Altering paths of optical photons passing through a scintillator

A method for altering paths of optical photons that pass through a scintillator. The scintillator includes a plurality of vertical sides. The method includes forming a reflective belt inside the scintillator by creating a portion of the reflective belt inside the scintillator on a vertical plane parallel with a vertical side of the plurality of vertical sides. Creating the portion of the reflective belt includes generating a plurality of defects on the vertical plane.

TECHNICAL FIELD

The present disclosure generally relates to nuclear radiation, and particularly, to nuclear imaging.

BACKGROUND

Radiation is used to acquire images in a variety of imaging systems, such as nuclear medicine that images internal organs of a human body. Radiopharmaceuticals are, generally, introduced into the body, either by injection or ingestion, and produce emissions of annihilation photons (such as gamma photons) emissions that originate from the body. Several detectors may be used to detect emitted annihilation photons and data collected from the detectors may be processed to locate a source of each of the emitted photons. An image may be produced by accumulating a large number of detected locations.

Scintillation crystals are commonly used in imaging systems that utilize radiation emitting materials. These crystals may emit visible light pulses when a high energy radiation (like annihilation photons) passes through the crystals. The pulses of emitted light (i.e., optical photons) may be detected by a photosensitive detector. Crystal surfaces may be coated with reflectors to direct light through internal reflections towards the photosensitive detector. However, significant losses of scintillation light may occur due to photon interactions with the crystal surfaces or reflective coatings on those surfaces. As a result, only a fraction of the scintillation light produced in the crystal may reach the photosensitive detector.

There is, therefore, a need for a method for increasing an amount of scintillation light that reaches a photosensitive detector. There is further a need for a processed scintillation crystal that is able to direct more optical photons towards a photosensitive detector.

SUMMARY

In one general aspect, the present disclosure describes an exemplary method for altering paths of optical photons that pass through a scintillator. The scintillator may include a plurality of vertical sides. An exemplary method may include forming a reflective belt inside the scintillator by creating a portion of the reflective belt inside the scintillator on a vertical plane parallel with a vertical side of the plurality of vertical sides. In an exemplary embodiment, creating the portion of the reflective belt may include generating a plurality of defects on the vertical plane.

In an exemplary embodiment, generating the plurality of defects may include generating the plurality of defects in a rectangular shape. In an exemplary embodiment, generating the plurality of defects in the rectangular shape may include determining a width of the rectangular shape, determining a vertical distance of a center of the rectangular shape from a top surface of the scintillator, and determining a horizontal distance of the rectangular shape from the vertical side based on optimal values for a sensitivity and a spatial resolution of a photosensitive detector. In an exemplary embodiment, the photosensitive detector may be associated with the scintillator.

In an exemplary embodiment, generating the plurality of defects may further include obtaining a curved surface by curving a surface of the rectangular shape. In an exemplary embodiment, the curved surface may include an elliptic curve on a side of the curved surface. The elliptic curve may include a center, a minor axis, and a major axis. In an exemplary embodiment, obtaining the curved surface may further include determining a location of the center of the elliptic curve, determining the minor axis, determining the major axis, and determining a width of the curved profile based on optimal values for a sensitivity and a spatial resolution of the photosensitive detector.

In an exemplary embodiment, creating the reflective belt inside the scintillator may include creating the reflective belt inside a monolithic crystal. In an exemplary embodiment, generating the plurality of defects may include generating each of the plurality of defects in a spherical shape by engraving a subsurface of the vertical plane utilizing a laser beam.

DETAILED DESCRIPTION

The following detailed description is presented to enable a person skilled in the art to make and use the methods and devices disclosed in exemplary embodiments of the present disclosure. For purposes of explanation, specific nomenclature is set forth to provide a thorough understanding of the present disclosure. However, it will be apparent to one skilled in the art that these specific details are not required to practice the disclosed exemplary embodiments. Descriptions of specific exemplary embodiments are provided only as representative examples. Various modifications to the exemplary implementations will be readily apparent to one skilled in the art, and the general principles defined herein may be applied to other implementations and applications without departing from the scope of the present disclosure. The present disclosure is not intended to be limited to the implementations shown, but is to be accorded the widest possible scope consistent with the principles and features disclosed herein.

