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Automated LED-Based Calibration Methodology and Apparatus for PET and PET–CT

IP.com Disclosure Number: IPCOM000012149D
Publication Date: 2003-Apr-11
Document File: 3 page(s) / 46K

Publishing Venue

The IP.com Prior Art Database

Abstract

PET is performed using block-based detector system in most good PET platforms. The front-end detector system, consisting of blocks of Bismuth Germinate (BGO) crystals are organized into M x M crystals optically coupled to dual or quad photomultiplier tubes (PMT). Signals from the individual crystals are decoded by taking ratios of the four PMT signals. Historically, front-end detector calibration for PET has consisted of the following four calibrations: Position Maps, PMT Gains, Energy, and Normalization. The method and apparatus described herein is based on the use of a well-controlled light source placed on the front end of each crystal in the PET system. With proper control, stability and sequencing, this system can allow pulsing of the crystals in such a manner as to produce the signals required to perform all of the aforementioned calibrations.

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FIELD OF TECHNOLOGY:

Positron Emission Tomography (PET)

INVENTION TITLE:

Automated LED-Based Calibration Methodology and Apparatus for PET and PET – Computed Tomography

PROBLEM/BACKGROUND:

Currently, PET is performed using block-based detector system in most good PET platforms. The front-end detector system, consisting of blocks of Bismuth Germinate (BGO) crystals are organized into M x M crystals optically coupled to dual or quad photomultiplier tubes (PMT). Signals from the individual crystals are decoded by taking ratios of the four PMT signals. Historically, front-end detector calibration for PET has consisted of the following four calibrations, briefly described below:

Position Maps. The purpose of the position map calibration in a block-detector based PET system is to allow proper crystal selection of an event in the block. This calibration is performed by flooding a PET detector block with a 511keV photon source. The resultant signals are surface-mapped, and a peak-finding algorithm is used to determine the locations of the N x N crystal peaks.

PMT Gains. The purpose of this calibration is to adjust the gain of a PMT such that it falls within performance specifications. This calibration is done also by flooding a block with 511keV photons.

Energy. This calibration determines, on a crystal-by-crystal basis, the settings to be used in electronics to determine whether an event positioned in the crystal has deposited an amount of energy such that it falls within the specified energy window. If the event in the crystal produces a detected signal within this range, the event becomes a qualified event.

Coincidence Timing. PET functions on the principle that from positron annihilation, the two resultant 511keV photons must be detected within a given amount of time of one another in order to form a valid true coincidence. This allotted time is referred to as the coincidence window, and that time is commonly set at 12 nanoseconds. The response of the detector system, however, is prone to have time delays due to lengths of cables, detector response times, and crystal response times. This effect is removed by determining the relative response time of all the blocks (crystals) to those in coincidence across the scanner, and a calibration factor (mean centered about 0) is determined to use as an offset to the block (crystal). To calibrate the system, a positron source is typically rotated around the PET detector ring, and the time of arrival of signals between allowed coincident blocks (crystals) are analyzed, which form this calibration.

Normalization. Currently, this calibration is performed by rotating a low-activity rod-source and measuring the coincidence response of the system. This calibration is used to normalize out any line-of-response (LOR) dependent system response. It should be noted that for 2D acquisitions, a rotating rod source method is used. While for 3D, the current method uses the 2D information as well as coincidence response...