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Image Reconstruction Using Filtered
Backprojection and Iterative Method: Effect on
Motion Artifacts in Myocardial Perfusion SPECT
Seyed Rasoul Zakavi
1
, Amin Zonoozi, NMT
1
, Vahidreza Dabbagh Kakhki
1
, Mohsen Hajizadeh
2
, Mehdi Momennezhad
1
,
and Kamran Ariana
1
1
Department of Nuclear Medicine, Mashad University of Medical Sciences, Mashad, Iran; and
2
Department of Medical Physics,
Mashad University of Medical Sciences, Mashad, Iran
Patient motion during myocardial perfusion SPECT is a common
source of errors. The extent and severity of motion artifacts have
been described for filtered backprojection (FBP) reconstruction.
In recent years, iterative reconstruction has been used increas-
ingly in reconstruction of myocardial perfusion SPECT images
and has been shown to be more accurate than FBP even in cases
of incomplete datasets. This study evaluated the effect of itera-
tive reconstruction on the extent and severity of motion artifacts.
Methods: Six normal, motion-free, and nongated
99m
Tc myo-
cardial perfusion SPECT scans were selected, and simulated
motion of 3 pixels was applied to the early, middle, and late
phases of acquisition in 2 types of movement, returning and non-
returning. The images were acquired by a single-head g-camera
in 32 steps at 30 s per step and in a 180arc from right anterior
oblique to left posterior oblique. All original and shifted images
were reconstructed using FBP and ordered-subset expectation
maximization (OSEM) techniques and interpreted by 2 nuclear
medicine specialists qualitatively and semiquantitatively (using
17 segments and a 5-point scoring system). Results: Overall,
68.1% and 70.8% of shifted images were categorized as defi-
nitely abnormal in the FBP and OSEM reconstructions, respec-
tively (P.0.5). The mean summed score was 11.9 (65.7) and
11.3 (65.2) for nonreturning shifted images (P50.13) and 5.2
(62.4) and 3.9 (62.0) for returning shifted images (P,0.001) in
the OSEM and FBP reconstructions, respectively. The incidence
of defects in different myocardial segments was similar with the
2 reconstruction methods. The summed score was higher with
shifting in the middle phase of acquisition than in the late or early
phase. Conclusion: Our study showed that the incidence of
abnormal findings and the location of defects were not different
between the 2 reconstruction types; however, with semiquantita-
tive assessment, the severity of defects increased with OSEM re-
construction. Although OSEM reconstruction has been reported
to be more tolerant to missing data than is FBP reconstruction,
our study showed that OSEM reconstruction may be less tolerant
to motion artifacts than is FBP reconstruction.
Key Words: myocardial perfusion SPECT; motion artifacts;
OSEM reconstruction; filtered backprojection reconstruction
J Nucl Med Technol 2006; 34:220–223
Patient or organ motion during myocardial perfusion
SPECT is believed to affect as many as 10%220% of all
cardiac SPECT studies and is a well-recognized source of
errors in scan interpretation (1–4). The pattern and severity
of motion artifacts have been studied for actual or sim-
ulated motion during SPECT acquisitions using single-head
and dual-head g-cameras (4–6). All previous studies have
used filtered backprojection (FBP) reconstruction for image
processing. However, statistical image reconstruction algo-
rithms, such as iterative methods, are becoming increas-
ingly popular in the reconstruction of myocardial perfusion
images. The most commonly used method is ordered-subset
expectation maximization (OSEM), which is an ordered-
subset implementation of the maximum-likelihood expec-
tation maximization algorithm (7).
Iterative techniques have been shown to provide more
accurate reconstructions than does FBP, even in cases of
sparse or incomplete datasets (8,9). However, the effect of
patient motion in the OSEM reconstruction method has not
been studied. This study compared motion artifacts in the
2 reconstruction methods (FBP vs. OSEM).
