Donald J. Roth

Glenn Research Center, Cleveland, Ohio

Richard W. Rauser

University of Toledo, Toledo, Ohio

The Effect of Experimental Variables on 

Industrial X-Ray Micro-Computed Sensitivity

NASA/TM—2014-218332

December 2014
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Donald J. Roth

Glenn Research Center, Cleveland, Ohio

Richard W. Rauser

University of Toledo, Toledo, Ohio

The Effect of Experimental Variables on 

Industrial X-Ray Micro-Computed Sensitivity

NASA/TM—2014-218332

December 2014

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NASA/TM—2014-218332 1 

The Effect of Experimental Variables on 

Industrial X-Ray Micro-Computed Sensitivity 

Donald J. Roth 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Richard W. Rauser 

University of Toledo 

Toledo, Ohio 43606 

Abstract 

A study was performed on the effect of experimental variables on radiographic sensitivity (image 

quality) in x-ray micro-computed tomography images for a high density thin wall metallic cylinder 

containing micro-EDM holes. Image quality was evaluated in terms of signal-to-noise ratio, flaw 

detectability, and feature sharpness. The variables included: day-to-day reproducibility, current, 

integration time, voltage, filtering, number of frame averages, number of projection views, beam width, 

effective object radius, binning, orientation of sample, acquisition angle range (180° to 360°), and 

directional versus transmission tube.

Background 

Industrial X-ray Computed Tomography (CT) has become a critical nondestructive evaluation method 

in the last decade due to advances in detector, tube, and computational technology. These advancements 

have led to a range of applications for CT from the ability of CT to scan and reconstruct practically-sized 

(on the scale of 10s of centimeters and even meters) components at higher resolutions and higher speeds, 

as well as to perform materials characterization at the sub-1 Pm level (Refs. 1 to 5). 

Radiographic sensitivity is the size of the smallest detail that can be seen in a radiograph or the ease 

with which small details can be detected. Sensitivity depends on the sharpness and the contrast of the 

resulting image. Several figures of merit related to CT system sensitivity are available. Equation (1) 

shows an estimate of the signal-to-noise ratio (SNR) in a voxel element as a function of various CT 

system characteristics/experimental variables for a reconstruction of a cylindrical object (Refs. 6 and 7): 

  R

p

nvqt SNR w  S

' 0.665P exp 2 1.5 (1) 

where μ is the linear attenuation coefficient, w is the x-ray beam width, v is the number of projection 

views, n is the number of frame averages, q is the photon intensity rate at the detector, t is the integration 

WLPHRIWKHGHWHFWRUVǻp is the ray spacing and R is the radius of the object. The number of frame 

averages (n) has been incorporated by the authors into the original equation found in (Ref. 6) since SNR 

is proportional to the square root of the number of frame averages. SNR increases as x-ray beam width, 

number of views, x-ray beam intensity, number of frame averages, and integration time increases. 

(Additionally, as the number of views increases, the ability to more precisely reconstruct the object 

increases (Ref. 7).) SNR also increases as ray spacing and object radius decreases. The photon intensity 

rate q will increase with increasing source voltage and / or current as more electrons per unit time 

bombard the target and subsequently produce more photons per unit time. Not indicated by Equation (1) 

but necessary to mention is that limiting/collimating the x-ray beam to a fan beam the height of one row 

of detector pixels, and increasing detector pixel size, also increase SNR. 
NASA/TM—2014-218332 2 

Several variables can be used to experimentally and quantitatively assess x-ray and CT image quality. 

The first one, SNR, is experimentally measured in a digital image by dividing the mean gray level by the 

standard deviation ı in a representative area of the image so that the effect of these variables can be 

measured. 

V mean SNR (2) 

Another parameter, contrast ratio (CR), is given by (Ref. 7): 

SNR Z

CR *

6 (3) 

where Z is the number of pixels over which the contrast is observed. Equation (3) shows that the larger 

the SNR and the greater the number of pixels over which contrast is observed, the lower (better) the 

contrast ratio that can be achieved. Computed tomographic systems often provide contrast sensitivity 

measurements on the order of 0.1 to 1.0 percent (Ref. 7). Using the measured SNR and knowing the 

number of pixels in the area over which SNR was measured, CR can also be determined experimentally. 

Related to SNR and CR, contrast-to-noise (CNR) ratio is a potential figure of merit for assessing flaw 

detectability in a CT image when the flaw presents a gray scale variation from the background of the 

image. Figure 1 illustrates the definitions for signal contrast (C) and noise (N) (Ref. 8). Signal contrast 

can be defined as the difference between the mean gray level in the line drawn through the center of the 

flaw (M) and the minimum gray level in the flaw area (maximum dip gray value) (dmax). Noise can be 

defined as the difference between M and the minimum gray value on either side of the flaw. CNR is then 

obtained from: 

N

C CNR . (4) 

Scatter from the object itself as well as external sources can add a background haze (noise) to 

radiation intensities so that the CNR is reduced. 

 

Figure 1.—Definitions for signal contrast (C) and noise (N).
NASA/TM—2014-218332 3 

Variables affecting SNR would be expected to have a similar effect on CNR. CNR, although desired 

as a measurement for probability of detection (POD) studies (Ref. 8), is problematic to implement for 

very small flaws covering a limited number of pixels. It requires a line profile to be drawn directly 

through the center of the indication, and the ability to discern where the flaw indication ends and noise 

begins. Incorrect line draws, even by one pixel, can cause dramatic error in calculation of CNR. One 

cumbersome way to overcome this is to draw many lines at different angles and obtain average values for 

contrast and noise, but this was not feasible for this investigation. 

