Validation of Voxman Monte Carlo code and calibration for ... - DiVA

Institutionen för medicin och vård Avdelningen för radiofysik Hälsouniversitetet

Validation of Voxman Monte Carlo code and calibration for digital systems

Gustav Ullman, Michael Sandborg and Gudrun Alm Carlsson

Department of Medicine and Care Radio Physics Faculty of Health Sciences

Series: Report / Institutionen för radiologi, Universitetet i Linköping; 95 ISRN: LIU-RAD-R-095 Publishing year: 2003

© The Author(s)

Report 95 Dec. 2003

ISRN ULI-RAD-R--95--SE

Validation of Voxman Monte Carlo code and calibration for digital systems. G Ullman, M Sandborg, and G Alm Carlsson Department of Radiation Physics, Linköping University

Full addresses: Department of Radiation Physics, IMV, Faculty of Health Sciences, Linköping University, SE-581 85 LINKÖPING, Sweden

1

1. Objective The objective of this work was to test the Monte Carlo model ‘Voxman’ against measurements on x-ray systems in the clinic. X-ray transmission experiments are performed to test of the accuracy of the Monte Carlo photon transport. Experiments were also performed with an image plate (CR) system in the clinic to compare the measured pixel values with calculated pixel values. Measurements were also performed with the automatic exposure control (AEC) chambers used in Linköping and Motala. The purpose for those measurements was to choose a normalisation of the entrance surface dose.

2. Materials and Methods 2.1 Computer model The Monte Carlo model (M. Sandborg et al.) simulates photon transport through tissue, grid and detector. The program is used for calculating quantities of two types; 1: quantities associated image quality, 2: quantities concerning patient dose. These two quantities can then be used for comparison of the performance of different imaging systems. This report concerns applications of the Voxman code for digital systems. Digital systems differ from analogue systems in the way that the image is made of pixel values on a monitor instead of variations in optical density in the film. In the former work with the Voxman code for analogue systems, the optical density was used for normalising the incident air kerma. In digital systems, this approach cannot be used, since it is possible to select different contrast windows when displaying the image on the monitor, so the pixel values are not fixed. The method used here is instead to normalise according to air kerma in the AEC chambers. Since the model is very sensitive to inertial conditions, uncertainties in the measured input parameters easily introduce significant errors. Therefore great care has to be taken to measure the experimental set with great precision, as well as care has to be taken to match the input data with the measured experimental ones. One very important thing is to check that the HVL of the spectra input file matches the measured HVL. The spectra were acquired by using a spectra-generating program (Birch et al).

2.2 Transmission experiments The transmission experiments were performed in the x-ray lab at the Radiation physics department. The X-rays were transmitted through Plexiglas slabs of different thicknesses. The air kerma was measured at the slab entrance as well as at the slab exit with the instrument Solidose 300 with the R 100 detector (RTI Mölndal Sweden). The fraction of the two measured air kermas (transmission) was compared with the corresponding fraction obtained from the Monte Carlo calculations. Three different

2

total slab thicknesses were used: 14.775 cm, 9.85 cm and 4.925 cm. Also three different tube voltages were used: 100 kV, 80 kV and 60 kV. The experiments were made both with and without a grid. 2.21 Geometries and data of the lab at radiation physics department Below is a description of x-ray lab 3 at the Radiation physics lab describing 1: system 2: tube voltages 3: HVL 4: filtration 5: focus detector distance 6: grid characteristics 7: detector characteristics and 8 slab dimensions.

1 A non-commercial system is used in the radio physics lab. 2 The experiments were performed at the tube voltages: 100 kV, 80 kV and 60 kV. 3 For these tube voltages the HVL was measured as: 3.00 mm Al for 100 kV, 2.65 mm Al for 80 kV and 2.02 mm Al for 60 kV. 4 This corresponds to total filtration, inner: 100 kV 1.5 mm Al, 80 kV 2.1 mm Al, 60 kV 2.0 mm Al, no added filtration. 5 The focus detector distance was 105 cm. 6 The grid characteristics: ratio 12, 70 cm-1, 43 µm lead strip thickness, interspace width 100 µm and 0.25 mm top and bottom cover thickness. Aluminium interspace and covers. 7 No detector was used. 8 The Plexiglas slab dimensions are: area 25x30 cm2, thickness 4.925 cm each.

