PET detector assembly for a combined PET and CT imaging system

Disclosed is a PET detector assembly in a combined PET/CT scanner system having a backplane structure for supporting two or more PET detector rings that provides substantially balanced load on the gantry backplane while accommodating the varying number of PET detector rings between short axial PET FOV system as well as long axial PET FOV system.

FIELD

This specification relates to combined positron emission tomography (PET) and computed tomography (CT) imaging systems and, more particularly, to a PET detector assembly wherein at least one PET detector is located on each side of a neutral axis of a backplane of the PET scanner.

BACKGROUND

In medical imaging, a computed tomography (CT) scanning modality can be used to provide images of the internal structures of a human body, such as the bones. In addition, a positron emission tomography (PET) scanning modality can be used to provide images of the functional aspects of the body, usually corresponding to the metabolic uptake of an internal organ or tissue. It is frequently desirable to combine the CT and PET modalities to provide a co-registered image or series of images to assist in non-invasively studying physiological processes and structures within the body.

In a CT scan, an x-ray source is passed around a patient. Detectors around the patient then respond to an x-ray transmission through the patient to produce an image of the area of study. In a PET scan, a short-lived radioisotope is injected into a patient. For example, one such radioisotope is fluorodeoxyglucose (FDG). During the PET scan, the patient is positioned in a tunnel within a PET scanner gantry. The radioisotope undergoes positron emission decay and emits a positron. The positron encounters and annihilates with an electron to produce a pair of gamma rays moving in approximately opposite directions. The gamma rays are then detected by a plurality of PET detectors (a.k.a. gamma detectors) arranged around the circumference of the tunnel.

A representative layout of a combined PET/CT scanner system500is shown inFIG. 1. The PET/CT scanner system500includes a PET scanner portion510, a CT scanner portion520, and a patient bed530. One metric of such PET/CT scanner system's capability is its ability to acquire serial images, in which, field-of-view (FOV) of both modalities are maximized. The extended stroke of the patient bed530defines the FOV of the CT scanner520. The PET FOV, however, because it is positioned farther from the patient bed530, inherently has a FOV scan that is much shorter than that of the CT scanner520. The co-scan, the effective overlapping scan area of the combined modalities PET and CT, is largely dependent on three parameters: (1) the extension length of the patient bed530; (2) the FOV separation distance550as taken from the center C1of the CT FOV and the first plane P1of the PET FOV; and (3) the axial length560of the PET FOV.

The spatial arrangement of the three components introduces a couple of challenges to the system design. In certain CT embodiments, the CT FOV is shifted to the patient side of the CT scanner520, assisting in interventional therapy. This accommodation increases the separation between the CT scanner520and the PET scanner510, thus increasing the FOV separation distance550.

Another challenge arises most often in newer PET/CT scanner system designs where a common PET gantry is tasked with structurally supporting both the short and long PET FOV axial length560designs.

In a conventional PET detector assembly10used in PET/CT scanner systems shown inFIG. 2, the rings20of PET detectors12are mounted on and supported by a gantry backplane14. Generally, a PET modality includes more than one detector rings but inFIG. 2, only one PET detector ring20is shown. The PET detector ring(s)20are all mounted on one side of the gantry backplane14. For the purposes of discussion, a neutral or vertical plane16of the gantry backplane14is defined and identified as the center of the gantry backplane14in the axial direction of the PET/CT scanner when viewed from the side as shown inFIG. 1. Because all of the PET detector ring(s)20are mounted on one side of the gantry backplane14, the PET detector rings20are on one side of the neutral vertical plane16. The PET detectors12are configured such that they extend from a face18of the backplane14in a cantilevered arrangement. The center of mass of such cantilevered PET detector ring20is positioned to one side of the neutral vertical plane16of the gantry backplane14. This results in a moment load W being on one side of the neutral vertical plane16as noted inFIG. 1. Therefore, when the PET/CT scanner system design calls for a long PET FOV axial length560, that requires more number of PET detector rings to cover the longer PET FOV axial length. This also produces a greater moment load W on the gantry backplane14. In turn, that requires more structurally robust gantry backplane14which increases the cost of the PET/CT scanner system.

