Ann Nucl Med (2014) 28:484–497
DOI 10.1007/s12149-014-0828-7

TECHNICAL NOTE

Performance evaluation of the eXplore speCZT preclinicalimaging system
Ichiro Matsunari • Yoshiharu Miyazaki • Masato Kobayashi • Kodai Nishi •Asuka Mizutani • Keiichi Kawai • Akiko Hayashi • Ryoko Komatsu •Shoko Yonezawa • Seigo Kinuya

Received: 31 March 2013 / Accepted: 19 February 2014 / Published online: 8 March 2014
Ó The Japanese Society of Nuclear Medicine 2014

Abstract
Objective The eXplore speCZT is a recently introduced
cadmium zinc telluride-based preclinical SPECT systemthat has a stationary detector design with interchangeablerotating collimators. Our aim was to evaluate the performance of the eXplore speCZT using 99mTc-sources. In
particular, the image quality was assessed using the
National Electrical Manufacturers Association NU-4 image
quality phantom as well as an in vivo mouse.
Methods Energy resolution, sensitivity and spatial resolution were measured using 99mTc sources. Image quality
was assessed using NU-4 image quality phantom. The
measurements were performed for 4 available collimators:(1) mouse 7-pinhole collimator (mouse PH); (2) mouse
8-slit collimator (mouse SL); (3) rat 5-pinhole collimator(rat PH); and (4) rat 5-slit collimator (rat SL). Furthermore,a mouse bone imaging study was performed using mouse
PH and mouse SL.
Results The system achieved the energy resolution of
5.5 % in full-width at half maximum (FWHM) at 140 keVusing a 99mTc source. Without resolution recovery function, the system provided a near millimeter transaxial andaxial spatial resolution using mouse PH. Mouse SL and rat
SL provided reasonably good transaxial (1.79–2.00 mm in
FWHM), but much worse axial resolutions (4.55–4.96 mm
in FWHM). The use of resolution recovery significantly
improved spatial resolution by in average 31 ± 3 or
35 ± 4 % in FWHM or full-width at tenth maximum,
respectively. In particular, a sub-millimeter resolution of
0.71 mm in FWHM was achieved in either transaxial oraxial direction with mouse PH. Using NU-4 phantom, theuniformity of slit collimators as expressed as percentage
standard deviation was generally better than that of pinhole
collimators. The use of resolution recovery substantially
improved uniformity for all the collimators tested, butcaused some overestimation in recovery coefficient.
Reconstruction settings such as iteration or subset number
significantly affected image quality measures. Finally,bone images of acceptable quality were obtained in in vivo
mouse using mouse PH with resolution recovery.
Conclusions The overall performance shows that theeXplore speCZT system is suitable for preclinical imagingbased research using small-animals.
Keywords

I. Matsunari (&)  Y. Miyazaki  A. Hayashi  R. Komatsu 
S. Yonezawa
Clinical Research Department, The Medical andPharmacological Research Center Foundation, Wo 32, Inoyama,
Hakui, Ishikawa 925-0613, Japane-mail: matsunari@mprcf.or.jp
M. Kobayashi  K. Nishi  A. Mizutani  K. Kawai
School of Health Sciences, College of Medical, Pharmaceuticaland Health Sciences, Kanazawa University, Kanazawa, Japan
S. Kinuya
Department of Nuclear Medicine, Kanazawa University
Hospital, Kanazawa, Japan

123

Preclinical  Cadmium zinc telluride  SPECT

Introduction
Preclinical molecular imaging such as SPECT and PET
using small-animals is increasingly recognized as animportant tool for biomedical research and new drug
development [1–3]. In particular, SPECT equipped withmulti-pinhole collimators enables small-animal imaging
with high spatial resolution and reasonable sensitivity [1].
There are several such systems that are commerciallyavailable for current use [4–8], most of which use NaI-Ann Nucl Med (2014) 28:484–497

based detectors. Alternatively, some newer detectors
developed for small-animal SPECT incorporate semiconductor materials such as cadmium zinc telluride (CZT) [9],which directly convert gamma-rays to electric signal. As
compared to conventional NaI-based detectors, CZT is
attractive because it can be segmented to provide a highintrinsic spatial resolution [9]. Furthermore, high-energy
resolution of CZT is beneficial for high contrast imaging by
reducing scatter and multi-isotope imaging [1]. More
importantly, the small size of CZT detectors enables
compact designs [9], by which it is possible to surround thesubject by an array of detectors. Such CZT-based cameras
are now commercially available for clinical cardiac
SPECT, resulting in improved image quality and shorter
acquisition time [10–12].
The eXplore speCZT (GE Healthcare, Milwaukee, WI,USA) is a recently introduced commercially available
preclinical SPECT system using a full ring 10 CZT-based
detectors equipped with interchangeable collimators. It canbe operated as a stand-alone SPECT system or can be
combined with micro-CT. To date, however, there are nodata available in literature that systematically assessed theimaging performance of the speCZT. More importantly,
image quality of CZT-based small-animal SPECT has notbeen addressed in a standardized manner such as that using
the National Electrical Manufacturers Association(NEMA) NU-4 image quality phantom [13].
The aim of this study was to evaluate the performance of
CZT-based preclinical imaging system (eXplore speCZT)
in view of energy resolution, sensitivity, spatial resolution,and image quality using 99mTc-sources. In particular, theimage quality was assessed using NU-4 image quality
phantom as well as an in vivo mouse.

Methods

485
Table 1 Features of the speCZT System
Detector configurations
Material

Cadmium–zinctelluride

Detector length, width, and thickness (mm)80, 80, 5

Type and number of detectors

Fixed-ring, 10

Number of arrays (size)

32 9 32 (2.46 mm)

Energy range

20–250 keV

Data collection

List-mode

ECG and respiratory gating capability

Yes

Collimator configurations
Material

Tungsten

Mouse PH
Number of pinholes

7

Diameter of pinhole (mm)Single-bed position FOV diameter (mm)132Single-bed position FOV axial (mm)25Bore diameter (mm)54

Radius of rotation/focal length (mm)32.5/87.5

Rat PH
Number of pinholes

5

Diameter of pinhole (mm)1Single-bed position FOV diameter (mm)76Single-bed position FOV axial (mm)38

Bore diameter (mm)89Radius of rotation/focal length (mm)50/70

Mouse SL
Number of slits

8

Width of slit (mm)1.23Single-bed position FOV diameter (mm)32Single-bed position FOV axial (mm)Bore diameter (mm)8064Radius of rotation/focal length (mm)37.5/82.5