2.3 Measurements and calculation of pixel values The experiments were performed in x-ray room 3 at the Radiology department with a Philips PCR 5000 system using a BaFBr image plate. The air kerma was measured with Solidose 300. 2.31 Relation between incident air kerma and pixel values The X-rays were transmitted through a 20 mm aluminium slab placed directly at the X-ray tube collimator. The air kerma was measured at the CR plate both with and without the aluminium slab. Photon transport for this geometry is simulated in the Monte Carlo program and the relation between energy imparted and entrance air kerma is calculated. This relation is used to determine the energy imparted in the detector. The S and L values were fixed to S=200 L=2.00. The pixel values were derived from energy imparted with the relation PV = PVmax − A ⋅ log( Eim ) + B

(1)

Where the constants A and B are estimated from the experiments with the aluminium slab. Eim is the energy imparted, PV and PVmax are the pixel value and the maximum pixel value: in this case, PVmax=1023.

3

2.32 Validation The relation between incident air kerma and pixel value was validated with a serie of Plexiglas slab experiments. Three different total slab thicknesses were used: 19.7 cm, 14.775 cm and 9.85 cm.

2.33 Geometries and data of lab 3 and the lab at radiation physics department Below is a description of x-ray room 3 at the Radiology department describing 1: system 2: tube voltages 3: HVL 4: filtration 5: focus detector distance 6: grid characteristics 7: detector characteristics and 8: slab dimensions. 1 2 3 4

X-ray room 3 uses the Philips PCR 5000 system. The tube voltages used were 60, 70, 81, 90 and 102 kV For these tube voltages the HVL was measured according to table 1 below The total filtration was achieved by testing which filtration in the Birch spectragenerating program that results in a calculated HVL that corresponds with the measured HVL. The resulting filtrations are given in table 1. Tube voltage kV 60 70 81 90 102

HVL mm Al 2.96 3.29 3.89 4.31 4.77

Filtration mmAl 4.7 4.3 4.3 4.3 4.0

Table 1. Showing measured HVL and filtration 5 The focus detector distance was 105 cm. 6 The grid characteristics: ratio 12, 70 cm-1, 43 µm lead strip width, interspace width 100 µm and 0.25 mm top and bottom cover thickness. Aluminium interspace and covers. 7 The detector consisted of BaFBr with surface density 100 mg/cm2. The image plate was covered with a layer of 2 mm carbon fibre 8 The Plexiglas slab dimensions are: area 25x30 cm2, thickness 4.925 cm each.

2.4 Measurements on AEC chambers in Motala and Linköping The automatic exposure control terminates the exposure for a specified air kerma to the AEC chambers. In order to simulate a specific system, knowledge of this air kerma can be used to normalize the incident air kerma to a realistic level; this is since the incident air kerma at the slab entrance is proportional to the air kerma in the AEC chambers for a given x-ray spectra and imaging geometry. This relation can be formulated as: K AEC = rK in

(2)

4

Where KAEC is the air kerma at the AEC chambers, Kin is the air kerma incident to the Plexiglas phantom and r is the transmission. To determine the KAEC in Linköping and in Motala, PMMA slab experiments were performed. The air kerma Kin was measured with the instrument r100 at the slab entrance. 2.41 Geometries and data of room 15 in Linköping and room 3 in Motala Below is a description of x-ray rooms in Linköping and Motala defining 1: system, 2: tube voltages, 3: HVL, 4: filtration, 5: focus detector distance, 6: grid characteristics, 7: detector characteristics and 8: slab dimensions. Linköping 1 2 3 4 5 6

7 8

The lab in Linköping uses the Siemens Vertix FD system. The tube voltages used in Linköping were 100, 133 and 150 kV. The HVL for 100 kV was measured to 4.36 mm Al without additional filtration. This corresponds to total filtration, inner: 3.4 mm Al, added: 0.3 mm Cu. The focus detector distance was 180 cm The grid used in this system is a LD grid with characteristics: ratio 15, strip density 80 cm-1, interspace width 0.105 mm, cover thickness 0.37 mm. The interspace and cover material was carbon fibre. The detector consisted of CsI with surface density 190 mg/cm2. The slab dimensions were approximately the same for all slabs: area 25x30 cm2, thickness 4.925.

