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Effects of Misalignment Between Transmission and
Emission Scans on Attenuation-Corrected Cardiac
SPECT
Ichiro Matsunari, Guido Boning, Sibylle I. Ziegler, Istvan Kosa, Stephan G. Nekolla, Edward P. Picaro and Markus Schwaiger
Nulearmedizinische Klinik und Poliklinik der Technischen Universität München,Klinikum rechts der Isar, München,
Germany; and Department of Internal Medicine, Division of Nuclear Medicine, University of Michigan Medical Center, Ann
Arbor, Michigan
Misalignment between transmission and emission scans in attenu
ation-corrected (AC) cardiac SPECT can introduce errors of mea
sured activity. The severity of these errors, however, has not yet
been fully elucidated. Methods: We performed a phantom mea
surement as well as a study of patients with low likelihood of
coronary artery disease. Transmission and emission scans were
acquired using a triple-head SPECT system with a collimated 241Am
line source and an offset fanbeam collimator. The left ventricular
myocardium was divided into five segments, and the mean regional
activity was calculated for each segment using a semiquantitative
polar map approach. Misalignment between transmission and emis
sion data was created by shifting the emission data along the x, y or
z axis. Results: In the heart phantom, a shift between the transmis
sion and emission data produced a decrease or increase in relative
regional activity in each segment resulting in heterogeneous activity
distribution. A 7-mm (1-pixel) shift produced up to 15% change in
relative regional activity, suggesting that even a small misalignment
between transmission and emission data can produce serious
errors in measured activity. In the clinical data, the effects of
misalignment were less significant than those observed in the
phantom data but were still measurable and visually identifiable.
Conclusion: The resultsindicatethat a small misalignmentbetween
the transmission and emission data can produce serious errors in
measured activity, and, thus, geometrical precision is essential for
accurate diagnosis of AC SPECT images.
Key Words: attenuation correction;SPECT; transmission;emission
scans; misalignment
J NucÃ-Med 1998; 39:411-416
rVttenuation artifacts are known to reduce the diagnostic
accuracy of cardiac SPECT imaging. Recently, approaches for
attenuation correction have been proposed (1-3), and clinical
results have shown its use for improving the detection of
coronary artery disease (4) as well as the identification of viable
myocardium (5). Misalignment between transmission and emis
sion scans, on the other hand, can introduce errors of measured
activity into attenuation-corrected (AC) SPECT images. This is
particularly true when a sequential acquisition of transmission
and emission scans is performed because such misalignment
can easily be introduced by patient motion between the trans
mission and emission scans.
Similar errors between transmission and emission data could
also occur in multidetector SPECT systems with simultaneous
transmission and emission scan capability if transmission and
emission images are acquired using separate heads or if one or
more detectors become misaligned. Considering the increasing
availability of attenuation correction techniques for cardiac
Received Dec. 12, 1996; revision accepted Jun. 12, 1997.
For correspondence or reprints contact: Markus Schwaiger, MD, Nuklearmediz
inische Klinik und Poliklinik der Technischen UniversitätMünchen,Klinikum rechts der
Isar, Ismaninger Str. 22, 81675 München,Germany.
SPECT imaging, it would be useful to know how such mis
alignment affects AC SPECT images.
Thus, this study was designed to evaluate the effect of errors
caused by misalignments between the transmission and emis
sion data on AC cardiac SPECT images acquired over 360°.
Because evaluation of SPECT images usually relies on relative
regional activity rather than absolute value, we only studied the
misalignment effects on relative regional activity.
MATERIALS AND METHODS
Cardiac Phantom
The phantom study was performed using an elliptical cylinder
chest phantom (32X23X 17.7 cm) (RTW, Freiburg, Germany) with
cardiac inserts (Model 7070, Data Spectrum Corp., Chapel Hill,
NC). A balloon filled with approximately 200 ml of radioinactive
water was also inserted in the right lower part of the chest phantom
to simulate diaphragmatic attenuation. The myocardium was filled
with 50 MBq (0.45 MBq/ml) 99mTc.The chest wall, as well as the
ventricular cavity, were filled with radioinactive water. The phan
tom had a chest cavity with air around the heart insert to simulate
lungs.