Rat SL

System description
Features of the speCZT system are summarized in Table 1.
This system has a stationary detector design with interchangeable rotating collimators. As illustrated in Fig. 1, thedetector consists of 10 CZT-based detector panels surrounding the field of view (FOV). Each CZT detector panel
consists of 32 9 32 arrays with the pixel size of
2.46 mm 9 2.46 mm. There are 4 collimators provided by
the manufacturer (Fig. 1): (1) mouse 7-pinhole collimator(mouse PH); (2) mouse 8-slit collimator (mouse SL); (3)rat 5-pinhole collimator (rat PH); and (4) rat 5-slit collimator (rat SL). The transaxial and axial FOVs varydepending on collimator geometry. The pinhole collimators
are designed for high resolution images with small regionsof interest, whereas slit collimators are designed for larger

Number of slits

5

Width of slit (mm)1.23Single-bed position FOV diameter (mm)76Single-bed position FOV axial (mm)80Bore diameter (mm)89Radius of rotation/focal length (mm)50/70

Mouse PH mouse 7-pinhole collimator, Rat PH rat 5-pinhole collimator, Mouse SL mouse 8-slit collimator, Rat SL rat 5-slit collimator,
FOV field of view

FOV (e.g., whole-body imaging) with reasonable spatial
resolution [14, 15]. Although the axial FOVs of pinhole
collimators are small, these can be set to a larger value(B250 mm) in the helical acquisition mode with bed
motion. The system saves each event of the acquired data
in a list-mode format including its energy, and gated

123

486
Fig. 1 Cadmium zinc telluridedetectors built in the system(upper) and collimators (lower).
Mouse PH indicates mouse
7-pinhole collimator; Rat PH rat
5-pinhole collimator, Mouse SL
mouse 8-slit collimator; Rat SL
rat 5-slit collimator. Star
denotes pinhole; arrow slit

Ann Nucl Med (2014) 28:484–497

Cadmium-Zinc Telluride
Detectors

Mouse PH

electrocardiogram (ECG)/respiratory motion trigger, forlater specialized analysis. Images can be reconstructed by
the three-dimensional maximum-likelihood expectation
maximization (MLEM) or ordered subset expectation
maximization (OSEM) method.
Energy resolution
Energy spectra within an energy window of 50–200 keVwere obtained using a 99mTc point source, which was
located in the center of FOV, and mouse PH. Energy resolution was measured based on the system as a whole, andwas expressed as the full-width at half maximum (FWHM)
as percentage of 140 keV energy peak, which was determined by direct measurement of histogram values.
Reconstruction settings
Unless specified, all the images were reconstructed using
MLEM with 50 iterations with and without resolution
recovery function. The number of iterations (n = 50) was
tentatively determined in this study after testing several
iteration numbers by visual inspection. The resolution
recovery feature in this system was based on the modelingof pinholes and slits [16], including the modeling of geometrical structure, the material and related photon physics
of the collimators. The energy window was set to125–150 keV. Because the image matrix voxel size had to
be varied according to collimator choice and imaging

123

Rat PH

Mouse SLRat SLprocedure, the voxel size used for each measurement was
detailed in the following sections. No post-reconstruction
filtering was applied. Neither attenuation nor scatter correction was performed.
To assess the effects of reconstruction settings on image
quality measures, we tested the effects of iteration numbers(n = 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, and 200) for
MLEM on image quality using NU-4 phantom. These
measurements were made for all types of collimators.
Images were reconstructed with and without resolution
recovery function.
Sensitivity and spatial resolution measurements
System sensitivity was measured for each collimator with a
99m
Tc point source that was mounted on the imaging bed in
the center of transaxial and axial FOVs. The activity of thepoint source was 3.5 MBq. The energy window was set to125–150 keV. Projection data were acquired in step-andshoot mode over 360° in 3° increment at 60 s/projection.
The photon counts were obtained from projection images.
System sensitivity was defined as the recorded counts per
second divided by the decay-corrected activity of the point
source.
Spatial resolution was measured for each collimatorusing a glass capillary line source. A glass capillary of
inner diameter 0.14 mm (outer diameter 0.55 mm) was
filled with 370 MBq/mL of 99mTc-solution and was placedin the center of the transaxial FOV and was aligned axially.

Ann Nucl Med (2014) 28:484–497

For axial resolution measurements, the glass capillary wasplaced horizontally in the center of the axial FOV. Projection data were acquired in step-and-shoot mode over
360° in 1.06° increment for mouse PH, which was the
manufactural standard setting, or 1° for the other 3 collimators at 30 s/projection. The image matrix voxel sizes fortransaxial spatial resolution measurements were 0.2 mm 90.2 mm 9 0.2 mm for mouse PH, 0.2 mm 9 0.2 mm 92.46 mm for mouse SL, 0.4 mm 9 0.4 mm 9 0.4 mm for
rat PH, or 0.4 mm 9 0.4 mm 9 2.46 mm for rat SL,
because reconstructed z-axis pixel size with slit collimators
was always fixed to 2.46 mm, which was equal to thedetector pixel size. For axial resolution measurements, wechanged z-axis pixel size to 0.2 mm for mouse SL or
0.4 mm for rat SL using trilinear interpolation function in
PMOD 2.7 (PMOD technologies Ltd., Zurich, Switzerland). Count profiles of the reconstructed transaxial images
across the voxel having the maximum voxel intensity wereplotted in the horizontal and vertical directions. The
FWHM and full-width at tenth maximum (FWTM) werethen determined by Gaussian fitting of the line profile. The
averages of the horizontal and vertical directions were
calculated over ten transaxial slices. FWHM and FWTM
values of the axial profiles were obtained in the same way
as the coronal images of the rotated source. The results ofresolution measurements were not corrected for the source
dimensions.
Image quality assessment
Image quality assessment was performed using the NU-4
image quality phantom [17], which had originally beendeveloped for evaluating animal PET system and subsequently applied to animal SPECT system [5, 13]. The
phantom has internal dimensions of 50-mm length and
30-mm diameter. The phantom consists of three compartments: a fillable uniform region, five fillable rods of different diameters (1, 2, 3, 4, and 5 mm) and two nonradioactive chambers filled with water or air (15-mm
length; outer diameter of 10 mm; and wall thickness of1 mm). The phantom was filled with 50 MBq of 99mTc
solution and was scanned for 30 min using each collimator;
350 views from 7 pinholes (50 views/pinhole) at 36 s/view
in 1.06 degree increment for mouse PH, 360 views from 8
slits (45 views/slit) at 40 s/view in 1 degree increment formouse SL, 360 views from 5 pinholes (72 views/pinhole)
25 s/view in 1 degree increment for rat PH, and 360 views
from 5 slits (72 views/slit) 25 s/view in 1° increment for rat
SL. These acquisition settings were determined so thatprojection data cover over 360° with 1 or near 1° increment, and with comparable acquisition time among thecollimators tested. For the mouse PH, where the axial FOVof 25 mm is smaller than the inner length (50 mm) of the487

