A, et at. In vivo quantitation using SPECT of radiopharmaceutical uptake by human meningiomas. Radiology 1987; 164:93-96. Ziessman HA, Silverman PM, Patterson J, Harkness BA, Zeman RK, Keyes JWJr. Improved detection of small cavernous hemangiomas of the liver with high-resotution SPECT (abstr). Radiology 1990; 177(P): 121.
Dordrecht,
November
1991
A. Birnbaum,
J. Megibow,
Alec Evan
M. Kaminer,
Hepatic ofMR, Blood
#{149} Marilyn
MD
MD MD
E. Noz, PhD Maguire, Jr, PhD L. Kramer, MD
Elissa
#{149}
of red
blood
with
cells
technetium-99m.
Image analysis was performed in 20 patients with 35 known hepatic hemangiomas. After section thickness and pixel sizes of the different studies were matched, intrinsic
landmarks
were
chosen
to
identify anatomically corresponding locations. Regions of interest (ROIs) drawn on the CT and/or MR images were translated, rotated, and reprojected to match the areas of interest on
the
corresponding
SPECT
images
by means of a two-dimensional polynomial-based warping algorithm. Analysis of ROIs on 30 SPECT-MR and
20 SPECT-CT
pairs
of registered
images provided absolute confirmation that 34 suspected hemangiomas identified on SPECT images correlated exacily with lesions seen on CT and/or
MR
images.
Accuracy
of fu-
sion was within an average of 1.5 pixels ± 0.8 (±1 standard deviation). The technique enabled diagnostic confirmation of hemangiomas as small as 1.0 cm and proved useful for evaluating lesions located adjacent to intrahepatic vessels. Index
terms:
Liver
neoplasms,
neoplasms,
plasms,
gastrointestinal tract, CT #{149} Images, analysis #{149} CT, 761.1211, 761.30 #{149} Liver
Angioma,
#{149} Emission
76i.3194
diagnosis,
761.3194
MR studies,
761.1214,
neoptasms,
radionucide
Radiology
1991;
ported
#{149} Liver
761.30
studies,
neo-
#{149} Liver
761.1299,
761.30
181:469-474
I From the Department York, NY 10016 (B.A.B., puter Science, Columbia
Received
#{149} Jeffrey
Chapnick, C. Weinreb,
Hemangiomas: Diagnosis CT, and Tc-99m-labeled Cell SPECT Images’
A method of image analysis was developed for correlation of hemangiomas detected at computed tomography (CT) and/or magnetic resonance (MR) imaging with increased blood pool activity evident at single photon emission CT (SPECT) performed after labeling
#{149}Jeffrey
Q.
#{149} Gerald
P
increasingly
HYSICIANS
required
and
April 25, 1991; revision
requested
pare data obtained with different cross-sectional imaging modalities. Quantitative nuclear medicine tomographic techniques such as single photon emission computed tomography (SPECT) and positron emission tomography are capable of providing unique functional information, but interpretation
may
be limited
by
high-
contrast, poor-resolution images that offer little anatomic detail. For instance, tissue-specific information regarding abnormal concentrations of tumor-associated antigen can be demonstrated with radiolabeled monoclonal antibody (MoAb) imaging with SPEd, but the images frequently lack sufficient anatomic landmarks for precise identification of the structure in which the abnormal concentration is localized (i). On the other hand, computed tomography (CT) and magnetic resonance (MR) imaging provide high-resolution structural anatomic detail, but currently lack functional tissue specificity. Image registration or “fusion”methods that enable precise matching and alignment of data from different imaging modalities-was developed in part to facilitate comparison of functional and structural cross-sectional images to complement and enhance the information provided by each modality. Improved interpretation of the results of brain imaging, with regard to quantitative analysis (2,3)
and
radiation
treatment
plan-
ning (4), has been achieved with use of this technique. Nonneurologic applications within the thorax have recently been explored; fusion of CT
May 17; revision Body
corn-
Tomography
received and
June General
1; accepted Electric
J. Sanger,
#{149}Joseph
with Red
are being
to interpret
of Radiology, New York University Medical Center, 560 First M.E.N.,J.C., J.J.S., A.J.M., J.C.W., E.M.K., ELK.) and Department University, New York (G.Q.M.). From the 1990 RSNA scientific
in part by the Society of Computed Address reprint requests to B.A.B. ( RSNA, 1991
MD MD
MD
Fusion
and SPECT images has been used in studies of lung physiology (5) and for mediastinal staging of patients with lung cancer (6). Although Kaplan reported use of CT-SPECT fusion in the chest and abdomen for analysis of imaging with gallium-67, indium-illlabeled leukocytes and technetium99m-labeled red blood cells (RBCs) (7), abdominal applications neventheless have been limited. We recently described the use of CT-SPEd fusion to correlate uptake of radiolabeted MoAb with abdominal CT findings in eight patients with colorectal adenocarcinoma (1). In the present study, we fused tabeled-RBC SPECT images with CT and MR images obtained in patients with known hepatic hemangiomas to explore the potential role of image registration in characterizing liver tissue. Because hemangiomas often demonstrate a well-defined region of increased activity at SPECT imaging, this patient population was thought to be an ideal clinical model for further investigation and refinement of this technique and would simultaneously allow us to improve and optimize our software design. MATERIALS
AND
METHODS
Subjects Twenty patients (ii men and nine women) with 35 known hepatic hemangiomas
were
studied.