Patients with Low Likelihood of Coronary Artery Disease
Ten patients (8 men, 2 women; age range 50-67 yr; mean age
58 ±5 yr) with low likelihood (^5%) of coronary artery disease
based on age, sex, history and exercise electrocardiogram were
studied (6).
Symptom-limited treadmill exercise was performed, and 740-
1110 MBq (20-30 mCi) 99mTc-sestamibi was injected at peak
exercise. Imaging was started 20-30 min postinjection using
identical acquisition parameters (e.g., scan time, energy window)
used for the phantom study.
Data Acquisition
Simultaneous transmission and emission measurement was per
formed using a triple-head SPECT system (MULTISPECT 3,
Siemens AG, Erlangen, Germany) equipped with a low-energy,
fanbeam collimator with a focal length of 53 cm with its focal line
offset by 17 cm for detector 1 and with low-energy, high-
resolution, parallel-hole collimators for detectors 2 and 3 (5.7,8).
The transmission line source consisted of a 5.55 GBq (150 mCi)
24'Am line source sealed in a stainless steel tube.
Transmission and emission projection data were acquired simul
taneously in 64X64 matrices. Images were acquired in 6°steps
over 360°for 20 sec per projection. An energy window of 59.0 ±
5.9 keV was used for the 241Amtransmission photons, and a 15%
window centered on the 140-keV peak was used for the emission
data. An 80-sec transmission blank scan was acquired to compute
attenuation maps from the transmission data.
MISALIGNMENTONATTENUATION-CORRECTEDSPECT •Matsunari et al. 411
FIGURE 1. Schematic representa
tion of polar map display. The left
ventricular myocardium was divided
into five segments. After normaliz
ing pixel values for the region show
ing the maximal activity in the myo
cardium, the mean relative regional
activity was calculated for each
segment.
Left Ventricular Myocardium
Septan Apex Lateral
Processing of SPECT Data
Image reconstruction of SPECT data was performed in a manner
similar to that previously reported (4,9) except for the use of
filtered backprojection (FBP) to reconstruct transmission data.
Rather than iterative methods, FBP was used to reconstruct the
transmission data in this study because the geometry used provided
a transmission imaging field-of-view of 39 cm, yielding little
truncation of the transmission data, and because FBP requires less
computational power compared to iterative reconstruction meth
ods. As previously described by Ficaro et al. (4), the main reason
for the use of iterative reconstruction for transmission data is to
minimize truncation problems, and, theoretically, the use of FBP to
reconstruct transmission data should not alter results. After the
correction of down scatter from the emission to transmission data,
misalignment between transmission and emission data was created
by manipulating emission projection data along the x- (right/left),
y- (up/down) and z-axis (cephalad/caudal) according to the shift
desired. This manipulation produced the effect that would have
been obtained had the subject actually moved to the new position
between transmission and emission scans. It also represents the
situation in which camera heads used for emission acquisition are
not aligned to the detector that acquires transmission data. No
manipulation was performed for transmission data. The magnitude
of the shift ranged from I to 5 pixels, where 1-pixel was 7-mm. The
emission images were then reconstructed using an iterative recon
struction method [penalized weighted least-squares algorithm (10)]
with reconstructed attenuation maps to correct the emission data
for photon attenuation.
In the setup used in this study, transmission and emission data
were acquired simultaneously in different heads. Careful alignment
procedure and calibration tools provided by the manufacturer were
used to assure data acquisition without misalignment. The calibra
tion tools used in this study included those for source-to-detector
distance, fanbeam offset and pixel sizes.
Data Analysis
Image data analysis was performed using a semiquantitative
polar map approach, which was developed in our laboratory. This
method involved two steps. First, the long axis of the left ventricle
was defined interactively in three dimensions. Second, an auto
matic volumetric radial search for activity maxima was performed
(/ /) creating 15 short-axis slices. This procedure was performed by
a single operator, and great care was taken to keep a consistent axis
in the aligned and misaligned image dataseis for each patient. The
left ventricular myocardium was then divided into five segments as
displayed in Figure 1. The SPECT images were normalized to the
pixel showing maximal value in the left ventricular myocardium.
