Depth of interaction system in nuclear imaging

Details Depth of interaction system in nuclear imaging Depth of interaction system in nuclear imaging

1999-10-26

2002-10-01

Epps, Georgia (Department: 2873)

Radiant energy

Invisible radiant energy responsive electric signalling

Semiconductor system

C250S370010, C250S370090, C250S591000, C250S367000

Reexamination Certificate

active

06459085

ABSTRACT:

The present invention is directed to a method and system for determining depth of interaction (DOI) of gamma rays with a scintillation crystal used in a nuclear imaging apparatus. More particularly the invention is directed to a method and system for establishing the depth of interaction of gamma rays by use of scintillating wavelength-shifting fibers disposed near the scintillation crystal. These fibers assess the distribution width of scintillation light received from the scintillation crystal to establish depth of interaction.
A variety of nuclear imaging methodologies are in wide spread use in the medical community. These systems include, for example, Anger (gamma) cameras and positron emission tomography (PET). In the case of the Anger gamma camera shown in an exploded view in
FIG. 1
, these systems have been an important part of nuclear medicine laboratories for many years. The Anger camera also determines the distribution of radio pharmaceuticals in a patient. Projection images are derived over a short time period by determining sequentially the coordinates (x,y) of the gamma ray interaction site of each legitimate photon in the scintillator crystal. The nuclear images which are produced have diminished resolution as a result of the uncertainty in depth of gamma ray interaction with the scintillator crystal.
PET has found significant clinical applications because of the favorable biodistribution of positron tracers, such as F-18 FDG. Because of the high costs associated with dedicated PET scanners, dual-head Anger (gamma) camera systems, normally used for single-photon emission computed tomography (SPECT) and whole body imaging, have attracted commercial interest as an alternative to PET systems for the imaging of positron tracers. The two opposed Anger cameras detect in coincidence the pair of 511 keV photons emitted from the annihilation of a positron and a local electron. Ideally, the annihilation site is localized to lie along a line connecting the detected interaction positions in the two opposed cameras. After collecting a large number of such coincidence events, image reconstruction techniques are used to derive the three-dimensional distribution of the positron tracer. Most PET systems use ring- or cylinder-shaped detectors to achieve 2&pgr; coverage and data sampling. With dual-head Anger cameras, although rotation of the cameras can achieve the same sampling, the detector coverage is only about ⅓ of the 2&pgr; circumferential coverage of dedicated PET systems.
Because dedicated PET systems are optimized for coincidence imaging, the quality of PET images will always be superior to that of images obtained with multi-purpose dual-head Anger cameras. Lower photon detection efficiency and modest reconstructed spatial resolution are the primary reasons why the image quality for coincidence imaging with dual-head Anger cameras lags behind that of dedicated PET systems. The detection efficiency for 511 keV gamma rays in a 9.5 mm (⅜″) thick NaI(Tl) crystal used in standard Anger cameras is slightly less than one-third that of a conventional 25 mm (1.0″) thick BGO crystal. Therefore, the intrinsic detection efficiency for coincidence measurements (proportional to the square of the detection efficiency of a single detector) with dual-head Anger cameras is only about one-tenth that of a BGO-based PET system of the same solid angle coverage. However, if the loss of image quality is within acceptable limits to provide diagnostic information, the much lower cost of a dual-head Anger camera system compared to a dedicated PET system is very attractive and should justify further investigation and optimization of coincidence imaging techniques with dual-head Anger cameras. Coincidence imaging with dual-head Anger cameras can be a practical way for many community hospitals to offer positron imaging capabilities to their patient population. All the major camera manufacturers now offer dual-head Anger camera systems with coincidence imaging capability, such as ADAC's MCD, Picker's &ggr;PET, GE's CoDe5 and Siemen's e-cam.
To boost detection efficiency and stopping power, camera manufacturers have since increased the thickness of the scintillation crystals of their dual-head systems from the previous ⅜″ to {fraction (8/5)}″. One manufacturer even goes up to ¾″ in crystal thickness. A direct consequence of using thick crystals is degraded intrinsic spatial resolution. The increased thickness causes greater spreading of scintillation light from interaction sites in the crystal. The effect of light spreading is less of a problem for coincidence imaging than for low-energy single-photon imaging because 511 KeV photons penetrate more deeply into the crystal—less spreading of light occurs for interactions deep in the scintillation crystal. The loss of intrinsic resolution for single-photon imaging is marginally acceptable, mainly because collimator resolution is more important than intrinsic resolution in single photon imaging. Therefore, the use of thicker crystals in Anger cameras is a practical, but not optimal, compromise for an imaging system designed for both single-photon and positron-coincidence imaging.
Another consequence of using thick crystals is parallax error, which manifests as degraded reconstructed image resolution in coincidence imaging. Parallax error occurs when gamma rays enter the detector obliquely. The true interaction position for gamma ray &ggr; is at depth z and transverse position x. In a detector that does not provide DOI information (e.g., a conventional Anger camera), all gamma rays that interact at the transverse position x are assumed to have the same depth coordinate z′ (the average depth of interaction). This assumption often misplaces the detection site of the gamma ray. For single-photon imaging, this mispositioning is not significant because gamma rays are constrained by most collimators to enter the crystal at normal or near-normal angles of incidence. For coincidence imaging, which does not use collimators, 511 keV photons can enter the detectors obliquely; mispositioning can occur for both detected annihilation photons in a coincidence event. PET reconstruction algorithms assume that the source activity is located along the line joining the sites of the two detected photons. Consequently, if the endpoints of the line are mispositioned, the reconstructed source distribution is inaccurate. The error in the reconstruction caused by this mispositioning is reflected by degraded spatial resolution. For example, for a cylindrical system, the spatial resolution is degraded in the radial resolution in the peripheral imaging field; and for planar detector-based systems (such as dual head Anger cameras) blurring due to parallax is more an elliptical blur at r=o, as opposed to r>o for cylindrical systems. Note that this parallax error is more significant when thick crystals are used, and when large-area plate detectors are used for coincidence imaging of positron sources distributed over large volumes, the situation encountered in using dual-head cameras for coincidence imaging of the body.
Therefore, the best detection sensitivity and reconstructed image resolution in coincidence imaging with dual-head Anger cameras can be achieved by using thick scintillation crystals and making real-time DOI measurements along with the conventional x and y positions.
Various techniques to obtain DOI information in scintillation detectors have been proposed in the last dozen years. Most of these techniques were for PET, and to date there has been no practical technique that is applicable to commercial Anger cameras.
In one method in 1986, two different layers of scintillation detectors were used which were optically coupled to each other. The interactions that occur in each layer are distinguished based on the different decay times of the two scintillators. A phoswich detector can be used and this consists of a layer of LSO optically coupled to a layer of YSO or NaI(Tl). The phoswi

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