The
mean
age
of the
patients was 53 years (range, 33-79 years). In i9 patients (34 lesions) previously reported, the diagnosis of hemangioma was confirmed by correlating findings at charactenistic Tc-99m-labeled RBC SPECT and MR imaging with results obtained at clinical and serial imaging follow-up (8). The remaining patient had a single lesion that
Aye, New of Cornassembly.
June Medical
17. SupSystems.
MoAb = monoctonal antibody, RBC = red blood cell, ROI = region of interest, SD = standard deviation, SPECT = single photon emission CT.
Abbreviations:
469
was characterized as hemangioma on the basis of results of confirmatory SPECT (perfusion-blood pool mismatch with delayed blood pool uptake), CT (a mass that appeared hypoattenuating at nonenhanced imaging but which demonstrated peripheral enhancement and progressive, complete isoattenuating fill-in after administration of contrast medium), and MR imaging
(hyperintense,
welt-marginated,
homogeneous mass on 12-weighted images with signal intensity equal to that of cerebrospinal fluid). Eleven patients had solitary lesions and nine had multiple tesions (range, two to six lesions). Alt patients underwent labeted-RBC SPECT and MR imaging studies, and 16 patients underwent CT studies, but digitized CT data could be retrieved in only 12 cases.
Imaging
Techniques
SPECT.-RBC labeling was performed by using the modified in vitro technique with 30 mCi (i,iiO MBq) of Tc-99m pertechnetate (9). After an interval of 90-120 minutes after the patient’s blood was reinjected, SPECT scans of the liver (360#{176} rotation, 64 view angles, 25-30 seconds per projection) were acquired with a largefield-of-view tomographic gamma camera fitted with a high-resolution, low-energy collimator (Starcam 400-AC/T cameracomputer system; GE Medical Systems, Milwaukee). After correction for center-ofrotation deviation and camera nonuniformity, the projections were reconstructed into 6-mm-thick transaxiat sections by using commercially available filtered-back projection and Chang’s method of attenuation correction (10). Sagittal and coronal sections were re-sorted from these transaxial slices and were merged pairwise to yield 1.2-cm-thick contiguous axial, sagittat, and coronal sections (64 x 64 pixel matrix). CT-Twelve CT studies performed at our institution were used for image registration; these examinations were initially performed for detection of lesions. For dynamic incremental CT scanning, a bolus
infusion of contrast medium was administered. A total of 200 mL of 43% contrast medium (Conray 43; Mallinckrodt, St Louis) was administered [50-mL botus (2.5 mL/s), followed by a 150-mL infusion (1 mL/s)] performed by means of a power injector (Angiomat CT injector; LiebetFlarsheim, Cincinnati). Scanning commenced approximately 10-15 seconds after initiation of the infusion phase. Preliminary
nonenhanced
and
delayed
images were not routinely obtained in these cases. Studies were performed with either of two scanners (CT/I 9800 Quick or CT/I 9800 HiLight, GE Medical Systems). Axial 10-mm-thick CI sections of the liver were obtained at 12-mm intervals in a 512 x 512-pixel matrix. A variety of equipment was used in CT studies performed at other institutions; because technique in these studies was not standardized, the results were not used for image registration. In the patient 470
#{149} Radiology