NU-4 phantom, the acquisition was performed twice tocover the entire phantom. Images were reconstructed withthe voxel size set to 0.5 mm 9 0.5 mm 9 0.5 mm formouse PH or rat PH and 0.5 mm 9 0.5 mm 9 2.46 mm
for mouse SL or rat SL. In addition, we performed acquisition of the NU-4 image quality phantom using mouse PH
and helical scan mode. The acquisition parameters to cover
the entire phantom were 165 steps at 11 s/step in 1°
increment, resulting in a total acquisition time of 30 min.
The image matrix voxel size was 0.33 mm 9 0.33 mm 90.33 mm. This was because the voxel size of helical
SPECT using mouse PH was always fixed to 0.33 mm,
which was automatically determined by table motion stepsize.
In this study, we performed the NU-4 phantom analyses based on NEMA recommendations for performance
evaluation of small-animal PET [18]. A 22.5-mm diameter and 10-mm (or 2.46 mm 9 4 mm for slit collimators)
depth cylindrical volume of interest (VOI) was drawn
over the center of the uniform region of the phantom. The
percentage standard deviations (%STD) in the VOI werereported as a measure of uniformity. For mouse PH,
where axial FOV was smaller than the length of phantom,%STDs from the 2 acquisitions, which covered the upperand lower portion of the uniform region (5-mm depth foreach portion) were averaged. Spill-over ratios (SORwat
and SORair) were calculated by drawing VOI’s ofdiameter 4-mm and 7.5-mm (or 2.46 mm 9 3 mm for slit
collimators) length on the water- and air-filled cylindricalchambers. Then, the ratio of the mean counts in each cold
region to the mean of the hot uniform area was reported
as SOR.
To obtain the recovery coefficients (RC) for the filledrods, the reconstructed image slices covering the central
10-mm (or 2.46 mm 9 4 mm for slit collimators) length of
the rods were averaged to obtain a single slice. Circularregions of interest (ROIs) were drawn on this image,around each rod and background, with diameters twice thephysical diameter of the rods. The background ROIs wereplaced outside the rod ROIs, where no activity was seen byvisual inspection, and were processed in a manner that wasused for the rod ROIs. The maximum values in each ofthese ROIs were measured on the averaged image. Then,the transverse image pixel coordinates of the locations withthe maximum ROI values were transferred to the original
unaveraged slices and were used to create line profiles
along the rods in the axial direction. The mean and standard deviation of the pixel values measured along each rod
or background line profile, based on the locations withmaximum pixel value on the averaged image, were calculated. Then, the RC for the filled rods were obtained as
the mean values in each rod line profile divided by themean value obtained in the uniformity test described above

123

488

Ann Nucl Med (2014) 28:484–497[17]. The standard deviation of the RC (STDRC) was calculated as follows [18]:Results
Energy resolution

ð1Þ

In Eq. (1), SQRT denotes square root, STDinterprofile andaverageinterprofile are the standard deviation and average of arod line profile, and STDbackground and averagebackground are
the standard deviation and average of background.
To assess the effects of reconstruction settings on RC,
we newly introduced an index of error in RC using a formula as follows:
Index of error in RC ¼ SQRTf ð1  RCrod1 Þ2 þ ð1
 RCrod2 Þ2 þ ð1  RCrod3 Þ2 þ ð1
 RCrod4 Þ2 þ ð1  RCrod5 Þ2 :
ð2ÞIn Eq. (2), SQRT denotes square root, RCrod1 is RC in
1-mm diameter rod, RCrod2 is RC in 2-mm diameter rod,
RCrod3 is RC in 3-mm diameter rod, RCrod4 is RC in
4-mm diameter rod and RCrod5 is RC in 5-mm diameterrod.
The idea behind this index is that (1) in ideal situation,
the RC is expected to be 1 irrespective of rod diameter; (2)therefore, the sum of absolute differences between ideal(RC = 1) and measured RC for each rod should provide anestimate of error in measured RC that reflects errors in allrods of NU-4 phantom. Such an index would be useful todetermine optimal reconstruction parameters based on RC
values.
Animal imaging

As illustrated in Fig. 2, the system achieved the energy
resolution of 5.5 % in FWHM at 140 keV using a 99mTc
source.
Sensitivity and spatial resolution
The results of sensitivity and spatial resolution measurements are summarized in Table 2. The highest sensitivity
was achieved with mouse SL, followed by mouse PH, rat
SL, and rat PH. Without resolution recovery function
during reconstruction, the system provided a near millimeter transaxial and axial spatial resolution using
mouse PH. Mouse SL and rat SL provided reasonably
good transaxial (1.79–2.00 mm in FWHM), but muchworse axial resolutions (4.55–4.96 mm in FWHM). The
use of resolution recovery significantly improved spatial resolution by in average 31 ± 3 or 35 ± 4 %
20000

16000

Count

STDRC ¼ SQRTfðSTDinterprofile =averageinterprofile Þ2þ ðSTDbackground =averagebackground Þ2 g:

12000

8000

4000

0
50

To assess the overall system performance, the imaging
capabilities of the scanner were assessed through additionalin vivo imaging studies. A normal mouse (SAMR1, 28.8 g,
Nihon SLC Co, Ltd, Hamamatsu, Japan) was injected with
40 MBq of 99mTc-methylene diphosphonate (99mTc-MDP)
via tail vein, and anesthetized with 1.0–2.0 % isoflurane.
At 2 h after injection, the mouse was imaged for 32 minusing the mouse PH. Immediately after the first scan, thesecond scan was performed for another 32 min using themouse SL. For imaging with pinhole collimator, the helicalscan mode was used to cover the body of mouse. The
image matrix voxel size was 0.33 mm 9 0.33 mm 90.33 mm for mouse PH, or 0.33 mm 9 0.33 mm 92.46 mm for mouse SL. All experiments were performed
with the approval of the institutional committee for animal
research in compliance with the position of the American
Heart Association on use of research animals.