Herein is disclosed an exemplary method and apparatus for directing optical photons that are created inside a scintillation crystal to a photosensitive detector. The optical photons may be created from an annihilation photon that passes through the crystal. A reflective belt may be engraved inside the crystal to reflect scintillation lights towards the photosensitive detector. The reflective belt may have a rectangular surface or a curved surface. Specifications of the reflective belt may be determined based on a desired sensitivity and spatial resolution of the detector. An exemplary apparatus may be utilized in various imaging techniques that exploit radioactive materials as sources of radiation of annihilation photons (such as gamma photons). Examples of such techniques include positron emission tomography (PET), single photon emission computed tomography (SPECT), computed tomography (CT), etc.

FIG. 1Ashows a schematic of an apparatus for altering paths of optical photons, consistent with one or more exemplary embodiments of the present disclosure. An exemplary apparatus100may include a scintillator102and a reflective belt104. In an exemplary embodiment, scintillator102may include a monolithic crystal. In an exemplary embodiment, scintillator102may include a plurality of vertical sides (including sides106aand106binFIG. 1A). In an exemplary embodiment, scintillator102may be configured to convert annihilation photons to optical photons by annihilating the annihilation photons which enter scintillator102at an annihilation point. In an exemplary embodiment, the annihilation point may be a point annihilation photons may lose enough energy after passing through scintillator102to be annihilated and generate optical photons. In an exemplary embodiment, the annihilation photons may be emitted from a radioactive material.

In an exemplary embodiment, reflective belt104may be located inside scintillator102and may include a plurality of portions (including portions108a,108b,108c, and108dinFIG. 1A). In an exemplary embodiment, a portion108aof plurality of portions may be created inside scintillator102on a vertical plane110that may be parallel with a vertical side106aof the plurality of vertical sides. In an exemplary embodiment, portion108amay include a plurality of defects which may be generated on vertical plane110. In an exemplary embodiment, each of the plurality of defects may have an approximately spherical shape.

FIG. 1Bshows a schematic of a side view of apparatus100, consistent with one or more exemplary embodiments of the present disclosure. Referring toFIGS. 1A and 1B, in an exemplary embodiment, a method may be utilized for altering paths of optical photons that pass through scintillator102. An exemplary method may include forming reflective belt104inside scintillator102by creating portion108aof reflective belt104inside scintillator102on vertical plane110. In an exemplary embodiment, creating portion108amay include generating a plurality of defects112on vertical plane110. In an exemplary embodiment, plurality of defects112may be engraved on a portion of scintillator102that may be located on vertical plane110utilizing a laser beam.

In an exemplary embodiment, different engraving methods may be utilized to generate plurality of defects112, such as subsurface laser engraving (SSLE) or laser-induced optical barriers (LIOB) methods. In these methods, one or more laser beams may be emitted to a specific point determined by an operator. By releasing laser beam energy to that point, a localized heat may be generated which may result in a deformation of a molecular arrangement in a small region around the point. In an exemplary embodiment, the region may be deformed in an approximately spherical form. In an exemplary embodiment, changes in molecular structures and crystalline networks may cause a change in a refractive index of changed regions. In an exemplary embodiment, the change in the refractive index may cause scattering of optical photons produced inside scintillator102, thereby causing plurality of defects112to act as an inner reflector. As a result, the path of the optical photons passing through the scintillator102may be altered after meeting reflective belt104.

In an exemplary embodiment, generating plurality of defects112may include generating plurality of defects112in a rectangular shape114. In an exemplary embodiment, generating plurality of defects112in rectangular shape114may include determining a width116of rectangular shape114and a vertical distance118of a center of rectangular shape114from a top surface120of scintillator102.

FIG. 1Cshows a schematic of a side view of rectangular shape114, consistent with one or more exemplary embodiments of the present disclosure. In an exemplary embodiment, generating plurality of defects112in rectangular shape114may further include determining a horizontal distance122of rectangular shape114from vertical side106a.