MATERIALS AND METHODS
Six normal, motion-free
99m
Tc-methoxyisobutylisonitrile myo-
cardial perfusion SPECT studies were selected according to the
review of 3 nuclear medicine specialists. Absence of motion was
documented by visual inspection of a rotating cinematographic
display of the raw projections and summed images (linogram and
sinogram). Scanning was performed on a single-head g-camera (DSX;
Sopha Medical Vision) equipped with a low-energy parallel-hole
high-resolution collimator. The images were acquired in 32 pro-
jections with a 180circular orbit from 45right anterior oblique
to 45left posterior oblique, using step-and-shoot technique and
30 s of imaging per frame in nongated mode. The matrix size was
Received Jan. 2, 2006; revision accepted May 15, 2006.
For correspondence or reprints contact: Seyed Rasoul Zakavi, MD, IBNM,
Department of Nuclear Medicine, Emam Reza Hospital, Mashad University of
Medical Sciences, Mashad, Iran.
E-mail: Zakavi@mums.ac.ir
COPYRIGHT ª2006 by the Society of Nuclear Medicine, Inc.
220 JOURNAL OF NUCLEAR MEDICINE TECHNOLOGY • Vol. 34 • No. 4 • December 2006
64 ·64, and the zoom factor was 1.33 (pixel size, 6.4 mm). No
scatter or attenuation correction was applied to image reconstruc-
tion. Simulated patient motion was applied to all raw data with a
displacement of 3 pixels in the x- and y-axes. The image set was
displaced once in the x-axis and again in the y-axis, in a positive
direction and in 2 types of shifting: returning and nonreturning
(Fig. 1). The returning shift was applied for 3 consecutive frames
only, resulting in 90 s of displacement. All simulated motion was
applied in the early (frame 7), middle (frame 16), and late (frame
24) phases of acquisition. Accordingly, for the returning type of
shifting, the early shift was in frames 729, the middle shift was
in frames 16218, and the late shift was in frames 24226. For the
nonreturning type, early, middle, and late shifting were from frames
7232, 16232, and 24232, respectively. All original images and
shifted images were saved as separate files and reconstructed
using the FBP and OSEM methods. We used the Metz filter with
a cutoff frequency of 4.8 (in full width at half maximum) and
order 8 for all images. For OSEM reconstruction, 8 iterations and
2 subsets were used.
The reconstructed images were reviewed by 2 nuclear medicine
specialists for the presence and locations of defects or the absence
of defects. Imaging findings were categorized as normal, abnor-
mal, or equivocal. The physicians were not aware of the recon-
struction method used for each image. In cases of disagreement,
consensus was reached with the review of the images by a third
nuclear medicine specialist. Semiquantitative analysis was done
for all images using a 17-segment model and a 5-point scoring
system. However, basal segments were excluded, and 11 segments
were considered in summation scores to exclude membranous
septum variance.
Data from the 2 reconstruction methods were compared using
the paired ttest. The McNemar test and Fisher exact test were
used for comparison of proportional data. Independent groups
were compared using an independent ttest or 1-way ANOVA. The
Duncan test was used for multiple comparisons. A Pvalue of less
than 0.05 was considered statistically significant.
RESULTS
Seventy-two sets of shifted images were created: 36 sets
for each axis, including 18 images with a returning shift
(6 in the early phase, 6 in the middle phase, and 6 in the late
phase of acquisition) and 18 images with a nonreturning
shift. All images were reconstructed by the FBP and OSEM
methods, yielding 144 images for interpretation.
The findings on the image sets were reported as either
abnormal or equivocal using visual interpretation. With
FBP reconstruction, 68.1% (49/72) of findings were con-
sidered abnormal, whereas 70.8% (51/72) of findings were
abnormal using OSEM reconstruction.
Of the images with a returning shift, 44.4% (16/36) were
reported as showing abnormal findings and 55.6% (20/36)
as showing equivocal findings using FBP reconstruction.
The percentage was 50% (18/36) in each group using OSEM
reconstruction. Of the images with a nonreturning shift,
91.7% (33/36) were considered to show abnormal findings
with both types of reconstructions (Table 1).
The paired ttest was used to compare the mean summed
score in the different types of reconstructions. In all shifted
images, the mean summed score was 7.6 (65.5) with FBP
reconstruction and 8.5 (65.5) with OSEM reconstruction
(P,0.001).
The mean summed score was lower with FBP recon-
struction than with OSEM reconstruction in images with a
returning shift (P,0.001) but was not statistically differ-
ent from OSEM reconstruction in images with a non-
returning shift (P50.13) (Fig. 2).