The mean relative regional activity was then calculated for each
segment. The mean relative regional activity data in each segment
without any misalignment served as the control data, while the
mean regional activity for each segment from images reconstructed
FIGURE 2. Effects of a 21 -mm (3-pixel) displacement of the emission data in
various directions on a homogeneous cardiac phantom. The control image
with no misalignment shows uniform myocardial distribution (center), while
misalignment in any direction causes apparent inhomogeneity in myocardial
distribution on the reconstructed emission image.
with various misalignments are referred to as the misaligned data.
The changes of the mean relative regional activity in each segment
between the control and misaligned data were calculated as indices
of errors.
Statistical Analysis
Data were expressed as mean ±s.d. The mean values were
compared using a Wilcoxon signed rank test. Statistical signifi
cance was defined as p < 0.05.
RESULTS
Phantom Study
Reconstructed short-axis slices of the cardiac phantom, with
a 21-mm (3-pixel) misalignment in various directions, are
displayed in Figure 2. The visual effect of misalignment is
readily apparent from the emission images. Changes in relative
regional activity from shifting the emission data by various
degrees and directions are shown in Figure 3. A shift between
the transmission and emission data, regardless of its direction,
produced a decrease or increase in relative regional activity in
all five segments. Notably, a right shift as small as 7-mm
(1-pixel) produced 15% change of relative regional activity in
the septal wall. For a given misalignment, the severity of such
changes differed from segment to segment resulting in inhomo
geneity of activity distribution in the myocardium.
Clinical Study
The effects of misalignment in the human heart from a
21-mm (3-pixel) shift, as was done in the phantom data, are
displayed in Figure 4. Similar to the phantom results, inhomo
geneity produced by the misalignment between transmission
and emission data are visually noted. The degree of change,
however, was less significant than that seen in the phantom
data. The relationship between the degree of displacement and
the severity of errors in relative regional activity on the AC
SPECT images is shown in Figure 5. The values are reported as
an average from all 10 patients. As was observed in the
412 THEJOURNALOFNUCLEARMEDICINE•Vol. 39 •No. 3 •March 1998
20
10
O
'S-10
¿-20
2 -30
t -40
m -50
Right/Left 20
10
O
0-10
¿-20
«2-30
t-40
LU
-42 -28 -14 O 14 28 42
Misalignment (mm) left
Up/Down
20
10
O
"5-10
¿-20
2-30
fc-40
m -50
-42 -28 -14 0 14 28 42
down Misalignment (mm) up
Cephalad/Caudal
-42 -28 -14 0 14 28 42
caudal Misalignment (mm) cephalad
FIGURE 3. Changes in relative regional
activity as a function of misalignment be
tween transmission and emission data on
a cardiac phantom are shown for each of
five segments. The directions shown on
each graph indicate the direction in which
the emission data have been shifted rel
ative to the transmission data.
phantom data, a shift of the emission data produced changes in
relative regional activity. Nevertheless, the effect of misalign
ment was less significant in patients than in the phantom. For
example, a 21-mm (3-pixel) shift produced errors of more than
10% in 1.0 ±0.6 of the five segments for each direction
compared to 3.8 ±0.4 of the segments in the phantom data
(p = 0.026). The effect of misalignment along the z-axis
FIGURE 4. Effects of a 21-mm (3-pixel) misalignment on "Tc-sestamibi
activity distribution from a patient with low likelihood of coronary artery
disease. A shift of the emission data relative to the transmission data
introduces inhomogeneity in myocardial distribution into the reconstructed
emission image.
(cephalad/caudal), especially in the caudal direction, was less
significant than that along the x- (right/left) or y-axis (up/
down). However, the effect of shifting the emission data was
measurable and visually identifiable in Figures 4 and 5. A 7-mm
(1-pixel) right shift, for example, produced a mean 7% ±5%
change in relative regional activity in the septal wall. This
increased to a mean of 14% ±6% when the emission data were
shifted by 14 mm (2 pixels).
Tables 1-3 show the effects of misalignment in 10 patients.
It is noted that, for a given misalignment, there was a consid
erable variation in measured errors from patient to patient
indicating that the severity of errors is difficult to predict from
the magnitude of misalignments in some patients.
