Patent Publication Number: US-2016220221-A1

Title: Apparatuses And Methods For Determining The Beam Width Of A Computed Tomography Scanner

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to co-pending U.S. Provisional Application Ser. No. 62/111,191, filed Feb. 3, 2015, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     Computed tomography (CT) is an imaging technology used in diagnostic radiology that uses x-rays to produce images that correspond to two-dimensional “slices” through a patient.  FIG. 1  schematically illustrates a typical CT scanner  10 . As shown in this figure, the scanner  10  includes a table  12  upon which the patient lies that can be passed through a circular gantry  14 . An x-ray tube and an imaging radiation detector (not shown) are mounted on the gantry  14  and rotate around the patient as the table  12  passes through the gantry. 
     Such CT scanners normally use a “fan-beam” geometry in which the x-rays are restricted to a volume that is relatively wide in a transverse (left/right) direction but relatively narrow in longitudinal (head/foot) direction.  FIG. 1  illustrates an example of such a beam  16  of radiation. The intensity of the radiation as a function D(x) of position x along the longitudinal axis of the scanner is referred to as the radiation profile.  FIG. 2  is a graph that shows an example of a measured radiation profile. The width of the radiation profile is referred to as the “radiation profile width” or simply the “beam width.” This dimension is identified in  FIG. 1 . A typical CT scanner may have anywhere from a half dozen to a dozen different beam widths that can be used. 
     Measurement of the beam width is considered an important part of quality assurance programs for CT scanners. This is because only the central part of the beam is used to produce the CT images, while the full width of the beam contributes to the radiation dose of the patient. Thus, a beam that is wider than necessary imparts more radiation to the patient than is necessary. The American College of Radiology (ACR) has for several years recommended that the beam width of CT scanners be measured as part of an annual quality assurance program for all of the beam widths in clinical use. 
     The standard technique for measuring the beam width of a CT scanner is to expose film to the beam. Standard radiographic film may be used, although self-developing radiochromic film is more commonly used. The film is placed at the isocenter of the scanner (i.e., the intersection of the central plane of the radiation field and the axis of rotation of the gantry) and a CT exposure is made without moving the patient table. The x-ray radiation darkens a stripe on the film and the width of this stripe is equal to the beam width. The width of the stripe can be measured using a ruler or regular markings on the film, or by using a magnifier device with a graticule. However, because the edge of the stripe is invariably blurred, these measurements are subjective in nature and therefore imprecise. Although the width can be measured more precisely by using an optical scanner to digitize the image of the film and using image processing software to measure the width of the stripe, this procedure is much more difficult than the former methods. 
     One variation that has been proposed is to use computed radiographic (CR) plates in place of the film. Unlike film, the plates are reusable. This technique, therefore, does not consume any supplies. However, the technique involves greater effort than measuring a stripe on radiochromic film and cannot be used over as wide a range of radiation levels as film. The CR plates will saturate at all but the lowest radiation levels available on a typical CT scanner. 
     At least one manufacturer has developed an electronic device to accurately measure CT beam width. The device comprises a thin radiation detector that is placed on the patient table. The table is moved continuously during a CT scan and the radiation dose rate as a function of time is recorded on a computer. The dose rate can be converted to a measurement of the radiation dose profile as a function of position, and the beam width can be calculated from the dose profile. Although this provides an accurate measurement of the beam width, it is a more cumbersome procedure than using radiochromic film. In addition, the device is much more expensive than the other alternatives. 
     The pencil ionization chamber is one of the standard tools used by medical physicists when performing quality assurance tests on CT scanners. This device measures the total radiation integrated over a standardized length (e.g., 10 cm) along the longitudinal axis of the CT scanner gantry. In some embodiments, the pencil ionization chamber is a 0.5 inch diameter rod made of radiolucent plastic having a hollow, air-filled chamber inside. The pencil ionization chamber is connected to a radiation dosimeter, which measures the average air kerma (i.e., the radiation dose) in the chamber. The average air kerma is then multiplied by the length of the chamber, either automatically by the dosimeter or manually by the user, to determine the “dose-length product,” or DLP. The DLP is the integral of the radiation profile D(x) over the length of the ionization chamber: 
       DLP=∫ D ( x ) dx  
 