In this study, SNR is used to quantitatively assess the effect of variables on CT image quality. 

Qualitative evaluations of cylinder wall sharpness and detectability of the most difficult-to-detect hole are 

also utilized for assessing image quality. (A quantitative measure of sharpness can also be obtained in the 

following manner. A line profile of an edge can be differentiated and Fourier-transformed to obtain 

modulation transfer function (Ref. 7), but this is beyond the scope of this article.). The variables to be 

evaluated for effect on image quality include day-to-day reproducibility, current, integration time, voltage, 

filtering, number of frame averages, number of projection views, beam width, effective object radius, 

binning, orientation of sample, angle range (180° to 360°), and directional versus transmission tube. 

CT Inspection Tradeoffs 

Tradeoffs are often required in performing inspections with CT and because they affect SNR, are 

mentioned here. For example 

 

x The greater the integration time (or lower number of frames per second) and the higher the 

number of frame averages, both of which improve SNR, the more time is required for the CT 

scan. 

x Larger detector element size increases SNR but reduces resolution and accuracy of representing 

an indication (Ref. 7). 

x Beam collimation to a fan beam the height of one row of detector pixels increases SNR over cone 

beam CT but increases scan time to obtain a full-height image. 

x Increased beam width increases SNR according to Equation (1), but would be expected to reduce 

sensitivity as the ratio of the beam width to lateral flaw size increases. 

x The effective radius R that the detector sees is decreased as the object moves closer to the 

detector for an object smaller than the detector. This is expected to increase SNR according to 

Equation (1), but will increase voxel dimension and thus reduce resolution [while also reducing 

geometric unsharpness] (the latter is less of an issue at very small focal spots). 

x Increased photon intensity rate via increased voltage and / or current increases SNR but increased 

voltage will also decrease contrast between different phases and thicknesses in a material as 

higher energy x-rays more similarly penetrate the different phases and thicknesses than less 

energetic x-rays (increased latitude). Additionally, increased voltage and current result in more 

power and a larger focal spot size to prevent the target from overheating. A larger focal spot 

decreases sensitivity. 

CT System and General Procedure 

Microfocus x-UD\&RPSXWHG7RPRJUDSK\ȝ&7LVDQRQ-destructive imaging technique designed to 

inspect complex-shaped parts for micron scale or larger flaws. Multiple x-ray projection images are 

acquired, followed by software reconstruction techniques using the projection images, to obtain crosssectional slices of the part. The cross-sectional images can be viewed individually or used to render a 

volume. Two X-ray WorX microfocus sources that produce a cone beam were used in this study. These 

were the XWT-225-THE 225 keV transmission tube (< 5 ȝm spot size at lowest powers, 25 W power) 
NASA/TM—2014-218332 4 

 (a) (b)

Figure 2.—NASA Glenn Research Center X-ray CT system hardware. (a) Shown with transmission tube. (b) Shown 

with reflection/directional tube.

and the X-ray WorX SE-NH9UHIOHFWLRQGLUHFWLRQDOWXEHȝPVSRWVL]HDWORZHVWSRZHUV

300 W power). The detector is a Dexela 2923 (true 14-bit dynamic range, 0.0748-mm pitch, 29- by 

23-cm area, and 3888u3072 pixel elements). This detector has nearly half of the pixel pitch of the 

commercial grade prior state-of-the-art detectors utilized for microfocus digital x-ray (typically 

0.127 mm). The acquisition, reconstruction, and visualization software is from Northstar Imaging, Inc. 

(NSI). Four-axis motion control allows automated positioning and scanning. The system, shown in 

Figure 2, has resolution capability easily > 30 line pair/mm. 

Prior to the experiments, the sample stage and detector were precision-leveled, and detector and 

geometric calibrations were performed for every scan. CT scans were performed with the long axis of the 

cylindrical part aligned vertically. All CT scans were accomplished over the full 360° except for the 

limited angle range experiments. A procedure to reduce unsharpness was accomplished prior to 

reconstruction using a software algorithm in the NSI software. Subsequently, fast Feldkamp (FDK) 

reconstruction was performed on the projection view data set (Ref. 9). 

Sample 

The cylinder sample was made of MarM 247 nickel-based superalloy with wall thickness ~ 300 μm. 

A micro-EDM procedure was performed to create a pattern of holes of varying diameter and depth (see 

Figure 3). The most difficult-to-detect hole examined in this investigation was 32 μm in diameter and 

50 μm in depth. The detectability of this hole was used to qualitatively assess CT image quality. 

(Diameters and depths were obtained by SEM and optical characterization, respectively, with an 

estimated uncertainty of ±10 percent). 

Analysis Methodology 

Flaw detection in top views of cylindrical CT data is very difficult if the walls of the cylinder are very 

thin. It can be advantageous to unwrap and reslice the 360° data so as to view two-dimensional “sheets” 

from the exterior to interior of the cylinder separated by the voxel dimension. The data analysis in this 

study was performed on sections of unwrapped/resliced CT images for two of the three measures. The 

X-ray Tube X-ray Tube Detector

Detector

Stage with 

Sample

Stage with 

Sample
NASA/TM—2014-218332 5 

unwrapping and reslicing of the top view slice set was done using NASA CT-CURS software (Ref. 1). A 

precision alignment procedure of the top view slice set is required to obtain best results and is included as 

part of the unwrap/reslice software procedure. The unwrapped reslice images were automatically contrast 

expanded between the minimum and maximum gray level values as part of the software procedure. The 

latter procedure will likely impact the subjective assessment of effect of variables on contrast versus 

analysis with images that all have a gray level between the same minimum and maximum. However, it 

was required to perform this procedure in order to characterize the most difficult-to-detect flaw.