DISCUSSION
The major findings of this study were: (1) a shift between
transmission and emission data, regardless of its direction,
produced a decrease or increase in relative regional activity in
all five segments; (2) in the cardiac phantom, a 7-mm (1-pixel)
shift produced up to 15% change in relative regional activity
and (3) the errors in the human heart were less significant than
those observed in the phantom but were still measurable.
Effects of Misalignment in the Phantom
Our data clearly show that misalignment between transmis
sion and emission data can cause serious errors in relative
regional activity on AC SPECT images. This is consistent with
published data by Murase et al. (12) who studied the effects of
misalignment in a thorax phantom as well as in clinical brain
data. A misalignment as small as a 7-mm (1-pixel) shift created
up to 15% change in measured activity suggesting that even a
small shift can lead to serious errors in measured activity in the
AC SPECT images. For a given misalignment, these effects are
different in different segments. Some segments decrease in
relative regional activity, and others increase resulting in rather
markedly inhomogeneous distribution in the myocardium.
Thus, geometrical precision between transmission and emission
data is critical for the accurate measurement of AC SPECT
MISALIGNMENTONATTENUATION-CORRECTEDSPECT •Matsunari et al. 413
FIGURE 5. Changes in relative regional
activity, as a function of misalignment
between transmission and emission data,
are shown for each of five segments and
are an average of all 10 patients. The
directions shown on each graph indicate
the direction in which the emission data
have been shifted relative to the transmis
sion data.
Right/Left
-42
right -28 -14 0 14 28 42
Misalignment (mm) left
20
ÃŽ10
8. 0
o -10
¿-20
«-30
§-40
"-50
Up/Down
20
10
o
•5-10
¿-20
«-30
2-40
UJ
-42 -28 -14 0 14 28 42
down Misalignment (mm) up
Cephalad/Caudal
-50-42 -28 -14 0 14 28 42
caudal Misalignment (mm) cephaiad
images. It should be noted, however, that these results are based
on a simple phantom without factors confounding the interpre
tation of data (e.g., variation in thorax geometry). Although
phantom studies would give useful information as to the effect
of such misalignment in a clean situation, clinical study is
important to confirm the phantom data.
Clinical Data
To date, there is little data available for the effects of
misalignment between transmission and emission data on AC
cardiac SPECT images in humans (13). As expected from the
phantom data, misalignment between the transmission and
emission data produced errors in measured activity on AC
emission images. The severity and extent of errors, however,
was less significant in humans than that observed in the
phantom. This is explained by blurring in both transmission and
emission data caused by cardiac motion, by breathing and by
different thorax structures of patients from that of the most
simplified chest phantom. In one PET study (14), estimates for
range of expected misalignments due to repositioning patients
between transmission and emission scans were reportedly x,
y < 6 mm and for z < 11 mm. If the misalignments are kept
small (within 7-mm or 1-pixel), the errors may be kept under
10%. However, a 14-mm (2-pixel) shift readily causes up to
14% error in the measurement. It should be noted that these
errors are an averaged value from all 10 patients. Therefore, the
severity of errors differs from patient to patient, depending on
details of body habitus, etc., and some patients show a greater
magnitude of errors than what was reported as an averaged
value (Tables 1-3).
In addition to these observations, a shift along the z-axis
(cephalad/caudal) appears to cause less distortion per pixel of
shift than does the equivalent x (right/left) and y (up/down)
shifts. This is in agreement with a PET study assessing errors in
the emission data caused by misalignment between the trans
mission and emission scans (75). This is probably because a
displacement along the z-axis does not produce as large a
TABLE 1
Error in Relative Regional Activity Caused by Misalignment (21 mm) Between Transmission and EmisiónData Along X-Axis
AnteriorPatient
no.12345678910SexMMMFFMMMMMRight-9.7-17.0-9.2-8.7-8.5-6.2-7.4-11.6-12.3-16.7Left-4.0-10.0-4.2-1.6-5.6-10.6-9.2-9.2-12.9-2.6SeptalRight-21.7-28.0-23.8-19.8-19.0-10.4-18.3-30.5-23.0-25.7Left0.41.53.91.0-1.43.60.6-7.4-1.64.1InferiorRight-6.7-12.2-7.4-3.5-8.0-6.2-7.8-10.4-11.1-11.7Left-1.6-8.91.2-3.1-3.0-1.4-4.1-10.1-8.63.5LateralRight3.5-5.80.33.0-0.75.31.96.14.0-2.6Left-13.5-21.4-20.5-11.3-18.6-14.8-16.3-18.8-25.2-12.3ApexRight-2.7-8.7-9.0-5.0-5.6-0.6-5.1-5.5-12.2-8.2Left-9.5-10.3-11.0-6.4-7.3-5.8-11.9-12.1-20.6-2.7
Mean ±s.d. 10.8 ±3.7 -7.0 ±3.9 -22.0 ±5.7 0.5 ±3.4 -8.5 ±2.8 -3.6 ±4.4 1.5 ±3.7 -17.1 ±4.5 -6.2 ±3.3 - 9.8 ±5.0
Values indicate errors in relative regional activity (% of peak activity) introduced by a 21-mm misalignment.
