Background: Positron Emission Mammography (PEM) is a nuclear medicine imaging tool, playing a significant role in the diagnosis of patients with breast cancer. These days, many research has been done in order to improve the performance of this system.
Objective: This study aims to propose a new method for optimizing the size of axial Field of View (FOV) in PEMs and improving the performance of the systems.
Material and Methods: In this analytical study, a conventional Inveon PET is simulated using GATE in order to validate the simulation. For this simulation, the mean relative difference is 2.91%, showing the precision and correction of simulation and consequently it is benchmarked. In the next step, for design of the new optimized detector, several validated simulations are performed in order to find the best geometry.
Results: The best result is obtained with the axial FOV of 101.7 mm. It has 1.6×1.6×15 mm3 lutetium yttrium orthosilicate (LYSO) crystals. The detector consists of 6 block rings with 30 detector blocks in each ring. In this paper, the performance of the scanner is improved and the geometry is optimized. Sensitivity and scatter fraction of the designed scanner are 4.65% and 21.2%, respectively, also noise equivalent count rate (NECR) is 105.442 kcps.
Conclusion: The results showed 1 up to 3% improvement in the sensitivity of this new detector compared with different PEMs.
Saeed Setayeshi (Google Scholar)
Positron Emission Mammography (PEM) is a highly accurate way to diagnose breast cancer in patients. When positrons collide with electrons, annihilation occurs and the arrival time of two 511 keV positron annihilation photons should be measured. Therefore, the location of positron annihilation can be constrained. This procedure has known as time of flight Positron Emission Tomography (TOF-PET) [ 1 - 5 ].
There are different parameters which are momentous in PET systems. One of them is sensitivity because higher sensitivity makes less dose of radiotracer for the patients. Sensitivity is defined as the number of counts per unit time detected by the detectors for each unit of activity present in a source. There are several items, affecting the sensitivity such as detection efficiency of 511 keV photons, detector spatial angular coverage, timing window and energy window. Other important parameters are noise equivalent count rate (NECR) and scatter fraction that NCER is equal to the square of the signal to noise ratio (SNR). Therefore, enhancing this parameter increases the quality of images and when the scatter fraction is low, the performance of PET is better, and high quality in images would be obtained [ 6 - 7 ].
The geometry of the detectors has an essential effect on the parameters of PET systems and various PETs have different geometries. The first PEM had two planar detectors to compress the breast to obtain better results in images like Clear-PEM that has lutetium yttrium orthosilicate (LYSO) crystals with the size of 2×2×20 mm3 in the 64×48 crystal arrays [ 8 ]. Also, some PEMs have detectors with the capability of rotating like PEM Flex Naviscan, which every detector module had LYSO crystals with the size of 2×2×13 mm3 [ 9 ]. Furthermore, ring detector was used in some cases such as MAMMI, designed with LYSO crystals with the size of 40×40×10 mm3 and twelve modules in each ring [ 10 ].
To investigate PET, NU 4—2008 Standard was used, designed for the performance evaluation of small animal PET scanners and used for the breast tissue. In this study, Inveon PET, which is one the best commercial tomography, has been simulated by GEANT 4 Application for Tomographic Emission (GATE) to validate simulations (the mean relative difference was 2.91%). Inveon PET had good sensitivity, 7.5% for energy window of 250-750 keV and timing window of 4ns (with LSO crystals). Peak of NECR was 538 kcps at 131400 kBq, and this peak was obtained for the rat phantom. Scatter fraction was 0.22 for this system. Therefore, it has good parameters among other PEMs [ 11 - 13 ].
Material and Methods
In this analytical study, a new designed PEM was simulated in GATE V7.2 open source. It consists of 6 rings in axial Field of View (FOV), each ring has 30 modules that every module has 10×10 crystal arrays. The size of a crystal is 1.6×1.6×15 mm3 with a pitch of 1.67 mm. A model of the designed PEM is shown in Figure 1.
At first, geometry of the scanner must be defined and then for some cases, phantom should be added (for example, this part is not necessary for calculation of the sensitivity). Setting up the physics processes must be defined. The digitizer for each particle’s physical observables, including energy, position, and time of detection, should be added. Next part is defining the source. The advantage of GATE is that desired format of the output can be selected, and after these steps, the acquisition could be started [ 14 - 15 ].
In the crystals, LYSO was used with excellent traits for detecting 511 keV gammas in PEM. For the physics part, photoelectric effect, Rayleigh, Compton, bremsstrahlung, multiple scattering, ionization and positron annihilation were added. Also, setting up the digitizer which has several parts (like an adder and readout) was added in GATE.
In order to benchmark the simulation, the Inveon PET that has high sensitivity and resolution has been simulated and consists of 64 detector blocks in 4 rings. Each block has 20×20 crystal arrays using lutetium orthosilicate (LSO) as material. Each crystal has 10.0 mm length. The crystal pitch is 1.59 mm in both axial and transaxial. The detector ring diameter is 16.1 cm while the axial FOV is 12.7 cm [ 16 ].