The unwrapped/reslice used for flaw detectability assessment and SNR determination was either the 

brightest and/or highest resolution of the series of unwrapped images in a data set. For SNR 

determination, an area just to the left the flaws shown in Figure 3 of at least 8000 to 10000 pixels was 

analyzed. The area analyzed for SNR was pure background—it did not contain any of the micro EDM 

holes. Screen captures of the most difficult-to-detect hole from the unwrapped/reslice image and of a wall 

area from a top (plan) view slice at the middle of the stack were obtained and compared within each series 

of experiments. 

 ______ 

 1 mm

Figure 3.—Micro-EDM hole pattern (unwrapped-resliced CT view) in thin wall 

section near outer diameter surface. Hole circled was the most difficult to 

detect hole in the pattern.

Increasing Diameter

Increasing Depth
NASA/TM—2014-218332 6 

Experiments and Results 

The tables below lists the experiments and range of these parameters studied. Photon counts at the 

detector were adjusted using current and frame rate so that mean counts value with no sample present was 

kept at 60 to 70 percent of the detector's saturation limit of 16,384 (except for variables of current and 

frame rate (integration time) which directly affect photon count).

Day-to-Day Reproducibility (Reflection/Directional Tube) 

CT scans were performed under nearly identical conditions given in Table 1 three days in a row, 

separated by ~ 24 hr. 

Day 1 showed a lower SNR and the hole was subsequently more difficult to detect than for days 2 and 

3 (see Table 2). It is likely that detector photon count at the detector was lower in day 1 versus days 2 and 

3. Inner cylinder wall was well-defined each day. Subsequent series of experiments where puzzling or 

unexpected results were observed were run at least twice to assure consistent results. 

TABLE 1.—EXPERIMENTAL PARAMETERS FOR 

DAY-TO-DAY REPRODUCIBILITY EXPERIMENTS

Parameter Value 

Reproducibility ............................................................ Day 1 to 3 

Voltage, kV ............................................................................. 170 

Current, μA ............................................................................. 130 

Estimated approximate focal spot, μm ...................................... 10 

Binning ...................................................................................1u1 

Frame rate, frames per second (fps) ............................................ 3 

Number of frame averages .......................................................... 3 

Filtering ...................................................................... 0.16 in. Cu 

Number of projection views .................................................. 1800 

Magnification factor .............................................................. 6.62 

Voxel dimension, μm ............................................................. 11.3 

 
NASA/TM—2014-218332 7 

TABLE 2.—RESULTS FOR DAY-TO-DAY REPRODUCIBILITY 

[The scale bar shown in the top row images is approximately the same for all images from this point forward in the manuscript.]

Day SNR Hole Wall 

1 23.4 

Circled area shows hole 

2 30 

3 29.5 

0.5 mm Inner 

Wall

0.3 mm
NASA/TM—2014-218332 8 

Varying Voltage and Filtering (Transmission Tube) 

All else being equal, the effect of increasing voltage (energy) in digital radiography is to increase 

penetration (Ref. 7). The effect of increasing voltage in combination with applying more beam hardening 

filtration increases latitude (the thickness range of inspection possible). The effect of decreasing voltage is 

generally greater contrast between discontinuities and background material as lower energy radiation is 

preferentially attenuated by thicker and / or more dense sections. 

Five basic voltages were utilized while keeping filtering constant using three different filters 

(Table 3). Current and integration time were adjusted to keep photon count through the sample at the 

detector within 10 percent for the different voltages. The steel and Cu filters were close in terms of 

radiographic equivalence and would be expected to yield similar results. (1.6 * 0.16 in. Cu = 0.26 in. 

Steel where 1.6 is the radiographic equivalence filtering factor for Cu at 150 kV as compared to Steel.) 

Only the results for 210, 170, and 130 kV are shown regarding the effect of filtering in Table 4 to 

Table 6. 

At 210 kV and using these filters, SNR is similar, the hole is detectable, and the inner cylinder wall is 

defined for these trials with poorest wall definition using the 0.24 in. steel filter. 

TABLE 3.—EXPERIMENTAL PARAMETERS RELATED TO 

VARYING FILTERING WHILE KEEPING VOLTAGE CONSTANT 

AND VARYING VOLTAGE WHILE KEEPING FILTERING CONSTANT 

[All other experimental parameters were the same as shown in Table 1.] 

Voltage, 

kV 

Filter a

Current/frame rate, 

μA/fps 

210 0.16 in. Cu 110 / 3 

 0.24 in. Steel 

 0.028 in. Pb 

190 0.16 in. Cu 120 / 3 

 0.24 in. Steel 

 0.028 in. Pb 

170 0.16 in. Cu 130 / 3 

 0.24 in. Steel 

 0.028 in. Pb 

150 0.16 in. Cu 150 / 3 

 0.24 in. Steel 

 0.028 in. Pb 

130 0.16 in. Cu 185 / 2 

 0.24 in. Steel 

 0.028 in. Pb 

a

Current (μA) and / or frame per second (fps) value adjusted to keep photon 

count through the sample at the detector within 10 percent for the different 

voltages. 
NASA/TM—2014-218332 9 

TABLE 4.—RESULTS FOR EFFECT OF FILTERING WITH 210 kV VOLTAGE 

Voltage 

kV 

Filter SNR Hole Wall 

210 0.16 in. Cu 23 

210 0.24 in. 