The proportion of defects in each segment was compared
for the 2 types of reconstructions using the McNemar test.
The reconstruction technique had no significant impact on
the location of defects (Table 2).
The Student ttest was used to compare the mean
summed score between x-axis and y-axis shifting. The
mean summed score was 7.93 (64.7) for x-axis shifting and
7.29 (66.2) for y-axis shifting (P50.629) using FBP
reconstruction. The numbers were 8.66 (65.1) and 8.48
(65.9) using OSEM reconstruction (P50.891). The
values for OSEM reconstruction were significantly higher
than the values for FBP reconstruction in both the x-axis
(P50.04) and the y-axis (P,0.001). However, the lo-
cation of the defects was not similar in x-axis and y-axis
shifting (Table 3). With x-axis shifting, apicoseptal defects
were seen more frequently, whereas with y-axis shifting,
apicolateral and midinferior defects were more commonly
seen.
Figure 3 shows the severity and extent of defects
(depicted as mean summed score) in relation to the frame
of shifting. With a returning shift, the mean summed score
was lower for early-phase shifting (frames 729) than for
middle-phase shifting (frames 16218) with both recon-
struction types (P,0.05). Similarly, the mean summed
score was lower for late-phase shifting than for middle-
phase shifting with FBP reconstruction (P50.02), but
FIGURE 1. Schematic display of differ-
ent kinds of motion used for simulation:
early-phase returning shift (A), middle-
phase returning shift (B), late-phase
returning shift (C), early-phase nonreturn-
ing shift (D), middle-phase nonreturning
shift (E), and late-phase nonreturning
shift (F).
EFFECT OF RECONSTRUCTION ON MOTION • Zakavi et al. 221
differences in mean summed score between these phases
did not reach statistical significance with OSEM recon-
struction (P50.07). Also there was no significant differ-
ence in mean summed score between early and late shifting
with either reconstruction method (P.0.22).
With a nonreturning shift, the mean summed score was
greater for shifting on middle- or late-phase frames than on
early frames (Fig. 3), with both OSEM reconstruction and
FBP reconstruction (P,0.001). Also, the mean summed
score was greater for middle-phase shifting than for late-
phase shifting with FBP reconstruction (P50.004), but
differences in mean summed score between these phases
did not reach statistical significance with OSEM recon-
struction (P50.39).
DISCUSSION
Patient motion commonly degrades SPECT myocardial
imaging. Because of arthritis, weakness, and postexercise
fatigue, patients usually have difficulty in hyperextending
their left arm and remaining still for about 20 min, espe-
cially when imaged by a single-head g-camera (10). Al-
though previous studies noted that the tolerance of
tomographic imaging to patient motion may be dependent
on image acquisition and processing methods, only FBP
reconstruction was used in those studies (4–6,10,11).
We studied normal, motion-free images and evaluated
the pattern and severity of motion artifacts using the FBP
and OSEM reconstruction methods.
We simulated patient motion by shifting planar images
by at least 3 pixels on 3 consecutive frames. Our pilot study
(12) and other studies showed that returning movements of
fewer than 2 pixels may not produce significant perfusion
defects (4,6,10,13). We also used at least 3 frames of
shifting in the returning type of shift because it has been
shown that even 20 pixels of movement on only 1 frame
may not result in any defect (5).
This study showed that the number of images with def-
initely abnormal findings was slightly greater with OSEM
reconstruction than with FBP reconstruction although not
statistically significant (P.0.5).
In addition, the summed mean score was larger with
OSEM reconstruction than with FBP reconstruction, but the
location of defects was not significantly different between
the 2 reconstruction techniques. This finding suggests larger
or more severe defects on images with OSEM reconstruction
than on images with FBP reconstruction.
TABLE 1
Qualitative Scan Interpretation
Reconstruction
method
All
images
Returning
shift
Nonreturning
shift
FBP 68.1% (49/72) 44.4% (16/36) 91.7% (33/36)
OSEM 70.8% (51/72) 50% (18/36) 91.7% (33/36)
Data are percentages and numbers of abnormal images. Mini-
mum Pvalue was more than 0.5 for all comparisons between
reconstruction techniques.