Motala 1 2 3 4 5 6

7 8

The lab in Motala uses the Philips PCR 5000 system. The tube voltages used in Motala were 100, 133 and 145 kV. The HVL for 100 kV was measured to 6.79 mm Al with filtration 1 mm Al plus 0.1 mm Cu. This corresponds to total filtration, inner: 4.9 mm Al, added: 1 mm Al plus 0.1 mm Cu. The focus detector distance was 150 cm. The grid used in this system is a Lysholm grid with characteristics: ratio 12, strip density 70 lp/cm, 43 mm lamella 100 µm interspace width, top and bottom cover thickness 0.2 mm. The interspace and cover material was aluminium. The detector consisted of BaFBr with surface density 100 mg/cm2. The slab dimensions varied slightly for the different slabs. Although the area was the same, 25x30 cm2, the slabs had different thicknesses. In order to separate the slabs of different thickness, the slabs were numbered 1-4. The thicknesses were: slab 1: 4.430 cm, slab 2: 5.195 cm, slab 3: 5.135, and slab 4: 5.450 cm. In the experiment, the 10 cm slab was the combination of slabs 3+4, totally 10.585 cm. The 20 cm slab was made of all four slabs, totally 20.21 cm.

5

Cassette holder The cassette holder in Motala had inner structures that has to be known in order to make a accurate simulation of the system. The cassette holder consisted of 6 different parts according to figure 1. 1 2 3 4 5 6

Protection layer, PMMA 3 mm Air 2 mm AEC chamber 8 mm Air 8 mm Grid 2 mm Tree 2 mm

12 3 4 56

Figure 1: Inner structures of the cassette holder in Motala

2.5 Error analysis When an input parameter to the Monte Carlo code is changed slightly, this can make a significant difference in the resulting data. Since the Monte Carlo code is depending on many input variables, this has the consequence that the uncertainty in the calculations not primarily is due to the precision of the code itself, but rather the uncertainty from estimating the input parameters. In order to test this uncertainty, the codes sensitivity on the input files for the four different parts on the image chain was tested: spectra, slab, grid and detector. Also uncertainties in the geometry were considered, as uncertainty in focus detector distance and the field size. The five file types tested were: the files for geometry, spectra, material, grid and detector. Each of these files was reproduced in three different versions: file+, the file that would result in the highest transmission, file0 the expected transmission and file- the lowest transmission. The files were defined by the variations: geom: FFD 104 +/- 2 cm, field size 195 +/- 3 mm (square) spect: tube voltage 81 +/- 2 kV, filter 4.3 +/- 2 mm Al mat: density 1.19 +/- 0.02 g/cm3 grid: lamella 43 +/- 7 µm, cassette front 0.12+/- 0.02 cm det: surface density 100 +/- 10 mg/cm2.

6

2.6 Validation against earlier versions The code that is used which is the version 2.04 was tested against an earlier version (1.2). If the versions give the same results, the validation experiments performed for the earlier versions also applies to this version of the code.

3. Results 3.1 Transmission experiments The measured and calculated transmissions are displayed in table 2 and 3.

Tube voltage (kV) 60 80 100 60 80 100 60 80 100

Thickness (cm) measured transmission Error +/- 1% 15 0.0032 15 0.0060 15 0.0094 10 0.0140 10 0.0220 10 0.0310 5 0.0600 5 0.0810 5 0.1000

calculated transmission Error +/- 10% 0.0033 0.0063 0.0095 0.0139 0.0231 0.0300 0.0650 0.0912 0.1040

difference % 1.56% 5.00% 1.06% -0.71% 5.00% -3.23% 8.33% 12.59% 4.00%

Table 2. Showing the measured and calculated transmissions from the experiments with grid.

Tube voltage (kV)

Thickness (cm)

60 80 100 60 80 100 60 80 100

15 15 15 10 10 10 5 5 5

measured transmission Error +/-1% 0.021 0.033 0.045 0.072 0.100 0.124 0.238 0.284 0.319

calculated transmission Error +/-10% 0.020 0.035 0.043 0.068 0.102 0.125 0.229 0.302 0.316

Table 3. Showing the measured and calculated transmissions from the experiments without grid.

