Engineering, 2013, 5, 114-117
http://dx.doi.org/10.4236/eng.2013.510B023 Published Online October 2013 (http://www.scirp.org/journal/eng)
A New Approach t o the Presentation of Myocardial
SPECT Images
—Radial Slices—Data Reduction without Loss of Information
Niloufar Darv ish1, Fatma Nadide Öçba1, Hamed Hamid Muhammed1*, Dianna Bone2
1School of Technology and Health STH, Royal Institute of Technology KTH, Alfred Nobels Allé 10,
SE-14152 Huddinge, Sweden
2Department of Clinical Physiology, Thoracic Clinics, Karolinska Hospital,
SE-17671 Stockholm, Sweden
Received December 2012
ABSTRACT
Objective: SPECT data from myocardial perfusion imaging (MPI) are normally displayed as a set of three slices or-
thogonal to the left ventricular (LV) long axis. For data presentation, the images are orientated about the LV long axis.
Therefore, radial slices provide a suitable alternative to standard orthogonal slices, with the advantage of requiring few-
er slices to adequately represent the data.In this study, a semi-automatic method is developed for displaying MPI
SPECT data as a set of radial slices orientated about the LV axis. The aim is to reduce the number of slices viewed
without loss of information and independently from the heart size. Method: Standard short axis slices, orientated per-
pendicular to the LV axis, are utilized. The skeleton of the segmented myocardium is found and the true LV axis is de-
termined in each central long slice. The LV axis of the whole volume is determined by aligning the axes of all
slices. Result: Radial slices centered about this axis were generated by integration over a sector equal to the resolution
of the imaging system which was of the order of 1.2 cm. Therefore, assuming a mean LV diameter of 8 cm, 20 slices
were sufficient to represent a non-gated study. Gated information could be adequately displayed with 4 slices integrated
over an angle of 45˚. Conclusion:
Keywords: Myocardial Perfusion SPECT; Cardiac Left Ventricle; Radial Slices; Left Ventricular Long Axis
1. Introduction
Myocardial perfusion imaging (MPI) is a non-invasive
nuclear medicine technique used to study blood flow in
the left ventricular (LV) heart muscle. Images are ac-
quired using a gamma camera system following the in-
jection of a radiopharmaceutical during a cardiac stress
test [1]. At a subsequent examination, an injection is
given when the heart is in a resting state. Images from
the stress and rest examinations are then compared to
identify differences that may indicate ischemia, or other
abnormalities [1]. The method of choice for acquiring the
myocardial data is single photon emission tomography
[2].
A semi-automatic method for generating radial slices from SPECT MPI short axis
slices has been developed.
SPECT performed without any reference to heart
rhythm (MSPECT) provides the best quality images for
determining perfusion. Additional useful information
about LV function can be obtained with gated-SPECT
(GSPECT) studies, when the patient’s ECG is used to
control the acquisition [3]. In modern gamma camera
systems MSPECT and GSPECT projections can be ac-
quired simultaneously. Sets of slices orientated abou t the
long axis of the LV are reconstructed from projection
images. The standard slices presented for interpretation
are horizontal long axis (HLA), ver tical long axis (VLA)
and short axis (SA) (Figure 1).
Depending on the size of the LV, more than 30 slices
from each acquisition are needed to represent the heart
muscle, thus a considerable number of images must be
compared when making a diagnosis. To assist in this
problem, the polar presentation (bull’s eye) has been de-
veloped [4], but the display results in loss of information
and cannot completely replace the standard sets of slices.
An alternative approach would be to reconstruct radial
slices. Radial slices are orientated p arallel to the LV long
axis and arranged diametrically.
The central HLA and VLA slices are examples of
radial slices at 0˚ and 90˚ respectively. A set of radial
slices would, therefore, include the central HLA and
*Corresponding a uthor.

N. DARVISH ET AL.
115
Figure 1. Example of the standard slices displayed for MPI.
From left to right: Short axis, horizontal long axis and ver-
tical long axis [3].
VLA slices together with diametrical slices at other an-
gles through 180˚. The number of radial slices required
to display the LV without loss of information should be
less than in the standard three view presentation and also
be independent of LV size. The aim of this pilot study
was to develop software for generating radial slices and
to assess the number of images required to present the
LV without loss of information.
2. Materials and Methods
The input volume data was a stack of SA slices (Figu re
2a) from an MPI patient study. SA slices were orientated
perpendicular to the LV axis chosen at the time of recon-
struction and not necessarily the true LV axis. The true
LV long axis was defined as a line passing through the
apex and the center of the cavity in two central and or-
thogonal long axis images. To determine the true LV
long axis, the central HLA (Figure 2b) and VLA slices
were chosen by visually identifying the slice from each
set in which the LV had the largest dimensions. Both
slices were then segmented using an adaptive threshold-
ing algorithm [5] to separate the background from the
LVmyocardium. The threshold was adjusted manually.
As a result of segmentation, images were converted
into binary images where pixels with a value greater than
or equal to the threshold were set to one, while values
less than the threshold were set to zero. The skeleton of
the binary image, representing the medial axis of the LV
wall, was traced automatically using a morphological
method (Figure 3). The skeleton was the pixels remain-
ing when pixels on the boundaries of an object have been
removed wi thout the object breakin g up .
The apex of the slice was determined by identifying
the coordinates of the point on the skeleton indicating a
change in direction of the skeletal line, assuming a para-
bola. The central axis was defined as the mean of the X
co-ordinates for points with the same Y coordinate, but
Figure 2. Reconstructed slices from a patient study. (a):
Central SA slice from input volume. (b): Central HLA slice
used to determin e t ru e L V axis.
Figure 3. Stages for generating the skeleton by using the
morphological method on HLA slice. (a) Segmented slice; (b)
Skeleton of the slice; (c) Skeleton on binary image of the
slice.
from opposite si de s of the skeleton (Equation (1)).
( )()
( ) ( )
X1=x1,y, X2x2,y
Xmeanx1x2/2, y
=
⇒=+
(1)
The true long axis was found by performing a linear
least squares fit to these points. And the tilt angle was
then the slope of the line.
The 3D matrix was aligned about the true LV axis by
rotating the HLA slices followed by the rotation of VLA
slices with the respective tilt angles. Finally, the radial
slices centered about the new axis were generated by
integrating over a sector determined by the system reso-
lution. Programs used for calculations and presentations
were developed in MatLab [6].
3. Results
The initial SA slices were reconstructed from a SPECT
rest acquisition of a normal heart. The central HLA and
VLA slices chosen by visual inspection were the slices
through the centre of the LV cavity. The skeleton for
each slice was generated from the binary image after
applying the segmentation method. The resulting skele-
tons for the respective HLA and VLA slices are shown in
Figures 4(a) and ( c) and superimpo sed on the myocardi-
al activity in Figures 4(b) and (d).
The shape of the resolution had been better and the
shape of the LV skeleton was approximately parabolic. If
the image myocardium a true parabola, the apex would
have been the point at which the direction of the skeletal
line changed. This was not the case as can be seen in
Figure 4. The position of the apex was defined, therefore,
as the location in the middle of the angle curve at which