Steel 21.8 

210 0.028 in. Pb 22 

 
NASA/TM—2014-218332 10 

TABLE 5.—RESULTS FOR EFFECT OF FILTERING WITH 170 kV VOLTAGE

Voltage, 

kV 

Filter SNR Hole Wall 

170 0.16 in. Cu 29 

170 0.24 in. Steel 24 

170 0.028 in. Pb 19.9 

At 170 kV and using these filters, SNR is significantly different, the hole is detectable, and the inner 

cylinder wall is most well-defined using the 0.16 in. Cu filter. 
NASA/TM—2014-218332 11 

TABLE 6.—RESULTS FOR EFFECT OF FILTERING WITH 130 kV VOLTAGE

Voltage, 

kV 

Filter SNR Hole Wall 

130 0.16 in. Cu 27 

130 0.24 in. Steel 26 

130 0.028 in. Pb 35 
NASA/TM—2014-218332 12 

At 130 kV and using these filters, SNR is similar for Cu and steel filters, the hole is detectable, and 

the inner cylinder wall is ill-defined in all cases. 

Table 7 reorganizes data to show the effect of voltage using the 0.16 in. Cu filter. 

SNR did not follow a defined trend as the voltage decreased which was unexpected. The hole is 

detectable for all voltages but inner cylinder wall clearly becomes more well-defined as voltage is 

increased due to a reduction in scatter and increased latitude. 

TABLE 7.—RESULTS FOR EFFECT OF VOLTAGE WITH 0.16 in. Cu FILTER 

Voltage, 

kV 

SNR Hole Wall 

210 23 

190 25.4 

170 29 

 
NASA/TM—2014-218332 13 

TABLE 8.—CONCLUDED 

Voltage, 

kV 

SNR Hole Wall 

150 27 

130 27 
NASA/TM—2014-218332 14 

Varying Voltage and Filtering (Reflection/Directional Tube) 

The set of experiments described by the parameters of Table 3 for the transmission tube was repeated 

for the reflection / directional tube and the results are shown in Table 8. 

As for the transmission tube, SNR did not follow a defined trend as the voltage decreased. The hole is 

detectable for all voltages but the inner cylinder wall clearly becomes more well-defined as voltage is 

increased above 130 kV due to a reduction in scatter. The inner wall appears to be more highly resolved at 

most voltages for the reflection tube versus transmission tube. This may be due to a higher respective 

photon flux during the reflection tube experiments. However, Figure 4 shows nearly identical top view 

section CT image results for Transmission versus Reflection tube at 170 kV. 

TABLE 9.—RESULTS FOR EFFECT OF VOLTAGE WITH 0.16 in. Cu FILTER 

Voltage SNR Hole Wall 

210 28 

190 30.5 

 
NASA/TM—2014-218332 15 

TABLE 8.—CONCLUDED 

Voltage SNR Hole Wall 

170 28 

150 28 

130 25.5 
NASA/TM—2014-218332 16 

 (a) (b)

Figure 4.—Top view section CT image at 170 kV for (a) transmission tube and (b) reflection tube.

TABLE 10.—EXPERIMENTAL PARAMETERS FOR EFFECT 

OF FRAME AVERAGING EXPERIMENTS

Parameter Value 

Frame averaging ....................................................... 1, 3, 5, 9, 15 

Voltage, kV ............................................................................... 70 

Current, μA ............................................................................. 200 

Estimated approximate focal spot, μm ........................................ 7 

Binning ...................................................................................1u1 

Frame rate, fps ............................................................................ 3 

Filtering .................................................................... 0.010 in. Cu 

Number of projection views .................................................... 360 

Magnification factor .............................................................. 6.62 

Voxel dimension, μm ............................................................. 11.3 

Varying the Number of Frame Averages (Transmission Tube) 

The effect of frame averaging is pronounced (see Table 9 for parameters and Table 10 for results). 

SNR increases significantly with increased frame averaging, as predicted by Equation (1), and both the 

hole and outer wall images are sharper. The inner cylinder wall is not resolved at 70 kV voltage. Similar 

sharpness increase would be expected with increasing number of views for the inner wall for CT scans 

run at higher voltages such as 190 kV. 
NASA/TM—2014-218332 17 

TABLE 11.—RESULTS FOR EFFECT OF FRAME AVERAGING 

No. of frame 

averages 

SNR Hole Wall 

1 10 

3 16.5 

9 22.5 

15 28.5 
NASA/TM—2014-218332 18 

Varying the Number of Projection Views (Transmission Tube) 

SNR increases with an increase in the number of projection views, as predicted by Equation (1) (see 

experiment parameters in Table 11). Detectability of the hole is reduced with a decrease in the number of 

projection views below 360. The inner cylinder wall is not resolved at 70 kV but the outer wall definition 

appears sharper with an increase in the number of projection views (see Table 12). Similar sharpness 

increase would be expected with increasing number of views for the inner wall for CT scans run at higher 

voltages such as 190 kV. 

TABLE 12.—EXPERIMENTAL PARAMETERS FOR EFFECT 

OF NUMBER OF PROJECTION VIEWS EXPERIMENTS 

Parameter Value 

Number of views ........... 9, 18, 36, 90, 180, 360, 720, 1080, 1440, 1800 

Voltage, kV ........................................................................................ 70 

Current, μA ...................................................................................... 200 

Estimated approximate focal spot, μm ................................................. 7 

Binning ............................................................................................1u1 

Frame rate, fps ..................................................................................... 3 

Number of frame averages ................................................................... 3 

Filtering .............................................................................. 0.010 in. Cu 

Magnification factor ........................................................................ 6.62 

Voxel dimension, μm ...................................................................... 11.3 

TABLE 13.—RESULTS FOR EFFECT OF NUMBER OF PROJECTION VIEWS 

No. of 

projection 

views 

SNR Hole Wall 

9 

6 

(analysis 

included 

striping in 

image) 

A number of holes resolved but most difficult-todetect hole not detected 

Entire top view slice. Aliasing lines clearly 

visible. 