In the conventional PET/CT scanner systems, the FOV separation distance550in a system is kept constant for design simplicity regardless of the axial length560of the PET FOV, for example, between a 3-ring PET detector system and a 4-ring PET detector system. This compromise was acceptable in the past because the difference in the PET FOV axial length560was on the order of 4-6 cm. With the advent of more modern PET/CT scanner systems, however, the trend has been toward longer PET FOV axial lengths, on the order of 16-40 cm, which requires many more cantilevered PET detector rings being mounted on the gantry backplane14with greater moment load W.

Therefore, there is a need for an improved PET/CT scanner system's PET detector assembly.

SUMMARY

According to an aspect of the present disclosure, a PET detector assembly in a combined PET/CT scanner system comprises a backplane structure for supporting two or more PET detector rings; two or more PET detector rings that are mounted on the backplane, wherein the two or more PET detector rings define a PET detector field of view (FOV); and a CT scanner defining a CT FOV; where the backplane defines a neutral vertical plane; where when there are an even number of PET detector rings, half of the PET detector rings are at least partially disposed on a first side of the neutral vertical plane such that, the half of the PET detector rings are at least partially between the neutral vertical plane and the CT FOV and the remaining half of the PET detector rings are at least partially disposed on a second side of the neutral vertical plane that is opposite the first side; and where when there are an odd number Xodd of PET detector rings, the side of the neutral vertical plane that is nearer the CT FOV has n=(Xodd+1)/2 number of PET detector rings at least partially disposed on a first side of the neutral vertical plane such that, the n number of PET detector rings are at least partially between the neutral vertical plane and the CT FOV and n−1 number of PET detector rings are at least partially disposed on a second side of the neutral vertical plane that is opposite the first side.

A combined PET/CT scanner system incorporating a PET detector assembly of the present disclosure is also provided.

DETAILED DESCRIPTION

The present disclosure describes combined PET and CT imaging systems in which PET detectors are positioned on each side of the backplane, or supporting structure, of the PET scanner. Arranging the PET detectors in this way reduces the cantilever load on the backplane, which may allow the backplane to include less material. The more balanced arrangement of the PET detectors described herein may also allow for easier access for servicing of the PET detectors. Further, the separation of the field of view (FOV) of the PET and CT imagers may be held constant while changing the FOV of the PET imager.

Referring toFIG. 3, a partial view of a combined PET/CT scanner system40is shown which depicts a backplane14and a ring20PET detectors12. The PET/CT scanner system40includes an enclosure42having a patient tunnel44and a bed46for holding a patient. The bed46is supported by a bed traversing mechanism47that serves to move the bed46and, thus, the patient relative to the patient tunnel44during scanning. In other embodiments, the bed46is stationary and the combined PET/CT scanner system40moves relative to the bed46. The combined PET/CT scanner system40utilizes known PET and CT scanning techniques and associated components to generate PET and CT scans of the patient either sequentially or simultaneously without requiring the patient to get off the bed46.

Referring toFIGS. 4 and 5, schematic illustrations of an embodiment of a PET detector assembly100and another embodiment of a PET detector assembly100A for a PET scanner portion of the combined PET/CT scanner system40are shown. The PET detector assembly100shown inFIG. 4includes a series of rings20a,20b,20cof detectors12, with each detector12supported by the backplane14. The PET detector assembly100A shown inFIG. 5includes a series of rings20a,20b,20c,20dof detectors12, with each PET detector12supported by the backplane14. The backplane14is configured to be mounted to the ground or an intervening structure to support the detectors12. The backplane14has a circular aperture that forms part of the patient tunnel44of the combined PET/CT scanner system40(shown inFIG. 3). The PET detectors12are circumferentially arranged on the backplane14about the patient tunnel44.