not included in a previous study, a dynamic characterization study with delayed images was used to help confirm the diagnosis of hemangioma. MR imaging-MR imaging was performed with a 0.5-1 superconducting magnet (Gyroscan; Philips Medical Systems, Shelton, Conn). Muttiecho 12weighted axial images were obtained through the entire liver with a repetition time of 2,000 msec, echo times of 50 and 100 msec, and two excitations. Section thickness was 10 mm with a 2-mm intersection gap. Sections were acquired with a 128 x 128 matrix and were ultimately reconstructed to a 256 x 256-pixel matrix. Flow compensation and motion reduction techniques were not employed. An echo time of 100 msec was chosen because it allowed an entire study of the liver in a reasonable amount of time, provided an optimal signal-to-noise ratio on our system, and enabled sufficient 12 weighting for characterization of tissue (8). In our experience,
use
of spin-echo
pulse
se-
quences with echo times longer than 100 msec stilt may not ensure that all hemangiomas are differentiated from hypervascular or necrotic neoptasms.
SPECT-CT/MR
Registration
The SPECT, were transferred workstation
CT, and MR imaging data off-line to a Sun 3/180 (Mountain
View,
Calif).
Im-
age fusion required accurate data matching. To minimize differences in image geometry, the CT and MR imaging was performed with a section interval of 12 mm, which matched the SPECT section thickness. Pixel dimensions were matched within the plane of reconstruction by interpolating
the
images
were
were
chosen
by
two
of the
authors
(B.A.B., J.C.) (15). Approximately eight to ten such internal landmarks (eg, liver edge, splenic tip, inferior vena cava, and aorta) were selected on matching sections; these
were
occasionally
point pairs seen on CT scatter seen polynomial from these
supplemented
by
that correlated skin surface and MR sections with edge on SPECT sections (Fig 2). A transformation generated landmarks allowed rotation,
translation,
scale,
and
skew
to be
applied
to either the image to be registered or the polynomial vertices of a previously drawn region of interest (ROI) (12,13). With the use of a standard Spectrum, Chapel
resulted ing
Jaszczak phantom (Data Hill, NC), this technique
in correspondence
fused
images
of the result-
to within
±1.3
pixels
(±3.9 mm) (16). Accurate landmark placement was first verified by using trial ROIs traced around normal anatomic structures (Fig 3). This allowed precise delineation and correlation with
of boundaries the various
on images modalities.
obtained To identify
the hemangiorna, ROIs were then drawn on either the CT or MR image. A simultaneous display of this ROI, generated by the program, was “warped” onto the corresponding registered SPECT image. The warped ROI on the SPECT image was then evaluated for an obvious focus of blood pool activity thought to represent hemangioma uptake (Fig 4). The fusion algorithm also functioned in a reversible manner; ROIs generated over focal “hot spots”
on
the
SPECT
images
could
also
be
warped onto the corresponding registered CT and/or MR images. For each pair of images, registration typically required approximately 20-30 minutes.
to a corresponding
matrix size. Standard bilinear interpolalion was used to enlarge the 64 x 64 SPECT images to a 128 x 128-pixel format, and the 512 x 512 CT images and 256 x 256 MR images were reduced in matrix size to 128 x 128. Data conversion, handling, and standardization was performed by using “qsh,” a hardware-independent image display and handling toolkit cornposed of multiple software modules previously described in detail (1 1). The SPECT, MR. and CT images were simultaneously displayed side by side with a raster display-type frame buffer. Individualized color scales were assigned to each image, and background and saturation values were optimized to facilitate hemangioma visualization. The images were reviewed, and sections (located at matching anatomic levels) that best demonstrated hemangioma(s)
istered
selected
(Fig
1).