123

100

150

200

Energy (KeV)
Fig. 2 Energy spectrum obtained from a 99mTc source

Table 2 Sensitivity and spatial resolution of the speCZT using eachcollimator

Sensitivity(cps/MBq)

Mouse PH

Rat PH

Mouse SLRat SL

233.1

138.5

312.8

151.5

Spatial resolution in FWHM/FWTM (mm)Transaxial RR-on
Axial RR-on

0.71/1.28
0.71/1.30

1.20/2.18
1.11/2.02

1.26/2.29
3.13/5.70

1.47/2.68
3.14/5.72

Transaxial RR-off

1.11/2.02

1.81/3.29

1.79/3.27

2.00/3.64

Axial RR-off

1.04/1.90

1.83/3.35

4.55/8.30

4.96/9.03

FWHM full-width half maximum, FWTM full-width tenth maximum,
RR resolution recovery filter

Ann Nucl Med (2014) 28:484–497

489(average ± standard deviation) in FWHM or FWTM,
respectively. In particular, a sub-millimeter resolution of
0.71 mm in FWHM was achieved in either transaxial oraxial direction with mouse PH.

with resolution recovery, whereas such an edge enhancement was not seen without resolution recovery. Line profiles of uniform region essentially confirmed theobservation described above.

Image quality assessment using NU-4 phantom

Effects of reconstruction settings on image quality
measures

The %STD of uniform region and SORs in the water andair compartments for each collimator are summarized inTable 3. The uniformity of slit collimators as expressed as%STD was generally better than that of pinhole collimators. The use of resolution recovery substantially improved
uniformity in all collimators. The %STD values for thehelical scan were higher than those for the single circularorbit scan using mouse PH. The SORs in water wereslightly (2–30 %) higher than those in air for all collimators. The use of resolution recovery slightly lessened the
SORs for each collimator. RCs and corresponding STDRC
as a function of rod diameter are shown in Fig. 3a, b,
respectively. Without resolution recovery, the RC generally increased as a function of rod diameter, although itreached plateau at 4 mm with mouse PH or mouse SL.
When resolution recovery function was used, the RC
peaked at 2- to 3-mm rod diameter with values exceeding
1, and showed a subsequent decline. The STDsRC weregenerally larger with pinhole collimators than those withslit collimators. The use of resolution recovery lessened the
STDRC with pinhole collimators, but its effect was not
significant with slit collimators. Both the RC and STDRC
for the helical scan were higher than those for the single
circular orbit scan using mouse PH.
Transaxial NU-4 phantom images showing the uniform
region along with line profiles, the region with water andair compartments, and the filled rods for each collimatorswith and without resolution recovery are shown in Fig. 4.
By visual inspection, the images of uniform region were
noisy with pinhole collimators, whereas those with slit
collimators were more homogeneous. The images became
more homogeneous with resolution recovery function.
However, some edge enhancements were observed in theuniform region as well as the larger filled rods (3–5 mm)Table 3 %STD of uniform region and spill-over ratios
Mouse
pinholeRatpinhole

Mouse
slit

Rat
slit

%STD RR-on (%)

19.3 (24.0)

10.3

6.0

8.1

%STD RR-off (%)

33.1 (50.6)

20.1

10.3

10.7

SOR-air RR-on

0.085 (0.073)

0.087

0.094

0.092

SOR-water RR-on

0.095 (0.095)

0.093

0.096

0.095

SOR-air RR-off

0.094 (0.090)

0.093

0.097

0.102

SOR-water RR-off

0.103 (0.108)

0.100

0.100

0.104

Data in parentheses are those by helical scan mode

Effects of iteration numbers for MLEM on the %STD of
uniform region or SORs are illustrated for each collimatorin Fig. 5a, b. The %STD increased as a function of iteration in all the collimators. The pinhole collimators showed
more rapid increase in %STD as compared with the slit
collimators. The use of resolution recovery function significantly lessened the %STD values in all the collimators,
but did not have a major impact on SORs. The %STD
values of \20 % were obtained with the iteration numberof B100 with resolution recovery for rat PH; B200 for rat
SL; B50 for mouse PH; or B200 for mouse SL. By contrast, the SORs in both water and air decreased as a function of iteration. With resolution recovery function, theSOR values of \0.1 in both water and air were obtainedwith the iteration number of C50 for any type of collimators tested in this study.
The RC values as a function of iteration numbers areillustrated in Fig. 6a, and indices of error in RC in Fig. 6b.
The RC values rapidly increased as the iteration number
increased, which was followed by gradual increase or
decrease depending on rod diameter, collimator choice, andthe use of resolution recovery function. The index of error
in RC rapidly decreased as the iteration number increased,
which was followed by gradual increase or decreasedepending on collimator choice and the use of resolution
recovery function. With resolution recovery function, theindex of error in RC values of \1 was obtained with theiteration number of 30–100 for rat PH; C30 for rat SL; C10
for mouse PH; or C50 for mouse SL.
When the %STD of \20 %, the SOR of \0.1 in bothwater and air, and the index of error in RC of \1 wereconsidered to be ‘‘good’’ image quality measures, theserequirements were met with the iteration numbers of
50–100 for rat PH; 50–200 for rat SL; 50 for mouse PH; or
50–200 for mouse SL, when resolution recovery function
was applied. Without resolution recovery function, thesewere met with 60–100 iterations for rat SL or 60–90 iterations for mouse SL, but were never met for rat PH ormouse PH.
In vivo bone images
Figure 7 shows 3D maximum intensity projection imagesof bone SPECT of a mouse acquired with mouse PH ormouse SL. Without resolution recovery, the image with

123

490

Ann Nucl Med (2014) 28:484–497

mouse PH was very noisy, which substantially improved
after the use of resolution recovery. The image with mouse
SL was less noisy, but detailed bone structures were notclearly seen because of lower axial resolution. The use ofresolution recovery again improved image quality.

