Megavoltage imaging with a photoconductor based sensor

Abstract

A photodetector for detecting megavoltage (MV) radiation comprises a semiconductor conversion layer having a first surface and a second surface disposed opposite the first surface, a first electrode coupled to the first surface, a second electrode coupled to the second surface, and a low density substrate including a detector array coupled to the second electrode opposite the semiconductor conversion layer. The photodetector includes a sufficient thickness of a high density material to create a sufficient number of photoelectrons from incident MV radiation, so that the photoelectrons can be received by the conversion layer and converted to a sufficient of recharge carriers for detection by the detector array.

1 . A photodetector comprising:
 a semiconductor conversion layer having a first surface and a second surface disposed opposite the first surface;   a first electrode coupled to the first surface of the semiconductor conversion layer;   a second electrode coupled to the second surface of the semiconductor conversion layer;   a low density substrate coupled to the second electrode opposite the semiconductor conversion layer, the low density substrate including a detector array;   a high density layer disposed over the first electrode opposite the semiconductor conversion layer;   wherein the high density layer includes a thickness of a high density material to create a number of photoelectrons caused by mega-voltage x-ray photons incident upon the high density layer, the photoelectrons to be received by the semiconductor conversion layer;   wherein the first electrode includes a thickness of another high density material to create a number of photoelectrons caused by mega-voltage x-ray photons incident upon the first electrode to be received by the semiconductor conversion layer;   wherein the semiconductor conversion layer includes a thickness of a photo-conductive material to convert photoelectrons received from one of the high density layer and the first electrode to free charge carriers for detection by the detector array, wherein detected free charge carriers provide an image, and   wherein the low density substrate has a density and thickness so that mega-voltage x-ray photons incident upon the low density substrate create no more than an insubstantial amount of photoelectrons to be converted by the semiconductor conversion layer.   
     
     
         2 . The photodetector of  claim 1 , wherein the detector array comprises: a plurality of pixels, each pixel comprising a pixel electrode coupled to a thin film capacitor; and wherein the low density substrate comprises one of a glass material and a plastic material. 
     
     
         3 . The photodetector of  claim 1 , wherein the high density layer comprises a sufficient thickness of the high density material to reduce detection of scattered mega-voltage x-ray photons from a subject object due to a longer path length of the scattered photons in the high density layer as compared to detection of unscattered mega-voltage x-ray photons from the subject object due to a shorter path length of the unscattered photons in the high density layer. 
     
     
         4 . The photodetector of  claim 1 , wherein the high density layer comprises:
 a thickness of between 0.1 and 10 millimeters.   
     
     
         5 . The photodetector of  claim 1 , wherein the high density layer comprises one of copper, lead, tungsten, tantalum, iridium, a leaded glass, a leaded plastic, and an alloy thereof. 
     
     
         6 . The photodetector of  claim 1 , wherein the high density layer comprises:  a plurality of layers. 
     
     
         7 . The photodetector of  claim 1 , wherein the high density layer comprises:  a thickness of between 0.5 and 1.5 millimeters of one of lead, copper, a lead alloy, and a copper alloy. 
     
     
         8 . The photodetector of  claim 1 , wherein the first electrode is a continuous upper conducting electrode covering the first surface of the semiconductor conversion layer. 
     
     
         9 . The photodetector of  claim 8 , wherein the continuous upper conducting electrode comprises: one of palladium, platinum, carbon, gold, copper, aluminum, and an alloy thereof having a thickness of between 0.1 and 1000 microns. 
     
     
         10 . The photodetector of  claim 8 , wherein the continuous upper conducting electrode is coupled to a voltage generator to place a voltage bias between the semiconductor conversion layer and the second electrode. 
     
     
         11 . The photodetector of  claim 8 , wherein the continuous upper conducting electrode comprises:
 a thickness of conductive material to produce high energy photoelectrons from mega-voltage x-ray photons incident upon the continuous upper conducting electrode, the photoelectrons having sufficient energy to generate a sufficient number of free charge carriers in the semiconductor conversion layer for detection by the detector array.   
     
     
         12 . The photodetector of  claim 1 , wherein the semiconductor conversion layer is a photoconductive layer having a thickness of between 50 and 2000 microns of one of mercuric iodide (HgI 2 ), lead iodide (PbI 2 ), bismuth triodide (BiI 3 ), amorphous selenium, and thallium bromide (TlBr). 
     
     
         13 . The photodetector of  claim 12 , wherein the photoconductive layer is a wide band gap photoconductive material having a sufficient thickness to generate a sufficient number of free charge carriers from high energy photoelectrons received from one of the high density layer and the first electrode to be detected by the detector array. 
     
     
         14 . The photodetector of  claim 1 , wherein the second electrode comprises:  an upper pixilated electrode having a plurality of pixels of conductive material, each pixel coupled to a voltage generator to generate a voltage bias between the pixel and one of the first electrode and the high density layer. 
     