Image registration and fusion were accomptished by means of intrinsic anatomic landmark identification and a two-dimensional polynomial-based warping algorithm (12-14). The first three terms of an nth-order polynomial was used for this purpose, as previously described in detail (14). To achieve cross correlation, anatomically corresponding point pairs on the reference image and on the image to be reg-
Methodology The SPECT viewed
Validation
registered CT-SPECT and MRimage pairs were concurrently by three of the authors (B.A.B.,
M.E.N.,
J.C.).
initially
successful
Image
analysis
of ROIs
fusion
was
if visual
re-
considered
qualitative
demonstrated
direct
cor-
relation of hemangiomas on the reference and registered images. To ascertain the precision with which the warping algonthm functioned, two quantitative methods
for
determining
the
accuracy
of ROI
placement were developed. ROl center-to-center distance-After an ROI was drawn to identify (outline) a hemangioma on a CT or MR image, the algonithm was used to warp the ROI onto the corresponding registered SPECT section. The pixels within this warped structural ROl were analyzed, and the center of the “warped ROI” was calculated by averaging
the
x and
y coordinates
of the
individ-
uat pixels that comprised the ROI. A second ROI was then drawn directly on the SPECT image to identify the hot spot thought to represent hemangioma blood pool activity. This was performed in a “blind” fashion at a separate session, during
which
played.
the
The
warped
center
ROI
of this
was
not
“direct”
November
dis-
SPECT
1991
identified
with
CT and MR was located
SPECT
and
fused
with
was 1.0 cm. This lesion at the hepatic periphery
in the midlaterat aspect of the right lobe. Seven hemangiomas were located adjacent to major intrahepatic blood vessels. cm)
Five were
tered
of these
easily
SPECT
of SPECT
readers
lesions
(1.8-8.0
identified on regisimages. In one case, images
had
diffi-
culty distinguishing a possible hemangioma from branching intrahepatic vessel. In this case, image fusion conclusively matched a suspected negion of increased blood poot activity depicted by means of SPECT with a i.3-cm hemangioma posterior to the
1.
right
portal
vein
identified
at CT
and
MR imaging (Fig 5). One 1.5-cm hemangioma located immediately adjacent to the left portal vein was not seen at SPECT, despite fusion with CT and MR sections that demonstrated the lesion. In this case, image fusion
confirmed
the
juxtavascutar
location of the suspected hemangioma; its blood pool activity, however, could not be discerned from that of the neighboring vessel. Nine patients had multiple heman-
2. Figures veal
1, 2.
(1) Matching
a 1.8-cm
sections
at corresponding
sections obtained at the same anatomic level rethe right and left branches of the portal vein. regions of internal anatomy on the SPECT and MR
cross
and
hemangioma
(2) Landmark
point
(arrows)
based
SPECT (arrows)
pairs enabled
warping
and
MR
between
correlation
alignment
of image
data
with
the
giomas. lesions liver,
fled
polynomial-
algorithm.
case, tions
In one
then
was
manner.
The
the centers ROIs was means
determined
in a similar
distance
of the calculated
in pixels warped by
and using
analysis. warped simultaneously
between
els/no.
the SPECT standard
were
differences ideal result
in their would
sizes include
directly superimposed ROl size differences “blooming” artifacts (may increase erat enhancement (may decrease
size size
and/or differences tion through the
and SPECT displayed, and were noted. equal-sized
ROIs. Whether were secondary on SPECT images of SPECT fill-in on of warped
The and to
ROl), peniphCT images CT ROl),
in section hemangiomas
level
acquisi(lesion
size variable depending on location of section) was noted. Although direct SPECT ROIs were drawn on SPECT images with similar background and saturation levels, no attempt was made to use standardized quantitative threshold techniques to define hemangioma activity on the basis of blood poot pixel intensity (17). After determining the number of pixels within the warped and SPECT ROIs, the number of pixels common to both regions was calculated (pixel overlap). The number of overlapping pixels was pressed as a percentage relative number of pixels of whichever
Volume
percentage
as follows: of pixels
no.
of overlapping
in smaller
of ROl
ROI
x 100
pix=
overlap.