7

difference % -4.76% 6.06% -4.44% -5.56% 2.00% 0.81% -3.78% 6.34% -0.94%

3.2 Relation between incident air kerma and pixel values The results for the aluminium slab calibration experiments are displayed in table 4 and figure 1. mAs

K CR al (uGy) K ent (uGy) F

Enimp

0.5 1 2 3.2 4 6.3 8 12.5 16 20

0.993 2.41 5.59 9.22 11.75 18.95 24.25 38.22 49.33 61.84

161.3 337.2 693.7 1134.0 1415.2 2219.0 2855.8 4421.9 5739.9 7126.8

205.5 429.5 883.7 1444.5 1802.7 2826.6 3637.8 5632.7 7311.6 9078.2

0.78504 0.78504 0.78504 0.78504 0.78504 0.78504 0.78504 0.78504 0.78504 0.78504

PV meas PV calc Err 4% Err 10% 884 852 683 677 503 505 384 388 333 335 223 228 173 168 64 64 7 2 0 0

diff 32 6 -2 -4 -2 -5 5 0 5 0

Table 4. Displaying mAs, air kerma at CR, entrance air kerma, Monte Carlo calculated conversion factor from entrance air kerma to energy imparted, energy imparted, measured pixel value, calculated pixel value, difference between measured and calculated pixel value. 1000 900

Calculated Pixel Value

800 700 600 500 400 300 200 100 0 0

100 200 300 400 500 600 700 800 900 1000 Measured Pixel Value

Figure 2. Measured pixel value to calculated pixel value for the aluminium slab calibration.

8

3.3 Measurements and calculation of pixel values The measured and calculated pixel values are displayed in table 5 as well as in figures 1, 2 and 3. Thickness kV

K entr (uGy) F

20 20 20 20 20 15 15 15 15 15 10 10 10 10 10

2617 1906 1679 1187 1117 908 664 466 464 386 251 183 163 142 117

60 70 81 90 102 60 70 81 90 102 60 70 81 90 102

PV meas Err 4% 661 632 582 614 582 613 597 603 564 570 595 599 574 580 600

PV calc Err 5% 660 632 574 594 563 608 588 596 549 553 602 591 564 553 567

diff 1 0 8 20 19 5 9 7 15 17 -7 8 10 27 33

Table 5. Displaying slab thickness, tube voltage, entrance air kema, conversion factor, measured pixel value, calculated pixel value, and the difference between them. Measured Calculated

700

600

Pixel Value

500

400

300

200

100

0

60

70

81

90

102

Measured

595

599

574

580

600

Calculated

602

591

564

553

567

Tube Voltage (kV)

Figure 3. Measured and calculated pixel values for a 10 cm Plexiglas slab.

9

Measured Calculated

700

600

Pixel Value

500

400

300

200

100

0

60

70

81

90

102

Measured

613

597

603

564

570

Calculated

608

588

596

549

553

Tube Voltage (kV)

Figure 4. Measured and calculated pixel values for a 15 cm Plexiglas slab.

Measured Calculated

800 700 600

Pixel Value

500 400 300 200 100 0

60

70

81

90

102

Measured

661

632

582

614

582

Calculated

660

632

574

594

563

Tube Voltage (kV)

Figure 5. Measured and calculated pixel values for a 20 cm Plexiglas slab.

10

3.4 Measurements on AEC chambers in Linköping and Motala The measured air kerma Motala Linköping lab 15

Air kerma at det. (µGy)

7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 90

110

130

150

170

Tube voltage (kV) Figure 6. Calculated air kerma at the AEC chambers in Linköping

Air kerma at det. (µGy)

7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 90

110

130

150

Tube Voltage (kV) Figure 7. Calculated air kerma at the AEC chambers in Motala

11

Linköping Tube voltage Slab thickness Energy imparted (cm) (kV) (µJ/m2) 100 100 133 133 150 150 Mean

10 20 10 20 10 20

1037 942 875 767 776 627 837

In Air Kerma at detector (µGy)

Entrance air kerma (µGy)

New normalised kerma (µGy)

E/K

6.09 5.49 5.67 5.15 5.48 4.56

65.71 530.4 52.02 354.6 45.1 284.5

58.34 522.35 49.60 372.27 44.50 337.32

170.28 171.58 154.32 148.93 141.61 137.50

5.41

Motala Tube voltage Slab thickness Energy imparted (cm) (kV) (µJ/m2) 100 100 133 133 145 145 Mean

10 20 10 20 10 20

538 494 460 431 431 382 456

In Air Kerma at det entr air kerma (µGy) (µGy)

new norm kerma (µGy)

5.95 5.58 5.77 5.65 5.75 5.34

105.08 929.29 83.78 598.16 76.65 548.10

110.2 914 85.21 595.7 77.69 515.9

5.67

Table 6. Displaying the values of the calculated air kerma and their mean values. 3.41 Mean values for normalisation The mean value on the air kerma incident to the detector was 5.41 in Linköping and 5.67 in Motala. Taking only the means for 150 and 145 kV, these mean values are 5.02 in Linköping and 5.55 in Motala.