In both embodiments of the PET detector assemblies100and100A, at least one of the rings20a,20b,20c,20dof PET detectors12is positioned on either side of the neutral vertical plane16of the backplane14. For example, in the illustrated example ofFIG. 4, two detector rings20a,20care positioned on a first side of the neutral vertical plane16of the backplane14(e.g., closer to the CT portion of the PET/CT scanner system40) and a detector ring20bis positioned on the opposite side of the neutral vertical plane16(e.g., farther from the CT portion of the PET/CT scanner system40). In the illustrated example ofFIG. 5, two detector rings20a,20care positioned on a first side of the neutral vertical plane16of the backplane14(e.g. closer to the CT portion of the PET/CT scanner system40) and two detector rings20b,20dare positioned on the opposite side of the neutral vertical plane16(e.g., farther from the CT portion of the PET/CT scanner system40).

Positioning at least one ring of PET detectors12on each side of the backplane14reduces the moment load on the backplane14caused by the PET detectors12. As a result, stresses on the backplane14are reduced, thus reducing the structural strength and/or stiffness requirement for the backplane14. This provides additional design flexibility and material choices for fabricating the backplane14and also can reduce the cost of the materials. For example, the backplane14can be fabricated from relatively inexpensive foam or cardboard material sandwiched between sheets of aluminum, thus reducing fabrication costs.

In addition, this arrangement of the PET detectors12enables fabrication of a lighter backplane14resulting in reduced transportation and installation costs. Further, this arrangement of the PET detectors12reduces the risk that vibration induced damage to the PET/CT scanner system.

The PET detector assembly100can comprise any number of PET detector rings odd or an even number. For example, the PET detector assembly100can include three PET detector rings20a,20b,20c(as shown inFIG. 4), four PET detector rings20a,20b,20c,20d(as shown inFIG. 5), etc.

In applications in which the PET detector assembly100A includes an even number of PET detector rings, same number of PET detector rings can be positioned on either side of the neutral vertical plane16defined by the backplane14. For example, as shown inFIG. 5, in applications in which the PET/CT scanner system40includes four rings20a,20b,20c,20dof PET detectors12, two rings20a,20care positioned on the side of the neutral vertical plane16that is nearer the CT scanner and rings20b,20dare positioned on the opposite side of the neutral vertical plane16that is farther from the CT scanner.

Thus, according to the present disclosure, to reduce the moment load on the backplane14caused by the PET detectors12, the PET detector assembly is to be designed so that (1) when the total number of PET detector rings in the PET/CT scanner system is an even number, equal number of PET detector rings are on either side of the neutral vertical plane16of the backplane14, and (2) when the total number of PET detector rings in the PET/CT scanner system is an odd number, the side of the neutral vertical plane16that is nearer the CT scanner has n number of PET detector rings and the opposite side of the neutral vertical plane16(i.e., the side farther from the CT scanner) has n−1 number of PET detector rings where n+(n−1)=the total number of PET detector rings. Thus, if the total number of PET detector rings in the PET/CT scanner system is an even number Xeven, the PET detector assembly would comprise Xeven/2 number of PET detector rings on either side of the neutral vertical plane16of the gantry backplane14. If the total number of PET detector rings in the PET/CT scanner system is an odd number Xodd, the PET detector assembly would comprise n=(Xodd+1)/2 number of PET detector rings on the side of the neutral vertical plane16and n−1 number of PET detector rings on the opposite side of the neutral vertical plane16. Thus, one side of the neutral vertical plane16will have one more PET detector ring than the other side. It does not matter, however, which side (i.e., the side farther from the CT scanner or the side closer to the CT scanner) ends up with one more PET detector ring.

An advantage of the PET detector assembly100,100A disclosed herein is that the distance from the CT FOV to the nearest PET detector ring's FOV (the axial FOV separation distance550between PET and CT) is the same for an assembly with three PET detector rings (shown inFIG. 4) and an assembly with four PET detector rings (shown inFIG. 5). This is also true for assemblies of five PET detector rings and six PET detector rings, as well as for assemblies of seven PET detector rings and eight PET detector rings. This makes it simpler to design the system level software that determines the patient bed stroke distance between the PET FOV and the CT FOV because every two PET detector assemblies have the same distance between the PET FOV and the CT FOV. This is further illustrated in Table 1 and described below. The specific arrangement of the PET detector rings can be provided to the system level software such that the software can calculate the proper patient bed stroke.