 
NASA/TM—2014-218332 19 

TABLE 12.—CONTINUNED 

No. of 

projection 

views 

SNR Hole Wall 

18 11 

A number of holes resolved but most 

difficult-to-detect hole not detected 

Entire top view slice. Aliasing lines clearly 

visible. 

36 19 

Hole barely detected 

90 22.5 

Hole not detected 

180 22.5 
NASA/TM—2014-218332 20 

TABLE 12.—CONTINUNED 

No. of 

projection 

views 

SNR Hole Wall 

360 30.5 

720 39.5 

1080 48 

1440 48 

 
NASA/TM—2014-218332 21 

TABLE 12.—CONCLUDED 

No. of 

projection 

views 

SNR Hole Wall 

1800 48 

3600 65 

Varying Binning (Transmission Tube) 

SNR increases (averaging effect), hole detectability decreases, and wall / features become more 

pixelated with an increase in binning size. The inner cylinder wall is not resolved at 70 kV voltage but 

would be expected to exhibit similar pixilation with increased binning for CT scan run at higher voltages 

such as at 190 kV. For experiment parameters see Table 13 and for results see Table 14. 

TABLE 14.—EXPERIMENTAL PARAMETERS FOR 

EFFECT OF BINNING EXPERIMENTS 

Parameter Value 

Binning ................................................................... 1u1, 2u2, 4u4 

Voltage, kV .............................................................................. 70 

Current, μA ............................................................................ 200 

Estimated approximate focal spot, μm ....................................... 7 

Frame rate, fps ............................................................................ 3 

Number of frame averages ......................................................... 3 

Number of projection views ................................................... 360 

Filtering ..................................................................... 0.010 in. Cu 

Magnification factor .............................................................. 6.62 

Voxel dimension, μm ............................................................ 11.3 
NASA/TM—2014-218332 22 

TABLE 15.—RESULTS FOR EFFECT OF BINNING 

Binning SNR Hole Wall 

1u1 30.5 

2u2 55 

4u4 72.8 

Varying Cylinder Orientation (Transmission Tube) 

The wall thickness seen by the x-rays through the cylinder walls would slightly increase with tilt and 

therefore a lower photon flux would be expected leading to a lower SNR (see Table 15 for experiment 

parameters). It is likely that photon flux varied between experiments causing the unexpected result. The 

hole was equally detectable at both angles. The outer cylinder wall was sharply defined for both angles 

and the inner cylinder wall is not resolved at 70 kV voltage in either case (see Table 16). It is expected 

that inner wall sharpness might be affected over a range of tilt angles from 0° to 45° for CT scans run at 

higher voltages such as at 190 kV. The series of holes in the reslice image are tilted as shown in Figure 5 

for the 3° tilt. 

TABLE 16.—EXPERIMENTAL PARAMETERS FOR EFFECT 

OF CYLINDER ORIENTATION EXPERIMENTS 
NASA/TM—2014-218332 23 

Parameter Value 

Cylinder Orientation, tilt ..................................................... 0°, 3° 

Voltage, kV .............................................................................. 70 

Current, μA ............................................................................ 200 

Estimated approximate focal spot, μm ....................................... 7 

Binning................................................................................... 1u1 

Frame rate, fps ........................................................................... 3 

Number of frame averages ......................................................... 3 

Number of projection views ................................................... 360 

Filtering ..................................................................... 0.010 in. Cu 

Magnification factor ...............................................................6.62 

Voxel dimension, μm .............................................................11.3 

TABLE 17.—RESULTS FOR EFFECT OF CYLINDER TILT 

Cylinder 

orientation 

SNR Hole Wall 

0° tilt 30.5 

3° tilt 35 

Figure 5.—Sample tilt results in angling of row of holes on the unwrap / reslice image.
NASA/TM—2014-218332 24 

Varying Angular Range of Scan (Reflection/Directional Tube) 

Gray scale banding (alternating light and dark bands) was apparent in all unwrap reslice images for 

angle range < 360°. The SNR and hole detectability results do not change in a predictable fashion with a 

decrease in angle range over which the CT scan was performed. This may indicate that the FDK 

reconstruction algorithm will show variable results from 180° to 360° (experiment parameters are shown 

in Table 17). Artifacts and reduced contrast prevail in some cases in the cylinder wall images as angle 

range is reduced (Table 18). However, the inner wall is reasonably well-resolved in most cases. 