Discussion
To the best of our knowledge, this is the first study that hasreported the imaging performance of speCZT system

Mouse PH

Rat PH

1.5

RR-off
RR-on
Helical RR-off
Helical RR-on

1.0

0.5

0.0
0

1

2

3

4

Recovery Coefficient

Recovery CoefficientA

particularly using NU-4 image quality phantom. The major
findings of this study were that (1) this system yielded ahigh-energy resolution of 5.5 % in FWHM; (2) a submillimeter resolution was achieved using mouse PH withreasonable sensitivity, but axial resolution was much worsewith slit collimators; (3) the NU-4 image quality phantom
results showed better uniformities with slit collimators ascompared with pinhole collimators; (4) resolution recovery
during reconstruction improved spatial resolution anduniformity, but caused some overestimation of recovery
coefficient with NU-4 phantom; (5) reconstruction settings

1.5

RR-off
RR-on

1.0
0.5
0.0
0

5

1

Recovery Coefficient

Mouse SL
1.5

RR-off
RR-on

1.0
0.5
0.0
1

2

3

4

Recovery Coefficient

Rod Diameter (mm)

0

1.0
0.5
0.0
1

2

3

4

5

Rod Diameter (mm)Mouse PH

Rat PH
1.5

RR-off
RR-on
Helical RR-off
Helical RR-on

1.0

STD RCSTD RC

5

RR-off
RR-on

0

0.5

RR-off
RR-on

1.0

0.5

0.0

0.0
0

1

2

3

4

0

5

1

2

3

4

5

Rod Diameter (mm)

Rod Diameter (mm)Mouse SLRat SL
1.5

1.5

RR-off
RR-on

RR-off
RR-on

STD RCSTD RC4

Rat SL

5

1.5

1.0

0.5

1.0
0.5
0.0

0.0
0

1

2

3

4

5

Rod Diameter (mm)Fig. 3 Recovery coefficients (a) and STDRC (b) for each collimatoras a function of diameter of 99mTc-filled rods with and without
resolution recovery. Dot lines denote values obtained by helical scanmode. Mouse PH mouse 7-pinhole collimator, Rat PH rat 5-pinhole

123

3

1.5

Rod Diameter (mm)B

2

Rod Diameter (mm)

0

1

2

3

4

5

Rod Diameter (mm)collimator, Mouse SL mouse 8-slit collimator, Rat SL rat 5-slit
collimator, RR-on with resolution recovery function, RR-off without
resolution recovery

Ann Nucl Med (2014) 28:484–497

such as iteration number significantly affected image
quality measures; and (6) bone images of acceptable
quality were obtained in an in vivo mouse particularly
using mouse PH with resolution recovery.
Energy resolution

491
Fig. 4 a Transaxial images of NU-4 image quality phantom acquired cusing 4 collimators reconstructed with (RR-on; right) and without(RR-off; left) resolution recovery. Images are normalized to the peak
activity on each image. b Line profile across the uniform region.
Mouse PH mouse 7-pinhole collimator, Rat PH rat 5-pinhole
collimator, Mouse SL mouse 8-slit collimator, Rat SL rat 5-slit
collimator, RR-on with resolution recovery function, RR-off without
resolution recovery

A major advantage of CZT detector system is its highenergy resolution, which enables high contrast imaging by
scatter rejection [10]. It should also be beneficial for
simultaneously acquired multi-isotope imaging. Our results
demonstrated that CZT detector system yielded a muchhigher energy resolution (5.5 % in FWHM) than thosegenerally reported for NaI (12–15 %) [5, 19]. Furthermore,
our result was comparable to that reported for a clinical
CZT-based camera with essentially identical detector units(5.4 % in FWHM) [10].

detector. This is particularly true when resolution recovery
during reconstruction was applied, which improved spatial
resolution by approximately 30 %. Slit collimator apertures
provide an opportunity for larger axial FOV imaging as
compared to pinhole apertures [1, 15]. However, the disadvantage of slit collimator is a loss of axial resolution
because there is no axial magnification. Therefore, slit
collimators (mouse SL and rat SL) provided reasonabletransaxial, but much worse axial resolution in this study.

Sensitivity and spatial resolution

Image quality assessment using NU-4 phantom

In general, sensitivity is the measure of rate at which eventsare detected in the presence of radioactive sources, anddepends on system configuration such as aperture size, thenumber of apertures, radius of rotation and detectormaterial. As mentioned earlier, this system has 10 CZTbased detectors with 5–7 pinhole or slit collimators with
1.0-mm diameter or slit width and 32.5- to 50-mm radius ofrotation. The results showed that the sensitivity was inorder of a couple of hundred cps/MBq, and varied
depending on collimator choice. The sensitivity achievedin this study was lower than those reported for U-SPECT II[8] that has 3 stationary NaI detectors equipped with acollimator having a total of 75 pinholes with 0.35- to 1.0mm diameter, but higher than that reported for Inveon
SPECT [5] that has 2 NaI detectors with single pinhole
collimators with 0.5- to 1.0-mm diameter (a total of 2
pinholes) or first generation CZT-based SPECT [9] that has
8 detectors with single pinhole collimators with 0.5-mm
diameter (a total of 8 pinholes), and comparable to thatreported for another CZT-based small-animal SPECT
system (FX3200, Gamma-Medica Ideas, CA, USA) [20]that has 4 detectors with single- or 5-pinhole collimatorswith 0.5-mm diameter (a total of 5 or 20 pinholes). Thus,
our results on sensitivity seem to be reasonable for thecommercially available animal SPECT cameras.
The spatial resolution is primarily determined by collimator aperture, detector size, magnification, and intrinsic
detector resolution. Despite the much smaller detector size
and thereby smaller magnification as compared to NaIbased detectors [5, 8], the mouse PH achieved spatial
resolution comparable to those reported for other commercially available small-animal SPECT systems [5],which is due to the small detector pixel size (2.46 mm) of

To date, no standardized method exists for assessing
imaging performance of small-animal SPECT. Therefore,
we used NU-4 phantom that had originally been developed
to assess small-animal PET [17], which has also turned out
to be useful for SPECT [13]. Our results showed that theuniformity as assessed by %STD was better with slit collimators than pinhole collimators. This is to some extent
because of the longer axial voxel size of slit collimators(2.46 mm) as compared with that of pinhole collimators(0.5 mm), which was unavoidable due to system configuration. Furthermore, the uniformity was significantly
improved for all the collimators by the use of resolution
recovery function. This is explained by the noise reduction
feature by resolution recovery [11, 21]. At present, it isdifficult to determine whether %STD values obtained inthis study are acceptable for practical use because there isno established threshold value available for this purpose.
However, the %STD values using U-SPECT-II (MILabs,
Utrecht, the Netherlands) and NU-4 phantom with radioactivity equal to that in our study (50 MBq) ranged fromapproximately 8–30 % without post-reconstruction filtering [13]. Other SPECT systems have also reported %STD
values of 9.2–20.5 % [5, 20], which are comparable tothose with resolution recovery observed in this study. Thus,
the sensitivity uniformity as expressed by %STD in oursystem seems to fall within the expected range.
The SORs largely reflect photon scatter from outside thecold chamber as well as that originating in the coldchamber itself. Therefore, the higher SORs-water thanSORs-air observed in this study is likely to be caused by
the stronger scatter in water than that in air. However, thedifference in SORs between water and air (2–12 %) seems
to be somewhat smaller than those reported for NaI-based

