Evaluating the spatial characteristics of a tilted-collimator scintillation camera

G. Baldazzi1, M. Bello2,3, P. Boccaccio2, D. Bollini1,G. Moschini2,3, N. M. Uzunov2,4

1 Department of Physics, University of Bologna and INFN, Section of Bologna, Italy; 2 INFN, National Laboratories of Legnaro, Italy; 3 Department of Physics, University of Padova; 4 Department of Natural Sciences, University of Shumen, Bulgaria

INTRODUCTION

In PET Laboratory at the National Laboratories of Legnaro, INFN, we are exploiting a recently developed gamma-ray imaging camera using tilted-collimator (TC) technique [1]. The parallel-hole lead collimator of the camera can be tilted at small angles around an axis parallel to the scintillator plane, thus enabling to determine the distance of imaged objects [1-3]. However, in order to allow sizeable tilt angles, the collimator-to-scintillator distance has been increased to 8mm. Moreover, when the collimator is tilted at a certain angle the distance between the collimator and the scintillator varies throughout the whole field of view (FOV). These conditions may affect the spatial resolution of the imaging camera. It is important to know how much the collimator-to-scintillator distance and the tilt angle influence the spatial characteristics of the TC camera throughout the whole FOV.

In this work the results of the spatial resolution measurements for the TC Yttrium Aluminum Perovskite (YAP) scintillation camera are presented.

EXPERIMENTAL SET-UP

A tilted-collimator scintillation camera has been constructed modifying an existing Yttrium-Aluminum Perovskite (YAP) camera previously used for small-animal imaging [4]. The camera is compounded from a position-sensitive photomultiplier tube (Hamamatsu Photonics, type R2486), a segmented YAP crystal with a parallel-hole lead collimator. The collimator with dimensions 40mm x 40mm x 20mm has holes with diameter of 0.5mm and center-to-center distances of 0.65mm. The collimator has been slightly retracted from the scintillator and placed in a cradle-like support providing maximum tilt angles ϕ1 = 5.50 and ϕ2 = -5.50 with respect to the scintillator plane. The resulting camera field of view (FOV) is 40mm x 40mm.

IMAGE PROCESSSING

To evaluate the spatial characteristics of the camera, a system of parallel capillary tubes with a circulating gamma-ray emitting solution was employed [2]. Eight capillary tubes laying on a plane surface at 5 mm mutual distances were filled with circulating 99mTc solution. The capillary tubes internal diameters were measured at several sections using an optical microscope. During the

measurements, the capillary-tubes layer was placed at 18mm and 68mm distances from the TC-YAP camera.

Images have been obtained at each camera-to-object distance for three fixed positions of the collimator: ϕ = -5.5°; ϕ = 0°; ϕ = 5.5°. Data acquisition yielded about 15x106 counts in the total FOV within the gamma-ray energy window (70-170keV). The images corresponding to the three collimator positions at 18 and 68mm distances are presented in figure 1 and figure 2, respectively.

FIG. 1: 99mTc-filled capillary tubes images obtained at 18mm distance: a) ϕ = -5.5°; b) ϕ = 0°; c) ϕ = 5.5°

FIG. 2: 99mTc-filled capillary tubes images obtained at 68mm distance: a) ϕ = -5.5°; b) ϕ = 0°; c) ϕ = 5.5°

In data analysis, the capillary tubes images were sliced orthogonally to their axes, then the total number of corresponding counts for each slice was evaluated. Introducing a coordinate system with x axis orthogonal to the capillaries’ axes, a fit to the count distributions from each slice of the capillary tubes images was obtained using the formula:

( )∑∑

= = ⎥⎥⎦

⎤

⎢⎢⎣

⎡ −−=

m

j

n

i j

ioj

j

ijj xxhAG

1 12

2

2 2exp

2 σπσ ( 1)

where: ojx – position of the j-th capillary center on the x axis; ix – position of the i-th segment obtained after the division of the capillary in a number of small segments; m – number of the capillary tubes; n – number of the segments in the capillary slice; hij - the height of the segments - 22)(2 jojiij rxxh −−= ; jr is the internal radius of the j-th capillary and jA are constants determining the distribution heights, related to the specific radioactivity in the capillary source.