     One technique that has been informally discussed is to use an established technique such as film to measure the beam width for a single collimation (beam width) setting, and then measure the DLP of the beam using a pencil ionization chamber for all available collimation settings. The ratio of the DLP for the first setting to the width of the beam measured using film is then used as a calibration constant that can be used to determine the beam width for other settings. This represents a significant simplification compared to using film to measure all beam widths, but still has the drawbacks associated with a single beam width measurement using film. 
     As can be appreciated from the above discussion, there is a need for an inexpensive and simple way to quickly and accurately determine the beam width of a CT scanner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure may be better understood with reference to the following figures. Matching reference numerals designate corresponding parts throughout the figures, which are not necessarily drawn to scale. 
         FIG. 1  is a schematic diagram of a typical computed tomography (CT) scanner. 
         FIG. 2  is a plot of an example radiation profile of a CT scanner. 
         FIG. 3A  is an end view of a first embodiment of a mask that can be used in a method for determining the beam width of a CT scanner. 
         FIG. 3B  is a side view of the mask of  FIG. 3A . 
         FIG. 4A  is a schematic view illustrating a pencil ionization chamber positioned at the isocenter of a CT scanner. 
         FIG. 4B  is a schematic view illustrating the pencil ionization chamber positioned at the isocenter of a CT scanner when the mask of  FIG. 3  is placed over the chamber. 
         FIGS. 5A and 5B  are graphs that plot the radiation profile of a CT scanner measured by a pencil ionization chamber without and with the mask of  FIG. 3 , respectively. 
         FIG. 5C  is a graph that plots the difference between the two graphs of  FIGS. 5A and 5B . 
         FIG. 6A  is an end view of a second embodiment of a mask that can be used in a method for determining the radiation profile width of a CT scanner. 
         FIG. 6B  is a partial, cross-sectional side view of the mask of  FIG. 6A . 
     
    
    
     DETAILED DESCRIPTION 
     As described above, there is a need for an inexpensive and simple way to quickly and accurately determine the beam width of a computed tomography (CT) scanner. Disclosed herein are apparatuses and methods for measuring beam width. The apparatuses include a radiopaque mask that can be placed over a pencil ionization chamber. In some embodiments, the radiation dose at the center of the x-ray beam is measured using the pencil ionization chamber both with and without the mask present. The beam width can then be calculated based upon these measurements and a further measurement performed at a different beam width setting. 
     In the following disclosure, various specific embodiments are described. It is to be understood that those embodiments are example implementations of the disclosed inventions and that alternative embodiments are possible. All such embodiments are intended to fall within the scope of this disclosure. 
       FIGS. 3A and 3B  illustrate a first radiopaque mask  20  that can be used in conjunction with a pencil ionization chamber for the purpose of determining the beam width. More particularly, the mask  20  can be slid over the pencil ionization chamber when the radiation dose of the x-ray beam is measured using the pencil ionization chamber. As used herein, the term “radiation dose,” or simply “dose,” is used to refer to any radiation quantity that is proportional to the intensity of radiation of a given beam quality, whether exposure, air kerma, or absorbed dose. As is apparent from  FIGS. 3A and 3B , the mask  20  comprises a continuous cylindrical body  22  that includes a cylindrical outer surface  24 , a concentric cylindrical inner surface  26  (which defines a passage through which the pencil ionization chamber can pass), a first end surface  28 , and a second end surface  30 . In some embodiments, the end surfaces  28 , 30  are generally perpendicular to the inner surface  26  and the longitudinal direction of the body  22 . The dimensions of the body  22  can depend upon the dimensions of the pencil ionization chamber with which the mask  20  is used. In some embodiments, the body  22  has an outer diameter (surface  24 ) of approximately 0.75 inches, an inner diameter (surface  26 ) of approximately 0.5 inches, and a length of approximately 0.4 inches (see  FIG. 3B ). Irrespective of its dimensions, the body  22  is made of a radiopaque material, such as tungsten. In some embodiments, the body  22  is hollow and is constructed of tungsten walls that are approximately 3 mm thick. The transmission of x-rays with photon energies typical of CT scanners through 3 mm of tungsten is much less than 0.1%. 
     With particular reference to  FIG. 3B , the ends of the body  22  can comprise chamfers  32  and  34  that minimize the effects of a tilt error when positioning the mask  20  and pencil ionization chamber within the CT scanner. In some embodiments, the chamfers  32 ,  34  form an angle, θ ( FIG. 3B ), of approximately 6 degrees relative to the end surfaces  28 ,  30 . As is further shown in  FIG. 3B , the body  22  can include a visible centerline  36  that denotes the center of the mask  20  along its longitudinal direction and aids in the centering of the mask within the x-ray beam. Most CT scanners are equipped with alignment lasers that mark the central plane of the x-ray beam. When the line  36  is illuminated by the alignment laser, the mask  20  is centered in the beam. 
     As noted above, the mask  20  can be used to determine the beam width of a CT scanner. To do this, the pencil ionization chamber can be placed at the isocenter of the gantry of the CT scanner such that it extends along the longitudinal axis of the gantry and the width direction of the x-ray beam when it is emitted. This is illustrated in  FIG. 4A . The radiation dose within the pencil ionization chamber can then be measured during a brief x-ray exposure.  FIG. 5A  shows an example radiation profile during such a measurement. The DLP when the mask is not present, i.e., DLP nomask , is the integral over this profile. 
     Next, the mask  20  is slid onto the pencil ionization chamber and positioned at the center of the x-ray beam, as illustrated in  FIG. 4B . At this point, the radiation dose within the pencil ionization chamber is measured during an x-ray exposure identical to the previous exposure.  FIG. 5B  shows an example radiation profile during such a measurement. The DLP when the mask if present, i.e., DLP mask , is the integral over this profile. 
     Once these two measurements have been made, the accumulated radiation dose D(0) at the center of the radiation profile D(x) can be calculated by subtracting DLP mask  from DLP nomask  and dividing the mask length, L mask : 
     
