Application Note # AP0102 Mar 2011 X-ray Dose and System Efficiency (including -ray and ß Efficiencies) in the Carestream DXS Digital X-ray System

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1 Application Note # AP0102 Mar 2011 X-ray Dose and System Efficiency (including -ray and ß Efficiencies) in the Carestream DXS Digital X-ray System Douglas Vizard Author Information: Bruker BioSpin, 15 Fortune Drive, Billerica, MA Application Overview The measure of X-ray dose for an imaging system is critical to those concerned about any physical or biological damage that a specimen or subject might incur during the imaging process. Further, the measured radiation dose of the In-Vivo DXS Digital X-ray System (DXS) enables a reasonable estimate of the efficiency with which an X-ray event is detected. The dose rate measured with precision instruments at the platen is 4.7 Rad/min, where a Rad is a measure of absorbed dose and presumes the subject or any detection devise (such as a phosphor screen) absorbs all of the X-ray energy delivered. The quoted dose rate is for unfiltered X-rays at 35 Kvp and the maximum current for the existing model of the X-ray head (150 µa). An accurate energy spectrum (from an X-ray spectrometer) of the unfiltered beam is presented in Figure 1. X-rays below 10 Kev (with multiple edge structures) contribute less to dose or detection measures given that they are significantly attenuated by air, but those lower energies will exert some effect upon the average energy detected by differing devices. A clearer understanding of the energy spectrum influence may be ascertained from filtration measures, or the extent to which dose is attenuated by known filters. Filter attenuation is shown in Figure 1 for qualified aluminum. Figure 1. X-ray Energy Spectra, Dose and Response for the DXS Digital X-ray System.

2 filters having of the stated thickness, where the data has been reduced to relative measures. If the measures were ideal (mono-energetic X-rays), the graphed response should be log-linear with negative slopes corresponding to the material characteristics as shown by the calculated data for aluminum attenuation of 10, 15 and 20 Kev X-rays. Both the dose and screen responses to filter thickness are clearly curved, indicating non-ideal measures as a result of the broad spectrum of X-ray energies. The changing slopes along the dose or screen response curves reflect the average energy detected by the sensor (dosimeter or screen) at the given levels of filtration, and corresponds to the energies cited for the aluminum calculations. The graph shows that lower energy X-rays contribute more to the dose response than and the screen response. Sensor window and other detector efficiency issues cause the dosimeter/screen differences, and there is no difference between the available radiographic screens designed for the system. The conclusions drawn from the dose and screen measures are that the stated maximum dose rate of 4.7 Rad/min (78 mrad/sec) includes very low energy X-radiation with very limited penetration. The weighted average energy that will contribute to a small animal dose and imaging is about Kev. The estimate of system X-ray detection efficiency is outlined below. The discussion focuses on the endpoint of estimating the output/input ratio, where the input is an X-ray event (or photon) and the output is a digital response. The input estimate is a significant overestimate, given that both very low and high energy X-rays do not participate in the detection/imaging or dose response. The resulting underestimate of efficiency is not known with certainty, but is likely a factor of 2-3. The assumptions are simply that a dose of 4.7 Rad/min of 35 Kvp X-rays (maximum output) impinges upon a 100 mm field of an DXS system using f4 optics, and the digital output (net mean signal) of the system is about 150 digits/sec for the Radiographic Screen designed for the system. 1. A Rad is a fully absorbed Roentgen (R, a measure of ionizing radiation), and the measured fluence is R/sec. 2. A Roentgen is one esu/cc of charge which is due to primary ionizations (pi), an electron is 4.8e -10 esu, a Roentgen/sec is 2.08e 9 electrons/sec/cc; R/sec is 1.6e 8 electrons (pi)/sec/cc. 3. Using an estimated 12 Kev as an average for 35 Kvp X- rays, the usually accepted 60 ev/pi implies 200 pi (electrons) per absorbed X-ray event. From 2 (above), 1.6e 8 pi/sec/cc at one event/200 pi implies 8e 5 events/sec/cc. 4. Applying a 2Kx2K array to a 100 mm field implies a mm pixel, or a pixel area of 2.4e -5 cm Projecting 8e 5 events/sec/cc onto a 1cm 2 footprint implies 8e 5 x 2.4e -5 = 19 events/sec per pixel. 6. An approximate signal of about 150 dig/sec as a mean response (per pixel) is nominal measure for the screen, yielding a response of about 150 digits/19 events=8 digits/event. 7. With a dark noise of about 13 digits (from the nominal camera read noise), a signal/noise ratio of about one per X-ray event is predicted actually S/N = 0.6 at f4, 1.2 at f2.8, both of which are underestimates. 8. Using a similar reasoning to estimate the dose required to attain the highest system resolution (a 40 µm microfocus X-ray spot size limitation), the minimum absorbed dose to resolve a 40x40 µm 2 feature at SN=10 will correspond to 100 absorbed events, which translates to 0.73 Rad. 9. These same arguments apply to a cone-beam CT. For the 40 µm voxel, each voxel of the specimen must be sampled in the same manner fir about 100 angles, implying a dose of 73 Rad. A conservative underestimate of system efficiency suggests that an X-ray event is detected at the inherent system noise level using the Radiographic Screen. Since the dark noise of the system (camera electronics) does not significantly increase for exposure times of less than thousands of seconds, practical exposures of 10 s to 100 s of seconds will have a significance that is determined by X-ray event statistics rather than any other system limitation. Other aspects of screen efficiency may be estimated in the following discussion and Figure 2 summarizes the estimates.. 2