TABLE 18.—EXPERIMENTAL PARAMETERS FOR EFFECT 

OF ANGULAR RANGE OF ACQUISITION EXPERIMENTS 

Parameter Value 

Angular range of acquisition ............. 360°, 300°, 270°, 240°, 210°, 180° 

Voltage, kV ........................................................................................ 190 

Current, μA ........................................................................................ 120 

Estimated approximate focal spot, μm ................................................. 10 

Binning ............................................................................................... 1u1 

Frame rate, fps........................................................................................ 3 

Number of frame averages ..................................................................... 3 

Angular increment ............................................................................. 0.2° 

Filtering .................................................................................. 0.16 in. Cu 

Magnification factor .......................................................................... 6.62 

Voxel dimension, μm ........................................................................ 11.3 

TABLE 19.—RESULTS FOR EFFECT OF ANGULAR RANGE OF ACQUISITION. UNWRAPPEDRESLICE IMAGE 

BRIGHTNESS RESULTS VARIED CONSIDERABLY FOR THIS SET OF EXPERIMENTS 

Angular range 

(q)/no. of views

SNR Hole Wall 

360° / 1800 27.5 

 
NASA/TM—2014-218332 25 

TABLE 20.—CONTINUED 

Angular range 

(q)/no. of views 

SNR Hole Wall 

300° / 1500 25 to 50 

(banding) 

270° / 1350 6 to 13 

(banding) 

240° / 1200 13 to 28 

(banding) 

 
NASA/TM—2014-218332 26 

TABLE 21.—CONCLUDED 

Angular range 

(q)/no. of views 

SNR Hole Wall 

210° / 1050 20 to 30 

(banding) 

180° / 900 3 to 45 

(banding) 

Varying Beam Width and Effective Object Radius 

(Via Source-to-Object (SO) Distance Variation) (Reflection/Directional Tube) 

Effective beam width in the test sample is a function of focal spot size, detector element size, and 

SO distance (Ref. 7), (see Table 19 for experiment parameters). The further the object from the source, 

the larger the effective beam width (see Figure 6). The object-to-detector distance will also affect the 

effective object radius that the detector sees. For objects smaller than the detector and moving them closer 

to the detector, the x-rays that penetrate the object will impinge on a smaller number of pixels (and a 

decrease in the effective object radius will result, see Table 20). Photon count reaching the detector, 

scatter conditions, magnification and geometric unsharpness also change for different SO distances.

SNR did not monotonically increase with increasing SO distance (increasing beam width and 

decreasing effective object radius) as predicted by Equation (1). SNR was significantly lower at 

SO = 480 mm than for the other two SO distances, indicating a possible change in scatter conditions. As 

expected, resolution decreases with increasing SO, as manifested by the hole being more difficult to 

detect and the cylinder inner wall becoming pixelated. 
NASA/TM—2014-218332 27 

Figure 6.—Effective x-ray beam width in test object as a function of source-to-object 

position. Focal spot estimated at 7 to 10 μm. Detector pixel element size ~ 75 μm.

TABLE 22.—EXPERIMENTAL PARAMETERS FOR EFFECT 

OF BEAM WIDTH AND EFFECTIVE OBJECT RADIUS 

Parameter Value 

SO distance, mm ..................................................... 120, 240, 480 

Voltage, kV ............................................................................ 190 

Current, μA ............................................................................. 120 

Estimated approximate focal spot, μm...................................... 10 

Binning ................................................................................... 1u1 

Frame rate, fps ............................................................................ 3 

Number of frame averages .......................................................... 3 

Number of projection views ................................................... 720 

Filtering ...................................................................... 0.16 in. Cu 

Magnification factor .......................... Will vary with SO distance 

Effective Beam Width at this end of 

sample
NASA/TM—2014-218332 28 

TABLE 23.—RESULTS FOR EFFECT OF BEAM WIDTH AND EFFECTIVE OBJECT RADIUS 

(VIA SOURCE-TO-OBJECT (SO) DISTANCE CHANGE) 

Beam 

width/magnification 

factor/SO 

SNR Hole Wall 

Smallest / 6.7x 

(SO = 120 mm) 25 

Larger / 3.33x 

(SO = 240 mm) 28 

Hole barely detected 

Largest / 1.67x 

(SO = 480 mm) 

18 

Hole not detected 

Varying Current (Reflection/Directional Tube) 

Current directly affects photon count at the detector. The mean counts value with no sample present 

was approximately 36, 49, and 64 percent of the detector’s saturation limit of 16,384 for currents of 80, 

120, and 160 PA, respectively (see Table 21 and Table 22).

SNR increased with increasing current as expected from Equation (1). Hole detectability appeared to 

best at lowest current which may be due to the smaller focal spot at lower power. Hole and wall images 

appear less noisy which agrees with the increasing SNR measure. The inner cylinder wall appears to be 

sharper with increasing current. 
NASA/TM—2014-218332 29 

TABLE 24.—EXPERIMENTAL PARAMETERS 

FOR EFFECT OF CURRENT 

Parameter Value 

Current, μA ............................................................... 80, 120, 160 

Voltage, kV ............................................................................. 170 

Estimated approximate focal spot, μm ...................................... 10 

Binning ................................................................................... 1u1 

Frame rate, fps ............................................................................ 3 

Number of frame averages .......................................................... 3 

Number of projection views ................................................... 720 

Filtering ...................................................................... 0.16 in. Cu 

Magnification factor .............................................................. 6.62 

TABLE 25.—RESULTS FOR EFFECT OF CURRENT (AFFECTING PHOTON INTENSITY RATE) 

Current, 

μA 

SNR Hole Wall 

80 16 

120 22.5 

160 28 
NASA/TM—2014-218332 30 

Varying Integration Time (Reflection/Directional Tube) 

Frame rate (integration time) directly affects photon count at the detector. The mean counts value 

with no sample present was approximately 88, 64, and 52 percent of the detector’s saturation limit of 

16,384 for 2, 3, and 4 fps, respectively (see Table 23 and Table 24).

SNR, hole detectability, and inner cylinder wall visibility increased with increasing integration time 

(decreasing frame rate) as expected as expected from Equation (1). 