of vectorial
ROI overlap-The ROIs
smaller,
181
#{149} Number
2
then
exto the ROI was
RESULTS Image fusion was successfully performed in every case in which it was attempted and resulted in a total of 50 registered structural-functional image pairs. Twenty CT-SPECT and 20 MRSPECT image pains obtained in 12 patients (20 hemangiomas) and 10 MRSPECT image pairs obtained in eight patients (15 hemangiomas) were aligned. Of the patients in whom MRSPECT image pairs were obtained, one had six hemangiomas on one section, and two each had two lesions located at different levels. Hemangioma size was 1.0-11.0 cm (mean, 3.5 cm). Analysis of ROIs drawn on corresponding registered sections
provided
absolute
confirma-
tion that 34 suspected hemangiomas identified as hot spots on axial SPECT images correlated precisely with hyperintense lesions seen on MR images and/or peripherally enhanced lesions seen at CT. The smallest hemangioma
with
multiple
the identiat the same anatomic level. In this individual SPECT and MR secwere fused. Use of the warping six
throughout
hemangiomas
algorithm ROI
patient
dispersed
resulted
were
in “global”
data
alignment throughout the image, which enabled correlation of each focus of increased blood pool activity with a hypenintense lesion on the registered MR section. In the other patients with multiple hemangiomas, the lesions were located at different anatomic levels, and multiple section levels were fused. Image landmarks in these cases were “translatable”; that
is, after
anatomic
landmarks
were
selected at a given level, these same landmarks were used to register subsequent sections without having to select new sets of point pains. In two patients with multiple lesions (hemangioma levels located 4.8 cm and 8.0 cm positions
apart, respectively), were modified
and
landmark reposi-
tioned to facilitate acceptable image registration. Hemangioma blood poot activity was identifiable in 48 of the 50 registered structural-functional image pairs (29 MR-SPECT and 19 ClSPECT image pairs), which permitted quantitative ROl comparison in these cases.
The
image cated
pains of the hemangioma adjacent to the left portal
MR-SPECT
and
CI-SPECT
Radiology
tovein #{149} 471
4.
3.
Figures
3, 4. (3) To assess on the matching the ROl becomes outlines its borders.
displayed gorithm, curately
displayed ing
on the matching
“hot
cisely
spot”
(arrow).
correlates
with
were
accuracy of landmark placement, an ROl SPECT section (top left), the ROl appears registered with the SPECT image (bottom (4) An ROl is drawn closely around the
the
included
not
SI’ECT After
section
application
focus
(top left), the ROl appears of
the
of hemangioma
in this
possible
ing,
15
with
studied
with
pool
MR
ROIs
In this
imaging
anatomic
±
2.4)
(±1
standard
ation [SD]). ln 31 of these 53 instances, warped ROl contained fewer than did the SPECT ROl. This differential was thought to be the blooming effects of SPECT cases,
peripheral
±
The warped
ranged
from
percentage
and 45.5%
the pixels size due to in 21
were
tive
472
#{149} Radiology
centers
of
permitted
hepatic
internal
or
(eg, level
it was
an ade-
essential
point
to correlate
skin
pairs
and
now
pre-
scatter
from
SPECT was
sets
the
sections.
This
necessary
oven
the
entire
points image
for the polynomial
algorithm to result tion of the locations
5. CT-SPECT image pair analysis of hemangioma adjacent to the vessel. Matching SPECT and CT sections demonstrate two hernangiomas within the right lobe of the liver. An ROl drawn on a CT image
(arrow,
1.3-cm
seen
top
lesion
right
vein.
appears
ROI onto the SPECT image
focus
on
edge
body
sun-
branch
(top
which
ROI this
aligns
(arrows
and
on
confirmation
correlated
at CT
the
the
registered the ROI spot
the
warping
enabled
of activity portal
left),
After
hot
seen of the
on
corresponding (bottom left),
images),
this
of
displayed
liver.
perivascular
hemangioma
with
the
a smaller,
to a branch
section
outside
the
identifies
When
SPECT
matching
that
right)
adjacent
portal
SPECT
with
was
not
the
a
vessel.
supplemarks.
because
of coordinate
Figure a small
with
were
surface
sections
originating
ac-
hepatic
to supplement
landmarks;
on
splenic
of the
CT or MR imaging
required of
seen
left)
possible RBCs pro-
and vascular was sufficient to anatomic borders
and
existed
eight
or percentage
was
paucity
mentation
relationship between size and either distance No correlation
of the correspond-
bottom
markwas
Tc-99m-labeled
organs
dome),
persed
ROl
location
(arrow,
intrinsic
number
used
aver-
only This
poot activity indicate the
tivity
no significant hemangioma overlap.