     
         15 . The photodetector of  claim 14 , wherein the first electrode is a continuous upper conducting electrode over the plurality of pixels, and the low density substrate comprises an array of pixilated electronic switches and capacitors having a plurality of detection pixels, each detection pixel comprising a switch and a capacitor, and associated with each pixel of the pixilated electrode. 
     
     
         16 . The photodetector of  claim 15 , wherein the detector array comprises:  analog to digital converters to create a digital image file from free charge carriers collected by the capacitors from the semiconductor conversion layer and switched to the analog to digital converters. 
     
     
         17 . The photodetector of  claim 1 , wherein the high density layer and the first electrode are combinatorially configured to convert 0.25-20% of mega-voltage x-ray photons incident upon the photodetector to free charge carriers. 
     
     
         18 . The photodetector of  claim 1 , wherein the high density layer, and the first electrode are combinatorially configured to convert 0.25-20% of mega-voltage x-ray photons incident upon the photodetector to free charge carriers. 
     
     
         19 . The photodetector of  claim 1 , wherein the high density layer comprises a density between 5 and 20 g/cm 3 . 
     
     
         20 . The photodetector of  claim 1 , wherein the first electrode is adhered to the semiconductor conversion layer. 
     
     
         21 . The photodetector of  claim 1 , wherein the low density substrate has a density selected to allow a majority of mega-voltage x-ray photons to pass through the low density substrate. 
     
     
         22 . The photodetector of  claim 1 , wherein the low density substrate has a density of between 0.1 and 8 g/cm 3 . 
     
     
         23 . The photodetector of  claim 1 , wherein the photodetector is configured to be a bidirectional detector by providing images from x-rays incident upon the first surface of the semiconductor conversion layer and images from x-rays incident upon the second surface of the semiconductor conversion layer. 
     
     
         24 . The photodetector of  claim 1 , wherein the high density layer and the first electrode are combinatorially configured to convert between 1 and 2% of incident mega-voltage x-ray photons to high energy photoelectrons for conversion by the semiconductor conversion layer to free charge carriers. 
     
     
         25 . The photodetector of  claim 1 , wherein the semiconductor conversion layer is a single continuous layer of semiconductor material. 
     
     
         26 . The photodetector of  claim 1 , wherein the high density layer is touching the first electrode. 
     
     
         27 . A photodetector comprising:
 a semiconductor conversion layer having a first surface and a second surface disposed opposite the first surface, the semiconductor conversion layer comprising a thickness of between 20 and 20000 microns of one of mercuric iodide and lead iodide;   a first electrode comprising a continuous upper conducting electrode of palladium or carbon coupled to the first surface of the semiconductor conversion layer;   a second electrode comprising an upper pixilated electrode of indium tin oxide coupled to the second surface of the semiconductor conversion layer;   a low density substrate coupled to the second electrode opposite the semiconductor conversion layer, the low density substrate comprising an unleaded glass plate having a detector array, the detector array including a plurality of amorphous silicon thin film transistor (TFT) pixilated electronic switches and amorphous silicon thin film transistor (TFT) pixilated electronic capacitors;   a high density layer comprising a thickness of between 0.2 and 1.5 millimeters of lead or copper over the first electrode opposite the semiconductor conversion layer; and   wherein the thickness of the semiconductor conversion layer is inversely proportional to the thickness of the high density layer.   
     
     
         28 . A method comprising:
 absorbing megavoltage X-ray photons incident upon a high density layer;   producing high energy photoelectrons from the megavoltage X-ray photons;   generating free charged carriers in a photoconductive layer from the photoelectrons; and   detecting the free charge carriers at a plurality of pixels on a low density substrate,
 wherein substantially all of the high energy photoelectrons are produced by the collision of the megavoltage X-ray photons and the high density layer, and no more than an insubstantial number of high energy photoelectrons are produced by the collision of the megavoltage X-ray photons and the low density layer. 
   
     
     
         29 . The method of  claim 28 , further comprising:
 applying a voltage bias between pixels of the upper pixilated electrode and one of the high density layer and the continuous upper conducting electrode;   depositing the free charge carriers generated in the photoconductive layer into pixilated capacitors of the low density substrate using the bias;   electronically switching the free charge carriers collected in the pixilated capacitor to detection electronics;   converting the free charge carriers switched to the detection electronics from analog to digital signals to create an electronic digital image file;   displaying the electronic digital image file on a display.   
     
     
         30 . The method of  claim 28 , further comprising: filtering out megavoltage x-ray photons scattered by a subject object from producing high energy photoelectrons. 
     
     
         31 . The method of  claim 28 , wherein no more than an insubstantial number of high energy photoelectrons comprise substantially no high energy photoelectrons.