       
         
           
             
               
                 
                   
                     D 
                      
                     
                       ( 
                       0 
                       ) 
                     
                   
                   = 
                   
                     
                       ( 
                       
                         
                           DLP 
                           nomask 
                         
                         - 
                         
                           DLP 
                           mask 
                         
                       
                       ) 
                     
                     
                       L 
                       mask 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
       FIG. 5C  shows the difference between the two radiation profiles of  FIGS. 5A and 5B . The value (DLP nomask −DLP mask ) is equal to the integral of this differential profile. The value at the center of the differential profile is equal to the integral divided by the length of the profile. 
     Once the value of D(0) has been determined, it is possible to determine the beam width W beam  for any nominal collimation (nominal beam width) by changing the collimation setting to a new setting (and therefore a new beam width) and acquiring a new DLP new  measurement without the mask. The beam width W beam  can then calculated by dividing the DLP new  by D(0) 
     
       
         
           
             
               
                 
                   
                     W 
                     beam 
                   
                   = 
                   
                     
                       DLP 
                       new 
                     
                     
                       D 
                        
                       
                         ( 
                         0 
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
     The above-described procedure requires less work than determining the beam width using any of the currently practiced methods. Experiments performed by the inventors indicate that the measured beam width is typically accurate to within 0.3 mm for a 40 mm wide beam, implying a 0.8% accuracy for the value of D(0). 
       FIGS. 6A and 6B  illustrate a second radiopaque mask  40  that can be used in conjunction with a pencil ionization chamber for the purpose of determining beam width. With reference to  FIG. 6A , the mask  40  comprises a body  42  having a cylindrical outer surface  44 , a concentric cylindrical inner surface  46  (which defines a passage through which the pencil ionization chamber can pass), a first end surface  48 , and a second end surface (not visible). The dimensions of the body  42  can depend upon the dimensions of the pencil ionization chamber with which the mask  40  is used. In some embodiments, however, the body  42  has an outer diameter (surface  44 ) of approximately 0.75 inches, an inner diameter (surface  46 ) of approximately 0.375 inches, and a length of approximately 4 inches. 
     With reference to  FIG. 6B , the body  42  can be formed by two radiopaque cylinders  50  and  52  that are joined by a central radiolucent cylindrical spacer  54 . In some embodiments, the cylinders  50 ,  52  are made of tungsten and the spacer  48  is made of a polymer material, such as poly(methyl methacrylate) (PMMA). As shown in  FIG. 6B , the cylinders  50 ,  52  can each have a circular outer notch  56 ,  58  that is adapted to receive an end of the spacer  54 . 
     To measure the beam width using the mask  40 , the mask is slid onto the pencil ionization chamber, positioned at the center of the x-ray beam, and the radiation dose within in the pencil ionization chamber is measured during an x-ray exposure. Because the only portion of the pencil ionization chamber that is exposed to the x-rays is the central part that aligns with the spacer  54 , the accumulated radiation dose D(0) at the center of the radiation profile D(x) can be calculated by dividing the DLP with the mask  40  in place, i.e., DLP mask , by L mask : 
     
       
         
           
             
               
                 
                   
                     D 
                      
                     
                       ( 
                       0 
                       ) 
                     