image
of
of internal anatomic landmarks to be chosen for cross conrelation of the SPECT, CT, and MR images. When sections of interest were at anatomic levels where a rela-
face
ROl
to the expected SPECT
x on y
external
parenchymal
these
age ROl percentage overlap was 83.8% ± 12.2% (±1 SD). Scatterplot analysis demonstrated
between
the
substantial blood poo1 activity the liver, spleen, kidneys, and retropenitoneal vessels. The
quate
be-
overlap
the
registration
using
placed.
the
vided within major
of
image by
not
blood clearly
fill-in
SPECT ROIs to 100%. The
either
landmarks;
resulting
CT in four cases, differences in section level acquisition through a hemangioma in one case, and a combination of differences in section level acquisition and the aforementioned factors in the remaining five intween
study,
because
at
stances.
placement
along
achieved
ers
dcvi-
enhancement
on
DISCUSSION
MR inlag-
The calculated distance between the centers of the warped and SPECT ROIs ranged from 0.2 to 3.9 pixels (0.6-11.7 mm). The average ROI centen-to-center distance was 1.5 pixels mm
superolaterat
direction.
was
(4.5
the liver, is registered
of preferential
warped
alone).
0.8
ROl
the
activity.
(19 le-
CT and
outside
algorithm,
in terms
assessment,
in 53 instances
studied
sions
warping
blood
because the hemangioma in this patient was not seen with SPECT. Quantitative analysis of image pairing was
is first drawn around the liver on the MR image (arrows, top right). When larger than the liver and overlaps it. After application of the warping alleft) such that the ROl now clearly matches the liver in size and more achyperintense hernangioma on the MR section (arrow, top right). When
diswere
warping
in precise correlaof anatomic land-
nat
Future image
registration clear intrinsic
identified
applications fusion of
medicine
are images studies
anatomic
(eg,
of
likely obtained in
in
which
landmarks
images
abdomi-
to involve nu-
few are
with
radiola-
November
1991
beled
monoclonal
antibodies).
these instances, landmarks atone to
accurately
cross-correlate
combination
and
of
Analysis
images;
internal
externally
necessary
a
landmarks
placed
markers
may
be
(1).
In our
ROI
In
intrinsic anatomic may be insufficient
study,
image
analysis
fusion
with
absolute conflnmation that suspected hemangiomas identified by means of SPECT with labeled RBCs correlated exactly with lesions seen on CT and MR images. The smallest hemangioma identified
by
provided
review
SPECT
and
of registered
MR-SPECT
1.0 cm in size. (triple-headed)
capable
images
New
SPECT
and
mented
cameras
fusion
a recent
tempts
study
(18)
SPECT
docu-
sensitivity
of hepatic hemangiomas 1.5 cm. We believe image
may
clinically
prove
diagnostic
are
3.5-mm-thick
improved
for detection smaller than
was
high-resolution
of acquiring
sections,
CT-
confirmation
useful of such
for small
lesions.
Image
fusion
the
evaluation
near
regions
may
potentially
of hot-spot of vascular
SPECT scanning with has limited sensitivity small near
heart this
(8). As seen study,
unsure
foci activity.
cm) vessels
SPECT a focus
whether
case
in
readers were of increased
uptake represented hemangioma or branching vessel, image fusion with ROI analysis matched a focus of suspicious
uptake
identified Small ately
a hemangioma
with
on CT and
MR images.
hemangiomas located adjacent to vessels are
SPECT
missed
on
Image
fusion
blood
may
pool
serve
to
the suspected juxtavascular of these hemangiomas;
per
se, however,
lesions
that
wilt
are
racy
ROI
enabled of
of registered
within
1.5 pixels accuracy
to that toms
Of the two developed,
anal-
the
images 0.8 (4.5
is favorable
achieved
with
with
and
images
is close of phan-
correspondence)
quantitative we believe
2.4).