FIG. 1Dshows a schematic of a side view of a curved surface, consistent with one or more exemplary embodiments of the present disclosure. Referring toFIGS. 1C and 1D, in an exemplary embodiment, generating plurality of defects112in rectangular shape114may further include obtaining a curved surface124by curving a surface of plurality of defects112in rectangular shape114according to a curved profile. In an exemplary embodiment, the curved profile may refer to the shape of the surface of rectangular shape114from the side view. In other words, curved surface124may conform to a segment of the curved profile. In an exemplary embodiment, the curved profile may include an elliptic curve126. In an exemplary embodiment, elliptic curve126may include a center128, a minor axis130, and a major axis132. In an exemplary embodiment, curving the surface of rectangular shape114may further include determining a location of center128of elliptic curve126, minor axis130, major axis132, and a width134of curved surface124. In an exemplary embodiment, determining the location of center128may include determining distances of center128from top surface120and vertical side106a. In an exemplary embodiment, determining minor axis130and major axis132of elliptic curve126may include determining lengths and orientations of these axes. In an exemplary embodiment, elliptic curve126may be located on a plane perpendicular to vertical side106a.

FIG. 2shows a schematic of an apparatus placed on a photosensitive detector, consistent with one or more exemplary embodiments of the present disclosure. In an exemplary embodiment, a photosensitive detector202may be associated with scintillator102. In an exemplary embodiment, photosensitive detector202may be placed under apparatus100. In an exemplary embodiment, each of plurality of vertical sides may be perpendicular to a surface of photosensitive detector202. In an exemplary embodiment, photosensitive detector202may be configured to convert optical photons to electrical signals.

In an exemplary embodiment, photosensitive detector202performance may be evaluated based on its sensitivity and spatial resolution. In an exemplary embodiment, a ratio of optical photons converted to electrical signals by photosensitive detector202to a total number of optical photons that may be created from annihilation photons entering scintillator102may be referred to as the sensitivity of photosensitive detector202. Therefore, increasing the number of optical photons that reach photosensitive detector202may increase the number of conversions and consequently the sensitivity of photosensitive detector202. In an exemplary embodiment, a minimum distance between two different annihilation points that may be recognized by detecting respective optical photons reaching photosensitive detector202may be referred to as the spatial resolution of photosensitive detector202. Since light rays associated with the optical photons may diverge as they pass through scintillator102, shortening a path that an optical photon may pass before reaching photosensitive detector202may enhance the spatial resolution. In an exemplary embodiment, the optical photon's path may be shortened by decreasing a depth of scintillator102. However, this approach may also decrease a distance that an annihilation photon may pass through scintillator102before exiting scintillator102, which may decrease a probability of the conversion of the annihilation photon to optical photons, thereby decreasing the sensitivity. Therefore, in an exemplary embodiment, there may be a tradeoff in determining different design aspects of apparatus100to obtain optimal values for the sensitivity and the spatial resolution.

Referring toFIGS. 1B-2, in an exemplary embodiment, width116, vertical distance118, and horizontal distance122may be determined based on optimal values for the sensitivity and the spatial resolution of photosensitive detector202. In an exemplary embodiment, variations of the sensitivity and the spatial resolution with respect to variations of each of width116, vertical distance118, and horizontal distance122may be obtained and values of width116, vertical distance118, and horizontal distance122that correspond to desired values of the sensitivity and/or the spatial resolution may be selected as the determined values of width116, vertical distance118, and horizontal distance122. In an exemplary embodiment, the location of center128, minor axis130, major axis132, and width134of curved surface124may also be determined based on the optimal values for the sensitivity and the spatial resolution of photosensitive detector202. In an exemplary embodiment, variations of the sensitivity and the spatial resolution with respect to variations of each of the location of center128, minor axis130, major axis132, and width134may be obtained and values of the location of center128, minor axis130, major axis132, and width134that correspond to desired values of the sensitivity and/or the spatial resolution may be selected as the determined values of the location of center128, minor axis130, major axis132, and width134. In an exemplary embodiment, the desired values of the sensitivity and the spatial resolution of photosensitive detector202may include values that are higher than about 0.9 of maximum values of the sensitivity and/or the spatial resolution.

FIG. 3shows a schematic of a side view of an imaging system, consistent with one or more exemplary embodiments of the present disclosure. In an exemplary embodiment, different elements of apparatus100may be utilized in an imaging system300. In an exemplary embodiment, imaging system300may include a medical imaging system, such as a PET, a SPECT, or a CT.