123

492

Ann Nucl Med (2014) 28:484–497

A
RR-off

RR-on

Mouse PH100%

Mouse SL

Rat PH0%

Rat SL

B

123

Ann Nucl Med (2014) 28:484–497

A With Resolution Recovery

Without Resolution Recovery
Rat PH
Rat SL
Mouse PH
Mouse SL

%STD

60
40

80

40
20

20

0

0
50

100

150

200

0

250

50

100

150

200

250

The number of iterations

The number of iterations

Rat PH

Rat SLB

SOR in water RR-on
0.3

SOR in air RR-on
SOR in water RR-off

0.2

SOR in air RR-off

0.1

Spill Over Ratio0

SOR in water RR-on
SOR in air RR-on

0.3

SOR in water RR-off

0.2

SOR in air RR-off

0.1
0.0

0.0
50

100

150

200

250

0

50

100

150

200

250

The number of iterations

The number of iterations

Mouse PH

Mouse SLSOR in water RR-on
SOR in air RR-on

0.3

SOR in water RR-off

0.2

SOR in air RR-off

0.1
0.0

Spill Over Ratio0

Spill Over Ratio

Rat PH
Rat SL
Mouse PH
Mouse SL

60

%STD

80

Spill Over Ratio

Fig. 5 a The %STD of uniform
region of NU-4 image quality
phantom as a function of
iteration numbers for
maximum-likelihood
expectation maximization(MLEM) plotted for eachcollimator with and without
resolution recovery function
during reconstruction. b Spillover ratios as a function of
iteration numbers for MLEM
plotted for each collimator withand without resolution recovery
function. Mouse PH mouse
7-pinhole collimator, Rat PH rat
5-pinhole collimator, Mouse SL
mouse 8-slit collimator, Rat SL
rat 5-slit collimator, RR-on withresolution recovery function,
RR-off without resolution
recovery, SOR spill-over ratios

493

SOR in water RR-on
SOR in air RR-on

0.3

SOR in water RR-off

0.2

SOR in air RR-off

0.1
0.0

0

50

100

150

200

250

The number of iterations

SPECT [13]. This may reflect differences in system configuration (CZT versus NaI) and reconstruction setting
between the studies. As demonstrated in this study, CZT
detector has a much higher energy resolution than NaIbased detector, resulting in less scatter fraction. Furthermore, it is known that reconstruction settings such as thenumber of iteration affect SORs as demonstrated in thisstudy and others [13]. These mechanisms may have contributed to our results. It is notable that the use of resolution
recovery lessened both SOR-water and SOR-air. Although
the precise mechanisms are not clear, this may also be
related to improvement of partial volume effect by resolution recovery. The RC serves as a surrogate measure of
spatial resolution that could be compared with otherimaging systems [13]. Our results showed that RC generally increased as a function of rod diameter without resolution recovery, but it exceeded 1 at 2- to 3-mm rod
diameter with resolution recovery, indicating that the resolution recovery function causes overestimation of activity
in some circumstances. This is likely to be because ofoverestimation of activity due to edge enhancement. In 2to 3-mm diameter rods, such edge enhanced regions could

0

50

100

150

200

250

The number of iterations

be overlapped, resulting in significant overestimation ofactivity. The edge enhancement phenomena with resolution
recovery were also notable in uniform region of NU-4
phantom images as well as its line profiles.
The STDsRC, which are recommended to measure in
NEMA NU-4 standards [18], were obtained as an index ofstability in RC that would reflect image noise. Our results
showed that the STDsRC were larger for pinhole collimators than those for slit collimators, and that the use ofresolution recovery significantly lessened STDRC values
particularly for pinhole collimators. These trends are similar to those observed for %STD of uniform region.
Pinhole versus slit collimator
In vivo imaging is important for overall assessment ofimage quality. The bone images of a mouse demonstrated
that acceptable image quality can be obtained with a
40 MBq of the administered tracer activity and reasonably
acceptable acquisition time (32 min) if resolution recovery
function is turned on. As expected from the results ofphantom studies, detailed structure was not visible with

123

494

Ann Nucl Med (2014) 28:484–497

0.5
0.0
0

50

100

150

200

250

1.0
0.5
0.0
0

50

100

150

200

250

Mouse PH
1.5
1.0
0.5
0.0
0

The number of iterations

The number of iterations

50

100

150

200

250

Recovery Coefficient

1.0

Rat SL
1.5

Recovery Coefficient

Rat PH
1.5

Recovery Coefficient

Recovery CoefficientWith Resolution Recovery
Mouse SL
1.5

1 mm
2 mm
3 mm
4 mm
5 mm

1.0
0.5
0.0
0

The number of iterations

50

100

150

200

250

The number of iterations

0.5
0.0
50

100

150

200

250

0.5
0.0
0

Index of Error in RC

With Resolution Recovery
Rat PH
Rat SL
Mouse PH
Mouse SL

1.5
1.0
0.5
0.0
100

150

200

150

200

250

0.5
0.0
0

50

100

150

200

250

The number of iterations

Mouse SL
1.5

1 mm
2 mm
3 mm
4 mm
5 mm

1.0
0.5
0.0
0

50

100

150

200

250

The number of iterations

Without Resolution Recovery

2.0

50

100

1.0

The number of iterations

The number of iterations

0

50

250

The number of iterations

Index of Error in RC0

1.0

Mouse PH
1.5

Recovery Coefficient

1.0

Rat SL
1.5

Recovery Coefficient

Rat PH
1.5

Recovery Coefficient

Recovery Coefficient

Without Resolution Recovery

2.0

Rat PH
Rat SL
Mouse PH
Without RR

1.5
1.0
0.5
0.0
0

50

100

150

200

250

The number of iterations

Fig. 6 a Recovery coefficients (RC) of NU-4 image quality phantom
as a function of iteration numbers for maximum-likelihood expectation maximization (MLEM) plotted for each collimator with (upper)
and without (lower) resolution recovery function during reconstruction. b Index of error in RC as a function of iteration numbers for