TABLE 26.—EXPERIMENTAL PARAMETERS FOR 

EFFECT OF INTEGRATION TIME (FRAME RATE) 

Parameter Value 

Frame rate, fps .................................................................... 2, 3, 4 

Voltage, kV ............................................................................ 190 

Current, μA............................................................................. 120 

Estimated approximate focal spot, μm ..................................... 10 

Binning ................................................................................... 1u1 

Number of frame averages ......................................................... 3 

Number of projection views ................................................... 720 

Filtering ...................................................................... 0.16 in. Cu 

Magnification factor .............................................................. 6.62 

 
NASA/TM—2014-218332 31 

TABLE 27.—RESULTS FOR EFFECT OF INTEGRATION TIME (FRAME RATE) 

Frame 

rate SNR Hole Wall 

2 25 

3 23 

4 20 

Summary 

A study was performed on the effect of experimental variables on radiographic sensitivity (image 

quality) in x-ray micro-computed tomography images for a high density thin wall metallic cylinder 

containing micro-EDM holes. Image quality was evaluated in terms of signal-to-noise ratio, flaw 

detectability, and feature sharpness. The variables included: day-to-day reproducibility, current, 

integration time, voltage, filtering, number of frame averages, number of projection views, beam width, 

effective object radius, binning, orientation of sample, angle range (180° to 360°), and directional versus 

transmission tube. Table 25 to Table 27 provide textual and pictorial summaries of the results of this 

study. 
NASA/TM—2014-218332 32 

TABLE 28.—SUMMARY OF STUDY RESULTS 

 SNR EDM hole detectability Inner cylinder wall sharpness Comment 

Reproducibility 

(3 trials) 

Variable. Variable. Reasonably Consistent. Photon count at detector likely not 

identical from day-to-day. 

Voltage 

(130, 150, 170, 190, 

210 kV) 

Variable and not 

well-defined 

trend. 

Reasonably consistent 

moderate detectability. 

Increase in wall sharpness with 

increase in voltage. 

Filtering at 210 kV 

(0.16 in. Cu, 0.24 in. 

Steel, 0.028 in. Pb) 

Reasonably 

Consistent. 

Reasonably consistent 

moderate detectability. 

Reasonably Consistent moderately 

sharp inner wall. 

Hole was less detectable at 210 kV 

vs. 170 kV indicating poorer 

contrast at higher voltage. 

Filtering at 170 kV 

(0.16 in. Cu, 0.24 in. 

Steel, 0.028 in. Pb) 

Variable. Reasonably consistent 

good detectability. 

Ill-defined inner wall except using 

0.16 in. Cu Filter. 

Hole was more detectable at 170 kV 

vs. 210 kV indicating better contrast 

at lower voltage. 

Filtering at 130 kV 

(0.16 in. Cu, 0.24 in. 

Steel, 0.028 in. Pb) 

Variable. Reasonably consistent 

good detectability. 

Ill-defined. 130 kV too low to resolve inner wall 

indicating poorer penetration at 

lower kV. 

Number of Frame 

Averages 

(1, 3, 5, 9, 15) 

Increase in SNR 

with increase in 

number of frame 

averages. 

Hole more easily 

resolved with increase 

in frame averaging. 

Wall not defined at 70 kV regardless 

of number of frame averages. Based 

on results for outer wall, inner wall 

definition likely would increase with 

increasing number of frame 

averages for CT scans run at 

190 kV. 

Outer wall more well-defined with 

increase in number of frame 

averages. 

Number of Projection 

Views 

(9, 18, 36, 90, 180, 360, 

720, 1080, 1440, 1800) 

Increase in SNR 

with increase in 

number of 

projection views. 

Hole less detectable 

below 360 projection 

views. 

Wall not defined at 70 kV regardless 

of number of projection views. 

Based on results for outer wall, inner 

wall definition likely would increase 

with increasing number of frame 

averages for CT scans run at 

190 kV. 

Outer wall more well-defined with 

increase in number of projection 

views. 

Beam Width and 

Effective Object Radius

via variation of SO 

distance 

Variable. Best detectability at 

smaller beam width and 

larger effective radius. 

Best wall definition at smaller beam 

width and larger effective radius. 

Appears to be an optimal SO 

distance to achieve best SNR. 

Features more pixelated with 

increase in SO distance. 

Binning 

 (1x1, 2x2, 4x4) 

Increase in SNR 

with increase in 

bin size. 

Poorer detectability 

with increase in bin 

size. 

Poorer definition with increase in 

bin size. 

Features more pixelated with 

increase binning. 

Sample Orientation / tilt 

(0q, 3q) 

Variable. Consistently good 

detectability over this 

small tilt range. 

Wall not defined at 70 kV. Inner 

wall sharpness likely would be 

affected by tilt angles over range 0q

to 45q for CT scans run at 190 kV. 

Would not expect variation unless 

tilt significantly changed. 

Angle Range of 

Acquisition (360q, 

300q, 270q, 240q, 210q, 

180q) 

Highly variable. Variable detectability, 

not always in 

predictable fashion. 

Variable definition but mostly 

resolved at all angular ranges of 

acquisition. 

Highly variable SNR within images 

< 360q angular range of acquisition. 

Results not predictable. 

Directional versus 

Transmission Tube 

(Varying Voltage) 

Variable for both 

and not welldefined trend. 

Reasonably consistent 

good detectability. 

Increase in wall sharpness with 

increase in voltage for both tube 

types. 

The inner wall appears to be more 

highly resolved at most voltages for 

the reflection tube versus 

transmission tube. This may be due 

to a higher respective photon flux at 

detector during the reflection tube 

experiments. 