N. DARVISH ET AL.
116
(a) (b)
(c) (d)
Figure 4. Skeletel curve generated for two long axis slices (a):
HLA, (c): VLA and superimposed on the respective slices (b)
& (d).
there was no change in the direction between two neigh-
boring points. The location of the apex was, therefore,
the mean of the X co-ordinates of two neighboring points
with t he same maximu m Y co-ordinate (Ymax) .
In the VLA image there were more than two points
with the same value as Ymax due to the presence of the
right ventricular wall. To avoid incorrect positioning of
the apex, extraneous points were eliminated by calculat-
ing the difference in X between points with the same
co-ordinates as Ymax. A point was rejected if the differ-
ence between adjacent points was >2. Points along the
axis were calculated as described by Equation (1), and
then a linear least square fit was applied to find the true
LV axis. The calculated points and the fitted lines are
shown in Figure 5.
The tilt angle for the HLA stack was 6.1˚ with respe ct
to the Y-axis, while for the VLA stack the angle was 6.9˚
in the same direction. Radial slices generated for presen-
tation were integrated over a sector angle of 18˚, based
on a system resolution for tomographic studies of ap-
proximately 1.2 cm. Thus a maximum of 20 radial slices
was sufficient to represent a MSPECT study.
Gated information could be adequately displayed with
four slices at 90˚ intervals integrated over an angle of 30˚,
equivalent to summing 5 standard slices. Fig ure 6 shows
the diastolic and systolic radial slices generated about 0˚
from a GSPECT study, with the standard HLA slice for
comparison.
4. Discussion and Limitations
This pilot study investigated the feasibility of using radial
slices for interpretation of data from MPI SPECT instead
of the three sets orthogonal slices normally used. The
Figure 5. GSPECT radial slices. Images (a) and (b): radial
slices summe d over 30° about 0° (HLA). Images (c ) and (d):
Standard HLA slices for comparison.
Figure 6. The fitted line as true Horizontal (a) and Vertical
(b) long axes.
morphological method employed to define the LV myo-
cardium and the true long ax is resulted in a good corre la-
tion to a straight line through the center of the LV cavity.
Radial slices from MSPECT were generated with the
same voxel size as standard orthogonal slices, whereas
GSPECT slices were integrated over 30˚ to improve the
signal to noise ratio. It was found that 20 radial slices
were sufficient to represent the LV myocardium perfu-
sion distribution without loss of resolution or information
for MSPECT. For GSPECT where wall motion is of
primary interest, four radial slices at 45˚ intervals were
sufficient.
Although the method has only been tested on normal
data, the morphological method can be combined with a
curve fitting method such as the Hough transform when
there is of a reduction in or absence of perfusion in the
LV wall.
Radial slices offer other advantages, apart from a re-
duction in the number of slices that must be assessed.
The number of slices required is independent of LV size
unlike in the standard orthogonal presentation, particu-
larly the SA orientation. Furthermore, radial slices can be
summed without significantly compromising the appear-
ance of the myocardium as can be seen in Figure 5.
Another advantage is that it is easier to assess the extent
and location of regions of reduced perfusion in radial
slices.
Diastole Systole
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dc
Diastole Systole
Diastole Systole
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dc
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dc

N. DARVISH ET AL.
117
At present, there are some limitation s with the method
used to generate radial slices. The initial central slice is
identified manually which can be a source of error, par-
ticularly when comparing two studies and the method is
not an integrated part of the standard SPECT processing
routine. These are, however, problems associated with
the pilot study rather than method itself and can be elim-
inated wi th furt he r development.
5. Conclusions
A semi-automatic method for generating radial slices
from standard SPECT MPI short axis slices has been
developed. When the radial slices were integrated over a
sector equivalent to the imaging resolution, MSPECT
data could be represented by 20 slices. GSPECT infor-
mation could be represented adequately with 4 slices
spaced at 45˚ intervals.
Radial slices provide an alternative display method to
standard orthogonal slices, with the advantage of requir-
ing fewer slices to adequately represent the data without
loss of information. Furthermore, the number of slices
required is independent of heart size.
6. Acknowledgements
We would like to thank all the staff working in the De-
partment of Clinical Physiology, Karolinska University
Hospital for providing the patient data used in this study
and also an excellent working environment.
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