Positron emission mammography (PEM) cameras are novel-dedicated PET systems optimized to image the breast. For these cameras it is essential to achieve an optimum trade-off between sensitivity and spatial resolution and therefore the main challenge for the novel cameras is to improve the sensitivity without degrading the spatial resolution. We carry out an analytical study of the effect of the different detector geometries on the photon sensitivity and the angle of incidence of the detected photons which is related to the DOI effect and therefore to the intrinsic spatial resolution. To this end, dual head detectors were compared to box and different polygon-detector configurations. Our results showed that higher sensitivity and uniformity were found for box and polygon-detector configurations compared to dual-head cameras. Thus, the optimal configuration in terms of sensitivity is a PEM scanner based on a polygon of twelve (dodecagon) or more detectors. We have shown that this configuration is clearly superior to dual-head detectors and slightly higher than box, octagon, and hexagon detectors. Nevertheless, DOI effects are increased for this configuration compared to dual head and box scanners and therefore an accurate compensation for this effect is required.
Breast cancer is one of the most commonly diagnosed cancers and one of the leading causes of cancer related deaths in women [
Metabolical imaging techniques such as positron emission tomography (PET) are increasingly being used in oncology [
Positron emission mammography (PEM) cameras are novel-dedicated PET systems optimized to image the breast. By reconstructing the radiotracer distribution inside the breast, tomographic images of breast lesions are obtained in a noninvasive procedure. Compared to conventional whole-body PET systems, PEM cameras cover a smaller field of view that is limited to a single breast. The detectors are arranged around the breast so that their performance can be higher at a lower cost. The photon sensitivity of a PET system is related to the ratio between detected and emitted photons (i.e., detected counts and injected activity) and is mainly determined by the system geometry and the type and volume (thickness) of the detector. Thus, the sensitivity is increased in PEM cameras compared to conventional PET due to the proximity of the detectors to the breast. Due to this, gamma rays penetrate a significant distance into the detector before detection. This distance is called depth of interaction (DOI) and it can produce an uncertainty in the calculation of the photon interaction point. This intrinsic uncertainty is also related to the positron range, photon noncollinearity, [
In breast PET cameras, it is essential to achieve an optimum trade-off between sensitivity and spatial resolution. Therefore the main challenge for the novel cameras is to improve the sensitivity without degrading the spatial resolution. Due to this, to increase the sensitivity by arranging the detectors closer to the breast, the DOI has to be measured and then its effects corrected, avoiding mis-positioning errors that decrease the spatial resolution. Recently, several studies described a number of detector designs with DOI information and different correction methods [
In last years, an increasing number of PEM prototypes and commercial systems were proposed [
The understanding of the properties of the detection systems used in PEM systems is essential for establishing appropriate operating criteria or designing schemes. In PET and PEM, two gamma photons (511 keV) from a positron-electron annihilation process are detected by means of a scintillator material. This material involves the conversion of the photon pair into visible light. Due to that the scintillator is optically coupled to a photomultiplier tube (PMT) the visible light can be converted to an electrical signal. This information is used to compute the spatial location of the photon interactions (photon interaction point) and the total energy deposited. When two photons from the same annihilation are detected in time coincidence then a line-of-response (LOR) can be defined and an event useful for the tomographic reconstruction is recorded. Nevertheless, some photons may escape after depositing only part of their energy into the crystal or even without interacting so that the event may be lost. The probability that a photon is detected depends on the scintillation material used and the crystal thickness.
Several authors have investigated the relation between the performance of a PEM scanner and its detector geometry configuration. Thus, Moses and Qi [
In this paper, we carry out an analytical study of the effect of different detector geometries on the photon sensitivity and on the angle of incidence of the detected photons which is related to the DOI effect. To this end, dual-head detectors are compared to box and different polygon detector configurations including rectangular parallelepiped crystals filling the intermodule gaps in order to avoid the drawback of these scanner geometries.
The most common PEM scanner geometry based on dual head detectors was compared to other geometries encircling the breast that included box and polygonal arrangements of panel detectors. As reported by Habte et al. [
Effect of intermodule wedge-shaped gaps (a) with respect to a camera with filled gaps (b).
Figure
Different arrangements of panel detectors for a PEM scanner. They are based on polygons (a) and different two detectors schemes (b).
The configuration based on two detectors makes possible to decrease this distance and therefore distances of 200 mm, 100 mm, 50 mm, and 25 mm are also considered.
The physical performance of a PET scanner can be studied by using different parameters.
Sensitivity of a PET scanner is defined as the rate in counts per second between detected true coincidence events and a given source activity. It depends on the material used as scintillator crystal, the geometry of the arrangements of the detectors, the energy threshold, and the time coincidence window.