414 THE JOURNALOFNUCLEARMEDICINE•Vol. 39 •No. 3 •March 1998
TABLE 2
Error in Relative Regional Activity Caused by Misalignment (21 mm) Between Transmission and Emisión Data Along Y-Axis
Anterior Septal Inferior Lateral Apex
Patient no. Sex Down Up Down Up Down Up Down Up Down Up
12345678g10MMMFFMMMMM0.33.40.9-4.2-3.2-0.3-1.42.84.1-1.8-3.0-6.8-6.1-9.6-6.7-3.3-8.1-4.3-2.1-9.3-9.4-1.4-5.0-2.5-7.2-3.9-9.8-7.3-5.4-11.7-13.0-10.9-12.0-15.1-11.7-4.1-11.4-14.7-3.2-10.0-7.7-2.5-3.7-15.9-13.7-2.7-6.5-11.6-5.5-6.83.81.80.4-3.90.77.11.03.07.31.6-12.8-8.9-18.6-24.4-21.8-8.7-16.3-14.7-10.0-17.04.6-2.8-1.1-0.2-1.13.3-1.46.44.3-2.56.07.08.2-4.15.28.06.46.49.611.6-2.0-12.4-12.2-12.7-11.4-5.8-12.2-4.3-9.8-11.9
Mean±s.d. 0.1±2.8-5.9±2.7-6.4±3.4-10.6±4.0 -7.7±4.7 2.3±3.3 -15.4±5.3 1.0±3.46.4±4.2 -9.4±3.9
Values indicate errors in relative regional activity (% of peak activity) introduced by a 21-mm misalignment.
movement of myocardium into the lung field as does x or y
movement depending on heart axis orientation.
Study Limitations
There are several limitations in our study. First, it is uncertain
to what extent the results with the uniform cardiac phantom we
used are applicable to the phantom with defects. We did not
investigate the effects of misalignment in patients having
perfusion defects. Second, the small number of patients studied
precluded us from performing a gender-specific analysis of
misalignment effects in a meaningful way. Third, patient
motion during acquisition was not considered in our study,
which may also cause additional artifacts. Finally, our current
software did not allow for the assessment of the effect of
rotational misalignment. These issues need to be addressed in
any further phantom and clinical studies.
Implications of the Study
Attenuation correction using measured attenuation maps is
one of the most important developments in recent SPECT
technology. When it is applied to cardiac SPECT imaging, an
improved diagnostic accuracy can be expected by reducing
attenuation artifacts. There are several approaches proposed for
the attenuation correction (1,2,9). Some of these use sequential
imaging of transmission and emission scans (16), and others use
simultaneous transmission and emission measurement (9).
When sequential imaging is performed, misalignment between
transmission and emission scans can be caused by patient
motion. As reported in this study, it is obvious that such patient
motion can cause serious errors in measured activity in AC
SPECT. In this regard, simultaneous transmission and emission
measurement, rather than separate acquisition, is preferable
because it virtually eliminates errors caused by patient motion
between the scans.