12

90.42 88.53 79.72 76.28 74.96 71.54

3.5 Sensitivity test The results from the sensitivity test are given in table 7 and 8. file geomgeom0 geom+ spectspect0 spect+ matmat0 mat+ gridgrid0 grid+ detdet0 det+ ∆geom ∆spect ∆mat ∆grid ∆det ∆tot

transmission 0.0095 0.0097 0.0096 0.0089 0.0097 0.0103 0.0091 0.0096 0.0101 0.0090 0.0097 0.0103 0.0096 0.0097 0.0096

% -2.11 0.00 -1.04 -8.99 0.00 5.83 -5.49 0.00 4.95 -7.78 0.00 5.83 -1.04 0.00 -1.04 1.57 7.41 5.22 6.80 1.04 11.49

AK behind grid 4.43 4.51 4.49 4.14 4.51 4.82 4.25 4.51 4.73 4.19 4.51 4.79 4.46 4.51 4.48

% -1.81 0.00 -0.45 -8.94 0.00 6.43 -6.12 0.00 4.65 -7.64 0.00 5.85 -1.12 0.00 -0.67 1.13 7.68 5.38 6.74 0.90 11.64

Energy imparted 396 401 404 369 401 432 379 401 422 376 401 432 369 401 429

% -1.26 0.00 0.74 -8.67 0.00 7.18 -5.80 0.00 4.98 -6.65 0.00 7.18 -8.67 0.00 6.53 1.00 7.92 5.39 6.91 7.60 14.08

Table 7 Displaying the results from the sensitivity test file geomgeom0 geom+ spectspect0 spect+ matmat0 mat+ gridgrid0 grid+ detdet0 det+ ∆geom ∆spect ∆mat ∆grid ∆det ∆tot

PV 638.42 635.44 633.66 655.23 635.44 617.71 648.87 635.44 623.29 650.76 635.44 617.71 655.23 635.44 619.37

0.47 0.00 -0.28 3.02 0.00 -2.87 2.07 0.00 -1.95 2.35 0.00 -2.87 3.02 0.00 -2.59 0.37 2.94 2.01 2.61 2.81 5.25

Norm incident AK 526 515 521 562 515 485 549 521 495 556 515 485 521 515 521

% 2.06 0.00 1.03 8.25 0.00 -6.19 5.21 0.00 -5.21 7.22 0.00 -6.19 1.03 0.00 1.03 1.55 7.22 5.21 6.70 1.03 11.29

SNR 11.78 10.75 11.10 11.81 10.75 11.31 10.47 10.75 12.56 12.11 10.75 10.84 12.36 10.75 11.31

Table 8 Displaying the results from the sensitivity test

13

% 8.74 0.00 3.15 8.98 0.00 4.95 -2.67 0.00 14.41 11.23 0.00 0.83 13.03 0.00 4.95 5.95 6.96 8.54 6.03 8.99 16.55

C 0.1719 0.1715 0.1717 0.1784 0.1715 0.1654 0.1704 0.1715 0.1731 0.1734 0.1715 0.1681 0.1720 0.1715 0.1718

% 0.23 0.00 0.12 3.87 0.00 -3.69 -0.65 0.00 0.92 1.10 0.00 -2.02 0.29 0.00 0.17 0.17 3.78 0.78 1.56 0.23 4.17

3.6 Test against earlier versions The simulations for the version 2.04 gave the same results as the version 1.2

4. Discussion 4.1 Sensitivity test The results from the sensitivity test show how entities like transmission and air kerma depend on variations in the input parameters. The results show that an error approximately 11% is due to the uncertainty in the input parameters. Since the precision in the calculation itself often is 1-3% and the precision of the measured input parameters is many times higher, it is hard to make a good validation of the program this way. In order to develop the code to correspond as good as possible to the clinical situation, the sources for possible errors should preferable lie mainly in the calculations rather than the input parameters. With large uncertainties in the input parameters, only errors of the same magnitude as those of the input parameters can be discovered. Smaller errors, i.e. about 1% will remain undetected. If the error in the estimation of input parameters can be as assumed to be random, a great number of experiments should make a much better validation of the program. Many such validations have been made throughout the years with satisfying results. In this sense the code has been well validated. A great number of experiments that are successfully predicted by the model give a greater reason to trust the model.