MLEM plotted for each collimator with (upper) and without (lower)
resolution recovery function. Mouse PH mouse 7-pinhole collimator;
Rat PH rat 5-pinhole collimator, Mouse SL mouse 8-slit collimator,
Rat SL rat 5-slit collimator

mouse SL because of much lower axial resolution thanmouse PH. Based on these results, we would recommend touse mouse PH rather than mouse SL together with resolution recovery in this setting. In this study, we used thehelical scan mode for in vivo imaging with mouse PH. A
drawback of helical scan is, however, that it usuallyrequires a large number of projections to scan entire mouseor rat body, because acquisition step angle and bed motion
per step are fixed to 1.0° and 0.33 mm for mouse PH or
0.67° and 0.5 mm for rat PH in this system. For this reason,
the helical scan with mouse PH presented in Fig. 4 required
242 scan steps as compared to 45 steps with mouse SL.
Furthermore, the number of steps with the slit collimator
can easily be reduced to, for example, 10 (increment angleof 4.5°). These indicate that a single scan time would bemuch shorter with slit collimator for a given acquisition
time per step. When the acquisition time per step is set tothe minimal (i.e., 1 s), for example, a single scan withmouse SL would require only 10 s as compared to 242 swith mouse PH. As such, slit collimators should be usefulfor dynamic imaging of an entire mouse or rat body, wheremultiple serial short acquisitions are required. However,the value of slit collimator for dynamic imaging needs tobe addressed in further studies, because we focused oncomparative evaluation of imaging performance withcomparable acquisition time among the collimators.

123

Effect of resolution recovery
We applied resolution recovery function during reconstruction based on the modeling of pinholes and slits [16]in this study, reflecting recent advances in iterative
reconstruction algorithms. The results showed that the useof resolution recovery improves spatial resolution, uniformity, SOR, and overall image quality. This is in line withthe published results in that the resolution recovery duringreconstruction generally provides improved resolution andlower noise magnitude in the images [11, 21]. Therefore, inAnn Nucl Med (2014) 28:484–497

495

Fig. 7 3D maximum intensity
projection images of a mousebone SPECT using either mouse
PH (left) or mouse SL (right)
with (RR-on) and without
resolution recovery (RR-off).
Mouse PH mouse 7-pinhole
collimator, Mouse SL mouse
8-slit collimator, RR-on withresolution recovery function,
RR-off without resolution
recoveryMouse PH
RR-offMouse SL
RR-off

RR-on

Head

RR-on

Head

100%

Post

Ant

0%

Tail

Tail

50 mm
practical settings, it is preferable to use resolution recovery
whenever possible as described earlier. However, ourresults also indicate that images reconstructed with resolution recovery should be interpreted with caution in thatthe pixel values may not necessarily guarantee quantitative
accuracy as discussed earlier related to RC in NU-4
phantom. Therefore, a care should be taken when measuring activity of small objects with diameter of 2–3 mm,or object edge. In such cases, one could probably use
images without resolution recovery function as an option.
Helical scan
In this study, we preferred to use a single circular orbit rather
than a helical scan. This was because the helical scan in oursystem was less flexible in terms of voxel size and acquisition
parameters such as step angle per view. For example, thevoxel size was always fixed to 0.33 mm 9 0.33 mm 90.33 mm and step angle was set to 1 degree for mouse PH
with no other options available. However, a helical scan has
an advantage over a single circular orbit scan in that it cancover long axial FOV. Furthermore, a single circular orbit
scan with pinholes for a long object may cause axial blurringand may affect analysis for performance evaluation. Therefore, we performed a helical scan of the NU-4 image quality
phantom using mouse PH in addition to the more conventional single circular orbit scan. We observed higher %STD
values of the uniform region with the helical scan than thosewith the single circular orbit scan. This was likely because ofsmaller voxel size for the helical scan (0.33 mm 90.33 mm 9 0.33 mm) than that for the single circular orbit

scan (0.5 mm 9 0.5 mm 9 0.5 mm). Moreover, imagingof the entire phantom using two 30-min scans of single circular orbit (a total of 60 min) was longer than that using
helical scan (30 min). These circumstances in the helicalscan may have caused a high noise level and thereby a high
%STD. We also observed higher RC and STDRC values withthe helical scan. The smaller voxel size for the helical scan
may have caused less partial volume effects than those for the
single circular orbit scan, resulting in the higher RCs. The
higher STDRC values for the helical scan were likely to be
explained by the higher noise due to the smaller voxel sizeand shorter acquisition time because, as mentioned earlier,the STDRC would reflect image noise. In practice, we would
recommend to use a single circular orbit scan when the target
object is small such as heart or brain, because it is more
flexible in terms of voxel size and acquisition parameters. On
the other hand, a helical scan would be useful in the circumstances where whole-body imaging is required, althoughmore validations are necessary to optimize the imaging
parameters.
Effects of reconstruction settings on image quality
measures
The results showed that the %STD of sensitivity uniformity
increased, while SORs decreased, as a function of iteration,
which is in line with a study by Harteveld et al. [13].
Furthermore, the index of error in RC rapidly decreased as
the iteration number increased, which was followed by
gradual increase or decrease depending on collimator
choice and the use of resolution recovery function. From a

123

496

practical point of view, an optimal reconstruction setting is
one that provides relatively low %STD (e.g., \20 %), low
SORs (e.g., \0.1), and low index of error in RC (e.g., \1).
Our results showed that the suitable range of iteration
numbers for MLEM reconstruction to meet such requirements is different depending on collimator choice. For
example, the suitable range of iteration numbers with resolution recovery function was considered to be 50–100 for
rat PH or 50 for mouse PH, whereas it was wider (50–200)
for rat SL or mouse SL. Without resolution recovery
function, it was considered to be 60–100 for rat SL ormouse SL. However, it was difficult to meet suchrequirements for pinhole collimators mainly due to higher%STDs. When higher %STDs (e.g., \40 %) were accepted, the suitable range of iteration numbers was considered
to be 50–100 for rat PH or 50–60 for mouse PH. Thus,
reconstruction settings should be optimized for each collimator with/without resolution recovery function.