Table 1 provides exemplary arrangements for rings of PET detectors according to embodiments described herein. Table 1 shows exemplary arrangements for a PET/CT scanner system having a co-scan length of 200 cm. Given that co-scan range, the maximum FOV separation between the PET scanner and the CT scanner is provided for a given number of rings of detectors. As shown in Table 1, in order to maintain the desired co-scan range, the maximum allowed FOV separation is reduced as more rings of PET detectors are added to the PET scanner. This is because the axial FOV of the PET scanner increases with the addition of each ring of detectors.

FIGS. 6 and 7show schematic illustrations of the PET/CT scanner systems40and40A, respectively. Each PET/CT scanner comprises a PET scanner portion510and a CT scanner portion520. The CT scanner portion520has a CT FOV202. The PET/CT scanner system40shown inFIG. 6has a PET detector assembly100comprising three PET detector rings20a-20c, defining the PET FOV, that are mounted on a gantry backplane14defining a neutral vertical plane16. The PET/CT scanner system40A shown inFIG. 7comprises four PET detector rings20a-20d, defining the PET FOV, that are mounted on a gantry backplane14defining a neutral vertical plane16. As shown inFIGS. 6 and 7, each PET/CT scanner system40,40A has the same axial FOV separation550. This is also shown in Table 1, wherein for a co-scan region of 200 cm, an imaging system in which the PET scanner has three or four rings of PET detectors has an axial FOV separation of 517 mm. Similarly, as shown in Table 1, an imaging system in which the PET scanner has five or six rings of detectors has an axial FOV separation of 490 mm. The five-ring assembly results from adding an additional PET detector ring next to ring20con the side of the neutral vertical plane16that is nearer to the CT scanner520. The six-ring assembly results from adding another PET detector ring next to ring20don the other side of the neutral vertical plane16that is opposite from the side of the ring20c. Further, as shown in Table 1, an imaging system in which the PET scanner has seven or eight rings of detectors has an axial FOV separation of 464 mm. The seven-ring assembly results from adding an additional PET detector ring next to the fifth PET detector ring on the side of the neutral vertical plane16that is nearer to the CT scanner520. The eight-ring assembly results from adding another PET detector ring next to the sixth PET detector ring on the other side of the neutral vertical plane16that is opposite from the side of the ring20c.

The PET detector assembly100,100A disclosed herein provides a balanced PET scanner gantry where the PET detectors span a gantry support backplane, represents the most efficient use of the space in the gantry, yielding the most compact design, and the axial length of the detectors alone drive the overall gantry width. Some of the benefits of the PET detector assembly of the present disclosure are: a balanced PET system for the range of the axial PET FOV lengths; achieving the required co-scan range without changing the patient bed and without adding system footprint; reduction in weight and cost of a balanced over a cantilevered gantry, as there is little to no moment to support; and ease of service access to the PET detectors without opening the gantry. To service the PET detectors, the gantry cover needs to be removed to access the PET detectors. However, in the PET/CT scanner system of the present disclosure, the PET detector assembly100,100A and the CT scanner520do not need to be separated to access the PET detectors.

In various embodiments, the PET detectors12can be of the type known as time-of-flight (TOF) detectors. A TOF detector has enhanced sensitivity due to the better positioning (through better electronic timing) of events along a line of coincidence. Alternatively, non-TOF types of detectors may be used. Further, the PET detectors12may include photo sensors such as photomultiplier tubes (PMTs), avalanche photo diodes (APDs) and/or silicon photo multipliers (SiPMs). It should be understood that these are only some examples of PET detectors and that other types of detectors may be used.

It will be understood that the foregoing description is of exemplary embodiments of this invention, and that the invention is not limited to the specific forms shown. Modifications may be made in the design and arrangement of the elements without departing from the scope of the invention.