In an exemplary embodiment, imaging system300may include scintillator102, photosensitive detector202, and reflective belt104. In an exemplary embodiment, scintillator102may be configured to receive an annihilation photon302, convert annihilation photon302to an optical photon304by annihilating annihilation photon302at an annihilation point306, and emit optical photon304from annihilation point306. In an exemplary embodiment, annihilation photon302may be emitted from a radiopharmaceutical308. In an exemplary embodiment, radiopharmaceutical308may include a radioactive material.

In an exemplary embodiment, the conversion of annihilation photon302to optical photon304may occur due to scintillation phenomena in scintillation crystals. In an exemplary embodiment, a large number of optical photons may be created responsive to the annihilation of annihilation photon302. However, a single optical photon is illustrated inFIG. 3to better demonstrate the conversion of annihilation photon302to optical photon304.

In an exemplary embodiment, photosensitive detector202may be configured to convert optical photon304to an electrical signal. In an exemplary embodiment, reflective belt104may be configured to reflect optical photon304towards photosensitive detector202.

In this example, an imaging system with a scintillator equipped with a rectangular reflective belt is demonstrated. An exemplary imaging system includes a lutetium-yttrium oxyorthosilicate (LYSO) scintillation monolithic crystal coupled to an exemplary photosensitive detector (analogous to photosensitive detector202) and a rectangular reflective belt (analogous to reflective belt104). The volume of the LYSO crystal is about 50.2×50.2×20 mm3. The photosensitive detector includes 12×12 light-sensitive cells and the thickness of the reflective belt is about 100 micrometers.

The first step for creating the rectangular reflective is placing the monolithic crystal inside a holder for decreasing probable movements. In the next step, a laser system is utilized to create the reflective belt by sending commands to laser engagement arms. The arms move in three dimensions, and consequently, points of the reflective belt are carved into the crystal. After creating the reflective belt, the crystal surface except the face opposite of the photosensitive detector is wrapped in a reflective sheet-like aluminum or barium sulfate sheet.

After carving the reflective belt with an initial set of specifications, the Monte Carlo simulation is used for evaluation and optimization of the designed reflective belt. The simulation is be performed based on GEANT4 and GATE. Next, the sensitivity and the spatial resolution are measured according to the international standard NEMA NU 4-2008. The design process is then repeated for different sets of specifications, and a set of specifications for the rectangular reflective belt corresponding to optimal values of the sensitivity and the spatial resolution is selected as optimal specifications of the rectangular reflective belt to achieve a high-quality image in terms of optimal values for the spatial resolution and the sensitivity of the photosensitive detector. The optimal specifications of the rectangular reflective belt include a width (analogous to width116) of about 8.6 mm and a center located at a depth (analogous to vertical distance118) of about 37.8 mm from the crystal top surface (analogous to top surface120).

In this example, a performance of an imaging system with a scintillator equipped with a curved reflective belt is evaluated. An exemplary imaging system, similar to the imaging system of EXAMPLE 1 is implemented, except of being equipped with a curved reflective belt with a surface that is curved according to an elliptic profile (analogous to curved surface124). The elliptic profile includes a major axis (analogous to major axis132) and a minor axis (analogous to minor axis130). In order to find optimized major and minor axes, a Monte Carlo simulation is used.

FIG. 4shows variations of the spatial resolution and the sensitivity of the imaging system for different ratios of the minor axis length to the major axis length of the elliptic profile, consistent with one or more exemplary embodiments of the present disclosure. As shown inFIG. 4, a sensitivity402increases as the ratio of the minor axis length to the major axis length increases. In other words, increasing the ratio of the minor axis length to the major axis length increases a number of optical photons that reach an exemplary photosensitive detector.

A spatial resolution404increases with the increase in the ratio of the minor axis length to the major axis length until the ratio reaches about 0.4, at which spatial resolution404is maximized. Increasing the ratio above 0.4 decreases spatial resolution404. Therefore, an optimal value for the ratio is obtained about 0.4, which results in a spatial resolution of about 7.6%.