Uniformity is defined as the maximum relative deviation of counts obtained from an acquisition by using an extended uniform source. It depends on multiple factors such as PMT performance, inhomogeneities of the scintillator crystal, or changes in sensitivity along the FOV.
Spatial resolution of a PET scanner is defined as its ability to distinguish between two points after image reconstruction, that is, it is the distance between adjacent detection points. The spatial resolution can be characterized by the full width half maximum (FWHM) in mm of the image of a point source in air. It depends on the interaction point estimation (intrinsic spatial resolution) and the tomographic reconstruction algorithm.
The photon sensitivity of a PET system is determined by the intrinsic efficiency (
Figure
Intrinsic efficiency.
Analytical estimation of the geometric efficiency.
An estimation of the total solid angle (
As can be observed in Figure
DOI and mis-positioning (
If the incoming photon direction or angle of incidence is
The mis-positioning originated from the DOI effect is obtained for the different detector configurations in order to evaluate the need of accurate methods to compensate this effect.
The photon sensitivity at a centered point was obtained for the different detector arrangements and it is shown in Table
Sensitivity at a centered point for a distance of 200 mm between opposing detectors.
Configuration | Sensitivity at center |
---|---|
Dual detector | 8.4% |
Box detector | 33.3% |
Hexagon detector | 34.0% |
Octagon detector | 34.2% |
Dodecagon detector | 34.5% |
Although the dual head configuration showed a lower sensitivity than the polygon configurations it has the advantage that the distance between opposing detectors can be decreased in order to adapt it to the object size. This is particularly interesting due to the variable size of breast in women. Thus, the distance between opposing detector can be decreased for small breast. This is clearly an advantage of dual head cameras with respect to fixed cameras. Table
Sensitivity at a centered point for dual head cameras for different distances between opposing detectors.
Distance (FOV) | Sensitivity at center |
---|---|
200 mm | 8.4% |
100 mm | 14.8% |
50 mm | 19.6% |
25 mm | 22.2% |
Figure
Sensitivity along the FOV for dual-head cameras (a) and polygon cameras (b).
Transverse profiles of sensitivity for dual-head cameras (left) and polygon cameras (right).
The averaged mis-positioning of the photon interaction point due to the DOI effect at a centered point is shown in Table
Mis-positioning of the photon interaction point due to the DOI effect at center and edge (80 mm) of the FOV.
Configuration | FOV (mm) | Mis-positioning center FOV (mm) | Mis-positioning edge FOV (mm) | ||
---|---|---|---|---|---|
Averaged | Maximum | Averaged | Maximum | ||
200 | 2.9 | 8.1 | 1.9 | 5.8 | |
Dual detector | 100 | 4.0 | 9.4 | 2.6 | 6.7 |
50 | 4.6 | 9.8 | 3.0 | 7.0 | |
25 | 4.8 | 10.0 | 3.1 | 7.1 | |
Box detector | 200 | 3.8 | 8.1 | 4.1 | 9.7 |
Hexagon detector | 200 | 5.0 | 7.5 | 2.9 | 9.2 |
Octagon detector | 200 | 4.7 | 7.3 | 2.5 | 9.3 |
Dodecagon detector | 200 | 4.4 | 7.2 | 1.7 | 9.3 |
These results show a significant effect of the DOI on the mis-positioning of the photon interaction point and therefore on the intrinsic spatial resolution for all detector geometries. Furthermore, this becomes an essential issue to achieve a high performance of the scanner since this effect is greater at the center than at the edge of the FOV.
An analytical study of the performance of different PEM detector geometries in terms of photon sensitivity and DOI effect was carried out in order to find an optimal arrangement of the detectors. To this end, dual-head detectors were compared to box and different polygon detector configurations.
Our results showed that higher sensitivity and uniformity are obtained for box and polygon detector configurations compared to dual-head cameras. For the polygon configurations the sensitivity is only moderately increased when the number of detectors is raised. The variable size of breast in women is an advantage for dual-head cameras with respect to to fixed cameras. Thus, for dual head cameras the sensitivity can be increased for small breasts by decreasing the distance between opposing detectors. Nevertheless this translates in an increase of the mis-positioning of the photon interaction point due to the DOI effect.
The optimal configuration in terms of sensitivity is a PEM scanner based on a polygon of twelve (dodecagon) or more detectors. We have shown that this configuration is clearly superior to dual-head detectors and slightly higher than box, octagon, and hexagon detectors. Nevertheless, DOI effects are increased for this configuration compared to dual-head and box scanners and therefore an accurate compensation for this effect is required.
This work was supported in part by Fondo de Investigaciones Sanitarias ISCIII PS09/01206 and Xunta de Galicia 10CSA918001PR. P. Aguiar was awarded a Sara Borrell fellowship 2010 by ISCIII.