Gamma Camera的linearity correction

今天看到一个patent的说明: http://www.freepatentsonline.com/20060065826.html, 里面概述了Gamma Camera的linearity correction问题.

Existing scintillation cameras experience spatial distortion that requires linearity correction (LC). The spatial distortion arises from the fact that the spatial coordinates of light events occurring either at the edges of or between adjacent photomultiplier tubes in a photodetector array will be computed differently than the coordinates of events occurring directly over the center of a photomultiplier tube, due to the physical limitations of the photomultiplier tube. A significant amount of effort has been made to developing correction schemes for spatial or linearity distortion (along with, e.g., the companion energy and flood corrections). Existing LC methods can be generally divided into two categories.

A first category is illustrated in U.S. Pat. No. 3,745,345 (the '345 patent) entitled Radiation Imaging Device, the entire disclosure of which is incorporated herein by reference. Here, a camera head is covered by a lead mask having a uniform grid of pinhole apertures. A sheet source of uniform radiation placed adjacent to the mask causes each aperture to illuminate a scintillation crystal located on the opposite side of the mask. The camera then records the detected location of events in the crystal. There is a difference between the (known) location of the pinholes and the detected location of the events as computed by the camera, which is representative of the degree of spatial distortion at the respective locations on the camera face. Accordingly, a correction factor is computed for each location point so as to move the apparent location of an event as detected to its actual location, as determined by the difference computed in the flood source calibration procedure. The correction factors are then stored in an array for later use during acquisition of clinical images.

[0007] A second category is illustrated in U.S. Pat. No. 4,212,061 entitled Radiation Signal Processing and U.S. Pat. No. 4,316,257 entitled Dynamic Modification Of Spatial Distortion Correction Capabilities Of Scintillation Camera, which pertain to spatial correction (both the '061 and '257 patents also are incorporated herein in their entirety by reference). For calibration, a lead mask having elongated slit apertures is used. The camera is exposed to a radiation source, first with the mask oriented in x lines and then with the mask oriented in y lines. For each such exposure orientation, a series of transverse peak measurements at select intervals is developed. An analytical expression is generated to represent event coordinates between calibration intervals. Each orientation exposure, thus, produces one of a pair of calibration coordinates, which in turn permit direct correspondence to associated spatial coordinates. Among other deficiencies in this method, this method can take more than one hour of time by itself. It also requires additional preparation such as `centering and gain`. Moreover, this method requires use of multiple masks wastes time and money and increases equipment downtime.

The present inventors have co-developed a new type of flood calibration mask having a much denser population of pinhole apertures in a non-uniform grid pattern, which is used in conjunction with a novel Gaussian fit algorithm to obtain a complete pinhole mask image model for LC coefficient generation. A LC coefficient represents a displacement vector of a point from its detected location in an acquired image to an ideal location.