With multidetector SPECT systems, transmission and emis
sion images are simultaneously acquired using separate heads
as is the case in our system and others (1,2). Misalignment
between the camera heads is minimized by routine quality
control, including center-of-rotation correction. When using
nonparallel-hole collimators, such as fanbeam or offset fanbeam
collimators, precision measurements must be performed to
determine the exact localization and stability of the transmis
sion source relative to the focus of the fanbeam collimator. If
this is not assured, misalignment between transmission and
emission projections will result after rebinning to parallel hole
geometry, and this will introduce errors in measured activity on
the reconstructed AC SPECT images. Depending on varying
geometrical errors, different distortions of the reconstructed
transmission data can be observed as shown in Figure 6. It is
also noteworthy that this effect is not constant for each location
in the transmission image. In addition, pixel size and homoge
neity of fanbeam and parallel-hole collimators must be deter
mined to avoid mismatching of reconstructed attenuation maps
and emission images. Routine quality assurance, including
fanbeam parameters, is important in systems with simultaneous
TABLE 3
Error in Relative Regional Activity Caused by Misalignment (21 mm) Between Transmission and Emisión Data Along Z-Axis
Anterior Septal Inferior Lateral Apex
Patient no. Sex Caudal Cephalad Caudal Cephalad Caudal Cephalad Caudal Cephalad Caudal Cephalad
12345678910MMMFFMMMMM6.63.52.65.00.20.33.51.42.4-0.2-10.3-11.6-8.8-14.1-12.7-15.5-13.3-8.0-9.4-9.6-2.5-0.8-4.4-3.4-4.7-2.6-5.2-1.8-1.2-6.71.70.5-0.4-4.6-3.43.5-1.3-3.90.34.61.5-0.7-5.9-1.6-0.1-4.0-5.61.61.7-4.06.13.66.52.20.80.0-0.2-2.00.88.92.6-1.5-0.7-1.5-0.40.20.12.40.2-0.7-4.0-11.1-10.6-13.6-11.4-12.0-9.9-7.2-5.7-6.20.5-1.7-1.3-3.2-3.4-3.9-5.3-1.2-3.5-3.7-5.1-13.8-11.5-10.2-6.5-2.7-4.6-13.9-5.50.3
Mean ±s.d. 2.5 ±2.2 -11.3 ±2.4 -3.3 ±1.9 -0.3 ±3.1 -1.7 ±3.0 2.7 ±3.5 0.1 ±1.4 -9.2 ±3.2 -2.7 ±1.7 -7.3 ±4.8
Values indicate errors in relative regional activity (% of peak activity) introduced by a 21-mm misalignment.
MISALIGNMENTONATTENUATION-CORRECTEDSPECT •Matsunari et al. 415
SOURCE Original image
teta- 180dig.
COR)*
*fan
beam projection / /
with angle teta /•l ••''/''">f"*/fan_offset
-172.2mmIen
tnxf-63
,^. eHn lan-oeam projection ' '
^^^^ / ,' corresponding to angletota^WLxp
- 4ft pixel In parallel dole projection
^"<^^ corresponding to angle teU+phlsrc2d
fan_osrcZd
, fan_oet
+10 mm srcZd
Tset +- 0 mmfan_o-4--•4-et +10
(Tset - 10rmm
mmat
-10mm src2det -10mm
Tset +-0mm fan_offset +10mm*"*-
FIGURE 6. (A)Schematic of the fanbeam
geometry for the transmission measure
ment. Errors in the rebinned parallel hole
projection data can be introduced by
either those in source/detector distance
(src2det) or in fan_offset. COR indicates
the center of rotation. (B) Effects of wrong
geometrical values on the reconstructed
transmission image of a lead rod. The
upper right image displays the recon
structed transmission image without er
rors. The ¡magesdisplayed in the middle
and lower rows represent misaligned
transmission data due to the wrong val
ues for the distance between the trans
mission source and detector (src2det)
and/or for the fan_offset. The wrong geo
metrical parameters introduce not only
translocation but also distortion of the
transmission data.
transmission and emission measurement capability in order to
avoid such artifacts.
CONCLUSION
The results of our study indicate that it is essential for the
accurate diagnosis of AC cardiac SPECT images to ensure that
there is no geometrical error between transmission and emission
data.
ACKNOWLEDGMENTS
We thank Dr. Claire S. Duvernoy for her comments during
manuscript preparation. Dr. Matsunari was supported by Mitsu
bishi Research Institute, Japan.
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