FIGURE 2. Mean summed score in returning and nonreturning
shifts by 2 different reconstruction techniques. Statistical
comparison was done between reconstruction techniques in
each group.
TABLE 2
Number of Defects in 72 Shifted Images
Reconstruction segment FBP OSEM P
Apex 23 21 0.62
Anteroapical 41 43 0.62
Apicolateral 5 5 1
Inferoapical 34 34 1
Apicoseptal 10 12 0.68
Mid anterior 26 28 0.72
Mid anteroseptal 23 29 0.14
Mid anterolateral 5 7 0.62
Mid inferoseptal 10 13 0.25
Mid inferolateral 8 7 1
Mid inferior 21 24 0.37
TABLE 3
Percentage of Defects in Each Segment
Motion axis segment xyP*
Apex 38.9 19.4 0.12
Anteroapical 69.4 50 0.15
Apicolateral 30.6 63.9 0.009
Inferoapical 8.3 5.6 1
Apicoseptal 30.6 2.8 0.003
Mid anterior 36.1 41.7 0.81
Mid anteroseptal 38.9 41.7 1
Mid anterolateral 11.1 8.3 1
Mid inferoseptal 22.2 13.9 0.54
Mid inferolateral 8.3 11.1 1
Mid inferior 11.1 57.1 0.001
*Fisher exact test was used for comparisons.
222 JOURNAL OF NUCLEAR MEDICINE TECHNOLOGY • Vol. 34 • No. 4 • December 2006
Matsumoto et al. (6) showed that a nonreturning shift
produced more defects than did a returning shift. Similarly,
our study confirmed their findings (Fig. 2) and showed that
defects are more severe with OSEM reconstruction for a
returning shift. For a nonreturning shift, the mean summed
score was slightly higher with OSEM reconstruction than
with FBP reconstruction but did not reach statistical sig-
nificance. This finding suggests that although the difference
between these 2 reconstruction methods is significant for
mild defects, the difference is not significant for severe
defects, which are seen easily with both methods.
Like previous studies (5,10), our study showed that the
location of defects may differ according to the axis of
motion (Table 3), but the severity and extent of defects
were similar in both axes.
Although some researchers (6) have found that, with
single-head g-cameras, movement in the early phase of a
study produces a greater number of false-positive defects
than does movement in the middle or late phases, Cooper
et al. (10) found that middle-phase movement was more
significant than movement in the early or late phases. Our
study showed that, with either type of reconstruction,
movement in the middle phase of imaging produced more
artifacts than did movement in the early phase of imaging.
This finding is not surprising because when motion occurs
toward the middle of the study, the images become more
evenly split between projections of 2 different distributions
of radioactivity, one before the motion and one after the
motion.
In a recent work, Hatton et al. (9) showed that in cases of
missing data (using a dual-head g-camera), OSEM recon-
struction demonstrates less severe artifacts than does FBP
reconstruction and can tolerate at least 4 missing projec-
tions with no apparent effect on clinical interpretation.
However, our study showed that motion artifacts are similar
in location with both reconstruction types and that the
extent and severity of artifacts (as depicted by summed
scores) are significantly larger in OSEM reconstruction.
Because missing data are fundamentally different from
motion, which can be considered misregistered data, we do
not expect to find similar results.
CONCLUSION
Motion artifacts do not differ in location between the
2 types of reconstructions (OSEM vs. FBP), but summed
score is larger in OSEM reconstruction than in FBP re-
construction, suggesting larger or more severe defects using
OSEM technique. OSEM reconstruction is not more toler-
ant than FBP reconstruction to motion artifacts.
ACKNOWLEDGMENTS
We thank the research vice chancellor of the Mashad
University of Medical Sciences for financial support of this
research. We appreciate Hadi Jabbari’s assistance in pre-
paring the manuscript.
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FIGURE 3. Mean summed score in shifted images compared
in early, middle, and late phases of acquisition with respect to
reconstruction technique and type of shifting. R 5returning
shift; NR 5nonreturning shift.
EFFECT OF RECONSTRUCTION ON MOTION • Zakavi et al. 223