Sensitivity is normally expressed in counts per second per microcurie (or megabecquerel) (cps/μCi or cps/kBq). For calculating this parameter, 22Na point source (0.3 mm diameter) was used. The sensitivity was calculated for 2 energy windows, 350-650 keV and 250-750 keV and for timing windows of 2.8 and 3.4 ns. In Table 1, the experimental and simulated data of Inveon PET were shown.
|Energy Window (keV)||Coincidence Window||Experimental Inveon PET (%) [ 13 ]||Simulated Inveon PET (%)||Relative Difference (%)|
|PET: Positron Emission Tomography|
For designed detector, sensitivity was calculated with LYSO crystals, timing window of 6 ns and energy window of 250-750 keV. This plot was obtained by putting a point source in different places in the direction of the axial FOV, as seen in Figure 2.
In this design, different parameters were considered. Indeed, beside sensitivity, the length of the axial FOV and the number of used crystals should be considered. Therefore, two parameters were defined for calculating optimized geometry. In Equations 1 and 2, these parameters were expressed.
SAFOV is the sensitivity regarding length of the axial FOV as shown in Equation. 1 and SNC is the sensitivity considering the number of crystals in Equation. 2. Based on these equations, the higher sensitivity and the shorter length of the axial FOV lead to the better parameters.
Indeed, optimized geometry should be calculated. Therefore, the plots of these two equations were shown for different axial FOVs and the best geometry was obtained for 10.17 cm as axial FOV. It was led to 6 repetitions during the Z axial.
In Figure 3, these two parameters were plotted, showing the peak of axial FOV in the 10.17 cm.
Comparing performance of count rate between different tomographs or one scanner, working in different situations is hard. Therefore, one parameter, regarding these features needs to be defined. Noise Equivalent Count Rate (NECR) is proportional to the signal-to-noise (SNR) in images, thus, it is a good parameter to compare the performances of different PET scanners. Some parameters cause different NECR such as the geometry of the detector, the size of the object and activity of the radiotracers.
Equation 3 was used for calculating the NECR.
In this equation, TC is the true coincidence, SC is the scatter coincidence and RC is the random coincidence [ 17 ]. For obtaining Figure 4 that is the plot of NECR with different activity concentrations, energy window of 350-650 keV with timing window of 4 ns was used.
For calculation of this parameter, it was necessary to regard a phantom with the same size as the breast. Based on NU 4—2008 Standards, a rat phantom that is a cylinder with 150 mm long, a 50 mm diameter and a hole was used that has a density of 0.96 g/cm3. Also, for this, a line source with Fludeoxyglucose (F-FDG or FDG), should be put [ 14 ].
Another parameter is the scatter fraction that was shown in Equation 4.
As stated, the RC and TC are random and true coincidences [ 18 ].
Based on Equation 4, when the scatter fraction is low, the performance of PET is better, and high quality in images would be obtained. Also, scatter fraction is plotted based on the activity concentrations (kBq/ml), as shown in Figure 5 that scatter fraction is 0.212 in the peak of NECR.
In Table 2, comparison of parameters for new design detector and other common detectors were provided.
|PEM||Energy Window (keV)||Axial FOV (cm)||Crystals size (mm3)||Sensitivity (%)|
|Clear PEM [ 8 ]||100-700||11||2×2×20||1.87|
|Inveon [ 13 ]||350-650||12.7||1.5×1.5×10||2.8|
|Mosaic HP [ 12 ]||385-665||11.9||2×2×10||1.77|
|ALBIRA [ 19 ]||350-650||40||40×40×10||2|
|PEM: Positron emission mammography, FOV: Field of View|
Improving the performance of the scanner in PEMs is significant to improve parameters of the system which results in less dose for the patients as well as better quality in images. In this study, the simulation of Inveon PET was done to prove the accuracy of the simulations, and the difference error was 2.91% based on Table 1; thus, it was confirmed that the simulations were valid. In the following, a new detector with an optimized size of axial FOV is proposed with simulations by using Equations 1 and 2 that the best scanner has the maximum sensitivity and minimum number of the crystals and length of axial. Based on Figure 3, the plot peaks at 10.17 cm, and this size is the best for axial FOV because maximum sensitivity was reached, as an important parameter. Also, based on Figure 4, NECR has the most value in this size and the scatter fraction is 21.2% that has less value compared to other systems. In Table 2, comparison of parameters for new design detector and other common detectors were provided. Besides, it is obvious that the new size for the scanner has good ability compared to other scanners.
The new detector was simulated in GATE based on NU 4 in 2008, which has an optimized geometry. The best size of axial FOV and repetitions were calculated, resulting in 10.17 cm and 6 repetitions, respectively, leading to high sensitivity and NECR. Furthermore, the scatter fraction was measured 0.212 in the peak of the NECR, which was low and the results illustrated good ability for this system. This scanner has good ability compared with other scanners according to the results. Sensitivity of this system was obtained 4.65% with LYSO crystal in a timing window of 6 ns and with an energy window of 250-750 Kev, which demonstrated good performance in comparison with other systems.
S. Setayeshi conceived the idea. D. Roshani wrote the text and he was responsible for the practical implementation of the project. S. Setayeshi supervised the implementation of the project. Both authors read, modified, and approved the final version of the manuscript.
This is a simulation study that no ethical approval is required.
Conflict of Interest
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