Limitations
There are limitations of the study to be mentioned. First,
although NU-4 phantom is successfully applied in smallanimal PET scanners [17], it has not yet been optimized foranimal SPECT with small FOV, which is necessary toachieve high spatial resolution and sensitivity. For example, NU-4 phantom cannot be used for very small radius ofrotation. Conversely, the phantom with larger diameter
would be better for the collimators for rat imaging. These
are the limitations of NU-4 phantom for evaluating smallanimal SPECT, where system design becomes morediverse in recent years. Second, we focused on 99mTc as aradionuclide to be tested in this study, because it is widelyused and many of imaging performance test results arereported based on it [5, 8, 9, 13], which provides anopportunity for comparison of systems. Other radionuclides such as 123I or 111In, which are also used for molecular imaging, should be tested in future.

Conclusions
The performance of the eXplore speCZT system has beencharacterized for each collimator using phantoms including
the NEMA NU-4 image quality phantom and an in vivo
mouse. The results indicate that the scanner has a highenergy resolution and reasonable sensitivity and spatial
resolution. The image quality phantom results indicate thatthe use of resolution recovery function significantly
improves the spatial resolution as well as image quality,
although images with resolution recovery should be interpreted with caution in that the pixel values may not

123

Ann Nucl Med (2014) 28:484–497

necessarily guarantee quantitative accuracy. The overall
performance shows that the eXplore speCZT system issuitable for preclinical imaging-based research using
small-animals.
Acknowledgmentstural government.

This work was supported by Ishikawa prefec-

Conflict of interest

None of the authors have conflicts to disclose.

References
1. Franc BL, Acton PD, Mari C, Hasegawa BH. Small-animal
SPECT and SPECT/CT: important tools for preclinical investigation. J Nucl Med. 2008;49:1651–63.
2. de Kemp RA, Epstein FH, Catana C, Tsui BM, Ritman EL.
Small-animal molecular imaging methods. J Nucl Med.
2010;51(Suppl 1):18S–32S.
3. Bravo PE, Bengel FM. The role of cardiac PET in translating
basic science into the clinical arena. J Cardiovasc Transl Res.
2011;4:425–36.
4. Beekman FJ, van der Have F, Vastenhouw B, van der Linden AJ,
van Rijk PP, Burbach JP, et al. U-SPECT-I: a novel system forsubmillimeter-resolution tomography with radiolabeled molecules in mice. J Nucl Med. 2005;46:1194–200.
5. Magota K, Kubo N, Kuge Y, Nishijima K, Zhao S, Tamaki N.
Performance characterization of the Inveon preclinical smallanimal PET/SPECT/CT system for multimodality imaging. Eur J
Nucl Med Mol Imaging. 2011;38:742–52.
6. Metzler SD, Jaszczak RJ, Patil NH, Vemulapalli S, Akabani G,
Chin BB. Molecular imaging of small animals with a triple-head
SPECT system using pinhole collimation. IEEE Trans Med
Imaging. 2005;24:853–62.
7. Umeda IO, Tani K, Tsuda K, Kobayashi M, Ogata M, Kimura S,
et al. High resolution SPECT imaging for visualization of intratumoral heterogeneity using a SPECT/CT scanner dedicated forsmall animal imaging. Ann Nucl Med. 2012;26:67–76.
8. van der Have F, Vastenhouw B, Ramakers RM, Branderhorst W,
Krah JO, Ji C, et al. U-SPECT-II: an ultra-high-resolution device formolecular small-animal imaging. J Nucl Med. 2009;50:599–605.
9. Kim H, Furenlid LR, Crawford MJ, Wilson DW, Barber HB, Peterson TE, et al. SemiSPECT: a small-animal single-photon emission
computed tomography (SPECT) imager based on eight cadmium
zinc telluride (CZT) detector arrays. Med Phys. 2006;33:465–74.
10. Bocher M, Blevis IM, Tsukerman L, Shrem Y, Kovalski G,
Volokh L. A fast cardiac gamma camera with dynamic SPECT
capabilities: design, system validation and future potential. Eur J
Nucl Med Mol Imaging. 2010;37:1887–902.
11. Garcia EV, Faber TL, Esteves FP. Cardiac dedicated ultrafast
SPECT cameras: new designs and clinical implications. J Nucl
Med. 2011;52:210–7.
12. Gimelli A, Bottai M, Giorgetti A, Genovesi D, Kusch A, Ripoli
A, et al. Comparison between ultrafast and standard single-photon emission CT in patients with coronary artery disease: a pilot
study. Circ Cardiovasc Imaging. 2011;4:51–8.
13. Harteveld AA, Meeuwis AP, Disselhorst JA, Slump CH, Oyen
WJ, Boerman OC, et al. Using the NEMA NU 4 PET image
quality phantom in multipinhole small-animal SPECT. J Nucl
Med. 2011;52:1646–53.
14. Walrand S, Jamar F, de Jong M, Pauwels S. Evaluation of novel
whole-body high-resolution rodent SPECT (Linoview) based on
direct acquisition of linogram projections. J Nucl Med.
2005;46:1872–80.

Ann Nucl Med (2014) 28:484–497
15. Zheng GL, Gagnon D. CdZnTe strip detector SPECT imaging
with a slit collimator. Phys Med Biol. 2004;49:2257–71.
16. Bal G, Acton PD. Analytical derivation of the point spreadfunction for pinhole collimators. Phys Med Biol.
2006;51:4923–50.
17. Bao Q, Newport D, Chen M, Stout DB, Chatziioannou AF.
Performance evaluation of the inveon dedicated PET preclinicaltomograph based on the NEMA NU-4 standards. J Nucl Med.
2009;50:401–8.
18. NEMA Standards Publication NU 4-2008: Performance measurements of small animal positron emission tomographs. Rosslyn: National Electrical Manufacturers Association; 2008.

497
19. Zeniya T, Watabe H, Aoi T, Kim KM, Teramoto N, Takeno T,
et al. Use of a compact pixelated gamma camera for small animal
pinhole SPECT imaging. Ann Nucl Med. 2006;20:409–16.
20. Higaki Y, Kobayashi M, Uehara T, Hanaoka H, Arano Y, Kawai
K. Appropriate collimators in a small animal SPECT scannerwith CZT detector. Ann Nucl Med. 2013;27:271–8.
21. Vanhove C, Andreyev A, Defrise M, Nuyts J, Bossuyt A. Resolution recovery in pinhole SPECT based on multi-ray projections: a phantom study. Eur J Nucl Med Mol Imaging.
2007;34:170–80.

123