3 Estimating Speed and Efficiency of DXS Screens Two screens are available for the DXS: Radiographic for high resolution X-ray imaging and Radio-isotopic for the efficient detection of radiation emitted from isotopic decays. The event-based efficiency (or speed) of the screens depends upon the event energy. The Absorbed Fraction roughly corresponds to the S/N of an event detection. The Radiographic Screen is efficient for <15 Kev X-rays. The Radio-isotopic Screen is efficient for some isotopes, where the gamma energies of selected isotopes are designated. The efficiency estimates of isotopes assumes that the source is on the screen surface. The spatial resolution of the Radio-isotopic Screen is significantly lower than the Radiographic Screen. Figure 2. System efficiencies based on screen absorption of X- and -rays. The above discussion focused on estimating the number of X-ray events that were directed toward the screen. The number of events associated with isotopic emission ( -rays) is more easily estimated since isotope preparations are measured in units of emitted events (Curies). The difficulty estimating isotope event efficiency arises from the fact that the emission of an isotope has no directional preference. If a sample of isotope were intimately associated with the surface of a screen, the screen would sample less than half of the emitted radiation. The further the sample is removed from the screen, the fewer events are absorbed by the screen and the more ill defined the sample image becomes. An example is shown in Figure 3, in which the event efficiency of 111 In emissions is estimated using the Isotopic Screen. Efficiency of 111 In Using Radio-isotope Screen/DXS System Figure 3. A syringe containing 111 In solution is placed on a Radio-isotope Screen; the emission image is captured; the object X-ray is captured; and the emission image is overlaid (in color) onto the X-ray image. The emission image is dispersed over a large area but is centered about the object footprint (the closest proximity of the isotope to the screen). The system counts (digits of response) are graphed for several areas centric to the object, where the border region including 50% of the counts are shown in the figure. The colored area over the object center was produced by simply thresholding the response, and it contains 30% of the counts. 3

4 The event efficiency estimate summarized in the above is for an approximate total counts per isotopic emission and may be experimentally useful for circumstances where an object to be imaged is near the screen. Clearly, if the object were closer to the screen (such as a surface tumor) a much larger fraction of the counts would appear as a more confined image of the real object. A better estimate of event efficiency considers the object geometry, where no more than a onesteradian fraction (about 1/12th) of the total emissions, are projected toward the screen, and this includes no more than 30-40% of the total counts detected. Correcting for the geometry increases the event efficiency estimate to more than 12%. Note that using the Absorbed Fraction graph estimate (Figure 2), an event efficiency of about 3.5% is estimated, indicating that the Absorbed Fraction is a significant underestimate. While the above estimates of event efficiency have focused on X- and -rays, the ß-particle is commonly used for isotopic emission imaging (e.g., 32 P autoradiography). Estimating ß efficiency has the same geometric problems as X-rays and is further complicated by a screen absorption mechanism that differs considerably from the X-ray. The ß emission is a charged particle that interacts more strongly with materials and is much less penetrating than an X-ray. The practical usage of many ß-emitting isotopes commonly used for autoradiography is seriously limited by the selfabsorption of the particle in the sample. Among the most commonly used ß-emitters is 32 P, which is among the highest energy (1.7 Mev) and most penetrating examples. Figure 4 summarizes event efficiency estimates for 32 P emissions, where the object is as close as possible to the sensor (screen or film). Figure 4. Analysis estimates that the event efficiency for 32 P using the Radio-isotope Screen is about 6% for S/N = 1. Assuming about 40% of the emissions are directed toward the screen, the event efficiency estimate rises to 15%. The event efficiency measured on the Radiographic Screen is 10-fold less, consistent with the relative thickness of the different screens. The speed estimate statement in the figure compares an autoradiographic film response to the digital system. Note the enhanced spatial resolution of the digital system compared to the film. 4

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