The parameters varied during the fitting procedure were: the positions of the capillaries xoj; the standard distributions σj of each series of the Gaussian functions and the maximum value of each Gaussian function Aj (j=1, 2, …,8). The values for the FWHM differential spatial resolution of the TC-YAP camera have been derived from the fitting parameters σj of (1). The digital resolution for a given region of the FOV was obtained by dividing the real distance between the capillary axes (in millimetres) by the coordinate difference of their images (in pixels). Then the values obtained for the differential spatial resolution and differential linearity were expressed in millimetres.

DATA ANALYSIS AND DISCUSSION

The differential spatial resolution of the TC-YAP camera has been obtained from the images taken for the three collimator positions at two camera-to-object distances. For each image fits of the ten equidistant image slices have been conducted thus obtaining a set of spatial resolution data for the whole FOV of the camera. The spatial resolution, for each capillary tube position in the FOV, determined by averaging the differential spatial resolutions from the ten slices along the tube, is given in Table 1.

TABLE 1. Spatial resolution of the TC-YAP calculated for 8 capillary tubes spaced by 5mm

FWHM spatial resolution (mm)

Capilary tube No ϕ = -5.5° ϕ = 0° ϕ = 5.5°

Camera-to-object distance: 18mm 1 1.5 ± 0.1 1.7 ± 0.2 1.8 ± 0.2 2 1.6 ± 0.1 1.7 ± 0.1 1.7 ± 0.1 3 1.8 ± 0.1 1.7 ± 0.2 1.7 ± 0.1 4 1.8 ± 0.2 1.9 ± 0.1 2.0 ± 0.1 5 2.1 ± 0.2 1.9 ± 0.1 1.8 ± 0.1 6 1.9 ± 0.4 1.8 ± 0.2 1.9 ± 0.1 7 1.9 ± 0.2 1.8 ± 0.1 1.6 ± 0.2 8 1.9 ± 0.2 1.6 ± 0.2

Camera-to-object distance: 68mm 1 3.0 ± 0.4 3.1 ± 0.3 2 2.6 ± 0.3 3.0 ± 0.2 3.2 ± 0.4 3 3.4 ± 0.3 3.0 ± 0.4 2.7 ± 0.2 4 2.9 ± 0.4 3.5 ± 0.2 4.0 ± 0.4 5 3.8 ± 0.3 3.5 ± 0.3 3.3 ± 0.3 6 3.2 ± 0.3 3.1 ± 0.2 3.2 ± 0.3 7 3.3 ± 0.2 3.1 ± 0.3 2.9 ± 0.3 8 2.8 ± 0.4

Possible changes in the spatial resolution were expected

along horizontal direction since the collimator tilt axis is parallel to the capillary tubes. The linear least-squares fits to the data values for a given collimator angle in Table 1 tend to be flat for all the six series of spatial resolution data. The t-test analysis conducted on the spatial resolution, averaged for each capillary tube, does not show

significant differences between the values for the three collimator positions at the imaged distance. Hence there is no influence of the collimator tilt angle on the spatial resolution. This conclusion is in agreement with the general formula for the spatial resolution of the parallel-hole position-sensitive detectors [5] deduced for larger ratios of detector-to-object distances versus the collimator parameters (dimensions, hole diameters, hole shape and septa).

TABLE 2. Total spatial resolution comparison between the YAP and TC-YAP

Spatial resolution (mm)

Camera-to-object distance

(mm) YAP TC-YAP 18 1.6 ± 0.2 1.8 ± 0.2 68 2.8 ±0.3 3.2 ± 0.3

A comparison between the total spatial resolution of the

TC-YAP and the previously used YAP camera is given in Table 2. The spatial resolution of the YAP has been taken from [1]. The total spatial resolution of TC-YAP is obtained averaging the overall differential spatial resolution data from the images acquired for the three collimator positions at the given distance. The 10% higher values obtained for the TC-YAP are due to the 8mm gap between the collimator and the scintillator.

ACKNOWLEDGEMENT

One of the authors (N. Uzunov) would like to acknowledge the support received from the ICTP, Trieste, Italy and in particular the Program for Training and research in Italian Laboratories.

[1] G. Baldazzi et al., present report [2] N. Uzunov et al., Phys. Med. Biol. 51, 2006, N11 [3] P. Boccaccio, M. Bello, D. Bollini, F. de Notaristefani, G. Moschini, N. Uzunov, patent RM 2006 A 000216, Ufficio Italiano Brevetti e Marchi, Rome, April 4, 2006 [4] N. Uzunov et al. LNL Annual Report 2004, 246 [5] H. Baret et al., Radiological imaging: The theory of the image formation, detection and Processing, Vol I, Academic Press, New York, (1981)