Current 

(80, 120, 160 μA) 

Increase in SNR 

with increase in 

current. 

Lowest current gave 

best visibility of hole. 

Increase in wall sharpness with 

increase in current. 

Would have expected hole 

detectability results to mimic those 

of integration time. 

Integration Time 

(2, 3, 4 fps) 

Increase in SNR 

with increase in 

integration time. 

Increase in detectability 

with increase in 

integration time. 

Increase in wall sharpness with 

increase in integration time. 

Figure of merit 

Variable 
NASA/TM—2014-218332 33 

TABLE 29.—PICTORIAL SUMMARY OF STUDY RESULTS FOR HOLE DETECTABILITY 

Parameter 

Filter 

(V = 170 kV) 

0.16 in. Cu 0.24 in. Steel 0.028 in. Pb 

Voltage, kV 

(0.16 in. Cu filter) 

210 170 130 

Tube Type 

(V = 170 kV, 0.16 in. Cu 

filter) 

Transmission Reflection 

 

Number of frame 

averages (V = 70 kV, 

0.010 in. Cu filter) 

1 3 9 15 

Number of projection 

views (V = 70 kV, 0.010 

in. Cu filter) 

36 180 1080 3600 

Binning 

(V = 70 kV, 0.010 in. Cu 

filter) 

1x1 2x2 4x4 

 
NASA/TM—2014-218332 34 

TABLE 26.—CONCLUDED 

Parameter 

Angular range of scan 

(V = 190 kV, 0.16 in. Cu 

filter) 

360° 300° 240° 180° 

Beam width 

(V = 190 kV, 0.16 in. Cu 

filter) 

Smallest Larger Largest 

Current (μA) (V = 170 

kV, 0.16 in. Cu Filter) 

80 120 160 

Integration time, fps 

(V = 190 kV, 0.16 in. Cu 

filter) 

2 3 4 

 
NASA/TM—2014-218332 35 

TABLE 30.—PICTORIAL SUMMARY OF STUDY RESULTS FOR INNER WALL SHARPNESS 

[Note: Outer wall shown where inner wall was not resolved.] 

Parameter 

Filter 

(V = 170 kV) 

0.16 in. Cu 0.24 in. Steel 0.028 in. Pb 

Voltage, kV 

(0.16 in. Cu Filter) 

210 170 130 

Tube type 

(V = 170 kV, 0.16 in. Cu filter) 

Transmission Reflection 

 

Number of frame averages 

(V = 70 kV, 0.010 in. Cu Filter) 

1 3 9 15 

Number of projection views 

(V = 70 kV, 0.010 in. Cu Filter) 

36 180 1080 3600 

 
NASA/TM—2014-218332 36 

TABLE 31.—CONCLUDED 

Parameter 

Binning 

(V = 70 kV, 0.010 in. Cu filter) 

1x1 2x2 4x4 

Angular range of scan 

(V = 190 kV, 0.16 in. Cu Filter) 

360° 300° 240° 180° 

Beam width via SO distance 

change 

(V = 190 kV, 0.16 in. Cu Filter) 

Smallest Larger Largest 

Current, μA 

(V = 170 kV, 0.16 in. Cu Filter) 

80 120 160 

Integration time, fps 

(V = 190 kV, 0.16 in. Cu Filter) 

2 3 4 
NASA/TM—2014-218332 37 

References 

1. D. J. Roth, R.W. Rauser, R. R. Bowman, R.E. Martin, A. Koshti, David S. Morgan, “The Critical 

Role Of High Resolution X-Ray Micro-Computed Tomography For Ultra-Thin Wall Space 

Component Characterization,” Materials Evaluation, March 2014, pp. 

2. Richard E. Martin, Don J. Roth, Jon Salem, Application of Advanced Nondestructive Evaluation 

Techniques for Cylindrical Composite Test Samples, NASA TM-. December, 2013. 

3. Ali Abdul-Aziz, D.J. Roth, R. Cotton, George F. Studor, Eric Christiansen, and P.C. Young; 

“Composite Material Characterization using Microfocus X-ray Computed Tomography Image-based 

Finite Element Modeling”, Materials Evaluation, pp. 167-174, Volume 71, No.2, February 2013 

4. Maire, E. and Withers, P.J., “Quantitative X-ray Tomography,” International Materials Reviews, Vol. 

59, No. 1, pp. 1 – 43, 2014. 

5. Howard, P., Portaz, J., Janning, J., Barthelemy, P. and Fauverte, F., “Testing Techniques for High 

Volume Production Applications,” Materials Evaluation, Vol. 68, No. 4, pp. 400–406, April 2010. 

6. Barret, H.H. and Swindell, W., Radiological Imaging: The Theory of Image Formation, Detection, 

and Processing. Vols. 1 and 2. New York, NY: Academic Press (1981). 

7. Nondestructive Testing Handbook, Radiographic Testing. Ed. Bossi, R.H., Iddlings, F.A., Wheeler, 

G.C., and Moore, P.O. Third Ed. Vol. 4. American Society of Nondestructive Testing. pp. 141-142, 

313, 316-317, 328 – 338. 2002. 

8. MIL-HDBK-1823, Department of Defense Handbook: Nondestructive Evaluation (NDE) System, 

Reliability Assessment (30 Apr. 1999). 

9. Feldkamp, L.A., Davis, L.C., and Kress, J.W., “Practical cone-beam algorithm,” J. Opt. Soc. Amer. 

A6 (1984), 612 – 619.