                   
                   = 
                   
                     
                       DLP 
                       mask 
                     
                     
                       L 
                       mask 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     3 
                   
                   ) 
                 
               
             
           
         
       
     
     where L mask  is the distance between the proximal ends  60 ,  62  of the cylinders  50 ,  52 . Once the value of D(0) has been calculated, it is possible to determine the beam width W beam  for any nominal collimation (nominal beam width) by changing the collimation setting, acquiring a new DLP measurement, DLP new  without the mask, and dividing DLP new  by D(0) as in Equation 2. This procedure also requires less work than measuring the beam width using any of the currently practiced methods. 
     It is noted that an alternative approach would be to use a removable spacer. In such a case, a user could place the two radiopaque cylinders on the pencil ionization chamber and use the removable spacer to set the distance between them. The spacer could then be removed before taking the radiation dose measurement. 
     The approaches described above require a pencil chamber whose active length is substantially longer than the radiation profile width. The active length of the chamber is the length that is sensitive to radiation. The standard pencil chamber is 10 cm long, so the approaches described above may not suitable for radiation profiles substantially wider than 8 cm. However, the method may be adapted for wider beams by acquiring a series of measurements by longitudinally moving the pencil chamber a fixed distance from one measurement to another. As long as the total distance moved plus the length of the chamber is substantially wider than the radiation beam, the DLP of the beam can be determined from the measurements. 
     For example, assume that the radiation beam width is set to 16 cm and the active length of the pencil chamber is 10 cm. The DLP can be determined by taking radiation dose measurements for three different x-ray exposures. All three measurements can be taken with the longitudinal axis of the pencil chamber coincident with the longitudinal axis of the scanner. The first exposure can be taken with the center of the pencil chamber 10 cm from the isocenter, the second exposure can be taken at the isocenter, and the third exposure can be taken 10 cm from the isocenter on the opposite side of the scanner from the first exposure. In such a case, the DLP is the sum of these three measurements. Alternately, five measurements can be taken by moving the chamber 5 cm from one measurement to the next and then dividing the sum of the five dose measurements by 2. 
     Another approach that would work with such wide profiles would be to measure D(0) directly by using a radiation dosimeter whose active length is significantly less than the width of the radiation profile, and then measure the DLP of the full radiation profile. The DLP could be measured by using the same dosimeter used to measure D(0), but move the dosimeter along the gantry axis at a fixed known velocity v. If the dose rate at position x is equal to {dot over (D)}(x) during an exposure of duration T, then the accumulated dose at any position x is equal to 
         D ( x )= T{dot over (D)} ( x )  (Equation 4)
 
     and the accumulated dose at isocenter is 
         D (0)= T{dot over (D)} (0)  (Equation 5)
 
     The exposure accumulated by the dosimeter moving through the radiation profile at a constant velocity v is: 
     
       
         
           
             
               
                 
                   
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                       Equation 
                        
                       
                           
                       
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                       9 
                     
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     If the exposure is long enough that the product vT is substantially larger than the radiation profile width, then 
     
       
         
           
             
               
                 
                   
                     D 
                     v 
                   
                   = 
                   
                     
                       1 
                       vT 
                     
                      
                     
                       
                         ∫ 
                         
                           - 
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                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     10 
                   
                   ) 
                 
               
             
             
               
                 
                   
                     D 
                     v 
                   
                   = 
                   
                     DLP 
                     vT 
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     11 
                   
                   ) 
                 
               
             
           
         
       
     
     and the radiation profile width can be calculated using the following equation: 
     
       
         
           
             
               
                 
                   W 
                   = 
                   
                     
                       DLP 
                       
                         D 
                          
                         
                           ( 
                           0 
                           ) 
                         
                       
                     
                     = 
                     
                       
                         
                           D 
                           v 
                         
                          
                         vT 
                       
                       
                         D 
                          
                         
                           ( 
                           0 
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     12 
                   
                   ) 
                 
               
             
           
         
       
     
     This derivation assumes that the active length of the radiation dosimeter is infinitesimally small; however, the result is the same for any radiation dosimeter with an active length substantially smaller than the beam width. The radiation dosimeter may be either an ionization chamber or a solid state sensor or any sensor capable of measuring a radiation exposure or dose. 
     This technique differs from the use of the electronic device to accurately measure CT beam width that exists in the prior art in that there is no need to measure and store the dose rate as a function of time to determine a dose profile. All that the dosimeter needs to do is measure the accumulated dose over the length of an exposure.