±
(16).
techniques that the ROI
distance method proprecise means of evalu-
center-to-center
a more
ating the ability of the warping algonithm to align and match data. The ROI percentage overlap method provides
complementary
that
is helpful
size
and
Volume
overlap
181
information
for evaluating of
#{149} Number
lesion
2
lesion borders.
suc-
and
MR
imag-
the
accuracy
of
the
transformation
apply
entire image, intrinsic landwere selected throughout the section and were suppleby points that defined the surface.
Slight
if the were
geometric
fiducial
not
errors
landmark
symmetrically
distnib-
uted, which is demonstrated in Figure 3, in which the definition of the tatenal aspect of the right lobe of the by
the
warped
ROl
outline
is
more accurate than is definition of the anterior aspect of the left lobe. A more exact match may have been obtained by
the
tify
was mm
CT
rule,
to make
to the marks chosen mented
tional
accu-
conrespon-
±
(1.3-pixel
vides
visible
comparison
technique;
fort
liver
demonstrate
us to validate
our
dence This
location technique
clearly
not
SPECT. Quantitative ysis
not
As a general
points
studies.
revealed
landmark placement together with the number of terms used in the polynomial determines the tightness of fit between registered images. In an ef-
occurred
confirm
the
at acquiring
body
immedioften
data
ing sections at levels designed to match SPECT section thickness and position, such factors as section misregistration due to variable nespiratory excursion may make it difficult for the center of a Cl section to coincide with that of an axial SPECT section. Nevertheless, it was encouraging that even in these cases, a portion of the warped ROl always overlapped a region of suspected hemangioma blood pool activity.
of
located on the
2.5
in a single
when
located
labeled RBC for detection
(
hemangiomas major intrahepatic
aid in
of our
cessful image fusion to be most dependent on careful placement of anatomic landmarks. Accurate landmark placement was verified by using trial ROIs drawn around normal anatomic borders or structures, and landmark point pairs were repositioned as needed to achieve a “good fit.” At times, landmark positioning was accurate, but image registration appeared to be less than perfect. We encountered this panticularly when attempts were made to fuse SPECT images with those obtained with CT and MR imaging at different levels through a hemangioma. Despite at-
the
placement
of multiple
landmarks hepatic
to specifically surface. Such
match, however, necessary. In this ROI center-to-center
only 86%;
0.5 pixel a 1.8-cm
addi-
idena perfect
is often not clinically particular case, the distance was
and ROI overlap was hemangioma was con-
firmed. The axial sections used for image registration in this study were assumed to be coplanar. A small component of geometric error may have nesulted from this assumption. The polynomial-based warping algorithm we used is capable, nevertheless, of transforming any two-dimensional section (provided landmarks are suit-
ably
chosen),
irrespective
of the
tilt
of
the section, ie, the angle formed between the section and the longitudinat axis of the body. When tilt in the coronal or sagittal planes exists, the sections can be reoriented in the transaxial plane. The details of such oblique nepnojection techniques and their application to clinical abdominal imaging have been previously reported (i,14). The time required for image registration varied with case difficulty and as changes
design.
were
made
Typically,
20-30
minutes
in our
software
approximately were
needed
to regis-
ten matching image pairs. This included time expended for patient setection; image display and review; section selection; assignment of optimized colon scales, windows and 1evels
for
individual
images;
CT,
landmark
MR.
assignment
editing (if necessary); and oma ROI analysis. Accurate positioning
was
the
most
and
SPECT
and hemangilandmark time-con-
suming aspect of image fusion. The time necessary for this should decrease in the future, however, because automated or semiautomated reproducible means of landmark selection are being devised to replace the manuat technique used in this study. The hemangiomas in our patients represented an excellent clinical model to use in refining our registration
methodology.
In most
hemangio-
studied, “eyebatling” the nonfused images would doubtlessly have been a much faster means of anatyzing our data, given the time required for image fusion. This technique would be most useful, therefore, for evaluating hemangiomas that are eithen small on located adjacent to negions of blood pool activity (eg, intrahepatic vessels); in such cases, a more objective means of aligning image data would be preferable to visual mas
assessment. these methods
Future applications appear encouraging
of
and are likely to include fusion of SPECT-CT and SPECT-MR imaging studies to facilitate tumor-specific imaging and/or quantitation of the uptake of nadiophanmaceuticals such as monoclonal antibodies, which would be useful mates.
for U
making
dosimetry
esti-
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1991