4.2 Transmission experiments At the x-ray room at the Radiation physics, 95% of the measured transmissions are inside estimated deviation of 11%. Even though the transmission experiments are very simple, it is hard to estimate all input parameters exact also in this case. Many input parameters are needed, such as cross-section data, lab geometry, slab density and grid characteristics. The transmission experiments have validated that the Monte Carlo code itself does not produce errors larger than 11%.

4.3 Pixel The pixel value experiments at room 3 are also contributes to the validation of the program. For these experiments, 93% of the measured pixel values are within the estimated deviation of 5.25%, and 100% are within a deviation of 6%.

14

4.4 AEC chambers The experiments in Linköping and Motala on the AEC chambers can be used for normalisation of incident air kerma. The mean value of air kerma in Linköping for 150 kV is well in accordance with the sensitivity of 200.

4.5 Version Since the results from the different versions agree, also the validation experiments for the earlier versions apply for this version (2.02) (G McVey et al., Dance et al.).

5. Summary Three experiments were made: 1: transmission experiments to test the Monte Carlo photon transport, 2: measurements and calculations of pixel values to further test for CR systems, 3: measurements on the AEC chambers in Linköping and Motala to determine a calibration suitable for digital systems. The Monte Carlo model produces results that are in agreement with the measured results inside the estimated uncertainties.

6. Acknowledgements I want to thank Jahlil Bahar for helping me with the measurements in Motala and Linköping. I also want to thank Jan Fahlgren in Motala for measuring dimensions on the chest stand in Motala.

7. References R. Birch, B. Marshall and G. M. Ardran. Catalogue of spectral data for diagnostic X-rays, The Hospital Physicists' Association, Scientific Report Series 30, 47 Belgrave Square (London, 1979). M. J. Berger and J. H. Hubbell. XCOM: Photon Cross Section on a Personal Computer, NBSIR 873597, U.S. Department of Commerce, National Bureau of Standards, Office of Standard Reference Data, Gaithersburg MD 20899 (1987). David Dance, Graham McVey, Michael Sandborg, Gudrun Alm Carlsson, Jan Persliden Calibration and validation of a voxel phantom for use in the Monte Carlo modeling and optimization of X-ray imaging systems (SPIE Medical Imaging San Diego Feb 1999) G McVey, M Sandborg, D R Dance, J Persliden, G Alm Carlsson, Calibration and validation of the voxel Monte Carlo code from slab phantom measurements, Report RMT97/1001 http://www.radiusgroup.org/publications/RMT971001.pd RTI Mölndal, Göteborgsv 97 / 50, SE-431 37 MÖLNDAL, Sweden

15

Sandborg M, McVey G, Dance D RA Voxel Phantom Based Monte Carlo Computer Program for Optimization of Chest and Lumbar Spine X-ray imaging systems, Radiation Protection Dosimetry vol 90 Nos. 1-2 2000. M. Sandborg, G. McVey, D.R. Dance and G. Alm Carlsson. Schemes for the optimization of chest radiography using a computer model of the patient and x-ray imaging system. Medical Physics 28 2007-2019, 2001. Sandborg M, Dance D R, Alm Carlsson G and Persliden J Selection of antiscatter grids for different imaging tasks: the advantage of low atomic number cover and interspace materials. Brit J Radiol 66, 1151-1163, 1993. Michael Sandborg, David R Dance and Gudrun Alm Carlsson. Calculation of contrast and signal-tonoise degradation factors for digital detectors in chest and breast imaging ISRN ULI-RAD-R-93—SE, 2003. I. G. Zubal, C. R. Harrell, E. O. Smith, Z. Rattner, G. Gindi and P. B. Hoffer. Computerized threedimensional segmented human anatomy, Med Phys. 299-302, 1994.

16