The image reconstruction influence in relative measurement in SPECT / CT animal

The objective of this study was to evaluate the influence of different reconstruction methods in a FLEX Triumph TM Pre-Clinical Imaging System images applied to mice C57BL6 dosimetry in a new radiopharmaceuticals development for humans use. CT and SPECT images were obtained from a homemade phantom containing four spheres with diameters (d) simulating technetium-accumulating lesions at University Hospital (HU / UFRJ). The SPECT images were reconstructed with filtered back projection method (FBP) and the influence in resolution and partial volume effect was evaluated for different filters: Hamming, Hann and Ramp. The reconstructed image resolution was found 9.3 to 9.4 mm, significantly lower than the value provided by the manufacturer, 1.6 mm. Thus, the protocol for mice can be optimized by FBP reconstruction method and Hamming filter, 0.5 cut-off, yielding a reconstructed image resolution from 9.3 to 9.4 mm. This value indicates that technetium-accumulating regions diameter below 9.3 mm is not quantifiable.


INTRODUCTION
Animal models are used in new pharmacological compounds development related to human health since the 1950s, being a useful and efficient practice to avoid adverse effects (Martic-Kehl et al., 2012).
The image from SPECT (Single Photon Emission Computed Tomography) technique, also known as molecular imaging, is a non-invasive method to detect the signals originating from the injection of a radioactive tracer, observing the interaction of this with the target molecule in vivo (Meikle et al., 2005).
To guarantee diagnostic images quality for internal dosimetry purposes, the better reconstruction method analyses is necessary.This approach comprises a process of creating transaxial sections displaying a projection -filtered back projection (FBP) for the protocol applied in the studied system-where the raw data is used to create several transaxial slices.Figure 1 is a representation of this process; ten projection views are cut into seven separate bands.The corresponding bands form a radius, and so on, completing the image reconstruction process (Powsner, 2006).Subsequently, the images were filtered -a mathematical technique used during reconstruction to enhance the image "appearance".In particular, for diagnostic and dosimetric purposes, the filters are used to increase signal to noise ratio between distinct activity concentration areas in the image, to reduce "star" artefacts removing noise due to scattered photons and statistical variations in the count.The term cut-off is related to the maximum frequency that the filter could be applied, often referred to as the filter´s "power".Low-pass filters (Butterworth, Hann, Hamming, Parzen) are used as pre-filters to remove high frequency noise.The Ramp filter, high-pass, is then applied during the backprojection for removing star and other low frequency data.The optimum filter selection depends on both the characteristics of the data and the personal preference of the user, influencing the final score obtained in the image and hence, the dosimetry.In general, the high-pass filter is more suitable for higher data counts, while the low-pass filter is best for data containing a small number of counts (Powsner, 2006).

MATERIALS AND METHODS
SPECT and CT (Computed Tomography) images were obtained from a homemade phantom (Figure 2) containing four spheres of different diameters (d), developed at the Institute of Radiation Protection and Dosimetry, based on the Micro Hollow Sphere Phantom™ Model ECT/HS/MMP (Data Spectrum Co.), as described in Table 1.These inserts simulate technetiumaccumulating lesions to be analysed by images acquired in FLEX Triumph ™ Imaging System used in preclinical studies at the University Hospital (HU / UFRJ).The local SPECT protocol was: matrix 128 x 128, 20 minutes acquisition total time, energy window 140 keV±15%.CT protocol was: matrix 512 x 512, pitch 1.90 mm, 75 KV, FBP reconstruction method.
To simulate a real animal study, the total volume of the phantom (body) was filled with Technetium-99m solution diluted in water at an activity concentration of 0.629 MBq / ml.The spheres were filled with a concentration four times higher compared to the body of the phantom.Soriano, et. Al. • Braz. J. Rad. Sci. • 2018 Comparative analyses between different cut-off frequencies and filters were then performed to obtain the reconstruction method that could provide the highest resolution to be used for the dosimetry study in mice pre-clinical study.

RESULTS AND DISCUSSION
The average relative score curves were studied in function of the lesion diameters for Hamming, Hann and Ramp filters analysing the better cut-off, as exemplified in Figure 4 for Hamming filter.Hann and Ramp filters presented the same pattern as Hamming filter.Subsequent analyses were performed with this cut-off frequency of 0.5 since it was the one with the least degraded quantification (Figure 5).

Figure 2 :
Figure 2: Phantom used in this study

FBP
method with different filters (Hamming, Hann and Ramp, varying cut-off frequencies) was performed from the raw SPECT data.Regions of interest (ROI) were selected from CT images to obtain the mean and maximum counts per pixel.All measurements were performed in the central section of the lesion (spheres), subtracted from the phantom's body concentration value (background) as show in Figure3.

Figure 4 :
Figure 4: Average relative quantification, Hamming filter according to cut-off frequencies

Figure 5 :Figure 5
Figure 5: Average relative quantification per filter for cut-off 0.5